Estimation of peak discharge by slope-area method for a channel reach encompassing a bridge

Abstract

The slope-area method is used in estimating flood peak discharge at un-gauged sites or when flows exceed the maximum stage at a gauged site. The method is based on the concept of uniform flow and requires a channel reach which is fairly straight and uniform, free from falls or any features that could generate non-uniform flow patterns. However, it is not always possible to find a suitable channel reach. In this study, a flood peak discharge is estimated using a channel reach encompassing a bridge that was overtopped in the flood. A review of the literature shows that for flows under a bridge, the effect of the constriction by the bridge piers on the flow extends for distances between one and two times the bridge opening before uniform flow is re-established. No reference was found for situations when the bridge has been submerged. It was surmised that its effect was to contribute to increased flow resistance and reflected in a new steeper slope of the water line. Field measurements and an existing flow rating curve were used to calculate the peak discharge. Uncertainty in the calculated discharge was estimated at 22% by performing a first-order analysis of variance on Manning’s equation with discharge as the principle variable, thus yielding a discharge of 7076 ± 1557 m3/s. Studies are recommended to verify the estimated discharge and to establish the impact of riverine structures such as bridges on flow patterns and on the slope-area method.