Duccio Bertoni

University of Pisa, Italy

Gravel beaches as a form of coastal protection: pros and cons

Coastal erosion is a natural process that affects large portions of littoral areas around the world. In the last decades it has been emphasized by human activities, such as sediment dredging from river beds, river damming and armouring, and many other factors. As coastal settlements were endangered by shoreline retreat, local administrations reacted in different fashion to protect the beach and significant sites. Hard structures such as breakwaters and groynes were rapidly built to counteract the most intense erosive processes, frequently triggering downside effects though. In that, softer approaches such as artificial replenishments were adopted where the processes were not as threatening. Coarse-grained beach fills have been increasingly used also where no coarse sediments were naturally present on the shore. They are more stable and resistant during storm events, guaranteeing a protection level almost akin to breakwaters. A coarse-grained beach is way more durable than a sandy beach, even though a 1-year-long topographic surveying at the artificial gravel beaches at Marina di Pisa showed high mobility of the sediments and, consequently, a retreat of about 10 m. Still, little is known about sediment transport patterns and mass loss, which leads to severe volume losses and a decrease in protection of the whole system. Two separate sediment tracing experiments performed at those beaches helped figure out the high abrasion rates of the pebbles (about 60% of mass loss in 13 months), which was an impressive result as the rates were consistent between rounded and angular pebbles. The decadal experience of the Marina di Pisa artificial gravel beaches seems to confirm the pros and cons of this approach to coastal management. The jury is still out: more studies about sediment transport and abrasion are needed on this kind of environment before stating that gravel beaches are the optimum – but temporary – solution.