MODELLING THE IMPACT OF DETACHED BREAKWATERS ON THE COAST

Abstract

Detached breakwaters alter the nearshore wave climate and hence the wave-driven current and sediment transport patterns. They obstruct a part or all of the longshore sediment transport and because of this they play a role in the large-scale sediment budget of the coasts where they are applied. Their local effect on the coast is to form single or double salients or tombolos. Which of these beach shapes will develop may be important for the attractiveness of the beach, in terms of visual aspects and water quality. Besides having a function as part of a coastal management scheme, detached breakwaters with a jetty to the shore may be designed as a low-maintenance port. Obviously, one hopes that in such cases a salient rather than a tombolo will develop. Clearly, engineers need to have means to assess the bypass-characteristics and the local impact of detached breakwaters if they want to apply them in a responsible manner. Several methods are at their disposal. First, there are rules-of-thumb, based on field and lab experience. These are quite useful to get a first rough idea of the possible impacts, but generally do not give answers that are conclusive enough: a typical result is of the form that two design rules say you'll get a salient and three say you'll get a tombolo. The most common tools applied at present are coastline models. They assume that the coastal profile is more or less constant in shape, and that the longshore sediment transport is related to the local angle of incidence of the waves. Several commercially available packages have special options to assess the local wave climate behind detached breakwaters. Effects that are not included in the current and transport models, such as circulations induced by set-up gradients, can be simulated by adjusting transport parameters locally. Generally, a lot of calibration is required, and the predictive capability is often uncertain. The calibrated models often show nice comparisons with data, especially for the initial stages. The development towards equilibrium is generally not represented at all. Recently, more sophisticated "area models" have matured to the point that fully dynamic wave, current, transport and bed evolution simulations can be carried out over a period which is long enough to approach equilibrium conditions (see for instance Johnson et al., 1994). The advantages are obvious: these models explicitly take into account the most important processes, and therefore the amount of heuristic modelling is reduced substantially. In the light of the ongoing efforts of devising coastal area morphological models, the performance of DELFT HYDRAULICS morphological model is tested on the simple situation of a detached breakwater in a coastal area with parallel iso-bathi. This also furnishes a test for the DELFT3D system and has proven an opportunity of improving the model.