**1. Introduction**

In the last 140 years, scientific research has established that average sea levels have significantly increased [1–3], and this phenomenon is accelerating. This is a critical issue as even small increases can have devastating effects on coastal habitats [4–7]. Rising sea levels have been identified as a major cause of flooding events across the world [8,9]. Flooding poses a threat to property, safety, and the economic wellbeing of coastal communities [10]. In fact, considering that coastal areas provide a great amount of economic and leisure activities, they contribute significantly to the local and national economy. Thus, more people are continuously attracted to coastal zones contributing to an intense urbanization of these areas. To aggravate this situation, the ecosystems are also threatened by the impact of human activities in coastal areas as well as by the increase of natural extreme weather events (e.g., intensity and duration of storms, floods) generated by climate change, which interfere with local wave climate and changes in morphological beach characteristics [11]. More frequently, high tides reach values that cause costal recession and high sediment transport deficit, and hence, it is necessary to protect these areas with various coastal structures to reduce or at least to mitigate coastal erosion problems. As a result, impacts of climatic variations are usually the greatest along the coast [12–14]. However, many of

the current coastal protections (e.g., groins, seawalls, and emerged breakwaters) were built with the single purpose of protecting the coast, without environmental or economic concerns, maintenance costs, or the negative consequences that such structures could cause up to considerable distances along the coast. Coastal regions and their managers consequently face ever-increasing challenges to accommodate safely both the growth of these areas and their development [15].

Traditionally, bulkheads, seawalls, and revetments have been the most commonly used type of shoreline infrastructure implemented as a primary response to coastal hazard. Other applications such as shoreline armouring have also been adopted to protect coastal property from hazards like erosion and flooding [16]. However, there has been a growing interest during the last decade in developing sustainable approaches to guarantee solutions that could deal with the daily and emergency issues in parallel with promoting downtown living [17–19]. For example, in Hong Kong, the land policy emphasizes ecological protection [20–23] and reclamation, enhancing the innovative value in sustainable coastal land use management.

In line with these new approaches, recent studies conducted by scientists and practitioners have demonstrated the benefits of nature-based strategies for restoring degraded coastal ecosystems and mitigating risks including natural defences and "living shorelines" [24,25]. Without any human interaction, shorelines are mainly comprised of biogenic habitats (e.g., saltmarshes, mangroves, oyster and coral reefs) in their natural conditions. These natural coastal habitats secure the provision of essential habitat for marine life, promotion of favourable water quality, and reduction of shoreline erosion and flooding by attenuating waves, stabilizing sediments, and dampening surge [24,26,27]. As such, they are widely valued for their environmental benefits. By adopting alternative sustainable approaches, it is possible to enhance the quality of natural environments along the coasts that can help reduce the impact of coastal hazards [28–32].

It is clear that a crucial goal is to identify nature-based structures that can protect coastal areas and provide a low-cost option to effectively reduce the damaging effects of extreme meteorological events on coastal populations by absorbing storm energy [33], thus enhancing the quality of lives of people living in the surrounding areas. These green areas (including vegetation such as coral reefs or aquatic plants) typical of nature-based solutions could aid the production of sediments (sea grass beds and coral reefs) or could store and hold the sand together (mangroves and coastal dunes) [34]. For example, the benefits provided by coastal herbaceous wetlands in helping to reduce economic damages generated by hurricanes and their impacts have already been demonstrated [34,35].

One type of solution that has not been considered is the mix of artificial and green solutions. Human design structures can guarantee resistance to strong wave impacts and reduce the amount of flooding in coastal areas. However, if mixed with natural ecosystems/green solutions that can still help to reduce wave energy, coastal erosion, and flood hazards [36–41], it could also be possible to recover the natural functioning of the entire coastal area and target future conservation and restoration processes [35–37]. In brief, this option promotes coastal protection through the recovery of the natural functioning of natural ecosystems by means of conservation and restoration actions [38,42]. The trade-offs between socioeconomic development and conservation can be integrated [43–45], which will help with improving coastal development and promoting a sustainable coastal development.

This study provides a comprehensive review of existing hard and soft solutions adopted for coastal protection. Furthermore, it will experimentally investigate and compare preliminary sustainable approaches that could deliver both protection from coastal flooding and the added benefit of conserving, sustaining, and restoring valuable ecosystem functions and services to local communities [46–51].

#### *Hard and Soft Engineering Solutions for Coastal Protection*

To identify structural designs that assess new sustainable approaches for coastal protection and to highlight the advantages and disadvantages of existing hard and soft engineering solutions adopted to protect coastal lines, a review was conducted on the techniques available to date. Table 1 summarises the results obtained.


**1.**Areviewofexistingcoastalprotectionmeasureswithadvantagesanddisadvantagesidentifiedforeach



#### *Water* **2020** , *12*, 2471

To date, as previously mentioned, natural solutions have been adopted to preserve and/or restore coastal areas. For example, the presence of wetlands has demonstrated to retard waves and the mass flux of water with the presence of vegetation [87]. Despite a few studies on the effect of these vegetated surface, there are not specific guidelines available to determine the optimal shape of the vegetation to consider, the density to be selected, or the height of the vegetation to make it fully under water or emergent. Therefore, to seek this information, this preliminary experimental study was conducted to propose an approach that could combine hard and soft engineering characteristics; thus, it can be the base for a sustainable solution to be adopted. Despite initially using non-real vegetation due to the limitations explained below, hard and soft engineering techniques should be combined in a more ecological way (e.g., facilitating the growth of aquatic plants next to artificial structures), to achieve a less invasive structure on the environment and mitigate the negative influence of hard engineering on ecosystems [49]. In order to identify a feasible "softer" hard sustainable engineered solution, the paper experimentally compared three solutions tested in a wave tank with a physical model, which are presented in Section 2, on the foreshore of the beach and thus did not impede the wave energy or prevented land to sea interaction. The main purpose of the submerged breakwater systems identified is wave attenuation, with the idea of creating splashing and hydraulic conditions that can support sediment capture, helping at the same time in the mitigation of storm surge [30].
