Abstract
Perched beach nourishments are popular measures to contrast beach erosion in the presence of steep beaches. Inspired by a real case-study, in the present work the performance of two configurations of the submerged barrier used to protect the toe of the beach are experimentally investigated. Measurements have been gathered concerning wave reflection and transmission, stability and scours around the submerged sill, and beach profile evolution. Both accretive and erosive wave conditions have been considered here. A comparative analysis on the evolution of the two configurations of the sill, shows that the armoring of the filter behind the rubble mound structure is a very effective measure to increase the level of beach protection (e.g. the shoreline retreat under erosive wave attack is reduced to a third). The comparison of the experimental results with an analytical model to predict the equilibrium profile of perched beaches shows that the key phenomena controlling the process are related not only to wave reflection but also to wave dissipation mechanisms at the sill. Moreover, the comparison of the experimental results with field data proves that notwithstanding the several simplifying assumptions considered in the physical modeling, several phenomena are reasonably predicted by the model (e.g. beach evolution, stability and scours).
Highlights
In the presence of eroded steep beaches, beach nourishment projects are often coupled with a submerged reef to retain the beach in a perched position
Such a model is based on the following hypotheses: (i) in the region offshore of the rectangular impermeable submerged sill (Region 1 in Fig. 1), i.e. starting from the water depth he in front of the breakwater, the beach profile is an equilibrium profile and a constant breaker-to-depth ratio is assumed; (ii) there is no dissipation in the breakwater domain (Region 2 in Fig. 1) and the flux of wave energy is partially reflected offshore and partially transmitted onshore; (iii) onshore of the submerged sill (Region 3 in Fig. 2), there is an equilibrium beach profile with water depth equal to hi
Based on Eq (1), assuming linear shallow water wave theory and that only the oscillating part of waves contribute to the reflected energy flux (Baquerizo, 1995), the relationship between the internal and the external water depth at the barrier depends just on the value of the reflection coefficient Kr and it can be written as hi he (1 Kr 2 )2 / 5 (2)
Summary
Perched beach nourishments are popular measures to contrast beach erosion in the presence of steep beaches. Measurements have been gathered concerning wave reflection and transmission, stability and scours around the submerged sill, and beach profile evolution. Both accretive and erosive wave conditions have been considered here. A comparative analysis on the evolution of the two configurations of the sill, shows that the armoring of the filter behind the rubble mound structure is a very effective measure to increase the level of beach protection (e.g. the shoreline retreat under erosive wave attack is reduced to a third). The comparison of the experimental results with an analytical model to predict the equilibrium profile of perched beaches shows that the key phenomena controlling the process are related to wave reflection and to wave dissipation mechanisms at the sill.
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