A HYBRID MODEL OF SWASH-ZONE LONGSHORE SEDIMENT TRANSPORT ON REFLECTIVE BEACHES

Angela Wenping Jiang,Michael Hughes,Juan Carlos Savioli,Roshanka Ranasinghe,Peter Cowell,Angus Gordon

doi:10.9753/icce.v32.sediment.29

Abstract

The hydrodynamics and sediment transport in the swash zone is currently outside the domain of coastal-area models, which is a significant limitation in obtaining littoral sediment-transport estimates, especially on steep reflective beaches where the waves practically break on the beachface. In this study, an existing process-based coastal model (MIKE 21) is combined with a theoretical derivation of swash processes, resulting in an innovative hybrid modelling approach that is capable of estimating longshore sediment transport in the swash zone. The method relies upon estimation of swash hydrodynamics from an extended ballistic swash model with friction included. The terminal bore and other incident wave properties were computed from the output of a spectral-wave model (MIKE 21 SW). The Bagnold-type equation was applied to estimate gross transport volumes and the longshore component was computed for the sand volume displaced during the up-rush. The newly developed hybrid modelling approach was applied to Jimmys beach, a steep reflective beach (D50 = 0.3 mm, gradient=0.1) along the northern shoreline of Port Stephens, Australia. The model results yield the alongshore swash transport pathways and the indicative transport volumes. A point of divergence is identified at the beach erosion area, which is of critical importance in terms of shoreline erosion and management. The preliminary results suggest that swash-zone transport can account for a large percentage of the total littoral drift for such beaches. However, further field or laboratory data are required to test model utility, as well as to tune calibration parameters based on the site-specific conditions.

Highlights

Quantitative prediction of coastal processes and coastal evolution via numerical modeling is possible due to the major advances that have been made in understanding physical processes and mathematical modeling techniques over the last few decades
Hydrodynamics and sediment transport in the swash zone is currently outside the domain of even the most state-of-the-art coastalarea models, which is a significant limitation in obtaining littoral sediment-transport estimates on steep reflective beaches due to their direct contributions to beachface morphodynamics especially at short time scales
An existing process-based coastal model (MIKE 21) is combined with a theoretical derivation of swash processes, resulting in an innovative hybrid modelling approach that is capable of estimating longshore sediment transport in the swash zone

Summary

Introduction

Quantitative prediction of coastal processes and coastal evolution via numerical modeling is possible due to the major advances that have been made in understanding physical processes and mathematical modeling techniques over the last few decades. While sediment transport mechanisms in the surf zone are well established, the importance of swash zone processes has just received increasing attention in recent years. A significant part of the longshore sediment transport occurs in the swash zone and may account for a large portion of the total littoral drift (Bodge and Dean, 1987, Kamphuis, 1991, Smith et al, 2009), especially on steep reflective beaches (Van Wellen et al, 2000), where the surf zone is typically narrow or even absent, with wave breaking occurring practically on the beachface. Alongshore sediment transport in the swash zone can be significantly greater than that in the surf zone on such beaches (Hughes et al, 2007)

Results

Discussion

Conclusion