Evaluation of beach response to submerged groin construction at Marina di Ronchi, Italy, using field data and a numerical simulation model

An experimental submerged groin was built in spring 1999 at Marina di Ronchi (Tuscany, Italy), where the erosion rate was 4 m/yr from 1985 to 1998. The groin is 180 m long and built with 2.5 × 1.8 × 0.7 m polypropylene bags filled with sand. It is buried under the backshore and extends to the -3m isobath with a mean elevation of approximately 1 m above the bed. Four sets of cross-shore and longshore bathymetric profile surveys conducted from February 1999 (preconstruction) to April 2000 reveal that the submerged groin enhanced seasonal displacement of the bar system inside the 4 m isobath. Eighty-five sediment samples collected from the swash zone to 6-m water depth reveal limited changes near the groin, except for a coarsening at the landward end and in a depositional area a few hundred meters downdrift of the seaward end. Calculated rates of longshore sediment transport range from 52,909 nr3/yr (KAMPHUIS formula) to 496,300 m3/yr (CERC formula). The submerged groin favors deposition near the structure under fair weather conditions but deep scour (nearly 2 m) during extreme events. Unlike emergent groins, no significant changes were observed on the upper beach at the structure or on adjacent beaches. The submerged groin did not fully cross the bar system, and the deep scour associated with strong currents concentrated at its tip could lead to future collapse of the structure. A numerical model established for the site conditions was run to test the consequence of lengthening the groin to reach a depth of 5 m to place it across the zone of bar migration. The model effectively represents observed processes and predicts less scour and more deposition at the seaward tip of the lengthened groin, combined with increased likelihood of a rip current near the structure.