Sediment dynamic processes in the vicinity of offshore breakwaters

Abstract

A composite approach is used to study hydrodynamic and sediment dynamic processes, in the vicinity of offshore breakwaters situated in a macro-tidal environment; this combines field measurements, numerical modelling and grain size trend analysis.

Field measurements were undertaken at an offshore breakwater scheme (Elmer, West Sussex), in order to investigate the key processes in the vicinity of the breakwaters. The investigation into the hydrodynamics included measurements offshore and inshore of the breakwaters, under various wave and tidal conditions. The dependence of sediment transport on wave asymmetries and water depth, especially during higher energy
conditions, was identified.

Numerical modelling was utilised for simulating the tidal and wave processes, at different stages of the tidal cycle. The interaction of the tidal currents with the structures, over the entire area of the scheme, was studied for a spring-neap tidal cycle. Significant alterations of the tidal currents in the landward area of the breakwater were observed; these included flow channelisation between the structures and the coastline, together with flow acceleration over the salients. The design characteristics of the scheme, (i.e. gap width, offshore distance, relative angle in respect to the tidal currents) are found to influence the magnitude of the acceleration. The accelerated currents increase the sediment transport locally over the top of the salients, regulating their growth. Wave energy immediately to seaward of the structures was regulated by the tidal elevation. However, in the shadow areas of the structure and over the salients wave height and associated sediment transport were controlled, by the water level (i.e. tidal stage) and the period of the incoming wave.

The identification of the sediment transport pathways in the vicinity of the offshore islands was undertaken using the ‘Gao and Collins Method’; this is based upon correlating spatial resolution of sediment parameters. A sensitivity analysis of the transport pathways, under different sampling strategies (regular vs. irregular grids and variable grid spacing) and the analytical techniques (settling velocity vs. sieving), was undertaken. The importance of settling velocity in determining the statistical parameters of the grain-size distribution was identified, especially for the coarser-grained sediments (fine-medium sand).

The combination of the results obtained using the different methodologies verify the processes controlling sediment dynamics in the vicinity of the structures; these are applicable to schemes located in similar environments.

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