Modulation of outer bank erosion by slump blocks: Disentangling the protective and destructive role of failed material on the three‐dimensional flow structure

Christopher Hackney,Julian Leyland,Andrew Nicholas,Stephen E Darby,Daniel Parsons,Jim Best,Rolf Aalto

doi:10.1002/2015gl066481

Abstract

AbstractThe three‐dimensional flow field near the banks of alluvial channels is the primary factor controlling rates of bank erosion. Although submerged slump blocks and associated large‐scale bank roughness elements have both previously been proposed to divert flow away from the bank, direct observations of the interaction between eroded bank material and the 3‐D flow field are lacking. Here we use observations from multibeam echo sounding, terrestrial laser scanning, and acoustic Doppler current profiling to quantify, for the first time, the influence of submerged slump blocks on the near‐bank flow field. In contrast to previous research emphasizing their influence on flow diversion away from the bank, we show that slump blocks may also deflect flow onto the bank, thereby increasing local shear stresses and rates of erosion. We use our measurements to propose a conceptual model for how submerged slump blocks interact with the flow field to modulate bank erosion.

Highlights

The erosion and deposition of sediment by fluvial bank erosion plays a pivotal role in maintaining the ecological and geomorphological diversity of fluvial channels [Florsheim et al, 2008; Camporeale et al, 2013]
In contrast to previous research emphasizing their influence on flow diversion away from the bank, we show that slump blocks may deflect flow onto the bank, thereby increasing local shear stresses and rates of erosion
The application of high-resolution 3-D flow and topographic survey techniques to bank embayments at different stages of their evolution has revealed the influence that slump blocks may have upon the instantaneous near-bank flow field

Summary

Introduction

The erosion and deposition of sediment by fluvial bank erosion plays a pivotal role in maintaining the ecological and geomorphological diversity of fluvial channels [Florsheim et al, 2008; Camporeale et al, 2013]. Recent advances in acoustic Doppler profiling [Kostaschuk et al, 2005; Szupiany et al, 2007, 2009; Vermeulen et al, 2014] and high-resolution topographic data collection [Parsons et al, 2005; Aalho et al, 2009; Nittrouer et al, 2011; Lotsari et al, 2014; Kasvi et al, 2015; Leyland et al, 2015] enable the instantaneous flow and morphology of the near-bank region to be fully quantified, allowing for the novel investigation of these complex process-form interactions. For the first time, the complex flow-form interactions that occur around these large-scale roughness elements and highlight how slump blocks may initially enhance bank erosion, through their steering of the 3-D flow field, before the role of larger bank embayments and large-scale outer bank flow separation act to reduce erosion rates. We use our observations to propose a new conceptual model for the role of bank roughness in controlling the evolution of bankline topography and modulating rates of bank erosion

Study Site

Methods

Observations

Discussion