How Flood Mapping Can Help You

When building near waterways, large and small, developers and towns need to consider the future. Rainwater events can rapidly change the depth and velocity in the waterway, leading to flooding and water damage. Fortunately, there’s a way to anticipate the future, as GZA’s Christine Suhonen explains: A flood map, which depicts the anticipated inundation zone. Ms. Suhonen frequently develops these maps using a software called HEC-RAS. 

Short for the Hydrologic Engineering Center (HEC) River Analysis System (RAS), “HEC-RAS is a computer software we use for hydraulic modeling. We use it to model flow through a river, stream or brook.” Suhonen  tells us. “You put in the flow rate, how many cubic feet per second, and the software computes the water surface elevation.” Originally developed for the US Army Corps of Engineers, HEC-RAS has become an industry standard for modeling waterways. You’ll often see it in floodplain management reports.

When you look at a flood map, you may see a set of lines crossing the waterway. “That’s where we extracted the terrain data. We put the terrain data into the model so that the model knows what the brook and the overbank look like,” Suhonen informed us. Once the model’s been developed, it can be used to look at many magnitudes of storms. “You can run a small storm or simulate a bigger storm. If you have a bigger storm, it’ll compute a higher flood elevation.”

The key to reading the rest of the map is understanding the probabilities it presents. Flood maps developed for the Federal Emergency Management Agency (FEMA) often present the Base Floodplain, the 0.2-Percent Floodplain, and the Regulatory Floodway. “The base floodplain is the 100-year flood, a flood that has a 1% chance of being equaled or exceeded in any given year. Some people say it’s the flood you expect every 100 years, but probability doesn’t work like that. Multiple 100-year floods could occur in the same year.  If you had a long period of data, say 1,000 years of data, then you would expect to see ten 100-year floods, but they may not be spaced apart evenly.” The 0.2 Percent Floodplain is the “500-year” flood.   The HEC-RAS model can be used to compute the Regulatory Floodway too. “That’s the part of the river that should not be developed in to prevent the water surface elevation from increasing more than a foot,” as Suhonen tells us. “As long as you develop outside that area, the development won’t cause a significant rise in the river.”

The Base Floodplain, 0.2-Percent Floodplain, and Regulatory Floodway may change due to developments in the upstream watershed and climate change. Developments with significant impervious surfaces, such as pavement, can also result in larger flows in waterways. The National Climate Assessment, a report on climate change published by various United States government agencies, reported increasing rainfall intensity in the Northeast United States, which suggests the 100-year floods and 500-year floods are increasing as well.

A flood map is a powerful data tool for town governments, citizens, and developers, allowing them to zone land and build projects while avoiding flood risk. These maps will help keep us from building in places the water most wants to go.