Impact of Bridge Rail Geometry on Floodplain Analysis

This paper describes a method for incorporating the hydraulics of various bridge rail geometries on a bridge structure to determine the impacts on the surrounding floodplain during extreme flood events. Typical floodplain analysis either does not include the geometry of the bridge rails or simply increases the thickness of the bridge roadway deck to account for the bridge rails. These methods either underpredict or overpredict, respectively, the impact of the bridge rails and do not account for open space in the rails. The use of a previously developed rail rating curve and submergence model can be incorporated in floodplain mapping software such as HEC-RAS. The rail rating curve model can be used to determine the upstream specific energy as a function of the flow rate passing over the rail. Floodplain analysis software typically models flow over bridge structures using the weir equation. Therefore, the weir coefficient is altered such that the weir equation and rail rating curve model result in the same upstream specific energy. This method results in an iterative approach for incorporating the hydraulics of various bridge railing systems into floodplain analysis software. The method is outlined and a simple single bridge example is provided.