Extension of shoreline under erosion (period between 1958 and 2010)

APA, 2019

Data reported in 2010, shows that the length of low-lying sandy coast and low rocky coast supported by dunes under critical erosion is about 180 km, with retreat rates with variable magnitude. The average narrowing rates in the period 1958-2010 range from 0.5 m/year to 9.0 m/year  (Coastline Working Group Report, 2014Lira et al, 2016), which account as a whole, to a territory loss of about 12 km2 (APA, 2015).

In that period, the magnitude of the erosive process reached its largest expression in the coastal cells of Espinho - Torreira, Praia da Barra - Mira, Figueira da Foz - Leirosa and Costa da Caparica, which show an average coastline retreat ranging from 200m to 300m over the last 60 years (Sediment Working Group Report, 2015). The identification of the coastal cells was done by the Littoral Working Group, in 2014, based on the different characteristics of the Mainland Portugal shoreline with regard to geomorphology and sediment dynamics (Coastline Working Group Report, 2014).

The comparison of the 2010 and 2018 coastlines (under the COSMO Program) shows that the length of the coastline affected by erosion remains relatively unchanged, i.e. no erosion was triggered in new areas. Nevertheless, the erosion process continues inland in some of the previously identified areas in 2010, with a loss of territory of around 1 km2 (100 ha) by 2018.

Regarding the position of the coastline, the preliminary analysis of the data obtained by COSMO suggests a change in the evolutionary trend in certain coastal sectors between 2010-2018 in relation to 1958-2010, namely:

  • Between the Castle of Neiva and Esposende - worsening of erosion rates by about 2x;
  • Between Ofir and Estela - worsening of erosion rates by about 2x;
  • Between Cortegaça and Furadouro - worsening of erosion rates by about 2x;
  • Between Furadouro and Torrão do Lameiro - slight decrease in erosion rates over the entire length of the section, but increase of about 3x in the 2.5 km to the south);
  • Between Costa Nova and Mira - decrease of erosion rates by about 1/3;
  • Between Cova-Gala and Lavos - worsening of erosion rates by about 3x;
  • Costa da Caparica - relative stability;
  • Faro beach - relative stability.

The observed differences in the evolutionary trend of some sectors of the coastline between 2010-2018 depend on a broad set of interactive and retroactive factors, namely oceanographic forcing, geomorphological context and anthropic intervention. In some of the sectors, the information currently available is not yet sufficient to clarify the totality of cause-and-effect relationships, including the clear separation of natural processes from the anthropic ones.

However, the analysis of the results of the COSMO programme shows the worsening of the erosion process between 2010 and 2018 at Figueira da Foz (Cova Gala - Lavos sector), with maximum retreats of around 50m, confirming the trend already observed by the Coastline Working Group, 2014; André & Cordeiro, 2013 and Oliveira & Oliveira, 2016.

The weakening of the erosive process in some of the identified sectors will be related to a series of artificial feeding interventions implemented since 2010 (Pinto et al., 2018) in the immersed, emergent and dune-corded domain of their respective areas of influence or in the same coastal cell (see Coastaline Working Group, 2014).

The results of the COSMO program seem to reinforce the ongoing protection strategy, based on the restoration and maintenance of the sedimentary balance in coastal cells with an installed erosive tendency, preceded by the identification of compatible sedimentary resources in the near continental shelf and by the optimization of dredging management by the Ports.

Sediment deficit (i.e. decrease in the supply of sediments to the shoreline) in the respective coastal cells, combined with a high-energy wave regime and intense longshore drift (i.e. the sediment transported parallel to the shoreline during a given time interval) contribute to the installed erosive trend.

The magnitude of coastal erosion, and the respective associated risk, determined that most of the investment made in the coastline over the last 16 years, amounting to approximately 295M€, was allocated to coastal protection and defence interventions.

The COSMO Programme will allow the quantification of the evolution of the coastline and nearshore seabed, always based on the extent of Mainland Portugal's coastline, according to the official cartography - the coastline orthophotomaps provided by the Directorate-General for Spatial Planning -, which will allow to periodically identify the length of coastline under critical erosion, according to the applicable criteria.

Ordem de aparecimento: 
Coastline under erosion