The influence of bed roughness on the dynamics of gravity currents

Abstract

The present work experimentally investigates the dynamics of unsteady gravity currents over rough beds performed by lock-release of saline water into a fresh water tank. Four experiments were performed by varying roughness throughout the channel bed, maintaining the initial density of the saline water in the lock, the lock length and the water depth. An image analysis technique was applied to visualize and characterize the current allowing thus the understanding of its general dynamics and, more specifically, of the current head dynamics. The convergence of local Froude number obtained at the head suggests that the dynamics, namely entrainment processes, are governed by local variables at the head. The temporal evolution of the head length and mass shows repeated cycles of stretching and breaking, and the average period of the breaking cycles is seen to increase with roughness. The growing rate of the head mass is seen to decrease in time, suggesting that the entrainment at the head is ruled by local buoyancy.