Flood Inundation Mapping for Flood Prediction, Analysis, and Emergency Management

The National Weather Service – Hydrologic Information Center estimates the 30-year average of flood losses are $8.17 billion in damages. Excessive precipitation, from tropical storms, snow-melt, convective thunderstorms, or frontal systems, contributes to a major portion of catastrophic inland flooding. While it is impossible to prevent a flood, preparation, mitigation, and insurance can lessen the impacts.

Preparation at the local level includes evacuations, sand-bagging, and road closures. With advance notice of probable flooding conditions, efforts could be focused to the areas in greatest need, whether evacuations, sand-bagging, or operating flood control facilities. Emergency managers and responders, as well as residents, would benefit from information to where and when major flooding may occur, particularly the extents and depth of flooding. Local officials making decisions concerning disaster recovery and rebuilding post-flood require knowledge of where the greatest damage occurred. Depths and extents of previous flooding are necessary for resilient rebuilding.

Maps convey critical information to a variety of users. Flood inundation maps relate gage stages to the extents that will flood on the 3-dimensional land surface. The concept of flood inundation mapping is not new; the U.S. Geological Survey (USGS), National Weather Service (NWS), State, and local governments have completed such work previously. Flood inundation maps available from the NWS and USGS are available for limited geographical areas and lack continuous coverage of a watershed.

Another drawback of the NWS and USGS maps are pre-computed inundation areas are typically tied to one river gage, rather than a group of gages. The user has to make a subjective guess for areas in between gages. During a large event, it would be useful to have one map depicting real-time or forecast inundation extents and depths for the entire series of gages for a watershed or area of interest.

Such a map can be created quickly using Geographic Information System (GIS) software and functions, once the required underlying information for an area of interest has been compiled. This process allows quick creation of the flood extent and depth layers and is an inexpensive solution for local and regional governments. The resulting flood inundation layers can be displayed with other critical data such as roads, hospitals, airports, and wastewater treatment plants. GIS layers can be converted into compatible files for use with Google Earth so that anyone can visualize their risk of flooding during a real-time event. The Federal Emergency Management Agency's (FEMA) National Flood Hazard Layer (NHFL) can be referenced concurrently to relate the observed or expected flooding to the 1-percent or 0.2-percent annual recurrence interval to determine the initial severity of an event. This process can also be utilized to pre-compute stages related to flood action levels as part of an emergency action plan. The closest gage stage can be related to predicting when a particular critical access road will flood aiding in evacuation efforts. Emergency responders can be proactive rather than reactive knowing when road overtopping is expected to occur.

Flood depth tables can be utilized in FEMA's HAZUS software. Flood inundation maps are required to receive credit for the National Flood Insurance Program (NFIP) Community Rating System (CRS) program under the flood warning and response activity.

Other potential users of real-time and forecast flood inundation maps are insurers and reinsurers. Insurers and reinsurers could use these maps as documentation for insurance claims and damage estimates.

My presentation will detail this process and underlying data required to create flood inundation maps. Future improvements and ideas will be discussed. Sometimes a quick answer is better than no answer at all for those areas that do not have maps available from NWS or USGS.