## 1 Introduction  
Coasts are land-ocean interfaces of high environmental and economic value; they provide important ecosystem services, from coastal buffering and inland protection to nutrient cycling, biodiversity, and recreational and cultural environments. As a consequence, coasts are often densely populated and modified, suffering increasing anthropogenic pressure [1]. This raises concerns about their increasing vulnerability [2], particularly in the light of potential climate change impacts, such as sea-level rise and changes in ocean wave characteristics [3; 4]. Furthermore, many coasts suffer from erosion due to a sediment deficit, mostly caused by human actions that disrupt sediment fluxes and retain sediments in reservoirs and constructions [5]. More than 20% of the European and 30% of the Portuguese coastline are estimated to suffer from coastal erosion [6]. Erosion causes land loss, threatens ecosystems and infrastructures through increased exposure to wave impacts, and increases flood risk [7], and it is expected to be aggravated by climate change [8]. For Portugal, for instance, the projected sea-level rise is 1.14 m (ranging between 0.39 m and 1.89 m, with a 95% probability) by 2100 [9]. Erosion and shoreline retreat are therefore likely to become an even more relevant problem in the coming decades. There is hence an urgent need for coastal zone management and maritime and coastal planning. The retreat of coastlines can be prevented by hard structures, such as groynes, seawalls and breakwaters, or through soft methods, such as beach nourishment, dune stabilization and bioengineering [10; 11; 12]. Given the high implementation and maintenance costs of hard defence structures and their negative impacts on ecosystems, as well as on neighbouring hydro-morphodynamics, soft solutions have gained in popularity [12; 13; 14]. This trend is also visible in Portugal, where hard defence structures were implemented from the late 1950s onward, often producing unexpected or unwanted effects and exacerbating downdrift erosion problems [15; 16]. Over the past two decades, management strategies have therefore favoured soft defence approaches through artificial beach nourishment, placement of fences, construction of footbridges and revegetation of dunes [17; 18]. Furthermore, also after 1950, important dams were built in the main river basins of the Iberian Peninsula, and large harbours were constructed at the river mouths. These two anthropic interventions, next to other significant soil-use changes in the river basins, greatly contributed to diminishing the volumes of sediments that are transported to the coastal platform or feed the alongshore dominant drift. The consequent sediment starvation at the coast implies that the design principles of groynes and breakwaters are not satisfied, making these defence solutions inefficient in most coastal environments. Sustainable coastal management requires an integrated evaluation of coastal protection through hard defence structures or soft interventions, such as beach nourishment. Long-term cost-benefits and feasibility depending on local conditions, vulnerabilities and values, as well as sediment availability, need to be assessed to decide on mitigation measures or the alternative of a managed retreat, sacrificing areas that are considered less valuable [11]. Coastal morphology reflects the local natural and man-made conditions. Beach morphological changes occur due to the influence of natural phenomena, such as ocean waves and coastal currents, wind and river-flow effects, as well as due to human activities, such as urbanisation, infrastructures and coastal defence intervention [19; 20; 21]. Coastal vulnerability may therefore differ locally, depending on exposure/protection and local geological features, as well as local hydrodynamic and meteorological conditions, suggesting that a local approach to monitoring and management is needed [22].