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What is NDNR's role in Flood Studies?

As explained in the "What goes into a flood study" page, FEMA is the agency responsible for producing Digital Flood Insurance Rate Maps (DFIRMs) and Flood Insurance Studies (FIS's). NDNR shares FEMA’s vision to communicate flood risk information. FEMA started their Cooperating Technical Partners (CTP) program in 1999, and NDNR joined this program shortly after its creation. Through this CTP program, NDNR creates DFIRMs and FIS's according to FEMA’s Guidelines and Technical Standards; FEMA quality-checks and publishes the DFIRMs and FIS reports. The below sections explain this relationship further, the products produced by this collaboration, and other products that NDNR creates to communicate flood risk.

Digital Flood Insurance Rate Maps and Flood Insurance Studies

  1. Why does NDNR produce DFIRMs and FIS's?
    • Risk MAP’s goals and visions align with NDNR’s statutory responsibilities to provide flood risk data and promote effective floodplain management to the citizens of Nebraska. NDNR has the most current data for the State of Nebraska, and the infrastructure capabilities to do large-scale mapping.
  2. How are Floodplain Boundaries Created? The basic components are:
    • A Digital Elevation Model (DEM) is a model of the surface of the earth which is created using known ground elevations (feet above sea level). The DEM displays elevations for the entire study area and is the basis for all calculations.
    • All streams within the study area with at least one-square mile drainages. Streams modeled are determined through a combination of calculations that measure each stream’s slope, flow, and depth, at any location along the entire stream length.
    • Surveyed cross sections are transects across the stream that are used to find the elevations of stream banks and beds. This information is used to supplement the DEM.
    • Hydrology calculations used to determine the amount of runoff that will travel through the stream channel(s) being studied.
    • Hydraulic calculations used to determine the height of the water based on the amount of runoff traveling through the stream.
    • Base Flood Elevations (BFEs) are the elevations of the 100-year flood event. Values are calculated using hydraulic calculations.
  3. What's included in the maps?
    • In areas with “Approximate Studies,” the flood zones are typically modeled using elevation data, regression equations, and normal depth calculations. The DFIRMs show the 100-year approximated floodplain boundaries: Zone A
    • In areas with “Detailed Studies,” the flood zones are typically modeled from physical surveys and other advanced engineering methods such as using the DEM, HEC-HMS modeling for the hydrology, and HEC-RAS modeling for the hydraulics. The 100-year and 500-year floodplain boundaries are shown, as well as Base Flood Elevations (BFE’s) and Cross Sections.
    • Additional base map layers, such as benchmarks, roads, and aerial photos, provide quality control for the data and help reference where the floodplains are located.
  4. What's included in the FIS's?
    • The reports are detailed accounts of flood history data; previous flood mitigation projects; and (if multiple flood studies exist) every flood study’s methodologies, data, and conclusions reached from each study.
  5. How long does it take to make DFIRMs?
    • The timeframe depends upon the size and complexity of the study area, but generally a typical Risk MAP project will take 3-5 years from start (the study of whether the area should be studied: Discovery phase) to finish (the official publication of the maps after a review phase: Effective Map date)
  6. Funding for the production of these maps typically comes from FEMA and State funds.
  7. NDNR uses FEMA’s Guidelines and Standards (see: http://www.fema.gov/media-library/assets/documents/34953) to produce DFIRMs and FIS Reports.
  8. For examples of DFIRMs and FIS’s, and more information about how to read DFIRMs and FIS’s, click here.
  9. For information on where to find these maps and reports, please click here to go to the Flood Risk section.

NDNR Work Maps

  1. Why does NDNR produce Work Maps?
    • Work Maps are produced with the intent that they will become FIRM’s in the future if FEMA identifies funding to allow them to be used as the basis to update FEMA FIRM maps. NDNR and FEMA share the goal of providing the best available flood risk information to Nebraskans. However, FEMA is tasked with mapping floodplains in all fifty United States. FEMA maps Nebraska floodplains as funding permits, but currently there are many areas left to be mapped in Nebraska. By providing the best available flood data using Work Maps, NDNR can expedite the process of mapping Nebraska’s floodplains, share some of the associated costs, and improve communication between FEMA and local administrators.
  2. NDNR produces these Work Maps to be used as best available data for communities who do not yet have FIRM’s or those that have outdated FIRM’s.
    • These maps can be used for zoning and planning purposes within the community. Because the maps are produced using FEMA’s mapping standards, it’s possible to adopt the Work Map’s flood risk information for local planning and building purposes. NDNR encourages local adoption of Work Maps because this can help avoid or alleviate flood risk for current development in the community. It can also prevent or lower future flood insurance costs to homeowners, business owners, and the community at large. In addition, the maps can also be used when planning flood mitigation projects.
  3. These projects are funded by sources other than FEMA; most often they are funded by State funds.
  4. Click here for an example of a Work Map panel produced by NDNR in 2009.

Base Flood Elevations

  • A Base Flood Elevation (BFE) is the height (above sea-level) that flood waters are anticipated to reach at a given location in the event of the Base (100-year, or 1% annual chance) flooding event.
  • The FIS report, not the DFIRM, should be used to determine BFEs in detailed study areas (areas that have been physically surveyed.)
  • Approximate areas (areas that have been studied using a model, not surveying) will need to have BFEs approximated based on the original model.
    • The methodology may be different based upon available data.
  • In certain circumstances, NDNR provides BFE determinations to Floodplain Administrators upon request for distribution to stakeholders applying for permits or Letters of Map Changes (LOMCs)
  • If you are a Floodplain Administrator and want to request a BFE, click here.
  • If you are a homeowner, lender, or contractor and wish to request a BFE, please contact your local Floodplain Administrator. Search for your Floodplain Administrator here.
  • Click here to view information about why building structures above the Base Flood Elevation can reduce flood risk and lower insurance premiums in your community.