2.2.7. Coastal Structures

Sediment offshore dispersion is favored by coastal structures, both in harbor and with shore protection projects. These, not only modify long-shore sediment distribution, favoring or penalizing different sectors, but through wave reflection, especially with shore parallel structures: [111,112] can induce topographically controlled rip currents through the structures' gaps [113]. Structures orthogonal or oblique to the coast [114,115] favor the shift of sediments to deeper areas, where they can be lost or from where they hardly return to the near-shore further downdrift [116]. This is well known and considered by researchers, but here we want to highlight the variability of their effect.

In front of shore parallel structures, beach profiles gradually lower [117,118], so wave energy dissipation reduces, and the structure becomes hit with increasing intensity, under a positive feedback-regulated process. However, when depth is so large that no shoaling processes are present and waves are fully reflected, no impact on sediment transport is to be expected; but this terminal condition is hard to be achieved. If this happens, a contributing cause is a regional negative sediment budget, e.g., at Marina di Pisa, Italy, where breakwaters located at an initial depth of 2.5 m, now have a depth of up to 7.0 m at the seaward side [112].

With respect to groins, trapping capacity reduces with time, until sediments bypass the obstacle; at this point, offshore dispersion is increased, although some material can overpass the structure (if it does not extend over the depth of closure). Therefore, updrift accumulation, offshore dispersion and input to the downdrift sectors change over time [119,120].
