Estimating sand bed load in rivers by tracking dunes: a comparison of methods based on bed elevation time series

Kate C P Leary,Daniel Buscombe

doi:10.5194/esurf-8-161-2020

Abstract

Abstract. Quantifying bed-load transport is paramount to the effective management of rivers with sand or gravel-dominated bed material. However, a practical and scalable field methodology for reliably estimating bed load remains elusive. A popular approach involves calculating transport from the geometry and celerity of migrating bedforms, extracted from time series of bed elevation profiles (BEPs) acquired using echo sounders. There are various echo sounder sampling methodologies to extract bed elevation profiles. Using two sets of repeat multibeam sonar surveys with high spatiotemporal resolution and coverage, we compute bed load using three field techniques (one actual and two simulated) for acquiring BEPs: repeat multibeam, single-beam, and multiple single-beam sonar. Significant differences in flux arise between repeat multibeam and single-beam sonar. Multibeam and multiple single-beam sonar systems can potentially yield comparable results, but the latter relies on knowledge of bedform geometries and flow that collectively inform optimal beam spacing and sampling rate. These results serve as a guide for design of optimal sampling and for comparing transport estimates from different sonar configurations.

Highlights

Bed load is usually a significant proportion of total transported load in rivers with sand and/or gravel-dominated bed material, and the relative importance of suspended load and bed load often changes with flow and the location within the channel (e.g., Gomez, 1991)
Reliable estimates of bed-load transport can be produced from application of the Exner equation (Simons et al, 1965; Engel and Lau, 1980) to time series of bed elevation profiles (BEPs) (Simons et al, 1965; Van Den Berg, 1987; Dinehart, 2002; Villard and Church, 2003; Wilbers and Ten Brinke, 2003; Claude et al, 2012; Guala et al, 2014) acquired with an echo sounder
We consider the repeat multibeam-derived bed-load estimates to be the most accurate because the superior spatiotemporal coverage of these data allows for simultaneous resolution of Vc, H, and λ

Summary

Introduction

Bed load is usually a significant proportion of total transported load in rivers with sand and/or gravel-dominated bed material, and the relative importance of suspended load and bed load often changes with flow and the location within the channel (e.g., Gomez, 1991). Simons et al (1965) showed that bed-load flux can be estimated by tracking the average celerity, Vc, of the downstream migration of dunes with a known average height, H , and average length, λ. These variables are averaged over a field of dunes to satisfy the necessary assumptions that suspended sediment load, qs, is in equilibrium (dqs/dx = 0), and with continuity of mass (dqb/dx + dη/dt = 0), where x and η are downstream distance and bed elevation, respectively (Simons et al, 1965). The Simons et al (1965) approach quantifies only the first-order bed-load flux due to dune translation, not accounting for any Published by Copernicus Publications on behalf of the European Geosciences Union

Methods

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Discussion

Conclusion