The Federal Emergency Management Agency (FEMA) and local governments are responsible for regulation of floodplains in the U.S. In the due diligence phase of development, engineers can use GIS to analyze the current floodplain, natural ground elevations, and other datasets to determine the maximum amount of developable land that can be removed from the floodplain while meeting all the requirements of the regulatory agencies. The main solution computed from this analysis is the volume of earthwork that needs to be moved to prepare the land for development. This, in turn, provides an estimate of construction cost.
Generally, a portion of land can be removed from the floodplain if the land is filled to an elevation that is above the regulatory 100-year water surface elevation (WSEL). The volume of fill placed is computed as the difference between the 100-year WSEL and natural ground. Additionally, this fill volume must be removed from elsewhere within the floodplain to compensate for flood storage volume lost due to development. This is called floodplain fill mitigation. GIS’s spatial analyst and 3D analyst tool sets are utilized to create a visual picture of the floodplain depths to aid in identifying areas that can be economically reclaimed from the floodplain.
In the example in figure-01, a maximum flood depth of 4-feet was determined as an economical point at which the land could be removed from the floodplain
Once the areas to fill have been establish, GIS terrain modeling tools are utilized to compute the volume of earthwork required to reclaim the land. This information is then used to compute construction cost estimates and demonstrate to regulatory agencies that the required floodplain fill mitigation volume has been provided. These methods allow engineers to find a balance between the cost of earthwork and the amount of land that can be reclaimed for development, maximizing the developer’s return on investment.
While GIS is not a substitute for sound engineering judgment, it can be an important tool that engineers use to provide responsible and profitable development within the floodplain.