Predicting shoreline response to changes in longshore sediment transport for the Rio Grande do Sul coastline

Resumo

Longshore sediment transport (LST) has been calculated using the CERC formula for the long (618 km), gently undulating, sandy shore of Rio Grande do Sul (RS) for wave climate scenarios typifying normal (i.e. non-ENSO years) and La Niña years. Predicted LST values have been validated with available field measurements. Whilst in most cases LST was directed to the north, simulations revealed large spatial variability in both the magnitude and direction of the net annual LST flux. This was related primarily to the orientation of the coastline relative to the incident waves and to the dominance of waves from different directions. Predicted net annual LST values were used in a simple model to calculate changes in the position of the shoreline. For normal years, results showed that areas of erosion predicted along the southern shores of the coastal projections and regions of accretion predicted in the embayments agreed well with shoreline surveys. The wavelength and amplitude of temporal and spatial oscillations in shoreline position measured by DGPS surveys were estimated well by a simple simulation of shoreline change. The simulation showed also that observed reversals in shoreline displacement from erosion to accretion or vice-versa could be explained by subtle changes in the annual wave climate. The present results provide strong evidence that slight changes to the wave climate can impact significantly on coastal processes along the RS shoreline. Similar impacts might be anticipated along other sandy shorelines worldwide.