Flexural Rigidity and Shoot Reconfiguration Determine Wake Length Behind Saltmarsh Vegetation Patches

T I Marjoribanks,S Puijalon,C Mony,R J Boothroyd,R J Hardy,J Leroux,D Lague

doi:10.1029/2019jf005012

Abstract

AbstractVegetation patches play an important role in controlling sediment deposition in shallow aquatic environments such as coastal saltmarshes and fluvial systems. However, predicting deposition around vegetation patches is difficult due to the complexity of patch morphology and their dynamic interaction with the flow. Here we incorporate a biomechanical model, parameterized using field data, within a 3‐D computational fluid dynamics model which allows prediction of individual shoot reconfiguration within patches due to flow forcing. The model predicts velocity attenuation and bed shear stresses within the wake of the patch which agree spatially with accretion patterns measured in the field using terrestrial lidar. The model is applied to sparse patches of Suaeda maritima, located in saltmarshes of coastal habitats, to explore the role of (I) shoot distribution, (II) patch geometry, (III) shoot flexural rigidity, and (IV) bulk flow velocity in determining the length of the predicted wake region. We demonstrate that for Suaeda maritima, with intermediate rigidity, the vertical shear layer over the vegetation controls the length of the predicted wake region. Consequently, reconfiguration due to flexural rigidity strongly impacts on wake length, confounding the relationship between patch height and wake length. A simplified model for predicting wake length based on shoot reconfiguration is applied to the simulation data and shows good agreement. The results demonstrate that the observed wake characteristics can be well explained by intraspecific variability in flexural rigidity, thus demonstrating the importance of biomechanical traits in determining flow‐vegetation‐sediment interactions.

Highlights

Vegetation has a major impact on flow, sediment dynamics, and ecological processes within shallow aquatic environments and can control the evolution of fluvial, deltaic, and coastal sedimentary systems including coastal saltmarshes (Neumeier & Amos, 2006; Temmerman et al, 2005)
Wake Deposition Length Measurement The field data presented here consist predominantly of high‐resolution sediment deposition records, and in order to compare these with the numerical flow data, we calculated wake lengths which we suggest correspond to potential regions of enhanced deposition
Both velocities and wake lengths, to see whether they fit within the observed parameter space and to validate the assumptions used in modeling

Summary

Introduction

Vegetation has a major impact on flow, sediment dynamics, and ecological processes within shallow aquatic environments and can control the evolution of fluvial, deltaic, and coastal sedimentary systems including coastal saltmarshes (Neumeier & Amos, 2006; Temmerman et al, 2005). Saltmarsh vegetation communities develop through a process of colonization by pioneer species, which alter flow and sedimentation for succession to occur. Previous research has shown that sediment deposition behind idealized vegetation patches may be explained using the hydraulic principle of flow separation and reattachment (Zong & Nepf, 2012; see Figure 1). Turbulent kinetic energy is higher, and vortex shedding may occur, which limits sediment deposition (Chen et al, 2012; Zong & Nepf, 2012). Previous studies have defined the spatial extent of the wake sedimentation region using threshold values of both velocity and turbulent kinetic energy

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