Failure of the nourishment intervention at Ladispoli Beach (Central Latium coast, Italy), Part 2: the causes

Abstract

This study focuses on the causes of the failure of a nourishment intervention that took place on a stretch of coast near Rome. Using data collected before and after this intervention, these causes have been researched through numerous inspections on selected aspects of possible relevance. The results obtained describe this nourishment as highly controlled by the grain size characteristics of the borrow sands, different from those of the local sediments. The harmonisation of post-nourishment profiles occurred through erosion of a large part of the sand mass originally placed on the beach, and external redeposition of the eroded sand with formation of a large nearshore deposit. This dynamic was modulated by the availability of water space for deposition. Within the general progradational trend, distinct processes occurred along three belts parallel to the coast: in the inner belt, the steep pre-fill topography (positive shoreface accommodation) was mostly filled during nourishment operations; in the middle belt, the migration of the bar trough caused erosion on pre-existing morphology (negative accommodation); in the outer belt with high rates of sediment redeposition, the sediments in excess were expelled from the system (zero accommodation). Most of the methods used to explore this nourishment indicate that its failure is due to the borrow sand used, similar in size to the native sediment but considerably better sorted and, most importantly, with a relevant shortage of those sandy fractions instead present in the original beach. As a result, fill sands fed insufficiently the beach and excessively the breaking zone.