Analysis and Evaluation of Flooding Predictions in the Transition Zone Using a State of the Art Coupled Hydrologic/Hydrodynamic Modeling System
Panagiotis Velissariou, NOAA/NWS/OWP National Water Center, Tuscaloosa, AL, United States, Roham Bakhtyar, NOAA / Office of Water Prediction / National Water Center, Tuscaloosa, AL, United States, Hassan Mashriqui, Office of Water Prediction, National Weather Service, NOAA, Silver Spring, MD, United States, Kazungu Maitaria, National Oceanic and Atmospheric Administration (NOAA), National Water Center (NWC), Tuscaloosa, AL, United States, Beheen Trimble, NOAA-NWC, Tuscaloosa, AL, United States, Ali Abdolali, University Corporation for Atmospheric Research, College Park, MD, United States, Saeed Moghimi, Coast Survey Development Laboratory, National Ocean Service, NOAA, Silver Spring, United States, Andre Jaco Van der Westhuysen, Nielsen, New York, NY, United States, Graeme R Aggett, Lynker, Boulder, United States and Trey Flowers, National Water Center, Office of Water Prediction, National Weather Service, NOAA, Tuscaloosa, AL, United States
Abstract:
Flooding damages in the transition zone caused by tropical / extra-tropical storms are the results of the combined effects of heavy rainfall, river discharge, tides, storm surge and waves. The prediction of the intensity and the extent of these flooding events requires the use of an advanced modeling system that represents properly the interacting atmospheric, hydrologic, hydraulic, and coastal hydrodynamic processes. The modeling system used in the present study consists of the National Water Model (NWM) that resolves the inland hydrologic processes and DFlow FM (DFlow Flexible Mesh, Deltares) that couples the hydrologic and hydraulic processes with the coastal processes at the transition zone. The boundary and initial conditions to DFlow FM are supplied by the Extratropical Surge and Tide Operational Forecast System (ESTOFS). The modeling system is applied for Hurricane Florence on the Eastern coast of the US with a focus on Delaware Bay, Chesapeake Bay and the Carolinas. The atmospheric forcings for both NWM and DFlow FM are supplied by the Global Forecast System (GFS).
In the present study, the coupled system predicts “total water levels”, “flow velocities / flowrates” and overbank flooding. Model outputs are presented in the form of gridded data across the storm’s inundation zones. To evaluate the model’s response to storm’s flooding events (inland, coastal and compound), an assessment needs to be made on the magnitude (intensity) and timing (to peak) of the total water levels as well as on the extent of overbank flooding and flood inundation. To this end, data used for this purpose are water level observations, high-water mark data, river stages and flowrates and satellite imagery. The predicted water levels are evaluated using standard statistical measures for all point observation data. The flooding maps produced by the model are statistically compared with satellite imagery to assess the quality of flooding predictions. Flooding components (inland and coastal) are differentiated to account for both inland and coastal flooding.
