Artificial coarse-clastic beaches are often used in a twofold way to support coastal management: a large replenishment can be either perceived as a form of protection from storm surge and erosion or as a mere reconstruction of a beach to promote summer resort activities and, in turn, revenues. As a matter of fact, the economy of most of the Italian littoral communities is tightly connected to tourism; on the other hand, most of the Italian coastal areas are also subjected to severe erosive processes and are in dire need of efficient defense schemes. Based on the higher level of stability during storms, coarse-clastic beaches have been progressively built in many sites to replace sandy beaches that were completely wiped out by erosion processes. Marina di Pisa, a small village 11 km west of the city of Pisa, is an example of this practice: during the last 10 years, five artificial coarse-clastic beaches have been realized (three in the 2006-2008 timespan; two in 2015). They were basically intended to protect the littoral promenade from dangerous storm surge events, but they ended up being frequently used by tourists in a recreational way. On the wake of this newfound utilization, the knowledge about coarse-clastic beaches, which once was poor compared to that of sandy beaches, is now richer; still, there are several aspects that are far from being completely understood. The artificial beaches at Marina di Pisa have been the test site of several scientific studies aimed to increase the understanding of this peculiar setting, which is not yet included in any existing classification scheme. These analyses concerned grain-size and morphometry of the pebbles, beach profile evolution, and sediment tracing experiments. The latter have been particularly crucial to improve the sedimentologic characterization of the beaches, both in terms of transport processes and of textural parameters. The experiments were conceived to enable the unambiguous identification of individual pebbles, to avoid the statistical approach often used to perform such experiments in the past. The technique that provided the best cost/efficiency ratio was the radio frequency identification technology (RFID), which consists of an antenna that transmits low frequency radio signals and of small transponders (2 mm thick discs of about 30 mm diameter) inserted into the pebbles. Each transponder is identified by a code, which is coupled to each marked pebble. Three separated experiments were carried out to address different aspects: i) tracer transport in the short term (6 and 24 hours); ii) tracer transport in the medium term (two months); iii) mass loss of the tracers in the long term (13 months). The first experiment involved the injection of 78 tracers to address the transport rate in short timespans (6 and 24 hours) and in complete absence of wave motion (fair-weather conditions). The results showed that pebbles of 10 cm diameter experienced significant displacement even though wave activity was at a low, which is a finding that had never been pointed out before. The second experiment was carried out injecting about 100 tracers and performing a recovery campaign after two months. The positions of the detected pebbles allowed to point out peculiar but univocal trajectories determined by the topographic features of the sea floor. The third experiment was intended to assess the abrasion rate of the tracers, because after frequent observations the volume of the beach was clearly reducing even though sediment loss out of the beach system was never detected. In fact, a progressively increasing mass loss was measured on marked pebbles at different times during a 13 months timespan. The final measurement showed a mass loss of more than 60% on all the pebbles recovered after the last recovery campaign, raising major doubts on the long term durability of such defense schemes. These results confirm that coarse-clastic beaches are highly dynamic also when wave motion is barely detectable, which means that in the foreshore pebble grazing is always active, not only during storms as it was previously thought. Therefore, the high abrasion rate measured on individual pebbles at Marina di Pisa is an aspect that must be carefully taken into account while setting up future artificial coarse-clastic beach construction, because the efficiency and durability of the intervention may be reduced if mass loss due to wearing would not be fully factored in.
The impact of coarse sediment tracing experiments for an in-depth characterization (management; planning) of artificial pebble beaches
Duccio Bertoni
;Giovanni Sarti;
2018-01-01
Abstract
Artificial coarse-clastic beaches are often used in a twofold way to support coastal management: a large replenishment can be either perceived as a form of protection from storm surge and erosion or as a mere reconstruction of a beach to promote summer resort activities and, in turn, revenues. As a matter of fact, the economy of most of the Italian littoral communities is tightly connected to tourism; on the other hand, most of the Italian coastal areas are also subjected to severe erosive processes and are in dire need of efficient defense schemes. Based on the higher level of stability during storms, coarse-clastic beaches have been progressively built in many sites to replace sandy beaches that were completely wiped out by erosion processes. Marina di Pisa, a small village 11 km west of the city of Pisa, is an example of this practice: during the last 10 years, five artificial coarse-clastic beaches have been realized (three in the 2006-2008 timespan; two in 2015). They were basically intended to protect the littoral promenade from dangerous storm surge events, but they ended up being frequently used by tourists in a recreational way. On the wake of this newfound utilization, the knowledge about coarse-clastic beaches, which once was poor compared to that of sandy beaches, is now richer; still, there are several aspects that are far from being completely understood. The artificial beaches at Marina di Pisa have been the test site of several scientific studies aimed to increase the understanding of this peculiar setting, which is not yet included in any existing classification scheme. These analyses concerned grain-size and morphometry of the pebbles, beach profile evolution, and sediment tracing experiments. The latter have been particularly crucial to improve the sedimentologic characterization of the beaches, both in terms of transport processes and of textural parameters. The experiments were conceived to enable the unambiguous identification of individual pebbles, to avoid the statistical approach often used to perform such experiments in the past. The technique that provided the best cost/efficiency ratio was the radio frequency identification technology (RFID), which consists of an antenna that transmits low frequency radio signals and of small transponders (2 mm thick discs of about 30 mm diameter) inserted into the pebbles. Each transponder is identified by a code, which is coupled to each marked pebble. Three separated experiments were carried out to address different aspects: i) tracer transport in the short term (6 and 24 hours); ii) tracer transport in the medium term (two months); iii) mass loss of the tracers in the long term (13 months). The first experiment involved the injection of 78 tracers to address the transport rate in short timespans (6 and 24 hours) and in complete absence of wave motion (fair-weather conditions). The results showed that pebbles of 10 cm diameter experienced significant displacement even though wave activity was at a low, which is a finding that had never been pointed out before. The second experiment was carried out injecting about 100 tracers and performing a recovery campaign after two months. The positions of the detected pebbles allowed to point out peculiar but univocal trajectories determined by the topographic features of the sea floor. The third experiment was intended to assess the abrasion rate of the tracers, because after frequent observations the volume of the beach was clearly reducing even though sediment loss out of the beach system was never detected. In fact, a progressively increasing mass loss was measured on marked pebbles at different times during a 13 months timespan. The final measurement showed a mass loss of more than 60% on all the pebbles recovered after the last recovery campaign, raising major doubts on the long term durability of such defense schemes. These results confirm that coarse-clastic beaches are highly dynamic also when wave motion is barely detectable, which means that in the foreshore pebble grazing is always active, not only during storms as it was previously thought. Therefore, the high abrasion rate measured on individual pebbles at Marina di Pisa is an aspect that must be carefully taken into account while setting up future artificial coarse-clastic beach construction, because the efficiency and durability of the intervention may be reduced if mass loss due to wearing would not be fully factored in.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
