Floodplain Development

Developing the Base Flood Elevation (BFE) for a 100-year floodplain is a key step in understanding flood risk and in creating accurate floodplain maps for floodplain management and flood insurance purposes. The 100-year flood (or 1% annual chance flood) is the flood level that has a 1% chance of being equaled or exceeded in any given year. It is the basis for FEMA’s flood maps and flood risk assessments. Below are the basic steps involved in developing BFE for a 100-year floodplain, along with the related forms required during the process:

Here are the steps to follow in getting approval for development in a 100-year floodplain:

Determine Floodplain Location

  • Check if the property is within a 100-year floodplain by reviewing FEMA’s Flood Insurance Rate Maps (FIRMs) or contacting local floodplain administrators.

Contact Local Floodplain Administrator

  • Reach out to the local floodplain administrator or building department for specific regulations, permit requirements, and guidance on development in flood-prone areas.

Obtain a Floodplain Development Permit

  • Submit a Floodplain Development Permit application with the necessary documentation, including site plans and details of the proposed development.

Provide an Elevation Certificate

  • Obtain an Elevation Certificate from a licensed surveyor to show the lowest floor elevation of the proposed structure relative to the Base Flood Elevation (BFE).

Ensure Compliance with Local Regulations

  • Verify that the proposed development complies with local floodplain management ordinances, zoning regulations, and building codes.

Assess Flood Impact

  • If applicable, conduct hydrologic and hydraulic studies to assess how the development might affect floodwaters and to determine whether the development will increase the risk of flooding.

Elevate Structures or Use Flood-Resistant Materials

  • If required, elevate the building above the BFE or use flood-resistant materials to reduce flood damage potential.

Submit a Letter of Map Revision (LOMR) Request (if necessary)

  • If the development changes the floodplain boundaries or Base Flood Elevation (BFE), request a Letter of Map Revision (LOMR) from FEMA to update the Flood Insurance Rate Maps (FIRMs).

Review and Approve Development Plans

  • Submit the development plans to the local governing authority or planning board for review and approval, ensuring compliance with floodplain regulations.

Obtain Required Flood Insurance

  • For properties within a Special Flood Hazard Area (SFHA), obtain flood insurance as required by FEMA and the local authorities.

Commence Development

  • Once the necessary approvals, permits, and insurance are obtained, proceed with construction while adhering to all floodplain management requirements to minimize flood risk.

1. Data Collection and Site Assessment

Key Tasks:

  • Collect Hydrologic and Hydraulic Data: Gather data related to rainfall, river flow, and other factors that contribute to flooding in the area. This includes historical flood data and streamflow information.
  • Obtain Existing FEMA Maps: If applicable, retrieve existing Flood Insurance Rate Maps (FIRMs) or Flood Hazard Boundary Maps (FHBMs) from FEMA. These provide the baseline information for flood risk and mapping.
  • Field Survey and Elevation Data: Conduct field surveys to obtain the elevation of the property and surrounding area, particularly the lowest floor elevation of any existing buildings in the floodplain.

Related Forms:

  • FEMA Flood Map Service Center (MSC): This is where you can access existing FEMA flood maps (e.g., FIRMs) to get the current flood zone designations and flood hazard information.
  • Surveyor’s Elevation Certificate: A licensed surveyor may need to provide elevation data for properties in the floodplain, which will be important when determining the BFE.

2. Hydrologic and Hydraulic Modeling

Key Tasks:

  • Conduct Hydrologic Analysis: Perform a hydrologic analysis to estimate the amount of runoff and the flow of water during a 100-year storm event. This includes understanding precipitation, watershed conditions, and flow rates.
  • Hydraulic Modeling: Use hydraulic models (e.g., HEC-RAS, XP-SWMM) to simulate how water behaves within the floodplain during a 100-year flood event. The goal is to determine how floodwaters will flow and where they will accumulate.
    • The model will help calculate the Base Flood Elevation (BFE) at specific points within the floodplain.
    • The BFE is defined as the water surface elevation of the 100-year flood.

Related Forms:

  • Riverine Hydrology and Hydraulics Form (Form 2): If the floodplain is riverine in nature, this form may be used to document the hydrologic and hydraulic calculations that lead to the determination of BFE.
  • Floodplain Mapping Data: This data will be used to create updated Flood Insurance Rate Maps (FIRMs).

3. Floodplain Boundary Determination

Key Tasks:

  • Identify the Floodplain Boundary: Using the hydraulic modeling results, determine the boundary of the floodplain for the 100-year flood event. The floodplain boundary is defined by areas that are likely to flood during a 100-year event.
  • Floodway Delineation: Identify the floodway, which is the channel of the river or stream and the adjacent land areas that must remain unobstructed to allow floodwaters to flow efficiently.
    • The Floodway is often associated with higher flood velocities and should be kept clear of development.

Related Forms:

  • Riverine Structures Form (Form 3): If structures (e.g., levees, dams) are present or have been proposed to alter the floodplain, this form is used to document their impact on the floodplain boundaries and BFEs.
  • Flood Hazard Boundary Map (FHBM): This map helps delineate the boundary of flood-prone areas and can be used to assess the extent of the floodplain.

4. Base Flood Elevation (BFE) Calculation

Key Tasks:

  • Determine BFE: Based on the hydraulic model’s results, calculate the Base Flood Elevation (BFE) at various locations along the river or floodplain. The BFE is the elevation above mean sea level (MSL) that floodwaters are expected to reach during the 100-year flood event.
  • Validate BFE: If available, compare the modeled BFE against existing data (e.g., from past flooding events or historical records) to validate the accuracy of the modeling.

Related Forms:

  • Elevation Certificate: This form, completed by a licensed surveyor, is used to document the lowest floor elevation of a building relative to the BFE. It is important for determining flood insurance rates and whether buildings fall within the floodplain.
  • Flood Insurance Study (FIS) Report: The FIS report provides the detailed technical data, including the BFE, used to develop FEMA flood maps.

5. Mapping and Documentation

Key Tasks:

  • Generate Flood Maps: Once the BFE and floodplain boundaries are determined, generate updated flood maps. These maps should clearly show the floodplain boundaries, BFEs, floodways, and other relevant flood risk information.
  • Prepare FEMA Submission: Submit the results of the floodplain analysis to FEMA, including the BFE data and floodplain boundary information. This submission may lead to the creation or modification of FEMA’s Flood Insurance Rate Maps (FIRMs).

Related Forms:

  • Letter of Map Amendment (LOMA) or Letter of Map Revision (LOMR): If the BFE or floodplain boundaries are being updated or changed, you may need to submit a LOMA or LOMR request to FEMA. These forms are used to request changes to FEMA’s official flood maps based on new data, modeling, or analyses.
  • FEMA’s Map Revision Request Form: If you believe the existing flood maps do not accurately reflect the BFE or floodplain, this form can be submitted to request a Map Revision.

6. Floodplain Regulation and Risk Management

Key Tasks:

  • Update Local Ordinances: Once the BFE is determined and floodplain boundaries are updated, local governments may need to update their floodplain management ordinances to ensure that construction and development comply with the new floodplain regulations.
  • Risk Assessment: Assess flood risk for the community, including developing flood mitigation strategies and ensuring that flood insurance requirements are based on accurate flood zone information.

Related Forms:

  • Floodplain Development Permit: This form is required before new construction can occur in a floodplain. It ensures that buildings comply with local floodplain management regulations based on the BFE and flood risk.
  • Floodplain Management Plan: In some communities, a formal floodplain management plan will be developed based on the new BFE and floodplain data to guide future land use and construction in flood-prone areas.

Most man-made changes to land within a floodplain require a permit to ensure that the development complies with floodplain management regulations and does not increase flood risks. Some common activities that require a permit in a floodplain include:

  • Constructing new buildings, including temporary or agricultural structures.
  • Additions to existing buildings that expand the footprint or structure.
  • Substantial improvements to existing buildings, such as interior renovations that increase value or area.
  • Repairs to substantially damaged buildings, restoring them to pre-damage condition or elevating them to meet floodplain requirements.
  • Placement of manufactured (mobile) homes within flood-prone areas.
  • Subdivision of land, which may create new lots in flood hazard zones.
  • Parking or storage of recreational vehicles in flood-prone areas, which can obstruct flood flow or add to flood risk.
  • Storing materials, including gas or liquid tanks, which may be hazardous in a flood event.
  • Construction of roads, bridges, and culverts, especially if they impact flood flow or drainage patterns.
  • Placement of fill, grading, excavation, mining, or dredging that can alter water flow or floodplain elevation.
  • Alteration of stream channels, which can change the natural flow of water and increase flood risk.
  • Oil and gas drilling, which may impact groundwater or floodplain stability.

COMPACTION OF FLOODPLAIN FILL


Earthen fill used to raise the ground above the flood elevation must be placed properly so that
it does not erode or slump when water rises. For safety and to meet floodplain requirements,
floodplain fill should:

  • Be good clean soil, free of large rocks, construction debris, and woody material (stumps,
    roots)
  • Be machine-compacted to 95 percent of the maximum density (determined by a design
    professional)
  • Extend 10 to 15 feet beyond the footprint of the structure
  • Have graded side slopes that are not steeper than 2:1 (one-foot vertical rise for every 2 feet
    horizontal extent); flatter slopes are recommended
  • Have slopes protected against erosion (vegetation for “low” velocities, durable materials for
    high velocities – determined by a design professional)
    Communities may ask for a professional engineer to certify the fill elevation, compaction,
    slope, and slope protection materials in order to determine that the proposed structure will be
    “reasonably safe from flooding.”

Glossary

  • EC—Elevation Certificate
  • BFE—Base Flood Elevation
  • FEMA—Federal Emergency Management Agency
  • FIRM—Flood Insurance Rate Map
  • NFIP—National Flood Insurance Program
  • SFHA—Special Flood Hazard Area
  • TWDB—Texas Water Development Board
  • TFMA—Texas Floodplain Management Association

Federal Resources

  • Federal Emergency Management Agency (FEMA)
  • U.S. Army Corps of Engineers (USACE)
  • National Weather Service (NWS)
  • U.S. Fish and Wildlife (USFWS)
  • National Park Service (NPS)
  • Association of State Floodplain Managers (ASFPM)
  • Environmental Protection Agency (EPA)
  • National Resource Conservation Service (NRCS)
  • National Highway Traffic Safety Administration (NHSTA)