The Validation of the Logarithmic Velocity Profiles Above Rough-bed Flows

Knowledge of rough-bed flows is important for river engineers and environmental scientists alike as most natural surface water flows are considered within the hydraulically rough flow regime. This study helps our understanding of flow characteristics above a rough gravel riverbed, which in-turn, affect fine sediment and nutrient transport which can have a large impact on aquatic organisms and the ecosystem at large. High velocity flows over rough beds result in increased turbulence intensity and contribute to the suspension of fine sediment. Therefore, the main aim of this study is to validate first-order turbulence statistics in the form of spatially-averaged streamwise velocity profiles of the flow over a highly-porous rough bed at intermediate relative submergence. The experiments were conducted under uniform flow conditions in Cardiff University’s Hydraulics Laboratory using a narrow flume, 10 m long, 0.3 m wide, and 0.3 m deep, equipped with a bed of natural gravel with an average diameter of 20 mm. A side looking Acoustic Doppler Velocimeter (ADV) was used to measure velocity profiles at 40 random locations within the sampling area. The results show that a logarithmic velocity profile is valid for rough-bed flows with a von Karman’s constant of 0.41 and a Nikuradse equivalent sand roughness of between 1.5d and 2d.