748.8 Development in Floodplains
Acting as the Federal Highway Administration’s (FHWA’s) agent, MoDOT must evaluate every project to determine whether it could have a negative impact on the base (1% or 100-year) floodplain and/or regulatory floodway. 23 CFR 650A requires that hydraulic conditions created by a project be consistent with Federal Emergency Management Agency (FEMA) standards established for the NFIP.
The Federal Emergency Management Agency (FEMA) through the National Flood Insurance Program (NFIP) requires the management of development within regulated floodplains. FEMA defines development as: Any man-made change to improved or unimproved real estate, including but not limited to buildings or other structures, mining, dredging, filling, grading, paving, excavation or drilling operations or storage of equipment or materials.
For NFIP requirements and Floodplain Development Permit information, see EPG 127.9 Floodplain Management.
- 1 748.8.1 Floodplain Management Tools
- 2 748.8.2 FEMA Floodplain Products
- 3 748.8.3 Design of Encroachments
- 4 748.8.4 Letters of Map Revision
- 5 748.8.5 Levees
748.8.1 Floodplain Management Tools
718.104.22.168 Floodplain and Special Flood Hazard Area
A floodplain is defined by the Federal Emergency Management Agency (FEMA) as any land area susceptible to being inundated by water. The 100-year flood, or a flood with a one percent annual chance of being equaled or exceeded in a given year, has been adopted by FEMA as the Base (regulatory) Flood for the National Flood Insurance Program (NFIP). The water surface elevation of the Base Flood is known as the Base Flood Elevation (BFE).
A Special Flood Hazard Area (SFHA) is land in the floodplain inundated by the Base Flood and is commonly referred to as the "100-year floodplain." Special Flood Hazard Areas are typically shown as "Zone A, (A1-A30), or AE" on flood insurance maps. A floodplain development permit is typically required for any construction in a Special Flood Hazard Area.
Encroachment in the floodplain, such as roadway fill, can reduce the flood-carrying capacity, increase the flood elevations in streams and increase flood hazards in areas beyond the encroachment itself. One aspect of floodplain management involves balancing the economic gain from floodplain development against the resulting increase in flood hazard. For the purposes of the NFIP, the floodway concept is used as a tool to assist in this aspect of floodplain management.
The 100-year floodplain is divided into a floodway and a floodway fringe. The floodway is the channel of the stream plus the portions of the adjacent overbanks which must be kept free of encroachment in order to pass the Base Flood without cumulatively increasing the water surface elevations by more than a designated height. The floodway fringe is the area between the floodway and floodplain boundaries.
748.8.2 FEMA Floodplain Products
The Federal Emergency Management Agency (FEMA) conducts studies to determine floodplain boundaries, floodway boundaries and Base Flood Elevations. Products produced from these studies include:
- Flood Insurance Study (FIS) report
- Flood Insurance Maps
- Hydraulic Models
- Hydrologic Models.
The FIS report and flood insurance maps are typically not updated until a revised FIS is conducted. Changes or corrections made after the FIS report is published are accomplished through a Letter of Map Changes (LOMC).
722.214.171.124 Community Status Book
A current list of communities for which flood insurance studies have been performed is available in FEMA’s Community Status Book (CSB).
This list may be consulted to determine if a flood insurance study has been performed for any community within the project limits. The CSB list is divided into two parts: communities participating in the NFIP and communities that are not participating. Both parts of the list must be reviewed, as permits are required by SEMA for projects in a special flood hazard area when a flood insurance study has been performed, regardless of whether the community participates in the NFIP.
The CSB also includes the effective date of the current flood insurance study for the community. It is important to compare this date with the effective date of the FIS and maps used; if the CSB shows a later date, a revised study report and maps is available.
7126.96.36.199 Flood Insurance Study (FIS) Report
FIS reports are produced on a county wide basis for all communities within the county (city wide for the City of St. Louis). Cities within the county that participate in the NFIP are considered communities. FEMA considers the county to be the community in unincorporated areas and in cities that do not participate in the NFIP. Some older studies produced separate FIS reports for each community in the county.
The FIS report is used to record the methods used to conduct the flood insurance study. Depending on the degree of flood hazard posed, a particular stream may have been analyzed by approximate methods or by detailed hydrologic and hydraulic methods. The level of information presented in the study can vary greatly depending on whether the stream in question was studied by detailed or approximate methods. Information for a studied stream may include:
- stream discharges,
- floodway widths,
- water surface elevations,
- hydrologic modeling method,
- hydraulic modeling method,
- method used to determine the beginning water surface elevation,
- flood profiles for streams with detailed studies,
- elevation datum of the FIS.
The FIS report for any communities within the project limits should be carefully reviewed.
The FIS report is typically not updated until a revised FIS is conducted. Changes or corrections made after the FIS report is published are accomplished through a Letter of Map Correction (LOMC).
FIS reports are available through the FEMA Map Service Center.
7188.8.131.52 Flood Insurance Maps
Flood insurance maps may be one of three types:
- Flood Insurance Rate Maps (FIRMs),
- Flood Boundary and Floodway Maps (FBFMs),
- or Flood Hazard Boundary Maps (FHBMs).
Originally, FBFMs were used to delineate the floodway and FIRMs or FHBMs were used to delineate the various insurance rating zones. The map types have been combined, and now only the FIRM is published. Newer FIRMs delineate both rating zones and floodways. Depending on the publication date of the flood insurance study, it may be necessary to look at either a FBFM or a FIRM to determine whether the project lies within a regulatory floodway.
FHBMs and FIRMs are used when detailed studies have not been performed, no floodway has been developed, and floodplain boundaries are approximate.
FIRMs and FBFMs are used when a detailed study has been performed and a floodway has been developed, and show the boundaries of both the floodplain and the floodway. Special flood hazard areas are typically shown as Zone A on FHBMs and as Zone A, Zone AE, or Zones A1 through A30 on FIRMs and FBFMs.
For all communities for which a flood insurance study has been performed, the maps that include a portion of the project should be checked to determine if the project is within a special flood hazard area. If so, a floodplain development permit may be required.
If a floodplain development permit is required and any portion of the project is to be constructed within a regulatory floodway, the portion of the construction within the floodway cannot cause an increase in the BFE and a No-Rise Certification will be required by SEMA.
Flood insurance map panels are available through the FEMA Map Service Center. The National Flood Hazard Layer (NFHL) shows all digital map data, FIRM panels for counties with digital maps may be printed from the NFHL. The NFHL is also available as a kmz file for use in Google Earth.
7184.108.40.206 Hydraulic Models
Hydraulic models used to develop the data for a flood insurance study may be available from FEMA. The most common models are HEC-2 and HEC-RAS.
FEMA HEC-RAS models for streams with detailed studies typically can be used as the base model for the existing conditions model. On streams studied using unsteady state flow, or 2D analysis, the FEMA model should be used as the base model due to the difficulty in reproducing the discharge hydrographs used in the model.
FEMA HEC-RAS models for streams with approximate studies typically are not used as base models. Approximate models do not include manmade structures or floodplain encroachments; typically the valley sections are not at appropriate locations for modeling structures and encroachments.
FEMA HEC-2 (predecessor of HEC-RAS) models are sometimes available, but often only the input and output data is available. Due to the age of these models it is typically not recommended that HEC-2 Models be used as the base model for the existing conditions model.
7220.127.116.11 Hydrologic Models
Hydrologic models used to develop stream discharges for a flood insurance study may be available from FEMA.
718.104.22.168 Letters of Map Changes
Letters of Map Changes (LOMC) are documents issued by FEMA that revise or amend the flood hazard information shown on the FIRM without requiring the FIRM to be physically revised and re-published. LOMCs can be one of the following types:
- Letter of Map Revision (LOMR)
- Letter of Map Revision Based on Fill (LOMR-F)
- Letter of Map Amendment (LOMA).
A LOMR is a modification to an effective Flood Insurance Map. LOMRs are generally based on physical changes in the floodplain that affect the hydrologic or hydraulic characteristics of a flooding source and thus result in the modification of the existing regulatory floodway, the effective Base Flood Elevations (BFE), or boundary of the Special Flood Hazard Area (SFHA). The LOMR officially revises the Flood Insurance Rate Map (FIRM) or Flood Boundary and Floodway Map (FBFM), and sometimes the Flood Insurance Study (FIS) report, and when appropriate, includes a description of the modifications. The LOMR is generally accompanied by an annotated copy of the affected portions of the FIRM, FBFM, and FIS report.
A LOMR-F is a modification of the Special Flood Hazard Area (SFHA) shown on the Flood Insurance Map based on the placement of fill outside an existing regulatory floodway.
A LOMA is an official amendment, by letter, to an effective National Flood Insurance Map. A LOMA establishes a property's location in relation to the Special Flood Hazard Area (SFHA). A LOMA is usually issued because a property has inadvertently been mapped as being in the floodplain, but is actually on natural high ground above the base flood elevation.
LOMRs should be reviewed to determine if any changes were made to the BFE, SFHA limits or floodway limits within the project area. LOMR-Fs and LOMAs do not apply to transportation infrastructure and typically do not need to be reviewed.
722.214.171.124 Review of FEMA Floodplain Products
Encroachments. To determine whether a project encroaches into a regulated floodplain or floodway on the current effective Flood Insurance Map or pending Flood Insurance Map, the following sources should be consulted:
- • Current Effective FIRMs
- • Digital Maps only (not all counties have digital maps available)
- • Historical, Current Effective, Pending and Prelim. Flood Insurance Maps
- • Digital and scanned paper (non-digital) maps
- • Current Effective, Pending and Prelim. Flood Insurance Maps
- • Digital Maps only.
To determine if a Floodplain Development Permit Application or “No-Rise” Certificate is needed, see EPG 127.9.2 Floodplain Development Permits.
For information on how a Pending or Preliminary FIRM may impact an upcoming project, review the Pending or Preliminary Flood Insurance Study report or contact the Structural Hydraulics Engineer in Bridge Division.
Design. The flood insurance maps noted above should be used to determine the Flood Hazard Zone(s) the project encroaches into and if a floodway is involved. The Current Effective Flood Insurance Study (FIS) report (Pending and Preliminary FIS report if applicable) should also be reviewed. The following data in the FIS report will aid in the project’s design and may be required to meet FEMA requirements (some FIS reports do not contain some of the items listed below):
- Flooding Sources Included in the Report
- Study type (approximate or detailed) of the flooding source
- Principle Flood Problems
- Flooding sources with accredited levees
- Hydrologic methods used to determine peak discharges
- Summary of peak discharges for flooding sources
- Stream gage information
- Hydraulic Models used for analysis
- Roughness Coefficients used for each flooding source
- Starting water surface determination method
- Vertical Datum information
- Floodway Data Tables
- Flood Profiles.
748.8.3 Design of Encroachments
7126.96.36.199 Construction Within a Floodplain
Construction in the floodplain that causes an increase of more than 1 foot over the natural condition in the BFE is prohibited, except when the hydraulic performance of an existing structure is being matched, see EPG 748.4.3 Existing Structures.
If it is not practical to limit the increase in BFE to 1 foot a Conditional Letter of Map Revision (CLOMR) can be submitted to FEMA to modify the FIS and Flood Insurance Maps, See EPG 748.8.4 Letters of Map Revision.
7188.8.131.52 Construction within a Floodway
Construction within a floodway that causes any increase in the BFE is prohibited. In order to issue a floodplain development permit for construction in the floodway, a "No-Rise Certification" must be provided by a registered professional engineer, which certifies that the proposed construction will cause no increase in the BFE.
Several methods can be used to demonstrate that a construction project within a floodway will not cause an increase in the BFE. The simplest method is to model both the existing conditions and the proposed conditions. Comparison of the water surface elevations from these two models will show any increase caused by the construction; generally, if the project as a whole causes no increase in the BFE, that portion within the floodway will also cause no increase.
Another method is to model the existing and proposed conditions using a floodway analysis. Comparison of these water surface elevations will directly show the impact of the proposed construction in the floodway.
It is generally not difficult to show no increase in BFEs for bridge replacements where the existing bridge is on or near the existing alignment and there is little or no roadway overtopping. For bridges on new alignment, or with significant roadway overtopping, additional steps can sometimes be taken to limit the length of the new bridge and to cause no increase in BFEs. Possibilities include modification of the roughness coefficients through the structure or excavation of material from the overbanks for some distance upstream and downstream of the structure. All such modifications must be justifiable.
If it is not practical to cause no increase to the BFEs a Conditional Letter of Map Revision (CLOMR) can be submitted to FEMA to modify the FIS and Flood Insurance Maps, See EPG 748.8.4 Letters of Map Revision.
7184.108.40.206.1 Temporary Bridges
Temporary bridges designed to pass the 10-year discharge with 1.0 ft. of backwater will typically result in an increase in base flood elevations. Permits for temporary bridges in floodways will be handled by SEMA on a case-by-case basis. The floodplain development permit application for temporary bridges must include the following:
- Hydraulic analysis of the effect of the temporary bridge on base flood elevations
- Determination of the effect of any increased flooding resulting from the temporary bridge on any upstream improvements
- Estimate of length of time temporary bridge will be in place.
For additional information, see Section 4.4 of FEMA Bulletin P-993, Variances and the National Flood Insurance Program, or consult the Structural Hydraulics Engineer in Bridge Division.
7220.127.116.11.2 Culvert Extensions
Culvert extensions in floodways can pose a particularly challenging problem depending on whether they operate under inlet control or outlet control.
Culverts operating under inlet control can generally be lengthened without increasing water surface elevations. In some cases, an improvement to the inlet may be required to compensate for increases in culvert length.
Culverts operating under outlet control generally cannot be lengthened without increasing water surface elevations upstream.
If it is not practical to cause no increase to the BFEs a Conditional Letter of Map Revision (CLOMR) can be submitted to FEMA to modify the FIS and Flood Insurance Maps. See EPG 748.8.4 Letters of Map Revision.
748.8.4 Letters of Map Revision
Where the proposed construction in an absolutely necessary, and such construction will cause an increase in the BFE, or increase in floodway width, of an existing floodway, or increase the BFE of a SFHA by more than 1 foot, the Flood Insurance Study (FIS) or floodway must be revised so that the proposed construction no longer causes an increase, greater than the amount allowed, in the BFE. Flood insurance study revisions for construction related revisions require the submission of a detailed hydraulic analysis, annotated FIRM panel(s) and FIS report be submitted to FEMA in the form of a Conditional Letter of Map Revision (CLOMR). Upon approval of FEMA’s review of the CLOMR and with concurrence of all communities involved, SEMA will approve a floodplain development permit for the project. One the project is complete the hydraulic model and annotated FIRM(s) and FIS are updated and submitted to FEMA. Once FEMA has reviewed and accepted the changes to the FIRM(s) and FIS they will issue a Letter of Map Revision (LOMR).
In general, obtaining a CLOMR/LOMR is a difficult and time-consuming process and should be avoided if at all possible. However, revising the floodway can be cost-effective in one situation. Floodway widths are determined precisely only at the locations of cross-sections in the hydraulic model used to create the FIS. At all other locations along the stream, floodway widths are determined by interpolation along topographic maps. When a stream crossing is located between cross-sections, at a significant distance from both the upstream and downstream cross-section, it may be beneficial to review the hydraulic model used in the FIS. In some cases, adding an additional cross-section to the model at the location of the proposed structure will allow the floodway width to be reduced at that location, especially if the floodway appears unusually wide at the structure location.
Information concerning levees on MHTC right of way and roadway embankments versus levees is located in EPG 203.2.5 Levee Attachments. Information on freeboard of FEMA accredited levees can be found in EPG 748.3.4 Levees.