Overwash vulnerability assessment based in long-term washover evolution. Application to the Ria Formosa barrier islands system (Southern Portugal)

Tese mest. , Estudos Marinhos e Costeiros, 2009, Universidade do AlgarveAn integrated methodology for evaluation of overwash vulnerability is developed with respect to the historical washover evolution of a barrier island system. Three different aspects of overwash are addressed in the vulnerability indices developed: Overwashed Shoreline Ratio, Maximum Overwash Intrusion Recurrence, and Complete Barrier Overwash. The indices were applied to the barriers in the Ria Formosa system (Southern Portugal) using an aerial photography catalogue covering the period 1947-2001. Historical trends of washover evolution were observed to be different between the barriers analysed, but generally there was a decrease in washover number and dimensions throughout the analysed period. The final overwash diagnostic obtained allowed an integrated overwash vulnerability rating to be defined for each barrier, with vulnerabilities ranging from low to extreme. The methodology has produced results that are able to assist coastal managers with information concerning barrier island system overwash hazard, through defining the temporal and geographical distributions of overwash, and providing reliable indications as to where overwash is most likely to occur in the future. The comparison between the result here presented and the scientific background supporting the most relevant land-use management plans valid for the Ria Formosa barriers showed that the later gives little relevance to historical overwash occurrence and washover evolution. The entire set of plans analysed presents nevertheless a satisfactory overwash diagnostic which in general terms is in agreement with the present study