Direct Measurements of Bed Shear Stress under Swash Flows on Steep Laboratory Slopes at Medium to Prototype Scales

Howe Howe,Turner Turner,Puleo Puleo,Blenkinsopp Blenkinsopp,Baldock Baldock

doi:10.3390/jmse7100358

Abstract

Robust measurements of bed shear stress under wave runup flows are necessary to inform beachface sediment transport modelling. In this study, direct measurements of swash zone bed shear stress were obtained in medium and prototype-scale laboratory experiments on steep slopes. Peak shear stresses coincided with the arrival of uprush swash fronts and high-resolution measurement of swash surface profiles indicated a consistently seaward sloping swash surface with minimal evidence of a landward sloping swash front. The quadratic stress law was applied to back-calculate time-varying friction factors, which were observed to decrease with increasing Reynolds number on smooth slopes, consistent with theory for steady flows. Additionally, friction factors remained relatively constant throughout the swash cycle (except around flow reversal), with a variation of approximately ±20% from the mean value and with only small differences between uprush and backwash. Measured friction factors were observed to be larger than expected when plotted on the Moody or wave friction diagram for a given Reynolds number and relative roughness, consistent with previous field and laboratory studies at various scales.

Highlights

When a fluid flows past a solid boundary, shear stresses are generated
The shear plates were deployed in multiple locations across the beachface under a wide r3a. nRgeesuolftsincident wave conditions, allowing bed shear stress to be compared with several other parameters, including bed roughness, experimental scale and cross-shore location
This is explained in part by the fact that the uprush flows are unsteady and exhibit a developing boundary layer and flow convergence at the swash tip leading to enhanced bed shear stress

Summary

Introduction

When a fluid flows past a solid boundary, shear stresses are generated. Where ρ is fluid density and U is the depth-averaged flow velocity. This formulation is well established for calculating bed shear stress in steady, uniform flow conditions, where f remains constant (e.g., [2,3]). Previous authors have used a range of direct and indirect measurement techniques to calculate friction factors in swash flows (a summary of selected studies is presented in Table 1 and Figure 1), but the results vary widely between studies (see review [4]), with mean values ranging from f = 0.001 [5]. Numerical models of the nearshore environment typically assume f remains constant through the sNwuamshercicyacllem(oed.ge.l,s[o1f9t]h).eTnheatrsahsosruemenpvtiioronnwmaesnttetystpeidcailnly tahsissuimnveefsrteigmaationns,cobnystbaanctkt-hcraolcuuglhattihneg svwaalushescoyfclfev(eia.gt.h, [e1q9]u)a. dTrhaatticasstruemsspltaiown, wusaisntgesdtedptihn-tahviesriangveedstvigealoticointy, bayndbadckir-eccatlcmuleaatsinugrevmaleunetss ooff fbvediasthheeaqrusatrdersast(ipc rsotrveisdseldaw,ituhsianfgludsehp-mtho-auvnetreadgsehdevaerlpolcaitey).and direct measurements of bed shear stress (provided with a flush-mounted shear plate)

Materials and Methods

ReRsu3lts

Influence of Cross-shore Position on Bed Shear Stress

Findings

Conclusions