Morphodynamic study of the Ondarreta Beach in San Sebastian (Spain)

Abstract

Ondarreta Beach is one of the most emblematic and visited beaches in the Basque Country. This beach is part of La Concha bay, with Santa Clara Island in the middle of the bay and a semi submerged dike and reef in front of it. The combination of this morphology together with the tidal range in the area (max. springs 4.6m) results in a very particular wave climate.The beach contains different strata of sand, natural rocks and the remains of artificial structures. Therefore, rocks sometimes appear at the surface, depending on the changes of the beach profile. This situation harms the normal use of the beach and can be a risk for the users. During recent years this process has increased and, especially in the summer, a big part of the intertidal profile of the West part of the beach shows a gravely and rocky profile.In 2003, the town council of San Sebastian entrusted AZTI Foundation with the study of this beach. For this purpose AZTI planned the data analysis and the morphodynamic study of the evolution of the beach in three different time scales, in order to clarify whether the recent problems at the beach are an increasing erosive process, or part of a natural cycle.The three different time scales of analysis of morphodynamic processes taking place in this beach are:-Short term scale: wave data measurement and analysis, modeling of wave, current and sediment transport.-Medium term scale: analysis of seasonal equilibrium shape and profile.-Long term scale: analysis of morphodynamic evolution of the whole system (the bay). This analysis has contributed to the management of the beach, implying artificial seasonal sand movements, trying to encompass the natural processes and shoreline protection with the recreational use of the beach.