Opere costiere per la difesa dei ripascimenti artificiali a basso impatto ambientale

The development of waterfronts constitutes a highly topical issue that involves large as well as small urban are-as facing the sea. Coastal areas represent the interfaces between the urban fabric and the sea, which is widely recognized as an engine for European economy. In order to exploit the potential that coastal areas offer, the planning and management strategies must be aimed at recovering the environmental quality which has seen its gradual decline, especially in recent years since the effects induced by climate changes exacerbate natural and anthropic pressures. In agreement with Horizon 2020 and recently highlighted by the Blue Growth strategy undertaken by the Euro-pean Commission, it is necessary to encourage and support sustainable long-term growth in the marine and maritime sectors, through the strengthening of the seas, oceans and coasts in order to create new job opportuni-ties and companies in the Blue Economy productive sectors. Indeed, the protection of coasts and maintenance of a good quality of marine environments become a priority, to guarantee their sustainable use and reduce the ex-posure against erosion and flood phenomena that alter their stability, environmental quality and economical value. In such a context, the rehabilitation of coastal areas often translates into the creation of artificial nourishments, which allow to restore an adequate extension of the emerged beach and the dune system, without compromis-ing the landscape and bathing activities. These interventions require an accurate management plan aimed at defining the financial sustainability of the work, mainly linked to its life-time. In the present work, an alternative approach for protecting sandy beach nourishment is proposed. The short-term morphological evolution of beach nourishment profile under erosive wave conditions is investigated in presence of a mixed defence system composed by a drain pipe placed inside the beach in the swash zone, paral-lel to the shoreline, and a submerged rubble mound breakwater characterized by a high freeboard, namely equal to 90% of the offshore significant wave height below the mean water level. A new experimental campaign is underway at the Coastal Engineering Laboratory of the Polytechnic University of Bari (Bari, Italy). The tests on 2-D small scale physical model (according to Froude analogy, 1:10) have been conducted in both unprotected and protected conditions starting from the same initial profile up to equilibrium conditions (16-19 hours in model scale). The drainage system (BDS) and the submerged barrier (BW) have been tested firstly separately and deployed together, in order to be able to separate the effects of each intervention on sediment transport. The experimental measurements have also allowed a preliminary calibration of the XBeach numerical model in reproducing the evolution of the nourishment profile in unprotected conditions.

Nel presente lavoro si riportano i principali risultati ottenuti durante una campagna sperimentale in corso presso il Laboratorio di Ingegneria Costiera (LIC) del Politecnico di Bari. Le attività sperimentali mirano ad ana-lizzare gli effetti del sistema di drenaggio spiagge (BDS) accoppiato ad una barriera sommersa ad elevata som-mergenza (c.a. - 1.80 m) sull’evoluzione trasversale a breve termine di un ripascimento artificiale, in condizioni ondose erosive. Le prove su modello fisico 2-D in scala geometrica indistorta di Froude 1:10 sono state condotte in condizioni non protette e protette, a partire dallo stesso profilo iniziale sino al raggiungimento del profilo di equilibrio (16-19 ore in scala modello). L’influenza dei due sistemi sui processi sottocosta è stata analizzata os-servando il comportamento del sistema di drenaggio spiagge (BDS) e della barriera sommersa (BW) prima se-paratamente e poi congiuntamente, al fine di poter scindere gli effetti di ciascun intervento sul trasporto dei se-dimenti. In via preliminare, i risultati sperimentali hanno inoltre consentito l’analisi delle capacità predittive del modello numerico XBeach (Roelvink et al., 2009) per lo studio dell’evoluzione del profilo trasversale di un ripa-scimento non protetto, nelle condizioni testate in laboratorio sul modello fisico. La sensitività del modello è sta-ta dapprima valutata in relazione alla discretizzazione spaziale della griglia e alla condizione ondosa al contor-no, consentendo la definizione di un modello base per la successiva fase di calibrazione rispetto all’evoluzione del profilo, considerando alcuni parametri coinvolti nella modellazione dei processi di idrodinamica e morfodi-namica. I primi risultati mostrano come, nonostante il modello restituisca un buon grado di accuratezza per la previsione dell’evoluzione morfodinamica del fondo e dell’idrodinamica, alcune incertezze risiedono nella mo-dellazione della forma della barra sommersa.