Experimental Analysis and Numerical Simulations of Waves and Current Flows Around Low-Crested Rubble-Mound Structures

Abstract

The objective of this paper is to analyze flow patterns and velocity fields around rubble-mound low-crested structures based on wave basin experiments and 2DH numerical simulations. Experiments were performed in the 12.5×9.7m2 wave basin at Aalborg University, Denmark, within the framework of the EU-funded project DELOS. Two different layouts were designed: two detached breakwaters with a gap in between and an oblique breakwater forming a rip channel with the basin side wall. Both cases of narrow and wide berm were tested under two- and three-dimensional regular and irregular wave attacks. Fluxes around and through the structures are examined by means of image analysis and measurements of free surface elevation and velocity at fixed locations. An original scheme for evaluating the overtopping discharge through a wave-by-wave analysis of wave gauge records over the barrier is described. Overtopping discharges are reconstructed and related to existing wave run-up models for emerged and zero-freeboard structures. Since no measurement was performed in the rubble mound, a “scale” model to estimate filtration flux based on other available datasets containing filtration velocity and set-up is proposed and verified. The analysis of the datasets serves to discuss different filtration mechanisms depending on structure submergence. The approach for separately evaluating the different flux terms is then globally checked for zero freeboard and emerged structures through experimental mass balance. For deeply submerged structures, for which this approach fails, numerical simulations are carried out and the accuracy of the commercial 2DH model MIKE 21 is tested in predicting parameters relevant to the design process as set-up, overtopping and returning flows. Simulations are also presented for a sample emerged case in order to show how some limitations in the tool can be surpassed to allow general applications. The numerical model furthermore invites a description of the physical processes over the whole area of interest, where extensive measurements cannot be performed due to the test imperfect repeatability in presence of irregular waves and thus the impossibility of moving the limited number of available instruments.