Video monitoring nearshore sandbar morphodynamics on a partially engineered embayed beach

Abstract

De Santiago, I., Morichon, D., Abadie, S., Castelle, B., Liria, P. and Epelde, I., 2013. Video observation of the morphodynamics of nearshore sandbars on a partially engineered embayed beach.Embayed beaches are prevalent environments along the northern Spanish coastline. The present work analyses the temporal and spatial variability of nearshore sandbars on the pocket beach of Zarautz using daily video observations over 2 years. This partially urbanized beach can be divided into two well defined distinct areas: a natural section backed by a sand dune system to the East and an engineered section with rigid seawalls to the West. The objective of the present work is to evaluate if the engineered and natural sections of the beach present different morphological behaviors. The video image analysis was performed in two steps: First, sand bars were characterized visually and classified following standard classification schemes. Then, the outer sand bar migration was measured using automatic bar detection. Results show that the nearshore sandbars evolution covers a wide range of temporal and spatial variability. A noteworthy exception is the persistent presence of headland rips. The beach is mostly double-barred, with both bars able to go through all the states within the intermediate classification. The most common double bar morphological configuration is an inner low tide terrace coupled to well-developed megacusps, with an outer subtidal crescentic bar. Various preferred locations of rip channel formation are identified along the beach, suggesting that the effects of the headlands can propagate far away from the headlands towards the center of the embayment. Interestingly, the western engineered and more sheltered section of the beach sometimes exhibits a different beach state to that of the eastern section.