Recent Evolution of the Natural Inlets of a Barrier Island System in Southern Portugal

Four natural inlets, existing in a multi-inlet, barrier island system, were studied for the period between the 1940s and 1996. Inlet width and position of the inlet channel were determined using a series of vertical aerial photos and charts. The objective of the work was to determine the association between inlet migration patterns and different hydrodynamic conditions, major storms and engineering interventions. Results indicate that natural inlet opening and evolution were mainly affected by three factors: (a) existence of sub-embayments (western and eastern) inside the system, (b) exposure to wave energy, and (c) inlet efficiency. Two distinctive eastward migration patterns were found by analysing the correlation coefficient (r) between inlet width evolution and inlet migration and by the comparison of the shape of the curves fitted to the inlet migration behaviour. Typical migration of the high-energy flank (on the west side of the system) is characterised by an initial stage of readjustment, with low migration rates, followed by a stage of high eastwards migration rates, up to a limiting position. Inlet width remains reasonably constant during the entire migration cycle, thus the correlation between inlet width and position is very low. Typical inlets on the low-energy flank (east side of the system) are formed by barrier breaching during major storms and produce initially very wide inlets. Eastward inlet migration on the low-energy flank follows a natural logarithmic curve where channel migration is accompanied by strong constructional processes on the updrift barrier. Due to subsequent inlet width reductions, the correlation between inlet width and position is significant.