Tropical Storm Allison Event Analysis

Over 70,000 cases of residential flooding were reported during Tropical Storm Allison. This aerial shows the Hunting Bayou watershed, just east of downtown Houston.

Tropical Storm Allison was the first tropical storm of the 2001 hurricane season. This storm dropped more than 35 inches of rain over the Houston metropolitan area between June 5 and 9 before moving across the country to leave more devastation in its wake, notably in southeast Louisiana (20 inches of rain) and in Pennsylvania (up to 10 inches of precipitation).  By the time the storm dissipated off the Atlantic coast on June 17, Tropical Storm Allison had claimed over 20 lives and had resulted an estimated $5 billion in damages in Harris County, Texas alone.

As a result, PBS&J was called upon to analyze the storm event for the Harris County Flood Control District and the Federal Emergency Management Agency. The PBS&J project team started by collecting and analyzing available rainfall data, flood elevations and flows, and storm-related damage across the county, documenting the flood's impact and comparing it to previous events in Harris County.  Using existing FEMA hydrologic and hydraulic models for the affected county watersheds, PBS&J analyzed rainfall and flood data to, ultimately, assess the performance of the models, to recommend potential refinements to them, and to establish the statistical significance of the storm.

Once conclusions were reached about the magnitude of the tropical storm and its associated flooding, PBS&J was able to assess the performance of drainage infrastructure in Harris County overall.  The findings of the project team were delivered to the Harris County Flood Control District in a report that documented the study methodology and its conclusions, which was used to provide information about the storm to the public.