Characteristics of density current dynamics over rough channel beds

Abstract

Density currents occur in a variety of natural and man-made scenarios, and this emphasises the importance of studying them. The density-driven currents are the main agent for sediment transportation in many dam reservoirs. In most cases, these currents flow over surfaces which are not smooth; nevertheless, the effect of bottom roughness on the body of these currents has not been fully understood. Hence, this study mainly aims to examine the structure of density currents propagating over rough beds. To achieve this, alterations in the velocity and concentration profiles of the density currents in the presence of different bottom roughness configurations are investigated. The influence of various bottom roughness configurations on entrainment of ambient fluid into these currents is also quantified. Initially, laboratory experiments were carried out with density currents flowing over a smooth surface to analyse the dynamics of the currents with a range of experimental conditions; this provided a baseline for comparison. Then, seven bed roughness configurations (λ/Kr=1, 4, 8, 16, 32, 64 and 128 where λ denotes the downstream spacing between each two subsequent roughness elements and Kr denotes the roughness height) were chosen to encompass both dense and sparse bottom roughness. The rough beds consisted of square cross-section beams which cover the full channel width and are perpendicular to the flow direction in a repeated array. The primary results of this research reveal that the bottom roughness causes deceleration of the currents, reduction of their excess densities and enhancement of water entrainment into them. A critical spacing of the roughness elements (λ/Kr=8) is found for which the currents demonstrate the lowest velocities. For the spacings which are more than the critical value, the controlling influence of the roughness is reduced, and the velocities are increased by expanding the cavities between the elements. The rough bed with λ/Kr=128 roughness has very little influence on the currents and maintained velocities resembling those of the smooth bed. The magnitude of the entrainment rates also varies depending on the roughness configurations with the most substantial entrainment rate occurring for the λ/Kr=8, which is 5.26 times higher than that of the plane surface. Using dimensional analysis, equations are proposed for estimating the mean velocities of the currents and their entrainment rates for various configurations of the bottom roughness. The findings of this research contribute towards a better parameterisation and improved knowledge of density currents flowing over non-plane surfaces. This can lead to a better prediction of the evolution of these currents in many practical cases as well as improved planning and design measures related to the control of such currents.