Distributed Hydrologic Modeling of Large Storm Events in the Houston-Galveston Region
Bedient, Philip B
Master of Science
In conjunction with the SSPEED Center, large rainfall events in the upper Gulf of Mexico are being studied in an effort to help design a surge gate to protect the Houston Ship Channel during hurricane events. When hurricanes hit Galveston Bay, there is a funneling effect and, depending on the track of the hurricane, the storm surge can vary by as much as 5 to 10 feet. For instance, Hurricane Ike produced a surge of about 13 feet in the bay; however, other tracks and higher winds could bring a worst case scenario of 20 to 25 feet of storm surge. Since the Houston Ship Channel is only protected from flooding up to 14-15 feet, and is currently the world’s second largest petrochemical complex, it is critical to understand the linkage between rainfall and storm surge to better protect the region. In this effort, rainfall events in the Houston-Galveston area are being examined. Given the large size of the watersheds flowing from the north and west, statistical methodologies, such as the Probable Maximum Precipitation (PMP) and Precipitation Depth Duration Frequency (PDDF), were employed to better design and predict the shape, pattern, size, and intensity of large rainfall events. Using Hydrometeorological Report (HMR) 52, as well as local hydrologic reports, the 24 hour PMP storm event was created for the upper Gulf of Mexico. In addition, large historic storms, such as Hurricane Ike, and simulated rainfalls from Hurricanes Katrina and Rita, were modeled over the Houston-Galveston region in a hydrologic/hydraulic model with the use of radar and rain gauge data. VfloTM, a distributed hydrologic model was used to model the aforementioned storms. The region was first calibrated to USGS stream gauge data from Greens Bayou Brays Bayou and Peach Creek, and the modeled results accurately depict key features of observed hydrographs, including time to peak, discharge, and the double peak discharge phenomenon caused by double rain bursts. Once calibrated, VfloTM, is used to quantify the effect that storm size, intensity, and location has on timing and peak flows in the upper drainage area. Results indicate that there is a double peak phenomenon with flows from the west draining earlier than flows from the north. With storm surge typically lasting 36-48 hours, this indicates the flows from the west and north are interacting with storm surge, with flows from the west arriving before flows from the north downstream. Gate operations were optimized in the model to account for the relative timing of upland runoff and hurricane surge, as well as the capability of the gate structure to protect the Ship Channel industry was quantified.
Hydrology; PMP; Hurricanes; Storm Surge