Urban Hydrologic Forecasting Application Issues Using the WSR-88D Radar

Flooding conditions along the Texas Gulf Coast and in the Houston area have increased significantly over the past thirty years due to a drastic increase in urbanization. Two urban watersheds, the Brays Bayou watershed and the White Oak Bayou watershed, are both channelized streams that cannot contain their current 100-year design storm and are now subject to flooding during small storms. Highly spatially and temporally variable rainfalls are common in the area. Level II reflectivity data from the WSR-88D NEXRAD radar was used to estimate rainfall rates over both watersheds for two storms (October 1994 and September 1998). The hydrologic modeling results from variable grid size radar-rainfall inputs (1 km², 4 km², and 16 km²) were compared to the results of the hydrologic modeling using the traditional method of Thiessen polygon weighting from the local rain gages. All modeling studies were compared to the observed flows at a downstream flow gage. Hydrologic modeling runs using radar-rainfall have been very encouraging and were used to create the real-time hydrologic forecasting capabilities of the Rice University - Texas Medical Center Flood Alert System (www.floodalert.org). The system, in place since 1998, serves the largest medical center in the world and uses WSR-88D rainfall averaged over the entire basin to make flood forecasts.