Adjustment of Reflective Beaches to Waves

Abstract

A new mode of beach response to changes in wave height is identified for reflective beaches where waves break close to the shoreline. This response involves vertical adjustment of the beach face morphology, especially the beach step. It is found that profile relief varies with wave height rather than inversely to it as expected from existing models. As a result the characteristic steep reflective profile can be maintained under certain conditions, during periods of beach erosion. A field program which included surveys of 67 beach profiles on a persistently reflective beach measured vertical adjustments of the beach step and associated wave and sediment conditions. Based on these data a model is developed to describe the changes in the amplitude of the beach step related to changes in breaker height and sediment size. It was found that as breaker height increases the step height increases, while the surf zone width remains constant. Larger grain sizes and changes in grain size across the beach face are also associated with larger step heights. This step adjustment model implies that if wave height increases, under the range of conditions observed, the resultant increase in step amplitude and associated deepening of the nearshore will be sufficient to delay wave breaking, allowing a reflective profile to be maintained. Therefore the model can account for the environmental conditions which produce changes in step heights and favour the maintenance of a reflective system. The model also implies that a threshold exists between this mode and the conventionally recognised mode of beach response and therefore differentiates between transient reflective profiles, and those of less mobile reflective beach systems.