Abstract
Coastal areas accommodate a great part of large metropolises as they support a great amount of economic and leisure activities. The attraction of people to coastal zones is contributing to an intense and continuous urbanization of these areas, while the ecosystems are threatened by the increase of natural extreme weather events (e.g., intensity and duration of storms, floods), which interfere with local wave climate and changes in morphological beach characteristics. Protection of coastal zones predisposed to coastline recession, due to the action of high tides, high sediment transport deficit, and high wave energy, may involve various coastal structures to reduce or at least to mitigate coastal erosion problems. Many of the current coastal protections (notably groins, seawalls, and emerged breakwaters) were built with a single purpose, which was to protect at all costs without environmental or economic concerns, especially maintenance costs, or the negative consequences that such structures could cause up to considerable distances along the coast. The current concept of integrated coastal zone management presupposes studies involving other types of concerns and more actors in the decision-making process for the implementation of coastal works. In this context, multifunctional structures emerge and are increasingly frequent, such as the so-called multifunctional artificial reefs (MFARs), with the aim of improving leisure, fishing, diving, and other sporting activities, in addition to coastal protection. MFARs are in fact one of the latest concepts for coastal protection. Behind the search for more efficient and sustainable strategies to deal with coastal retreat, this study focused on a comparison between the performance of two traditional coastal protection solutions (submerged detached breakwater and emerged detached breakwater) and an MFAR on a particular coastal stretch. In order to analyse the hydro- (wave height and wave energy dissipation) and morphodynamics (sediment accumulation and erosion areas, and bed level) of the structures and beach interactions, two numerical models were used: SWAN (Simulation WAves Nearshore) for hydrodynamics and XBeach for hydrodynamics and morphodynamics. In addition, a comparison between SWAN and XBeach hydrodynamic results was also performed. From the simulations conducted by SWAN and XBeach, it can be concluded that amongst all structures, the emerged detached breakwater was the most efficient in reducing significant wave heights at a larger scale due to the fact that it constituted a higher obstacle to the incoming waves, and that, regarding both submerged structures (detached breakwater and the MFAR), the MFAR presented a more substantial shadow zone. Regarding morphodynamics, the obtained results presented favourable tendencies to sediment accretion near the shoreline, as well as at the inward areas for the three structures, especially for the emerged detached breakwater and for the MFAR in both wave directions. However, for the west wave direction, along the shoreline, substantial erosion was observed for both structures with more noticeable values for the emerged detached breakwater. For all the northwest wave direction scenarios, no noticeable erosion areas were visible along the shoreline. Overall, considering the balance of erosion and accretion rates, it can be concluded that for both wave predominance, the submerged detached breakwater and the MFAR presented better solutions regarding morphodynamics. The MFAR storm wave condition performed in XBeach indicated substantial erosion areas located around the structure, which added substantial changes in the bed level.
Highlights
In the past, coastal engineering was essentially based on empirical approaches and physical concepts of the hydrodynamic and morphodynamic processes involved [1]
The following sections present the key results of hydrodynamics and morphodynamics using SWAN and XBeach numerical models, as these results present an adequate clarification of the methodology developed without compromising the extension of this research work
The XBeach model for the analysis of significant wave height dissipation for a frequent wave condition was used for each structure in the west wave direction: submerged detached breakwater (Figure 6a1), emerged detached breakwater (Figure 6b1), and multifunctional artificial reefs (MFARs) (Figure 6c1); and for the northwest wave direction: submerged detached breakwater (Figure 6a2), emerged detached breakwater (Figure 6b2), and MFAR (Figure 6c2)
Summary
Coastal engineering was essentially based on empirical approaches and physical concepts of the hydrodynamic and morphodynamic processes involved [1]. Coastal interventions were based on structural projects without public consultation, and without a sound assessment of their environmental impact [1,2]. The need and urgency of such projects were a consequence of the large sedimentary deficits that occurred mainly as a result of activities in the river systems, notably, the increase in dam construction after World War II [3]. This has led to a large reduction of beaches in coastal areas and the destruction of many natural protections such as dunes [3]. By allowing urban expansions in marginal areas, often over dune fields, they contributed to negative consequences locally [2,5]
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