Desembocadures fluvialsentre el mar i la terra : el cas del riu Millars

Abstract

The mouth of the River Millars-Rambla de la Viuda is part of a large prograding alluvial fan, formed throughout the Quaternary period. Currently, the mouth is formed by three channels separated by two fluvial bars. The main objective of this paper is to analyze the evolution of the mouth between 1956 and 2021 by means of photographs and orthoimages. To achieve this objective, we analyze river flow and wave series, in order to reckon the influence of river floods and marine storms on their evolution. In the 1960 and 1970 decades, the 1956 coastline retreated, and it was not recovered until 2020. The scarce progradation along the study period is due to the fluvial hydro-sedimentary deficit caused by gravel extraction and sediment retention in river reservoirs. The little shoreline advance is made of numerous bars that close the river channels. River floods open the bars and later, during the storms, the waves form spits and beach barriers closing microlagoons. The bars are pebble ridge-shaped, 2-3 meters high. During major storms, these ridges are likely to be overtopped by the waves. They form washover fans which end up filling the micro-lagoons. The washover processes are the primary mechanism by which ridges increase in width and migrate landward, expanding the deltaic fan. The existence of washover fans and the low fan delta progradation suggests that nowadays this is a fan delta dominated by marine processes.