GRSG 35th Conference 2024 Abstract

Title: Monitoring Shoreline Position at a Multi-Scales Using Earth Observations and Cloud Computing: A Case Study of the Portuguese Coast

Author: Andreia Silva

Organisation: : CoLAB +ATLANTIC

Mapping and analysing shoreline positions at a national level presents critical logistical challenges for coastal managers, who require this information to take decisions. Quantifying shoreline changes, such as the rate of change (measured in meters per year) is crucial to support coastal management. In this context, remote sensing tools, such as CASSIE (https://cassiengine.org), represent a promising solution to address this challenge.

CASSIE is an open-source free web-tool utilizes co-registered historical satellite imagery to automatically extract the waterline and perform several statistical analysis, including the determination of the Linear Regression Rate (LRR). With this parameter, it is possible to determine which coastal sectors are in erosion, stability, or accretion conditions, thus performing an assessment of the shoreline evolution at a national scale. Within the present work, we have applied CASSIE tool to all the beaches of Mainland Portugal, using Sentinel-2 imagery for a recent period (since 2015), to generate the waterline and to create transects with a spacing of 100 meters.

The LRR was quantified, and shoreline trends analysis at different administrative levels: i) National; ii) Regional and iii) Municipal level. Portugal is a coastal country with an extensive (~ 987 Km) and dynamic coastline, subject to variations over the years (e.g. coastal erosion and changes in coastal sediment transport). The main results reveal that 41% of the analysed transects show shoreline stability (LRR > -0.5 & <0.5 m/yr), 31% show accretion, while 19% are experiencing erosion and 9% critical erosion (LRR<-2 m/yr).

Using a more detailed analysis scale, municipalities aligning with national trends and those most affected by coastal erosion were identified, based on municipal averages of LRR. From all the analysed municipalities, Ovar is the one with the highest average erosion rate (-2.35 m/yr). The 10 Portuguese beaches identified with the highest erosion rates for this recent period were Praia das Furnas, Odemira (-8.4 m/yr), Praia de São Pedro de Maceda, Ovar (-6.1 m/yr), Praia das Dunas de Ovar, Ovar (-5.4 m/yr), Praia da Cova Gala (south), Figueira da Foz (-4.1 m/yr), Praia da Cova Gala (north), Figueira da Foz (-3.6 m/yr), Praia do Furadouro north, Ovar (-2.9 m/yr), Praia da Costa de Lavos, Figueira da Foz (-2.8 m/yr), Praia da Falca, Alcobaça (-2.7 m/yr), and Praia do Furadouro south, Ovar (-2.6 m/yr).

The comprehensive data approach reveals that municipalities with the highest erosion rates may not necessarily correspond to those with the most beaches with critical erosion rates. Analysing the results by municipality, we emphasize the importance of always considering the geomorphological context of the area, as the results may: 1) Experience critical rates in beaches strongly influenced by fluvial dynamics of sediment transport, as seen with Praia das Furnas, situated at the mouth of the Mira River; 2) Present stable municipalities with beaches undergoing critical erosion: case of Figueira da Foz, where two beaches are experiencing critical erosion, which, in spite of significant areas of coastal erosion within the municipality, there are other sectors characterized by stability and accretion, resulting in a stable average rate of 0.5 m/y, underscoring the importance of examining with a more detailed scale of analysis; 3) Be similar to the existing knowledge for the municipality, such as with Ovar, which has the highest erosion rate (-2.6 m/year), encompassing some of the beaches experiencing the most critical coastal erosion rates.

The CASSIE tool can be applied globally, is freely accessible and serves as an essential tool for addressing coastal dynamics, playing an important role in coastal management. Additionally, it is worth noting that CASSIE can generate results very close to the analysis that have been conducted. However, its use should be complemented with a geomorphological analysis of the area.