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What Goes Into a Flood Study?

Flood Studies and Flood Maps are vital parts of any Floodplain Management program. Flood Studies are conducted to provide Nebraskans with the best available flood risk information so that they can make more-informed decisions. Flood Work Maps, Digital Flood Insurance Rate Maps (DFIRM’s), Flood Insurance Studies (FIS’s), and other non-regulatory products are used to communicate this risk in simple formats.

In the following sections you can find the following information: flood risk, flood risk products, and flood studies are explained; digital and physical places to find flood risk products and resources are identified; data commonly used in flood studies is listed and explained; and FEMA’s Risk MAP program is summarized.

Flood Risk: Basic Questions Answered

  1. Who Needs Flood Studies and Why?
    • Everyone! Flooding can affect everyone because wherever it rains, water can collect and cause flooding. In the past 5 years, all 50 states have experienced floods or flash floods. From 2003-2012, floods caused nearly $4 billion dollars in damages to infrastructure per year. In addition, more than 20% of flood insurance claims come from areas mapped outside of the “high-risk” (Zones A, AE, AO, and AH) areas. (Source: Floodsmart.gov)
    • Current Flood Studies that incorporate precise data are the best way to forecast and communicate flood risk.
    • More information is available on FEMA’s FloodSmart Website (Floodsmart.gov) and in NDNR’s Digital Desk Reference section.
  2. Who Studies Flood Risk?
    • FEMA is the agency responsible for providing flood risk information to all citizens in the United States and administering the National Flood Insurance Program. To learn more about FEMA’s Risk MAP Program click here: http://www.fema.gov/risk-mapping-assessment-planning
    • State programs can be responsible for providing flood risk information or best available information as well. NDNR is the official state agency in Nebraska for all matters pertaining to floodplain management. The Floodplain Management Section is responsible for coordinating a program promoting the wise use of land subject to flooding. For more information about NDNR’s mission and Nebraska statutes regarding floodplain management, click here: http://dnr.ne.gov/fpm/what-is-floodplain-management
    • Local watershed programs, such as those developed by natural resource districts or communities, also sometimes contribute resources to flood studies. Links to some example organizations: http://www.nrdnet.org/ and http://lincoln.ne.gov/city/pworks/watrshed/index.htm
    • Nonprofits whose missions align with wise floodplain management can contribute.
    • Contractors hired by any of the above have the resources to study flood risk.
  3. What Products Communicate Flood Risk?
    • FEMA’s Digital Flood Insurance Rate Maps (DFIRMs or FIRMs): DFIRMs typically display the 1% annual chance and 0.2% annual chance floodplain boundaries and are used for both floodplain management and for administering the National Flood Insurance Program (NFIP). The 1% annual chance flood, or 100-year flood, is the standard that insurance rates are based upon. Structures in the 1% annual chance flood zones (Zones A, AE, AO, AH, and AR) are known to be at “high risk” of flooding. High risk structures with loans backed by the Federal Deposit Insurance Corporation (FDIC) require flood insurance. Purchasing flood insurance for structures without loans that are in high risk areas is also recommended.
    • FEMA’s Flood Insurance Study Reports (FIS): The FIS is a document that explains the study methods used in the creation of floodplain boundaries shown in associated DFIRMs. It also lists or displays on profiles the Base Flood Elevations (BFEs), which are the calculated elevations of the 1% annual chance flood in detailed study areas.
    • Letters of Map Change (LOMCs): LOMC’s are documents that list or display changes made to DFIRMs because of revised flood studies or updated information in a localized area, such as a part of a community. This could be due to new engineering data, or a community activity such as a flood risk reduction project. LOMC’s can also be used to clarify the floodplain map at locations that are too small to see on a map, such as the size of one house.
    • Other non-regulatory products: products used to convey flood risk information, but not used to administer flood insurance rates.
      • NDNR’s Flood Work Maps: NDNR produces floodplain maps which can be used as “best available data” for counties and cities throughout Nebraska. The goal of producing these maps is to give local jurisdictions and their floodplain administrators a tool to avert flood losses and mitigate areas in danger of frequent flooding. NDNR encourages communities to consider adoption of Flood Work Maps for local floodplain management purposes. Contact NDNR to learn more about how Flood Work Maps can benefit your community. Nebraska Counties with existing Flood Work Maps can be seen on the ‘Floodplain Mapping Status in Nebraska’ figure.
      • FEMA’s non-regulatory products: As part of new flood mapping projects, non-regulatory products are created that show further detail than just the 1% annual chance flood zones. These products give communities more information on depths of water during floods, velocities of flowing water, and other flood information. Click here to view FEMA’s Risk MAP Products Fact Sheet: https://www.rampp-team.com/documents/region3/RiskMapProducts_Fact_Sheet.pdf
  4. Why Can DFIRMs and Flood Risk Change?
    • Typically when DFIRMs change, new and more accurate data becomes available in an area. This allows for more accurate Flood Risk Assessments to take place. Examples may include more accurate elevation data or data from a recent storm event.
    • Another possibility is that the landscape may have changed resulting in increased or decreased flood risk. For instance, development in the floodplain upstream of a mapped area could increase water flowing into the mapped area. Or, the installation of new flood control structures near or upstream of the mapped area could lower flood risk. Also, a reduction in wetlands, or other natural permeable surfaces (surfaces that absorb water) could increase or change water flow into an area downstream, and therefore increase flood risk.
  5. Where Can I Find Flood Maps or Other Risk Products?
    • FEMA’s Map Service Center distributes maps and reports. Digital downloads are available in the form of a full FIRM or FIS (for a fee), or a FIRMette (a small portion of the FIRM, which is available for free). Click to go to the Map Service Center: http://msc.fema.gov/
    • FEMA’s Library distributes digital downloads of many other educational products. Click here (http://www.fema.gov/library/) to go to FEMA’s Library.
    • NDNR’s Floodplain Interactive Map is a map viewer available to the public to review flood risk information for their local area. NDNR maintains the map and provides useful layers like the following: FEMA floodplain boundaries, FEMA Flood Insurance Rate Map panel index, FEMA map changes (LOMC’s), NDNR Work Map floodplain boundaries, local Floodplain Administrator information, political boundaries, Public Land Survey boundaries, recent Aerial Photos, BFE determinations by section (if they exist in the area of interest), roads, and contours. Click here to go to the map: http://dnr.nebraska.gov/fpm/using-the-floodplain-interactive-map
    • NDNR's Digital Desk Reference: NDNR has many helpful documents on a wide range of topics including: assistance with Elevation Certificates, understanding flood risk, what to do after a flood, etc. Click here to go to the Desk Reference: http://dnr.nebraska.gov/fpm/digital-desk-reference
    • Local Cities and Villages that participate in the NFIP have an identified repository site, such as a zoning office, where it’s possible to look at paper maps. Floodplain Administrators in NFIP participating communities can help you find maps, information about flood insurance, and apply for permits. Click here to search for your local Floodplain Administrator: http://dnr.nebraska.gov/fpm/community-floodplain-information-2

Flood Study Requirements

  1. Quality Topographic Information
    • Current and accurate topographic information (elevation data, which gives a realistic model of the earth’s surface) can greatly improve floodplain studies. About 59% of the state of Nebraska is covered by LiDAR (Light Detection And Ranging) data whose accuracy varies from a few centimeters to 6 feet (NOAA’s explanation of LiDAR data: http://oceanservice.noaa.gov/facts/lidar.html). The rest of the state has either 10 meter or 30 meter elevation data available from USGS topographic surveys. Much of the USGS data for Nebraska is more than twenty years old.
      • Click here to find out more about LiDAR data in NDNR's online repository and other data formats available online.
      • Click here to find out more about the USGS elevation data in the form of Digital Raster Graphics (DRGs).
      • Click here to find out more about how to obtain USGS Digital Elevation Models.
  2. Past Flood Information
    • History of repeated flooding
    • Storm event specific information
      • Streamgaging data
      • High water marks
      • Community records and data
    • Past Flood Studies
  3. Past Mitigation Activities
    • Any emergency, comprehensive, stormwater, or hazard mitigation plans
    • History of mitigation activities from local sources
    • History of federal disaster funds
    • Click here for more information on flood mitigation
  4. Recent Engineering Studies and Models
    • Acceptable engineering data and computer models include, but are not limited to:
      • Hydrology models, such as HEC-HMS
      • Hydraulic models, such as HEC-RAS
      • HAZUS data
      • Survey data
    • The level of detail required for engineering models varies depending on the circumstances, such as whether the area is urban or rural and the amount of infrastructure at risk
  5. Base Map Information
    • Examples of this data include: aerial photos, roads, Public Land Survey boundaries, political boundaries, flood control structure locations, community parcel data, geographic control data (such as benchmark locations), and stream location data.
  6. The more data FEMA or their mapping partners have, the more accurate flood risk information can be. Because NDNR’s goal is to provide communities with the best available flood risk information, local data is always welcomed.

What is Risk MAP and its role in flood studies?

Risk MAP (Risk Mapping Assessment and Planning) is FEMA’s vision to deliver quality data that increases public awareness and leads to action that reduces risk to life and property. See the following page for FEMA’s Risk MAP Process: http://www.fema.gov/risk-mapping-assessment-planning. NDNR participates in FEMA’s Risk MAP process because it aligns with NDNR’s goal to help Nebraskans make better informed floodplain management choices. Risk MAP provides the framework for assessing risk. NDNR assesses the risk, communicates the risk to local officials and the public, and coordinates between Nebraskans and FEMA.

The typical products of this process are updated or new floodplain maps (DFIRMs), Flood Insurance Studies (FISs), flood risk datasets, and other non-regulatory educational products such as:

  • "Changes since last FIRM," a product that shows where the floodplain has increased or decreased in size since the last Flood Study.
  • Depth and Analysis Grids showing the estimated depth of water at the 10-year, 25-year, 50-year, 100-year, and 500-year flood events.
  • "Areas of Mitigation Interest," a dataset that identifies areas that may benefit from mitigation measures.