Effect of non‐migrating bars on dune dynamics in a lowland river

T V De Ruijsscher,S Naqshband,A J F Hoitink

doi:10.1002/esp.4807

T V De Ruijsscher, S Naqshband + Show 1 more

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https://doi.org/10.1002/esp.4807

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Abstract

AbstractAs dunes and larger‐scale bed forms such as bars coexist in rivers, the question arises whether dune dynamics are influenced by interaction with the underlying bed topography. The present study aims to establish the degree to which dune characteristics in two and three dimensions are influenced by an underlying topography dominated by non‐migrating bars. As a case study, a 20 km stretch in the Waal River in the Netherlands is selected, which represents a sand‐bed lowland river. At this location, longitudinal training dams (LTDs) have recently been constructed to ensure sufficient navigation depth during periods with low water levels, and to reduce flood risk. By using data covering 2‐year‐long periods before and after LTD construction, the robustness of the results is investigated. Before LTD construction, dune characteristics show large variability both spatially and temporally, with dunes being longer, lower, less steep and having a lower lee side angle when they are located on bar tops. The correlation between dune characteristics and the underlying bed topography is disrupted by unsteady conditions for which the dunes are in a state of transition. The bar pattern causes tilting of dune crest lines, which may result from a transverse gradient in bedload sediment transport. As a result of LTD construction, the hydraulic and morphological conditions have changed significantly. Despite this, the main conclusions still hold, which strengthens the validity of the results. ©2020 The Authors. Earth Surface Processes and Landforms published by John Wiley & Sons Ltd.

Highlights

Bed forms have been studied extensively for decades, in particular because they cause hydraulic resistance
As dunes and larger-scale bed forms such as bars are known to coexist in rivers (Ashworth et al, 2000; Villard and Church, 2005; Wintenberger et al, 2015; Rodrigues et al, 2015; Le Guern et al, 2019), the question arises whether dune dynamics are influenced by the underlying topography, which may manifest itself as a lagged spatial response to a change in depth
We focus on the effect of a spatially varying bed topography dominated by non-migrating bars on dune characteristics and hydraulic roughness, making use of a spatially and temporally extensive morphological field data set in the lowland sand-bedded Waal River, the Netherlands

Summary

Introduction

Bed forms have been studied extensively for decades, in particular because they cause hydraulic resistance. We define dunes as migrating bed forms adopting length scales of 10–200 m. As dunes and larger-scale bed forms such as bars are known to coexist in rivers (Ashworth et al, 2000; Villard and Church, 2005; Wintenberger et al, 2015; Rodrigues et al, 2015; Le Guern et al, 2019), the question arises whether dune dynamics are influenced by the underlying topography, which may manifest itself as a lagged spatial response to a change in depth. We test the hypothesis that dunes and the underlying bed topography can be treated independently, taking the Dutch Waal River as our study area

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