USGS crews documenting extent of flooding from Hurricane Irma

This green disc identifies a USGS High-Water Mark, which was found south of Jacksonville, Fla., Sept. 27, on a canal that feeds the St. Johns River. Photo by Kendra Smith, USGS. (Public domain.)

Eight U.S. Geological Survey (USGS) field crews are traveling around the Jacksonville, Tampa and Fort Myers, Fla., areas looking for evidence that will tell scientists how high the flood waters and storm surge from Hurricane Irma reached.

Around Jacksonville, the crews will initially be focusing on the St. Johns River, and the Julington, Sixmile, and Black Creeks, while crews based out of Tampa will be looking for marks on the Peace and Alafia Rivers. In Fort Myers, crews will be looking at the Imperial and Caloosahatchee Rivers.

The USGS specialists will be scouting these areas looking for High-Water Marks – telltale signs of how high waters reached. During a flood event or with storm surge, rising waters are laden with floating debris that can stick to trees, buildings, or other structures. Once flood waters recede, the line of debris left behind is a high-water mark and these delicate lines will indicate to scientists the highest point the flood reached. The work is expected to last up to two weeks.

As with most major flood events, the USGS is partnering with the Federal Emergency Management Agency and other state and federal agencies to flag and survey HWMs elevations in areas that were flooded to determine the extent and severity of the flooding Hurricane Irma caused.

The data associated with HWMs have many different uses and the location of the HWM plays a role in how the information is used. High-water marks connected to inland river flooding and coastal flooding can be used for future flood forecasting, predicting the severity of future floods and also for delineating the FEMA floodplain maps.

High water mark data collected from Hurricane Irma will allow FEMA to revise its current maps for the affected areas. This data is also part of the flood frequency calculations that FEMA uses to identify areas that are likely to experience high water in the event of a flood that has a 1 percent chance of happening in any given year. These floods, known as 100-year floods, serve as the foundation for flood management planning.

Another significant use for these high-water marks is the USGS Flood Inundation Mapping effort. A flood inundation map shows the extent and depth of flooding that occurred in various communities as a result of a major storm or flood event.

Inundation maps are one factor used to determine where changes should take place in building codes to help communities be more resilient; where evacuation routes should be; where (and how high) a bridge or road should be; and other community planning efforts. Once these flood inundation maps are complete, they will be documented in a USGS-series report and the associated data will be publically available online.

To learn more about USGS’ role providing science to decision makers before, during and after Hurricane Irma, visit the USGS Hurricane Irma page at https://www.usgs.gov/special-topic/hurricane-irma.