Simulating the Upper St. Johns River for extreme events

Christopher J. Brown and Amanda Tancreto


The St. Johns River is one of the most important river systems in Florida, USA.  The watershed begins in East-central Florida and discharges to the Atlantic Ocean east of Jacksonville, Florida.  The watershed is susceptible to large rainfall events including tropical storms/hurricanes and is topographically “flat” such that flooding is a real concern.

The Upper St. Johns River Basin (USJRB) encompasses an area of approximately 4,530 km2. USJRB is mainly comprised of marsh and agricultural land types including man-made storage areas used for flood control and environmental management and includes numerous water control structures.  The Middle St. Johns River Basin (MSJRB) is downstream of the USJRB and covers an area of approximately 3,100 km2.  The land use in this region is dominated by more urbanized areas including parts of Orlando, Florida.  Recently, researchers at the University of North Florida developed a preliminary HEC-HMS rainfall-runoff model of the USJRB and a portion of the MSJRB.  The model domain covers roughly 5,200 km2 and includes numerous sub-basins.  

The model was calibrated and verified against observed data recorded between 2007 and 2011 including a large tropical storm event.

This new study expands the use and scope of the model by examining extreme rainfall events and resulting flows that may occur in the future under possible climate change scenarios.  First, the model is used to simulate a hypothetical “probable maximum precipitation” or PMP event.  Then, the model is used to simulate the same PMP event but with future climate change forcings added.  Lastly, the model is used to simulate two, 100-year rainfall events occurring in the same week.  The study provides the simulation results, compares them to historical flow data at key locations within the model domain, and then finishes with a discussion of water resources planning considerations for the future.

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