A Froude-scaled model of a bedrock-alluvial channel reach: 1. Hydraulics

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Hodge, Rebecca A. ; Hoey, Trevor B. (2016)

The controls on hydraulics in bedrock-alluvial rivers are relatively poorly understood, despite the importance of the flow in determining rates and patterns of sediment transport and consequent erosion. To measure hydraulics within a bedrock-alluvial channel, we developed a 1:10 Froude-scaled laboratory model of an 18 x 9 m bedrock-alluvial river reach using terrestrial laser scanning and 3D printing. In the reported experiments, water depth and velocity were recorded at 18 locations within the channel at each of 5 different discharges. Additional data from runs with sediment cover in the flume were used to evaluate the hydraulic impact of sediment cover; the deposition and erosion of sediment patches in these runs is analysed in the companion paper. In our data: 1) spatial variation in both flow velocity and Froude number increases with discharge; 2) bulk flow resistance and Froude number become independent of discharge at higher discharges; 3) local flow velocity and Reynolds stress are correlated to the range of local bed topography at some, but not most, discharges; 4) at lower discharges, local topography induces vertical flow structures and slower velocities, but these effects decrease at higher discharges and, 5) there is a relationship between the linear combination of bed and sediment roughness and local flow velocity. These results demonstrate the control that bedrock topography exerts over both local and reach-scale flow conditions, but spatially distributed hydraulic data from bedrock-alluvial channels with different topographies are needed to generalise these findings.
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