In a follow-up to the severe flooding experienced last year due to Tropical Storm Allison, LJA Engineering was retained by the Harris County Flood Control District (HCFCD) to compile and evaluate the available rainfall, flooding, and damage data, and draw conclusions regarding the significance of the storm.
For four days in June 2001, Tropical Storm Allison battered the Houston area, dropping from 10-35 inches of rain in some regions. Records at some measuring points for the 12-hour period on June 8-9 show rainfall of 25 inches, which approaches a 1,000-year (0.1 percent exceedance probability) event. The resultant flooding damaged more than 70,000 homes and businesses, temporarily shut down major facilities at the Texas Medical Center and inundated parts of Houston's downtown tunnel system.
Over the past year, LJA has compiled and analyzed both rainfall and flood flow data for the event at approximately 100 HCFCD and U.S. Geological Survey gaging stations throughout Harris County.
In conjunction with another firm, part of LJA's responsibilities were:
- To assess the current flood control infrastructure and analyze how it performed during the storm. According to Steven Johnson, P.E., LJA Chief Hydrologist, "Allison was an extremely rare event, so virtually none of the flood control facilities were designed for this type of rainfall. We are interested in the areas where rainfall was closer to design values and seeing how the facilities performed in those areas."
- To compare the existing digital watershed models to the flooding that was actually observed during Allison. Part of LJA's responsibility is to
make recommendations on what changes to the models may be necessary to improve their accuracy.
LJA is preparing a report for HCFCD, including analysis and recommendations, to be delivered in August of this year. All data and recommendations on model calibration are being provided to FEMA for use in its ongoing Harris County Flood Insurance re-study.
