Assessment of the Anthropogenic Sediment Budget of a Littoral Cell System (Northern Tuscany, Italy)

Abstract

In the present study we describe a straightforward and highly replicable methodology to assess the anthropogenic sediment budget within a coastal system (the Northern Tuscany littoral cell, Italy), specifically selected in a partially natural and partially highly urbanized coastal area, characterized by erosion and accretion processes. The anthropogenic sediment budget has been here calculated as an algebraic sum of sediment inputs, outputs and transfer (m3) within a 40 year time interval (1980–2020). Sediment management strongly influences the sediment budget and, even if its evaluation is crucial to assess the efficiency of a coastal management policy, it is often difficult to quantify the anthropogenic contribution to sedimentary processes. Different types of intervention are carried out by a variety of competent authorities over time (Municipalities, Marinas, Port Authorities), and the correct accountability of sediment budget is no longer known, or possible, for the scientific community. In the Northern Tuscany littoral cell, sedimentation is concentrated in a convergent zone and updrift of port structures, which have determined a series of actions, from offshore dumping and disposal into confined facilities (sediment output), to bypassing and redistribution interventions (sediment transfer); conversely, river mouths and coastal areas protected by groins and barriers are subjected to severe erosion and coastline retreat, resulting in many beach nourishments (sediment input). The majority of coastal protection interventions were carried out to redistribute sand from one site to another within the study area (2,949,800 m3), while the sediment input (1,011,000 m3) almost matched the sediment output (1,254,900 m3) in the considered time interval. A negative anthropogenic sediment budget (−243,900 m3) is here documented.

Anthropogenic Impact on Beach Heterogeneity within a Littoral Cell (Northern Tuscany, Italy)

“…The detailed reconstruction of hard (e.g., breakwaters, groins, port structures) and soft interventions (e.g., nourishments, artificial dunes) along the Northern Tuscany littoral cell has been completed through a careful revision of the scientific literature [44][45][46][47][48]. As an impressive variety of activities was made on this sector of the Tuscany coast in the last 150 years, and in particular in the last 4 decades [20], many details about them were not available on scientific papers. The archives of the Region of Tuscany have been combed through to collect any document reporting information about protection schemes and nourishments.…”

“…A second convergence zone generates farther to the south, in Calambrone, where the southward-directed drift coming from the Arno River faces a northward-directed drift (Figure 1), whose characteristics have yet to be clearly defined. Sea floor profile is not uniform in terms of slope along the cell, as the sectors mainly affected by human activities are locally characterized by configurations owed to the presence of hard protection structures [20,41]. The natural nearshore slope is about 1% [36].…”

“…However, once the coastline reached the tip of the breakwater, sediments began overpassing the structures [54], thus creating an additional issue related to the siltation of the harbor. Regular dredging operations are required to maintain the access way to the harbor clear [20,55]. The downdrift beach suffered especially in the earlier years, but the erosion processes in Viareggio were never as harsh as in Marina di Carrara.…”

“…Once the past experiences of hard approach application along the littoral cell made it apparent that fixing the coastline would be often achieved at the expense of sediment deposition and generating downdrift erosion, a lot of interventions involving the redistribution of sand and gravel along the Northern Tuscany coast have been completed, especially in the last decades [20]. Nourishment activities have been realized according to many project designs [68,70]: Using sand from inland sites or gravel and pebbles from the nearby Carrara quarries; or rather, dredging the sand from accumulation areas within the littoral cell and redistributing it to the starving spots along the cell.…”

“…The anthropogenic impact involves many concurrent factors that are drastically changing the natural morphodynamic condition along the coasts. The presence of ports and beach facilities, the decrease of sediments supplied by rivers caused by riverbed quarrying and dam construction, the increase of subsidence due to excessive water extraction from the aquifer, the proliferation of protection structures, the sediment dredging from harbors, and the artificial nourishment at sites hit by erosion have permanently changed the natural characteristics of many inhabited coasts [18][19][20]. In-depth understanding of how all these human-induced factors combine in determining the morphology and the evolution of the coast is essential to understand the issue of sea-level rise and requires a multi-disciplinary approach [21].…”