Editorial Type: RESEARCH ARTICLES
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Online Publication Date: 21 Oct 2025

Experimental Investigation of Wave Transmission Influenced by Submerged Oyster Shell Artificial Reefs

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Article Category: Research Article
Page Range: 1069 – 1078
DOI: 10.2112/JCOASTRES-D-24-00079.1
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ABSTRACT

Raju, R.D. and Arockiasamy, M., 2025. Experimental investigation of wave transmission influenced by submerged oyster shell artificial reefs.

Oyster reefs are widely used for coastal protection and marine life enhancement. This experimental study focused on the investigation of wave transmission over an oyster reef made up of oyster shells in biodegradable bags. The experimental study was carried out on an oyster reef having 85 cm crest width and 110 cm base length for three water levels of 28 cm, 29 cm, and 30 cm for various wave conditions. The biodegradable bag was made from sinamay fibers. The sinamay fibers are made from the abacá plant (Musa textilis), which is a type of banana plant. Since the bag material is biodegradable, the oyster reef discussed here is environmentally friendly. The rate of decay of the biodegradable bag encasing the individual reef unit is also discussed in this study. The results show that the artificial oyster reef can reduce the incident wave height by about 57%. The wave reduction by the submerged oyster reef is maximum at lower water depth conditions. The reduction in wave height depends mainly on the wave steepness and submergence depth. The wave transmission coefficient shows a decreasing trend with increasing wave steepness. The degradation of the bag encasing the individual reef unit can be controlled by increasing the number of bags encasing an individual reef unit.

Keywords: Biodegradable; submergence depth; wave steepness; water depth; sinamay; abacá

INTRODUCTION

Artificial reefs are man-made coastal structures used for coastal protection and marine life improvement. Coastal areas are subjected to erosion due to wind, waves, tides, currents, and human activities (Lokesha, Sundar, and Sannasiraj, 2013). Artificial reefs mimic the characteristics and performance of natural reefs. Historically, artificial reefs were constructed for recreational fishing. These artificial reefs were made using discarded scraps and waste materials. Accidental shipwrecks also created artificial reefs (Ramm et al., 2021). This created the possibility of scuttling decommissioned ships for constructing artificial reefs (Raju, 2023). The Japanese were the pioneers in building artificial reefs when they started building artificial reefs for recreational fishing (Ino, 1974; Stone 1974, 1982). In the year 1952, the first designed and planned artificial reef for improving commercial fishing was documented in Japan (Lee, Otake, and Kim, 2018; Ramm et al., 2021). Artificial reefs have multipurpose benefits and are used for various reasons. Artificial reefs can be used for coastal protection, marine life enhancement, prevention of the use of trawling nets, recreational diving, fish farming, and promoting tourism (Baine, 2001; Ditton et al., 2002; Raju et al., 2020; Relini, 2000; Tsumura, Kakimoto, and Noda, 1999). Most artificial reefs are made from materials like concrete, used tires, decommissioned ships, stones, steel, plastics, oyster shells, wood, coir logs, coir mats, and other non-biodegradable materials. Artificial reefs made from non-biodegradable materials can lead to serious environmental pollution in the long term. The Osborne reef, which was installed on the Florida coast with the purpose of improving marine life in the year 1974, failed in its function as an artificial reef. The Osborne reef is made of used tires (Cabral, 2015). Studies show that there is no marine life near the Osborne reef (Allahgholi, 2014; Cabral, 2015; Morley et al., 2008; Piazza, Banks, and La Peyre, 2005; Raju and Arockiasamy, 2022). In recent years, there has been a shift from the traditional coastal protection structures like seawalls, bulkheads, revetments, breakwaters, geotextiles, groins, etc., to sustainable coastal protection structures. Sustainable coastal protection structures are made from materials that are environmentally friendly and do not cause environmental pollution. These sustainable coastal protection structures are called living shorelines. Living shorelines provide better habitat for marine organisms along with shore protection, promotion of tourism, carbon sequestration, etc. (Barbier et al., 2013; Mcleod et al., 2011; Scyphers et al., 2011; Silliman et al., 2019; Smith et al., 2020). Materials like oyster shells, seagrass, mangroves, coir logs, coir mats, sand, rock, etc., are used for the construction of sustainable coastal protection structures (Lekha and Kavitha, 2006; Piazza, Banks, and La Peyre, 2005; Raju, 2023; Spalding et al., 2014).

Sustainable coastal protection structures are finding their way into coastal protection and beach restoration projects. Coastal protection structures made of materials like steel, polymers, rubber, and concrete are comparatively cheap, but these materials create serious environmental problems in the long run (Allahgholi, 2014; Cabral, 2015; Morley et al., 2008; Piazza, Banks, and La Peyre, 2005; Raju and Arockiasamy, 2022). Coastal protection structures made of sustainable materials like rocks, plants (mangroves and seagrass), oyster shells, coir, banana plant fiber, sand, etc., are environmentally friendly (Lekha and Kavitha, 2006; Piazza, Banks, and La Peyre, 2005; Spalding et al., 2014). These materials increase the biodiversity around the coastal protection structures.

Oyster reefs are widely used these days for sustainable coastal protection and marine life enhancement due to their multipurpose benefits (Jones, Lawton, and Shachak, 1994; Tolley and Volety, 2005; Wells, 1961). Oyster reefs are considered as a natural breakwater due to their ability to buffer the incoming waves and protect the shoreline from erosion. Oyster reefs are used as living shorelines, since oysters are naturally available materials that are environmentally friendly. Oysters are effective in improving the quality of nearby water (Cressman et al., 2003; Hoellein, Zarnoch, and Grizzle, 2015; Nelson et al., 2004) and providing a source of habitat for marine organisms (Cohen and Zabin, 2009; Huang et al., 2019; Scanes et al., 2020). Salinity of water plays a major role in the growth of oysters (Kennedy, Newell, and Shumway, 1996; La Peyre et al., 2015). Higher salinity conditions are not favorable for the growth of oysters (Ewart and Ford, 1993; Garland and Kimbro, 2015; Kennedy, Newell, and Eble, 1996). Ultraviolet type B radiation does not support oyster growth, and it increases the mortality rate of oysters (Kett et al., 2022). Oysters have the ability to adhere to hard substrate like rocks and form a large colony of oysters, which make the oyster reef stable (Nestlerode, Luckenbach, and O’Beirn, 2017; Seilacher, Matyja, and Wierzbowski, 1985; Wasson, 2010). Biodegradable materials like biodegradable ecosystem elements (BESEs; e.g., potato waste polymer) (Herbert et al., 2018; Nitsch et al., 2021), jute (Coen, 2008; Manley et al., 2010), coconut coir (Dunlop, 2016; Moody et al., 2020), etc., are used for constructing oyster reefs to protect the beach from the action of waves. The experimental study discussed here focused on the effectiveness of an oyster reef in resisting incoming waves. Experimental studies were carried out for an oyster reef made up of oyster shells in biodegradable bags. The experimental study was conducted for three water levels, 28 cm, 29 cm, and 30 cm, for different wave conditions.

METHODS

The experiments on the oyster reef were conducted in a wave tank of length 7.57 m, width 0.78 m, and height 0.762 m. The reef tested in the wave tank was a two-dimensional reef with crest width (length of crest) of 0.85 m and base length of 1.10 m. The height and width of the reef were 0.25 m and 0.716 m. In total, 36 individual reef units were used to make the oyster reef. The oyster reef was tested in the wave tank at the Hydrodynamics Laboratory at the Florida Atlantic University, Boca Raton campus, Florida. The wave tank used for the experimental study has a flap-type wave maker on the wave-generation side and a wave absorber on other side for absorbing the waves. Regular waves were considered herein for the study. The experimental setup of the oyster reef in the wave tank is shown in Figure 1.

Figure 1.Figure 1.Figure 1.
Figure 1.Artificial reef in wave tank.

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Two wave gauges were used in front of the reef, and an additional two wave gauges were placed after the submerged reef as shown in Figure 2. The wave gauges were arranged as per the guidelines by Goda and Suzuki (1976). Figure 2 shows the schematic diagram of the experimental setup. The wave transmission coefficient was calculated for three water levels of 28 cm, 29 cm, and 30 cm under different wave conditions. Wave height measured using wave gauge 3 and the methods suggested by Ahrens (1985, 1987) were used here for calculating the wave transmission coefficient. The wave transmission coefficient is defined as the ratio of transmitted wave height to incident wave height. The transmitted wave height was measured using wave gauge 3 with the submerged oyster reef in place, whereas the incident wave height was measured without the reef using wave gauge 3.

Figure 2.Figure 2.Figure 2.
Figure 2.Schematic diagram of experimental setup (Raju, 2023).

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Material Selection

The size of oysters selected for the experiment were in the range of 7.6 to 10.1 cm (3 to 4 in.). Individual oyster reef units were arranged in a trapezoidal-shaped cross section to make the oyster reef. The individual unit (Figure 3) was constructed using oyster shells (Figure 4) in a biodegradable bag (Figure 5). The biodegradable bags were made from sinamay. Sinamay is a natural straw fabric made from abacá fibers. Abacá fibers, also known as Manila hemp, are extracted from the abacá plant, which is a variety of banana plant. The properties of the individual units are shown in Table 1. The wave parameters used for the study are presented in Table 2. Figure 6 shows a wave passing over the submerged oyster reef.

Figure 3.Figure 3.Figure 3.
Figure 3.Artificial reef unit.

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Figure 4.Figure 4.Figure 4.
Figure 4.Oyster shells.

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Figure 5.Figure 5.Figure 5.
Figure 5.Biodegradable bag made out of sinamay.

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Figure 6.Figure 6.Figure 6.
Figure 6.Wave passing over the submerged oyster reef.

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Table 1.Properties of individual oyster unit.
Table 1.
Table 2.Wave parameters.
Table 2.

RESULTS

The experimental study was conducted for three water levels, 28 cm, 29 cm, and 30 cm, for different wave conditions to calculate the wave transmission coefficient. Figure 7 shows the plot of wave transmission coefficient for different wave steepness conditions for 30 cm water depth. The wave transmission coefficient shows a decreasing trend with increasing wave steepness. This trend agrees with the earlier published literature (Abdul Khader and Rai, 1980; Armono, 2004; Dattatri, Raman, and Shankar, 1978; Goda, 2010; Murakami and Maki, 2011; Shirlal and Rao, 2007). The waves with higher steepness break more effectively over the submerged oyster reef than the waves with lower wave steepness. For 30 cm water depth, the oyster reef reduced the wave height by around 13% to 32%. For wave steepness less than 0.08, the wave height reduction was around 20%. For low values of wave steepness less than 0.08, the wave transmission coefficient was in the range of 0.76 to 0.87.

Figure 7.Figure 7.Figure 7.
Figure 7.Plot of wave transmission coefficient vs. wave steepness for different wave steepness conditions for 30 cm water depth (Raju, 2023).

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Figure 8 shows the wave transmission coefficient vs. wave steepness for 29 cm water depth. The wave transmission coefficient shows a decreasing trend for increasing wave steepness. This trend agrees with the earlier published literature (Abdul Khader and Rai, 1980; Armono, 2004; Dattatri, Raman, and Shankar, 1978; Goda, 2010; Murakami and Maki, 2011; Shirlal and Rao, 2007). The wave transmission coefficient for 29 cm water depth is lower compared to 30 cm water depth, as the submergence depth over the reef is also reducing. As the submergence depth reduces, the wave breaking becomes more intense, and more energy is released through wave breaking. For water depth of 29 cm, the wave height reduction was around 23 to 36%. For wave steepness greater than 0.08, the wave height reduction was around 30%. The wave transmission coefficient for 29 cm water depth was in the range of 0.64 to 0.77, whereas for 30 cm water depth, it was in the range of 0.76 to 0.87.

Figure 8.Figure 8.Figure 8.
Figure 8.Plot of wave transmission coefficient vs. wave steepness for different wave steepness conditions for 29 cm water depth (Raju, 2023).

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Figure 9 shows the wave transmission coefficient vs. wave steepness for 28 cm water depth condition. The submergence depth over the oyster reef for this condition was 3 cm. The wave transmission coefficient shows a decreasing trend with an increase in wave steepness. This trend agrees with the earlier published literature (Abdul Khader and Rai, 1980; Armono, 2004; Dattatri, Raman, and Shankar, 1978; Goda, 2010; Murakami and Maki, 2011; Shirlal and Rao, 2007). The maximum wave height reduction for the submerged oyster reef was observed at the 28 cm water depth condition, where the wave attenuation was 57%. As the water depth decreased from 30 cm to 28 cm, the submergence depth over the oyster reef also decreased from 5 cm to 3 cm. The intensity of wave breaking, and submergence depth have a direct dependence on the wave transmission coefficient. The intensity of wave breaking increases as the submergence depth over the reef gets reduced. As the water depth or the submergence depth increases, the intensity of wave breaking is less, and the transmitted wave height is higher. Since the transmitted wave height is higher, the wave transmission coefficient is also higher. Another parameter that influences wave transmission is wave steepness. As the wave steepness gets higher, the wave transmission coefficient shows a decreasing trend. For waves with higher wave steepness, the wave breaking intensity is higher, and more energy is released during wave breaking. So, the wave transmission coefficient shows a decreasing trend with increasing wave steepness.

Figure 9.Figure 9.Figure 9.
Figure 9.Plot of wave transmission coefficient vs. wave steepness for different wave steepness conditions for 28 cm water depth (Raju, 2023).

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Degradation of Biodegradable Bags

To check the rate of degradation of the sinamay biodegradable bags, two individual oyster units were kept inside buckets filled with tap water. Three buckets filled with tap water were used to check the degradation rate for a duration of 6 months. Three buckets were filled up with tap water, and two units were added to each of the buckets (Figures 10, 11, and 12). The buckets were opened and checked every 2 months to check the rate at which the Sinamay bags are degrading.

Figure 10.Figure 10.Figure 10.
Figure 10.Three buckets filled with individual reef units.

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Figure 11.Figure 11.Figure 11.
Figure 11.Plan view of buckets filled with individual reef units (Raju, 2023).

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Figure 12.Figure 12.Figure 12.
Figure 12.Two individual reef units placed one over the other.

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Two individual units were placed inside the bucket one over the other. The bags were taken out from the bucket and thoroughly examined every 2 months. After 2 months, the first bucket was opened to check the rate at which the sinamay bag enclosing the oyster shells was degrading. Unit 1 is the individual reef unit that was kept in the bottom of the bucket, and unit 2 is the individual reef unit that was on the top. Some of the mesh units of unit 1 (in red circle, Figure 13), which were located under unit 2, were broken (Figure 14). The weight of unit 2 was acting on unit 1 and exerted force on the mesh units of unit 1.

Figure 13.Figure 13.Figure 13.
Figure 13.Units 1 and 2 after 2 months.

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Figure 14.Figure 14.Figure 14.
Figure 14.Unit 1 after 2 months.

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

After 4 months, the second bucket was opened to check the degradation of sinamay bags. Some of the mesh units of unit 2, which were placed on the top of unit 1, were broken (Figure 15). The mesh units of unit 1 kept on the bottom were broken, and the oyster shells had fallen into the bucket (Figure 16). The individual oyster reef units of the third bucket were discarded after 4 months, since it was understood that mesh units of unit 1 will break completely in 4 months. The mechanical strength of abacá fibers used for making Sinamay bags decreases when submerged in water (Paglicawan et al., 2022).

Figure 15.Figure 15.Figure 15.
Figure 15.Unit 2 after 4 months.

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

Figure 16.Figure 16.Figure 16.
Figure 16.Unit 1 after 4 months.

Citation: Journal of Coastal Research 41, 6; 10.2112/JCOASTRES-D-24-00079.1

DISCUSSION

Sustainable coastal protection structures contribute a significant extent to the attenuation of the incident wave. An oyster reef constructed using oyster shells in biodegradable bags made up of sinamay is effective in coastal protection without causing environmental pollution. Since the bags encasing the oyster shells are biodegradable, the oyster reef is environmentally friendly. The wave height attenuation by the oyster reef depends mainly on two parameters—submergence depth and wave steepness. When the submergence depth is kept constant, the wave transmission coefficient shows a decreasing trend with increasing wave steepness. As the wave steepness increases, the intensity of the wave breaking increases, and the energy released during the wave breaking also increases. So, the wave transmission coefficient for waves with higher steepness is lower. As the submergence depth of the reef reduces, the wave breaking over the reef becomes more intense, and the transmitted wave height is less. The oyster reef considered herein was able to reduce the wave height up to 57%. An artificial reef made up of oyster shells in biodegradable bags can protect a beach from incoming waves. An oyster spat (oyster offspring) will attach to the oyster shells and grow into a living oyster. These oysters will start to grow through the individual units and grow into a colony of oysters, which will increase the overall stability of the reef. If the chances of oyster spat getting attached to the oyster shells are scarce, then oyster spat can be provided artificially to the oyster reef. A single individual artificial reef unit takes around 4 months to degrade when it is kept inside a closed bucket filled with tap water. The mesh units of the sinamay bag were broken after 4 months under static conditions. This is due to the weight exerted by the individual unit that was placed over the bottom unit and degradation of meshes of the sinamay bag. The degradation of the bag encasing the individual reef unit can be controlled by increasing the number of bags encasing an individual reef unit. Oyster reefs have low initial cost of construction compared to other sustainable coastal protection methods like seagrass, mangroves, coir bags and mats, etc. Among these sustainable coastal protection methods, oyster reefs provide better wave attenuation next to mangroves. Due to their complex root system and canopy, mangroves provide maximum wave attenuation among sustainable coastal protection methods. Oyster reefs can be constructed using locally sourced oyster shells from restaurants with the help of volunteer labor from local communities. Oyster reefs and mangroves have a larger life span compared to other sustainable coastal protection methods. Oyster reefs provide economic feasibility and several other benefits compared to traditional structures used for coastal protection. Some of the benefits are:

  1. Traditional coastal protection structures are made of concrete, steel, rocks, wood, geotextiles, etc. Oyster reefs are made up of oyster shells in biodegradable or non-biodegradable bags, which contribute less to environmental pollution.

  2. High initial construction costs and skilled labor are needed for making traditional coastal structures. Oyster shells used for making oyster reefs can be sourced from local restaurants. Volunteer labor from local communities can be used in constructing oyster reefs.

  3. Oyster reefs are self-sustaining as oysters grow naturally and do not require yearly maintenance. Oyster reefs can grow naturally along with the rise in sea level. Traditional coastal protection structures require yearly maintenance and need upgrades for rising sea level.

  4. Oyster reefs provide better habitat for marine life compared to traditional coastal protection structures (Jones, Lawton, and Shachak, 1994; Tolley and Volety, 2005; Wells, 1961).

  5. Oyster reefs have the ability to provide water filtration, seafood sources, carbon sequestration, ecotourism promotion, etc. (Cressman et al., 2003; Cohen and Zabin, 2009; Hoellein, Zarnoch, and Grizzle, 2015; Huang et al., 2019; Nelson et al., 2004; Scanes et al., 2020).

CONCLUSIONS

This study showed that an oyster reef constructed using oyster shells in biodegradable bags made up of sinamay was effective in coastal protection without causing environmental pollution. The study carried out here used only regular waves for understanding the wave transmission and breaking over a submerged oyster reef. However, in real-life conditions, a submerged oyster reef will encounter irregular waves most of the time. The wave breaking location of irregular waves over a submerged reef changes due to the randomness of irregular waves, and the energy dissipation during irregular wave breaking is variable. Further studies need to be carried out for irregular waves in a wave flume and wave basin to understand the wave transmission and breaking over a submerged oyster reef in real-life conditions.

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Copyright: Coastal Education and Research Foundation, Inc. 2025 2025
Figure 1.
Figure 1.

Artificial reef in wave tank.

Figure 2.
Figure 2.

Schematic diagram of experimental setup (Raju, 2023).

Figure 3.
Figure 3.

Artificial reef unit.

Figure 4.
Figure 4.

Oyster shells.

Figure 5.
Figure 5.

Biodegradable bag made out of sinamay.

Figure 6.
Figure 6.

Wave passing over the submerged oyster reef.

Figure 7.
Figure 7.

Plot of wave transmission coefficient vs. wave steepness for different wave steepness conditions for 30 cm water depth (Raju, 2023).

Figure 8.
Figure 8.

Plot of wave transmission coefficient vs. wave steepness for different wave steepness conditions for 29 cm water depth (Raju, 2023).

Figure 9.
Figure 9.

Plot of wave transmission coefficient vs. wave steepness for different wave steepness conditions for 28 cm water depth (Raju, 2023).

Figure 10.
Figure 10.

Three buckets filled with individual reef units.

Figure 11.
Figure 11.

Plan view of buckets filled with individual reef units (Raju, 2023).

Figure 12.
Figure 12.

Two individual reef units placed one over the other.

Figure 13.
Figure 13.

Units 1 and 2 after 2 months.

Figure 14.
Figure 14.

Unit 1 after 2 months.

Figure 15.
Figure 15.

Unit 2 after 4 months.

Figure 16.
Figure 16.

Unit 1 after 4 months.

Contributor Notes

*Corresponding author: rr2017@fau.edu
Received: 03 Nov 2024
Accepted: 24 May 2025
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