Geologically constrained morphological variability and boundary effects on embayed beaches

Abstract

Headlands, rock outcrops and engineering structures impact beach and nearshore dynamics of coastal embayments, inducing boundary effects that constrain the lateral and vertical beach variability. This study analyses morphological change in six embayed beaches with diverse levels of exposure to wave action and various degrees of geological control in the mesotidal coast of southwestern Portugal. The aim is to identify whether geological boundaries constrain the morphological behaviour of embayed beaches and assess whether their effects can be decoupled from datasets of morphological change. Topographic data, obtained over a two-year period on each of the six embayments, were analysed using empirical orthogonal functions (EOF) to decompose temporal and spatial variability in the datasets. First and second mode eigenfunctions were explored using time-variable linear correlation analysis with several nearshore parameters that include hydrodynamic variables, sedimentary and geometric characteristics of each embayment in order to derive forcing–response relationships. Our results demonstrate that natural geological boundaries constrain the morphological behaviour of embayed beaches, producing conspicuous alongshore variability in all embayments. Localised responses induced by lateral and vertical boundary interference with beach and nearshore dynamics include beach rotation, topographically-controlled rip circulation and restrained profile fluctuation. Spatial decoupling in cross- and longshore responses is accompanied by a temporal decoupling in response times, both of which are slower in more constrained embayments (from 1 day in exposed embayments to 1 week or more in the most sheltered ones). Normalised wave power was correlated at the 99% confidence level with the primary mode of morphological variability at most embayments, which represent 67% to 94% of the variance in the datasets. This correlation stresses the importance of combined parameterization of wave and tide forcing in process–response relations between hydrodynamics and morphological change for mesotidal coastal environments. Lateral and vertical geological boundaries exert their effects fundamentally by restraining longshore sediment transport, inducing cellular surf zone circulation and by impacting cross-shore sediment transport. It is postulated that decreasing sediment abundance and substrate depth intensify vertical boundary effects, while higher indentation and wave obliquity enhance the effects of lateral boundaries.