Determination of Equivalent Roughness of Bridge Piers’ Flow Resistance

AbstractThe Manning equation has been widely used in rivers, with the flow resistance caused by bed roughness (n) to account for all energy loss influences associated with channel characteristics. Flow resistance also arises from bridge piers, which are common features in rivers, but the Manning equation is inappropriate for estimating overall flow resistance caused by the drag force on bridge piers. Therefore, it is necessary to study the relationship between flow resistance and bridge pier parameters, especially when a large number of bridge piers are present in rivers. Two new methods are proposed, in which the alternative and equivalent roughness nt is related to bridge pier characteristics and can incorporate bed roughness n. The first method (namely measured data–based method) is derived from the analysis of local head loss and frictional head loss and can be used in the case of field-measured data. The second method (namely, the multiparameter empirical method) is derived from the drag coefficient of a single pier, the spatial distribution form of the piers, and the difference of the incoming flow velocities and can be used practically without a comprehensive field site data survey. The calculation of the equivalent roughness of 30 bridges in Jiangsu shows that the results from two methods are in good agreement with each other. The two methods developed in this study provide conveniences for assessing the flow resistance caused by pier drag forces.