Learn about how flood risk layers are created for map visualization.
About the process
The First Street Foundation Flood Model allows for the understanding of risk from multiple types of flood events by taking into account flooding that may be caused by fluvial (river), pluvial (rain), storm surge, and tidal sources. Flood risk types are created for map visualization through a multipart process that includes specific approaches to incorporate each risk type focused on data inputs, the incorporation of adaptation, and a review feedback loop for validation.
River (fluvial) flooding
The model used to map river flooding uses an approach that focuses on the changing river width, stream gauge flow information, and a localized set of river characteristics to capture the likelihood of the river to exceed its carrying capacity at any given location along the river or stream’s bank. Where that capacity is exceeded, the hydraulic model simulates the flow of water over the terrain using a high-precision digital elevation model.
Rain (pluvial) flooding
Flooding due to rain is mapped by applying rainfall rates, obtained from the National Oceanic and Atmospheric Administration’s (NOAA) Atlas 14 rainfall records, directly to the terrain using an elevation model surface and modeling surface runoff. This approach accounts for things like land cover and impervious surfaces to understand the absorption rate of the soil or pooling effects on concrete. The rain risk layer visualized in the Flood Model is the maximum inundation depth occurring in any extreme rain event based on the historic records in any local area.
Along the coast, flood risks are mapped to take into account high tides and storm surge. Extreme high tides can cause flooding in some low lying coastal areas. The Flood Model utilizes decades of readings from NOAA’s tide gauge stations to identify regular patterns in the levels of water along the coast. By analyzing those tide gauge readings, it is possible to isolate the flood risk associated with high tides, storm surge, and sea-level rise; as well as to incorporate each into the flood risk models. High tides and sea-level rise are regular patterns in the tide gauge readings, whereas the surge from hurricanes, tropical storms, and nor’easters tends to be much less prevalent due to the lower probability of the events. To add storm surge to the Flood Model, hundreds of thousands of coastal storms are simulated and used to generate the likelihoods of different water levels occurring at any location along the coast.
For the inland portions of the country, only riverine and rainfall hazards exist and the risk shown in the Flood Model is the maximum from those two sources for any given location. Outside of this inland area, coastal, riverine, and rainfall flood risk exist and are coupled in order to map their combined effects. These sources were modeled separately, and then coupled together in several different combinations based on historic records in order to capture the likelihood of any combination of these events occurring at any given time for any given location.
Adaptation and infrastructure
In order to create a national model with complete coverage of the contiguous United States, the Flood Model relies on nationally available data across the country. In addition to the environmental data, First Street has spent considerable time and effort to build a database of “grey” and “green” infrastructure and adaptation projects that affect the flow of water and therefore flooding. Grey flood control projects include a variety of traditional infrastructure solutions like levees, pump stations, and flood control channels. Many green infrastructure projects also contribute to flood reduction, such as wetland restoration, floodable open space, retention basins, and creek rehabilitation projects. This data is collected from state, county, and city agencies across the United States and digitized for inclusion in the mapping of flood risk.
Hazard layer validation
The Flood Model has been validated through a thorough review of the output in all areas and for all mapped flood risk layers. Comparisons are made against FEMA flood maps, hazard layers are reviewed for errors and inconsistencies, and other data sources are utilized to gauge flood risk based on past occurrences of flooding.