USGS evaluates Withlacoochee River, Georgia, high water marks

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Eight USGS field crews traveled around the Jacksonville, Tampa and Fort Myers areas last week looking for evidence that tell scientists how high the flood waters and storm surge from Hurricane Irma reached. A crew was able to make it to Hernando County to evaluate the Withlacoochee River. Andy Knaak, USGS hydrologist worked his way down river from Trilby, stopping in Ridge Manor at Cyril Drive and Highway 50. (The Withlacoochee runs north.) On that day, the river had already receded a couple feet from the high water mark.

Knaak evaluated seed lines, high water marks on trees as well as the grass line. A seed line is a what it sounds like: a line of seeds attached to a surface such as a fence post. Seeds floating on the surface of the water, adhere to objects and remain there for a short time after the water recedes. A seed line is considered to be a primary source for high water mark information, since the seeds do not stay in place for a long period of time and the mark cannot be confused with other periods of high water. A grass line delineates the living grass and the dead grass caused by flooding.

The USGS has provided the following information on how the high water mark data will be used,

“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 publicly available online.”