Enhancing a Distributed Hydrologic Model: Addition of Storm Water Pipe Analysis within a GIS Framework in an Urban Area

The City of Houston has been facing flood problems of a serious nature, some of which occurred in 1989, 1992, 1994, 1998, 2001, and 2003. Tropical Storm Allison in June 2001 created the most extensive urban flood damage in U.S. history ($5 billion). Urban flood problems have repeatedly brought attention to the need for improved urban drainage design and flood prediction, as in the case of the Texas Medical Center flooding in 2001. In order to improve flood alert capabilities for highly urbanized areas with dense drainage systems, the physics-based distributed model, VfloTM, was chosen because it preserves the spatial variability by dividing the model domain into small, interconnected cells to better represent watershed features. VfloTM has been tested with a number of rainfall events that affected the Houston area from 1998 to 2003. Incorporation of NEXRAD radar rainfall data, GIS databases, and detailed storm water system data with enhanced distributed hydrologic modeling tool (VfloTM) enables us to develop an advanced storm water modeling system in highly urbanized Harris Gully area to provide decision makers of TMC with more accurate flooding warning information in order to initiate timely evacuation plans under severe weather conditions. The original VfloTM model represented this network as overland flow model elements with only a short, trapezoidal channel reach draining into Brays Bayou. An updated hydrologic model for Harris Gully has been set up by feeding both higher spatial and temporal resolution precipitation data into an enhanced distributed hydrologic model of the same spatial resolution that more fully represents the storm water drainage network for Harris Gully. In the updated model, a much more extensive representation of the actual drainage network has been used by including channels to represent the sewer and road systems within the basin. Using channel elements to simulate overland flow along the road and sewer network has typically resulted in a better fit with respect to peak discharge magnitude and timing. It has been carefully calibrated against 6 historical rainfall events. This innovative flood modeling system will provide more accurate data on problematic flow areas in storm sewer systems and associated watersheds. The increased information and accuracy of flood levels will provide TMC decision-makers with more accurate flood warning information, enabling them to initiate timely evacuation plans under severe weather conditions. In addition, it will build a solid foundation for a future real-time flood alert system for areas as small as Harris Gully.