Bridge Scour by HEC-RAS Model: A Case Study over Ganga Bridge

Abstract

Lacey’s equation is often used to find out the scour depth in rivers in India. Lacey’s regime formula (1930) was developed on the basis of limited field data from irrigation canals in Punjab Province having discharge ranges from 0.70 to 173 m3/s. The total scour at a highway crossing is comprised of three mechanisms: long-term aggradation-degradation, contraction of reach, and local scours at piers and abutments. In the present study, HEC-RAS model is used to estimate the bridge scour for the Ganges road bridge and it is then compared with Lacey’s formula. Apparently, more realistic results are obtained by HEC-RAS model as it considers more related factors as angle of attack, bed conditions, shape of piers, flow concentrations, flow depth and Froude No. as against discharge intensity and silt factor considered in case of Lacey’s equation. Comparative study is carried out for different angles of attack and restricted flow condition near the bridge site. From the restriction of waterway by 3.6 km to 2 km, abutment scour and contraction scour change 1.6 times and 3.9 times respectively. So, the waterway has significant role for the bridge foundation design. By changing angle of attack by 5 °, the change in pier scour is 10%.

Keywords: Scour, mathematical modelling, bridge, angle of attack