The present study has been carried out to evaluate the morphodynamic behaviour of an artificial pebble beach during a prolonged period of ti me. In this span of about 1O months, a series of storms has hit the coast, which have made possible to define the response of the beach after each high-energy event. The study area is located on the Pisan coast, exactly at Marina di Pisa. The shores of Marina di Pisa are about 6 km in length and are delimited by Arno River mouth to the north and the city of Tirrenia to the south. The littoral drift is directed to the south throughout this area (Gandolfi and Paganelli, 1975; Aiello et al., 1975; Mazzanti, 1983; Pranzini, 2004). Tides are not overly significant, rarely exceeding 30 cm. The only source of sediments input is represented by Arno River's solid discharge, which is definitely not sufficient to naturally feed the coast. As a matter of fact, the Pisan coast has been struck by serious erosion processes since early xxi Century, which have caused a huge retreat of the coastline in several sectors. The decrease in Arno River sediments discharge and the massive use of hard protection systems (breakwaters, groynes) are considered as the main reasons to explain the erosion processes, which stili affect the coast. Breakwaters and groynes were built as a first defence against the strong wave processes acting on this coast: they were fit to prevent severe flooding, but they hampered the natural nourishment, hence the existence, of the beaches as well. Therefore, coarse sediments have been used to artificially feed the shore: during the mid-Nineties few gravel (1O to 30 mm diameter) beaches were built, but they resulted not well suited to that particular setting. Hence, pebble-to-cobble grain size has been used in subsequent beach fills. The Province of Pisa has recently completed the construction of three pebble beaches at Marina di Pisa. Due to the relative lack of knowledge on this kind of beaches, it has been stressed (Buscombe and Masselink, 2006) a continuous and in-depth monitoring of the morphodynamics (sedimentology, geomorphology, physical processes). However, the research on gravel beaches has been pushed lately by the large spreading of gravel replenishments as a form of coastal protection. Thus, repeated and intensive surveys of gravel beach systems should improve the understanding of this no more uncommon environment. The beach under study, named Celi 7, is characterized by very coarse grain size (mostly 30 to 70 mm spoils of Carrara marble quarries), two groynes at both ends and a submerged breakwater 60 m off the coastline. lt is about 250 m in length and 40 m wide. The nearshore is very steep and it reaches a 3 m plus depth just 20 m seaward of the coastline. The geomorphological aspect of the research has been addressed by the use of a highaccuracy GPS instrument. The observations were periodically carried out through July 2007 and Aprii 2008 and intensified in particular after significant high-energy events. This approach has allowed to define the beach response under changing sea-weather conditions: besides, it has made possible to appraise the rate the beach width narrows and, consequently, the effectiveness of the nourishment. Five profiles were outlined on the beach: along each profile, any single gradient variation was pinpointed and tracked in order to obtain highly detailed beach sections. The comparison between these sections has clearly showed a remarkable retreat of the beach right after the most powerful storms. The retreat is measurable to the tune of 11 m. Given the originai width of about 40 m, this value is even more noteworthy. Subsequently, the study has been extended to the sea-bottom fronting the beach through a series of surveys performed with a single beam instrument installed on a boat. The echo-sounder tracked points every second, so less than ten back-and-forth routes have been sufficient to complete the survey and, through data processing, to reconstruct the sea-bottom geomorphology. These surveys have been necessary to complete rough estimates of volumes of sediments moved during the period of observation. The research has been rounded out with two grain size analyses carried out to evaluate the sedimentologic variations occurring before and after a series of storm. Any morphologic element has been sampled, namely the step, the beachface, the ordinary berm, the storm berm and the backshore. The results have showed i) slight variations in mean grain size on the step and, in particular, on the beachface, where values almost coincide; ii) the Mean (Mz) plot of the second sampling shows exactly the same trend for each profile, that is a decrease moving from the step to the ordinary berm and a subsequent increase towards the backshore. In conclusion, the activities carried out in this research have been directed towards a better definition of this matter, which is of paramount importance not only to increase the knowledge on a topic not overly debated and studied in the literature, but even for a successful construction of enormous protection structures like these ones.

Morphodynamic evolution of an artificial pebble beach in a ten month span; Marina di Pisa, Tuscany, Italy: preliminary data

Bertoni D;SARTI, GIOVANNI
2008-01-01

Abstract

The present study has been carried out to evaluate the morphodynamic behaviour of an artificial pebble beach during a prolonged period of ti me. In this span of about 1O months, a series of storms has hit the coast, which have made possible to define the response of the beach after each high-energy event. The study area is located on the Pisan coast, exactly at Marina di Pisa. The shores of Marina di Pisa are about 6 km in length and are delimited by Arno River mouth to the north and the city of Tirrenia to the south. The littoral drift is directed to the south throughout this area (Gandolfi and Paganelli, 1975; Aiello et al., 1975; Mazzanti, 1983; Pranzini, 2004). Tides are not overly significant, rarely exceeding 30 cm. The only source of sediments input is represented by Arno River's solid discharge, which is definitely not sufficient to naturally feed the coast. As a matter of fact, the Pisan coast has been struck by serious erosion processes since early xxi Century, which have caused a huge retreat of the coastline in several sectors. The decrease in Arno River sediments discharge and the massive use of hard protection systems (breakwaters, groynes) are considered as the main reasons to explain the erosion processes, which stili affect the coast. Breakwaters and groynes were built as a first defence against the strong wave processes acting on this coast: they were fit to prevent severe flooding, but they hampered the natural nourishment, hence the existence, of the beaches as well. Therefore, coarse sediments have been used to artificially feed the shore: during the mid-Nineties few gravel (1O to 30 mm diameter) beaches were built, but they resulted not well suited to that particular setting. Hence, pebble-to-cobble grain size has been used in subsequent beach fills. The Province of Pisa has recently completed the construction of three pebble beaches at Marina di Pisa. Due to the relative lack of knowledge on this kind of beaches, it has been stressed (Buscombe and Masselink, 2006) a continuous and in-depth monitoring of the morphodynamics (sedimentology, geomorphology, physical processes). However, the research on gravel beaches has been pushed lately by the large spreading of gravel replenishments as a form of coastal protection. Thus, repeated and intensive surveys of gravel beach systems should improve the understanding of this no more uncommon environment. The beach under study, named Celi 7, is characterized by very coarse grain size (mostly 30 to 70 mm spoils of Carrara marble quarries), two groynes at both ends and a submerged breakwater 60 m off the coastline. lt is about 250 m in length and 40 m wide. The nearshore is very steep and it reaches a 3 m plus depth just 20 m seaward of the coastline. The geomorphological aspect of the research has been addressed by the use of a highaccuracy GPS instrument. The observations were periodically carried out through July 2007 and Aprii 2008 and intensified in particular after significant high-energy events. This approach has allowed to define the beach response under changing sea-weather conditions: besides, it has made possible to appraise the rate the beach width narrows and, consequently, the effectiveness of the nourishment. Five profiles were outlined on the beach: along each profile, any single gradient variation was pinpointed and tracked in order to obtain highly detailed beach sections. The comparison between these sections has clearly showed a remarkable retreat of the beach right after the most powerful storms. The retreat is measurable to the tune of 11 m. Given the originai width of about 40 m, this value is even more noteworthy. Subsequently, the study has been extended to the sea-bottom fronting the beach through a series of surveys performed with a single beam instrument installed on a boat. The echo-sounder tracked points every second, so less than ten back-and-forth routes have been sufficient to complete the survey and, through data processing, to reconstruct the sea-bottom geomorphology. These surveys have been necessary to complete rough estimates of volumes of sediments moved during the period of observation. The research has been rounded out with two grain size analyses carried out to evaluate the sedimentologic variations occurring before and after a series of storm. Any morphologic element has been sampled, namely the step, the beachface, the ordinary berm, the storm berm and the backshore. The results have showed i) slight variations in mean grain size on the step and, in particular, on the beachface, where values almost coincide; ii) the Mean (Mz) plot of the second sampling shows exactly the same trend for each profile, that is a decrease moving from the step to the ordinary berm and a subsequent increase towards the backshore. In conclusion, the activities carried out in this research have been directed towards a better definition of this matter, which is of paramount importance not only to increase the knowledge on a topic not overly debated and studied in the literature, but even for a successful construction of enormous protection structures like these ones.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11568/124287
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