Oyster breakwater reefs promote adjacent mudflat stability and salt marsh growth in a monsoon dominated subtropical coast

Chowdhury, M. S. N. W., et al., 2019. Scientific Reports

Original research (primary data)
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Oyster reefs have the potential as eco-engineers to improve coastal protection. A field experiment was undertaken to assess the benefit of oyster breakwater reefs to mitigate shoreline erosion in a monsoon-dominated subtropical system. Three breakwater reefs with recruited oysters were deployed on an eroding intertidal mudflat at Kutubdia Island, the southeast Bangladesh coast. Data were collected on wave dissipation by the reef structures, changes in shoreline profile, erosion-accretion patterns, and lateral saltmarsh movement and related growth. This was done over four seasons, including the rainy monsoon period. The observed wave heights in the study area ranged 0.1-0.5 m. The reefs were able to dissipate wave energy and act as breakwaters for tidal water levels between 0.5-1.0 m. Waves were totally blocked by the vertical relief of the reefs at water levels <0.5 m. On the lee side of the reefs, there was accretion of 29 cm clayey sediments with erosion reduction of 54% as compared to control sites. The changes caused by the deployed reefs also facilitated seaward expansion of the salt marsh. This study showed that breakwater oyster reefs can reduce erosion, trap suspended sediment, and support seaward saltmarsh expansion demonstrating the potential as a nature-based solution for protecting the subtropical coastlines.

Case studies

Basic information

  • Case ID: INT-239-1
  • Intervention type: Created habitats
  • Intervention description:

    Three oyster reefs were constructed on a tidal mud at of Kutubdia Island using precast concrete rings. these reefs were deployed parallel to the coastline (~0.5 m above mean lower low water, MLLW) as wave-break structures to attenuate wave energy. Prior to the deployment of the reefs, ECOBAS project used the concrete rings on the intertidal mud- at adjacent to the experimental site (at the same tidal exposure) for two years to allow oyster larvae settlement and grow50. During the rst year, settlement of oysters was low (<100 spat m−2). However, successful spat fall (>300 spat m−2) was observed in year two, when rings covered with high densities of oysters S. cucullata (~1200 individuals m−2; size class 5–47 mm shell length) and other marine organisms such as barnacles, sea anemones, gastropods and polychaetes. e overgrown rings were transported to the experimental site in March 2016 and termed as “oyster breakwater reefs”.

  • Landscape/sea scape ecosystem management: No
  • Climate change impacts Effect of Nbs on CCI Effect measures
    Coastal erosion  Positive Adaptation/DRR: climate impacts is coastal erosion: wave attenuation / sedimentation and erosion rates / sediment deposition / resultant elevation profiles of mudflats, mangroves, sea grass / changes in tidal flat morphology / sediment grain size
  • Approach implemented in the field: Yes
  • Specific location:

    Kutubdia island (Fig. 7), located in the southeast coast of Bangladesh.

  • Country: Bangladesh
  • Habitat/Biome type: Deltas and estuaries |
  • Issue specific term: Nature-based (general)


  • Notes on intervention effectivness: Effectiveness determined by comparing outcome measures at reef site to control sites where reefs had not been added "oyster break- water reefs appear to be e ective locally at reducing tidal at erosion (Figs 3 and 4). e deployed structures enhanced sediment deposition at the leeward (landward) side of the reefs, increasing the bed level up to 29 cm." "Wave dissipation was noted when the water level is less than a meter above the reefs." ... "Our study clearly indicates that breakwater reefs can successfully reduce the wave heights (i.e. wave energy). It provided better stability in sediment movement from intertidal bed and reduces erosion, which was not the case for control site without reefs. "
  • Is the assessment original?: Yes
  • Broadtype of intervention considered: Not applicable
  • Compare effectivness?: No
  • Compared to the non-NBS approach: Not applicable
  • Report greenhouse gas mitigation?: No
  • Impacts on GHG: Not applicable
  • Assess outcomes of the intervention on natural ecosystems: Yes
  • Impacts for the ecosystem: Positive
  • Ecosystem measures: lateral salt marsh movement "Alteration of the physical con- ditions by the deployed reef structures protected the salt marsh during the monsoon season, resulting in the sea- ward expansion and fast regeneration of the marsh a er the monsoon season….oyster breakwater reefs not only have the potential for stabilizing the tidal ats but also useful for restoring or enhancing other ecologically important habitats such as salt marsh, o ering favorable conditions for many organisms to dwell and grow "
  • Assess outcomes of the intervention on people: No
  • Impacts for people: Not reported
  • People measures: n/a
  • Considers economic costs: No
  • Economic appraisal conducted: No
  • Economic appraisal described:
  • Economic costs of alternative considered: No
  • Compared to an alternative: Not reported

Evaluation methodology

  • Type of data: Quantitative
  • Is it experimental: Yes
  • Experimental evalution done: In-situ/field
  • Non-experimental evalution done: Not applicable
  • Study is systematic: