Monitoring beach nourishment evolution using satellite data: the case of Vale do Lobo (Portugal)

Increasing pressure on the coastal zone, driven by urbanization and related adoption of hard engineering protection structures, has frequently contributed to a gradual amplification of beach erosion. This is the case of Vale do Lobo beach (Algarve, Portugal), where sand retention caused by the Quarteira groin field and Vilamoura jetties led to soft cliff recession and reduction of the beach width downdrift (Teixeira, 2019). To mitigate these effects, an artificial beach nourishment program along with a monitoring plan have been implemented by the predecessor institution of the Portuguese Environment Agency since 1997 (Pinto & Teixeira, 2022), which involves systematic surveys of six beach profiles and has limited spatial scope and temporal resolution. These limitations could be overcome by satellite remote sensing (RS), which has been recognized as an alternative.

We aim to verify whether RS is suitable for measuring changes of beach width after beach nourishment operations, contributing to cost-effective monitoring with greater spatial and temporal coverage. The study was conducted along the Vale do Lobo coastline, focusing on the evolution of the average beach width from February 2000 to February 2024. During this period, the beach evolution was marked by a rapid increase in beach width following two beach nourishments and a gradual narrowing driven by a sediment deficit imposed by the updrift retention structures.

Images from the Landsat 5, 7, 8 and 9 satellites and Sentinel-2 Level 1C were obtained and classified, using the python toolkit CoastSat (Vos et al., 2019), which also made it possible to obtain the shorelines of the beach during the study period. The USGS DSAS (Himmelstoss et al., 2024) software was used to acquire beach width values, at the six profiles surveyed in the monitoring program. Although the relatively low spatial resolution of the images (30m and 10m), and the existing differences between the measured shoreline indicators (beach width at MSL and instantaneous water line in RS, which includes the effects of tide and swash signals), the relatively high temporal resolution of RS images allowed for the filtering of uncertainties. As a result, the time-averaged RS values were found to closely match those obtained from field monitoring. In response to the 2006 nourishment, the beach advanced 29m (33m for RS) followed by a gradual beach width reduction of 5.8m/yr (5.7m/yr for RS), while in the 2010 nourishment the beach advanced 29m (28m for RS) followed by a gradual reduction of 1.8m/yr (1.9m/yr for RS). The comparison between the data obtained showed congruence of field and RS results, proving evidence that remote sensing techniques and semi-automatic methods can be an asset for monitoring beach nourishment evolution. This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025, UIDB/50019/2020 (https://doi.org/10.54499/UIDB/50019/2020) and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020). The work is a contribution to the CREST project, funded by FCT through Grant 2022.05392.PTDC (doi:10.54499/2022.05392.PTDC). Authors also recognize the support of national funds through FCT, under the project LA/P/0069/2020 (doi:10.54499/LA/P/0069/2020), granted to the Associate Laboratory ARNET, and UID/00350/2020 (doi:10.54499/UIDB/00350/2020) granted to CIMA.