National geological and hazardous systematic cartography of the coastal zone as a contribution to the promotion of sustainability, defence and valorisation of the Portuguese coastal area

The Portuguese coastal zone, where ¾ of the population lives and where the contribution to GNP is estimated at 85%, assumes an important role in the national economic context, which is not only presently reflected on the budget dedicated to the management and mitigation of current risks associated to climate change, but also for its strategic importance in environmental, social and leisure industry perspectives.

The geological and hazardous cartography of the coastal zone, of mainland Portugal, on a 1:3000 scale, has been developed, at LNEG, as an instrument to support the sustainability, protection and enhancement of the coastline [2018, JCC, 22:1031-1043].

The increase in knowledge concerning coastal hazard, based on the historical evolution of the shoreline, expressed on a systematic and digital cartographic basis at a scale of detail, in addition to the important contribution to the development of regional geology, is a vital contribution for the correct use and sustainable development of the coast. So far, shoreline evolution evaluation has been determined for two coastal sectors covering approximately 140 km of coastline: the western sector of Figueira da Foz to Nazaré and the southern sector of Algarve between Faro and Vila Real de Santo António [2021, JCC, https://doi.org/10.1007/s11852-020-00791-3].

The western sector evolutionary trend, from 1947 to 2015, shows an overall erosional behaviour, even though a prograding tendency is observed in some areas. Coastline evolution assessment reveals an average retreat of -13.6 m and a 702,558 m² land loss area. However, when looking only for the sectors where erosion occurred, a total of 1,164,888 m² of land loss was observed. Erosion, that is more severe in the northern part, reaching a maximum coastal retreat of -145 m and an erosion rate of 2.46 my^-1, seems to be induced by a reduction of the littoral drift, but also by human interference in coastal dynamics, namely by the introduction and enlargement of the original rigid constructions and groins installation.

Regarding the south eastern Algarve coastline displacement, from 1950 to 2015, a seaward shift prevails, with a prograding coastline occupying approximately 54% of the studied sector. However, this progradation is mostly associated to human intervention on the coast, being related to up-drift accumulation against inlet jetties / groins. The erosional trend prevails predominantly in the central barrier island system of Ria Formosa, namely in the Culatra (with a maximum displacement of -163 m), Armona (maximum displacement of -83 m), the Tavira, (maximum displacement of -116 m), and the Cabanas islands, where maximum displacement observed is up to -235 m. Regarding land area changes, some sectors lost and others gained area due to coastline displacement. However, the overall analysis showed an area increase of 1.05 × 10⁶ m² for this south-eastern coastal fringe.

The achievement of high-resolution, continuous and updated data, at a regional scale, likely favour successful application of the needed mitigation measures (as beach/dune nourishment, sand-shots and others) at the exact key target locations.