Shoreline and Beach Cusps Dynamics at the Low Tide Terraced Grand Popo Beach, Bénin (West Africa): A Statistical Approach

Abstract

Bonou, F.; Angnuureng, D.B.; Sohou, Z.; Almar, R.; Alory, G., and du Penhoat, Y., 2018. Shoreline and beach cuspsdynamics at the low tide terraced Grand Popo beach, Benin (West Africa): A statistical approach. In: Almar, R.; Almeida, L.P.; Trung Viet, N., and Sall, M. (eds.), Tropical Coastal and Estuarine Dynamics. Journal of Coastal Research, Special Issue No. 81, pp. 138–144. Coconut Creek (Florida), ISSN 0749-0208.This paper presents the description of a 2-dimensional shoreline and 3-dimensional shoreline behaviour around a ‘storm free’ microtidal beach from a 3-year video observation using empirical orthogonal function (EOF) analysis. The EOF method is applied successfully to delineate 2D and 3D developments that show the beach evolution influenced by the action of waves and tides. The EOF model results are in agreement with measurements that reflect the morphological concepts for the 2D and 3D cases: the first temporal EOF mode which accounts for an overwhelming 80% of shoreline variation reflects the alongshore-averaged cross-shore migration (2D) largely influenced by waves while the higher eigenfunctions with 20% shoreline variation reflect beach deformation/irregularities (3D). Shoreline location is correlated to cross-shore energy flow from waves, while 3D is associated to the longshore energy flow. The influence of wave height increases cross-shore uniform migration and beach seasonality. The results show that with EOF method, beach cusps are clearly identified for 3D developments to be evaluated. Beach cusps are well observed at low tides on video images. Tide and cusps increase the importance of short term events and deformation; at the lowest high tide mark, beach cusps were observed on higher spatial functions of EOF. This indicates that a study of these morphological features as a coupled system, promotes their contribution in predicting 2D and 3D shoreline changes.