Natural vs. Anthropic Influence on the Multidecadal Shoreline Changes of Mediterranean Urban Beaches: Lessons from the Gulf of Cagliari (Sardinia)

Abstract

Urban Mediterranean beaches are often characterized by a fragile and unstable equilibrium that can be easily altered by ongoing climate change and by the increase in human pressure. This may pose serious threats to the survival of beach systems that cannot accommodate these modifications. In this paper, the spatio-temporal shift of the shoreline was investigated along two urban beaches in the Gulf of Cagliari (Poetto and Giorgino; southern Sardinia, western Mediterranean Sea) across a time frame of 62 years (1954–2016). The Digital Shoreline Analysis System (DSAS) ArcGIS™ extension was used to extract different statistical parameters which allowed us to quantify the erosion and accretion rates. These data were further examined in relation to a number of anthropic and natural forcings in order to disentangle the factors controlling shoreline evolution. Eight sectors with interchanging net erosive and accretion trends were identified along the Poetto and Giorgino beaches. In six decades, some sectors of the two study sites appeared to have undergone great shoreline modification as a result of the intense anthropogenic activities impacting these coastal areas. The westernmost portions of both beaches were found to be the most vulnerable to erosion processes; such conditions were likely controlled by the interplaying of local hydrodynamics and by the intense coastal development which affected these sectors. The highest retreat rates (mean end point rate (EPR) = −0.51/year) were recorded in the western limit of Giorgino beach. Along the western limit of Poetto beach, EPR erosion rates (mean EPR = −2.92/year) considerably increased in the years after the artificial beach nourishment carried out in 2002, suggesting that the majority of the nourished material was lost offshore or partly redistributed along the beach. Coastal structures, urban development, river catchment modification, industrial and port activities, beach cleaning and touristic and recreational activities have been identified as the ongoing causes of coastal alteration. If these factors remain constant, under projected climate change scenarios, these beaches are at risk of further increased flooding and erosion. In this context, the application of DSAS appeared as an essential tool, supporting a monitoring system able to provide understanding and, potentially, predictions of the short- to long-term evolution of these beach systems.

The Application of GIS and Remote Sensing in a Spatiotemporal Analysis of Coastline Retreat in Rufisque, Senegal

“…Ultimately, Bakhoum et al [38] revealed a strong spatial temporal variability of the coastline of Dakar from 1954-2015, with a dominance of eroded surfaces over accreted areas attesting to a negative sedimentary budget. On the same note, Biondo et al [39] recorded EPR values ranging from −0.51 to −2.92 m/year along the Urban Mediterranean beaches such as the gulf of Cagliary from 1954 to 2016.…”

A Four-Year Video Monitoring Analysis of the Posidonia oceanica Banquette Dynamic: A Case Study from an Urban Microtidal Mediterranean Beach (Poetto Beach, Southern Sardinia, Italy)

“…The carbonate fraction of the sediment has increased since 2002 due to nourishment carried out in the southwestern sector of the beach by a dredger, which dumped bioclastic sediment on the beach with a different grain size from the original sediments. The nourished material was redistributed and levelled with bulldozers [44,45].…”

Ecosystem services of reed and seagrass debris on a urban Mediterranean beach (Poetto, Italy)

“…A nourishment project carried out in 2002 in the Cagliari municipality sector has significantly modified the textural, compositional and morphological features of the backshore, shoreline and shoreface [12]. Besides, an increasing anthropic pressure, mainly related to the touristic sector, is responsible for an impact on the beach system, significantly affecting morphodynamic processes [13].…”