This page provides information related to the analysis of the elevated water level component of coastal flood risk. For the Atlantic and Gulf Coasts, water level is determined in one of two ways:
- Statistical analysis of observed tide gauge data; or
- Two-dimensional modeling.
In New England, due to the length of available tide gauge records (sometimes extending over 100 years) and the density with which this information is available, as well as the fact that the risk is often caused by storms with typical characteristics (large nor’easters), statistical analysis of the observed water levels is the preferred approach.
In the remaining portions of the Atlantic Ocean shoreline and the Gulf of Mexico coast, water level is more dependent on the track of tropical storms. In order to project these tropical events and understand how alternative tracks may impact flood risk, sophisticated two-dimensional computer models use wind speed, barometric pressure, and storm direction to calculate the water levels resulting from many different storms. These modeled results are verified using observed data and then sampled to determine the mapped water level. Technical details on the approaches used to determine water levels on the Atlantic and Gulf Coasts are provided in FEMA's guidance. Information on the organization of coastal guidance can be found on the Coastal Flood Hazard Analysis and Mapping page.