Local Head Loss Coefficients of Riffle Pools in Gravel-Bed Rivers

Case studies of energy loss over bed forms in gravel-bed rivers are needed to determine local loss coefficients for modeling and design. In this study, an intense field campaign in two rivers is used to determine energy loss and calculate local energy loss coefficients ($K_e$) for riffle-pool units. Velocity head, water surface elevations, and energy coefficients ($\alpha$) are determined at up to eight cross sections at each site over a range of discharges including, at one site, the bank-full discharge. The water surface and energy grade line are relatively flat as flow enters the pool and relatively steep as flow leaves the pool. In both watercourses and at all discharges, $\alpha$ is higher in pools (i.e., velocity is more heterogeneous) and lower in riffles (velocity is more homogeneous). In a short but deep pool forced by a large fallen tree (Moras Creek), $K_e=1.7–2.0$. This high value is explained by a pool obstruction that results in a series of gradual and sudden flow transitions. In a long and relatively shallow pool (Ditton River), $K_e\sim 0$. This low value is explained by the uncertainty in resistance partitioning and the variability of models for skin friction. Future research should determine the range of local energy loss coefficients in natural and constructed bed forms in rivers.