Medium-term morphodynamic evolution of a small coastal inlet

Abstract

Despite numerous qualitative and quantitative studies, the morphodynamic evolution of coastal inlets is far from being completely understood due to the complexity and different time-scales of the physical processes involved. A small coastal inlet system (Aljezur stream, Portuguese Southwest coast) was chosen to implement a monitoring program in order to investigate the inlet and adjacent beach morphologic evolution and response to the incident factors on a monthly time-scale. Topo-bathymetric surveys and video-images were acquired between April 2008 and September 2010 and morphologic parameters and their possible relation with forcing mechanisms (tide, waves and fluvial discharge) was evaluated. Results show that the system reveals different time-scale response to the forcing factors and a remarkable high morphologic resilience to tidal prism and wave action, only altered when strong fluvial discharges occur. Inlet channel morphologic evolution was expressed mainly by the parallel displacement of the northern bank forced by the beach plan form oscillations. The channel displayed a flat bottom with an almost constant depth, as well the straight alignment into the beach (inlet channel configuration A). When extreme fluvial discharges occur the inlet channel widens and its pathway freely migrates crossing the foreshore (configuration B). The present study allowed a detailed morphologic characterization of Aljezur coastal system and enhanced the understanding of the morphodynamics and its relationship with the controlling factors. This knowledge provides important information to support the development of future inlet monitoring programs and to forecast system evolution.