Analysis of scour depth in the case of parallel bridges using HEC-RAS

Scour is now one of the main problems for river as well as for coastline engineering. Bridges are the vital structures which must be designed to prevent failure against the effects of scour. Scour holes can occur without warning and cause the failure of a bridge. The main significant issues in hydraulic and river engineering are to determine the connection between parameters affecting the maximum and minimum depth of scour. The scour depth in the alluvial stream below a river bed differs based on the flows, pier shape, pier size and sediment characteristics. Dual bridges of basically the same structure are placed parallel to and only a small distance away from an existing bridge, either on the upstream or downstream side. Naturally, the backwater generated by dual bridges is bigger than that of a single bridge but lower than the value resulting from separate consideration of the two bridges. In the present work, an hydraulic model is used to simulate the stability of a bridge in the study area, namely 'Sardar Bridge' on the Tapi river. Scour profiles for various flood events have been assessed for a particular bridge. The velocity of flow is used to estimate depths of scour at different piers and abutments. Estimating depth of the scour during the design can significantly decrease the overall cost of bridge foundation construction. Results from the present study show that construction of a new bridge should be proposed on the upstream side rather than downside side of the existing bridge. By doing so, hydraulic stability of the existing bridge is ensured.