Analysis of Alternatives for Computing Backwater at Bridges

Abstract

The performance and reliability of four of the more popular methods for one-dimensional flow analysis of bridges were investigated. These four methods were the HEC-2 Normal Bridge Method, HEC-2 Special Bridge Method, WSPRO, and Modified Bradley Method. The study was based on models of 13 flood events at nine different bridge sites in the southeastern United States. The sites were located on wide, flat, heavily vegetated floodplains where only free-surface, subcritical flow conditions occurred. The results showed that the HEC-2 Normal Bridge Method was able to accurately simulate the measured backwater values when the recommended 4:1 expansion ratio assumption was not applied. The application of the 4:1 expansion ratio assumption generally resulted in water surface elevations and backwater values that were higher than the measured values. The HEC-2 Special Bridge Method generally underpredicted the measured backwater, and WSPRO tended to compute the backwater to be slightly greater than the measured values. The Modified Bradley Method consistently underpredicted the measured backwater amount at each site.