Dynamics of Sediment Transport and Erosion-Deposition Patterns in the Locality of a Detached Low-Crested Breakwater on a Cohesive Coast

Arniza Fitri,Soroush Abolfathi,Roslan Hashim,Khairul Nizam Abdul Maulud

doi:10.3390/w11081721

Abstract

Understanding the dynamics of sediment transport and erosion-deposition patterns in the locality of a coastal structure is vital to evaluating the performance of coastal structures and predicting the changes in coastal dynamics caused by a specific structure. The nearshore hydro-morphodynamic responses to coastal structures vary widely, as these responses are complex functions with numerous parameters, including structural design, sediment and wave dynamics, angle of approach, slope of the coast and the materials making up the beach and structures. This study investigated the sediment transport and erosion-deposition patterns in the locality of a detached low-crested breakwater protecting the cohesive shore of Carey Island, Malaysia. The data used for this study were collected from field measurements and secondary sources from 2014 to 2015. Sea-bed elevations were monitored every two months starting from December 2014 to October 2015, in order to quantify the sea-bed changes and investigate the erosion-deposition patterns of the cohesive sediment due to the existence of the breakwater. In addition, numerical modelling was also performed to understand the impacts of the breakwater on the nearshore hydrodynamics and investigate the dynamics of fine sediment transport around the breakwater structure. A coupled two-dimensional hydrodynamics-sediment transport model based on Reynolds averaged Navier-Stokes (RANS) equations and cell-centered finite volume method with flexible meshing approach was adopted for this study. Analysis of the results showed that the detached breakwater reduced both current speed and wave height behind the structure by an average of 0.12 m/s and 0.1 m, respectively. Also, the breakwater made it possible for trapped suspended sediment to settle in a sheltered area by approximately 8 cm in height near to the first main segment of the breakwater, from 1 year after its construction. The numerical results were in line with the field measurements, where sediment accumulations were concentrated in the landward area behind the breakwater. In particular, sediment accumulations were concentrated along the main segments of the breakwater structure during the Northeast (NE) season, while concentration near the first main segment of the breakwater were recorded during the Southwest (SW) season. The assessment illustrated that the depositional patterns were influenced strongly by the variations in seasonal hydrodynamic conditions, sediment type, sediment supply and the structural design. Detached breakwaters are rarely considered for cohesive shores; hence, this study provides new, significant benefits for engineers, scientists and coastal management authorities with regard to seasonal dynamic changes affected by a detached breakwater and its performance on a cohesive coast.

Highlights

Detached breakwaters are parallel barrier structures placed in the shallow nearshore water column, to protect any landform area behind them from the direct impacts of wave attack, currents, tides and storms surges [1,2]
The numerical modelling study showed that without having the breakwater in the degraded environment of the intertidal area, waves and curernts directly reach to the bare area at the study site with higher turbulent energy which means that in absence of the breakwater more intense degredation will occur in the coastal mangrove reserve area
The numerical model found that the pattern of suspended sediment around the breakwater during both seasons is strongly affected by the local hydrodynamic conditions

Summary

Introduction

Detached breakwaters are parallel barrier structures placed in the shallow nearshore water column, to protect any landform area behind them from the direct impacts of wave attack, currents, tides and storms surges [1,2]. Detached breakwaters are being used increasingly worldwide in the intertidal zones of non-cohesive (sand) coasts, to provide protection measures and mitigate erosion problems [4,5,6,7]. Such coastal structures cause local changes to nearshore flow hydrodynamics and sediment dynamics in the coastal zone [7,8,9]; these are site-specific and affected by parameters such as sediment characteristics, climate conditions, structural configurations (design, shape, dimension, and material) and nearshore hydro-morphodynamics [2,5,10]. The complexity of morphodynamic changes due to the presence of coastal structures is higher when coastal structures are located on intertidal areas of cohesive shores, where significant temporal and spatial variation of the water depths exist [11]

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