ICE - Institution of Civil Engineers - Virtual Library

This paper examines the hydrodynamic processes associated with detached breakwater schemes. It is based on a physical model study of the Elmer breakwater scheme, undertaken in the UK Coastal Research Facility. The main objectives were to investigate the influences on the hydrodynamics of wave directionality and directional spread, irregular bathymetry and breakwater permeability. In contrast to previous studies, it was found that one circulation cell was dominant across the middle of the bays for both random and monochromatic waves. Refraction due to the irregular bathymetry had a significant influence on the wave-induced currents behind the breakwaters. In addition, transmission through and overtopping of the breakwaters were responsible for the observed current patterns. Long-period waves were formed in the near-shore for multidirectional waves, resulting in long period oscillations of the currents. These complex hydrodynamic processes are not included in most current numerical models used in the design of offshore breakwater schemes. The paper concludes that a new generation of models is needed to incorporate these processes, but recommends the most appropriate techniques to use in the meantime.

Wave overtopping is the critical response of most sea defence structures and one of the more important responses for many coast defences around the UK and other developed shorelines. Sea defences in rural areas are commonly provided by embankment seawalls. Steep or vertical seawalls are more commonly used in urban areas to protect against erosion, flooding and local overtopping hazards, and to protect the base of eroding cliffs in urban or rural areas. Vertically faced breakwaters have been common around many European countries, and caisson-type breakwaters are heavily used in Japan. For simple slopes or embankments, overtopping performance can be predicted by simple monotonic empirical formulae. Overtopping of vertical or slightly battered walls is however rather more complicated, with substantial differences in overtopping volumes and velocities depending on the form of wave interaction at or close to the wall. The present paper draws together results from a number of UK and European research projects over the last 10 years. Their results improve and validate prediction methods for wave overtopping discharges and velocities for steep battered, composite and vertical seawalls/breakwaters. The methods presented herein support, extend and qualify guidance given in the UK Environment Agency overtopping manual. The present paper shows how the use of these methods can now explain why overtopping of some seawalls/breakwaters can be greatest at mid-water level, rather than at the highest water levels.

A study to investigate the feasibility of modifying the current engineering design of coastal structures, in order to enhance their ability to provide marine habitats is described. In particular, a detailed account of the wave modelling undertaken to develop guidelines for the conceptual design of structures that can enhance the marine habitat is presented. Illustrative examples are used to demonstrate how wave and flow modelling can be used, in conjunction with ecological and engineering constraints, to derive performance design curves for different defence scheme layouts. Conclusions and implications for the design of future schemes are discussed.

Overtopping of seawalls presents a considerable hazard to people and property near the coast and accurate predictions of overtopping volumes are essential in informing seawall construction. The methods most commonly used for the prediction of time-averaged overtopping volumes are parametric regression and numerical modelling. In this paper overtopping volumes are predicted using artificial neural networks. This approach is inherently non-parametric and accepts data from a variety of structural configurations and sea-states. Two different types of neural network are considered: multi-layer perceptron networks and radial basis function networks. It was found that the radial basis function networks considerably outperform both the multi-layer perceptron networks and the curve-fitting (parametric regression) regime, and approach bespoke numerical simulations in accuracy. Unlike numerical simulation, the neural network approach gives generic prediction across a range of structures and sea-states and therefore incurs considerably less computational cost.