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

Abstract

Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Abstract 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 × 9 m bedrock-alluvial river reach using terrestrial laser scanning and 3-D printing. In the reported experiments, water depth and velocity were recorded at 18 locations within the channel at each of five 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 are analyzed 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 generalize these findings.

A Froude‐scaled model of a bedrock‐alluvial channel reach: 2. Sediment cover

“…The relative importance of topography and hydraulics can be ascertained through a comparison of the sediment erosion data set with the hydraulic data collected in the runs without sediment cover (presented in Figure and Hodge and Hoey []). These two data sets are independent and therefore not strictly comparable as the hydraulic data do not include the changes in local roughness and velocity caused by the sediment cover (as demonstrated in Hodge and Hoey []). However, on the assumption that the presence of sediment increases roughness and so will decrease velocities, the hydraulic data can be used as an upper estimate.…”

“…Two sets of experiments were performed, focusing on hydraulics and sediment, respectively. In the first set (reported in the companion paper [ Hodge and Hoey , ]), 3‐D velocity data were collected from each of 18 locations, at discharges ranging from 20 to 60 L s −1 . These data show that hydraulic properties become more spatially variable as discharge increases and that a core of supercritical flow develops along the model domain.…”

“…The printed surfaces, comprising six individual tiles, were installed in the flume 3.5 m downstream from the inlet, and the rest of the flume was filled with sediment of a comparable roughness to the printed bed. Full details of the model creation, and a comparison of flume and field data that demonstrates the Froude scaling, are presented in the companion paper [ Hodge and Hoey , ].…”

“…This model is Froude‐scaled, which is an advance over previous models of bedrock‐alluvial channels that have had Froude numbers significantly higher than are found in many bedrock channels [ Chatanantavet and Parker , ; Johnson and Whipple , ]. Furthermore, the dimensions of the model (0.9 m wide) mean that spatially distributed hydraulics can be measured, and these data are presented in the companion paper [ Hodge and Hoey , ]. Although these hydraulic data are mostly independent of the data presented here, they provide a useful approximation for the hydraulic conditions in these experiments; here rapidly changing flow and sediment cover prevented the collection of spatially distributed hydraulic data.…”

“…This paper reports the use of a Froude‐scaled physical model of a specific bedrock‐alluvial channel to analyze the factors that control the spatial pattern and stability of sediment cover. This is the second in a pair of papers; the companion paper [ Hodge and Hoey , ] demonstrates the Froude scaling of the model and quantifies relationships between the bed topography and hydraulics.…”