Erosion depth of sand from an immobile gravel bed

R Kuhnle,E Langendoen,D Wren

doi:10.5194/piahs-367-117-2015

R Kuhnle, E Langendoen + Show 1 more

Open Access

https://doi.org/10.5194/piahs-367-117-2015

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Abstract

Abstract. This study was conducted to improve prediction of the depth of erosion of sand (D50 = 0.3–0.9 mm) from immobile gravel (D50 = 36.1 mm) under steady uniform flows with bed shear stresses from 0.1 to 0.9 of that required to entrain the gravel. This situation, often encountered downstream of dams, has important implications for habitat restoration. Steady uniform flows were imposed on a flume channel containing a mixture of sand and gravel until sediment concentrations in the flow exiting the channel became small. The elevation of sand relative to gravel was measured after each experiment and compared poorly to calculated depths from published relationships whose predictions were based in part on the equivalent grain roughness of the bed. An improved predictive relationship was developed by using the cumulative distribution function of the surface gravel elevations to scale the shear velocity available for transporting sand from the gravel substrate.

Highlights

Channels downstream of dams generally have reduced flood flows due to storage in their reservoirs
Based on a series of measurements of the pressure distribution within a gravel bed, Detert and Parker (2010) developed the following expression to predict the washout depth of sand from an immobile gravel bed:
Equation (1) yielded predictions of zero scour for the coarser sand, because the shear velocity was less than the sand fall velocity in all seven of the experiments

Summary

Introduction

Channels downstream of dams generally have reduced flood flows due to storage in their reservoirs. This may cause gravel in the bed of the channel downstream of the dam to become immobile and filled with sand delivered from tributaries downstream of the dam. Accurate predictions of the depth of the sand that can be eroded out of the gravel without causing gravel mobilization are currently difficult to make. Where λc (m) is the depth below the top of gravel (negative) from which sand is cleaned out, ks (m) is the equivalent grain roughness, u* (m/s) is the shear velocity of the flow at the bed, and vf (m/s) is the fall velocity of the sand. Ambiguities in the determination of the equivalent grain roughness makes implementation of equation (1) problematic

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Conclusion