Regional scale flood modeling and watershed investigation, using NEXRAD rainfall, GIS, and HEC-HMS/RAS: a case study for the San Antonio River Basin, Texas

Abstract

This research develops a methodology and framework for regional scale flood modeling that integrates NEXRAD Level III rainfall, GIS, and a hydrological model (HEC-HMS/RAS). The case study surrounds a summer storm event over the San Antonio River Basin (about 4000 square miles, 10000 square kilometers) in central Texas, USA, a region subject to frequent occurrences of severe flash flooding. The basic model design connects a rainfall-runoff model (HEC-HMS) with a hydraulic model (HEC-RAS) to model unsteady state flow. The infrastructure presented in this study extends the prototype Map to Map GIS tool to a regional scale. The preliminary model system is driven by NEXRAD 4km rainfall grids. Subsequent model experiments investigate the potential to use numerical weather forecasts of precipitation to drive the rainfall-runoff model. Results demonstrate that despite some significant errors, the calibrated model is capable of producing flood forecasts comparable to observed conditions. Addressing the heterogeneous nature of basin geomorphology and hydrology may aid in improving model accuracy. To this end, the variation in river and floodplain characteristics and their relationship to hydrologic behavior are investigated, and implications for future modeling efforts discussed. The regional scale flood model and watershed study will provide a useful tool for future flood studies over the San Antonio River Basin or similar catchments, and demonstrates the potential of increasing forecast lead time by incorporating numerical weather forecasts of rainfall into the modeling system.