Monitoring the evolution of nearshore nourishments along Barra-Vagueira coastal stretch, Portugal

Abstract

Maintenance of existing harbors implies regular dredging activities. Where the combined use of dredging and disposal of dredged material on nearby sediment-starved beaches can induce major changes in the beach morphology and generate unexpected impacts in the environment, monitoring becomes a concern. This paper was designed to analyze, interpret and evaluate a set of monitoring data collected along a regular-nourished coastal stretch with dredged sand (Barra-Vagueira coastal stretch, northwest coast of Portugal), surrounded by an energetic hydrodynamic environment with a scarce natural sediment input. Based on a field data set, collected between 2009 and 2015, the present study brings together a set of correlated analyses, intended to assess the morphodynamic evolution of the fills as well as their impact to the adjacent coast. The available data set encompasses topo-hydrographic surveys collected for 12 cross-sections (with 1 km spacing) distributed along the coastal stretch and bathymetric measurements collected for the dumping areas. Considering the concurrent offshore wave forcing, dominant temporal and spatial patterns, morphological changes, evolution trends, sediment budgets, and short- and medium-term responses of the fills are investigated by the use of ArcGIS tools and application of a multivariate statistical method based on Empirical Orthogonal Functions (EOFs). Overall, during the monitoring period, almost 2.8 Mm3 of sand was dumped in different locations and periods to control the erosion observed downdrift of the inlet. However, bathymetric surveys and profile indicators still point out the erosional longshore pattern diagnosed decades ago as a result of a negative longshore sediment balance. Observations also revealed that short-term changes arising from the seasonal cycles of cross-shore material exchange are mainly linked to the largest variations in the beach profile shape, also affecting the sediment budget. Profiling indicated cross-shore volume variations ranging from ±250 m3/m and ±1500 m3/m in the subaerial and subaqueous portion of the profile, respectively, along the monitored period. After the first completed seasonal cycle the sand bar, artificially created by the nourishments, could not be visually detected in the profiles, suggesting a cross-shore redistribution of the fill material. All the analyses developed in this paper stress the importance of establishing proper monitoring programs based on adequate surveying instruments and data collection strategies, in order to ensure high-density data that could be used in support to the decision-markers.