Artificial Reefs: A Review - SAGE Journals

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Artificial Reefs: A Review Lokesha1, V.Sundar2 and S.A.Sannasiraj3 Scholar, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu, 600036, INDIA, E-mail: [email protected] 2Professor, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu, 600036, INDIA, E-mail: [email protected] 3Professor, Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai, Tamil Nadu, 600036, INDIA, E-mail: [email protected] 1Research

Received: Feb. 11, 2013; accepted: April 30, 2013 Abstract A comprehensive literature review on the material characteristics, design of size and shape, application and management of artificial reef has been carried out. Multipurpose Artificial Surfing Reefs (MPASR) are increasingly being adopted for coastal protection because of several advantages associated with them such as coastal protection by reducing the wave energy, recreation of beaches (surfing, fishing, and diving), habitat for marine organisms and increase in socioeconomic prosperity. The most important characteristic is that they are soft barriers. It is concluded that although artificial reefs do have the ability to fulfill many objectives, for which they are meant, their success will depend mainly on the quality of planning and management prior to their implementation. Keywords: MPASR, coastal protection, breakwaters, soft barriers, soft measures.

1. INTRODUCTION The natural processes due to the action of waves, tides, currents, sediment deficit due to natural hazards and human impact such as, sand mining and coastal engineering works are all the possible causes for erosion. All over the world, the coastal community faces the problem of coastal erosion, either short term or perennial that often affects their livelihood. The Coastal Engineers and Scientists have been continuously working on the measures to control/combat this problem through hard and soft measures. Of these, the later has become a topic of great interest. The conventional methods (hard measures) such as seawalls, breakwaters, groins and gabions are short lived, expensive, non eco-friendly and often are an eyesore. The trend in the coastal zone management including coastal erosion mitigation and protection has been shifting towards soft and novel eco-friendly solutions. In general, there is a growing interest in low cost methods of shoreline protection. As there is a shortage of natural rock in certain geographical regions, there is a need of alternative materials and systems for shore protection. Further, for certain locations, the lead distance may be longer which increases the cost of the project significantly. Geo-systems such as geo-bags, geo-tubes, geo-containers, geo-curtains, geo-grids, etc and other systems such as Reef Balls, Aqua-reef, prefabricated units, and beach drainage have gained popularity in the recent years because of their simplicity in placement, cost effectiveness and less environmental adverse effects. 2. ARTIFICIAL REEFS 2.1 General Artificial reefs are man-made underwater obstructions that are submerged below the sea surface. These obstructions are like any natural ones acting as buffers in diffracting the incident wave energy, offer friction, thus leading to attenuation. The degree of attenuation of incident wave energy depends on the shape, size and material of the artificial reefs. The artificial reefs are also expected to enhance

Volume 4 · Number 2 · 2013

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Artificial Reefs: A Review

recreational benefits like diving, fishing and surfing. The geometry and design of the artificial reefs varies as it depends on the main functions of the structure and materials used. All kinds of materials such as steel, reinforced or pre-stressed concrete, fiber glass or a variety of composite materials have been used in construction of artificial reefs. Old wrecked cars, airplanes, military tanks, used truck or car tires, junked appliances, docks, old boats, ballistic missiles, decommissioned ships and obsolete oil rigs have been sunk and designated as artificial reefs. At a few locations, they serve as tourist attractions. For instance, the submarine ride in Honolulu, USA provides such views. 2.2 Geo-systems Coastal erosion can be controlled in several ways. Pilarczyk (2005) reported that geo-systems such as geo-bags, geo-tubes, geo-containers, geo-grids and geo-curtains have been adopted for controlling the beach erosion due to their simplicity in placement and construction, cost effectiveness and for being eco-friendly. Hard measures such as seawalls, dikes, revetments provide direct protection to the beaches where as groins and offshore breakwaters provide indirect method of shore protection. 2.3 Reef units Reef units with different shapes and constructive characteristics can be used to produce artificial reefs. Reef units are usually fabricated on land according to particular design specifications. The strength, stability and method of construction of reefs are the physical principles involving factors such as material science, civil engineering and physical oceanography. There have been various variations in the shape, size and complexity of reef units used in artificial reefs throughout the world. Several shapes that promote interlocking between the individual units as well as serve as a good habitat for marine life are being considered for implementation in the marine environment by several researchers. 3. REVIEW OF THE LITERATURE Harris (2009) introduced Reef Ball units that have been used to construct submerged breakwaters along the southern Caribbean shore of the Dominican Republic. Reef Ball units were designed to invite and provide environment for marine life. The construction of submerged breakwater with Reef Ball units helped in beach equilibrium, environment growth and eco-tourism. The Reef Ball units installed were of heights 1.2m and 1.3m, having base diameters of 1.5m and 1.6m respectively. Koerner (2000) reported that geo-tubes a growing technology can be used for protection of beach erosion. Geo-tubes made up of woven or knitted high strength fabric of diameter up to 3m have been effectively used to control beach erosion. Based on ease in handling/placing and filling, the length of geo-tubes is decided. The main tube is attached by the smaller diameter subsidiary tube on the upstream side which anchors in resisting lateral pressures. Also, a thin layer of soil is used to cover geo-tubes which protect them from degradation/damage. Shin et al. (2002) studied the performance of geo-tubes filled by hydraulically pumping dredged silty clay mineral into it by conducting pilot scale tests. The changes in geo-tube shape, dry unit weight, moisture content and vane shear strength of the soil at different places in the geo-tube with time were synopsized. It was concluded that the use of geo-tubes is a feasible alternative for coastal engineering works. Shin and Oh (2007) conducted 2-D limit equilibrium analysis and hydraulic model study tests of geo-tubes for Young-Jin beach along the east coast of Korea. A double-lined geo-textile tube was installed based on the results of stability analysis and hydraulic model tests. It was stated that sand gradually accumulates around areas covered by the geo-tubes as water depth in the near-shore area decreases with elapsed time. It was reported that compared to the extension of shoreline, the magnitude of re-erosion is relatively smaller. It was also concluded that use of geo-tubes with zero-water depth above crest was found to be most stable and effective for wave absorption than any other design plans. Alvarez et al. (2007) designed low-crested structures using geo-tubes to reduce the incident wave energy on the beaches of the Northern coast of Yucatan in Mexico. The geo-textile tubes were installed for a stretch of 4km along the beach. The geo-textile tubes were found to be an effective alternative for

International Journal of Ocean and Climate Systems

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shoreline stabilization. The dynamic balance on the shoreline was maintained by controlling the wave breaking process to the required level. Ranasinghe et al. (2006) have reported a comprehensive review of literature on the shoreline response to submerged breakwater. The numerical modeling was conducted for both one-line models and two-dimensional, depth averaged (2DH) coastal area models to find the shoreline response to submerged breakwater. The shoreline response to an artificial surfing reef was studied using 2DH numerical and 3D physical modeling tests. Depending on the offshore distance to the artificial reef, mode of shoreline response can vary between erosion and deposition of sand on beaches. The crest levels of the structure and predominant wave incident angle also have significant implications only on the magnitude but not on the mode of shoreline response. Based on the results, a predictive empirical relationship was proposed as a primary tool to determine shoreline response to submerged structures. The Narrow-neck Artificial Reef is located on the Gold Coast of Queensland, Australia (Jackson, 2004, 2007), and approximately 2 km north of Surfers Paradise. The design dimensions are 400m long (cross-shore), B=175m wide (alongshore), with the base of the reef positioned about S=150m offshore (B/S ratio = 1.16). The variable gradients of 1:18 for the focus, 1:2 for the reef face, and 1:8 for the fast sections were considered for reef design. At 1m below Australian Height Datum (AHD), the maximum crest elevation was set which equates to 1m below sea level which was chosen to insure that the mean wave (Hs=1.0m) would break at high tide. The crest width was about 300m (measured shore normal) and design reef volume is 128,000m3. The geo-textile containers of about 20 m long and upto 3-5 m diameter were filled in the hull of a dredge and then dropped to the seabed to form the Narrow-neck Reef. A distinct salient of about 30m wider than the adjacent beaches has formed in the leeside of the reef. The actual salient formations were found to be less than 78m predicted by the designers. Blenkinsopp and Chaplin (2008) conducted experiments over a submerged reef with an offshore gradient of 1:10 to measure the intensity, transmission and reflection of waves in a wave flume. The most important factor affecting the breaking characteristics is the relative water depth over the reef crest, hc/Ho (where, hc is the water depth above the crest and Ho is the deepwater wave height). In particular, a noticeable increase in the intensity of wave breaking over the reef with a reduction in the relative crest submergence was noticed. It was stated that as the submergence is reduced, resulted in a comparative decrease in wave transmission and reflection. It was suggested that a submerged reef with a relatively low seaward gradient is more significant in dissipating wave energy through wave breaking compared to sloping breakwaters. Bicudo et al. (2008) conducted a study of an artificial reef meant to enhance the surfing quality in the Sao Pedro do Estoril Beach, Caseais, Portugal. The hydrodynamics of the various geometry of the reef was determined by carrying numerical and physical model studies. The environmental impact of reef on surrounding region was also assessed. Lee et al. (2008) carried out studies to increase the immigration of marine underwater communities on artificial reef structures. The five chemoattractants such as ferrous sulphate, zinc oxide, ammonium nitrate, sodium phosphate and ferrous lactate were screened against spores of a fouling alga to increase the algal immigration. The performances of coating formulations using chemoattractants at East coast and South coast of Korea were investigated and the maximum fouling coverage has been estimated from ferrous lactate coating. The different composition of coating formulations and their chemoattractive properties were also evaluated. Strusinska and Oumeraci (2008) examined the feasibility of an artificial reef as coastal protection against tsunami impact. The reef workability and its hydraulic efficiency for different obstacle arrangements and changing incident wave conditions were investigated through numerical studies. Comparing the evidences of coastline in the presence and absence of reef, discussed the consequences of the reef on tsunami damping. Recio and Oumeraci (2009) developed analytical stability formula that account for the effect of the deformation of the individual Geo-textile Sand Containers (GSCs) for sliding and overturning stability. For each type of coastal structure made up of GSCs, stability formulae have been proposed. The recommendations were given with respect to the practical application of the proposed hydraulic stability formulae, including their limitations.


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Fig.1a. Coastal Protection Systems using Geo-tubes (Sundar et al 2009). Sundar et al (2009) considered sand filled geo-tubes to construct a seawall at Shankarpur of West Bengal, India as a coastal protection measure to withstand the wave climate. The geo-tubes of 20m length and 3m diameter, two at bottom and one at top were installed with a proper toe arrangement. It was the first time along the Indian Coast, that a geo-tube structure of two layers in exposed condition was being tried. The geo-tubes installed were successful in arresting waves till day of occurrence of highest high water level (HHWL) in September 2008. The severe climate scooped out the sand filling between the cliff and geo-tubes resulting in the damages of a few geo-tubes. The reason behind the damage of geo-tube was, the HHWL considered for design of geo-tube was only 3.84m and the HHWL recorded during tide was 4.89m which was under estimated. The pilot study with the exposed toe (Fig.1a) was found to be ineffective and hence the excavated toe (Fig.1b) was implemented which yielded satisfactory results.

Fig.1b. Cross-section of improved toe protection system with Geo-tubes (Sundar et al 2009).



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Duzbastilar and Senturk (2009) investigated the interaction between waves and artificial reefs made up of hollow cube and water pipe weighing 8.24kN (0.84t) and 1.27kN (0.13t) respectively. The study was limited to shallow waters upto 20m depth varying design weight, orientation of cube and pipe, and bottom slope of 10-1, 30-1, and 50-1. The resisting and mobilizing forces, and drag coefficients were estimated using physics equations and FLUENT software. Drag coefficients of 0.76 at 45o and 0.85 at 90o angle to the currents for the hollow cube, and 0.97 at 0o, 0.38 at 90o and 1.42 at 180o angle to the currents for the water pipe were estimated. The blocks with angles 45o and 90o were safely deployed at the water depth larger than 12 and 16m respectively. It was estimated that out of 720 cases at all stations 365 were unstable for the water pipes laid at angles of 90o and 180o to the currents and also found that water pipes laid at 0o angle were found to be stable in all 360 cases. It was concluded that study on interaction between waves and artificial reefs provides an important reference for engineers to increase the performance and life of artificial reefs. Mead et al. (2010) designed multipurpose artificial reef structure mainly for the enrichment of surfing at Boscombe, Poole Bay, England. Fifty four sand filled geo-textile containers of diameter 1 to 5m and length of 15 to 70m with a total volume of about 13,000 m3 adopted for the reef construction. The formation of inshore salient indicates the benefits of artificial reef in protecting the shore though it was not designed as a coastal protection structure. Boc and Burg. (2010) described an innovative shore protection method via an offshore reef that could be constructed at the Sacred Fall on the island of Oahu, Hawaii utilizing the shelf material. A physical model study with a scale 1:16 for the various artificial shapes and materials such as vertical lengths of high-density polyethylene (HDPE) 0.6m pipe, traffic barriers, and large storage units were conducted. It was stated that this technology was found to be effective in erosion reduction which has applicability in emergency and short term situations in shallow water island environments to protect infrastructure. Qin et al. (2011) studied the effects of artificial reefs construction in Yangmeikeng region of Shenzen on the marine ecosystem services. It was claimed that compared to other coastal areas, the tourism service value decreased from 87% to 42% and food supply service value increased from 7% to 27%. They effectively promote the development of artificial reefs and improvement in management of ecosystem which benefits the ecosystem service value. Autunes et al. (2011) described the influences of main relevant parameters such as height, submergence, length, and slope of the reef which protects the local coastline of Leirosa, Portugal by using COBRAS-UC numerical model. The initial values of height, length, seaward slope and submergence of the reef were demonstrated from the numerical modeling. Mendonca et al. (2012) conducted numerical investigation on multifunctional artificial reefs, a new alternative measure to protect coastal zone and to increase the surfing possibilities in the Leirosa area of Portugal. The Boussinesq-type COULWAVE model was used to investigate the hydrodynamics in the vicinity of artificial reefs. The breaker type, peel angle, wave height at breaking, and currents around the artificial reefs were the primary surfing parameters considered for analysis and design. Considering the design wave conditions (common and storm), medium and low tide levels, the reef geometries with different reef angles of 45o and 66o were tested. Based on the simulation done, it was concluded that both reef geometries were acceptable for surfing. But, the reef geometry with an angle of 45o was more acceptable for advanced/professional surfers. Dhinakaran et al. (2012) have reported a comprehensive review of literature on the hydrodynamic characteristics of emerged and submerged composite semi circular reefs. Considerable experiments were carried out to study the hydrodynamic characteristics of a semicircular breakwater and to optimize the parameters such as the height of rubble mound, the water depth and the size of perforations of breakwater. The experiments were conducted for both the emerged and submerged condition subjected to regular and random waves with various wave heights and periods to find the suitability of a semicircular breakwater in the field. Dassanayake et al. (2009, 2012 a &b) carried out studies on stability of GSCs through detailed experimental investigations and also identified important properties of GSCs such as type of material


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used for geo-textile and fill, sand fill ratio, friction between GSCs, seaward slope and sand displacement within the container influencing its stability. Pullout tests were conducted to study the properties of GSCs. Out of five different sand fill ratios such as 80%, 90%, 100%, 110%, and 120% used, the ratio between 90-100% were found to be optimal in terms of pullout resistance of GSCs. For all the sand fill ratios considered, 30-50% pullout resistance was observed compared to absence of sand fill. The nonwoven GSC structures of -0.2m freeboard were tested for sand fill ratios increased from 80 to 100% , which showed an increased stability numbers from 32% to 16% for the surf similarity parameter of 5-25. Comparing the inclined and horizontally placed GSCs with a freeboard -0.2m, an average value of 5% increase in hydraulic stability was seen in case of inclined GSCs. Damage was classified for single GSC based on the 3 stages such as detachment, with and without incipient motion as shown in Table1. For GSC structure, damage was classified as no damage, beginning of motion, minor, medium, severe and failure Table 2.

Table 1.Damage classification for Single GSC

Table 2.Damage classification for GSC

Note: GSC Structure was tested for regular waves of 100, and subjected to min of 80 waves.

The stability formula mentioned below was derived based on analysis of wave data and damage classification. Relationship between Stability Number and Surf Similarity

Ns =

c1

ξ o0.9

+ c2 ξ o



Where, Stability Number,

Surf Similarity Parameter,

Ns =

ξo =

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Hm ∆lc sin α Hm H m L0

Hm = Mean wave height, ∆ = Relative density of submergence of GSCs, lc = Length of the critical container, α = Slope of the structure, L0 = Deepwater wave length, c1 and c2 are coefficients for adjusting the freeboard, sand fill ratio, material and inclination of GSC. 4. SUMMARY A good design and construction of artificial reef provides considerable coastal protection, surfing and ecological enrichment. An understanding of the existing coastal processes at the site during design is important for artificial reef to produce the desired results. The comprehensive review of literature reveals that while significant work on the hydrodynamic characteristics of hard reefs have been reported, the literature on the said aspects for geo-synthetic materials is rather limited. This includes the behavior of the materials as well its performance characteristics. The materials and method of construction of artificial reefs plays a crucial role towards its benefits. The artificial reefs were built to its design specification using accurate methods which were good enough. The success of artificial reefs depends on the correlation between the designers and contractors. REFERENCES Alvarez, I.E., Ramiro Rubio and Herbert Ricadle. 2007. “Beach restoration with geotextile tubes as submerged breakwaters in Yucatan, Mexico” Journal of Geotextiles and Geomembranes 25, pp.233-241. Antunes do Carmo, J.S., Neves, M.G. and Voorde, M.t. 2011. “Designing a multifunctional artificial reef: Studies on the influence of parameters with most influence in the vertical plane” (2011) Journal of Coastal Conservation, 15 (1), pp.99-112. Bicudo, P., Custódio, M., Cardoso, N., Fortes, C., Da Graça Neves, M., Mendes, L., Monteiro, P., Pallia, A., Nogueira, M.J. and Carvalho, L.M. 2008. “Viability study of an artificial surf reef in S. Pedro do Estoril, Caseais, Portugal” Proceedings of the International Offshore and Polar Engineering Conference, pp.833-840. Blenkinsopp, C.E. and Chaplin J.R. 2008. “The effect of relative crest submergence on wave breaking over submerged slopes”, Coastal Engineering, Vol.55, Issue 12, pp.967-974. Boc, S.J. and Burg, E.C. 2010. “Innovative shore protection for island communities” WIT Transactions on Ecology and the Environment, 130, pp.209-220. Dassanayake, D.T. and Oumeraci, H. 2009. “Planned on the Hydraulic Stability of Geotextile Sand Containers”, 7. FZK-Kolloquium, pp.45-49. Dassanayake, D.T. and Oumeraci, H. 2012a. “Hydraulic Stability of Coastal Structures made of Geotextile Sand Containers (GSCs): Effect of engineering properties of GSCs”, Coastal Engineering Proceedings 1 (33), structures. 55. Dassanayake, D.T. and Oumeraci, H. 2012b. “Important Engineering Properties of Geotextile Sand Containers and their effect on Hydraulic Stability of GSC Structures”, Terra et Aqua Journal, pp.3-11.


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Dhinakaran.G., Sundar.V. and Sundaravadivelu.R. 2012. ”Review of the research on emerged and submerged semicircular breakwaters”. Journal of Engineering for the Maritime environment, Proc..IMechE, Part M:226 (4). pp.397-409. Düzbastılar F.O. and ?entürk U. 2009. “Determining the weights of two types of artificial reefs required to resist wave action in different water depths and bottom slopes” Ocean Engineering, Vol.36, Issues 12–13, pp.900-913. Harris, L.E. 2009. “Artificial Reefs for Ecosystem Restoration and Coastal Erosion Protection with Aquaculture and Recreational Amenities” Jackson, L.A., R.E. Reichelt, S. Restall, B. Corbett, R.B. Tomlinson and J. McGrath .2004. “Marine ecosystem enhancement on a geotextile coastal protection reef: Narrowneck Reef Case Study.” Proc. 29th Int. Conf. Coastal Eng. Jackson.L.A., Corbetet.B.B., McGrath. J.E., Stuart.G. and Tomlinson.R.B. 2007. “Narrowneck Reef : Review of Seven Years of Monitoring”, Shore and Beach, Vol.75 no.4, pp.67-79. Koerner, R.M. 2000. “Emerging and future developments of selected geosynthetic applications” Journal of Geotechnical and Geoenvironmental Engineering, 126(4), pp.293-306. Lee, H.S., Sidhartham, M., Shim, C.S., Kim, Y.D., Lim, C.Y., Ko, J.W., Han, M.D., Rang. M.J., Bim, L.S., Cho, H.S. and Shin, H.W. 2008. “Screening and formulation of chemoattractant coatings for artificial reef structures” Journal of Environmental Biology, 29 (4), pp.605-612. Mead, S.T., Blenkinsopp, C., Moores, A. and Borrero, J. 2010. “Design and construction of the boscombe multi-purpose reef” Proceedings of the Coastal Engineering Conference, No32, pp.58. Mendonca, A., Fortes, C.J., Capitão, R., Neves, M.G., Antunes do Carmo, J.S. and Moura, T. 2012. “Hydrodynamics around an artificial surfing reef at Leirosa, Portugal” Journal of Waterway, Port, Coastal and Ocean Engineering, 138 (3), pp.226-235. Pilarczyk, K.W. 2005. “Coastal stabilization and alternative solutions in international perspective” ArabianCoast 2005 Key Note address, 1-26. Qin, C.-X, Chen, P.-M., Jia, X.-P. 2011. “Effects of artificial reef construction to marine ecosystem services value: A case of Yang-meikeng artificial reef region in Shenzhen” Chinese Journal of Applied Ecology, 22 (8), pp.2160-2166. Ranasinghe, R., I. Turner and G. Symonds. 2006. “Shoreline response to multi-functional artificial surfing reefs: A numerical and physical modeling study.” Coastal Engineering, 53: pp.589-611. Ranasinghe, R.I. and Turner. 2006. “Shoreline response to submerged structures: a review.” Coastal Engineering, 53: pp.65-79. Recio, J. and Oumeraci, H. 2009. “Process based stability formulae for coastal structures made of geotextile sand containers” Coastal Engineering, 56 (5-6), pp.632-658. Shin, E. C., Ahn, K.S., Oh, Y.I. and Das, B.M. 2002. “Construction and monitoring of geotubes”.Proceedings of The Twelfth International Offshore and Polar Engineering Conference, Kitakyushu, Japan, May 26–31, 2002. Shin, E. C., and Oh, Y.I. 2007. “Coastal erosion prevention by geotextile tube technology” Journal of Geotextiles and Geomembranes 25, pp.264-277. Soysa V.A.N., Dassanayake, D.T. and Oumeraci, H. 2012. “Hydraulic Stability of Submerged GSC Structures”, ENGINEER 45 (04), pp.31-40. Strusinska, A. and Oumeraci, H. 2008. “Geotextile reef as a coastal protection against tsunami” Geotechnical Engineering for Disaster Mitigation and Rehabilitation - Proceedings of the 2nd International Conference GEDMAR08, pp.742-747. Sundar, V., Maiti, D.K., Sannasiraj, S.A. and Venkatraman, M. 2009. “Geosynthetic Application for Coastal Protection at Shankarpur, West Bengal, India” Proc.5th International Conference on Asian and Pacific Coasts (APAC) 23-25 Sep, Singapore, pp.58-64.
