Dynamics of the Douro estuary sand spit before and after breakwater construction

Abstract

The Douro estuary sand spit (NW Portugal) is an important natural defence against the impact of storm events. Aiming to improve navigation safety, a breakwater was built to stabilise the spit. In 2001, a monitoring programme was set up to understand the dynamics of the sand spit and, later, to evaluate breakwater efficiency on stabilising the spit's shape and volume. Nine years of periodic, accurate 3D digital elevation models obtained from kinematic differential Global Positioning System surveys were used to assess spatial and temporal changes in the spit morphology before and after the breakwater construction. Temporal and spatial spit dynamics were further related to river discharge flows, wave action and wind energy. Although the breakwater had a stabilising effect on most of the spit's area and volume, it caused a clear change in the morphology of the spit's head and in the previously observed patterns of erosion and accretion. Before breakwater construction, erosion was significantly correlated with extreme river discharge and wind conditions; accretion was significantly correlated with extreme wind conditions; and spit area and volume at 1–2 m elevation with extreme wave conditions. After breakwater construction, these correlations were not found, but the eastern (inland) face of the spit and particularly its southern part became more susceptible to the observed extreme river discharge flows, wave and wind conditions. The instability in the SE part of the spit may pose a threat to an existing local Nature Reserve. Furthermore, before breakwater construction, the spit often breached during severe floods favouring river discharges. The strengthening of the spit after breakwater construction may reduce the likelihood of breaching and lead to more severe flood effects in the estuary.