Short- and medium-term grain size changes in deltaic beaches (Ebro Delta, NW Mediterranean)

The beach sediment of the Ebro Delta shows significant temporal grain size changes unrelated with seasonal processes. The evolution of the grain size during the 1988-1991 period shows coarsening and fining variations, with a general coarsening trend which ranges from 2.10 to 1.96 phi (about 8 . 10(-3) mm/year). This low-magnitude grain size change is considered significant because it is indicated by both mean values from the data set (textural zones) and individualized sampling points. The coarsening trend of the beach sediment in the Ebro Delta is mainly caused by the sedimentary deficit affecting the Ebro coast. The finer sand fractions of the sediment are progressively winnowed from the initial beach deposit without being replaced by new river sediment supplies. Superimposed on the general coarsening trend there exist grain size variations that may be related with processes of a shorter period such as changes in the sediment supplied from the river into the nearshore zone, variations in the wave energy affecting the beach, and man-induced actions, such as beach nourishment. The mean grain size of beach sediment sampled 20 years ago indicates that the coarsening gradient measured during the study period cannot be extrapolated to longer periods of time (decades). This work shows that processes of a different temporal scale but a similar magnitude must be integrated in order to explain the evolution of sediment grain size in deltaic beaches.