Airflow Dynamics over a Beach and Foredune System with Large Woody Debris

Michael Grilliot,Ian Walker,Bernard Bauer

doi:10.3390/geosciences8050147

Abstract

Airflow dynamics over beach-foredune systems can be complex. Although a great deal is known about the effects of topographic forcing and vegetation cover on wind-field modification, the role of large woody debris (LWD) as a roughness element and modifier of boundary layer flow is relatively understudied. Individual pieces of LWD are non-porous elements that impose bluff body effects and induce secondary flow circulation that varies with size, density, and arrangement. Large assemblages of LWD are common on beaches near forested watersheds and collectively have a degree of porosity that increases aerodynamic roughness in ways that are not fully understood. A field study on a mesotidal sandy beach with a scarped foredune (Calvert Island, British Columbia, Canada) shows that LWD influences flow patterns and turbulence levels. Overall mean and fluctuating energy decline as flow transitions across LWD, while mean energy is converted to turbulent energy. Such flow alterations have implications for sand transport pathways and resulting sedimentation patterns, primarily by inducing deposition within the LWD matrix.

Highlights

Natural roughness elements protrude into the near-surface flow field, increasing aerodynamic drag and extracting momentum from the wind as well as acting as a physical barrier to aeolian sediment transport [1,2,3,4,5]
This paper examines the influence of a recently emplaced large woody debris (LWD) matrix on the flow dynamics fronting an established, scarped foredune
Differences in time between the various be assessed relative to the overall mean for all runs, and the relative deviation of T1 away runs can be assessed relative to the overall mean for all runs, and the relative deviation of from T2 for all runs provides an indication of the influence of LWD on flow dynamics

Summary

Introduction

Natural roughness elements (e.g., plants, rocks, wrack, logs) protrude into the near-surface flow field, increasing aerodynamic drag and extracting momentum from the wind as well as acting as a physical barrier to aeolian sediment transport [1,2,3,4,5]. There are many studies on the effects of porous and pliable roughness elements (i.e., grassy vegetation) on flow interactions and sediment transport over coastal foredunes [6,7,8,9,10,11,12,13], as well as around discrete, artificial non-porous roughness elements [11,14,15,16] and roughness arrays [17,18,19]. There remains a gap in understanding as to how natural non-porous objects, like fallen trees and large woody debris (LWD), affect flow and sedimentation on and in front of coastal foredunes [3,13,14,20]. Lee flow reversals create an area of converging flow with reduced velocity where shadow dunes form [16,26,27]

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