Review of Nature-Based Echo-Hydraulic Aqua-Forest Technology for Coastal Resilience and Sea Level Rise Climate-Induced Adaptation

1. Introduction

Extreme events, including tsunamis, flooding, erosion, and the subsequent cost of recovery in the EU and global coastal economic losses, have increased substantially. Hurricane Nadine caused flooding and erosion along hundreds of miles of shoreline, impacting communities and critical infrastructure. Climate change is expected to increase the number of coastal extreme events, and rising sea levels could exacerbate their impacts [1]. Coastal water quality is declining due to microbial pathogens, fertilizers, pesticides, and heavy metal contamination resulting from the aftermath of coastal events that threaten the ecosystem and human health. The physical, biological, and chemical processes that impact human and ecosystem health in nearshore regions include disruptive economic activities involving recreation, tourism, habitat, and ecosystem services, which must be sustained for future generations [2]. Addressing this research theme will improve understanding of the physical processes during extreme events, leading to improved nature-based flooding, erosion, and recovery models. The solution will require collaboration between scientists and engineers, nearshore communities, academicians, governments, and the coordinated development of nearshore coastal process observational and modelling research infrastructure to create novel nature-based solutions. This will lead to a new understanding and improved models of nearshore processes and green mitigation solutions. Collaborative transfer research involves stakeholders from knowledge institutes, communities, industries, and governments, using resources like a flume lab for testing and a scale-up to living lab sites that will provide the needed solutions to develop the best nature and soft engineering technology to protect future coasts.

Traditional hard engineering to protect the coast using concrete did not account for climate impact during the design. It is more likely that most structures will not live through their lifetime design year [3]. Therefore, there is a need to develop a system for climate change adaptation that will provide solutions to climate-induced risk. Growing seaweed to damp incoming waves to the coast is one way to address and mitigate this risk [112].

This review focuses on technology to plant seaweed as a nature-based solution that leverages efforts, avoids redundancy, and moves science and engineering rapidly forward, improving technology while protecting the natural ecosystem. A model test experiment followed the review, and the finding was published in [112]. Seaweed and seagrass green eco-engineering eco-technology systems will be designed for future deployment and tested in selected troubled coasts to assess mitigation that a natural-base and ecological approach can provide coastal protection and ensure environmental conservation. Nearshore processes research that intersects societal needs and scientific challenges have been organized into three broad themes, involving coupling and feedback between hydrodynamics, morpho-dynamics, and anthropogenic interactions, as well as between geological, meteorological, hydrological, and biological processes [3].

To develop the nature-based infrastructure that addresses current climate-induced coastal and nearshore challenges, the paper proposes building a sustained integrated nature-based solution system. This system involves nearshore processes research to address the challenges of climate and ocean: Variability, Predictability, and Change via the development of novel nature-based infrastructure that will foster understanding and prediction through observations and modelling of long-term coastal change, flooding; erosion from extreme storm events, and nearshore pollution and water quality evolution. Incorporating community participation and awareness will help foster and sustain the operation of the novel system and technology [4]. Besides using sea space for transportation and natural gas exploration, farming the sea and the ocean for other nature-based products is necessary. Cultivating seaweed and seagrass protects the coast from erosion, mitigates climate change, and provides bioremediation [5]. There are many seaweed species, and distinct species grow in different waters, so cultivation is essential.

This paper explores the cultivation of macroalgae that can provide ecosystem services, including eco-hydraulic coastal protection and bioremediation for coastal cities and islands and circular use of the harvested plants for bio-based raw materials (i.e., bioplastics and bio-textiles). [6,7,8]. This paper presents the applications of seaweed farms as an advanced nature-based mitigation approach, which furthermore provides additional farming space, facilitating the increasing demand for seaweed and seagrass as a natural resource. Wave impact damping by seaweed types and farming structures in controlled laboratory experiments was conducted [112].

2. Methodology

The study comprised a thorough, methodical search of academic, peer-reviewed literature on state-of-the-art in climate change adaptation, terminology, publication classification, and empirical and non-empirical investigations. A preliminary search was done using the Scopus and Google Scholar citations, Springer, Taylor & Francis, JSTORE, IEEE, MDPI, and Emerald databases to find relevant publications. The publications found in the search were then narrowed down by reading each article's entire text after studying its abstract. After being thoroughly reviewed, irrelevant materials were eliminated using scholarly judgment. The articles were reviewed, improved, and filtered to establish a breadth and width coverage as well as to capture key components of the overall picture of the literature on climate change adaptation and nature-based solutions (Figure 1).

3. Extreme Events, Climate Stressors, and Nearshore Problems

The UN Atlas of the Oceans shows that over 40% of the world's population lives within 150 km of a coastline, and population growth and tourism are accelerating in coastal regions (inland and sea) [9]. Worldwide, almost 1 billion people live at elevations within 10 m of the present sea level. Long-term erosion threatens communities, infrastructure, ecosystems, and habitats, and extreme storms can cause billions of dollars in damage and degraded water quality, which impacts the ecosystem and human health [10,11]. Nearshore processes involve the complex interactions between water, sediment, biota, and humans. Understanding the nearshore processes helps to predict and manage the vulnerable nearshore environment. This includes the causes of additional hazards leading to flooding inland. Coastal flooding and sea levelrise are expected to accelerate due to the warming climate [12,13]. Over the last five decades, observing nearshore processes and different solutions has provided insight into improved methods. Societal needs are growing with increased coastal urbanization and future climate change threats, and significant scientific challenges remain [14]. Climate change can further lead to an increase in coastal extreme events, sea level rise, and a reciprocal increase in the impact of extreme events [1,15]. The challenges of today are increased pressure on biodiversity due to environmental pollution, climatic change, and coastal squeeze, while the growing population needs higher productivity and protection from disasters. The integrity of terrestrial and aquatic ecosystems and their capacity to deliver a wide range of essential services to people [16,17] is expected to be undermined by the effects of unavoidable climate change emanating from nature’s response [10].

Therefore, it is imperative to assess the risk and vulnerability of coastal hazards and biodiversity losses, including current and future typhoons, monsoons, tsunamis, sea level rise scenarios, associated impact and damage to critical infrastructure, and analysis in terms of the damage that are detrimental to the economy and disruptive to the local community.

This is especially true in the context of global climate change and sea level rise. To limit warming to 1.5° C and consequential sea level rise, several intergovernmental organizations and governments have agreed to restrict global greenhouse gas (GHG) emissions [18]. Nonetheless, annual greenhouse gas (GHG) emissions are continuously rising [19]. Emanating climate change and coastal changes are an increased damage potential on critical infrastructure and detrimental effects on biodiversity, as well as socioeconomic effects and the disruption of local community activities. Some challenges facing today’s society include risk and resilience of coastal communities, species biodiversity and inclusiveness, growing population, an increasing need for biomaterial, and damages detrimental to the economy and disruptive to the local community. On top of it all, estuarine and coastal waters are particularly susceptible to non-point/point source pollution conveyed by rivers and streams [20,21]. Riverine transport is the primary mechanism for the direct impact of terrestrial human activities on the nearshore marine environment [22,23]. Excess nutrients and sediments transported by rivers, among other pollutants, seriously threaten the coastal and marine ecosystem [24,25]. The discharge of wastewater, alterations of physiographic features, and alterations in the distribution and amount of freshwater inflow are critical parameters [26,27]. Figure 2 shows the IPCC report warns about rising sea level and coastal ecosystem threats.

4. Threat of Climate Change, SLR, and Threat to Sustainable Development

Recently time has seen environmental calamity and abnormal environmental behaviour where the consensus of scientists has agreed to be linked to human activities. The connectivity and interdependency of the planetary system revealed uncertainty in the dynamics of air, water, and soil [28]. Especially the wave setup from the middle of the ocean involves a combination of temperature and wind, which in one way or another, imposes more energy on waves propagating inland. The impact of global warming includes rising sea levels, changing precipitation, and expansion of deserts in the subtropics. Increased temperature may also induce additional societal stress related to natural hazards, including earthquakes and extreme water events. Climate change involves climate variability and the statistical distribution of weather over a long time. Human causal factors of climate change affect greenhouse gas increases in the atmosphere (water vapor, carbon dioxide (CO2), methane, and nitrous oxide). Ozone GHG emissions have increased over the years; however, the accuracy of the increase is limited because of the environmental differences associated with all places. [29], reported the potential of seaweed and seagrass carbon sequestration.

The causes of climate change are atmospheric composition, including concentrations of greenhouse gases. The motion of tectonic plates results in changes in the relative location and amount of continental and oceanic crust on the Earth's surface, which affects wind and ocean currents; variations in solar output; the orbital dynamics of the Earth-Moon system; the impact of large meteorites and volcanism including eruptions of impact volcanoes [30]. Changes in climate along shorter time scales are reported by remaining oral traditions and climate proxies like ice cores, tree rings, sub-fossil pollen, boreholes, corals, and lake and ocean sediments [31]. The escalating greenhouse gas emissions cause ocean warming, by which coral is bleached, ecosystems lost, extreme weather events fomented, and sea levels made to rise ever upwards. The heating of the ocean has also been causing glaciers to melt and snow cover to shrink, warm-adapted plant and animal species to migrate upslope, and a decrease of cold and snow-adapted species increasing the risk of their extinction. Also, the retreat of the cryosphere is expected to affect recreational activities, tourism, and cultural assets. Ocean warming reduces the mixing between water layers, reducing the supply of oxygen and nutrients for marine life.

Climate change is affecting people, ecosystems, and livelihoods, creating a need to put a system in place to keep warming to 1.5° C rather than 2° C. Most coastal ecosystems, including seagrass meadows and kelp forests, are at moderate to high risk at this temperature. Since pre-industrial times, human activities have caused approximately 1.0° C of global warming, which has already caused consequences for people, nature, and livelihoods [32]. The ocean has taken up more than 90% of the excess heat in the climate system, causing increases in ocean acidity. Oceans have taken up 20 to 30% of these emissions, and continued uptake will exacerbate this. Marine heatwaves are harmful to warm-water corals, kelp forests, and the distribution of marine life. The projected ecosystem responses include losses of species’ habitat and diversity and the degradation of ecosystem functions. Today warming water corals are already at considerable risk. The heated water also evaporates from oceans into the atmosphere leading to heavy rainfall and flooding inland; alternatively, in other locations, the heat has caused dry land, heatwaves, drought, and wildfires. In this mechanism, oceans control the temperature, humidity, and weather [31,33].

Sea level rise is caused by ocean temperature, which is, in turn, caused by the emission of GHG, a phenomenon with a reciprocal impact on marine life. The global mean sea level rose during the 20th century, creating an international mean sea level rise of around 15cm. The sea level is rising more than twice as fast, and if emissions are not sharply reduced, it will further accelerate, reaching up to 1.10m by 2100. Extreme sea level events rarely occur during high tides, and intense storms will become more common. Many low-lying coastal cities and small islands will be exposed to flooding risks and land loss annually by 2050, especially without solid adaptation [18]. These events have led to an Intergovernmental Panel on Climate to provide the world with an objective scientific view of climate change and its political and economic impacts. The climate tipping point is the change from a stable state to an irreversible condition. To mitigate the tipping points, the United Nations Climate Change Conference (COP). Figure 3 shows that sea level rise risk outpaces current technology.

Natural variability has an impact and strong connectivity on the water cycle. Knowledge of uncertainty is essential to support the engineering design of climate change mitigation and adaptation actions to improve understanding and deduce efficient ways forward. Vegetated terrestrial systems such as forests sequester, capture and store carbon in their biomass and the soil beneath them [34]. A similar process occurs in the marine environment where vegetated marine systems, such as salt marshes, mangroves, and seagrass meadows, capture and store carbon [35]. The blue-green future approach to adaptation is becoming popular, the ocean covers 70 percent of the planet, and its system provides a buffer against climate change, providing food, energy, medicine, and employment, along with the oxygen for every second breath we take. The world’s ocean and cryosphere have been absorbing climate change for decades, and this has been causing high consequences to nature and society. Protecting and restoring ecosystems and carefully managing natural resources can reduce risks and provide multiple societal benefits.

For example, statistical analysis of flash flood events shows an increasing trend regarding their reoccurrence intervals [36,41]; (Figure 4). Between 2012 and 2016, the number of yearly flash floods resulting from extreme weather events increased by 100%, a statistically robust increasing trend [37,38]. The year 2020, for example, is only the second year in history in which the standard labelling system for hurricanes (containing 21 labels) is exceeded by the actual occurrences of 30 events [39,40]. Besides the frequency of storms and connected flash flood events, their impact on coastal areas is expected to increase with a rising sea level, regarding damage to critical infrastructure and ecological injuries like salinization [41,42]. Anthropogenic interferences can increase local risks if shorelines, river regulations, or extended land use alterations can influence natural floodplains or the nearshore hydrodynamic behaviour of waves [43,44]. The demand for alternative approaches is rising. Nature-based methods like tree belts and mangrove forests or the here proposed application of seaweed can not only protect high-energy wave events and flash floods but also maintain local ecosystems by regulating the local biodiversity or providing fish habitats, for example [17,45,46]. Figure 5 shows our closeness to the ocean and treatment of coastal events, the increase in coastal flooding, the risk of damage in coastal areas due to population growth, climate change, and the shortcomings of standard mitigation measures [45,46].

5. Ecosystem and Biodiversity Degradation

Since 1970, humanity’s ecological footprint has been higher than the world’s biocapacity, i.e., more environmental resources and services are in demand that can be regenerated by natural systems [47]. Regarding coastal ecosystems, large-scale seagrass losses have been reported for decades, usually due to eutrophication or turbidity from industries, dredging, catchment run-off, and natural disturbances [48]. For instance, worldwide seagrass loss between the mid-1980s and mid-1990s was estimated at 12,000 km² [49]. The loss of seagrass has led to numerous replanting restoration programs [49,50]. Traditional guidelines for restoration suggest that it is necessary to reverse habitat degradation by selecting transplantation habitats carefully and optimizing the transplantation techniques [51,52]. Thus, they lack recommendations for ecosystem services for native species. Recently, the importance of ecosystem engineering for seagrass beds was studied with the suggestion of native species ecosystem services to improve the efficiency described by [53,54,55], and, as a new guideline, these studies should be accounted for new restoration projects [56]. Other issues related to ecosystems and biodiversity are presented in Table 1.

6. Coastal Process and Wave Impact

The dynamic water-level characteristics in the nearshore include wave setup, run-up, and overtopping of sandy beaches and natural or constructed barriers at different meteorological forcing (wind, air pressure), sea, and swell. Waves are periodic deformations of an air-sea and internal interface. When the wind blows over the vast expanses of open water, it transfers energy to the water’s surface and creates water waves. The energy imparted from wind to water is high and proportional to the fourth wind speed power.

Because of a tsunami's long wavelength, which can be hundreds of miles, it is barely noticeable in the deep ocean and is rarely more than three feet (one meter) high; however, when it enters shallow water close to land, most tsunamis slow down. Earthquakes generate tsunamis, which are shallow water or long waves. Tides (astronomical forcing) can be shallow water or long waves [78,79]. Wave energy and dynamic frequency range are presented in Figure 6.

The coastal process represents the feedback response to energy-driven waves to the shore. Wave setup propagation and run-up contribute to severe wave forces and damage potential along the coast. Predictive real-time water and sediment-based pollutant modelling capabilities should be developed to ensure sustainable nearshore regions. This requires expanding our knowledge of the physics, chemistry, and biology of the nearshore wave dynamic and process. They need to be more resilient coastal communities. The resulting benefits will come from implementing the best solutions, including improved beach safety, healthier ecosystems, and improved mitigation and regulatory policies [80]. Assessment of long-term coastal evolution due to natural and anthropogenic processes includes global climate change impact on sea level rise, change in storm patterns, and increase in coastal urbanization. Improving knowledge of long-term morphological, ecological, and societal processes and their interactions is essential. Modelling and simulation based on assessing the requirements for enhanced coastal change will help deduce proactive solutions for resilient coasts and better guidance for reducing coastal vulnerability [81].

Figure 5 shows that the energy in the wave is stored between the top of the wave and a depth of about one-half the wavelength. When the water depth decreases to about half the wavelength, then the wave becomes a shallow-water wave. As the water becomes shallower, the wave rises and becomes higher; eventually, its potential energy is converted into kinetic energy, and we get a breaking wave. The wave’s energy travels, but the water does not. The water particles move in small circular motions as each wave passes by. The size of the circular motion decreases as we get deeper below the wave and dies out at a depth equal to half the wavelength [82]. Wind waves are propagating waves; during propagation, all points on the sea surface undergo periodic uplift and sinking and experience horizontal movement. A progressive wave is reflected at the barrier and produces a second wave of equal amplitude, moving in the opposite direction. The two waves combine to produce a standing wave. Tide is standing waves in deep water; they involve oscillatory movement that does not create horizontal displacement [83].

7. Coastal System Breaking Wave and Impact

Coastal regions are vital to the national economy, security, and commerce. Recreation that imposes dynamic evolution due to natural and land stressors activities, including dense population, can increase threats from sea level rise, long-term erosion, extreme storms, and anthropogenic influences [12,85]. Wave action in sandy areas or Long Strand can lose up to 10 m of the dune in a single storm and have short-term damage because accretion allows soft sediments to be replenished. Rocky, hard soil areas are subject to long-term effects of coastal erosion [86]. Figure 7 shows the wave-breaking Iribarren number [87].

The setup oscillates, consisting of a steady and dynamic component inshore wave; and conditions are influenced by shoaling and wave breaking. These processes are affected by several parameters, such as the sea steepness and the slope of the bathymetry. To take all the crucial parameters into account, [87] provided a series of graphs to determine the largest and the most significant wave heights (Hmax and Hs) for 1:10, 1:20, 1:30, and 1:100 sloping bathymetries [46]. The breaker parameter, surf similarity, or Iribarren number is defined as Jm–1,0 = tanA/ (Hm0/Lm–1,0) ½, where A is the slope of the front face of the structure and Lm–1,0 is the deep-water wavelength gT2m–1,0/2π. The combination of structure slope and wave steepness gives a specific type of wave breaking.

Coastal problems can lead to the disruption of economic activities near the coast, and this includes recreation, commerce, and safety. Many coasts have struggled to live with the natural variability and associated hazards over the years. The predictive system and physics of the coastal system have been used to define the dynamics of waves and wave-driven hydrodynamics on a sloping beach, as well as the response of an erodible bottom to those motions. Coastal research usually focuses on fluid dynamics or the sediment response to those motions, and essential processes result from the dynamic wave energy propagating across the beach profile and topology [88].

The modelling of nature-based infrastructure includes improved process representation, better bio-physics model coupling, incorporation of data assimilation techniques, and testing of real-time models. Recently, we have witnessed failures of cliffs in the North Sea predominantly happening in the winter because the trees cannot withdraw soil water during that season, causing wetter ground conditions. The study found the dramatic effects of wetter and drier-than-average summers. In 2017, when 126 % of the typical rain amount was measured, 65 failures happened. In contrast, the exceptional drought summer of 2018 (51 % of the average rain amount) resulted in only 11 cliff failure events during the following winter [89]. Figure 8 shows the erosion impact in the South China Sea and North Seas.

8. Hard Coastal Structures vs. Nature-based Solutions

Hard coastal structures, such as sea dikes and seawalls, build a barrier between sea and land, thus protecting the hinterland against flooding. Hard building materials, such as concrete or stone revetments or grass covers, provide surface protection of the coastal structures against the prevailing hydraulic loads: wave impacts, run-up, overtopping, and currents [91,92]. Coastal dunes provide a similar flood protection function but are more prone to the loads from the sea because the building materials (sand, often vegetated) are less erosion resistant. They are regularly nourished before and after storm surges to restore the coastal protection function. Coastal dunes are considered a (managed) natural coastal protection solution. Sea dikes and seawalls are classified as “grey” infrastructure. While incorporating nature-based solutions, grey infrastructure can be ecologically enhanced or integrated into a hybrid system, i.e., a combination of grey and natural structures [93]. However, there is little experience in establishing and managing coastal ecosystems and uncertainties concerning the constant, long-term coastal protection function, as shown in Figure 9 [94,95].

As the waves propagate towards the shore and encounter shallower water, the wavelength and hence, the wave speed decreases. However, the wave energy and height increase [96]. Friction between the water particles and the seabed results in energy loss. The presence of a vegetation meadow near the surface attenuates the wave energy and causes wave breaking. The presence of pockets of mud, large stands of seaweed, pile clusters, or submerged trees interferes with the wave orbital velocities, which causes an increase in turbulence and loss of energy. Previous studies to establish the effectiveness of coastal vegetation on wave attenuation include studies on internal wave attenuation by coastal kelp stands [97], wave damping as a result of local energy losses due to a cluster of cylinders, which represents a dense stand of giant kelp [98], flume observations of velocity and turbulence intensity profiles in eelgrass beds in a large seawater flume, and field investigations of water flow in a Spartina maritime salt-marsh in southern Portugal [99,100] investigated the processes of turbulence reduction and attenuation of orbital velocities in a Spartina Anglica salt marsh in east England. [101] Conducted laboratory experiments to measure wave attenuation resulting from synthetic and nearly emergent wetland vegetation under wave conditions and plant stem densities. The effects of submergence ratio on wave attenuation, transmission, and energy dissipation over the model of seagrass Posidonia Oceanica, were investigated in a large-scale flume facility [102]. A meta-analysis by [103] shows average wave height reductions of 72% for salt marshes and 70% for coral reefs, followed by average wave height reductions of 36% for seagrass and 31% for mangroves. The wave-damping capacities of coastal ecosystems depend, amongst others, on the wave conditions (wave height and length) and the ecosystem properties (width, height, and habitat density). As average values for water depths and wave heights were used within the study, further analyses for extreme events are required to consider design cases.

Wave damping is highest when the plants occupy the entire water column. Seagrass grows shallow so that the relative height (ratio of vegetation height/water level) is relatively low. It is expected that the wave-damping capacities of seagrass are low during extreme events with increased water levels and high waves. However, seagrass can also affect morphodynamics and thus contributes to coastal protection. Seagrasses increase sediment deposition by flow reduction, trap particles, and stabilize sediments [104,105]. Kelp beds may reduce the longshore current and wave energy, in addition to creating onshore currents, which can promote the movement of sediment onshore [106]. Figure 10A shows the concept of seaweed and seagrass, and Figure 9B shows the concept seaweed bed system for coastal protection.

vSea dikes, seawalls, coastal dunes, and associated coastal protection structures present the main coastal protection structures along the North Sea. With increasing design conditions due to climate change [107] and the aging of the infrastructure [108], a continuous costly adaptation of the structures is required. Recently, rising environmental awareness among the population has led to a reassessment of the typical coastal protection approaches towards more nature-based solutions, including hybrid solutions, i.e., a combination of natural and built infrastructure [93].

Foreshore ecosystems provide ecosystem services with coastal protection functions, such as wave damping [103], and thus can reduce the wave loads on coastal structures and the maintenance required [109]. Risk and economic analyses for future climate scenarios reveal the necessity and costs for prospective coastal adaptations at the investigated site. The influence of the foreshore structures on the hydraulic loads at the coastal structure is analyzed, providing innovative design guidance for coastal structures regarding foreshore ecosystem services. Cost-benefit analyses can give important information on the cost-effectiveness of hybrid coastal protection systems and provide a planning tool for design processes.

Climate change calls for sustainable adaptation strategies for coastal protection. Using nature-based solutions presents an innovative measure that integrates the three pillars of sustainability (society, economy, ecology) into coastal engineering [45]. At this, foreshore ecosystems provide ecosystem services that lead to ecological and socio-economic effects by strengthening existing coastal protection and thus reducing flood risk [110]. Yet, using foreshore ecosystems for coastal protection is limited to small-scale projects. With the current project, the basics for the design and upscaling of foreshore seagrass, seaweed, and an offshore aquaculture platform shall be obtained to promote the large-scale implementation of nature-based solutions for coastal adaptation with positive effects on the environment and humankind.

The coastal protection function and effects of foreshore seagrass, seaweed, and an offshore aquaculture platform on hydrodynamics and, thus, on the design and maintenance of coastal structures are investigated in physical, numerical, and in situ experiments with natural and artificial structures. The results are the input for elaborating design guidance for hybrid coastal structures. Wave damping capacities of the foreshore ecosystems and the effect on the hydraulic loads at the structure are analyzed and processed to provide planning instructions for engineers. The tasks are completed with standard design manuals [46] and in consultation with the site authorities to ensure practical handling and implementation. Figure 10A,B show the concept of integrated hard and soft engineering for coastal protection. Attenuating the hydraulic loads on the coastal structure from foreshore ecosystems reduces the stress on the infrastructure and thus reduces maintenance requirements and prolongs lifetime. The effects on maintenance efforts and the structure's lifetime are quantified in cooperation with the accountable site authorities. Furthermore, considering the coastal protection function of the investigated foreshore structures, the costs for future adaptation of the coastal structure are determined and compared to those without ecosystem-based foreshore engineering using cost-benefit analyses. The findings provide a new decision-making tool for planning processes and facilitate the broader use of nature-based solutions under consideration of socio-economic factors.

Coastal engineers are facing new challenges regarding nature-based solutions for coastal protection as design guidance still needs to be provided, and knowledge gaps exist [95]. The long-term, continuous coastal protection function of ecosystems and the effect on adjacent infrastructure is mainly unexplored. The review provides a new understanding of the interaction of ecosystem engineers, the environment, and coastal structures. It offers coastal engineers’ quantifiable information on the coastal protection services of the investigated foreshore ecosystems as a base for the design process of coastal structures and a decision-making tool for future planning processes.

Seaweed is a water plant with many applications. Growing seaweed can provide a natural way for adaptation that can help curb the increasing sea level rise and associated risk. Seaweed aquaculture can also help reduce the emissions from agriculture by improving soil quality feeding to minimize methane emissions [96,97],). Seaweed aquaculture helps climate change adaptation by damping wave energy to protect shorelines and by elevating pH and supplying oxygen to the waters, thereby locally reducing the effects of ocean acidification and de-oxygenation. Seaweed aquaculture faces challenges of the site, suitable areas, engineering systems that can withstand offshore conditions, and increasing market demand for seaweed products [98,99]. The ongoing climate change and extensive anthropogenic interferences in the ecosystems of coastal areas will further amplify the risk and damage of high-energy wave events, e.g., due to sea levelrise and increased frequency of extreme weather events like cyclones.

9. Result - Hydrodynamic Model Test of Storm Surge Mitigation using Seaweed

Model tests are conducted to provide valuable information on the behaviour of the system to deduce the hydrodynamic coefficient that can be used for the design of a working system. The model test is conducted at the IWW Flume lab to examine the damping effect of seaweed. Samples of seaweed obtained from the North Sea (Figure 12) are attached to the flume and the fluid pump at different speeds is imposed on the model.

The hydrodynamic force that is exerted on the seaweed is defined as the sum of drag force and inertia force and can be described by the Morison equation.

$$F=\frac{1}{2}{C}_D\rho A_v\left|u\right|u+C_M\rho V_v\frac{du}{dt}$$

(1)

where F is the total force on the vegetation, C_D is the drag coefficient, ρ is the water density, A_v is the area of the vegetation that is normal to the streamwise flow direction, u is the flow velocity, C_M is the inertia coefficient and V_v is the volume of the vegetation element. The first part of the equation refers to the drag force, the second part describes the inertia force. Usually, the second part, inertia force is smaller than the drag force and is therefore often neglected in studies.

Figure 12 shows sample of seaweed used for the experiment. The model seaweed mimic the number (Fr) Froude-scaled properties.

$$Fr=\frac{v}{\sqrt{g·l}}[-]$$

(2)

where v is the velocity [m/s], g the gravitational acceleration [m/s²] and l the length [m]. Models that are scaled with Froude scaling law, keep inertial and gravity forces similar.

Figure 13 shows the finding of the experiment involving a model of 2 lines of seaweed plantation. Local operating environment based on the North Sea site conditions that consider metaocean and hydrodynamic interaction of the sites. Metaocean data include historical data (tidal elevation, velocity, velocity, current speed, current direction, temperature, and salinity). Figure 14 shows the equivalent sensitivity force ratio.

10. Discussion

The hydrodynamic model test is to determine the hydrodynamic loads due to waves and currents acting on the seaweed platform and wave damping emanating from introducing the macroalgae system as impeding structure in the direction of the wave. There is a lack of available data on hydrodynamic loading for seaweed platform design. So, the best way to quickly obtain such data is through the use of model tests. For this purpose, samples of seaweed cultures are obtained, and these are subjected to dynamic tests to determine the hydrodynamic force and damping from seaweed.

According to the length of the model, up to 5 clumps of seaweed can be hung on the line rope of 1 m. The material used for the model can be wood instead to reduce the noise and vibration that can disturb the reading measurements. The tsunami-like bore is generated with the help of two pumps. Each pump is capable of generating a discharge of 400 litres/s. The bore of different strengths was generated by controlling the opening valve positions of the two pumps. The period of bore generation is controlled in the current experimental facility. Impose environmental condition includes bore celebrity - 1.5 to 2.2 m/s, wave height - 0.1 to 0.2 m. The wave period is interpolated from changing the period of bore generation in the experimental flume (See Figure 10).

During the hydrodynamic model test, samples (clumps) of fresh seaweed (see Figure 12), collected from the North Sea weighing about 1.5 kg in air, when fully grown. Froude’s law is introduced as the scaling method. Froude’s law is the most appropriate scaling law for the free and floating structure tests [112]. However, the natural buoyancy of the seaweed, makes its weight in water almost insignificant. But the movement of the plant normally drives the dynamics of the system. The samples are subject to a series of tests to determine the hydrodynamic loading coefficients in several different configurations. Suitable materials such as plastic ribbons and rubber tubing can be used to build a sufficient quantity of scaled seaweed. The model with seaweed and without seaweed is compared, and it is clearly observed that the model with seaweed damped the wave and has a lower force amplitude (Figured 13). The bore is generated for 25 s to 30 s. 15% wave damping is achieved in (Figure 14). More percentage can be achieved by adding more rows and increasing the density of seaweed, this can create shielded patterns for better wave damping.

11. Conclusion and Recommendation

The study involves reviews of topics related to climate change vulnerability, required adaptation, and the use of seaweed as a nature-based solution. The use of natural vegetation for coastal protection continues to develop in coastal and risk communities. Thus, there are only a few experiments and minimal data that have been accomplished in this area, especially, the use of seaweed for wave damping and coastal adaptation. The study also involves the conduct of experiments to validate the need to use seaweed as a nature-based solution for coastal adaptation. This paper provides a review of the state-of-the-art and presents experimental results for the need to design a future vegetated protection system using seaweed. This adds value to the spectrum of the ecosystem and coastal infrastructure protection and adaptation.

The review refers to problems associated with existing hard engineering coastal protection, the fact that most of them are designed without consideration for climate change impact, which exposed them to the risk of sustaining them through their design lifetime. The use of vegetated soft engineering can help existing coastal infrastructure to meet its design life and prepare the system against climate-induced SLR rise. Seaweed growth has the potential to provide a solution to major contemporary challenges facing the coast. The natural ecosystem and critical infrastructure on land face threats from climate change SLR-induced events. Growing seaweed can help open up new space for farming to meet the growing need for bio-based food and non-food material. Collaborative future work will enable the efficient transfer of results and predictive tools to stakeholders, supporting informed decisions that will improve diverse aspects of coastal management. The experiment limitation is that the model test static analysis of the effect of an immediate change to the system without respect to the longer-term response of the system change. Dynamic analysis of how the system responds to the change and environmental load. The model test is used to derive the coefficient to use for the modelling of the system. The test provides insight into the performance of vegetation as a natural-based solution for coastal protection. Material deterioration over time is an important factor to consider in the holistic study. The study recommends the incorporation of holistic risk and comprehensive modelling of robust design of vegetated floating structures as nature-based solutions aimed at dampening waves and providing sustainable adaptation for coastal structure and biodiversity.