Shallow gravel-bed river flows: The behaviour of the free surface and the flow information contained within

In this study several uniform shallow flows are considered, over an initially non-mobile, rough gravel sediment deposit, with no appreciable bedforms, in a laboratory flume. Conductance probe data is used to measure the free surface dynamics in a vertical streamwise plane along the centerline of the flume. A spatial correlation function is derived which captures the spatial and spectral nature of the free surface. Properties of this correlation function are shown to be governed by the bulk hydraulic conditions, confirming a link between the free surface behavior, the sub-surface turbulence properties, and the bulk flow and boundary conditions. Such observations suggest that the hydraulic properties of shallow flows may be measured remotely by investigation of the dynamics of the free surface. Further experiments were then conducted involving flow conditions whereby the sediment bed was caused to become mobile. It is shown that the bulk flow properties relate differently to one another, compared to the static bed conditions. This results in properties of the free surface spatial correlation function being markedly different to those of the static bed conditions, suggesting that remote measurement of the free surface may also facilitate the determination of bed material mobility.