A Numerical Model for Offshore Mound Evolution

Jie Zhang,Magnus Larson

doi:10.3390/jmse8030160

Abstract

A numerical model was developed to simulate the evolution of a mound placed in the offshore (i.e., outside the zone of wave breaking), exposed to varying non-breaking waves and water levels. The net sediment transport rate is assumed to be mainly dominated by bed load transport, where wave asymmetry plays an important role. The net transport over a wave cycle is expressed with reference to an equilibrium profile, which ensures model reliability and robustness. In order to validate the model, data collected at two field sites, Cocoa Beach and Perdido Key Beach in Florida, USA, were employed. The numerical results show good agreement with the measured data from the two sites in terms of the profile evolution. It demonstrates that the model has the capability to simulate the evolution of mounds placed in the offshore. In addition, several scenarios with different mound volume and location designs were investigated to indicate potential uses for the model. The results illustrate how the mound evolution is influenced by the volume and location of the mound placement.

Highlights

With climate change, sea levels are rising and extreme storms may occur more frequently, leading to shoreline retreat and beach erosion, which threaten the stability and sustainability of coastal areas
After one year of action by waves and tides, the simulated profile is in good agreement with the measured profile; both show a distinct spreading of the placed mound, but not as significant as Cocoa Beach, which is due to the different wave conditions at the two mounds and owing to the initial shape of the two mounds
If the placed mound is approximated with a triangle at the Cocoa Beach it is clear that it is steeper and narrower than the mound tending to a trapezoid at the Perdido Key

Summary

Introduction

Sea levels are rising and extreme storms may occur more frequently, leading to shoreline retreat and beach erosion, which threaten the stability and sustainability of coastal areas. Conventional approaches for coastal protection are mainly to employ hard structures [1,2,3], such as revetments, breakwaters, and seawalls. One of the most popular soft measures implying placement of large quantities of dredged sand, has been extensively used in recent years [7,8,9,10,11]. One merit of sand nourishment is that the placement of the material needs not be strictly compatible with the beach since the waves and currents change the coastal patterns by sorting the material. According to the placement of the sand, nourishment can be employed following different strategies, including shoreface nourishment, backshore nourishment, and beach nourishment. Shoreface nourishment through the construction of offshore mounds from dredged sand, often extracted from navigation channels, is emphasized. Offshore refers to a zone where the waves are not breaking, but are still the main agent for the transport

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Results

Conclusion