Floodway Vs. Floodplain

One of the first things that Engineers should do, when a client comes to them with land that they want to develop or build on, is to check if there is floodplain on the property. Floodplains represent the limits of the Base Flood Elevation or BFE and there are limits as to what can be done within these areas. The Floodplain is comprised of two parts: Regulatory Floodway and Floodplain Fringe.

What Does Base Flood Elevation Mean?

The Base Flood Elevation is equivalent to a 100-year or 1% Chance Storm event and is determined by hydrologic and hydraulic analysis. This means that the chance of having a 100-year event is 1%. This is a significant storm event and rarely occurs. The most common rainfall events are lower storm events such as a 1-Year (100% Chance), 2-Year (50% chance), and 5-year (20% chance). The higher the storm, the stronger the rainfall intensity. Rainfall intensity is the amount of rain that falls in a period of time, typically in inches/hour. For the Tulsa Area, this is around 9.0 in./hr. for a 100-year event.  While 100-year events are rare, this is the base that the vast majority of stormwater systems and floodplains are designed and modeled to, hence the name Base Flood.

 

The Regulatory Floodway

The National Flood Insurance Program (NFIP) defines “regulatory floodway” as the channel of a river or other watercourse and the adjacent land areas that must be reserved in order to discharge the base flood without cumulatively increasing the water surface elevation more than a designated height. Typically, this designated height is 1 foot. Depending on the community you are in, this designated height, also known as freeboard, could be as little as 6 inches or even no increase allowed. You can reach out to your local government to see what is required.

The National Flood Insurance Program (NFIP) is a program created through the National Flood Insurance Act of 1968. The program enables property owners in participating communities to purchase insurance protection, administered by the government, against losses from flooding, and requires flood insurance for all loans or lines of credit that are secured by existing buildings, manufactured homes, or buildings under construction, that are located in the Special Flood Hazard Area (Floodplain) in a community that participates in the NFIP. The U.S. limits the availability of National Flood Insurance to communities that adopt adequate land use and control measures with effective enforcement provisions to reduce flood damages by limiting development in areas exposed to flooding.

While development is discouraged in the Floodway, it is possible. If the proposed development changes are modeled through hydrologic and hydraulic modeling and the results show that the development will not cause an increase in the base flood elevation, then development may be allowed, but this depends on the community you are in. This analysis is called a “no-rise” analysis and should be analyzed and certified by a licensed Civil Engineer.

An increase in the Base Flood Elevation is not allowed to negatively impact anyone. Even if the increase does not directly affect your property, it could be affecting someone upstream or downstream of you. The floodway is determined by mathematics and river modeling and accounts for future development in the floodway fringe, and should not be modified if possible.

Communities also will typically have restrictions on how far above the base flood the lowest floor of the building can set. Many communities have used 1 foot above the BFE, but more are changing to higher amounts. For example, Tulsa County requires finished floors to be at least 2 feet above the BFE and the City of Broken Arrow requires at least 28” above the BFE. This requirement protects buildings/structures from future BFE increases that may occur if floodways are encroached upon.

Another purpose of the floodway and floodplain fringe is to protect wetlands, reduce sedimentation, and protect wildlife and wildlife habitat. To see if there are endangered species within your project limits, go to U.S. Fish & Wildlife Services for more information.

U.S. Fish & Wildlife Project Review Process

 

The Floodway Fringe

The floodway fringe is the remaining portion of the floodplain. FEMA & State regulations allow communities to allow the flood fringe to be modified and developed if certain requirements are met. These lowland areas provide additional storage for floodwaters. Flows in the floodplain fringe are slower moving and have shallow flood depths. Communities usually require Development Permits in these areas when modifications to the floodplain are proposed.

 

How Do I Know If There Is Floodplain On My Property?

To find out if there is floodplain on your property go to the FEMA Map Service Center and type in your address. Click the link provided below.

FEMA Map Service Center

You will see a map that looks something like this:

The red hatched area is the Floodway Limits and the blue shaded area is the Floodplain limits. The black lines with elevations on them are the elevation of the Base Flood. The 500-Year or 0.2% chance limits are also shown on FEMA maps as the gold areas above, but there are currently no requirements for those limits locally. If your property falls within the red or blue shaded areas, you will need hire an engineer to apply for the appropriate permits and complete the modeling to build or grade in these areas.

Now That I Know My Property Has Floodplain On It, What Do I Do So I Can Build On It?

Depending on your property there are several different Revisions and Amendments you can fill out to send to FEMA. The differences between an amendment and a revision is mapping of the floodway/floodplain. If you believe the flood map is inaccurate, a LOMA can be filled out and submitted to FEMA to remove the property from the floodplain. Amendments do not have any fees associated with the forms. To qualify for an amendment, if the property has an existing structure, the lowest adjacent grade (the lowest ground touching the structure) must be at or above the BFE or the lowest point on the lot must be at or above the BFE.

What does that mean if you don’t meet those requirements but still want to remove floodplain from the property to build? The next step is a LOMR-F. Hydraulic modeling software is used to model the impacts of placing fill (raising the natural ground) on your property. If the results reflect “no-rise”, then the LOMR-F form is submitted to FEMA for review. If there is a rise, a CLOMR/LOMR will be required. Please note that map revisions have significant fees associated with the review and processing of these requests. The time required to go through the process can be lengthy, up to 8 months.

Letter of Map Amendment (LOMA): A letter from FEMA stating that an existing structure or parcel of land that has not been elevated by fill (natural ground) would not be inundated by the base flood. The FEMA maps may not pick up that there is a knoll that a building sets on, a detailed survey can verify that.

Conditional Letter of Map Amendment (CLOMA): A letter from FEMA stating that a proposed structure that is not to be elevated by fill (natural ground) would not be inundated by the base flood if built as proposed.

Letter of Map Revision Based on Fill (LOMR-F): A letter from FEMA stating that an existing structure or parcel of land that has been elevated by fill would not be inundated by the base flood. But along with this, a study must be done to certify No Adverse Impact to other properties.

Conditional Letter of Map Revision Base on Fill (CLOMR-F): A letter from FEMA stating that a parcel of land or proposed structure would not be inundated by the base flood if fill is placed on the parcel as proposed or the structure is built as proposed, and as detailed in an engineering study.

Conditional Letter of Map Revision (CLOMR): A letter from FEMA stating that a proposed project, if built as proposed, or the proposed hydrology changes would meet minimum NFIP standards.

Letter of Map Revision (LOMR): A letter from FEMA officially revising the current NFIP map to show changes to floodplains, regulatory floodways, or flood elevations.

If you need help filling a Letter of Map Revision or Letter of Map Amendment or just want to discuss development options for your property which may or may not have floodplain on it, please contact us here at TEP, and we would be happy to assist you!

Stay tuned for our next blog posting on more about the NFIP, the Community Rating System (CRS), and Substantially Damaged/Substantially improved buildings within the floodplain.