Study of the responsible factors for the closure of an intermittent washout during a storm surge, Rio Grande do Sul, Brazil

Abstract

The washouts are water courses essential to the drainage of the water accumulated in the backshore zone, and are responsible for great ruptures in the dunes field. They supply the swash zone with large amounts of sediment. The study area is located a few kilometers south of the Patos Lagoon Inlet. This study measures the contribution of the wind, the waves, the atmospheric pressure and the tide on the elevation of the sea level in a period when the beach has suffered the impact of a storm surge. Field campaigns were performed in October and November of 2007. This data was combined with local wind and pressure data and modeled waves data from WAVEWATCH III, used by NOAA. The results showed that the southwest winds acting at the sea surface might be the main responsible for the abrupt elevation of the sea level during the period of study, because their capability of throwing sea water to the near shore by Ekman effect. However, the wave set up due to the high swell waves coming straight to the coast in the same time is a fundamental component in this elevation. In addition, there are evidences of a cyclone near the study area, which can explain some of the abnormalities studied in this work. The washout systems have been shown to be highly sensitive to this kind of storm surge, and all the sampled and modeled data suggest that the opening and closure of the washouts may occurs due to such phenomena.