Morphological Monitoring of Artificial Nourishment at Cova-Gala Beach (western coast of Portugal)

The southern region of Figueira da Foz (western coast of Portugal) faces a severe sediment deficit and high coastal vulnerability, largely due to the port jetties, which have partially blocked the southward longshore sediment transport, causing substantial shoreline retreat. To mitigate these effects, Cova-Gala beach (immediately north of the Figueira da Foz port) has undergone several small scale beach nourishment interventions. In July–August 2025, the Portuguese Environment Agency (APA) carried out the largest intervention in this area, depositing approximately 3.3 million m³ of sand, distributed between the subaerial beach (1.8 million m³) and the shoreface (1.5 million m³).

This study focuses only on the geomorphological and volumetric analysis of the Cova-Gala beach section between groynes 3 and 5 (cells 3-4 and 4-5). In this area, approximately 180,000 m³ of sediment was initially deposited in the dry beach above Chart Datum, representing the maximum retention capacity of the local groyne field. Digital Elevation Models (DEM) were generated from UAV and GNSS-RTK surveys to analyze coastal retreat and volumetric erosion. The analysis focused on the period between August 22, 2025 (post-filling) and January 7, 2026 (post-storm), using GNSS-RTK to ensure positional accuracy and to validate the January data through topographic profiles. This period presented a particular intense succession of storms. In December alone, three major storms occurred (Davide, Emilia and Francis). Under these synoptic conditions, atmospheric pressure ranged from 978 to 994 hPa, with peak winds of 90-124 km/h and h_s ranging from 5 to 11 m. Analysis of the UAV data quantified the sediment volume loss to approximately 90,000 m³. Subsequently, surface interpolation in QGIS, comparing the August DEM with the January GNSS survey, determined a loss of approximately 88,000 m³, a value that shows strong convergence with the UAV. To ensure estimate reliability, vertical accuracy was assessed using an independent GNSS-RTK control point. This validation yielded a maximum cumulative error of 5 cm and a Root Mean Square Error (RMSE) of approximately 2.7 cm. The observed dry-beach erosion suggests a rapid cross-shore sediment transport mechanism, moving sand from the beach face/berm to the foreshore/shallow nearshore in response to wave action. Although the subaerial beach experienced significant retreat, partial recovery is expected in the coming months, driven by milder wave action that favors onshore sediment transport. These results demonstrate the effectiveness of UAV photogrammetry for high-precision, rapid assessment of artificial nourishment performance under storm wave conditions.

This work is supported by FCT, I.P./MCTES through a PhD scholarship (2024.03765.BDANA) and national funds through (PIDDAC): LA/P/0068/2020 - https://doi.org/10.54499/LA/P/0068/2020 , UID/50019/2025 and  https://doi.org/10.54499/UID/PRR/50019/2025, UID/PRR2/50019/2025. Finally this work is a contribution to project iCoast (project 14796 COMPETE2030-FEDER-00930000).