ON THE SYSTEMATIC OFFSHORE DECAY OF BREAKER BARS

Kathelijne M. Wijnberg

Abstract


Multiple breaker bar systems along the Holland coast exhibit long term cyclic behaviour. Over periods of many years, all bars migrate in a net seaward direction; the outer bar decays offshore, and a new bar is generated near the shoreline. This behaviour is essentially a cross-shore redistribution of sediment; the sediment of the offshore decaying outer bar is transported back onshore. This paper presents a tentative evaluation of a part of the hypothesis formulated by Wijnberg (1995) to explain the cyclic bar system behaviour. The systematic offshore decay of the outer bar is considered to be the key to explaining the cyclicity in this behaviour. The offshore decay is assumed to occur due to a change in the long term balance of bar maintaining conditions (breaking waves) and bar diminishing conditions (very asymmetric waves). This balance will change as a result of offshore migration of the outer bar to deeper water during single storm events. The calculations presented in this paper support the hypothesis, but field observations of the hydrodynamic conditions during the proces of bar decay are required for a more rigorous verification.

Keywords


breaker bars; bar decay; offshore decay; systematic decay

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