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Concept Paper

Coastal Protection Using Integration of Mangroves with Floating Barges: An Innovative Concept

by
Rahul Dev Raju
1,* and
Madasamy Arockiasamy
2
1
Department of Ocean and Mechanical Engineering, Florida Atlantic University, Boca Raton, FL 33431, USA
2
Department of Civil, Environmental and Geomatics Engineering, Florida Atlantic University, Boca Raton, FL 33431, USA
*
Author to whom correspondence should be addressed.
J. Mar. Sci. Eng. 2022, 10(5), 612; https://doi.org/10.3390/jmse10050612
Submission received: 17 March 2022 / Revised: 23 April 2022 / Accepted: 29 April 2022 / Published: 30 April 2022
(This article belongs to the Section Coastal Engineering)
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Abstract

:
Mangroves and moored barges are used individually for coastal protection and beach restoration. This conceptual paper discusses about the integration of mangroves with moored floating barges for coastal protection. The concept involves towing of a barge to a particular location, mooring it to the seafloor and planting mangroves along the shore or beach. The barges will be unmoored and towed away once the mangroves attain certain growth and are well rooted in the soil. Mangroves can protect the beach from incoming waves using their roots and branches. The incoming waves can be reduced by 50% to 99% using mangroves of 500 m width. Mangroves have a life span of 20–100 years, and they do not need any yearly maintenance as do any other conventional coastal protection measures. Mangroves are considered as soft coastal protection structures and are environmentally friendly. Mangroves will also improve the aesthetic appearance of the beach. This paper discusses about some of the research methodologies for the development of the barge-assisted mangroves coastal protection method. The dimensions of the barge, gap width between the moored barges and the environmental condition at the location determines the performance of the barge-assisted mangroves coastal protection method. The gap width between the barges, draft of the barge and breadth of the barge influence the resonant frequency of the fluid between the barges. The shielding effect of the floating barges can be used for other applications, such as berthing of ships and growing living shorelines using oysters, rocks, sand, plants, coir, etc. for coastal protection.

1. Background

Coastal areas are subjected to serious erosion due to man-made and natural activities. The sea level rise caused by the melting of glaciers also adds to the coastal erosion. Coastal protection structures like seawalls, reefs, bulkheads, groins, breakwaters etc. are being used extensively for protecting the coastal areas from becoming eroded. One of the main drawbacks of these conventional structures is that the life span of these structures is comparatively less, and yearly maintenance is also needed. These structures in some cases give a temporary solution to the erosion at the location where it is installed while accelerating the erosion at another location. The initial cost of installation of these structures is high and requires yearly maintenance. Some of the coastal structures are made from non-biodegradable materials which cause serious environmental problems.
For the past few years, there has been a shift towards sustainable coastal protection methods which are environmentally friendly. Mangroves are effective in protecting the beach and have life span of 20–100 years. Mangroves reduce the waves approaching the beach with the help of their roots. These roots act as a barrier to incoming waves and reduce the wave height before reaching the beach. Mangroves can trap sediments in the waves which stabilize the beach from getting eroded. Mangroves are environmentally friendly and require less yearly maintenance. Mangroves have multipurpose benefits such as coastal protection, aquaculture, tourism, water purification, etc.

2. Introduction

Floating breakwater mimics the functions of a natural or artificial breakwater. Breakwaters are coastal protection structures which reduce the impact of waves or provide the space for safe docking of ships and other vessels in a harbor. Breakwaters are normally made with rocks and concrete. To overcome this issue of variation in water levels, floating breakwaters are used. Floating breakwaters are used mainly in places where the breakwaters are needed for a short period of time or in situations where a conventional breakwater cannot be constructed. Floating breakwaters provide protection from incoming waves by using a floating member. The sea floor contours, and other environmental conditions have less impact on the installation of floating breakwaters. The main disadvantage of the conventional breakwater is that it interferes with natural water circulation, whereas the floating breakwater has less influence on water circulation. Floating breakwaters can be aligned and moved to desired locations easily, compared to conventional breakwater [1]. Generally, floating breakwaters are classified based on geometric shape and performance [1,2]. Floating breakwaters are used in the United States for protecting small boat docks or basins [2]. Briggs et al. used ‘V’-shaped floating breakwater in the offshores of Cape Canaveral, Florida [3]. The ‘V’-shaped breakwater reduces the wave conditions at the site from sea state 3 to sea state 2. The wave data from the site was verified with the numerical results from WaveAnalysisMIT (WAMIT) software [3]. Wave height reduction of up to 50% was obtained using a ‘V’-shaped floating breakwater. McCartney classified the floating breakwaters mainly into four types: “box type floating breakwater, mat type floating breakwater, tethered float type floating breakwater and pontoon type floating breakwater” [2]. Dai et al. added three more types of floating breakwaters to the classification—“frame type floating breakwater, horizontal plate type floating breakwater and other type floating breakwater” [1].
Sannasiraj et al. conducted an experimental and theoretical study on pontoon type floating breakwater with three types of mooring layouts [4]. The three types of mooring configurations were: (i) “mooring attached to pontoon’s bottom, (ii) cross mooring at pontoon’s bottom, and (iii) mooring at pontoon’s water level”. The results show that the mooring configurations do not have any significant influence on the wave transmission coefficient. The cross moored floating pontoon type breakwater configuration had a higher wave transmission coefficient compared to other two configurations. Sun et al. numerically compared box type floating breakwater with an “I”-shaped floating breakwater and found that an “I”-shaped floating breakwater has better wave reduction properties [5]. “I”- shaped floating breakwater could save around 10% to 20% of the material, compared to box type breakwater. In tethered float type breakwater, the floating object is individually tethered to a submerged ballast system [6]. The ballast system helps to hold the floating object to a particular depth. The ballast system can be deballasted and moved to another location for reusing. The floating object can be properly arranged with effective spacing between them. Candle & Piper studied mat type floating breakwater made of scrap tires [7]. The required buoyancy for floating was provided by filling buoyant material inside the tires. The tires are interlocked using ropes and steel rods. Even though the mat type floating breakwater was made up of scrap tires, which are inexpensive, these types of floating breakwaters pose a serious environmental hazard. A classic example for this is the Osborne reef in Florida, in the United States, which not only failed functioning as an artificial reef but also created serious environmental problems [8,9,10]. Among the floating breakwaters discussed, the pontoon type floating breakwater and box type floating breakwater are most commonly used because of the ease of installation and fabrication [1]. These two types of floating breakwaters attenuate the waves by damping. Wave attenuation by reflection is also observed as a secondary effect. The fabrication of these two types of floating breakwaters is easy compared to other types.
Floating breakwaters are not only used for coastal protection but also extensively used for aquaculture these days [11,12,13]. Floating breakwaters are also used with floating storage units for storing fuel and hydrocarbons [14,15] and floating energy generation devices for protection from the action of incoming waves [16,17]. Murali & Mani experimentally investigated a taut moored cage type floating breakwater for 1:15 model scale in a wave-current flume [18]. The study was conducted to investigate the effect of waves and currents in the wave attenuation properties of the floating breakwater. The cage type floating breakwater consisted of two pontoons connected by pipes. The cage type floating breakwater was moored to the bed using eight taut moored lines. The wave reflection coefficient, wave transmission coefficient, water surface elevation and velocity of water particles were measured for different wave and current conditions. The cage type floating breakwater was found successful in lessening the incident wave height by half [18]. Dong et al. conducted an experimental investigation on three different configurations of box type floating breakwaters [19]. Six catenary type mooring chains were used for mooring the floating breakwater to the bed. The studies show that the span of the catenary mooring chain which is laid on the ocean floor and width of the floating breakwater influences the wave transmission coefficient. For achieving a wave height reduction of more than 50 percentage for long waves, the width of the floating breakwater should be large [19]. Liang et al. conducted experimental and numerical study on a spar buoy type floating breakwater to study the wave transmission coefficient and wave reflection coefficient [20]. The floating breakwater was made of submerged pipes which are moored to the bed using taut mooring lines [1]. During the parametric study, a wave transmission coefficient of less than 0.3 is obtained when the ratio of distance between the adjoining rows to the wavelength is 0.3 and the ratio of buoy diameter to spacing between adjacent individual components in the identical row is 7 [20].
Mangrove is a type of plant that grows at the intersection of ocean and land [21]. Mangrove is a halophytic plant which can tolerate high salinity. Around the globe, 70 varieties of mangrove species are available [22]. Globally, 1% to 2% of mangroves are disappearing every year [23]. Mangroves reduce the height of waves approaching the beach and protect the beach. The roots of the mangroves reduce the ocean current and accelerate the siltation of sediments [24]. McIvor et al. studied the effectiveness of mangroves to reduce wind and swell waves [25]. The study shows that mangroves could reduce the wave height by 50% to 99%. The studies were conducted for wave heights less than 70 cm. Mangroves with roots above the soil are effective in shallow water, and where these types of roots are absent the mangrove branches help in wave attenuation. Mazda et al. studied the wave reduction by Kandelin candel, a type of Rhizophoraceae mangrove in the Tong King delta in Vietnam [26]. The studies show that the Kandelin candel mangrove species were able to reduce the swell of a period of 5 s–8 s significantly. Mangrove trees were extended to a width of 1.5 km. Six-year-old mangrove trees reduce wave heights of 1 m in the open sea to 0.05 m near the coastline. The study also shows that the wave attenuation by mangrove depends on the spacing of the trees, type of mangroves, diameter of mangroves and wave properties, such as wave height, wave period and water depth at the location [26,27]. Putz & Chan studied the diameter growth rate of Rhizophora apiculate in the Matang mangrove forest in Malaysia from 1920 to 1950 [28]. The studies show that the annual diameter growth rate Rhizophora apiculate was 3.2 mm per year and the biomass above the ground also increased considerably every year.
Mangroves not only contribute to coastal protection but also to other areas, such as fisheries, fuel, tourism, timber, carbon storage and water purification [29]. The fish population around the mangroves also benefit from the mangroves as the ecosystem also gets improved. “Fish, oyster, crab, shrimp population also grow abundantly around mangrove forests” [29]. The biomass around the mangrove forests provides food source for the fisheries. One of main problems the wood industry is facing is the decay of wood by termites. The wood produced by mangroves is resistant to the action of termites. “The leaves of the mangroves are used for roof making, alcohol and sugar production” [29]. Mangroves are also considered as good carbon sinks. Mangroves can fix the carbon dioxide from the atmospheric air to the soil and act as a carbon source to the ocean [22,30]. Mangroves store carbon in their biomass from the atmospheric air [29,31]. Mangroves are effective in purifying wastewater. The complex root system of the mangroves is effective in trapping sediments in the wastewater [29]. Mangroves can fix the nitrogen and other substances in the wastewater. Mangroves are also coupled with aquaculture for water purification [32,33]. Apart from the above-mentioned uses, mangrove areas are also used for recreational tourism activities. The tourism around mangroves generates money which significantly contributes to the local economy. One such example is the mangroves in the state of Florida, which significantly contribute to the economy of the state [29,34].
Integration of fixed coastal protection structures like breakwaters, sea walls, geo bags, artificial reefs, etc. is used extensively for coastal protection. Coastal structures are constructed initially to resist the incoming waves so that the mangroves can grow effectively behind the coastal protection structures.
Coastal structures such as breakwaters, artificial reefs, sea walls, geo-bags, dikes, etc. are used for this integrated coastal protection method. Figure 1 shows the integration of a dike made up of geotextile geo-bags and mangroves for coastal protection [35]. The method presented herein is the integration of mangroves with floating barges for coastal protection, which is a similar type of methodology used in the integration of mangroves with fixed coastal structures. Yuanita et al. conducted an experimental investigation on combining mangroves as the main natural coastal protection structure and geotextile geo-bag dikes as a temporary fixed structure for coastal protection (Figure 1) [35]. Experiments were conducted on a 1:10 model scale in a wave flume to study the effect of geo-bag dike mangrove integration. Mangroves were made using iron bars, and geo-bags of different weights were used to make dikes of different slopes.
The effective configuration of the geotextile geo-bag dike configuration for the reduction of wave height was found in the experimental studies. The wave height reduction depends on the incident wave height and wave steepness. Figure 2 shows the plot between the wave transmission coefficient of the geo-bag dike and wave steepness for various incoming wave conditions. The geo-bag dike has a slope 1:2 and the unit weight of geo-bag is 0.5 kg. Wave measuring gauges are placed in front of the geo-bag dike, after the geo-bag dike and behind the mangroves, to measure the wave conditions at that location. The wave transmission coefficient of the geo-bag dike is the ratio of wave height recorded at the location between the geo-bag dike and the mangroves to the incident wave height. The plot between the wave transmission coefficient and wave steepness shows a decreasing trend with increasing wave steepness. Figure 3 shows the plot between the wave transmission coefficient of mangroves and wave steepness for various incoming wave conditions. The wave transmission coefficient of the mangroves is the ratio of wave height recorded at the location after the mangroves to the incident wave height. The plot between the wave transmission coefficient and wave steepness shows a decreasing trend with increasing wave steepness. The temporary structure was able to reduce the wave height up to 65 percent and the mangroves were able to reduce the wave height up to 71 percent. The combination of geo-bag dikes and mangroves is effective in protecting the beach from incoming waves. Hashim et al. reported the coastal rehabilitation method by integrating mangroves with detached breakwaters in Sungai Haji Dorani, Malaysia [36,37,38]. The breakwaters used for coastal protection and the effective growth of mangroves were emerged structures without filter layers and were made using homogeneous stones. The foundation was constructed using bamboo stems. The detached emerged breakwater consists of homogenous stones in the seaward side of the structure and stones in gabion basket in the leeward side, to promote stability [36,37,38]. The emerged breakwater consisted of three individual segments of breakwaters separated by small segments. A considerable amount of sand was deposited behind the detached breakwater after the installation. Around 30% of the transplanted mangrove saplings were able to survive after one year of transplanting to the site [36,37,38]. The emerged detached breakwaters were able to reduce the effect of wave action on beach and provide an effective condition for the growth of mangroves. Larsen & Hubeli studied the performance of floating lightweight concrete modules in the shape of root system integrated with mangroves in Isla Grande, Columbia [39]. A total of 10–20 individual concrete floating modules were integrated to form a floating island called ‘Rhizolith Island’. The mangroves were planted in the floating lightweight concrete modules. Since the mangrove-floating lightweight concrete modules will act as a floating breakwater, sand deposition will occur behind floating concrete modules. Once mangroves in the concrete module attain a certain growth, the mangroves will break through the modules and start growing through the water below it, finally developing roots into the ocean bed soil.
Akbar et al. compared the effectiveness of breakwater in protecting the shore and growth of mangroves before and after its existence in West Kalimantan, Indonesia [40]. The study shows that the breakwater was effective in reducing the coastal erosion by 70 percent. The mangroves were able to grow effectively by the wave shielding effect of the breakwater and the density of mangroves increased substantially. Fitri et al. studied the effectiveness of low-crested detached breakwater in trapping soil nutrients needed for the growth of mangroves in the mangrove-depleted area of Carey Island, Malaysia [41,42]. The detached low crested breakwater is made using homogenous rubble mount stones. The submergence of the detached low-crested breakwater depends on the tidal conditions. The low-crested breakwater was able to deposit soil at an average of 20 cm height behind the structure after 6 years of construction at the site. The deposition of soil with essential nutrients for the growth of mangroves made the soil rich with nutrients in the mangrove-depleted area suitable for planting mangrove seeds. Sreeranga et al. did field investigations to study the growth and endurance of mangroves to incoming ocean waves [43]. The growth of mangroves for six months was closely observed in laboratory for different soil conditions. Based on field and laboratory investigations, a temporary reef breakwater was designed, which would act as a barrier to the waves. The temporary reef breakwater was designed based on the low-crested breakwater proposed by Ahrens [44]. Homogenous stones with an average stone weight of 15 kg each can be used for constructing the proposed reef breakwater. The temporary reef breakwater is needed only for six months to facilitate the effective growth of the mangroves. After six months from the date of installation of temporary reef, it can be dismantled from the site for reuse. Bao studied the wave data of 32 mangrove locations in Vietnam [45]. The locations considered for the study had mainly six varieties of mangrove species. The wave height at different places of the mangrove locations along the cross shore was compared to the study of the effect of mangroves in reducing wave height. The wave height measured at different locations showed a decreasing trend along the cross-shore distance which indicates, that as the width of the mangrove forest increases, the reduction in wave height increases.
Elsheikh et al. carried out an experimental investigation on fixed and floating breakwaters to study the effect of draught, length, and crest width of floating breakwaters on the wave transmission coefficient [46]. Floating box type pontoon breakwater is used for the experimental study and three different draughts were considered for the analysis. The results from the experimental study showed that the wave transmission coefficient for floating breakwater decreases with an increase in draught and width when the wave height and wave period are kept constant. As the draught of the floating breakwater increases, the surface in contact with the wave also increases, which reduces the transmitting wave energy. The wave transmission coefficient shows a decreasing trend with increasing wave steepness. Hashim & Catherine conducted experimental studies to understand the effect of mangroves in tandem and in a staggered arrangement [47]. Rhizophora mangroves on a 1:10 model scale was selected for the study and the mangroves were arranged in tandem and in a staggered style. The results show that the tandem style Rhizophora mangroves were able to reduce the waves by about 78%, whereas the reduction in the wave height was about 85% in the case of the staggered-style Rhizophora mangroves, for a width of 100 m.
Sabari et al. conducted experiments to study the effect of Xbloc and mangrove roots in wave attenuation [48]. Xbloc and mangrove root models were used individually and in combination to study its effect on wave attenuation. Xbloc units and mangrove root models were 3D-printed for the study. The experimental study shows that as the mangrove root units are arranged in series making the roots complex, the wave reduction is higher than a single unit of mangrove root model. The combination of Xbloc and mangrove roots in series were able to give the maximum wave attenuation.
The coastal protection method discussed herein uses similar methodology discussed in the integration of mangroves with fixed coastal structures. Instead of fixed coastal structures, floating coastal structures are used. Taut moored barges are used as floating structures. The research methodologies needed for the effective development of the barge-assisted mangroves coastal protection method are also discussed here.

3. Methodology

The barge-assisted mangroves coastal protection method presented here uses the integration of mangroves with moored floating barges for coastal protection. This coastal protection technique is implemented in different stages. Rectangular barges are used here for this method. The first step in this method is the towing of the barge to the location (Figure 4).
This technology can only be used in areas where the wave action is low to moderate. It cannot be used in areas where the wave action is high or rough. After the barge is towed to the location, it is moored to the ocean bed using taut mooring (Figure 5). Taut mooring helps the barge to stay in the location and the response of the barge in normal sea conditions is less. The draft of the barge, dimensions, type, and thickness of barge materials depend on the environmental conditions at the site. After the barge is moored, the barge can be ballasted to the required draft and the mangroves can be planted to the required length and width (Figure 6). The mangroves take around 1 to 2 years to get well rooted in the soil. Mangroves reach a height of 2 m during this period [49,50]. Within this period, the mangroves are well rooted in the soil and the waves will not be able to induce beach erosion. The complex root system of the mangroves will hold the soil from becoming eroded [51,52].
The barges will act as a barrier for incoming waves travelling to the shoreline. The barges will effectively break the waves before reaching the beach. The effect of the waves on the beach is less or negligible due to the shielding effect of the moored barges [1,2]. Based on the length of the beach to be protected, the number of the barges can be increased to cover the shoreline (Figure 7). Floating cage type breakwaters in series have been proposed to protect the harbor, based on experimental studies [18].
It has been proven experimentally that barges being used as floating breakwaters can protect the leeward side from incoming waves [5,18,19,20]. Floating breakwaters can reduce the transmitted wave height to 30–50% of the incoming wave height [19,20]. Floating breakwaters can therefore be used to protect the beach from incoming waves, so that the mangroves can grow effectively. Barges can be moored in series along the direction of the wave action to keep the leeward side of the barge calm (Figure 8 and Figure 9). As the number of moored barges in series along the direction of the wave action increases, the transmitted wave height reduces. After the barges are moored to the ocean floor, the leeward side of the barge adjacent to the beach has a negligible effect on the waves and mangrove seeds can now be planted in the beach soil.
Once the mangroves attain sufficient growth in the beach soil, the barges can be unmoored and towed away from the site. The effect of mangroves in protecting the beach has already been reported in the published literature [25,26,27]. Mangroves of 500 m width can reduce the wave height by 50% to 99% [25]. Mangroves use their complex root systems to reduce incoming waves and protect the beach from erosion [29]. For example, if half- kilometer beach needs to be protected, then four barges of 120 m × 150 m in length and width can be arranged side by side along the beach with minimal spacing between them (Figure 8 and Figure 9). The minimal space between the barges can be determined based on the surge, sway, and heave motions of the barge. These motions can be determined based on the results from experimental and numerical studies on the moored barges in series [53]. The height and draft of the moored barge can be decided based on the environmental conditions at the location. If the wave action at the location is high, then the barges can be arranged in series towards the sea, as shown in Figure 8 and Figure 9, to reduce the wave impact and make the leeward side of the barges calm [1,2].
The beneficial shielding effect of the floating barges can also be utilized for other applications, such as berthing of ships and growing living shorelines using oysters, rocks, sand, plants, coir, etc. for coastal protection. Beaches which are badly eroded can be restored back to their normal form with the integration of this technology along with sand nourishment [54,55]. If the beach or shore does not have enough soil, then soil can be provided using sand nourishment for the effective growth of mangroves in the soil. After the barges are moored to the ocean floor, beaches can be nourished with sand to increase the width of the existing beach. Once the width of the beach is increased through sand nourishment, mangrove seeds can be planted in the beach soil.

4. Proposed Experimental Research and Preliminary Design Methodology

Figure 10 shows the proposed two dimensional (2D) experimental setup of two moored barges in series in a wave flume. Six wave gauges can be used to measure the wave heights. Wave gauges 1 and 2 will measure the incident wave height and wave reflection from barge 1. Wave gauges 3 and 4 will measure the transmitted wave height by barge 1 and wave reflection from barge 2. Wave gauges 5 and 6 will measure the transmitted wave height behind barge 2. The barges are taut-moored to the floor of the wave flume. The slope of the beach can be varied to study the effect of beach slope on the transmitted wave heights. The motion response of the two barges and the mooring line forces can be measured for regular and random wave conditions. The experimental study in the wave flume will lead to a better understanding of the effect of incident wave height on the barge responses, mooring line forces and transmitted wave heights. It is expected that the wave transmission coefficient will shed information on the reduction of the wave height behind barge 2, which in turn will be beneficial for the effective growth of mangroves providing protection from beach erosion.
Figure 11 shows the three dimensional (3D) experimental setup in a shallow wave basin. The results from this study will reflect the performance of the barges proposed to be installed at the site. Here three moored barges are considered for the study. Four wave gauges are proposed to measure the wave height at the respective location. Wave gauges 1 and 2 (WG 1 & WG 2) will measure the incident wave height and reflected wave height, respectively. Wave gage 3 (WG 3) will measure the wave height between barges 1 and 2. Wave gage 4 (WG 4) will measure the wave height behind barge 3. The barges will be taut- moored to the shallow wave basin floor. The slope of the beach can be varied to match the site conditions. This study will consider the phenomenon of wave diffraction, wave reflection and wave transmission by the barges. The motion response of the three barges and mooring line forces can be measured for regular and random wave conditions. The experimental studies in the wave flume and shallow wave basin will give a better understanding of the performance of the moored barges with waves. The results from the proposed 2D and 3D experimental studies would be utilized in qualitative evaluation of the effectiveness of barges in contributing to the growth of the mangroves and hence minimize beach erosion.
The preliminary design of the barges for the barge-assisted mangroves coastal protection system considers two barges each with a length of 100 m, width of 100 m and depth of 5 m (Figure 12). The draft of both the barges is 2.5 m. The water depth considered herein is 10 m. The two barges are arranged in a series along the incoming wave direction. The gap between the barges is an important factor in the design, which influences whether the motion of the fluid between the barges is a piston-type or a sloshing-type. The gap width between the barges, draft of the barge and breadth of the barge also influence the resonant frequency of the fluid between the barges [56]. As the gap between the barges increases, the fluid oscillation changes from a sloshing-type to a piston-type when the water depth and the draft of the barge remain constant [57]. Table 1 shows the dependency of gap width on resonant frequency of the fluid between the barges. Resonant frequency is calculated using the following equation, which is a modification of the pumping mode frequency suggested by Moradi et al. [56,57].
ω = g d ∗ B h − D + D  
ω is the resonant frequency, g is the acceleration due to gravity, B is the width of the barge, d is the gap width between the barges, h is the water depth and D is the depth of the barge.
As the gap width between the barges increases, the resonant frequency decreases. The fluid motion between the barges’ changes from a sloshing-type motion to a piston- type motion as the gap width between the barges increases. This increase in the gap width will contribute to a reduction in the fluid motion as well as the barge motions.

5. Conclusions

This conceptual paper discusses the integration of mangroves and moored barges for coastal protection and beach restoration. In this method the barges are towed to the location and moored to the sea floor using taut mooring. The dimensions of the barges are decided based on the results from the experimental and numerical studies of moored barges. The number of barges along the coast can be decided based on the length of the beach to be protected. The number of barges along the seaward side can be increased based on the environmental conditions at the site. Once the barges are moored to the seafloor at the location, the mangroves can be planted along the beach or shore. The type of mangroves that need to be planted can be decided based on the type of the soil and the water quality at the site. The mangroves take around 1 to 2 years to get well rooted in the soil and reach a height of 2 m during this period. If there is not enough soil in the beach, then soil for the effective growth of mangroves can be provided through sandy soil nourishment. Once the mangroves are well rooted in the soil and reach a height of 1–2 m the barges can be removed from the location. The advantages of using this method are:
  • Mangroves have a life span of 20–100 years. Once the mangroves are well rooted in the soil, the mangroves will be there in the beach for the next 20–100 years.
  • Barge assisted mangrove coastal protection method is environmentally friendly. As mangroves are biodegradable it does not pose a threat to the environment.
  • Mangroves improve the aesthetic appearance of the beach.
  • This method is a long-term solution to the soil erosion in beaches or shores. Once the mangroves are fully grown it requires less or negligible maintenance compared to other hard coastal protection structures.
  • This method can also be used to protect the shores of intracoastal waters where waves are created from the passage of the vessels.
This conceptual paper provides only the basic concept and idea of the barge-assisted mangrove coastal protection method. This method needs to be studied extensively, both numerically and experimentally, before installing at any location. Experimental methods suggested in this paper (Figure 10 and Figure 11) can be used to get an idea of how this innovative coastal protection methodology can be adapted to minimize beach erosion.

6. Limitations

Even though the barge-assisted mangroves coastal protection method is environmentally friendly, and has a comparatively longer life span compared to conventional coastal structures, this method has some limitations, such as:
  • When extreme waves approach a coast where the barges are moored to the seabed, the incoming waves can damage the mooring of the barges.
  • The mangrove barge integration method cannot be used for beaches where the waves encountering the beaches are rough.
  • Mangroves can create problems to native plant species by absorbing the nutrients of native plants.
  • Mangroves can pose environmental problems, creating a source for breeding of mosquitoes.

7. Patents

This innovative concept is based on the US patent application being filed by the authors: U.S. Patent Application Serial No.: 63/223,590.

Author Contributions

Conceptualization, R.D.R.; methodology, R.D.R.; resources, R.D.R.; data curation, R.D.R.; writing—original draft preparation, R.D.R.; writing—review and editing, R.D.R. and M.A.; visualization, R.D.R.; supervision, M.A. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Jmse 10 00612 g001 550
Figure 1. Integration of mangroves with fixed coastal protection structure (adapted from Yuanita et al.) [35].
Figure 1. Integration of mangroves with fixed coastal protection structure (adapted from Yuanita et al.) [35].
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Figure 2. Plot between the wave transmission coefficient and wave steepness for geo-bag dike structures (adapted from Yuanita et al.) [35].
Figure 2. Plot between the wave transmission coefficient and wave steepness for geo-bag dike structures (adapted from Yuanita et al.) [35].
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Figure 3. Plot between the wave transmission coefficient and wave steepness for mangroves structure (adapted from Yuanita et al.) [35].
Figure 3. Plot between the wave transmission coefficient and wave steepness for mangroves structure (adapted from Yuanita et al.) [35].
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Figure 4. Towing of barge to the location.
Figure 4. Towing of barge to the location.
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Figure 5. Barge moored to the seafloor.
Figure 5. Barge moored to the seafloor.
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Figure 6. Schematic diagram of mangroves grown on the beach after the barge has been moored to the location.
Figure 6. Schematic diagram of mangroves grown on the beach after the barge has been moored to the location.
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Figure 7. Moored barges and mangroves along the length of the beach in plain view.
Figure 7. Moored barges and mangroves along the length of the beach in plain view.
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Figure 8. Moored barges in a series with mangroves planted on the beach.
Figure 8. Moored barges in a series with mangroves planted on the beach.
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Figure 9. Moored barges in series with mangroves in plain view.
Figure 9. Moored barges in series with mangroves in plain view.
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Figure 10. Schematic diagram of an experimental setup in a wave flume.
Figure 10. Schematic diagram of an experimental setup in a wave flume.
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Figure 11. Schematic diagram of the experimental setup in a shallow wave basin in plain view.
Figure 11. Schematic diagram of the experimental setup in a shallow wave basin in plain view.
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Figure 12. Preliminary design of barges.
Figure 12. Preliminary design of barges.
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Table
Table 1. Effect of gap width between the barges on resonant frequency.
Table 1. Effect of gap width between the barges on resonant frequency.
Gap Width (m)Resonant Frequency (Rad/s)
80.3
50.38
20.58
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Raju, R.D.; Arockiasamy, M. Coastal Protection Using Integration of Mangroves with Floating Barges: An Innovative Concept. J. Mar. Sci. Eng. 2022, 10, 612. https://doi.org/10.3390/jmse10050612

AMA Style

Raju RD, Arockiasamy M. Coastal Protection Using Integration of Mangroves with Floating Barges: An Innovative Concept. Journal of Marine Science and Engineering. 2022; 10(5):612. https://doi.org/10.3390/jmse10050612

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Raju, Rahul Dev, and Madasamy Arockiasamy. 2022. "Coastal Protection Using Integration of Mangroves with Floating Barges: An Innovative Concept" Journal of Marine Science and Engineering 10, no. 5: 612. https://doi.org/10.3390/jmse10050612

APA Style

Raju, R. D., & Arockiasamy, M. (2022). Coastal Protection Using Integration of Mangroves with Floating Barges: An Innovative Concept. Journal of Marine Science and Engineering, 10(5), 612. https://doi.org/10.3390/jmse10050612

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