Latest revision as of 14:04, 23 April 2019

1 749.1 General
2 749.2 Design Frequency and Return Period
3 749.3 Design Data
4 749.4 Rural vs. Urban Hydrology
5 749.5 The Rational Method
6 749.6 USGS Regression Equations
- 6.1 749.6.1 Rural Regression Equations
  - 6.1.1 749.6.1.1 2014 Rural Regression Equations
  - 6.1.2 749.6.1.2 1995 Rural Regression Equations
- 6.2 749.6.2 Urban Regression Equations
7 749.7 Historical USGS Stream Gage Data
- 7.1 749.7.1 Analysis
- 7.2 749.7.2 Ungaged Sites Near Stream Gage
8 749.8 NFIP Flood Insurance Study Discharges
9 749.9 Flood Hydrographs
- 9.1 749.9.1 NRCS Unit Hydrograph
10 749.10 Detention Storage
- 10.1 749.10.1 Data Needs
  - 10.1.1 749.10.1.1 Stage-Storage Curve
  - 10.1.2 749.10.1.2 Stage-Discharge Curve
- 10.2 749.10.2 Routing Calculations
11 749.11 Stream Confluences
12 749.12 References

749.1 General

Hydrology is that branch of the applied earth sciences which deals with the occurrence and distribution of surface and ground waters. Highway drainage design is concerned with the surface water hydrology of small watersheds. In particular, the peak rate of runoff and/or the runoff hydrograph produced by flood events are of interest in analysis of highway drainage problems. Peak runoff rates are a parameter of design and their estimation is a prerequisite to hydraulic computations. Flood hydrographs are a necessary parameter of stormwater detention basin design, which can be used to reduce peak runoff rates for design of certain highway drainage structures.

In general, the design of drainage facilities may be described as follows:

Select the design frequency
Estimate the peak rate of runoff or runoff hydrograph resulting from the design and review floods
Design the drainage structure to pass the design flood peak or runoff hydrograph
Check the hydraulic capacity of the proposed system for the base flood
Check that the final design meets the requirements of the National Flood Insurance Program (NFIP)

Criteria for determination of the design flood frequency and methods for computing peak rates of runoff and runoff hydrographs are presented in this article.

749.2 Design Frequency and Return Period

The frequency of occurrence or return period of an event may be defined as the average period of time between events equal to or greater than a given magnitude. The annual probability of occurrence of an event is equal to the reciprocal of the event's return period. For example, a flood with a return period of 100 years has a 1% chance of occurring in any year; whereas a flood with a return period of 25 years has a 4% chance of occurring in any year.

749.2.1 Design Frequency Criteria

The return period used as a criterion of design is known as the design frequency. The design frequency to be used in the design of drainage structures is a function of roadway type (major or minor), traffic volume and type of drainage facility.

749.2.1.1 Crossroad Structures

The design flood magnitude, design frequencies and corresponding water surface elevation(s) shall be included on the project plans for all crossroad structures.

749.2.1.2 Roadside Ditches and Culverts

The design frequency for roadside drainage ditches should be selected based on the function served. Ditches with a primary function of removal of water from the pavement should be based on the design frequency of the pavement drainage. Ditches with a primary function of carrying water to or from a crossroad structure should be based on the design frequency of the crossroad structure.

Culverts not considered crossroad structures should be designed to the same frequency as the ditch.

749.2.1.3 Pavement Drainage

See EPG 640.1.2.1 Design Frequency.

749.2.1.4 Median Drainage

See EPG 640.2.3 Design Discharge.

749.2.2 Base Flood

The base flood is defined as the 100-year flood or the flood with a 1% chance of being exceeded in any given year.

After sizing a drainage facility using the design flood criteria, the ability of the proposed facility to pass the base (100-yr) flood shall also be evaluated, with special attention to any risks to people or property. This is done to ensure that there are no unexpected flood hazards inherent in the proposed facility for flood events exceeding the design discharge.

749.2.3 Overtopping Flood

The overtopping flood is defined as the discharge and corresponding return period and water surface elevation at which flow occurs over the roadway. The overtopping flood shall be determined for all crossroad structures. The resulting overtopping flood discharge and approximate return period shall be included on the project plans. When the overtopping flood is greater than the 100-year flood, the return period should be noted as “>100 yr”. If the overtopping flood is less than the 100-year flood, the magnitude and water surface elevation of the 100-year flood shall also be included on the project plans. For reporting overtopping for bridges, refer to EPG 750.3.2.4.5 Overtopping Discharge and Frequency.

749.2.4 National Flood Insurance Program Criteria

Meeting the requirements of the National Flood Insurance Program (NFIP) regulations can necessitate larger structures than those designed to meet the above design frequencies. The designer shall ensure that the final design meets all requirements of the NFIP. Federal Emergency Management Agency (FEMA) Flood Insurance Study (FIS) maps should be consulted to determine the applicable NFIP requirements.

See the following for definitions and additional information on the NFIP:

Floodplain Management and the Regulatory Floodway for roadway construction
National Flood Insurance Program (NFIP) for structures

749.3 Design Data

In order to carry out the hydrologic analysis of a watershed, it is necessary to assemble certain data. This data includes the drainage area, the length of the hydraulically longest drainage path, the elevation of the watershed ridge, the elevation of the watershed outlet, the hydrologic soil group, the type of terrain, the land use of the watershed, and information on the extent of development in urban areas. The topographic data should be obtained from suitable maps when maps are available. Suitable maps are defined as department manuscripts and 7-1/2 or 15 minute USGS topographic maps or maps of equal quality. Only when such maps are not available should field measurements be made for the purpose of obtaining hydrologic information. The land use and terrain of the watershed should be evaluated in the field by the designer. The hydrologic soil group is either given on the soil survey performed by the Materials Division or is to be determined from county soil survey maps or from information from the soil survey and site visits.

749.4 Rural vs. Urban Hydrology

Small watersheds may be divided into rural and urban classifications.

Three methods are presented in this article for estimating peak rates of runoff from small watersheds. The Rational Method may be used on all watersheds smaller than 200 acres. On watersheds within the area limits shown in the table below, United States Geological Survey (USGS) Regression Equations may be used. The Natural Resource Conservation Service (NRCS) Unit Hydrograph may also be used for drainage areas larger than 6.40 acres.

For more information on watershed area limits for the above methods see Summary of Small Watershed Peak Runoff Calculation Method Drainage Area Limits table.

Summary of Small Watershed Peak Runoff Calculation Method Drainage Area Limits

Calculation Method				Minimum Drainage Area	Maximum Drainage Area
Rational Method				0	200 acres
NRCS Unit Hydrograph				6.4 acres	16,000 acres
USGS Regression Equations	Rural	2014	Region I	70.4 acres	>10,000 acres
			Region II	108.8 acres	>10,000 acres
			Region III	1,356.8 acres	>10,000 acres
		1995	Region III	307.2 acres	>10,000 acres
	Urban	2010		180 acres	>10,000 acres

749.4.1 Rural Watersheds

A rural watershed is a drainage basin whose natural response to rainfall has not been substantially altered by urban land activity. Rural watersheds may be either natural or agricultural.

The Rational Method may be used on rural watersheds smaller than 200 acres. On watersheds larger than 200 acres, the USGS Regression Equations should be used. The 2014 USGS Rural Regression Equations may be used for Watersheds that lie in Region I and Region II if the drainage area is greater than 70 acres and 110 acres respectively. Watersheds that lie within Region III for the 2014 USGS Rural Regression Equations and have drainage areas between 200 to 1350 acres do not fall within the limits for either set of equations. In this case one of the following approaches may be used:

a. Use an alternate method such as Flood Hydrographs;

b. for drainage areas greater than 300 acres use the 1995 USGS Rural Regression Equations;

c. for drainage areas between 200 and 300 acres calculate the peak rate of runoff using the Rational Method and the 1995 USGS Rural Regression Equations, and use the one that is most appropriate.

The NRCS Unit Hydrograph may also be used for drainage areas larger than 6.40 acres.

For more information on watershed area limits for the above methods see Summary of Small Watershed Peak Runoff Calculation Method Drainage Area Limits table.

749.4.2 Urban Watersheds

An urban watershed may be defined as a drainage basin in which manmade developments in the form of impervious surfaces and/or storm drainage systems have substantially altered the basin's natural response to rainfall. Urbanization of a natural watershed progresses in one of two ways. First, the addition of impervious surfaces in the form of roads, streets, parking lots and roofs will prevent infiltration of rainfall into the covered soil surface, thus increasing the total volume and peak rate of runoff from a given rainfall volume. Second, to protect the now valuable property in a developed watershed from this increased peak and volume of runoff, storm drainage systems are installed. The installation of a storm drainage system does not increase the volume of runoff, but modifies the time distribution of runoff. Thus, when storm water drainage systems are installed, the time of concentration of the watershed is decreased. Therefore, storm water drainage systems have the effect of removing a given volume of runoff in a shorter period of time, thus further increasing the peak rate of runoff.

All hydraulic design in urban areas should consider the effect of increasing development throughout the projected life of the structure. Information on planned future development may be available from local agencies.

The Rational Method should be used on urban watersheds smaller than 200 acres. On watersheds larger than 200 acres, the USGS Urban Regression Equations should be used. The NRCS Unit Hydrograph may also be used for drainage areas larger than 6.40 acres.

For more information on watershed area limits for the above methods see Summary of Small Watershed Peak Runoff Calculation Method Drainage Area Limits table.

749.5 The Rational Method

The Rational Method was developed as early as 1889, and despite its limitations is one of the most widely used methods of estimating peak flows. Several assumptions are implicit in application of the Rational Method:

The maximum runoff rate occurs when the rainfall intensity lasts as long or longer than the time of concentration
The frequency of the discharge is the same as that of the rainfall intensity
The fraction of the rainfall that becomes runoff is independent of the rainfall intensity or volume

The first assumption implies that a homogeneous rainfall event is applied uniformly to the entire drainage area, and may not be valid for larger watersheds where constant rainfalls of high intensity do not occur simultaneously over the entire watershed. This assumption also provides the basis for using the watershed's time of concentration as the duration of the design storm. The second assumption again limits the size of the drainage area because for larger basins, factors other than rainfall frequency can play a large role in determining the flood frequency. Finally, the third assumption is reasonable for highly impervious areas, but less reasonable for pervious areas where the antecedent moisture condition plays a large role in determining the amount of rainfall that becomes surface runoff. For these reasons, use of the Rational Method is limited to small watersheds.

749.5.1 Equation

The Rational Method is expressed by the following formula.

Q=k_{c}CIA\,

where:

Q = the peak rate of runoff, ft³/s

k_c= 1.0

C = runoff coefficient, representing the ratio of direct runoff to rainfall

I = rainfall intensity of the design storm, in/hr

A = drainage area of the watershed, acres

749.5.2 Runoff Coefficient

The runoff coefficient represents the ratio of runoff to the total rainfall and combines the effects of several watershed characteristics such as land use, soil type, cover condition, and general terrain or watershed slope.

749.5.2.1 Rural Runoff Coefficients

For rural watersheds, the soil type, land use, and terrain are used to determine runoff coefficients.

The Natural Resource Conservation Service (NRCS) has classified about 4,000 major soils found in the United States into four basic hydrologic groups as follows:

Group A (Low runoff potential) - Soils having high infiltration rates even when thoroughly wetted and consisting chiefly of deep, well-drained sands or gravels. These soils have a high rate of water transmission.

Group B - Soils having moderate infiltration rates when thoroughly wetted and consisting chiefly of moderately deep to deep, moderately well to well drained soils with moderately fine to moderately coarse textures. These soils have a moderate rate of water transmission.

Group C - Soils having slow infiltration rates when thoroughly wetted and consisting chiefly of soils with a layer that impedes downward movement of water, or soils with moderately fine to fine texture. These soils have a slow rate of water transmission.

Group D (High runoff potential) - Soils having very slow infiltration rates when thoroughly wetted and consisting chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly impervious material. These soils have a very slow rate of water transmission.

The NRCS has performed modern soil surveys for many counties in Missouri, classifying the soils by a soil series name. The hydrologic soil group is given in the NRCS soil survey for these soils. If a modern soil survey map is not available for the county in question, the hydrologic soil group should be determined by the designer. This may be done by evaluating information available from the soil survey performed by the Construction and Materials Division and/or from a site visit and choosing the appropriate hydrologic soil group from the descriptions given above. The district soils and geology technologist may provide assistance in determining the hydrologic soil group.

For smaller projects, the soil series name and hydrologic soil group for soils along the right of way are generally given on the soil survey performed by the Construction and Materials Division. For larger projects, this soil survey does not name individual soil series, but rather gives "soil associations" along the right of way. These soil associations are groupings of soil series that have similar physical characteristics; however, the soils in an association may not have the same hydrologic soil group. Additionally, in many instances it will be necessary to determine a hydrologic soil group for soils located some distance from the right of way. In these cases, the hydrologic soil group should be determined by the designer from the NRCS county soil survey map, if available, or by evaluating information available from the soil survey performed by the Construction and Materials Division and/or from a site visit.

Land use and terrain can be determined by inspection of appropriate topographic maps or by a field inspection. After the hydrologic soil group, land use, and terrain have been determined, the runoff coefficient(s) to be used for estimating the 10-year peak flow for the rural watershed can be found in the following table.

**Rural Runoff Coefficients for 5 and 10-Yr Frequency Flows**
Topography and Vegetation	NRCS Hydrologic Soil Group
Topography and Vegetation	A	B	C	D
Woodland:
Flat	0.10	0.20	0.30	0.40
Rolling	0.25	0.35	0.40	0.50
Hilly	0.30	0.40	0.50	0.60
Pasture¹:
Flat	0.10	0.20	0.30	0.40
Rolling	0.16	0.30	0.40	0.55
Hilly	0.22	0.32	0.42	0.60
Cultivated¹:
Flat	0.30	0.40	0.50	0.60
Rolling	0.40	0.50	0.60	0.70
Hilly	0.52	0.62	0.72	0.82
¹Flat 0-5% slope; Rolling 5-10%; Hilly 10-30%
Source: Adapted from Ponce (1989)

749.5.2.2 Urban Runoff Coefficients

For urban watersheds, the runoff coefficient is primarily a function of land use and watershed slope. The following table gives a range of "C" values for various land use types in urban areas. The value selected by the designer should reflect the watershed slope, with steeper slopes having higher "C" values.

**Urban Runoff Coefficients for 5 and 10-Yr Frequency Flows**
Description	Runoff Coefficients
Business:
Downtown Areas	0.70 - 0.95
Neighborhood Areas	0.50 - 0.70
Residential:
Urban Single-Family	0.30 - 0.50
Urban Apartments	0.40 - 0.70
Suburban	0.25-0.40
Industrial:
Light	0.50 - 0.80
Heavy	0.60 - 0.90
Railroad Yards	0.20 - 0.40
Parks, Cemeteries	0.10 - 0.25
Playgrounds	0.20 - 0.40
Unimproved Areas	0.10 - 0.30
Streets and Roofs	0.70 - 0.90
Lawns:
Sandy Soil	0.10 - 0.20
Clay Soil	0.15 - 0.35
Sodded Roadway Slopes (1V:6H to 1V:1H)	0.40 - 0.60
Source: Ponce (1979)

749.5.2.3 Design Frequency

The rural and urban runoff coefficients given in the tables above are appropriate for 5-year or 10-year design frequencies. Estimation of peak flows for less frequent storms requires the use of a higher runoff coefficient because infiltration and other abstractions have proportionately less effect on the amount of rainfall that becomes runoff. To obtain runoff coefficients for other frequency events, the "C" value is multiplied by a frequency correction factor. The frequency correction factor is 1.10 for the 25-year event, 1.20 for the 50-year event, and 1.25 for the 100-year event. However, the resulting runoff coefficient (original "C" multiplied by the frequency correction factor) may not be greater than 1.0.

749.5.2.4 Composite Runoff Coefficients

The land use and soil type may vary from one portion of a watershed to another. In this case an average runoff coefficient, weighted by area, should be used. For example, consider a flat watershed with soils of average infiltration capacity (soil group B) and with 1/3 of the drainage area cultivated (C=0.40) and 2/3 of the drainage area in woodland cover (C=0.20). The weighted average runoff coefficient to be used to estimate the 10-year flood is:

C=[(0.40)(1/3)+(0.20)(2/3)]/1.0=0.267\,

The frequency correction factor may be applied to the weighted average runoff coefficient to estimate lower frequency flows: in the above example, a "C" value of (1.10)(0.267)=0.294 would be used for the 25-year flood, and a "C" of (1.20)(0.267)=0.320 would be used for the 50-year flood.

749.5.3 Time of Concentration

In order to determine the rainfall intensity used in the Rational Method, the time of concentration of the watershed must be estimated. The time of concentration of a watershed is defined as the time required for water to travel from the most hydraulically distant point of the watershed to the watershed outlet. This is also the time required before the entire watershed begins to contribute flow to the watershed outlet. This characteristic response time of the watershed is used as the duration of the design storm and thus influences the value of rainfall intensity used in the Rational Method.

Note that the location of the most hydraulically distant point in the watershed is a function of travel time and depends on both velocity and distance. The point in the watershed used to determine time of concentration may not necessarily be the point furthest from the watershed outlet.

In general, the time of concentration of urban drainage basins will be shorter than the time of concentration of rural basins. This is true when the natural drainage channels have been altered by storm sewers, main channel straightening, paving or similar modifications.

The time of concentration of a watershed will consist of the summation of travel times from one or more of the following flow regimes:

overland flow,
ditch or open channel flow,
and storm sewer system flow.

Overland Flow. The overland flow regime consists of very shallow sheet flow over the watershed surface. The velocities in overland flow are typically much lower than in the other flow regimes. Overland flow will become concentrated after a short distance and transform into one of the other flow regimes.

Open Channel Flow. The open channel flow regime consists of:

flow that has concentrated into swales or ditches, or
an outlet discharge from a storm sewer system
If open channel flow is a combination of the two, both contributors should be evaluated and engineering judgment used to determine the appropriate time of concentration.

Storm Sewer System Flow. Velocities are typically greater in sewer systems than in natural ditches or channels. The storm sewer system flow regime consists of:

Overland flow entering drain inlets, or
open channel flow entering drain inlets.
If sewer system flow is a combination of the two, both contributors should be evaluated and engineering judgment used to determine the appropriate time of concentration.

749.5.3.1 Rural Watersheds

For rural drainage basins the Kerby-Kirpich Method described below is a recommended method of determining time of concentration. Other methods such as those provided by the Natural Resource Conservation Service (NRCS) may be used as deemed necessary or appropriate by engineering judgment. See Velocity Method in Chapter 2 of FHWA publication Hydraulic Design Series No. 2 (HDS-2) for details on the NRCS method.

749.5.3.2 Urban Watersheds

For urban drainage basins the Kerby-Hathaway Equation for Overland Flow plus the appropriate Channel or Storm Sewer Travel Time as described below is a recommended method of determining time of concentration. Other methods such as those provided by the Natural Resource Conservation Service (NRCS) may be used as deemed necessary or appropriate by engineering judgment. See Velocity Method in Chapter 2 of FHWA publication Hydraulic Design Series No. 2 (HDS-2) for details on the NRCS method.

749.5.3.3 Kerby-Kirpich Method

For rural drainage basins the Kerby- Hathaway and Kirpich methods can be used in conjunction to estimate the time of concentration.

T_c = t_o + t_c

where:

T_c = the total time of concentration (min)

t_o = the overland flow time of concentration from the Kerby-Hathaway equation (min)

t_c = the channel flow time of concentration from the Kirpich equation (min)

749.5.3.3.1 Low (Flat) Slopes

In watersheds with low topographical relief, time of concentration calculations will often result in unreasonably large values. Cleveland, et al. 2012 recommends the following adjustment to the slope in the Kirpich and Kerby-Hathaway equations to provide a more realistic estimate of the time of concentration:

For slopes flatter than 0.002 ft/ft (0.2%) the adjusted slope should be increased by 0.0005 (S_adj = S + 0.0005).
For slopes between 0.002 ft/ft (0.2%) and 0.003 ft/ft (0.3%) engineering judgment should be used to determine if the adjustment to the slope should be made.

749.5.3.3.2 Kirpich Equation

Open channel flow travel time for streams with natural beds, or that mimic natural beds, can be calculated using the Kirpich Equations If flow regimes are combined the upstream end of the flow length (L) should extent to the basin divide.

t_{c}=KL^{0.77}S^{-0.385}\,

where:

t_c = the time of concentration (min)

K = 0.0078

L = the flow length of the principal watercourse, ft.

S = the slope between the minimum and maximum elevation, ft./ft.

749.5.3.3.3 Kerby-Hathaway Equation for Overland Flow

Overland flow travel time can be calculated using the Kerby-Hathaway equation:

t_{o}=K(Ln)^{0.47}S^{-0.235}\,

where:

t_o = overland travel time (min)

K = 0.8262

L = the overland flow length, ft.

n = roughness coefficient

S = the overland flow slope, ft./ft.

The roughness coefficient, n, used in this equation is similar in meaning to that used in Manning's equation for open-channel flow; however, for a given type of surface, the roughness coefficient for overland flow will be considerably larger than for open-channel flow. Typical values for the roughness coefficient are given in the table below. The overland flow length should be limited to less than 500 ft.;it is usually much less and engineering judgment should be used for lengths of more than 200 ft. After a short distance, overland flow usually changes flow regimes and begins to concentrate in swales or ditches, or into a storm sewer system.

**Roughness Coefficients for Overland Flow**
Type of Surface		N
Dense Growth		0.40 to 0.50
Pasture		0.30 to 0.40
Lawns		0.20 to 0.30
Bluegrass Sod		0.20 to 0.50
Short-Grass Prairie		0.10 to 0.20
Sparse Vegetation		0.05 to 0.13
Bare Clay-Loam Soil (Eroded)		0.01 to 0.03
Concrete - Asphalt
	Very Shallow Depths < ¼”	0.10 to 0.15
	Shallow Depths > ¼”	0.05 to 0.10
Deciduous Timberland		0.50 to 0.70
Conifer Timberland		0.60 to 0.80
Source: Ponce et al. (1989)

749.5.3.4 Storm Sewer Travel Time

The storm sewer travel time is the summation of travel times in each component of the sewer system between the uppermost inlet and the outlet. These times may be estimated by use of the open channel flow charts presented in Open Channels. The proper procedure is to solve for the velocity of flow in each component assuming the pipe is flowing at maximum capacity. The velocity of flow is then divided into the length of flow to obtain the travel time. In order to use this method, it is necessary to obtain detailed information on the existing storm sewer system. This information includes conduit sizes, materials, lengths and slopes. If this information is not available or if the structure being designed is not considered important enough to warrant the effort required for such a detailed analysis, the storm sewer travel time may be estimated by use of the Kirpich Equation. In this case, the travel time given by the Kirpich Equation should be multiplied by 0.20 to obtain the travel time in the storm sewer system.

749.5.3.5 Channel Flow Travel Time

The travel time in the open channel flow phase can be estimated by first estimating the velocity of flow in the channel. The velocity of flow is then divided into the length of flow to obtain the travel time. For subcritical flow conditions, the velocity should be determined for bank-full conditions at the mid-point of the channel's length. If the channel is improved, the average velocity may be estimated from the flow charts or flow computations presented in Open Channels. If the channel is natural, the travel time may be estimated by open channel flow computations. Care should be taken when estimating channel flow travel time for supercritical flow conditions. In some cases, the bank-full velocity under supercritical conditions may be too high to provide a reasonable estimate of travel time and a more reasonable estimate of flow depth should be used instead.

749.5.4 Rainfall Intensity

The design rainfall intensity is a function of the storm duration, the design frequency and the geographic location. The storm duration is taken as the time of concentration of the watershed or five minutes, whichever is greater. Knowing the storm duration and the design frequency, the rainfall intensity may be read from the appropriate Intensity-Duration-Frequency (IDF) curve for each district.

749.6 USGS Regression Equations

The USGS Regression equations were developed by the United States Geological Survey. Data from stream gage sites were analyzed to determine flood magnitudes with various recurrence intervals. The resulting magnitudes were then related to basin characteristics through a statistical analysis to provide the regression equations. Three sets of equations are currently available: 2014 and 1995 Rural Regression Equations, and Urban Regression Equations.

Stream flows for ungaged sites on streams that have a stream gage(s) may be determined using the area weighted method shown in EPG 749.7, Historical USGS Stream Gage Data.

749.6.1 Rural Regression Equations

For ungaged natural flood flow sites, flood magnitudes having recurrence intervals of 2, 5, 10, 25, 50, 100, 200 (2014 only) and 500 years are computed by using appropriate values of the contributing drainage basin area (A) and basin shape (B) (2014) or slope (S) (1995) in the equations shown in the Rural Regression Equations tables. A computer program is available to assist in performing these calculations. The state is divided into three hydrologic regions, each with its own set of regression coefficients. The three regions are described as follows:

Region I - Central Lowlands - Characterized by meandering stream channels in wide and flat valleys with local relief generally between 50 to 150 feet. Elevations range from about 600 ft. above sea level near the Mississippi River to about 1200 ft. above sea level in the northwestern part of the state.

Region II - Ozark Plateaus - - Characterized by streams that have narrow valleys 200 to 500 ft. deep, with local relief generally ranging from 100 to 500 feet. Main channel gradients generally are steeper than elsewhere in Missouri, and karst features are locally prominent in much of the region. Elevations (generally) range from 800 to 1700 ft. above sea level. Region II also includes the geological formation known as Crowley’s Ridge which rises 250 to 550 ft. above the Mississippi Alluvial Plain and reflects hydrologic and hydraulic characteristics similar to the Ozark Plateau.

Region III - Mississippi Alluvial Plain - This is a relatively flat area and is predominately agricultural cropland. Virtually all the area is drained by a series of man-made drainage ditches with average slopes of about 1.5 ft./mile. Elevations range from 200 to 300 ft. above sea level with local relief seldom exceeding 30 feet.

749.6.1.1 2014 Rural Regression Equations

Engineering judgment should be exercised when using these equations to ensure that the subject drainage area is similar in characteristics to the USGS region being used. Portions of eastern Butler and Ripley counties are especially sensitive as some streams will be consistent with slopes for Region 2 and others with Region 3.

Valid data ranges for each of the regional equations are listed in the Valid Data Ranges table below. The Supplemental Data Ranges table may be used to help determine which regional equation to use, but should not be used to determine the validity of the equations.

Valid Data Ranges					Supplemental Data Ranges
Drainage Area (mi²)		Basin Shape			Basin Slope (ft/ft)		Valley Slope (ft/mi)
Min.	Max.	Min.	Max.		Min.	Max.	Min.	Max.
0.11	8212.38	2.25	26.59	Region I	0.0141	0.1812	1.53	198.02
0.17	4008.92	2.04	26.89	Region II	0.0133	0.1999	0.34	257.13
2.12	2177.58	na	na	Region III	0.0017	0.0388	0.61	2.07
Source: Southard and Veilleux (2014)

2014 Rural Regression Equations
Flood Freq. (years)	Flood Magnitude ft³/s	Std. Error of Prediction (%)
Region I
2	(10^2.594)(A^0.618)(BS^-0.282)	38
5	(10^2.861)(A^0.593)(BS^-0.266)	31
10	(10^2.990)(A^0.580)(BS^-0.258)	29
25	(10^3.120)(A^0.568)(BS^-0.248)	29
50	(10^3.199)(A^0.560)(BS^-0.242)	29
100	(10^3.266)(A^0.554)(BS^-0.236)	30
200	(10^3.324)(A^0.550)(BS^-0.231)	31
500	(10^3.391)(A^0.544)(BS^-0.226⁾	33
Region II
2	(10^2.493)(A^0.686)(BS^-0.222)	44
5	(10^2.801)(A^0.679)(BS^-0.251)	32
10	(10^2.955)(A^0.676)(BS^-0.268)	28
25	(10^3.113)(A^0.673)(BS^-0.287)	24
50	(10^3.205)(A^0.671)(BS^-0.296)	24
100	(10^3.282)(A^0.669)(BS^-0.302)	24
200	(10^3.349)(A^0.668)(BS^-0.307)	25
500	(10^3.422)(A^0.667)(BS^-0.311)	27
Region III
2	(10^1.933)*(A^0.665)	30
5	(10^2.026)*(A^0.681)	26
10	(10^2.070)*(A^0.689)	26
25	(10^2.113)*(A^0.698)	26
50	(10^2.139)*(A^0.703)	27
100	(10^2.162)*(A^0.708)	27
200	(10^2.182)*(A^0.713)	28
500	(10^2.204)*(A^0.718)	29
Source: Southard and Veilleux (2014)

749.6.1.1.1 Drainage Area

Drainage area (A) in mi², can be obtained by determining the area contributing surface flows to the site as outlined along the drainage divide on the best available topographic maps or computer program.

749.6.1.1.2 Basin Shape

The Basin Shape (BS) is dimensionless and is a statistical method used to determine the magnitude and arrival time of a peak discharge. Basins with elongated shapes will have longer duration hydrographs and lower peak discharges than a circular basin that will have shorter duration hydrographs and higher peak discharges. The value used for the Basin Shape (BS) is calculated by dividing the square of the longest flow path or stream length by the Drainage Area (A). Stream length can be determined using the best available topographic maps or computer program.

749.6.1.1.3 Limitations of Equations

The USGS Rural Regression Equations may be used to estimate magnitude and frequency of floods on most Missouri streams. Caution should be used when using these equations when the drainage area and basin shape are outside of the limits of the Valid Data Ranges shown in EPG 749.6.1.1 2014 Rural Regression Equations.

However, the equations are generally not applicable for:

a) basins where manmade changes have appreciably changed the flow regimen such as:

Regulation of discharge (i.e. dams);
Channelization;
Diversion;
Urbanization

b) the main stems of the Mississippi and Missouri Rivers;

c) basins with greater than 5% impervious area;

d) areas near the mouth of streams draining into larger rivers where backwater effect is experienced may require special consideration, see EPG 749.11 Stream Confluences.

749.6.1.2 1995 Rural Regression Equations

Valid data ranges for each of the regional equations are listed in the Valid Data Ranges table below:

	Valid Data Ranges
	Drainage Area (mi²)		Valley Slope (ft/mi)
	Min.	Max.	Min.	Max.
Region I	0.13	11,500	1.35	150
Region II	0.13	14,000	1.20	279
Region III	0.48	1040	na	na
Source: Alexander and Wilson (1995)

**1995 Rural Regression Equations**
Flood Freq. (years)	Flood Magnitude ft³/s	Std. Error of Prediction (%)
Region I
2	69.4A^0.703S^0.373	34.00
5	123A^0.690S^0.383	32.00
10	170A^0.680S^0.378	34.00
25	243A^0.668S^0.366	36.00
50	305A^0.660S^0.356	38.00
100	376A^0.652S^0.346	40.00
500	569A^0.636S^0.321	45.00
Region II
2	77.9A^0.733S^0.265	43.00
5	99.6A^0.763S^0.355	36.00
10	117A^0.774S^0.395	34.00
25	140A^0.784S^0.432	32.00
50	155A^0.789S^0.453	31.00
100	170A^0.794S^0.471	32.00
500	203A^0.804S^0.503	34.00
Region III
2	88A^0.658	34.00
5	145A^0.627	36.00
10	187A^0.612	38.00
25	244A^0.595	41.00
50	288A^0.585	44.00
100	334A^0.576	46.00
500	448A^0.557	54.00
Source: Alexander and Wilson (1995)

749.6.1.2.1 Drainage Area

Drainage area (A) in mi², can be obtained by determining the area contributing surface flows to the site as outlined along the drainage divide on the best available topographic maps or computer program.

749.6.1.2.2 Valley Slope

Valley slope (S) in ft./mile is the average slope between points 10 percent and 85 percent of the distance along the main-stream channel from the site to the basin divide. The main channel is defined above stream junctions as the one draining the largest area. The elevation difference between the 10- and 85-percent points is divided by the distance between the points to evaluate the slope.

749.6.1.2.3 Limitations of Equations

The USGS Rural Regression Equations may be used to estimate magnitude and frequency of floods on most Missouri streams Caution should be used when using these equations when the drainage area and slope are outside of the limits of the Valid Data Ranges shown in EPG 749.6.1.2 1995 Rural Regression Equations.

However, the equations are generally not applicable for:

a) basins where manmade changes have appreciably changed the flow regimen such as:

Regulation of discharge (i.e. dams);
Channelization;
Diversion;
Urbanization

b) the main stems of the Mississippi and Missouri Rivers;

c) basins with greater than 5% impervious area.

d) areas near the mouth of streams draining into larger rivers where backwater effect is experienced may require special consideration, see EPG 749.11 Stream Confluences.

749.6.2 Urban Regression Equations

The USGS Rural Regression Equations given above are not applicable to urban watersheds where manmade changes have appreciably changed the flow regimen. A set of equations were developed in 2010 by the United States Geological Survey. Data from 35 gaged sites in urban locations in and adjacent to Missouri were analyzed to determine flood magnitudes with recurrence intervals of 2, 5, 10, 25, 50, 100 and 500 years. The resulting magnitudes were then related to drainage area and urban basin characteristics to provide the regression equations. A computer program is available to assist in performing these calculations.

An urban watershed may be defined as a drainage basin in which manmade developments in the form of impervious surfaces and/or storm drainage systems have substantially altered the basin's natural response to rainfall. Urbanization of a natural watershed progresses in one of two ways. First, the addition of impervious surfaces in the form of roads, streets, parking lots and roofs will prevent infiltration of rainfall into the covered soil surface, thus increasing the total volume and peak rate of runoff from a given rainfall volume. Second, to protect the now valuable property in a developed watershed from this increased peak and volume of runoff, storm drainage systems are installed. The installation of a storm drainage system does not increase the volume of runoff, but modifies the time distribution of runoff. Thus, when storm water drainage systems are installed, the time of concentration of the watershed is decreased. Therefore, storm water drainage systems have the effect of removing a given volume of runoff in a shorter period of time, thus increasing the peak rate of runoff.

749.6.2.1 Equations

Peak discharges can be estimated at urban locations using the equations presented in the Urban Regression Equations table, below. The Equations give peak discharge as a function of drainage area (A) and percentage of impervious area (I).

All hydraulic design in urban areas should consider the effect of increasing development throughout the projected life of the structure. Information on planned future development may be available from local agencies.

The range of valid data for USGS Urban Regression Equations are:

a) Drainage Area – 0.28 mi² to 189 mi²

b) Impervious Area - 2.3% to 46%

**Urban Regression Equations**
Flood Freq. (years)	Flood Magnitude ft³/s	Std. Error of Prediction (%)
2	188A^0.59910^0.014(I)	-23.1 to 30.0
5	352A^0.59210^0.011(I)	-20.6 to 25.9
10	484A^0.59410^0.010(I)	-19.6 to 24.4
25	671A^0.59910^0.008(I)	-19.6 to 24.4
50	826A^0.60410^0.008(I)	-20.6 to 25.9
100	991A^0.60810^0.007(I)	-22.3 to 28.7
500	1,420A^0.61910^0.006(I)	-28.9 to 40.7
Source: Southard (2010)

749.6.2.2 Drainage Area

Drainage area (A) in mi², can be obtained by determining the area contributing surface flows to the site as outlined along the drainage divide on the best available topographic maps or computer program.

749.6.2.3 Percentage of Impervious Area

The percentage of impervious area (I) is the portion of the drainage area that is impervious because of buildings, parking lots, streets and roads, and other impervious areas within an urban basin. The variable, I, is determined from the best available maps, satellite imagery or aerial photos showing impervious surfaces. Field inspection to supplement the imagery may be useful.

749.6.2.4 Limitations of Equations

The USGS Urban Regression Equations may be used to estimate magnitude and frequency of floods on most urban Missouri streams within the limits noted in EPG 749.6.2.1 Equations provided that the floodflows are relatively unaffected by manmade works such as dams or diversions.

749.7 Historical USGS Stream Gage Data

Numerous USGS recording stream gages have been maintained for many years on selected Missouri streams. For proposed structures at or near one of these gages, the gage data can be used in estimating discharge. When sufficient years of data have been collected at a stream gage, the data may be statistically analyzed to estimate discharge for the selected design flood frequency.

Stream gage data is available on the Internet at http://waterdata.usgs.gov/mo/nwis/.

749.7.1 Analysis

Gage data is analyzed by Log-Pearson Type III regression analysis to determine the discharges associated with the relevant return periods. See Water Resources Council Bulletin #17B in References for details on this analysis method. A computer program for the analysis is available.

One statistical parameter computed in the Log-Pearson analysis is the skew coefficient of the distribution of the stream gage data. Skew coefficients for the data from stream gages in Missouri are typically between -0.1 and -0.4 when sufficient years of record are available. Skew coefficients outside this range may indicate an insufficient length of record or an analysis affected by outliers in the data. In this case, other methods of determining discharges will likely provide better estimates.

749.7.2 Ungaged Sites Near Stream Gage

Stream gage data from gages at some distance from the site on the same watershed and stream gage data from nearby hydrologically similar watersheds may also be used to estimate discharges. Discharges obtained from this type of data should be compared with discharges obtained by other methods and not given the same weight as discharges obtained from data from a stream gage at the proposed site. Better estimates of discharge using this method may be obtained by repeating the procedure for several nearby gages and averaging the results. This method should not be used when drainage areas differ by more than 50% or at sites more than 50 miles from the stream gage(s).

Transposition of discharges from one basin to another, or from one location to another within the same watershed, is accomplished using the following Area-Weighting equation:

Q_{1}=Q_{2}\left({\frac {A_{1}}{A_{2}}}\right)^{k}

where:

Q1 = discharge for ungaged drainage basin (cfs)

A1 = drainage area for ungaged drainage basin (mi²)

Q2 = discharge for gaged drainage basin (cfs)

A2 = drainage area for gaged drainage basin (mi²)

k = exponent = see table

**Drainage-Area Ratio Exponent, k**
Flood Freq. (years)	Region I	Region II	Region III
2	0.580	0.654	0.665
5	0.557	0.643	0.681
10	0.546	0.637	0.689
25	0.536	0.632	0.698
50	0.529	0.629	0.703
100	0.524	0.626	0.708
200	0.520	0.625	0.713
500	0.515	0.623	0.718
Source: Southard and Veilleux (2014)

749.8 NFIP Flood Insurance Study Discharges

NFIP Flood Insurance Studies (FIS) typically include estimates of 10-, 50-, 100- and 500-year discharges for streams studied by detailed methods. These discharges may be more accurate than those obtained by other methods if the FIS discharges were determined through a detailed hydrologic study, such as an HEC-1, HEC-HMS or TR-20 hydrologic model. In some instances, the FIS discharges may have been determined using an older version of the USGS regression equations. Engineering judgment should be exercised to determine if the FIS discharges determined from older USGS regression equations should be used for the hydraulic design. When a No-Rise Certificate is required, and the FIS discharges are not used for hydraulic design, the 100-year FIS discharge should be used to verify the no-rise condition based on the Base Flood elevations in the FIS. Careful review of the FIS report will disclose the level of detail used in the hydrologic study.

749.9 Flood Hydrographs

For certain design problems it may be necessary to determine flood hydrographs associated with the peak discharge for a desired frequency. A hydrograph gives flow rate as a function of time at a particular location in a watershed, usually at the watershed outlet.

749.9.1 NRCS Unit Hydrograph

Techniques developed by the Natural Resource Conservation Service (NRCS), formerly known as the Soil Conservation Service (SCS), for calculating rates of runoff require the same basic data as the Rational Method: drainage area, a runoff factor, time of concentration, and rainfall information. The NRCS method is more sophisticated in that it considers also the time distribution of rainfall, the initial rainfall losses to interception and depression storage, and an infiltration rate that decreases during the course of a storm. The NRCS Unit Hydrograph method is applicable for all watersheds between 6.4 and 16,000 acres. Additional details on the NRCS methodology can be found in the National Engineering Handbook, (NEH), Part 630, Chapter 10 and Chapter 16(USDA/NRCS).

749.9.1.1 Rainfall-Runoff Equation and Concepts

The NRCS method is based on a 24-hour storm event. NRCS has developed four synthetic rainfall distributions typical of storms for various geographical regions in the United States. The Type II distribution is the appropriate distribution for using the NRCS method in Missouri.

The NRCS rainfall-runoff equation was developed by the NRCS from experimental plots for numerous soil types and vegetative cover conditions. The experimental data consisted mainly of daily rainfall totals on small watersheds, and did not include information on the time distribution of rainfall. The NRCS rainfall-runoff equation is therefore used to estimate the depth of runoff resulting from a given depth of 24-hour rainfall assumed to be spatially distributed uniformly over the watershed. The equation is given by:

Q={\frac {(P-I_{a})^{2}}{(P-I_{a})+S}}

where:

Q = accumulated direct runoff, in

P = accumulated rainfall (potential maximum runoff), in

I_a = initial abstractions including surface storage, interception and infiltration prior to runoff, in

S = potential maximum retention, in

The potential maximum retention, S, is a measure of the maximum amount of water a given watershed could retain, and is a function of land use, interception capacity, infiltration capacity, depression storage, and antecedent moisture. A relationship between Ia and S was also developed from experimental data:

I_{a}=0.2S\,

Substituting the equation for I_a into the equation above, the NRCS rainfall-runoff equation becomes:

Q={\frac {(P-0.2S)^{2}}{P+0.8S}}

It is important to note that while both P and Q are given in units of depth, they actually represent volumes of rainfall and runoff, respectively. This is because the indicated depth of rainfall or runoff is assumed to be applied uniformly to the entire watershed area.

The NRCS made additional empirical analyses to estimate the value of S for various watersheds. The principal physical characteristics affecting the relationship between rainfall and runoff are land use, land treatment, soil types and land slope. These characteristics which affect the maximum potential retention are represented by a runoff factor called the curve number (CN), which is related to S by:

S={\frac {1000}{CN}}-10

The curve number is an index that represents the combination of hydrologic soil group, land use, hydrologic condition, and antecedent moisture condition. The table(s) below, Runoff Curve Numbers, provide curve numbers for different land uses, treatments and hydrologic conditions; separate values are given for each soil group. The NRCS provides methods of adjusting the curve numbers based on varying antecedent moisture conditions; however, for design purposes, average antecedent moisture conditions are normally assumed and the values given in the table can therefore be used for design.

**Runoff Curve Numbers - Urban Areas¹**
Cover Description	Curve Numbers for Hydrologic Soil Groups
Cover Type and Hydrologic Condition	Average Percent Impervious Area²	A	B	C	D
Fully Developed Urban Areas (Vegetation Established)
Open Space (lawns, parks, golf courses, cemeteries, etc.)³
Poor Condition (grass cover<50%)		68	79	86	89
Fair Condition (grass cover 50% to 75%)		49	69	79	84
Good Condition (grass cover>75%)		39	61	74	80
Impervious Areas:
Paved Parking Lots, Roofs, Driveways, etc. (excluding right-of-way)		98	98	98	98
Streets and Roads:
Paved; Curbs and Storm Drains (excluding right-of-way)		98	98	98	98
Paved; Open Ditches (including right-of-way)		83	89	92	93
Gravel (including right-of-way)		76	85	89	91
Dirt (including right-of-way)		72	82	87	89
Western Desert Urban Areas:
Natural Desert Landscaping (pervious areas only)		63	77	85	88
Artificial Desert Landscaping (impervious weed
barrier desert shrub with 25- to 50 mm sand
or gravel mulch and basin borders)		96	96	96	96
Urban Districts:
Commercial and Business	85	89	92	94	95
Industrial	72	81	88	91	93
Residential Districts by Average Lot Size:
1/8 Acre or Less (town houses)	65	77	85	90	92
1/4 Acre	38	61	75	83	87
1/3 Acre	30	57	72	81	86
1/2 Acre	25	54	70	80	85
1 Acre	20	51	68	79	84
2 Acres	12	46	65	77	82
Developing Urban Areas
Newly Graded Areas (pervious areas only, no vegetation		77	86	91	94
Idle Lands (CNs are determined using cover types similar to those in Runoff Curve Numbers - Urban Areas
¹Average runoff condition, and I_a=0.2S
²The average percent impervious area shown was used to develp the composite CNs. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. If the impervious area is not connected, the NRCS method has an adjustment to reduce the effect.
³CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combinations of open space cover type.

**Runoff Curve Numbers - Cultivated Agricultural Areas¹**
Cover Description		Curve Numbers for Hydrologic Soil Groups
Cover Type	Treatment²	Hydrologic Condition³	A	B	C	D
Fallow	Bare Soil		77	86	91	94
	Crop Residue Cover (CR)	Poor	76	85	90	93
	Crop Residue Cover (CR)	Good	74	83	88	90
Row Crops	Straight Row (SR)	Poor	72	81	88	91
	Straight Row (SR)	Good	67	78	85	89
	SR + CR	Poor	71	80	87	90
	SR + CR	Good	64	75	82	85
	Contoured (C)	Poor	70	79	84	88
	Contoured (C)	Good	65	75	82	86
	C + CR	Poor	69	78	83	87
	C + CR	Good	64	74	81	85
	Contoured & Terraced (C&T)	Poor	66	74	80	82
	Contoured & Terraced (C&T)	Good	62	71	78	81
	C&T + CR	Poor	65	73	79	81
	C&T + CR	Good	61	70	77	80
Small Grain	SR	Poor	65	76	84	88
	SR	Good	63	75	83	87
	SR + CR	Poor	64	75	83	86
	SR + CR	Good	60	72	80	84
	C	Poor	63	74	82	85
	C	Good	61	73	81	85
	C + CR	Poor	62	73	81	84
	C + CR	Good	60	72	80	83
	C&T	Poor	61	72	79	82
	C&T	Good	59	70	78	81
	C&T + CR	Poor	60	71	78	81
	C&T + CR	Good	58	69	77	80
Close-Seeded or Broadcast Legumes or Rotation Meadow	SR	Poor	66	77	85	89
	SR	Good	58	72	81	85
	C	Poor	64	75	83	85
	C	Good	55	69	78	83
	C&T	Poor	63	73	80	83
	C&T	Good	51	67	76	80
¹Average runoff condition, and I_a=0.2S
²Crop residue cover applies only if residue is on at least 5% of the surface throughout the year.
³Hydrologic condition is based on a combination of factors that affect infiltration and runoff, including (a) density and canopy of vegetative areas, (b) amount of year-round cover, (c) amount of grass or closed-seeded legumes in rotations, (d) percent of residue cover on the land surface (good>20%) and (e) degree of roughness. Poor: Factors impair infiltration and tend to increase runoff. Good: Factors encourage average and better than average infiltration and tend to decrease runoff.

**Runoff Curve Numbers - Other Agricultural Lands¹**
Cover Description		Curve Numbers for Hydrologic Soil Groups
Cover Type	Hydrologic Condition³	A	B	C	D
Pasture, Grassland, or Range — Continuous Forage for Grazing	Poor	68	79	86	89
	Fair	49	69	79	84
	Good	39	61	74	80
Meadow - Continuous Grass, Protected from Grazing and Generally Mowed for Hay		30	58	71	78
Brush - Brush-Weed-Grass Mixture with Brush the Major Element	Poor	48	67	77	83
	Fair	35	56	70	77
	Good	⁴30	48	65	73
Woods - Grass Combination (Orchard or Tree Farm)⁵	Poor	57	73	82	86
	Fair	43	65	76	82
	Good	32	58	72	79
Woods⁶	Good	45	66	77	83
	Fair	36	60	73	79
	Good	⁴30	55	70	77
Farmsteads - Buildings, Lanes, Driveways and Surrounding Lots		59	74	82	86
¹Average runoff condition, and I_a=0.2S
²Poor: <50% ground cover or heavily grazed with no mulch Fair: 50 to 74% ground cover Good: >75% ground cover and lightly or only occasionally grazed
³Poor: <50% gound cover Fair: 50 to 75% ground cover Good: >75% ground cover
⁴Actual curve number is less than 30; use CN=30 for runoff computations.
⁵CNs shown were computed for areas with 50% grass (pasture) cover. Other combinations of conditions may be computed from CNs for woods and pasture.
⁶Poor: Forest litter, small trees and brush are destroyed by heavy grazing or regular burning. Fair: Woods grazed but not burned, and some forest litter covers the soil. Good: Woods protected from grazing, litter and brush adequately cover soil.

749.9.1.2 NRCS Dimensionless Unit Hydrograph

The NRCS unit hydrograph was developed based on analysis of a large number of natural unit hydrographs from a wide range of drainage basin sizes and geographic locations. The NRCS unit hydrograph is given in a dimensionless form and provides a standard unit hydrograph shape.

**NRCS Dimensionless Unit Hydrograph**
Time Ratio	Discharge Ratio	Time Ratio	Discharge Ratio	Time Ratio	Discharge Ratio	Time Ratio	Discharge Ratio
t/T_p	q/q_p	t/T_p	q/q_p	t/T_p	q/q_p	t/T_p	q/q_p
0.0	0.000	0.9	0.990	1.8	0.390	3.4	0.029
0.1	0.030	1.0	1.000	1.9	0.330	3.6	0.021
0.2	0.100	1.1	0.990	2.0	0.280	3.8	0.021
0.3	0.190	1.2	0.930	2.2	0.207	4.0	0.011
0.4	0.310	1.3	0.860	2.4	0.147	4.5	0.005
0.5	0.470	1.4	0.780	2.6	0.107	5.0	0.000
0.6	0.660	1.5	0.680	2.8	0.077
0.7	0.820	1.6	0.560	3.0	0.055
0.8	0.930	1.7	0.460	3.2	0.040
Source: McCuen (1996)

Use of the NRCS unit hydrograph requires calculation of the unit hydrograph peak discharge and the time to peak. The unit hydrograph peak discharge is given by:

q_{p}={\frac {484AQ}{T_{p}}}

where:

q_p = unit hydrograph peak discharge, cfs

A = drainage area, mi²

Q = runoff in inches

T_p = time to peak, hrs

The time to peak is assumed to be equal to the basin lag time plus one-half the duration of rainfall. Basin lag time is estimated as 0.6 times the time of concentration, leading to the following equation for time to peak:

{\frac {\Delta D}{2}}+0.6T_{c}=T_{p}

where:

t_p = time to peak, hrs

ΔD = duration of rainfall (unit hydrograph duration) = 0.133 t_c, hrs

t_c = time of concentration, hrs

749.9.1.3 Application of NRCS Methodology

Unit hydrograph theory depends on the principles of linearity and superposition. Given a unit hydrograph, the runoff hydrograph for a runoff depth other than unity can be obtained by multiplying the unit hydrograph ordinates by the runoff depth using the principle of linearity. The flood hydrograph for a particular storm event can be obtained by dividing the storm event into incremental periods of runoff, then applying the unit hydrograph to each incremental runoff and summing the resulting hydrographs together using the principle of superposition to obtain the total runoff hydrograph.

The unit hydrograph duration (and the corresponding duration of the period of incremental runoff used in applying the unit hydrograph method) is estimated as 0.133t_c. Since the NRCS Type II rainfall distribution has a 24-hour time base, application of the NRCS unit hydrograph methodology to typical watersheds by hand requires calculation of runoff hydrographs for a large number of increments. This can be cumbersome and time-consuming and a computer-based implementation is recommended.

749.10 Detention Storage

The traditional purpose of storm drainage systems has been to collect and convey storm runoff as rapidly as possible to a suitable location where it can be discharged. As areas urbanize this type of design may result in major drainage and flooding problems downstream. Under favorable conditions, the temporary storage of some of the storm runoff can decrease downstream flows and often the cost of the downstream conveyance system. Detention storage facilities can range from small facilities contained in parking lots or other on-site facilities to large lakes and reservoirs. This article provides general procedures for detention storage analysis.

An easement must be purchased for any land, outside of the right of way, that will be flooded by water from a detention storage structure.

749.10.1 Data Needs

The following data will be needed to complete storage calculations.

Inflow hydrograph for all selected design storms. The inflow hydrograph for the detention basin can be determined using the methods in Flood Hydrographs.
Stage-storage curve for storage facility.
Stage-discharge curve for the facility.

Using these data, the inflow hydrograph is routed through the storage facility to develop the outflow hydrograph.

749.10.1.1 Stage-Storage Curve

A stage-storage curve defines the relationship between the depth of water and storage volume in a reservoir. The data for this type of curve are usually developed using a topographic map and the conic formula for irregular shaped basins, or the prismoidal formula for trapezoidal basins. The conic formula is expressed as:

V_{1,2}={\frac {1}{3}}d\left(A_{1}+A_{2}+{\sqrt {A_{1}A_{2}}}\right)

(Equation 1)

where:

V_1,2 = storage volume, ft³ (m³), between elevations 1 and 2

A₁ = surface area at elevation 1, ft²

A² = surface area at elevation 2, ft²

d = change in elevation between points 1 and 2, ft

The prismoidal formula for trapezoidal basins is expressed as:

V=LWD+(L+W)ZD^{2}{\frac {4}{3}}Z^{2}D^{3}

(Equation 2)

where:

V = volume of trapezoidal basin, ft³

L = length of basin at base, ft

W = width of basin at base, ft

D = depth of basin, ft

Z = side slope factor, ratio of horizontal to vertical

749.10.1.2 Stage-Discharge Curve

A stage-discharge curve defines the relationship between the depth of water and the discharge or outflow from a storage facility. If the detention facility has both principal and emergency spillways the stage-discharge curve should take both into account. The following equations can be used to help develop the stage-discharge curve.

Sharp-crested weir flow equations for no end contractions, two end contractions, and submerged discharge conditions are presented below, followed by equations for broad-crested weirs, v-notch weirs and orifices, or combinations of these facilities.

Sharp-Crested Weirs. A sharp-crested weir with no end contractions is illustrated in Weir Configurations. The discharge equation for this configuration is (Chow, 1959):

English:

Q=\left[3.27+0.4\left({\frac {H}{H_{c}}}\right)\right]LH^{1.5}

(Equation 3)

Metric:

Q=0.55\left[3.27+0.4\left({\frac {H}{H_{c}}}\right)\right]LH^{1.5}

(Equation 4)

where:

Q = discharge, ft³/s

H = head above weir crest excluding velocity head, ft.

H_c = height of weir crest above channel bottom, ft.

L = horizontal weir length, ft.

A sharp-crested weir with two end contractions is illustrated in Weir Configurations. The discharge equation for this configuration is (Chow, 1959):

English:

Q=\left[3.27+0.4\left({\frac {H}{H_{c}}}\right)\right](L-0.2H)H^{1.5}

(Equation 5)

where: Variables are the same as for the previous equation.

A sharp-crested weir will be affected by submergence when the tailwater rises above the weir crest elevation. The result will be that the discharge over the weir will be reduced. The discharge equation for a sharp-crested submerged weir is (Brater and King, 1976):

Q_{s}=Q_{f}\left[1-\left({\frac {H_{2}}{H_{1}}}\right)^{1.5}\right]^{0.385}

(Equation 6)

where:

Q_s = submergence flow, ftA³/s

Q_f = free flow, ft³/s

H₁ = upstream head above crest, ft.

H₂ = downstream head above crest, ft.

Broad-Crested Weirs The equation generally used for the broad-crested weir is (Brater and King, 1976):

English:

Q=CLH^{1.5}\,

(Equation 7)

where: Q = discharge, ft³/s

C = broad-crested weir coefficient (see Broad-Crested Weir Coefficients)

L = broad-crested weir length, ft

H = head above weir crest, ft., measured at least 2.5H upstream of the weir

For weir flow over embankments with sloped sides, a C value of 3.0 should be used. For weir flow over embankments with vertical sides, a minimum C value of 2.6 should be used.

V-Notch Weirs A V-Notch weir is illustrated in Weir Configurations. The discharge through a v-notch weir can be calculated from the following equation (Brater and King, 1976):

English:

Q=2.5tan\left({\frac {\theta }{2}}\right)H^{2.5}

(Equation 8)

where:

Q = discharge, ft³/s

q = angle of v-notch, degrees

H = head on apex of notch, ft.

Orifices Pipes smaller than 12 in. may be analyzed as a submerged orifice if H/D is greater than 1.5. For square-edged entrance conditions,

English:

Q=0.6A(2gH)^{0.5}=3.78D^{2}H^{0.5}\,

(Equation 9)

where:

Q = discharge, ft³/s

A = cross-section area of pipe, ft²/s

g = acceleration due to gravity, 32.2 ft/s²

D = diameter of pipe, ft.

H = head on pipe, from the center of pipe to the water surface, ft.

Culverts If culverts are used as outlets works, procedures presented in the Culverts should be used to develop stage-discharge data.

749.10.2 Routing Calculations

The following procedure is used to perform routing through a reservoir or storage facility (Storage Indication or Puhls Method of storage routing).

Routing a flood through a reservoir results in an attenuation of the peak of the inflow hydrograph and an associated change in timing of the peak. Storage of flood waters within the reservoir causes the peak outflow from the reservoir to be lower than the peak inflow, and causes the peak outflow to occur at a later time than the peak inflow. The continuity equation relates the change of storage within the detention storage basin to the inflow and outflow for the basin:

$I-O={\frac {\Delta S}{\Delta T}}$ (Equation 10)

where:

I = inflow, ft³/s

O = outflow, ft³/s

DS = change in storage, ft³

DT = change in time, seconds

The Storage Indication method of reservoir routing uses a simple finite-difference form of the continuity equation. For any two points in time, the continuity equation can be written as:

{\Big (}{\frac {2S_{n+1}}{\Delta T}}+O_{n+1}{\Big )}=(I_{n}+I_{n+1})+{\Big (}{\frac {2S_{n}}{\Delta T}}-O_{n}{\Big )}

(Equation 11)

where:

S = storage

If the values at time step n are known, the only unknowns in equation 8 are on the left-hand side.

Substituting

U_{n+1}={\frac {2S_{n+1}}{\Delta T}}+O_{n+1}

(Equation 12)

W_{n}={\frac {2S_{n}}{\Delta T}}-O_{n}

(Equation 13)

U is known as the Storage Indication Number. With these substitutions, equation 8 becomes:

U_{n+1}=(I_{n}+I_{n+1})+W_{n}\,

(Equation 14)

For the first time step, W_n is calculated using the initial values of S and O, and equation 13. For subsequent time steps the following equation can be used as a shortcut.

W_{n+1}=U_{n+1}-2O_{n+1}\,

(Equation 15)

The procedure for using the storage-indication method of reservoir routing is as follows:

1) Develop an inflow hydrograph, stage-discharge curve, and stage-storage curve for the proposed storage facility.

2) Select a routing time period, t, to provide at least five points on the rising limb of the inflow hydrograph.

3) Use the stage-storage and stage-outflow data from Step 1 to develop a plot of U versus outflow.

4) Calculate W₁ using equation 22 and the initial values of S and O

5) Calculate U_n+1 using equation 23.

6) Using U_n+1 calculated in step 5 pick O_n+1 from the plot of U vs. outflow.

7) Using U_n+1 and O_n+1 calculate W_n+1 using equation 24

8) Start over at step 5 with n = n+1. Continue repeating until inflow ceases or the outflow peak discharge has been determined.

9) From the stage discharge curve, determine the stage for the peak outflow.

749.11 Stream Confluences

Water surface elevations at stream crossings near the confluence with another stream are often affected by backwater from the other stream. Several methods may be used to determine the water surface elevations at such crossings, such as:

Coincident Peaks (i.e., 100-yr event on both streams)
Exclusive Peaks (i.e., 100-yr event on one stream, no flow from other stream)
Flow Combinations (i.e., 86-yr Event on Main Reach, 62-yr Event on Tributary)

749.11.1 Coincident Peaks

When both streams reach their peak flow, for a given return period, at the confluence of the streams at or near the same time the peak flows can be considered to happen at the same time. The water surface elevation of each stream is effected by the other. This method may provide overly conservative water surface elevations if applied to confluent streams that do not have similar peak times.

FEMA, Guidelines and Specifications for Flood Hazard Mapping Partners, Appendix C: Guidelines for Riverine Flood Analysis and Mapping gives the following guidance on when coincident peaks are appropriate to use:

749.11.2 Exclusive Peaks

Hydraulic analysis is conducted individually for each stream, for a given return period, without inclusion of any flow from the other stream, as each stream is assumed to reach its peak flow at different times. Analysis of the crossed stream will yield water surface elevations for the headwater flow of the crossed stream. If a structure is included in this model the effects of the structure on the water surface elevations will be included in the model. Analysis of the confluent stream will yield water surface elevations for the headwater flow of the confluent stream which can be used to determine the backwater surface elevation on the crossed stream. This backwater surface elevation does not include any effects from the flow of the crossed stream or any structures on the crossed stream. Since there would likely be some flow from the crossed stream this method may provide un-conservative water surface elevations in some cases.

This method is commonly used in hydraulic modeling for FEMA Flood Insurance Studies.

749.11.3 Flow Combinations

Similar to Coincident Peaks this method accounts for effects of one streams water surface elevation on the other, but uses flows from different return periods for each stream. Using a combination of flows can more closely simulate actual flooding conditions; the difficulty is in selecting a flow combination that will mimic those conditions. A comparison of hydrographs of both watersheds to determine the maximum peak flow at a given return period, or the probability based procedure shown below may be used to estimate flow combinations.

For streams that are within the influence reach of the confluent streams backwater the procedure provided in NCHRP 199, Estimating Joint Probabilities of Design Coincident Flows at Stream Confluences, produces a probability based estimate for combinations of return periods for confluent streams that will produce the maximum water surface elevation and/or velocity for a design flood event. An excerpt below from NCHRP 199 provides information on which cases this procedure is applicable to (Note: For Case A this procedure may still be useful to determine flow downstream of the junction based on flow combinations).

749.12 References

AASHTO, 2014 AASHTO Drainage Manual, American Association of State Highway and Transportation Officials.

Southard, R.E. and Veilleux, A.G.2014, Methods of Estimating Annual Exceedance-Probability Discharges and largest Recorded Floods for Unregulated Streams in Rural Missouri, USGS Scientific Investigations Report 2014-5165.0000

Alexander T.W. and Wilson, G.L., 1995, Technique for Estimating the 2- to 500-Year Flood Discharges on Unregulated Streams in Rural Missouri, USGS Water-Resources Investigations Report 95-4231

Southard, R.E., 2010, Estimation of the Magnitude and Frequency of Floods in Urban Basins in Missouri, USGS Scientific Investigations Report 2010-5073.

McCuen, R.H., et al., 2002 Highway Hydrology – Hydraulic Design Series No. 2 (HDS-2), Federal Highway Administration, Publication No. FHWA-NHI-02-001

Ponce, V.M., 1989, Engineering Hydrology, Englewood Cliffs, New Jersey 07632, Prentice-Hall, Inc.

United States Department of Agriculture: National Resource Conservation Service, National Engineering Handbook. NEH part 630 – Hydrology & Hydraulics

US Army Corps of Engineers, 2000, Hydrologic Modeling System HEC-HMS Technical Reference Manual, Hydrologic Engineering Center, Davis, CA

Kilgore, R.T., et. al., 2013, Estimating Joint Probabilities of Design Coincident Flows at Stream Confluences, NCHRP 199.

Roussel, M. C., et al, 2005. Time-Parameter Estimation for Applicable Texas Watersheds, U.S. Geological Survey, Texas Water Science Center, Research Report No. 0-4696-2

Cleveland, T. G., et al, 2012, Guidance for Estimation of Time of Concentration in Texas for Low-Slope Conditions, Texas Tech University, Lubbock Texas, Report No. 0-6382-P1

@@ Line 1: / Line 1: @@
-==749.1 General==
+=749.1 General=
 Hydrology is that branch of the applied earth sciences which deals with the occurrence and distribution of surface and ground waters. Highway drainage design is concerned with the surface water hydrology of small watersheds. In particular, the peak rate of runoff and/or the runoff hydrograph produced by flood events are of interest in analysis of highway drainage problems. Peak runoff rates are a parameter of design and their estimation is a prerequisite to hydraulic computations. Flood hydrographs are a necessary parameter of stormwater detention basin design, which can be used to reduce peak runoff rates for design of certain highway drainage structures.
@@ Line 9: / Line 10: @@
 *Check that the final design meets the requirements of the National Flood Insurance Program (NFIP)
-Criteria for determination of the design flood frequency and methods for computing peak rates of runoff and runoff hydrographs are presented in this section.
+Criteria for determination of the design flood frequency and methods for computing peak rates of runoff and runoff hydrographs are presented in this article.
+=749.2 Design Frequency and Return Period=
-==749.2 Design Frequency and Return Period==
 The frequency of occurrence or return period of an event may be defined as the average period of time between events equal to or greater than a given magnitude. The annual probability of occurrence of an event is equal to the reciprocal of the event's return period. For example, a flood with a return period of 100 years has a 1% chance of occurring in any year; whereas a flood with a return period of 25 years has a 4% chance of occurring in any year.
-===749.2.1 Design Frequency Criteria===
+==749.2.1 Design Frequency Criteria==
-The return period used as a criterion of design is known as the design frequency. The design frequency to be used in the design of drainage structures is a function of roadway type (major of minor), traffic volume and type of drainage facility. The design flood magnitude, design frequency and corresponding water surface elevation(s) shall be included on the project plans for all crossroad structures.
+The return period used as a criterion of design is known as the design frequency. The design frequency to be used in the design of drainage structures is a function of roadway type (major or minor), traffic volume and type of drainage facility.
+===749.2.1.1 Crossroad Structures===
+The design flood magnitude, [[:Category:748 Hydraulics and Drainage|design frequencies]] and corresponding water surface elevation(s) shall be included on the project plans for all crossroad structures.
+===749.2.1.2 Roadside Ditches and Culverts===
+The design frequency for roadside drainage ditches should be selected based on the function served. Ditches with a primary function of removal of water from the pavement should be based on the [[640.1 Pavement Drainage#640.1.2.1 Design Frequency|design frequency of the pavement drainage]]. Ditches with a primary function of carrying water to or from a crossroad structure should be based on the [[748.2 Roadway Design Criteria#748.2.2 Crossroad Structure Frequency Criteria (Roadway Design Frequency)|design frequency of the crossroad structure]].
+Culverts not considered crossroad structures should be designed to the same frequency as the ditch.
+===749.2.1.3 Pavement Drainage===
+See [[640.1 Pavement Drainage#640.1.2.1 Design Frequency|EPG 640.1.2.1 Design Frequency]].
-The design frequency for roadside drainage ditches should be chosen based on the function served. Ditches with a primary function of removal of water from the pavement should be based on the [[640.1 Pavement and Bridge Deck Drainage|design frequency of the pavement drainage]]. Ditches with a primary function of carrying water to or from a crossroad structure should be based on the [[:Category:748 Hydraulics and Drainage|design frequency of the crossroad structure]].
+===749.2.1.4 Median Drainage===
+See [[640.2 Median Drainage#640.2.3 Design Discharge|EPG 640.2.3 Design Discharge]].
-===749.2.2 Base Flood and Overtopping Flood===
+==749.2.2 Base Flood==
-The base flood is defined as the 100-year flood or the flood with a 1% chance of being exceeded in any given year. The overtopping flood is defined as the discharge and corresponding return period and water surface elevation at which flow occurs over the roadway.
+The base flood is defined as the 100-year flood or the flood with a 1% chance of being exceeded in any given year.
 After sizing a drainage facility using the [[:Category:748 Hydraulics and Drainage|design flood criteria]], the ability of the proposed facility to pass the base (100-yr) flood shall also be evaluated, with special attention to any risks to people or property. This is done to ensure that there are no unexpected flood hazards inherent in the proposed facility for flood events exceeding the design discharge.
-The overtopping flood shall be determined for all crossroad structures, and the resulting overtopping flood magnitude and approximate return period shall be included on the project plans. When the overtopping flood is greater than the 100-year flood, the return period should be noted as “>100 yr”. If the overtopping flood is less than the 100-year flood, the magnitude and water surface elevation of the 100-year flood shall also be included on the project plans.
+==749.2.3 Overtopping Flood==
+The overtopping flood is defined as the discharge and corresponding return period and water surface elevation at which flow occurs over the roadway. The overtopping flood shall be determined for all crossroad structures. The resulting overtopping flood discharge and approximate return period shall be included on the project plans. When the overtopping flood is greater than the 100-year flood, the return period should be noted as “>100 yr”. If the overtopping flood is less than the 100-year flood, the magnitude and water surface elevation of the 100-year flood shall also be included on the project plans.  For reporting overtopping for bridges, refer to [[750.3 Bridges#750.3.2.4.5 Overtopping Discharge and Frequency|EPG 750.3.2.4.5 Overtopping Discharge and Frequency]].
-===749.2.3 National Flood Insurance Program Criteria===
+==749.2.4 National Flood Insurance Program Criteria==
-Meeting the requirements of the National Flood Insurance Program (NFIP) regulations can necessitate larger structures than those designed to meet the above design frequencies. The designer shall ensure that the final design meets all requirements of the NFIP. Federal Emergency Management Agency (FEMA) Flood Insurance Study (FIS) maps should be consulted to determine the applicable NFIP requirements. Development in a mapped regulatory floodway can cause no increase in Base Flood Elevations (BFE’s). Development in a mapped Special Flood Hazard Area where no floodways have been defined can cause no more than one-foot (0.30 m) increase in BFE’s.
+Meeting the requirements of the National Flood Insurance Program (NFIP) regulations can necessitate larger structures than those designed to meet the above design frequencies. The designer shall ensure that the final design meets all requirements of the NFIP. Federal Emergency Management Agency (FEMA) Flood Insurance Study (FIS) maps should be consulted to determine the applicable NFIP requirements.
 See the following for definitions and additional information on the NFIP:
@@ Line 33: / Line 48: @@
 *[[748.9 National Flood Insurance Program (NFIP)|National Flood Insurance Program (NFIP)]] for structures
-==749.3 Design Data==
+=749.3 Design Data=
 In order to carry out the hydrologic analysis of a watershed, it is necessary to assemble certain data. This data includes the drainage area, the length of the hydraulically longest drainage path, the elevation of the watershed ridge, the elevation of the watershed outlet, the hydrologic soil group, the type of terrain, the land use of the watershed, and information on the extent of development in urban areas. The topographic data should be obtained from suitable maps when maps are available. Suitable maps are defined as department manuscripts and 7-1/2 or 15 minute USGS topographic maps or maps of equal quality. Only when such maps are not available should field measurements be made for the purpose of obtaining hydrologic information. The land use and terrain of the watershed should be evaluated in the field by the designer. The hydrologic soil group is either given on the soil survey performed by the Materials Division or is to be determined from county soil survey maps or from information from the soil survey and site visits.
-==749.4 Rural vs. Urban Hydrology==
+=749.4 Rural vs. Urban Hydrology=
-Small watersheds may be divided into rural and urban classifications. A rural watershed is a drainage basin whose natural response to rainfall has not been substantially altered by urban land activity. Rural watersheds may be either natural or agricultural.
+Small watersheds may be divided into rural and urban classifications.
+Three methods are presented in this article for estimating peak rates of runoff from small watersheds. The Rational Method may be used on all watersheds smaller than 200 acres. On watersheds within the area limits shown in the table below, United States Geological Survey (USGS) Regression Equations may be used. The [[#749.9.1 NRCS Unit Hydrograph|Natural Resource Conservation Service (NRCS) Unit Hydrograph]] may also be used for drainage areas larger than 6.40 acres.
+For more information on watershed area limits for the above methods see Summary of Small Watershed Peak Runoff Calculation Method Drainage Area Limits table.
+<center>
+==Summary of Small Watershed Peak Runoff Calculation Method Drainage Area Limits==
+{| border="1" class="wikitable" style="margin: 1em auto 1em auto" style="text-align:center"
+|-
+!style="background:#BEBEBE"  colspan="4"|Calculation Method	!!style="background:#BEBEBE" |Minimum Drainage Area	!!style="background:#BEBEBE" |Maximum Drainage Area
+|-
+|colspan="4"|Rational Method	||0	||200 acres
+|-
+|colspan="4"|NRCS Unit Hydrograph||	6.4 acres||	16,000 acres
+|-
+|rowspan="5"|USGS Regression Equations||rowspan="4" width="80"|Rural||rowspan="3"|	2014||	Region I||70.4 acres||	>10,000 acres
+|-
+|Region II||	108.8 acres||	>10,000 acres
+|-
+|Region III||	1,356.8 acres||	>10,000 acres
+|-
+|1995||	Region III||	307.2 acres||	>10,000 acres
+|-
+|Urban||colspan="2"|2010||	180 acres||	>10,000 acres
+|}
+</center>
+==749.4.1 Rural Watersheds ==
+A rural watershed is a drainage basin whose natural response to rainfall has not been substantially altered by urban land activity. Rural watersheds may be either natural or agricultural.
+The Rational Method may be used on rural watersheds smaller than 200 acres. On watersheds larger than 200 acres, the USGS Regression Equations should be used. The 2014 USGS Rural Regression Equations may be used for Watersheds that lie in Region I and Region II if the drainage area is greater than 70 acres and 110 acres respectively. Watersheds that lie within Region III for the 2014 USGS Rural Regression Equations and have drainage areas between 200 to 1350 acres do not fall within the limits for either set of equations. In this case one of the following approaches may be used:
+:a.	Use an alternate method such as [[#749.9 Flood Hydrographs|Flood Hydrographs]];
+:b.	for drainage areas greater than 300 acres use the 1995 USGS Rural Regression Equations;
+:c.	for drainage areas between 200 and 300 acres calculate the peak rate of runoff using the Rational Method and the 1995 USGS Rural Regression Equations, and use the one that is most appropriate.
+The [[#749.9.1 NRCS Unit Hydrograph|NRCS Unit Hydrograph]] may also be used for drainage areas larger than 6.40 acres.
+For more information on watershed area limits for the above methods see Summary of Small Watershed Peak Runoff Calculation Method Drainage Area Limits table.
+==749.4.2 Urban Watersheds==
 An urban watershed may be defined as a drainage basin in which manmade developments in the form of impervious surfaces and/or storm drainage systems have substantially altered the basin's natural response to rainfall. Urbanization of a natural watershed progresses in one of two ways. First, the addition of impervious surfaces in the form of roads, streets, parking lots and roofs will prevent infiltration of rainfall into the covered soil surface, thus increasing the total volume and peak rate of runoff from a given rainfall volume. Second, to protect the now valuable property in a developed watershed from this increased peak and volume of runoff, storm drainage systems are installed. The installation of a storm drainage system does not increase the volume of runoff, but modifies the time distribution of runoff. Thus, when storm water drainage systems are installed, the time of concentration of the watershed is decreased.  Therefore, storm water drainage systems have the effect of removing a given volume of runoff in a shorter period of time, thus further increasing the peak rate of runoff.
@@ Line 43: / Line 101: @@
 All hydraulic design in urban areas should consider the effect of increasing development throughout the projected life of the structure. Information on planned future development may be available from local agencies.
-Two methods are presented in this section for estimating peak rates of runoff from small watersheds. The Rational Method should be used on all watersheds less than 200 acres in size. On watersheds greater than 200 acres in size, the USGS Regression Equations should be used. Watersheds that lie within Region III for the USGS Rural Regression Equations and have drainage areas between 200 to 300 acres do not fall within the limits for either set of equations. In this case, both sets of equations should be calculated, and the designer should decide which is appropriate.
+The Rational Method should be used on urban watersheds smaller than 200 acres. On watersheds larger than 200 acres, the USGS Urban Regression Equations should be used.  The [[#749.9.1 NRCS Unit Hydrograph|NRCS Unit Hydrograph]] may also be used for drainage areas larger than 6.40 acres.
-==749.5 The Rational Method==
+For more information on watershed area limits for the above methods see [[#Summary of Small Watershed Peak Runoff Calculation Method Drainage Area Limits|Summary of Small Watershed Peak Runoff Calculation Method Drainage Area Limits table]].
+=749.5 The Rational Method=
 The Rational Method was developed as early as 1889, and despite its limitations is one of the most widely used methods of estimating peak flows. Several assumptions are implicit in application of the Rational Method:
@@ Line 54: / Line 114: @@
 The first assumption implies that a homogeneous rainfall event is applied uniformly to the entire drainage area, and may not be valid for larger watersheds where constant rainfalls of high intensity do not occur simultaneously over the entire watershed. This assumption also provides the basis for using the watershed's time of concentration as the duration of the design storm. The second assumption again limits the size of the drainage area because for larger basins, factors other than rainfall frequency can play a large role in determining the flood frequency. Finally, the third assumption is reasonable for highly impervious areas, but less reasonable for pervious areas where the antecedent moisture condition plays a large role in determining the amount of rainfall that becomes surface runoff. For these reasons, use of the Rational Method is limited to small watersheds.
-===749.5.1 Equation===
+==749.5.1 Equation==
 The Rational Method is expressed by the following formula.
-:<math>Q=k_cCIA\,</math> (Equation 1)
+:<math>Q=k_cCIA\,</math>
 :where:
@@ Line 67: / Line 127: @@
 :A = drainage area of the watershed, acres
-===749.5.2 Runoff Coefficient===
+==749.5.2 Runoff Coefficient==
 The runoff coefficient represents the ratio of runoff to the total rainfall and combines the effects of several watershed characteristics such as land use, soil type, cover condition, and general terrain or watershed slope.
-====749.5.2.1 Rural Runoff Coefficients====
+===749.5.2.1 Rural Runoff Coefficients===
 For rural watersheds, the soil type, land use, and terrain are used to determine runoff coefficients.
-The Soil Conservation Service (SCS) has classified about 4,000 major soils found in the United States into four basic hydrologic groups as follows:
+The Natural Resource Conservation Service (NRCS) has classified about 4,000 major soils found in the United States into four basic hydrologic groups as follows:
 :'''Group A (Low runoff potential)''' - Soils having high infiltration rates even when thoroughly wetted and consisting chiefly of deep, well-drained sands or gravels. These soils have a high rate of water transmission.
@@ Line 80: / Line 140: @@
 :'''Group D (High runoff potential)''' - Soils having very slow infiltration rates when thoroughly wetted and consisting chiefly of clay soils with a high swelling potential, soils with a permanent high water table, soils with a claypan or clay layer at or near the surface, and shallow soils over nearly impervious material. These soils have a very slow rate of water transmission.
-The SCS has performed modern soil surveys for many counties in Missouri, classifying the soils by a soil series name. The hydrologic soil group is given in the SCS soil survey for these soils. If a modern soil survey map is not available for the county in question, the hydrologic soil group should be determined by the designer. This may be done by evaluating information available from the soil survey performed by the Materials Division and/or from a site visit and choosing the appropriate hydrologic soil group from the descriptions given above. The district soils and geology technologist may provide assistance in determining the hydrologic soil group.
+The NRCS has performed modern soil surveys for many counties in Missouri, classifying the soils by a soil series name. The hydrologic soil group is given in the NRCS soil survey for these soils. If a modern soil survey map is not available for the county in question, the hydrologic soil group should be determined by the designer. This may be done by evaluating information available from the soil survey performed by the Construction and Materials Division and/or from a site visit and choosing the appropriate hydrologic soil group from the descriptions given above. The district soils and geology technologist may provide assistance in determining the hydrologic soil group.
-For smaller projects, the soil series name and hydrologic soil group for soils along the right of way are generally given on the soil survey performed by the Construction and Materials Division. For larger projects, this soil survey does not name individual soil series, but rather gives "soil associations" along the right of way. These soil associations are groupings of soil series that have similar physical characteristics; however, the soils in an association may not have the same hydrologic soil group. Additionally, in many instances it will be necessary to determine a hydrologic soil group for soils located some distance from the right of way. In these cases, the hydrologic soil group should be determined by the designer from the SCS county soil survey map, if available, or by evaluating information available from the soil survey performed by the Construction and Materials Division and/or from a site visit.
+For smaller projects, the soil series name and hydrologic soil group for soils along the right of way are generally given on the soil survey performed by the Construction and Materials Division. For larger projects, this soil survey does not name individual soil series, but rather gives "soil associations" along the right of way. These soil associations are groupings of soil series that have similar physical characteristics; however, the soils in an association may not have the same hydrologic soil group. Additionally, in many instances it will be necessary to determine a hydrologic soil group for soils located some distance from the right of way. In these cases, the hydrologic soil group should be determined by the designer from the NRCS county soil survey map, if available, or by evaluating information available from the soil survey performed by the Construction and Materials Division and/or from a site visit.
 Land use and terrain can be determined by inspection of appropriate topographic maps or by a field inspection. After the hydrologic soil group, land use, and terrain have been determined, the runoff coefficient(s) to be used for estimating the 10-year peak flow for the rural watershed can be found in the following table.
@@ Line 88: / Line 148: @@
 {| border="1" class="wikitable" style="margin: 1em auto 1em auto"
 |+ '''Rural Runoff Coefficients for 5 and 10-Yr Frequency Flows'''
-! rowspan="2" style="background:#BEBEBE"|Topography and Vegetation !! style="background:#BEBEBE" colspan="4"|SCS Hydrologic Soil Group
+! rowspan="2" style="background:#BEBEBE"|Topography and Vegetation !! style="background:#BEBEBE" colspan="4"|NRCS Hydrologic Soil Group
 |-
 ! style="background:#BEBEBE"|A !! style="background:#BEBEBE"|B !! Style="background:#BEBEBE"|C
@@ Line 119: / Line 179: @@
 |colspan="5"|<sup>1</sup>Flat 0-5% slope; Rolling 5-10%; Hilly 10-30%
 |-
-|colspan="5" style="background:#99FFFF"|''Source'': Adapted from Ponce (1989)
+|colspan="5" |''Source'': Adapted from Ponce (1989)
 |}
-====749.5.2.2 Urban Runoff Coefficients====
+===749.5.2.2 Urban Runoff Coefficients===
 For urban watersheds, the runoff coefficient is primarily a function of land use and watershed slope. The following table gives a range of "C" values for various land use types in urban areas. The value selected by the designer should reflect the watershed slope, with steeper slopes having higher "C" values.
@@ Line 167: / Line 227: @@
 |Sodded Roadway Slopes (1V:6H to 1V:1H)||0.40 - 0.60
 |-
-|colspan="2" style="background:#99FFFF"|''Source'': Ponce (1979)
+|colspan="2" |''Source'': Ponce (1979)
 |}
-====749.5.2.3 Design Frequency====
+===749.5.2.3 Design Frequency===
 The [[#749.5.2.1 Rural Runoff Coefficients|rural]] and [[#749.5.2.2 Urban Runoff Coefficients|urban]] runoff coefficients given in the tables above are appropriate for 5-year or 10-year design frequencies. Estimation of peak flows for less frequent storms requires the use of a higher runoff coefficient because infiltration and other abstractions have proportionately less effect on the amount of rainfall that becomes runoff. To obtain runoff coefficients for other frequency events, the "C" value is multiplied by a frequency correction factor. The frequency correction factor is 1.10 for the 25-year event, 1.20 for the 50-year event, and 1.25 for the 100-year event. However, the resulting runoff coefficient (original "C" multiplied by the frequency correction factor) may not be greater than 1.0.
-====749.5.2.4 Composite Runoff Coefficients====
+===749.5.2.4 Composite Runoff Coefficients===
-The land use and soil type may vary from one portion of a watershed to another. In this case an average runoff coefficient, weighted by area, should be used. For example, consider a flat watershed with soils of average infiltration capacity (soil group B) and with 1/3 of the drainage area cultivated (C=0.40) and 2/3 of the drainage area in woodland cover (C=0.20). The weighted average runoff coefficient to be used to estimate the 10-year flood is.
+The land use and soil type may vary from one portion of a watershed to another. In this case an average runoff coefficient, weighted by area, should be used. For example, consider a flat watershed with soils of average infiltration capacity (soil group B) and with 1/3 of the drainage area cultivated (C=0.40) and 2/3 of the drainage area in woodland cover (C=0.20). The weighted average runoff coefficient to be used to estimate the 10-year flood is:
-<math>C=[(0.40)(1/3)+(0.20)(2/3)]/1.0=0.267\,</math> (Equation 2)
+:<math>C=[(0.40)(1/3)+(0.20)(2/3)]/1.0=0.267\,</math>
 The frequency correction factor may be applied to the weighted average runoff coefficient to estimate lower frequency flows: in the above example, a "C" value of (1.10)(0.267)=0.294 would be used for the 25-year flood, and a "C" of (1.20)(0.267)=0.320 would be used for the 50-year flood.
-===749.5.3 Time of Concentration===
+==749.5.3 Time of Concentration==
 In order to determine the rainfall intensity used in the Rational Method, the time of concentration of the watershed must be estimated. The time of concentration of a watershed is defined as the time required for water to travel from the most hydraulically distant point of the watershed to the watershed outlet. This is also the time required before the entire watershed begins to contribute flow to the watershed outlet. This characteristic response time of the watershed is used as the duration of the design storm and thus influences the value of rainfall intensity used in the Rational Method.
@@ Line 187: / Line 247: @@
 In general, the time of concentration of urban drainage basins will be shorter than the time of concentration of rural basins. This is true when the natural drainage channels have been altered by storm sewers, main channel straightening, paving or similar modifications.
-The time of concentration of an urban watershed may consist of the summation of travel times involved in three different flow regimes; overland flow, ditch or channel flow, and storm sewer system flow. The overland flow regime consists of very shallow sheet flow over the watershed surface. The velocities in overland flow are typically much lower than in the other flow regimes. After a short distance, the sheet flow becomes concentrated in swales or ditches, which begins the open-channel flow phase. The last phase consists of the travel time through the storm sewer system, where velocities are typically greater than through natural ditches or channels. Note that the flow often returns to open-channel flow after exiting the storm sewer system. The time of concentration is then the overland flow travel time plus the storm sewer travel time plus the channel flow travel time.
+The time of concentration of a watershed will consist of the summation of travel times from one or more of the following flow regimes:
+:*	overland flow,
+:*	ditch or open channel flow,
+:*	and storm sewer system flow.
+'''Overland Flow.''' The overland flow regime consists of very shallow sheet flow over the watershed surface. The velocities in overland flow are typically much lower than in the other flow regimes. Overland flow will become concentrated after a short distance and transform into one of the other flow regimes.
+'''Open Channel Flow.''' The open channel flow regime consists of:
+:*	flow that has concentrated into swales or ditches, or
+:*	an outlet discharge from a storm sewer system
+:*	If open channel flow is a combination of the two, both contributors should be evaluated and engineering judgment used to determine the appropriate time of concentration.
+'''Storm Sewer System Flow.''' Velocities are typically greater in sewer systems than in natural ditches or channels. The storm sewer system flow regime consists of:
-The sections below provide methods for estimating time of concentration for both rural and urban watersheds. Other methods such as those provided by the Soil Conservation Service (SCS) may be used as deemed necessary or appropriate by engineering judgment.
+:*	Overland flow entering drain inlets, or
+:*	open channel flow entering drain inlets.
+:*	If sewer system flow is a combination of the two, both contributors should be evaluated and engineering judgment used to determine the appropriate time of concentration.
-====749.5.3.1 Kirpich Equation====
+===749.5.3.1 Rural Watersheds===
-For small rural watersheds, all flow regimes may be combined into a single equation used to calculate time of concentration.  The Kirpich equation is used for these watersheds:
+For rural drainage basins the Kerby-Kirpich Method described below is a recommended method of determining time of concentration. Other methods such as those provided by the Natural Resource Conservation Service (NRCS) may be used as deemed necessary or appropriate by engineering judgment. See Velocity Method in Chapter 2 of FHWA publication ''Hydraulic Design Series No. 2 (HDS-2)'' for details on the NRCS method.
-:<math>t_c=KL^{0.77}S^{-0.385}\,</math> (Equation 3)
+===749.5.3.2 Urban Watersheds===
+For urban drainage basins the Kerby-Hathaway Equation for Overland Flow plus the appropriate Channel or Storm Sewer Travel Time as described below is a recommended method of determining time of concentration. Other methods such as those provided by the Natural Resource Conservation Service (NRCS) may be used as deemed necessary or appropriate by engineering judgment. See Velocity Method in Chapter 2 of FHWA publication ''Hydraulic Design Series No. 2 (HDS-2)'' for details on the NRCS method.
+===749.5.3.3 Kerby-Kirpich Method===
+For rural drainage basins the Kerby- Hathaway and Kirpich methods can be used in conjunction to estimate the time of concentration.
+:T<sub>c</sub> = t<sub>o</sub> + t<sub>c</sub>
+:where:
+:T<sub>c</sub> = the total time of concentration (min)
+:t<sub>o</sub> = the overland flow time of concentration from the Kerby-Hathaway equation (min)
+:t<sub>c</sub> = the channel flow time of concentration from the Kirpich equation (min)
+====749.5.3.3.1 Low (Flat) Slopes====
+In watersheds with low topographical relief, time of concentration calculations will often result in unreasonably large values. Cleveland, et al. 2012 recommends the following adjustment to the slope in the Kirpich and Kerby-Hathaway equations to provide a more realistic estimate of the time of concentration:
+:*	For slopes flatter than 0.002 ft/ft (0.2%) the adjusted slope should be increased by 0.0005 (S<sub>adj</sub> = S + 0.0005).
+:*	For slopes between 0.002 ft/ft (0.2%) and 0.003 ft/ft (0.3%) engineering judgment should be used to determine if the adjustment to the slope should be made.
+====749.5.3.3.2 Kirpich Equation====
+Open channel flow travel time for streams with natural beds, or that mimic natural beds, can be calculated using the Kirpich Equations If flow regimes are combined the upstream end of the flow length (L) should extent to the basin divide.
+:<math>t_c=KL^{0.77}S^{-0.385}\,</math>
 :where:
@@ Line 200: / Line 298: @@
 :t<sub>c</sub> = the time of concentration (min)
 :K = 0.0078
-:L = the length of the principal watercourse from outlet to divide, ft.
+:L = the flow length of the principal watercourse, ft.
 :S = the slope between the minimum and maximum elevation, ft./ft.
-====749.5.3.2 Kerby-Hathaway Equation for Overland Flow====
+====749.5.3.3.3 Kerby-Hathaway Equation for Overland Flow====
 Overland flow travel time can be calculated using the Kerby-Hathaway equation:
-:<math>t_o=K(Ln)^{0.47}S^{-0.235}\,</math> (Equation 4)
+:<math>t_o=K(Ln)^{0.47}S^{-0.235}\,</math>
 :where:
@@ Line 216: / Line 314: @@
 :S = the overland flow slope, ft./ft.
-The roughness coefficient, n, used in this equation is similar in meaning to that used in Manning's equation for open-channel flow; however, for a given type of surface, the roughness coefficient for overland flow will be considerably larger than for open-channel flow. Typical values for the roughness coefficient are given in the table below. The overland flow length should be limited to less than 500 ft., and is usually much less. After a short distance, overland flow usually begins to concentrate in swales or ditches, which begins the open channel flow phase.
+The roughness coefficient, n, used in this equation is similar in meaning to that used in Manning's equation for open-channel flow; however, for a given type of surface, the roughness coefficient for overland flow will be considerably larger than for open-channel flow. Typical values for the roughness coefficient are given in the table below. The overland flow length should be limited to less than 500 ft.;it is usually much less and engineering judgment should be used for lengths of more than 200 ft. After a short distance, overland flow usually changes flow regimes and begins to concentrate in swales or ditches, or into a storm sewer system.
 {| border="1" class="wikitable" style="margin: 1em auto 1em auto"
 |+ '''Roughness Coefficients for Overland Flow'''
-! style="background:#BEBEBE"|Type of Surface !! style="background:#BEBEBE"|N
+! style="background:#BEBEBE" colspan="2" width="225"|Type of Surface !! style="background:#BEBEBE"|N
 |-
-|Smooth Surfaces (concrete, asphalt, gravel or bare soil)||0.011
+|colspan="2"|Dense Growth||	0.40 to 0.50
 |-
-|Fallow (No Residue)||0.05
+|colspan="2"|Pasture||	0.30 to 0.40
 |-
-|colspan="2"|Cultivated Soils:
+|colspan="2"|Lawns||	0.20 to 0.30
 |-
-|text align="center"|Residue Cover<20%||0.06
+|colspan="2"|Bluegrass Sod||	0.20 to 0.50
 |-
-|text align="center"|Residue Cover >20%||0.17
+|colspan="2"|Short-Grass Prairie||	0.10 to 0.20
 |-
-|colspan="2"|Grass:
+|colspan="2"|Sparse Vegetation||	0.05 to 0.13
 |-
-|text align="center"|Short Grass Prarie||0.15
+|colspan="2"|Bare Clay-Loam Soil (Eroded)||	0.01 to 0.03
 |-
-|text align="center"|Dense Grasses -||
+|colspan="3"|Concrete - Asphalt
 |-
-|text align="right"|Bluegrass, Buffalo, Native Mixture||0.24
+|	||Very Shallow Depths < ¼”||	0.10 to 0.15
 |-
-|text align="center"|Bermudagrass||0.41
+|	||Shallow Depths > ¼”||	0.05 to 0.10
 |-
-|Range||0.13
+|colspan="2"|Deciduous Timberland||	0.50 to 0.70
 |-
-|colspan="2"|Woods:
+|colspan="2"|Conifer Timberland||	0.60 to 0.80
 |-
-|text align="center"|Light Underbrush||0.40
+|colspan="3" style="background:#f5f5f5"|Source: Ponce et al. (1989)
-|-
-|text align="center"|Dense Underbrush||0.80
-|-
-|colspan="2" style="background:#99FFFF"|''Source'': U.S. Army Corps of Engineers (2000)
 |}
-====749.5.3.3 Storm Sewer Travel Time====
+===749.5.3.4 Storm Sewer Travel Time===
 The storm sewer travel time is the summation of travel times in each component of the sewer system between the uppermost inlet and the outlet. These times may be estimated by use of the open channel flow charts presented in [[750.1 Open Channels|Open Channels]]. The proper procedure is to solve for the velocity of flow in each component assuming the pipe is flowing at maximum capacity. The velocity of flow is then divided into the length of flow to obtain the travel time. In order to use this method, it is necessary to obtain detailed information on the existing storm sewer system. This information includes conduit sizes, materials, lengths and slopes.  If this information is not available or if the structure being designed is not considered important enough to warrant the effort required for such a detailed analysis, the storm sewer travel time may be estimated by use of the [[#749.5.3.1 Kirpich Equation|Kirpich Equation]]. In this case, the travel time given by the Kirpich Equation should be multiplied by 0.20 to obtain the travel time in the storm sewer system.
-====749.5.3.4 Channel Flow Travel Time====
+===749.5.3.5 Channel Flow Travel Time===
 The travel time in the open channel flow phase can be estimated by first estimating the velocity of flow in the channel. The velocity of flow is then divided into the length of flow to obtain the travel time. For subcritical flow conditions, the velocity should be determined for bank-full conditions at the mid-point of the channel's length. If the channel is improved, the average velocity may be estimated from the flow charts or flow computations presented in [[750.1 Open Channels|Open Channels]]. If the channel is natural, the travel time may be estimated by open channel flow computations. Care should be taken when estimating channel flow travel time for supercritical flow conditions. In some cases, the bank-full velocity under supercritical conditions may be too high to provide a reasonable estimate of travel time and a more reasonable estimate of flow depth should be used instead.
-===749.5.4 Rainfall Intensity===
+==749.5.4 Rainfall Intensity==
 The design rainfall intensity is a function of the storm duration, the design frequency and the geographic location. The storm duration is taken as the time of concentration of the watershed or five minutes, whichever is greater. Knowing the storm duration and the design frequency, the rainfall intensity may be read from the appropriate Intensity-Duration-Frequency (IDF) curve for each district.
-:[[media:749 IDF Curve District 1.pdf|District 1]]
+:[[media:749 IDF NW.docx|Northwest District]]
-:[[media:749 IDF Curve District 2.pdf|District 2]]
+:[[media:749 IDF NE.docx|Northeast District]]
-:[[media:749 IDF Curve District 3.pdf|District 3]]
+:[[media:749 IDF KC.docx|Kansas City District]]
-:[[media:749 IDF Curve District 4.pdf|District 4]]
+:[[media:749 IDF Cen 2019.docx|Central District]]
-:[[media:749 IDF Curve District 5.pdf|District 5]]
+:[[media:749 IDF SL.docx|St. Louis District]]
-:[[media:749 IDF Curve District 6.pdf|District 6]]
+:[[media:749 IDF SW.docx|Southwest District]]
-:[[media:749 IDF Curve District 7.pdf|District 7]]
+:[[media:749 IDF SE.docx|Southeast District]]
-:[[media:749 IDF Curve District 8.pdf|District 8]]
-:[[media:749 IDF Curve District 9.pdf|District 9]]
+=749.6 USGS Regression Equations=
-:[[media:749 IDF Curve District 10.pdf|District 10]]
+The USGS Regression equations were developed by the United States Geological Survey.  Data from stream gage sites were analyzed to determine flood magnitudes with various recurrence intervals.  The resulting magnitudes were then related to basin characteristics through a statistical analysis to provide the regression equations.  Three sets of equations are currently available: 2014 and 1995 Rural Regression Equations, and Urban Regression Equations.
+Stream flows for ungaged sites on streams that have a stream gage(s) may be determined using the area weighted method shown in [[#749.7 Historical USGS Stream Gage Data|EPG 749.7, Historical USGS Stream Gage Data]].
+==749.6.1 Rural Regression Equations==
+For ungaged natural flood flow sites, flood magnitudes having recurrence intervals of 2, 5, 10, 25, 50, 100, 200 (2014 only) and 500 years are computed by using appropriate values of the contributing drainage basin area (A) and basin shape (B) (2014) or slope (S) (1995) in the equations shown in the Rural Regression Equations tables. A computer program is available to assist in performing these calculations. The state is divided into [[media:749.6.1.pdf|three hydrologic regions]], each with its own set of regression coefficients. The three regions are described as follows:
+'''Region I - Central Lowlands''' - Characterized by meandering stream channels in wide and flat valleys with local relief generally between 50 to 150 feet. Elevations range from about 600 ft. above sea level near the Mississippi River to about 1200 ft. above sea level in the northwestern part of the state.
-==749.6 USGS Regression Equations==
+'''Region II - Ozark Plateaus''' - - Characterized by streams that have narrow valleys 200 to 500 ft. deep, with local relief generally ranging from 100 to 500 feet. Main channel gradients generally are steeper than elsewhere in Missouri, and karst features are locally prominent in much of the region. Elevations (generally) range from 800 to 1700 ft. above sea level. Region II also includes the geological formation known as Crowley’s Ridge which rises 250 to 550 ft. above the Mississippi Alluvial Plain and reflects hydrologic and hydraulic characteristics similar to the Ozark Plateau.
-The USGS Regression equations were developed by the United States Geological Survey Office in Rolla.  Data from stream gage sites were analyzed to determine flood magnitudes with various recurrence intervals.  The resulting magnitudes were then related to basin characteristics through a statistical analysis to provide the regression equations.  Three sets of equations are currently available: Rural Regression Equations, Urban BDF Regression Equations and Urban Percentage of Impervious Area Regression Equations.
-===749.6.1 Rural Regression Equations===
+'''Region III - Mississippi Alluvial Plain''' - This is a relatively flat area and is  predominately agricultural cropland. Virtually all the area is drained by a series of man-made drainage ditches with average slopes of about 1.5 ft./mile. Elevations range from 200 to 300 ft. above sea level with local relief seldom exceeding 30 feet.
-These equations were developed in 1995 by the [http://mcmcweb.er.usgs.gov/index.html United States Geological Survey in Rolla]. Data from 278 gaged sites in Missouri were analyzed to determine flood magnitudes with recurrence intervals of 2, 5, 10, 25, 50, 100 and 500 years. The resulting magnitudes were then related to hydrologic region, drainage area and average main-channel slope by a statistical analysis to provide the regression equations.
-====749.6.1.1 Equations====
+===749.6.1.1 2014 Rural Regression Equations===
-For ungaged natural floodflow sites, flood magnitudes having recurrence intervals of 2, 5, 10, 25, 50, 100 and 500 years are computed by using appropriate values of the contributing drainage basin area (A) and slope (S) in the equations shown in the [[#Rural Regression Equations|Rural Regression Equations]] table. A computer program is available to assist in performing these calculations. The state is divided into [[media:749 Missouri's Hydrologic Regions.pdf|three hydrologic regions]], each with its own set of regression coefficients. The three regions are described as follows:
+Engineering judgment should be exercised when using these equations to ensure that the subject drainage area is similar in characteristics to the USGS region being used. Portions of eastern Butler and Ripley counties are especially sensitive as some streams will be consistent with slopes for Region 2 and others with Region 3.
-'''Region I - Central Lowlands''' - "Characterized by meandering stream channels in wide and flat valleys resulting in long and narrow drainage patterns with local relief generally between 50 to 150 ft.  Elevations range from about 600 ft. (180 m) above sea level near the Mississippi River to about 1200 ft. above sea level in the northwest parts of the region"
+Valid data ranges for each of the regional equations are listed in the Valid Data Ranges table below. The Supplemental Data Ranges table may be used to help determine which regional equation to use, but should not be used to determine the validity of the equations.
+<center>
-'''Region II - Ozark Plateaus''' - "Characterized by streams that have cut narrow valleys 200 to 500 ft. deep, resulting in sharp rugged ridges that separate streams, with local relief generally ranging from 100 to 500 ft. The drainage patterns are described as dendritic (tree shaped) with main-channel gradients steeper than elsewhere in Missouri, and karst features are locally prominent in much of the region. Elevations (generally) range from 800 to 1700 ft. above sea level."
+{| border="1" class="wikitable" style="margin: 1em auto 1em auto" style="text-align:center"
+|-
+!style="background:#BEBEBE"  colspan="4" width="350"|Valid Data Ranges!!style="background:#BEBEBE" | !!style="background:#BEBEBE" colspan="4" width="450"|Supplemental Data Ranges
+|-
+|colspan="2"|Drainage Area (mi<sup>2</sup>)||colspan="2"|Basin Shape||style="background:#BEBEBE" |	||colspan="2"|Basin Slope (ft/ft)||colspan="2"|Valley Slope (ft/mi)
+|-
+|Min.||	Max.||	Min.||	Max.||style="background:#BEBEBE" | ||	Min.||	Max.||	Min.||	Max.
+|-
+|0.11||	8212.38||	2.25||	26.59||'''	Region I'''||	0.0141||	0.1812||	1.53||	198.02
+|-
+|0.17||	4008.92||	2.04||	26.89||'''	Region II'''||	0.0133||	0.1999||	0.34||	257.13
+|-
+|2.12||	2177.58||	na||	na||'''	Region III'''||	0.0017||	0.0388||	0.61||	2.07
+|-
+|colspan="9" align="left"|Source: Southard and Veilleux (2014)
+|}
+</center>
-'''Region III - Mississippi Alluvial Plain''' - "A relatively flat area of excellent farmland.  Virtually all the area is drained by a series of man-made drainage ditches that slope southward at an average of about 1.5 ft./mile.  Elevations range from 200 to 300 ft. above sea level with local relief seldom exceeding 30 ft."
-<div id="Rural Regression Equations"></div>
+::::::::{| border="1" class="wikitable" style="margin: 1em auto 1em auto" style="text-align:center"
-{| border="1" class="wikitable" style="margin: 1em auto 1em auto"
+|-
-|+ '''Rural Regression Equations'''
+!style="background:#BEBEBE"  colspan="3"|2014 Rural Regression Equations
-! style="background:#BEBEBE"|Flood Freq. !! style="background:#BEBEBE"|Flood Magnitude !! style="background:#BEBEBE"|Std. Error of Prediction !! style="background:#BEBEBE"|Area Limits !! style="background:#BEBEBE"|Slope Limits
+|-
+!style="background:#BEBEBE"| Flood Freq.<br/>(years)!!style="background:#BEBEBE" |Flood Magnitude<br/>ft<sup>3</sup>/s!!style="background:#BEBEBE" |Std. Error of Prediction<br/>(%)
+|-
+!colspan="3"|Region I
+|-
+|2|| 	(10<sup>2.594</sup>)*(A<sup>0.618</sup>)*(BS<sup>-0.282</sup>)||	38
 |-
-! (years) !! ft<sup>3</sup>/s !! (%) !! (mi<sup>2</sup>) !! (ft/mi) !!
+|5|| 	(10<sup>2.861</sup>)*(A<sup>0.593</sup>)*(BS<sup>-0.266</sup>)||	31
 |-
-|colspan="8" text align="center" | Region I
+|10|| 	(10<sup>2.990</sup>)*(A<sup>0.580</sup>)*(BS<sup>-0.258</sup>)||	29
 |-
-| text align="center"| 2 ||69.4A<sup>0.703</sup>S<sup>0.373</sup> || text align="center"| 34.00 || 0.13 to 11,500  || 1.35 to 150
+|25|| 	(10<sup>3.120</sup>)*(A<sup>0.568</sup>)*(BS<sup>-0.248</sup>)||	29
 |-
-| text align="center"| 5 ||123A<sup>0.690</sup>S<sup>0.383</sup> || text align="center"| 32.00 || 0.13 to 11,500  || 1.35 to 150
+|50|| 	(10<sup>3.199</sup>)*(A<sup>0.560</sup>)*(BS<sup>-0.242</sup>)||	29
 |-
-| text align="center"| 10 ||170A<sup>0.680</sup>S<sup>0.378</sup> || text align="center"| 34.00 || 0.13 to 11,500  || 1.35 to 150
+|100|| 	(10<sup>3.266</sup>)*(A<sup>0.554</sup>)*(BS<sup>-0.236</sup>)||	30
 |-
-| text align="center"| 25 ||243A<sup>0.668</sup>S<sup>0.366</sup> || text align="center"| 36.00 || 0.13 to 11,500  || 1.35 to 150
+|200||	(10<sup>3.324</sup>)*(A<sup>0.550</sup>)*(BS<sup>-0.231</sup>)||	31
 |-
-| text align="center"| 50 ||305A<sup>0.660</sup>S<sup>0.356</sup> || text align="center"| 38.00 || 0.13 to 11,500  || 1.35 to 150
+|500|| 	(10<sup>3.391</sup>)*(A<sup>0.544</sup>)*(BS<sup>-0.226<sup>)||	33
 |-
-| text align="center"| 100 ||376A<sup>0.652</sup>S<sup>0.346</sup> || text align="center"| 40.00 || 0.13 to 11,500  || 1.35 to 150
+!colspan="3"|Region II
 |-
-| text align="center"| 500 ||569A<sup>0.636</sup>S<sup>0.321</sup> || text align="center"| 45.00 || 0.13 to 11,500  || 1.35 to 150
+|2|| 	(10<sup>2.493</sup>)*(A<sup>0.686</sup>)*(BS<sup>-0.222</sup>)||	44
 |-
-|colspan="8" text align="center" | Region II
+|5|| 	(10<sup>2.801</sup>)*(A<sup>0.679</sup>)*(BS<sup>-0.251</sup>)||	32
 |-
-| text align="center"| 2 ||77.9A<sup>0.733</sup>S<sup>0.265</sup> || text align="center"| 43.00 || 0.13 to 14,000 || 1.2 to 279
+|10|| 	(10<sup>2.955</sup>)*(A<sup>0.676</sup>)*(BS<sup>-0.268</sup>)||	28
 |-
-| text align="center"| 5 ||99.6A<sup>0.763</sup>S<sup>0.355</sup> || text align="center"| 36.00 || 0.13 to 14,000 || 1.2 to 279
+|25|| 	(10<sup>3.113</sup>)*(A<sup>0.673</sup>)*(BS<sup>-0.287</sup>)||	24
 |-
-| text align="center"| 10 ||117A<sup>0.774</sup>S<sup>0.395</sup> || text align="center"| 34.00 || 0.13 to 14,000 || 1.2 to 279
+|50|| 	(10<sup>3.205</sup>)*(A<sup>0.671</sup>)*(BS<sup>-0.296</sup>)||	24
 |-
-| text align="center"| 25 ||140A<sup>0.784</sup>S<sup>0.432</sup> || text align="center"| 32.00 || 0.13 to 14,000 || 1.2 to 279
+|100|| 	(10<sup>3.282</sup>)*(A<sup>0.669</sup>)*(BS<sup>-0.302</sup>)||	24
 |-
-| text align="center"| 50 ||155A<sup>0.789</sup>S<sup>0.453</sup> || text align="center"| 31.00 || 0.13 to 14,000 || 1.2 to 279
+|200||	(10<sup>3.349</sup>)*(A<sup>0.668</sup>)*(BS<sup>-0.307</sup>)||	25
 |-
-| text align="center"| 100 ||170A<sup>0.794</sup>S<sup>0.471</sup> || text align="center"| 32.00 || 0.13 to 14,000 || 1.2 to 279
+|500|| 	(10<sup>3.422</sup>)*(A<sup>0.667</sup>)*(BS<sup>-0.311</sup>)||	27
 |-
-| text align="center"| 500 ||203A<sup>0.804</sup>S<sup>0.503</sup> || text align="center"| 34.00 || 0.13 to 14,000 || 1.2 to 279
+!colspan="3"|Region III
 |-
-|colspan="8" text align="center" | Region III
+|2|| 	(10<sup>1.933</sup>)*(A<sup>0.665</sup>)||	30
 |-
-| text align="center"| 2 ||88A<sup>0.658</sup> || text align="center"| 34.00 || 0.48 to 1,040 || na
+|5|| 	(10<sup>2.026</sup>)*(A<sup>0.681</sup>)||	26
 |-
-| text align="center"| 5 ||145A<sup>0.627</sup> || text align="center"| 36.00 || 0.48 to 1,040 || na
+|10|| 	(10<sup>2.070</sup>)*(A<sup>0.689</sup>)||	26
 |-
-| text align="center"| 10 ||187A<sup>0.612</sup> || text align="center"| 38.00 || 0.48 to 1,040 || na
+|25|| 	(10<sup>2.113</sup>)*(A<sup>0.698</sup>)||	26
 |-
-| text align="center"| 25 ||244A<sup>0.595</sup> || text align="center"| 41.00 || 0.48 to 1,040 || na
+|50|| 	(10<sup>2.139</sup>)*(A<sup>0.703</sup>)||	27
 |-
-| text align="center"| 50 ||288A<sup>0.585</sup> || text align="center"| 44.00 || 0.48 to 1,040 || na
+|100|| 	(10<sup>2.162</sup>)*(A<sup>0.708</sup>)||	27
 |-
-| text align="center"| 100 ||334A<sup>0.576</sup> || text align="center"| 46.00 || 0.48 to 1,040 || na
+|200||	(10<sup>2.182</sup>)*(A<sup>0.713</sup>)||	28
 |-
-| text align="center"| 500 ||448A<sup>0.557</sup> || text align="center"| 54.00 || 0.48 to 1,040 || na
+|500|| 	(10<sup>2.204</sup>)*(A<sup>0.718</sup>)||	29
 |-
-|colspan="8" style="background:#99FFFF"|''Source'': Alexander and Wilson (1995)
+|colspan="3" align="left"|Source: Southard and Veilleux (2014)
 |}
-====749.6.1.2 Drainage Area====
-Drainage area (A) in mi<sup>2</sup>, can be obtained by determining the area contributing surface flows to the site as outlined along the drainage divide on the best available topographic maps.
-====749.6.1.3 Valley Slope====
+====749.6.1.1.1 Drainage Area====
-Valley slope (S) in ft./mile is the average slope between points 10 percent and 85 percent of the distance along the main-stream channel from the site to the basin divide. Distance is measured by setting draftsman's dividers at 0.1 mile spread and stepping along the main channel. The main channel is defined above stream junctions as the one draining the largest area. The elevation difference between the 10- and 85-percent points is divided by the distance between the points to evaluate the slope.
+Drainage area (A) in mi<sup>2</sup>, can be obtained by determining the area contributing surface flows to the site as outlined along the drainage divide on the best available topographic maps or computer program.
-====749.6.1.4 Limitations of Equations====
+====749.6.1.1.2 Basin Shape====
-The USGS Rural Regression Equations]] may be used to estimate magnitude and frequency of floods on most Missouri streams providing the drainage area and slope are within the limits shown in the.
+The Basin Shape (BS) is dimensionless and is a statistical method used to determine the magnitude and arrival time of a peak discharge. Basins with elongated shapes will have longer duration hydrographs and lower peak discharges than a circular basin that will have shorter duration hydrographs and higher peak discharges. The value used for the Basin Shape (BS) is calculated by dividing the square of the longest flow path or stream length by the Drainage Area (A). Stream length can be determined using the best available topographic maps or computer program.
-However, the equations are not applicable for:
+====749.6.1.1.3 Limitations of Equations====
+The USGS Rural Regression Equations may be used to estimate magnitude and frequency of floods on most Missouri streams. Caution should be used when using these equations when the drainage area and basin shape are outside of the limits of the Valid Data Ranges shown in [[#749.6.1.1 2014 Rural Regression Equations|EPG 749.6.1.1 2014 Rural Regression Equations]].
-*basins where manmade changes have appreciably changed the flow regimen
+However, the equations are generally not applicable for:
-*the main stems of the Mississippi and Missouri Rivers
-*areas near the mouth of streams draining into larger rivers where backwater effect is experienced
-===749.6.2 Urban Regression Equations===
+:a)	basins where manmade changes have appreciably changed the flow regimen such as:
-The USGS Rural Regression Equations given above are not applicable to urban watersheds where manmade changes have appreciably changed the flow regimen A set of equations were developed in 1986 by the [[United States Geological Survey in Rolla]]. Data from 37 gaged sites in both urban and rural locations in Missouri were analyzed to determine flood magnitudes with recurrence intervals of 2, 5, 10, 25, 50, and 100 years. The resulting magnitudes were then related to drainage area and urban basin characteristics to provide the regression equations.
+::*	Regulation of discharge (i.e. dams);
+::*	Channelization;
+::*	Diversion;
+::*	Urbanization
+:b)	the main stems of the Mississippi and Missouri Rivers;
+:c)	basins with greater than 5% impervious area;
+:d)	areas near the mouth of streams draining into larger rivers where backwater effect is experienced may require special consideration, see [[#749.11 Stream Confluences|EPG 749.11 Stream Confluences]].
-An urban watershed may be defined as a drainage basin in which manmade developments in the form of impervious surfaces and/or storm drainage systems have substantially altered the basin's natural response to rainfall.  Urbanization of a natural watershed progresses in one of two ways.  First, the addition of impervious surfaces in the form of roads, streets, parking lots and roofs will prevent infiltration of rainfall into the covered soil surface, thus increasing the total volume and peak rate of runoff from a given rainfall volume.  Second, to protect the now valuable property in a developed watershed from this increased peak and volume of runoff, storm drainage systems are installed.  The installation of a storm drainage system does not increase the volume of runoff, but modifies the time distribution of runoff.  Thus, when storm water drainage systems are installed, the time of concentration of the watershed is decreased.  Therefore, storm water drainage systems have the effect of removing a given volume of runoff in a shorter period of time, thus increasing the peak rate of runoff.
+===749.6.1.2 1995 Rural Regression Equations===
+Valid data ranges for each of the regional equations are listed in the Valid Data Ranges table below:
-A computer program is available to assist in performing these calculations.
-====749.6.2.1 Equations====
+<center>
-Peak discharges can be estimated at urban locations using either of the two sets of equations presented in the [[#Peak Discharge for Urban Basins Based on Basin Development Factor|Peak Discharge for Urban Basins Based on Basin Development Factor]] table or the [[#Peak Discharge for Urban Basins Based on Percent Impervious Area|Peak Discharge for Urban Basins Based on Percent Impervious Area]] table. Both sets give peak discharge as a function of drainage area (A) and a characteristic of urbanization: either basin development factor (BDF) or percentage of impervious area (I). Choice of which set of equations to use should depend on whether it is easier to determine BDF or percentage of impervious area for a given basin. Either set of equations should provide comparable results.
+{| border="1" class="wikitable" style="margin: 1em auto 1em auto" style="text-align:center"
+|-
+!style="background:#BEBEBE" width="170"| !!style="background:#BEBEBE"  colspan="4" width="650"|	Valid Data Ranges
+|-
+!style="background:#BEBEBE" width="170"| !! colspan="2"|Drainage Area (mi<sup>2</sup>)!!colspan="2"|Valley Slope (ft/mi)
+|-
+|!style="background:#BEBEBE" width="170"| ||	Min.||	Max.||	Min.||	Max.
+|-
+|Region I	||0.13	||11,500	||1.35	||150
+|-
+|Region II	||0.13	||14,000	||1.20	||279
+|-
+|Region III	||0.48	||1040	||na	||na
+|-
+|align="left" colspan="5"|Source: Alexander and Wilson (1995)
+|}
+</center>
-All hydraulic design in urban areas should consider the effect of increasing development throughout the projected life of the structure.  Information on planned future development may be available from local agencies.
-====749.6.2.2 Drainage Area====
+<div id="Rural Regression Equations"></div>
-Drainage area (A) in mi<sup>2</sup>, can be obtained by determining the area contributing surface flows to the site as outlined along the drainage divide on the best available topographic maps.
-====749.6.2.3 Basin Development Factor====
-The basin development factor (BDF) is determined by dividing the drainage basin into thirds (subareas). Each subarea of the basin is then evaluated for four aspects of urbanization. For each of the four criteria, a value of either 1 (if the subarea meets the criteria) or 0 (if the subarea does not meet the criteria) is assigned. The BDF is the sum of the values for each of the four criteria and for each third of the basin. A maximum BDF of twelve results when each of the three subareas meets each of the four criteria for urbanization described below:
-'''Channel Improvements''' - channel improvements such as straightening, enlarging, deepening, and clearing have been made to at least 50 percent of the main channel and principal tributaries.
-'''Channel Linings''' - more than 50 percent of the main channel and principal tributaries has been lined with an impervious material. (Note that the presence of the channel linings also implies the presence of channel improvements)
-'''Storm Drains or Storm Sewers''' - more than 50 percent of the secondary tributaries of a subarea consists of storm drains or storm sewers.
-'''Curb-and Gutter Streets''' - more than 50 percent of a subarea is urbanized and more than 50 percent of the streets and highways in the subarea are constructed with curbs and gutters.
-The valid range for BDF is 0 to 12. See [[Media:749 Typical Drainage Basin Shapes.pdf|Typical Drainage Basin Shapes]] and the method of subdivision into thirds.
-<div id="Peak Discharge for Urban Basins Based on Basin Development Factor"></div>
 {| border="1" class="wikitable" style="margin: 1em auto 1em auto"
-|+ '''Peak discharge for Rural Basins Based on Basin Development Factor'''
+|+ ''' 1995 Rural Regression Equations'''
-! style="background:#BEBEBE"|Flood Freq. !! style="background:#BEBEBE"|Flood Magnitude !! style="background:#BEBEBE"|Std. Error of Estimate !! style="background:#BEBEBE"|Area Limits !!  style="background:#BEBEBE"|Slope Limits
+! style="background:#BEBEBE"|Flood Freq.<br/>(years) !! style="background:#BEBEBE"|Flood Magnitude<br/>ft<sup>3</sup>/s !! style="background:#BEBEBE"|Std. Error of Prediction<br/>(%)
+|-
+|colspan="3" text align="center" | Region I
 |-
-! (years) !! ft<sup>3</sup>/s !! (%) !! (mi<sup>2</sup>) !! (ft/mi)
+| text align="center"| 2 ||69.4A<sup>0.703</sup>S<sup>0.373</sup> || text align="center"| 34.00
 |-
-| text align="center"| 2 ||801A<sup>0.747</sup>(13-BDF)<sup>-0.400</sup> || text align="center"| 32.90 || 0.25 to 40  || 8.7 to 120
+| text align="center"| 5 ||123A<sup>0.690</sup>S<sup>0.383</sup> || text align="center"| 32.00
 |-
-| text align="center"| 5 ||1150A<sup>0.746</sup>(13-BDF)<sup>-0.318</sup> || text align="center"| 29.40 || 0.65 to 100 || 8.7 to 120
+| text align="center"| 10 ||170A<sup>0.680</sup>S<sup>0.378</sup> || text align="center"| 34.00
 |-
-| text align="center"| 10 ||1440A<sup>0.755</sup>(13-BDF)<sup>-0.300</sup> || text align="center"| 28.40 || 0.25 to 40 || 8.7 to 120
+| text align="center"| 25 ||243A<sup>0.668</sup>S<sup>0.366</sup> || text align="center"| 36.00
 |-
-| text align="center"| 25 ||1920A<sup>0.764</sup>(13-BDF)<sup>-0.307</sup> || text align="center"| 27.30 || 0.25 to 40 || 8.7 to 120
+| text align="center"| 50 ||305A<sup>0.660</sup>S<sup>0.356</sup> || text align="center"| 38.00
 |-
-| text align="center"| 50 ||2350A<sup>0.773</sup>(13-BDF)<sup>-0.319</sup> || text align="center"| 26.50 || 0.25 to 40 || 8.7 to 120
+| text align="center"| 100 ||376A<sup>0.652</sup>S<sup>0.346</sup> || text align="center"| 40.00
 |-
-| text align="center"| 100 ||2820A<sup>0.783</sup>(13-BDF)<sup>-0.330</sup> || text align="center"| 26.40 || 0.25 to 40|| 8.7 to 120
+| text align="center"| 500 ||569A<sup>0.636</sup>S<sup>0.321</sup> || text align="center"| 45.00
 |-
-|colspan="8" style="background:#99FFFF"|''Source'': Becker (1986)
+|colspan="3" text align="center" | Region II
+|-
+| text align="center"| 2 ||77.9A<sup>0.733</sup>S<sup>0.265</sup> || text align="center"| 43.00
+|-
+| text align="center"| 5 ||99.6A<sup>0.763</sup>S<sup>0.355</sup> || text align="center"| 36.00
+|-
+| text align="center"| 10 ||117A<sup>0.774</sup>S<sup>0.395</sup> || text align="center"| 34.00
+|-
+| text align="center"| 25 ||140A<sup>0.784</sup>S<sup>0.432</sup> || text align="center"| 32.00
+|-
+| text align="center"| 50 ||155A<sup>0.789</sup>S<sup>0.453</sup> || text align="center"| 31.00
+|-
+| text align="center"| 100 ||170A<sup>0.794</sup>S<sup>0.471</sup> || text align="center"| 32.00
+|-
+| text align="center"| 500 ||203A<sup>0.804</sup>S<sup>0.503</sup> || text align="center"| 34.00
+|-
+|colspan="3" text align="center" | Region III
+|-
+| text align="center"| 2 ||88A<sup>0.658</sup> || text align="center"| 34.00
+|-
+| text align="center"| 5 ||145A<sup>0.627</sup> || text align="center"| 36.00
+|-
+| text align="center"| 10 ||187A<sup>0.612</sup> || text align="center"| 38.00
+|-
+| text align="center"| 25 ||244A<sup>0.595</sup> || text align="center"| 41.00
+|-
+| text align="center"| 50 ||288A<sup>0.585</sup> || text align="center"| 44.00
+|-
+| text align="center"| 100 ||334A<sup>0.576</sup> || text align="center"| 46.00
+|-
+| text align="center"| 500 ||448A<sup>0.557</sup> || text align="center"| 54.00
+|-
+|colspan="3" |''Source'': Alexander and Wilson (1995)
 |}
-====749.6.2.4 Percentage of Impervious Area====
+====749.6.1.2.1 Drainage Area====
-The percentage of impervious area (I) is the portion of the drainage area that is nonpervious because of buildings, parking lots, streets and roads, and other impervious areas within an urban basin. The variable, I, is determined from the best available maps or aerial photos showing impervious surfaces. Field inspection to supplement the maps may be useful.
+Drainage area (A) in mi<sup>2</sup>, can be obtained by determining the area contributing surface flows to the site as outlined along the drainage divide on the best available topographic maps or computer program.
+====749.6.1.2.2 Valley Slope====
+Valley slope (S) in ft./mile is the average slope between points 10 percent and 85 percent of the distance along the main-stream channel from the site to the basin divide. The main channel is defined above stream junctions as the one draining the largest area. The elevation difference between the 10- and 85-percent points is divided by the distance between the points to evaluate the slope.
+====749.6.1.2.3 Limitations of Equations====
+The USGS Rural Regression Equations may be used to estimate magnitude and frequency of floods on most Missouri streams Caution should be used when using these equations when the drainage area and slope are outside of the limits of the Valid Data Ranges shown in [[#749.6.1.2 1995 Rural Regression Equations|EPG 749.6.1.2 1995 Rural Regression Equations]].
+However, the equations are generally not applicable for:
+:a)	basins where manmade changes have appreciably changed the flow regimen such as:
+::*	Regulation of discharge (i.e. dams);
+::*	Channelization;
+::*	Diversion;
+::*	Urbanization
+:b)	the main stems of the Mississippi and Missouri Rivers;
+:c)	basins with greater than 5% impervious area.
+:d)	areas near the mouth of streams draining into larger rivers where backwater effect is experienced may require special consideration, see [[#749.11 Stream Confluences|EPG 749.11 Stream Confluences]].
+==749.6.2 Urban Regression Equations==
+The USGS Rural Regression Equations given above are not applicable to urban watersheds where manmade changes have appreciably changed the flow regimen. A set of equations were developed in 2010 by the [http://mcmcweb.er.usgs.gov/ United States Geological Survey]. Data from 35 gaged sites in urban locations in and adjacent to Missouri were analyzed to determine flood magnitudes with recurrence intervals of 2, 5, 10, 25, 50, 100 and 500 years. The resulting magnitudes were then related to drainage area and urban basin characteristics to provide the regression equations. A computer program is available to assist in performing these calculations.
+An urban watershed may be defined as a drainage basin in which manmade developments in the form of impervious surfaces and/or storm drainage systems have substantially altered the basin's natural response to rainfall.  Urbanization of a natural watershed progresses in one of two ways.  First, the addition of impervious surfaces in the form of roads, streets, parking lots and roofs will prevent infiltration of rainfall into the covered soil surface, thus increasing the total volume and peak rate of runoff from a given rainfall volume.  Second, to protect the now valuable property in a developed watershed from this increased peak and volume of runoff, storm drainage systems are installed.  The installation of a storm drainage system does not increase the volume of runoff, but modifies the time distribution of runoff.  Thus, when storm water drainage systems are installed, the time of concentration of the watershed is decreased.  Therefore, storm water drainage systems have the effect of removing a given volume of runoff in a shorter period of time, thus increasing the peak rate of runoff.
+===749.6.2.1 Equations===
+Peak discharges can be estimated at urban locations using the equations presented in the Urban Regression Equations table, below. The Equations give peak discharge as a function of drainage area (A) and percentage of impervious area (I).
-If the percentage of impervious area cannot be determined directly, a reasonable estimate may be obtained using 7-1/2 minute topographic maps and a relationship between developed area and impervious area. The drainage divide is outlined on the map, then the drainage area is divided into two subareas, open area and developed (urban) area. Open area consists of all undeveloped land, which may include scattered farmhouses and buildings, scattered single-family housing, and paved roads without significant development along the road. Developed areas include single- or multi-family housing structures, large business and office buildings, shopping centers, extensively industrialized areas, and schools. When delineating developed areas, it is important to include those areas devoted to paved parking lots around buildings. Once the amount of developed area has been determined, it can be converted into a percentage developed area (PDA) by dividing by the basin drainage area and multiplying by 100. The percentage of impervious area can then be obtained using the following equation:
+All hydraulic design in urban areas should consider the effect of increasing development throughout the projected life of the structure. Information on planned future development may be available from local agencies.
-<math>I=2.03PDA^{0.618}\,</math> (Equation 5)
+The range of valid data for USGS Urban Regression Equations are:
-The valid range for I is 1.0 percent to 40 percent. The values for both I and PDA are entered as percents (i.e. I = 29 for 29% impervious area and PDA = 75 for 75% developed area.)
+:a)	Drainage Area – 0.28 mi<sup>2</sup> to 189 mi<sup>2</sup>
+:b)	Impervious Area - 2.3% to 46%
-<div id="Peak Discharge for Urban Basins Based on Percent Impervious Area"></div>
+<center>
-{| border="1" class="wikitable" style="margin: 1em auto 1em auto"
+{|  border="1" class="wikitable" style="margin: 1em auto 1em auto"
-|+ '''Peak discharge for Rural Basins Based on Percent Impervious Area'''
+|+ ''' Urban Regression Equations'''
-! style="background:#BEBEBE"|Flood Freq. !! style="background:#BEBEBE"|Flood Magnitude !! style="background:#BEBEBE"|Std. Error of Estimate !! style="background:#BEBEBE"|Area Limits !! style="background:#BEBEBE"|Slope Limits
+! style="background:#BEBEBE"|Flood Freq.<br/>(years) !! style="background:#BEBEBE"|Flood Magnitude<br/>ft<sup>3</sup>/s !! style="background:#BEBEBE"|Std. Error of Prediction<br/>(%)
 |-
-! (years) !! ft<sup>3</sup>/s !! (%) !! (mi<sup>2</sup>) !! (ft/mi)
+|align="center"| 2 ||align="center"|	188A<sup>0.599</sup>10<sup>0.014(I)</sup> ||align="center"|	-23.1 to 30.0
 |-
-| text align="center"| 2 ||224A<sup>0.793</sup>(I)<sup>0.175</sup>|| text align="center"| 32.30 || 0.25 to 40|| 8.7 to 120
+|align="center"|5 ||align="center"|	352A<sup>0.592</sup>10<sup>0.011(I)</sup>|| align="center"|	-20.6 to 25.9
 |-
-| text align="center"| 5 ||424A<sup>0.784</sup>(I)<sup>0.131</sup> || text align="center"| 29.50 || 0.25 to 40 || 8.7 to 120
+|align="center"|10||align="center"| 	484A<sup>0.594</sup>10<sup>0.010(I)</sup>|| align="center"|	-19.6 to 24.4
 |-
-| text align="center"| 10 ||560A<sup>0.791</sup>(I)<sup>0.124</sup> || text align="center"| 28.60 || 0.25 to 40 || 8.7 to 120
+|align="center"|25|| align="center"|	671A<sup>0.599</sup>10<sup>0.008(I)</sup>|| align="center"|	-19.6 to 24.4
 |-
-| text align="center"| 25 ||729A<sup>0.800</sup>(I)<sup>0.131</sup> || text align="center"| 27.20 || 0.25 to 40 || 8.7 to 120
+|align="center"|50|| align="center"|	826A<sup>0.604</sup>10<sup>0.008(I)</sup>|| align="center"|	-20.6 to 25.9
 |-
-| text align="center"| 50 ||855A<sup>0.810</sup>(I)<sup>0.137</sup> || text align="center"| 26.10 || 0.25 to 40 || 8.7 to 120
+|align="center"|100||align="center"|	991A<sup>0.608</sup>10<sup>0.007(I)</sup>||align="center"|	-22.3 to 28.7
 |-
-| text align="center"| 100 ||986A<sup>0.821</sup>(I)<sup>0.144</sup> || text align="center"| 25.90 || 0.25 to 40 || 8.7 to 120
+|align="center"|500||align="center"|	1,420A<sup>0.619</sup>10<sup>0.006(I)</sup>||align="center"|	-28.9 to 40.7
 |-
-|colspan="8" style="background:#99FFFF"|''Source'': Becker (1986)
+|colspan="3" align="left"|Source: Southard (2010)
 |}
+</center>
+===749.6.2.2 Drainage Area===
+Drainage area (A) in mi<sup>2</sup>, can be obtained by determining the area contributing surface flows to the site as outlined along the drainage divide on the best available topographic maps or computer program.
+===749.6.2.3 Percentage of Impervious Area===
+The percentage of impervious area (I) is the portion of the drainage area that is impervious because of buildings, parking lots, streets and roads, and other impervious areas within an urban basin. The variable, I, is determined from the best available maps, satellite imagery or aerial photos showing impervious surfaces. Field inspection to supplement the imagery may be useful.
-====749.6.2.5 Limitations of Equations====
+===749.6.2.4 Limitations of Equations===
-The USGS Urban Regression Equations may be used to estimate magnitude and frequency of floods on most urban Missouri streams within the limits shown in the two tables above provided that the floodflows are relatively unaffected by manmade works such as dams or diversions.
+The USGS Urban Regression Equations may be used to estimate magnitude and frequency of floods on most urban Missouri streams within the limits noted in [[#749.6.2.1 Equations |EPG 749.6.2.1 Equations]] provided that the floodflows are relatively unaffected by manmade works such as dams or diversions.
-==749.7 Historical USGS Stream Gage Data==
+=749.7 Historical USGS Stream Gage Data=
 Numerous USGS recording stream gages have been maintained for many years on selected Missouri streams. For proposed structures at or near one of these gages, the gage data can be used in estimating discharge. When sufficient years of data have been collected at a stream gage, the data may be statistically analyzed to estimate discharge for the selected design flood frequency.
 Stream gage data is available on the Internet at [http://waterdata.usgs.gov/mo/nwis/ http://waterdata.usgs.gov/mo/nwis/].
+==749.7.1 Analysis==
 Gage data is analyzed by Log-Pearson Type III regression analysis to determine the discharges associated with the relevant return periods. See Water Resources Council Bulletin #17B in [[750.3 Bridges#750.3.1.9 List of References|References]] for details on this analysis method. A computer program for the analysis is available.
-One statistical parameter computed in the Log Pearson analysis is the skew coefficient of the distribution of the stream gage data. Skew coefficients for the data from stream gages in Missouri are typically between -0.1 and -0.4 when sufficient years of record are available. Skew coefficients outside this range may indicate an insufficient length of record or an analysis affected by outliers in the data. In this case, other methods of determining discharges will likely provide better estimates.
+One statistical parameter computed in the Log-Pearson analysis is the skew coefficient of the distribution of the stream gage data. Skew coefficients for the data from stream gages in Missouri are typically between -0.1 and -0.4 when sufficient years of record are available. Skew coefficients outside this range may indicate an insufficient length of record or an analysis affected by outliers in the data. In this case, other methods of determining discharges will likely provide better estimates.
+==749.7.2 Ungaged Sites Near Stream Gage==
 Stream gage data from gages at some distance from the site on the same watershed and stream gage data from nearby hydrologically similar watersheds may also be used to estimate discharges. Discharges obtained from this type of data should be compared with discharges obtained by other methods and not given the same weight as discharges obtained from data from a stream gage at the proposed site. Better estimates of discharge using this method may be obtained by repeating the procedure for several nearby gages and averaging the results. This method should not be used when drainage areas differ by more than 50% or at sites more than 50 miles from the stream gage(s).
-Transposition of discharges from one basin to another, or from one location to another within the same watershed, is accomplished using the following equation:
+Transposition of discharges from one basin to another, or from one location to another within the same watershed, is accomplished using the following Area-Weighting equation:
 :<math>Q_1=Q_2\left(\frac{A_1}{A_2}\right)^k</math>
@@ Line 459: / Line 653: @@
 :where:
-:Q1 = discharge for drainage basin 1 (cfs)
+:Q1 = discharge for ungaged drainage basin (cfs)
-:A1 = drainage area for drainage basin 1 (mi<sup>2</sup>)
+:A1 = drainage area for ungaged drainage basin (mi<sup>2</sup>)
-:Q2 = discharge for drainage basin 2 (cfs)
+:Q2 = discharge for gaged drainage basin (cfs)
-:A2 = drainage area for drainage basin 2 (mi<sup>2</sup>)
+:A2 = drainage area for gaged drainage basin (mi<sup>2</sup>)
-:k = exponent = 0.5 to 0.7
+:k = exponent = see table
+<center>
+{|  border="1" class="wikitable" style="margin: 1em auto 1em auto"
+|+ ''' Drainage-Area Ratio Exponent, ''k'''''
+! style="background:#BEBEBE" width="170"|Flood Freq.<br/>(years) !! style="background:#BEBEBE" width="130"|Region I !! style="background:#BEBEBE" width="130"|Region II !! style="background:#BEBEBE" width="130"|Region III
+|-
+|align="center"| 2 ||align="center"|0.580||align="center"|	0.654||align="center"|	0.665
+|-
+|align="center"|5||align="center"|	0.557||align="center"|	0.643||align="center"|	0.681
+|-
+|align="center"|10||align="center"|	0.546||align="center"|	0.637||align="center"|	0.689
+|-
+|align="center"|25||align="center"|	0.536||align="center"|	0.632||align="center"|	0.698
+|-
+|align="center"|50||align="center"|	0.529||align="center"|	0.629||align="center"|	0.703
+|-
+|align="center"|100||align="center"|	0.524||align="center"|	0.626||align="center"|	0.708
+|-
+|align="center"|200||align="center"|	0.520||align="center"|	0.625||align="center"|	0.713
+|-
+|align="center"|500||align="center"|	0.515||align="center"|	0.623||align="center"|	0.718
+|-
+|colspan="4"|Source: Southard and Veilleux (2014)
+|}
+</center>
-==749.8 NFIP Flood Insurance Study Discharges==
+=749.8 NFIP Flood Insurance Study Discharges=
-NFIP Flood Insurance Studies (FIS) typically include estimates of 10-, 50-, 100- and 500-year discharges for streams studied by detailed methods. These discharges may be more accurate than those obtained by other methods if the FIS discharges were determined through a detailed hydrologic study, such as an HEC-1 or TR-20 hydrologic model. In some instances, the FIS discharges may have been determined using an older version of the USGS regression equations. These discharges should not be used. Careful review of the FIS report will disclose the level of detail used in the hydrologic study.
+NFIP Flood Insurance Studies (FIS) typically include estimates of 10-, 50-, 100- and 500-year discharges for streams studied by detailed methods. These discharges may be more accurate than those obtained by other methods if the FIS discharges were determined through a detailed hydrologic study, such as an HEC-1, HEC-HMS or TR-20 hydrologic model. In some instances, the FIS discharges may have been determined using an older version of the USGS regression equations.  Engineering judgment should be exercised to determine if the FIS discharges determined from older USGS regression equations should  be used for the hydraulic design. When a No-Rise Certificate is required, and the FIS discharges are not used for hydraulic design, the 100-year FIS discharge should be used to verify the no-rise condition based on the Base Flood elevations in the FIS.  Careful review of the FIS report will disclose the level of detail used in the hydrologic study.
-==749.9 Flood Hydrographs==
+=749.9 Flood Hydrographs=
-For certain design problems it may be necessary to determine flood hydrographs associated with the peak discharge for a desired frequency. A hydrograph gives flow rate as a function of time at a particular location in a watershed, usually at the watershed outlet. Two techniques are available for obtaining synthetic hydrographs for ungaged sites. The SCS Unit Hydrograph method is applicable for all watersheds up to 1000 acres, and the USGS synthetic hydrograph method is applicable for drainage areas greater than 200 acres.
+For certain design problems it may be necessary to determine flood hydrographs associated with the peak discharge for a desired frequency. A hydrograph gives flow rate as a function of time at a particular location in a watershed, usually at the watershed outlet.
-===749.9.1 SCS Unit Hydrograph===
+==749.9.1 NRCS Unit Hydrograph==
-Techniques developed by the Natural Resource Conservation Service (NRCS), formerly known as the Soil Conservation Service (SCS), for calculating rates of runoff require the same basic data as the Rational Method: drainage area, a runoff factor, time of concentration, and rainfall information. The SCS method is more sophisticated in that it considers also the time distribution of rainfall, the initial rainfall losses to interception and depression storage, and an infiltration rate that decreases during the course of a storm. Additional details on the SCS methodology can be found in the SCS National Engineering Handbook, Section 4 (NEH-4) (SCS, 1985).
+Techniques developed by the Natural Resource Conservation Service (NRCS), formerly known as the Soil Conservation Service (SCS), for calculating rates of runoff require the same basic data as the Rational Method: drainage area, a runoff factor, time of concentration, and rainfall information. The NRCS method is more sophisticated in that it considers also the time distribution of rainfall, the initial rainfall losses to interception and depression storage, and an infiltration rate that decreases during the course of a storm. The NRCS Unit Hydrograph method is applicable for all watersheds between 6.4 and 16,000 acres. Additional details on the NRCS methodology can be found in the ''National Engineering Handbook'', (NEH), Part 630, Chapter 10 and Chapter 16(USDA/NRCS).
-====749.9.1.1 Rainfall-Runoff Equation and Concepts====
+===749.9.1.1 Rainfall-Runoff Equation and Concepts===
-The SCS method is based on a 24-hour storm event. SCS has developed four synthetic rainfall distributions typical of storms for various geographical regions in the United States. The Type II distribution is the appropriate distribution for using the SCS method in Missouri.
+The NRCS method is based on a 24-hour storm event. NRCS has developed four synthetic rainfall distributions typical of storms for various geographical regions in the United States. The Type II distribution is the appropriate distribution for using the NRCS method in Missouri.
-The SCS rainfall-runoff equation was developed by the SCS from experimental plots for numerous soil types and vegetative cover conditions. The experimental data consisted mainly of daily rainfall totals on small watersheds, and did not include information on the time distribution of rainfall. The SCS rainfall-runoff equation is therefore used to estimate the depth of runoff resulting from a given depth of 24-hour rainfall assumed to be spatially distributed uniformly over the watershed. The equation is given by:
+The NRCS rainfall-runoff equation was developed by the NRCS from experimental plots for numerous soil types and vegetative cover conditions. The experimental data consisted mainly of daily rainfall totals on small watersheds, and did not include information on the time distribution of rainfall. The NRCS rainfall-runoff equation is therefore used to estimate the depth of runoff resulting from a given depth of 24-hour rainfall assumed to be spatially distributed uniformly over the watershed. The equation is given by:
-:<math>Q=\frac{(P-I_a)^2}{(P-I_a)+S}</math> (Equation 6)
+:<math>Q=\frac{(P-I_a)^2}{(P-I_a)+S}</math>
 :where:
@@ Line 494: / Line 713: @@
 The potential maximum retention, S, is a measure of the maximum amount of water a given watershed could retain, and is a function of land use, interception capacity, infiltration capacity, depression storage, and antecedent moisture. A relationship between Ia and S was also developed from experimental data:
-:<math>I_a=0.2S\,</math> (Equation 7)
+:<math>I_a=0.2S\,</math>
-Substituting the equation for I<sub>a</sub> into equation 9, the SCS rainfall-runoff equation becomes:
+Substituting the equation for I<sub>a</sub> into the equation above, the NRCS rainfall-runoff equation becomes:
-:<math>Q=\frac{(P-0.2S)^2}{P+0.8S}</math> (Equation 8)
+:<math>Q=\frac{(P-0.2S)^2}{P+0.8S}</math>
 It is important to note that while both P and Q are given in units of depth, they actually represent volumes of rainfall and runoff, respectively. This is because the indicated depth of rainfall or runoff is assumed to be applied uniformly to the entire watershed area.
-The SCS made additional empirical analyses to estimate the value of S for various watersheds. The principal physical characteristics affecting the relationship between rainfall and runoff are land use, land treatment, soil types and land slope. These characteristics which affect the maximum potential retention are represented by a runoff factor called the curve number (CN), which is related to S by:
+The NRCS made additional empirical analyses to estimate the value of S for various watersheds. The principal physical characteristics affecting the relationship between rainfall and runoff are land use, land treatment, soil types and land slope. These characteristics which affect the maximum potential retention are represented by a runoff factor called the curve number (CN), which is related to S by:
-:<math>S=\frac{1000}{CN}-10</math> (Equation 9)
+:<math>S=\frac{1000}{CN}-10</math>
-The curve number is an index that represents the combination of hydrologic soil group, land use, hydrologic condition, and antecedent moisture condition. The table(s) below, Runoff Curve Numbers, provide curve numbers for different land uses, treatments and hydrologic conditions; separate values are given for each soil group. The SCS provides methods of adjusting the curve numbers based on varying antecedent moisture conditions; however, for design purposes, average antecedent moisture conditions are normally assumed and the values given in the table can therefore be used for design.
+The curve number is an index that represents the combination of hydrologic soil group, land use, hydrologic condition, and antecedent moisture condition. The table(s) below, Runoff Curve Numbers, provide curve numbers for different land uses, treatments and hydrologic conditions; separate values are given for each soil group. The NRCS provides methods of adjusting the curve numbers based on varying antecedent moisture conditions; however, for design purposes, average antecedent moisture conditions are normally assumed and the values given in the table can therefore be used for design.
 <div id="Runoff Curve Numbers - Urban Areas"></div>
@@ Line 517: / Line 736: @@
 | colspan="6" text align="center"| ''Fully Developed Urban Areas (Vegetation Established)''
 |-
-| colspan="6"| Open Space (lawns, parks, golf courses, cemetaries, etc.)<sup>3</sup>
+| colspan="6"| Open Space (lawns, parks, golf courses, cemeteries, etc.)<sup>3</sup>
 |-
 | text align="center"| Poor Condition (grass cover<50%) || ||68 ||79 ||86 ||89
@@ Line 575: / Line 794: @@
 | colspan="6"| Idle Lands (CNs are determined using cover types similar to those in [[#Runoff Curve Numbers - Urban Areas|Runoff Curve Numbers - Urban Areas]]
 |-
-| colspan="6" style="background:#FF6699"| <sup>1</sup>Average runoff condition, and I<sub>a</sub>=0.2S (see equation 7)
+| colspan="6" | <sup>1</sup>Average runoff condition, and I<sub>a</sub>=0.2S
 |-
-| colspan="6" style="background:#FF6699"| <sup>2</sup>The average percent impervious area shown was used to develp the composite CNs. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. If the impervious area is not connected, the SCS method has an adjustment to reduce the effect.
+| colspan="6" | <sup>2</sup>The average percent impervious area shown was used to develp the composite CNs. Other assumptions are as follows: impervious areas are directly connected to the drainage system, impervious areas have a CN of 98, and pervious areas are considered equivalent to open space in good hydrologic condition. If the impervious area is not connected, the NRCS method has an adjustment to reduce the effect.
 |-
-| colspan="6" style="background:#FF6699"| <sup>3</sup>CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combinations of open space cover type.
+| colspan="6" | <sup>3</sup>CNs shown are equivalent to those of pasture. Composite CNs may be computed for other combinations of open space cover type.
 |}
@@ Line 655: / Line 874: @@
 | Good|| 51 ||67 ||76 ||80
 |-
-| colspan="7" style="background:#FF6699"| <sup>1</sup>Average runoff condition, and I<sub>a</sub>=0.2S (see equation 7)
+| colspan="7" | <sup>1</sup>Average runoff condition, and I<sub>a</sub>=0.2S
 |-
-| colspan="7" style="background:#FF6699"| <sup>2</sup>Crop residue cover applies only if residue is on at least 5% of the surface throughout the year.
+| colspan="7" | <sup>2</sup>Crop residue cover applies only if residue is on at least 5% of the surface throughout the year.
 |-
-| colspan="7" style="background:#FF6699"| <sup>3</sup>Hydrologic condition is based on a combination of factors that affect infiltration and runoff, including (a) density and canopy of vegetative areas, (b) amount of year-round cover, (c) amount of grass or closed-seeded legumes in rotations, (d) percent of residue cover on the land surface (good>20%) and (e) degree of roughness.
+| colspan="7" | <sup>3</sup>Hydrologic condition is based on a combination of factors that affect infiltration and runoff, including (a) density and canopy of vegetative areas, (b) amount of year-round cover, (c) amount of grass or closed-seeded legumes in rotations, (d) percent of residue cover on the land surface (good>20%) and (e) degree of roughness.
 Poor: Factors impair infiltration and tend to increase runoff.
@@ Line 700: / Line 919: @@
 |Farmsteads - Buildings, Lanes, Driveways and Surrounding Lots || || 59 ||74 ||82 ||86
 |-
-| colspan="6" style="background:#FF6699"| <sup>1</sup>Average runoff condition, and I<sub>a</sub>=0.2S (see equation 7)
+| colspan="6" | <sup>1</sup>Average runoff condition, and I<sub>a</sub>=0.2S
 |-
-| colspan="6" style="background:#FF6699"| <sup>2</sup>Poor: <50% ground cover or heavily grazed with no mulch
+| colspan="6" | <sup>2</sup>Poor: <50% ground cover or heavily grazed with no mulch
 Fair: 50 to 74% ground cover
@@ Line 708: / Line 927: @@
 Good: >75% ground cover and lightly or only occasionally grazed
 |-
-|colspan="6" style="background:#FF6699"|<sup>3</sup>Poor: <50% gound cover
+|colspan="6" |<sup>3</sup>Poor: <50% gound cover
 Fair: 50 to 75% ground cover
@@ Line 714: / Line 933: @@
 Good: >75% ground cover
 |-
-|colspan="6" style="background:#FF6699"|<sup>4</sup>Actual curve number is less than 30; use CN=30 for runoff computations.
+|colspan="6" |<sup>4</sup>Actual curve number is less than 30; use CN=30 for runoff computations.
 |-
-|colspan="6" style="background:#FF6699"|<sup>5</sup>CNs shown were computed for areas with 50% grass (pasture) cover. Other combinations of conditions may be computed from CNs for woods and pasture.
+|colspan="6" |<sup>5</sup>CNs shown were computed for areas with 50% grass (pasture) cover. Other combinations of conditions may be computed from CNs for woods and pasture.
 |-
-|colspan="6" style="background:#FF6699"|<sup>6</sup>Poor: Forest litter, small trees and brush are destroyed by heavy grazing or regular burning.
+|colspan="6" |<sup>6</sup>Poor: Forest litter, small trees and brush are destroyed by heavy grazing or regular burning.
 Fair: Woods grazed but not burned, and some forest litter covers the soil.
@@ Line 725: / Line 944: @@
 |}
-====749.9.1.2 SCS Dimensionless Unit Hydrograph====
+===749.9.1.2 NRCS Dimensionless Unit Hydrograph===
-The SCS unit hydrograph was developed based on analysis of a large number of natural unit hydrographs from a wide range of drainage basin sizes and geographic locations.  The SCS unit hydrograph is given in a dimensionless form and provides a standard unit hydrograph shape.  Table 9-02.9 gives the ordinates of the SCS dimensionless unit hydrograph.
+The NRCS unit hydrograph was developed based on analysis of a large number of natural unit hydrographs from a wide range of drainage basin sizes and geographic locations.  The NRCS unit hydrograph is given in a dimensionless form and provides a standard unit hydrograph shape.
+<center>
 {| border="1" class="wikitable" style="margin: 1em auto 1em auto" style="text-align:center"
-|+ '''SCS Dimensionless Unit Hydrograph'''
+|+ '''NRCS Dimensionless Unit Hydrograph'''
-! style="background:#BEBEBE"|Time Ratio !! style="background:#BEBEBE"|Discharge Ratio !! style="background:#BEBEBE"|Time Ratio !! style="background:#BEBEBE"|Discharge Ratio !!style="background:#BEBEBE"|Time Ratio !! style="background:#BEBEBE"|Discharge Ratio !!style="background:#BEBEBE"|Time Ratio !! style="background:#BEBEBE"|Discharge Ratio !!
+! style="background:#BEBEBE"|Time Ratio !! style="background:#BEBEBE"|Discharge Ratio !! style="background:#BEBEBE"|Time Ratio !! style="background:#BEBEBE"|Discharge Ratio !!style="background:#BEBEBE"|Time Ratio !! style="background:#BEBEBE"|Discharge Ratio !!style="background:#BEBEBE"|Time Ratio !! style="background:#BEBEBE"|Discharge Ratio
 |-
-! t/t<sub>p</sub> !! Q/Q<sub>p</sub> !! t/t<sub>p</sub> !! Q/Q<sub>p</sub> !!t/t<sub>p</sub> !! Q/Q<sub>p</sub> !!t/t<sub>p</sub> !! Q/Q<sub>p</sub> !!
+!t/T<sub>p</sub> !! q/q<sub>p</sub> !! t/T<sub>p</sub> !! q/q<sub>p</sub> !!t/T<sub>p</sub> !! q/q<sub>p</sub> !!t/T<sub>p</sub> !! q/q<sub>p</sub>
 |-
 |0.0||0.000||0.9||0.990||1.8||0.390||3.4||0.029
@@ Line 752: / Line 972: @@
 |0.8||0.930||1.7||0.460||3.2||0.040|| ||
 |-
-|colspan="8" style="background:#99FFFF"|''Source'': McCuen (1996)
+|colspan="8" |''Source'': McCuen (1996)
 |}
+</center>
-Use of the SCS unit hydrograph requires calculation of the unit hydrograph peak discharge and the time to peak.  The unit hydrograph peak discharge is given by:
+Use of the NRCS unit hydrograph requires calculation of the unit hydrograph peak discharge and the time to peak.  The unit hydrograph peak discharge is given by:
-:<math>Q_p=\frac{K_qA}{t_p}</math> (Equation 10)
+:<math>q_p=\frac{484AQ}{T_p}</math>
 :where:
-:Q<sub>p</sub> = unit hydrograph peak discharge, cfs
+:q<sub>p</sub> = unit hydrograph peak discharge, cfs
-:K<sub>q</sub> = constant, 484
 :A = drainage area, mi<sup>2</sup>
-:t<sub>p</sub> = time to peak, hrs
+:Q = runoff in inches
+:T<sub>p</sub> = time to peak, hrs
 The time to peak is assumed to be equal to the basin lag time plus one-half the duration of rainfall. Basin lag time is estimated as 0.6 times the time of concentration, leading to the following equation for time to peak:
-:<math>t_p=\frac{t_r}{2}+0.6t_c</math> (Equation 11)
+:<math>\frac{\Delta D}{2}+0.6T_c = T_p</math>
 :where:
 :t<sub>p</sub> = time to peak, hrs
-:t<sub>r</sub> = duration of rainfall (unit hydrograph duration) = 0.133 t<sub>c</sub>, hrs
+:ΔD = duration of rainfall (unit hydrograph duration) = 0.133 t<sub>c</sub>, hrs
 :t<sub>c</sub> = time of concentration, hrs
-====749.9.1.3 Application of SCS Methodology====
+===749.9.1.3 Application of NRCS Methodology===
 Unit hydrograph theory depends on the principles of linearity and superposition. Given a unit hydrograph, the runoff hydrograph for a runoff depth other than unity can be obtained by multiplying the unit hydrograph ordinates by the runoff depth using the principle of linearity. The flood hydrograph for a particular storm event can be obtained by dividing the storm event into incremental periods of runoff, then applying the unit hydrograph to each incremental runoff and summing the resulting hydrographs together using the principle of superposition to obtain the total runoff hydrograph.
-The unit hydrograph duration (and the corresponding duration of the period of incremental runoff used in applying the unit hydrograph method) is estimated as 0.133t<sub>c</sub>. Since the SCS Type II rainfall distribution has a 24-hour time base, application of the SCS unit hydrograph methodology to typical watersheds by hand requires calculation of runoff hydrographs for a large number of increments. This can be cumbersome and time-consuming and a computer-based implementation is recommended.
+The unit hydrograph duration (and the corresponding duration of the period of incremental runoff used in applying the unit hydrograph method) is estimated as 0.133t<sub>c</sub>. Since the NRCS Type II rainfall distribution has a 24-hour time base, application of the NRCS unit hydrograph methodology to typical watersheds by hand requires calculation of runoff hydrographs for a large number of increments. This can be cumbersome and time-consuming and a computer-based implementation is recommended.
-===749.9.2 USGS Synthetic Hydrograph===
-A technique for generation of synthetic flood hydrographs for small basins in Missouri was developed in 1990 by the USGS in Rolla. Data from 341 floods recorded at 41 gaging stations located on small rural and urban streams in Missouri were analyzed. An average dimensionless hydrograph applicable to rural and urban basins with drainage areas from 0.25 to 0.40 mi<sup>2</sup> was developed from this data. This procedure is applicable to flood flows that are not significantly affected by storage or diversions.
-The dimensionless hydrograph developed for Missouri is given in the table below and [[Media:749 Dimensionless Hydrograph for Small Basins in Missouri.pdf|shown graphically]]. The dimensionless hydrograph is given in terms of flow divided by peak flow (Q<sub>p</sub>) versus time divided by basin lag time (LT). Expansion of the dimensionless hydrograph is accomplished by multiplying each ordinate value (Q/Q<sub>p</sub>) by Q<sub>p</sub> and each abscissa value (T/LT) by LT, resulting in a flood hydrograph for the desired flood frequency.
+=749.10 Detention Storage=
+The traditional purpose of storm drainage systems has been to collect and convey storm runoff as rapidly as possible to a suitable location where it can be discharged. As areas urbanize this type of design may result in major drainage and flooding problems downstream. Under favorable conditions, the temporary storage of some of the storm runoff can decrease downstream flows and often the cost of the downstream conveyance system. Detention storage facilities can range from small facilities contained in parking lots or other on-site facilities to large lakes and reservoirs. This article provides general procedures for detention storage analysis.
-{| border="1" class="wikitable" style="margin: 1em auto 1em auto" style="text-align:center"
-|+ '''Dimensionless Hydrograph for Small Basins  in Missouri'''
-! style="background:#BEBEBE"|Time Ratio !! style="background:#BEBEBE"|Discharge Ratio !! style="background:#BEBEBE"|Time Ratio !! style="background:#BEBEBE"|Discharge Ratio !!
-|-
-! T/LT !! Q/Q<sub>p</sub> !! T/LT !! Q/Q<sub>p</sub>
-|-
-|0.25||0.11||1.35||0.59
-|-
-|0.30||0.14||1.40||0.53
-|-
-|0.35||0.18||1.45||0.48
-|-
-|0.40||0.23||1.50||0.44
-|-
-|0.45||0.29||1.55||0.40
-|-
-|0.50||0.37||1.6||0.37
-|-
-|0.55||0.46||1.65||0.34
-|-
-|0.60||0.55||1.70||0.31
-|-
-|0.65||0.65||1.75||0.28
-|-
-|0.70||0.74||1.80||0.26
-|-
-|0.75||0.83||1.85||0.24
-|-
-|0.80||0.89||1.90||0.22
-|-
-|0.85||0.95||1.95||0.20
-|-
-|0.90||0.98||2.00||0.19
-|-
-|0.95||1.00||2.05||0.17
-|-
-|1.00||0.98||2.10||0.16
-|-
-|1.05||0.95||2.15||0.15
-|-
-|1.10||0.90||2.20||0.14
-|-
-|1.15||0.84||2.25||0.13
-|-
-|1.20||0.77||2.30||0.12
-|-
-|1.25||0.71||2.35||0.11
-|-
-|1.30||0.65||2.40||0.10
-|-
-|colspan="8" style="background:#99FFFF"|''Source'': Becker (1990)
-|}
-====749.9.2.1 Peak Flow Estimation====
-Peak flows of the desired frequency are estimated using the appropriate method given in [[#749.6 USGS Regression Equations|USGS Regression Equations]].
-====749.9.2.2 Basin Lag Time====
-Lag time is usually defined as the time from the center of the rainfall hyetograph to the centroid of the hydrograph. A hyetograph is a plot of rainfall intensity with time. The basin lag time (LT) can be estimated using either of two equations:
-:English: <math>LT=1.46A^{0.34}I^{-0.19}\,</math> (Equation 12)
-:Metric: <math>LT=1.06A^{0.34}I^{-0.19}\,</math> (Equation 12)
-:English: <math>LT=0.34A^{0.37}(13-BDF)^{0.52}\,</math> (Equation 13)
-:Metric: <math>LT=0.24A^{0.37}(13-BDF)^{0.52}\,</math> (Equation 13)
-:where:
-:LT = basin lag time (hrs)
-:A = basin drainage area, mi<sup>2</sup>
-:I = percentage impervious area as defined in [[#749.6.2.4 Percentage of Impervious Area|Percentage of Impervious Area]]
-:BDF = basin development factor as defined in [[#749.6.2.3 Basin Development Factor|Basin Development Factor]]
-The choice of which equation to use should again depend on whether it is easier to determine BDF or percentage of impervious area for a given basin.
-==749.10 Detention Storage==
-The traditional purpose of storm drainage systems has been to collect and convey storm runoff as rapidly as possible to a suitable location where it can be discharged. As areas urbanize this type of design may result in major drainage and flooding problems downstream. Under favorable conditions, the temporary storage of some of the storm runoff can decrease downstream flows and often the cost of the downstream conveyance system. Detention storage facilities can range from small facilities contained in parking lots or other on-site facilities to large lakes and reservoirs. This section provides general procedures for detention storage analysis.
 An easement must be purchased for any land, outside of the right of way, that will be flooded by water from a detention storage structure.
-===749.10.1 Data Needs===
+==749.10.1 Data Needs==
 The following data will be needed to complete storage calculations.
-*Inflow hydrograph for all selected design storms. The inflow hydrograph for the detention basin can be determined using the methods in [[#749.7 Flood Hydrographs|Flood Hydrographs]].
+*Inflow hydrograph for all selected design storms. The inflow hydrograph for the detention basin can be determined using the methods in [[#749.9 Flood Hydrographs|Flood Hydrographs]].
 *Stage-storage curve for storage facility.
 *Stage-discharge curve for the facility.
@@ Line 875: / Line 1,016: @@
 Using these data, the inflow hydrograph is routed through the storage facility to develop the outflow hydrograph.
-====749.10.1.1 Stage-Storage Curve====
+===749.10.1.1 Stage-Storage Curve===
 A stage-storage curve defines the relationship between the depth of water and storage volume in a reservoir. The data for this type of curve are usually developed using a topographic map and the conic formula for irregular shaped basins, or the prismoidal formula for trapezoidal basins. The conic formula is expressed as:
-:<math>V_{1,2}=\frac{1}{3}d\left(A_1+A_2+\sqrt{A_1A_2}\right)</math> (Equation 14)
+:<math>V_{1,2}=\frac{1}{3}d\left(A_1+A_2+\sqrt{A_1A_2}\right)</math> (Equation 1)
 :where:
@@ Line 889: / Line 1,030: @@
 The prismoidal formula for trapezoidal basins is expressed as:
-:<math>V=LWD+(L+W)ZD^2\frac{4}{3}Z^2D^3</math> (Equation 15)
+:<math>V=LWD+(L+W)ZD^2\frac{4}{3}Z^2D^3</math> (Equation 2)
 :where:
@@ Line 899: / Line 1,040: @@
 :Z = side slope factor, ratio of horizontal to vertical
-====749.10.1.2 Stage-Discharge Curve====
+===749.10.1.2 Stage-Discharge Curve===
 A stage-discharge curve defines the relationship between the depth of water and the discharge or outflow from a storage facility. If the detention facility has both principal and emergency spillways the stage-discharge curve should take both into account. The following equations can be used to help develop the stage-discharge curve.
@@ Line 907: / Line 1,048: @@
 A sharp-crested weir with no end contractions is illustrated in [[Media:749 Weir Configurations.pdf|Weir Configurations]]. The discharge equation for this configuration is (Chow, 1959):
-:English: <math>Q=\left[3.27+0.4\left(\frac{H}{H_c}\right)\right]LH^{1.5}</math> (Equation 16)
+:English: <math>Q=\left[3.27+0.4\left(\frac{H}{H_c}\right)\right]LH^{1.5}</math> (Equation 3)
-:Metric: <math>Q=0.55\left[3.27+0.4\left(\frac{H}{H_c}\right)\right]LH^{1.5}</math> (Equation 16)
+:Metric: <math>Q=0.55\left[3.27+0.4\left(\frac{H}{H_c}\right)\right]LH^{1.5}</math> (Equation 4)
 :where:
@@ Line 920: / Line 1,061: @@
 A sharp-crested weir with two end contractions is illustrated in [[Media:749 Weir Configurations.pdf|Weir Configurations]]. The discharge equation for this configuration is (Chow, 1959):
-:English: <math>Q=\left[3.27+0.4\left(\frac{H}{H_c}\right)\right](L-0.2H)H^{1.5}</math> 	(17)
+:English: <math>Q=\left[3.27+0.4\left(\frac{H}{H_c}\right)\right](L-0.2H)H^{1.5}</math> 	(Equation 5)
-:Metric: <math> Q=0.55\left[3.27+0.4\left(\frac{H}{H_c}\right)\right](L-0.2H)H^{1.5}</math> (Equation 17)
 :where: Variables are the same as for the previous equation.
@@ Line 928: / Line 1,067: @@
 A sharp-crested weir will be affected by submergence when the tailwater rises above the weir crest elevation. The result will be that the discharge over the weir will be reduced. The discharge equation for a sharp-crested submerged weir is (Brater and King, 1976):
-:<math>Q_s=Q_f\left[1-\left(\frac{H_2}{H_1}\right)^{1.5}\right]^{0.385}</math> (Equation 18)
+:<math>Q_s=Q_f\left[1-\left(\frac{H_2}{H_1}\right)^{1.5}\right]^{0.385}</math> (Equation 6)
 :where:
@@ Line 940: / Line 1,079: @@
 The equation generally used for the broad-crested weir is (Brater and King, 1976):
-:English: <math>Q=CLH^{1.5}\,</math> (Equation 19)
+:English: <math>Q=CLH^{1.5}\,</math> (Equation 7)
-:Metric: <math>Q=0.55CLH^{1.5}\,</math> (Equation 19)
 :where: Q = discharge, ft<sup>3</sup>/s
-:C = broad-crested weir coefficient (see [[Media:749 Broad-Crested Weir Coefficients.pdf|Broad-Crested Weir Coefficients]]
+:C = broad-crested weir coefficient (see [[Media:749 Broad-Crested Weir Coefficients.pdf|Broad-Crested Weir Coefficients]])
 :L = broad-crested weir length, ft
 :H = head above weir crest, ft., measured at least 2.5H upstream of the weir
@@ Line 954: / Line 1,091: @@
 A V-Notch weir is illustrated in [[Media:749 Weir Configurations.pdf|Weir Configurations]]. The discharge through a v-notch weir can be calculated from the following equation (Brater and King, 1976):
-:English: <math>Q=2.5tan\left(\frac{\theta}{2}\right)H^{2.5}</math> (Equation 20)
+:English: <math>Q=2.5tan\left(\frac{\theta}{2}\right)H^{2.5}</math> (Equation 8)
-:Metric: <math>Q=1.38tan\left(\frac{\theta}{2}\right)H^{2.5}</math> (Equation 20)
 :where:
@@ Line 966: / Line 1,101: @@
 Pipes smaller than 12 in. may be analyzed as a submerged orifice if H/D is greater than 1.5. For square-edged entrance conditions,
-:English: <math>Q=0.6A(2gH)^{0.5}=3.78D^2H^{0.5}\,</math> (Equation 21)
+:English: <math>Q=0.6A(2gH)^{0.5}=3.78D^2H^{0.5}\,</math> (Equation 9)
-:Metric: <math>Q=0.6A(2gH)^{0.5}=2.09D^2H^{0.5}\,</math> (Equation 21)
 :where:
@@ Line 981: / Line 1,114: @@
 If culverts are used as outlets works, procedures presented in the [[750.2 Culverts|Culverts]] should be used to develop stage-discharge data.
-===749.10.2 Routing Calculations===
+==749.10.2 Routing Calculations==
 The following procedure is used to perform routing through a reservoir or storage facility (Storage Indication or Puhls Method of storage routing).
 Routing a flood through a reservoir results in an attenuation of the peak of the inflow hydrograph and an associated change in timing of the peak. Storage of flood waters within the reservoir causes the peak outflow from the reservoir to be lower than the peak inflow, and causes the peak outflow to occur at a later time than the peak inflow. The continuity equation relates the change of storage within the detention storage basin to the inflow and outflow for the basin:
-<math>I-O=\frac{\Delta S}{\Delta T}</math> (Equation 22)
+<math>I-O=\frac{\Delta S}{\Delta T}</math> (Equation 10)
 :where:
@@ Line 997: / Line 1,130: @@
 The Storage Indication method of reservoir routing uses a simple finite-difference form of the continuity equation. For any two points in time, the continuity equation can be written as:
-:<math>\Big(\frac{2S_{n+1}}{\Delta T}+O_{n+1}\Big)=(I_n+I_{n+1})+\Big(\frac{2S_n}{\Delta T}-O_n\Big)</math> (Equation 23)
+:<math>\Big(\frac{2S_{n+1}}{\Delta T}+O_{n+1}\Big)=(I_n+I_{n+1})+\Big(\frac{2S_n}{\Delta T}-O_n\Big)</math> (Equation 11)
 :where:
@@ Line 1,003: / Line 1,136: @@
 :S = storage
-If the values at time step n are known, the only unknowns in equation 20 are on the left-hand side.
+If the values at time step n are known, the only unknowns in equation 8 are on the left-hand side.
 Substituting
-:<math>U_{n+1}=\frac{2S_{n+1}}{\Delta T}+O_{n+1}</math> (Equation 24)
+:<math>U_{n+1}=\frac{2S_{n+1}}{\Delta T}+O_{n+1}</math> (Equation 12)
-:<math>W_n=\frac{2S_n}{\Delta T}-O_n</math> (Equation 25)
+:<math>W_n=\frac{2S_n}{\Delta T}-O_n</math> (Equation 13)
-U is known as the Storage Indication Number. With these substitutions, equation 20 becomes:
+U is known as the Storage Indication Number. With these substitutions, equation 8 becomes:
-:<math>U_{n+1}=(I_n+I_{n+1})+W_n\,</math> (Equation 26)
+:<math>U_{n+1}=(I_n+I_{n+1})+W_n\,</math> (Equation 14)
-For the first time step, W<sub>n</sub> is calculated using the initial values of S and O, and equation 22. For subsequent time steps the following equation can be used as a shortcut.
+For the first time step, W<sub>n</sub> is calculated using the initial values of S and O, and equation 13. For subsequent time steps the following equation can be used as a shortcut.
-:<math>W_{n+1}=U_{n+1}-2O_{n+1}\,</math> (Equation 27)
+:<math>W_{n+1}=U_{n+1}-2O_{n+1}\,</math> (Equation 15)
 The procedure for using the storage-indication method of reservoir routing is as follows:
-*Develop an inflow hydrograph, stage-discharge curve, and stage-storage curve for the proposed storage facility.
+:1) Develop an inflow hydrograph, stage-discharge curve, and stage-storage curve for the proposed storage facility.
-*Select a routing time period, t, to provide at least five points on the rising limb of the inflow hydrograph.
-*Use the stage-storage and stage-outflow data from Step 1 to develop a plot of U versus outflow.
+:2) Select a routing time period, t, to provide at least five points on the rising limb of the inflow hydrograph.
-*Calculate W<sub>1</sub> using equation 22 and the initial values of S and O
-*Calculate U<sub>n+1</sub> using equation 23.
+:3) Use the stage-storage and stage-outflow data from Step 1 to develop a plot of U versus outflow.
-*Using U<sub>n+1</sub> calculated in step 5 pick O<sub>n+1</sub> from the plot of U vs. outflow.
-*Using U<sub>n+1</sub> and O<sub>n+1</sub> calculate W<sub>n+1</sub> using equation 24
+:4) Calculate W<sub>1</sub> using equation 22 and the initial values of S and O
-*Start over at step 5 with n = n+1. Continue repeating until inflow ceases or the outflow peak discharge has been determined.
-*From the stage discharge curve, determine the stage for the peak outflow.
+:5) Calculate U<sub>n+1</sub> using equation 23.
+:6) Using U<sub>n+1</sub> calculated in step 5 pick O<sub>n+1</sub> from the plot of U vs. outflow.
+:7) Using U<sub>n+1</sub> and O<sub>n+1</sub> calculate W<sub>n+1</sub> using equation 24
+:8) Start over at step 5 with n = n+1. Continue repeating until inflow ceases or the outflow peak discharge has been determined.
+:9) From the stage discharge curve, determine the stage for the peak outflow.
+=749.11 Stream Confluences=
+Water surface elevations at stream crossings near the confluence with another stream are often affected by backwater from the other stream. Several methods may be used to determine the water surface elevations at such crossings, such as:
+:* Coincident Peaks (i.e., 100-yr event on both streams)
+:* Exclusive Peaks (i.e.,  100-yr event on one stream, no flow from other stream)
+:* Flow Combinations (i.e., 86-yr Event on Main Reach, 62-yr Event on Tributary)
+==749.11.1 Coincident Peaks==
+When both streams reach their peak flow, for a given return period, at the confluence of the streams at or near the same time the peak flows can be considered to happen at the same time.  The water surface elevation of each stream is effected by the other. This method may provide overly conservative water surface elevations if applied to confluent streams that do not have similar peak times.
+FEMA, ''Guidelines and Specifications for Flood Hazard Mapping Partners'', Appendix C: Guidelines for Riverine Flood Analysis and Mapping gives the following guidance on when coincident peaks are appropriate to use:
+[[image:749.11.1.jpg|center|750px]]
+==749.11.2 Exclusive Peaks==
+Hydraulic analysis is conducted individually for each stream, for a given return period, without inclusion of any flow from the other stream, as each stream is assumed to reach its peak flow at different times. Analysis of the crossed stream will yield water surface elevations for the headwater flow of the crossed stream.  If a structure is included in this model the effects of the structure on the water surface elevations will be included in the model.  Analysis of the confluent stream will yield water surface elevations for the headwater flow of the confluent stream which can be used to determine the backwater surface elevation on the crossed stream. This backwater surface elevation does not include any effects from the flow of the crossed stream or any structures on the crossed stream. Since there would likely be some flow from the crossed stream this method may provide un-conservative water surface elevations in some cases.
+This method is commonly used in hydraulic modeling for FEMA Flood Insurance Studies.
+==749.11.3 Flow Combinations==
+Similar to Coincident Peaks this method accounts for effects of one streams water surface elevation on the other, but uses flows from different return periods for each stream.  Using a combination of flows can more closely simulate actual flooding conditions; the difficulty is in selecting a flow combination that will mimic those conditions.  A comparison of hydrographs of both watersheds to determine the maximum peak flow at a given return period, or the probability based procedure shown below may be used to estimate flow combinations.
+For streams that are within the influence reach of the confluent streams backwater the procedure provided in [http://onlinepubs.trb.org/onlinepubs/nchrp/nchrp_w199.pdf NCHRP 199], ''Estimating Joint Probabilities of Design Coincident Flows at Stream Confluences'', produces a probability based estimate for combinations of return periods for confluent streams that will produce the maximum water surface elevation and/or velocity for a design flood event. An excerpt below from NCHRP 199 provides information on which cases this procedure is applicable to (Note: For Case A this procedure may still be useful to determine flow downstream of the junction based on flow combinations).
+[[image:749.11.3.jpg|center|750px]]
-==749.11 References==
+=749.12 References=
-American Association of State Highway and Transportation Officials, 1991, ''Model Drainage Manual''.
+''AASHTO, 2014 AASHTO Drainage Manual'', American Association of State Highway and Transportation Officials.
-Alexander, T.W. and Wilson, G.L., 1995, ''Technique for Estimating the 2- to 500-Year Flood Discharges on Unregulated Streams in Rural Missouri'', USGS Water-Resources Investigations Report 95-4231
+Southard, R.E. and Veilleux, A.G.2014, ''Methods of Estimating Annual Exceedance-Probability Discharges and largest Recorded Floods for Unregulated Streams in Rural Missouri'', USGS Scientific Investigations Report 2014-5165.0000
-Becker, L.D., 1986, ''Techniques for Estimating Flood-Peak Discharges from Urban Basins in Missouri'', USGS Water-Resources Investigations Report 86-4322.
+Alexander T.W. and Wilson, G.L., 1995, ''Technique for Estimating the 2- to 500-Year Flood Discharges on Unregulated Streams in Rural Missouri'', USGS Water-Resources Investigations Report 95-4231
-Becker, L.D., 1990, ''Simulation of Flood Hydrographs for Small Basins in Missouri'', USGS Water-Resources Investigations Report 90-4045.
+Southard, R.E., 2010, ''Estimation of the Magnitude and Frequency of Floods in Urban Basins in Missouri'', USGS Scientific Investigations Report 2010-5073.
-McCuen, Richard, et al., 1996, ''Highway Hydrology – Hydraulic Design Series No. 2'', Federal Highway Administration Report No. FHWA-SA-96-067
+McCuen, R.H., et al., 2002 ''Highway Hydrology – Hydraulic Design Series No. 2'' (HDS-2), Federal Highway Administration, Publication No. FHWA-NHI-02-001
-Missouri State Highway Department, 1972, ''Review of Hydraulic and Drainage Design Criteria'', Missouri Cooperative Highway Research Program Report 72-3.
+Ponce, V.M., 1989, ''Engineering Hydrology'', Englewood Cliffs, New Jersey 07632, Prentice-Hall, Inc.
-Newton, D.W., and Herrin, J.C., 1982, ''Assessment of Commonly Used Methods of Estimating Flood Frequency'', Transportation Research Record 896.
+United States Department of Agriculture: ''National Resource Conservation Service, National Engineering Handbook. NEH part 630 – Hydrology & Hydraulics  ''
-Ponce, V.M., 1989, ''Engineering Hydrology'', Prentice-Hall, Inc., Englewood Cliffs, New Jersey 07632.
+US Army Corps of Engineers, 2000, ''Hydrologic Modeling System HEC-HMS Technical Reference Manual'', Hydrologic Engineering Center, Davis, CA
-Soil Conservation Service, 1985, ''National Engineering Handbook. Section 4 – Hydrology'', Washington, DC
+Kilgore, R.T., et. al., 2013, ''Estimating Joint Probabilities of Design Coincident Flows at Stream Confluences'', NCHRP 199.
-Southard, R.E., 1986, ''An Alternative Basin Characteristic for Use in Estimating Impervious Area in Urban Missouri Basins'', USGS Water-Resources Investigations Report 86-4362.
+Roussel, M. C., et al, 2005. ''Time-Parameter Estimation for Applicable Texas Watersheds'', U.S. Geological Survey, Texas Water Science Center, Research Report No. 0-4696-2
-US Army Corps of Engineers, 2000, ''Hydrological Modeling System HEC-HMS Technical Reference Manual'' Hydrologic Engineering Center, Davis, CA
+Cleveland, T. G., et al, 2012, ''Guidance for Estimation of Time of Concentration in Texas for Low-Slope Conditions'', Texas Tech University, Lubbock Texas, Report No. 0-6382-P1

Difference between revisions of "Category:749 Hydrologic Analysis"

Latest revision as of 14:04, 23 April 2019

Contents

749.1 General

749.2 Design Frequency and Return Period

749.2.1 Design Frequency Criteria

749.2.1.1 Crossroad Structures

749.2.1.2 Roadside Ditches and Culverts

749.2.1.3 Pavement Drainage

749.2.1.4 Median Drainage

749.2.2 Base Flood

749.2.3 Overtopping Flood

749.2.4 National Flood Insurance Program Criteria

749.3 Design Data

749.4 Rural vs. Urban Hydrology

Summary of Small Watershed Peak Runoff Calculation Method Drainage Area Limits

749.4.1 Rural Watersheds

749.4.2 Urban Watersheds

749.5 The Rational Method

749.5.1 Equation

749.5.2 Runoff Coefficient

749.5.2.1 Rural Runoff Coefficients

749.5.2.2 Urban Runoff Coefficients

749.5.2.3 Design Frequency

749.5.2.4 Composite Runoff Coefficients

749.5.3 Time of Concentration

749.5.3.1 Rural Watersheds

749.5.3.2 Urban Watersheds

749.5.3.3 Kerby-Kirpich Method

749.5.3.3.1 Low (Flat) Slopes

749.5.3.3.2 Kirpich Equation

749.5.3.3.3 Kerby-Hathaway Equation for Overland Flow

749.5.3.4 Storm Sewer Travel Time

749.5.3.5 Channel Flow Travel Time

749.5.4 Rainfall Intensity

749.6 USGS Regression Equations

749.6.1 Rural Regression Equations

749.6.1.1 2014 Rural Regression Equations

749.6.1.1.1 Drainage Area

749.6.1.1.2 Basin Shape

749.6.1.1.3 Limitations of Equations

749.6.1.2 1995 Rural Regression Equations

749.6.1.2.1 Drainage Area

749.6.1.2.2 Valley Slope

749.6.1.2.3 Limitations of Equations

749.6.2 Urban Regression Equations

749.6.2.1 Equations

749.6.2.2 Drainage Area

749.6.2.3 Percentage of Impervious Area

749.6.2.4 Limitations of Equations

749.7 Historical USGS Stream Gage Data

749.7.1 Analysis

749.7.2 Ungaged Sites Near Stream Gage

749.8 NFIP Flood Insurance Study Discharges

749.9 Flood Hydrographs

749.9.1 NRCS Unit Hydrograph

749.9.1.1 Rainfall-Runoff Equation and Concepts

749.9.1.2 NRCS Dimensionless Unit Hydrograph

749.9.1.3 Application of NRCS Methodology

749.10 Detention Storage

749.10.1 Data Needs

749.10.1.1 Stage-Storage Curve

749.10.1.2 Stage-Discharge Curve

749.10.2 Routing Calculations

749.11 Stream Confluences

749.11.1 Coincident Peaks

749.11.2 Exclusive Peaks

749.11.3 Flow Combinations

749.12 References

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