The anthropogenic pressure in coastal zones is constantly increasing [1
]. The socioeconomic values and the benefits characterizing such areas are attracting people that increasingly choose to live there. As a matter of fact, worldwide the majority of megacities are located close to the shoreline [3
]. Consequently, socioeconomic benefits and population increase feed off each other causing an inevitable escalation in human pressure. In Europe, around 86 million people live within 10 km from the coastline and about 200 million people within 50 km from the coastline, respectively, [8
] despite the shoreline variations that inherently characterize such a dynamic environment [10
In recent decades, coastline modifications have become a chronic problem for coastal communities, and the anthropogenic impact due to human activities (e.g., harbor maintenance, beach facilities, coastal protection schemes) is increasingly becoming a driver of shoreline changes [15
]. The mismatch between natural and anthropogenic influence on coastal and human activities and interests is evidenced by many protection schemes characterizing the most inhabited coastal regions. Despite such effort, a large number of European coastal zones still show a high degree of exposure to erosion processes [19
] causing significant impact on economic activities and the Gross Domestic Product (GDP) of several countries. Furthermore, since the 1960s climate changes have dramatically increased the vulnerability of coastal zones moving the focus from coastal erosion emergency to coastal flooding risk [20
In this picture, the in-depth comprehension of coastal processes and their driving factors is one of the most relevant tools to support conscious coastal management [24
]. The final target should be the ability to elaborate accurate morpho-dynamic models able to predict coastline changes over time [25
], and to select correct sediment management options in order to regulate the use of natural resources so as to maintain the width of the backshore and to support the medium-to-long term sustainability of coastal protection schemes [28
]. To achieve this goal, knowledge of the source (input) to sink (output) sand cycle in terms of total sediment volume stored within the littoral cell (i.e., the sediment budget) is crucial. Although from a theoretical point of view the concept of a sediment budget is relatively simple (e.g., a positive sedimentary balance means that sand input is higher than sand output, leading to coastline accretion; vice versa, a negative sedimentary balance means that sand output is higher than sand input, resulting in coastline retreat), it is not easy to calculate with accuracy [31
Furthermore, with the transition to the still debated Anthropocene epoch [37
] human activities are increasingly capable of influencing and modifying natural processes [41
]. Both the sediment budget and the longshore distribution of sediments can be significantly modified by human actions, and worldwide this is a usual practice of coastal management [45
]. Anthropogenic modifications are: (i) the input of new volumes of sediment coming from sources located outside of the littoral cell in order to mitigate erosion processes (e.g., beach nourishments); (ii) the output of volumes of sediment from within the littoral cell and their destination to sites that do not belong to the system (e.g., dredging of harbor areas and allocation of sediments to utilization other than nourishments, or offshore dumping); (iii) transfer of sediments within the littoral cell (e.g., sand bypassing and back-passing) to restore stretches of coast characterized by shoreline retreat while redistributing sediment from accreting areas (e.g., littoral drift convergence zones, updrift accumulation at port and defense structures).
Usually, the anthropogenic actions of sediment input, output, and transfer are standard operations in need of periodical recurrence (in periods ranging from months to decades). A quantitative assessment of the anthropogenic sediment budget in terms of sand volumes is crucial information to complement the total value of a “natural sediment budget” and to understand the efficiency of a coastal management policy. The lack of detailed quantitative information about the anthropogenic sediment budget is due to a series of reasons: (i) mismatch between the extension of the littoral cell and local authority jurisdiction, (ii) change of administrations over time, and (iii) tendency to take measures out of a local emergency situation. As a consequence, the total anthropogenic budget is often unknown, and this seriously hampers sustainability analysis both for the environment (inability to optimize the use of non-renewable natural resources) and for economic development (tourism accounts for about 10% of the Gross Domestic Product in Italy). The economic assessment of coastal defense interventions is crucial, and its evaluation is fundamental to estimate the sustainability of management strategies in the medium to long term.
The aim of this paper is to make a quantitative assessment of the anthropogenic sediment budget along a sector of the northern coast of Tuscany (Italy), situated between the Magra and Arno river mouths. This area represents an ideal natural laboratory because it encompasses a northern sector characterized by strong human pressure (presence of beach resort facilities, port and industrial activities, coastal defense structures) and a southern sector devoid of anthropic settlements and still preserving the natural aspects of the coast. Thanks to an efficient cooperation with the main authority of the area, the Region of Tuscany, a detailed evaluation of the anthropogenic sediment budget over the last 40 years (1980–2020) has been calculated for the first time, and the methodological approach is highly replicable in other locations.
The results here presented show the spatial and temporal distributions of sediment management activities along a sector of the Northern Tuscany littoral cell in the 1980–2020 timespan and, for the first time, they provide a quantitative evaluation of the total volume of anthropogenic sand input, output, and transfer occurring in the area during this time interval. Two main key points should be highlighted.
The first is that the total volume of the anthropogenic sediment budget resulted in a loss. As a matter of fact, the algebraic sum of the volume of sediments used as artificial nourishments (1,011,000 m3) and those removed from the system in the same timespan (1,254,900 m3) indicates a deficit of 243,900 m3. As transfers exclusively involved the sediments displaced within the boundaries of the study area (2,949,800 m3), they are irrelevant for the computation of the total budget, but they do play an important role by deceiving the local community that the origin and effects of coastal erosion have been solved. However, transfer data are crucial in redesigning the management of sand resources within the littoral cell over time, revealing the significant amount of sand redistributed as part of coastal management practices.
The second relevant key-point to highlight is the difference in the order of magnitude between the value of the anthropogenic sediment input (about 1 million m3
) and the quantity of sediment dredged from the Magra and Arno Rivers immediately after the Second World War. Riverbed excavation along the lower reach of the Magra River generated a sediment deficit of ~24,400,000 m3
between 1950 and 1980 [59
]. This corresponds to an annual extraction of about ~1,600,000 m3
/y, which is higher by one or two orders of magnitude to the present-day annual bedload transport estimation [60
]. At present, no quantitative data nor estimation are available for the Arno River, but investigation is in progress (Bini; personal communication); intense dredging activities and sand and gravel exploitation are documented in the 1960–1970 timespan [61
]. No information is available for the Serchio River and the other minor streams flowing into the sea in the area.
These data are likely to be underestimated, because of the lack of a detailed and continuous integration into the database of any riverbed quarrying activity, which sometimes was not even authorized and, as such, not recorded. They also do not take into account the decrease of river sediment supply occurring in recent decades, related to the construction of dams and weirs along rivers, to levee stabilization, to the increase in vegetation cover, and to several other factors closely linked to human activities [e.g., 50]. The extensive erosive processes occurring in recent decades are substantially due to such an abrupt reduction of sediments supplied by rivers, as confirmed by the Italian Ministry of Environment, Land and Sea Protection, which certified on July 2020 that, over the last 50 years, Italian coastline areas have been reduced by 40 × 106
. Clearly such a structural problem cannot be faced only by turning to the good practice of injecting sediments as artificial nourishment. The offices dedicated to coastal management need to take this aspect into serious consideration while laying down future strategies for mitigation and adaptation to global changes in the mid-to-long term. However, this is an issue that Italy shares with the whole of Europe, as confirmed by the high degree of exposure to erosion processes that characterizes many European coasts [19
]. All these considerations allow us to correctly frame the erosion problem against artificial beach nourishing activities, which is valid for the study area.
An in-depth analysis of sediment transfer data within the Northern Tuscany littoral cell provides an additional characterization of the results in terms of sediment redistribution, which helps us in defining the areas where most of the activities have been carried out. The study area has been subdivided into 5 sub-cells, which are each characterized by a different anthropogenic pressure (Figure 7
; Table 8
Sub-cell 1 was only characterized by sediment input over the considered timespan (517,900 m3
), whereas all three types of contribution (input, transfer and output) were involved within sub-cell 2 (total reworked volume of 1,654,000 m3
of sediments, for a net loss of about 156,000 m3
). These are the sub-cells where coastal erosion has been tackled both with beach nourishments and with the construction of barriers and groins (Figure 5
). No activities have been carried out within sub-cell 3; it is the only one without breakwaters and groins. No sediment input intervention occurred within sub-cell 4, which is characterized by a total reworked volume of 2,944,300 m3
of sediments and by a net loss of 692,900 m3
. In fact, sediment transfer observed within sub-cells 2 and 4 is generated by sediment transfer from updrift to downdrift of the Marina di Carrara and Viareggio ports. Part of this volume moved to sub-cell 3 under the effect of waves and longshore currents, contributing to the accretion of the beach and to contribute to the exclusive nature of the world-famous resorts at Forte dei Marmi. Finally, just 97,300 m3
of sediments have been reworked within sub-cell 5, where no output has been reported in the considered timespan.
An additional examination of the results allows us to get a clear look at the sediment redistribution activities in the period 1980–2020 along the study area. If transfer operations between different sub-cells are considered as input and output and not just as a mere mobilization of sediments within the same littoral cell, the resulting data change significantly: inputs and outputs increase to 3,371,000 and −3,614,800 m3
respectively, while transfers take a hit, decreasing to 587,800 m3
Such data underline the huge effort made by the competent Authorities to redistribute sediments in the last 40 years, often pre-empting the modern strategy of managing sand as a resource and not just as an inert material. This is also backed up by the impressive symmetry of the timeline curves of inputs and outputs in such a timespan (Figure 9
), which is an indication that major operations were carried out as a result of emergency rather than in accordance with a strategic, long-term plan. Unfortunately, this is clearly not enough, otherwise the need for recurrent replenishments would not be as imperative as it still is.
The present study revealed another important issue. Many groins and breakwaters were built to combat erosion in the study area in recent decades, but ended up modifying the natural morphology of the littoral sedimentary deposits, changing the grain-size distribution and the profile of the emerged and submerged beach (Figure 2
). This approach has increased the need for artificial beach nourishments because of the downdrift migration of the erosive processes. As a consequence, the northern part of the study area (sub-cells 1 and 2) cannot be considered as a natural beach anymore: emerged and submerged structures of different shape and size are continuously designed and implemented, but they have generated a new concept of anthropogenic littoral deposits, the artificial beach. However, breakwaters do not prevent sediment loss and the expectations of stakeholders and tourists are not fully satisfied even though many replenishments have been completed (Table 1
However, sediment redistribution from accreting areas (e.g., updrift of port structures) to eroding beaches is a good compromise in terms of socioeconomic and environmental issues. In fact, while the touristic appeal of beaches affected by erosion processes is reduced or null, excessively wide backshores are not the best for beach goers either. This is the case with the beach at the ports of Marina di Carrara and Viareggio, whose updrift sectors have a width of about 150 and 350 m respectively (Figure 1
; Figure 10
). Likewise, the sand accumulating at the convergence area at Marina di Pietrasanta (Figure 1
; Figure 10
) might be redistributed to re-nourish the nearby suffering beaches.
Another complementary option, related more to environmental issues, would be the back-passing of sediments from the Port of Viareggio to the natural reserved area of the Migliarino-San Rossore-Massaciuccoli Regional Park. This zone is located in the southern part of the study area (sub-cell 5) and is characterized by strong erosion processes that recently led to the destruction of a wide portion of the natural beach-dune system [70
] (Figure 11
Sand redistribution is a delicate practice that requires more studies, analyses and strict monitoring to evaluate the efficiency and the performance of completed projects. It also needs to be repeated over time within the framework of a defined maintenance program. A crucial point is represented by the accurate definition of the characteristics of grain movement, considering the prevailing path of transport flows. In particular, the assessment of the depth of closure needs detailed consideration. The estimate of this value determines the depth seaward at which morpho-dynamic processes are no longer active [95
]. In this sense, the depth of closure has relevant implications in determining whether the volume of dredged sediments dumped offshore should be considered as sediment output or transfer. Currently, this is a matter of debate in the scientific community [97
] and several methods have been presented in the last 40 years [100
]. Each approach needs an adequate dataset, collected over time in a constant manner: (i) sedimentological and lithological data for the application of a geologically oriented approach, (ii) beach–offshore profiles for a morphological methodology, and (iii) a wave–hydrodynamics dataset for a mathematical modeling approach. The dissimilar values of the depth of closure obtained for the same area using different approaches have been recently highlighted by [106
]. This underlines the need for caution in giving an absolute value to the depth of closure, especially in sites frequently subjected to beach nourishments [107
]. However, the choice of approach basically depends upon the set of data available. Within the study area, the dataset existing for regular intervals of time comes from a buoyant wave-meter located offshore of the Gorgona Island (Figure 1
a). For this reason, the depth of closure has been estimated by applying Hallermeier’s formula [86
]. In agreement with [71
], the resulting value indicates a depth of −14 m for the entire study area. More recently, [85
] applied Hallermeier’s formula [86
], suggesting a lower value, of about −9 m. However, such a debate is beyond the purpose of the present paper and we have decided not to dwell on it as depth of closure does not alter the results of the present study.
An additional issue that is gaining interest among the scientific community and stakeholders concerns the possibility of optimizing and increasing the volumes of sand available for redistribution [108
]. So far, part of the sediments dredged in the harbors cannot be utilized for nourishment purposes due to its high contamination level, ending up discarded on confined facilities or dumped offshore [71
]. While legislation tends to mitigate the environmental impact derived from the movement of sediments in the coastal-marine environment, it should also tend to avoid the wrong disposal of dredged material, and should be applied to limit or prevent sediments from being removed from the sites where they are most polluted (e.g., navigation channels, harbors) where hot spots are more frequent. The expansion and the upgrade of methods allowing in situ decontamination of sediments would represent a good strategy in order to increase the volume of available sands [111
]. In the last 40 years the volume of sediments discarded due to contamination in the study area corresponds to an output of about 576,100 m3
, which is more than half of the total anthropogenic sediment input (1,011,000 m3
). This implies that a significant amount of the anthropogenic input (e.g., replenishments) has served to cover the anthropogenic outputs (e.g., dredging activities). Such operations do not come cheap, and inland and offshore sediment disposal contributes to the total expense. The balance is clearly negative in terms of costs and benefits: charges for dredging operations, sediment disposal and replenishments are factors that must be considered as a summation, not only from the financial point of view but also for the environment; at the same time, the available sand resource is halved. Cost assessment is far beyond the purpose of this study, but it is a relevant issue when the effects of coastal erosion must be mitigated. The cost of coastal defense interventions, including the inflation factor, are variable, but are mainly related to the origin of the sand and gravel and their final destination/disposal (e.g., ~10 €/m3
for sediment supplied by dredging operation, ~30–50 €/m3
for sediment supplied from onshore quarries or rivers by trucks, and ~100 €/m3
for sediment treatment or disposal into CDF and landfill due to the presence of contaminants). A conservative estimation of the cost of sediment management and exploitation within the study area is about 800 million Euros, with an average of more than 10 million Euros spent every year during the Anthropocene period.
The present study also provides insights relative to other cases [36
Location, length of coastal area subjected to coastal interventions, time interval and total amount of sediment management operations are reported in Table 9
. Such comparisons must be considered with caution due to the many natural and anthropogenic conditions that may affect the results from different study areas. A standard methodology to assess sediment budget along a coastal area is still not available, but it is clear that the scientific community is trying to find a solution by following different methodological approaches. The competent Authorities, along with academics, are asked to find a sustainable solution to the geomorphological evolution of coastal areas and to better understand which factors are responsible for coastal erosion in order to provide correct information and support coastal management. The present study demonstrates that the rate of sediment management in Northern Tuscany (Italy) is comparable with the results provided by using other methodologies in other parts of the world, where beach nourishment was carried out to mitigate natural, anthropogenic and climate changes impacts.