Evaluation of Alternatives For Hydraulic Design of Bridges With HEC-RAS

On average, flooding causes over five billion dollars of damage and 99 fatalities per year in the United States alone. These ever-increasing numbers indicate a need for rigorous design procedures for any structures which exacerbate flood risk. The U.S. Army Corps of Engineers Hydraulic Engineering Center (HEC) has developed a computer program called River Analysis System, and more commonly referred to as HEC-RAS or RAS.

HEC-RAS is the best and most recent method for hydraulic design of bridges, and available documentation provides guidance for its use. The author is experienced in hydraulic design of bridges and has developed this thesis in order to address certain generalities and gaps in the HEC-RAS documentation. Factors examined included: location of the transition and approach sections, effects of choice of bridge calculation methods for low flow events, effects of bridge calculation methods for high flow events, effects of interpolated cross-sections, and effects of choice of inappropriate boundary conditions.

It was found that transition reach lengths recommended by HEC-RAS documentation were the most accurate, and recommendations made as part of the HEC-2 program result in over calculation of water surface elevations. It was also found that use of the energy method for bridge calculations during high flow events which experience only pressure flow results in calculated water surface elevations that are much higher than observed data indicates.