Seasonal to Decadal Variability of Longshore Sand Transport at the Northwest Coast of Portugal

Longshore sediment transport (LST) is a major driver of coastal evolution. However, despite the recognition that it presents an unsteady behavior at the seasonal scale, the variability at longer term scales (interannual and decadal) is still far from being properly acknowledged. The present work contributes to the understanding of the seasonal to decadal variability of the LST, benefiting from recent developments in wave hindcast modeling. This work was developed for the northwest coast of Portugal, which is fully exposed to the highly energetic wave regime generated in the Northeast Atlantic Ocean that induces unusually large LST rates. Hindcast offshore waves, between 1953 and 2010, were used as offshore forcing to deduce LST estimates. The mean annual LST, between 1953 and 2010, shows an irregular and noncyclic pattern. Computed mean annual LST, for this coastal stretch, is around 1 million cubic meters directed to the south, with yearly averages ranging from 108,000 to 2.24 million m³ year⁻¹ always directed to the south. The maximum observed annual LST magnitude exceeds the mean magnitude by more than 100%. The variability in LST magnitude was found to be mainly related to the LST magnitudes of autumn/winter months in response to the wave regime seasonality. Results show that to estimate the long-term LST within an error of about 20%, a period of about 10 years of data/observations is required. Interannual variability in the magnitude of the LST was found to be correlated (${\mathit{R}}^2=0.55$) with the annual North Atlantic Oscillation (NAO) index. In the years where the NAO index was higher, the mean annual LST was generally greater than the long-term average, while negative NAO index corresponds to lower than average LST estimates.