748.8 Development in Floodplains
A list of acronyms is provided at the end of this article.
- 1 748.9.1 Floodplain Development Permit
- 2 748.9.2 Floodplain and Special Flood Hazard Area
- 3 748.9.3 Floodway
- 4 748.9.4 Review of Flood Insurance Study and Maps
- 5 748.9.5 Floodplain Development Permit Application and No-Rise Certification
- 6 748.9.6 Levee Attachments
- 7 748.9.7 Acronyms
748.9.1 Floodplain Development Permit
|Floodplain Development Permit|
Communities (cities, counties or states) participating in the National Flood Insurance Program (NFIP) are required to regulate construction in the floodplain. Communities accomplish this by requiring permits for development in special flood hazard areas. The State Emergency Management Agency (SEMA) has been granted authority (Executive Order No. 98-03) to regulate floodplain development by state agencies and to issue floodplain development permits for state projects. SEMA requires a floodplain development permit for any development in special flood hazard areas, regardless of whether the community is participating in the NFIP.
The Central office will obtain the necessary floodplain development permit(s) from SEMA for construction in a regulated floodplain. The Bridge Division will obtain permits for projects which include structures in a regulated floodplain and the Design Division will obtain permits for other projects involving roadway fill in a regulated floodplain. The district will be responsible for determining whether a floodplain development permit is required on the project, and for providing to the appropriate Central Office any project information necessary to obtain the permit.
748.9.2 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 flood for the NFIP. The water surface elevation of the base flood is known as the base flood elevation (BFE). A special flood hazard area is land in the floodplain inundated by the 100-year flood and is commonly referred to as the "100-year floodplain." A floodplain development permit is required for any construction in a special flood hazard area. Special flood hazard areas are typically shown as "A zones" on flood insurance maps.
Encroachment on the floodplain, such as roadway fill, reduces the flood-carrying capacity, increases the flood heights of streams and increases 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.
7188.8.131.52 Construction within a Floodway
Construction in the 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 include only that portion of the project within the floodway in a "proposed conditions" model. Comparison of these water surface elevations to the existing conditions 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 BFE's for bridge replacements where the existing bridge is on or near the existing alignment; new bridges are usually longer and cause less obstruction to the 100-year discharge than existing bridges. For bridges on new alignment, additional steps must sometimes be taken to cause no increase in BFE's. 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.
7184.108.40.206 Temporary Bridges
Temporary bridges designed to pass the 10-year discharge with 1.0 foot (0.3 m) 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
7220.127.116.11 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 can not be lengthened without increasing water surface elevations upstream.
718.104.22.168 Floodway Revisions
Where construction in an existing floodway is absolutely necessary, and such construction will cause an increase in the BFE, the flood insurance study or floodway must be revised so that the proposed construction no longer causes an increase in the BFE or is no longer in the floodway. Flood insurance study revisions are obtained from FEMA through the community or communities with jurisdiction. The revision process requires a detailed hydraulic analysis and the cooperation and approval of all communities involved.
In general, obtaining a revision is a difficult and time-consuming process and should be avoided if at all possible. However, revising the floodway can be particularly 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.
748.9.4 Review of Flood Insurance Study and Maps
The Bridge Division and appropriate district office maintain copies of the FEMA Flood Insurance Study (FIS) reports and associated maps for streams subject to the National Flood Insurance Program.
722.214.171.124 Community Status Book
A current list of communities for which flood insurance studies have been performed is available in the Community Status Book (CSB).
This list should 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 must be obtained.
7126.96.36.199 Flood Insurance Study
The study report contains valuable information regarding discharges, floodway widths, water surface elevations, and other items that may be pertinent to hydraulic design. 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. The report for any communities within the project limits should be carefully reviewed.
7188.8.131.52 Flood Insurance Maps
The FIS maps may be one of three types: Flood Insurance Rate Maps (FIRMs), Flood Boundary and Floodway Maps (FBFMs), or Flood Hazard Boundary Maps (FHBMs). FHBMs 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.
Originally, FBFMs were used to delineate the floodway and FIRMs were used to delineate the various insurance rating zones. Recently, however, the two were combined, and now only the FIRM is published. The 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.
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 is required.
If any portion of the project is to be constructed within a regulatory floodway, the portion of the construction within the floodway can not cause an increase in the BFE and a No-Rise Certification will be required by SEMA.
7184.108.40.206 Summary of FIS Review Process
The process for reviewing floodway maps is summarized below:
- Check all communities within project limits to see if a flood insurance study has been performed.
- If study exists, check maps (FIRMs, FBFMs, FHBMs).
- If in special flood hazard area, floodplain development permit is required.
- If in regulatory floodway, can cause no increase in BFE. No-Rise Certification is required.
- If it is not possible to achieve no increase in BFE, a flood insurance study or floodway revision may be required.
748.9.5 Floodplain Development Permit Application and No-Rise Certification
- determination of the quarter-quarter section, township and range
- floodway/floodway fringe designation
- 100-year flood elevation and datum – the FIS base flood elevation should be given if available
- current map date – check the community status book
- project description - must include all aspects of the proposed construction, including grading, fill, and pavement in addition to the proposed bridge
The application for permit shall include a photocopy or Firmette of the section of the relevant flood map.
748.9.6 Levee Attachments
Information concerning levees on MHTC right of way and roadway embankments versus levees is located in 203 Roadway and Drainage Excavation, Embankment and Compaction.
|BFE||Base Flood Elevation|
|CSB||Community Status Book|
|FBFM||Flood Boundary and Floodway Map|
|FEMA||Federal Emergency Management Agency|
|FHBM||Flood Hazard Boundary Map|
|FIRM||Flood Insurance Rate Map|
|FIS||Flood Insurance Study|
|NFIP||National Flood Insurance Program|
|SEMA||State Emergency Management Agency|