Experimental analysis and 2DH numerical simulations of morphodynamics around Low-Crested Structures.

Low crested structures are frequently used for beach defence purposes in form of cells. The aim of this paper is to experimentally investigate the morphological response of such defence schemes; experimental observations are also compared with formulae and numerical simulations. Experiments are carried out at Politecnico di Bari in a large wave basin with mobile bed. Segmented structures with wide and narrow, protected and non-protected gaps are tested in submerged, zero freeboard and emerged conditions. A typical storm of the Northern Adriatic sea is simulated in 6 steps. Waves and currents are measured in front and behind the structures, at gaps and roundheads. The bed is surveyed along 27 profiles across the structures and the shoreline position is monitored by means of image analysis. Evaluation of volume changes shows that sand is in average trapped within the protected area, being transported off-shore from gaps and returning inshore over the structures. Time evolution of bed level changes is calculated using Bijker formula, which allows to obtain a correct prediction of maximum erosion but not of its localization, since the transport process spatial delay is not accounted for. For the submerged set of tested conditions, morphodynamic simulations performed with MIKE 21 CAMS are presented showing a good agreement among simulated and surveyed erosion/deposition patterns, both in these experiments and in the field.