Modeling an Urban Drainage System with Large Tailwater Effects Under Extreme Rainfall Conditions

摘要

The City of Houston has a history of massive flooding and is listed third on the list ofrnRepetitive Loss Communities ranked by payments from the National Flood InsurancernProgram. The remainder of Harris County (outside of Houston) is ranked fourth on thernabove list. The Harris Gully watershed, located about 3 miles southwest of downtownrnHouston, is one area that has received significant attention due to the frequency ofrnflooding over the years. Harris Gully drains approximately 13 km~2 (5 mi2) of developedrnurban land use. One of the main causes of flooding in the Harris Gully watershed is arnhigh tailwater condition in the receiving stream, Brays Bayou. Due to development in thern1960s and 1970s, Brays Bayou reaches near bank full conditions more frequently thanrnwas expected when the bayou was channelized. When the gully’s outlet becomesrnsubmerged, the outflow capacity can be reduced by 60-80%.rnRice University has developed and calibrated a model to understand the drainage andrnflooding issues in the Harris Gully watershed during extreme rainfall. With the use ofrnadvanced technologies such as NEXRAD radar and GIS, as well as the unsteady staterncapabilities of SWMM, a dual drainage model was created to evaluate the sub-grade pipernnetwork (minor system) and overland flow and storage network (major system) as itrnrelates to large tailwater conditions at Brays Bayou. The resulting SWMM modelrnsimulates the flow through the pipes as well as ponding and sheet flow in the streets.rnWhen Tropical Storm Allison deluged southeast Texas in June 2001 it caused anrnestimated $5 billion in damages to the 28 counties declared as disaster areas.rnApproximately $2 billion of this damage occurred at the Texas Medical Center in thernHarris Gully watershed. The data collected during and after the storm, however, providesrnone of the most extensive databases ever available for the analysis of an urban flood. Therncalibrated SWMM model successfully simulated the observed T.S. Allison high waterrnlevels in Harris Gully. In a continuing study, the model is being used to evaluate potentialrnmitigation options for the watershed.