In a fascinating study conducted by a team of world-renowned researchers from Griffith University and the City of Gold Coast, Australia, the impact of Multi-purpose Artificial Reefs (MPARs) on coastal sediment transport and morphology was examined, particularly focusing on the ICM led Narrowneck Reef project, two decades post-construction. This research is crucial as it sheds light on the long-term effects of MPARs, which have been designed to offer coastal protection while enhancing marine ecology and recreational activities such as surfing.
The Study's Findings
The research utilized a combination of high-resolution topo-bathymetric surveys and numerical modeling to investigate how the Narrowneck reef has influenced sediment transport and morphological changes around the structure.
Key findings include:
Sand Bypassing: Contrary to initial expectations, the study revealed that sand can bypass the MPAR around its offshore end. This was not anticipated during the reef's design phase and has not been widely reported in literature on similar structures.
Current Deflection and Sediment Deposition: The presence of the Multi Purpose Artificial Reefs causes longshore currents to be deflected as they pass the reef, creating a "shadow zone" on the down drift side where sand accumulates. This finding is significant as it demonstrates the reef's role in sediment storage and coastal protection, aligning with its initial design objectives.
Stabilisation of Coastal Bars: The research also found that the Multi Purpose Artificial Reefs can help stabilise coastal bars as they move onshore, with a notable downdrift offset of the inner bar due to low oblique wave incidence. This effect contributes to the stabilisation of the coastal environment around the reef.
Implications and Future Directions
This study highlights the multifaceted role of Multi Purpose Artificial Reefs in coastal management, offering insights into their impact on sediment transport pathways and coastal morphology. The findings suggest that MPARs can indeed fulfill their dual purpose of providing coastal protection while enhancing recreational outcomes, such as surfing conditions. However, the research also underscores the importance of long-term monitoring and data analysis to fully understand the implications of such structures on coastal environments.
Future research should continue to focus on the long-term performance of MPARs, exploring their impacts under varying environmental conditions and their potential role in climate change adaptation strategies for coastal communities as costal erosion solutions. The insights gained from studies like this are invaluable for policymakers, and environmental managers in designing and implementing effective coastal protection measures that harmonize with recreational and ecological objectives.
The study can be found, on Research Gate.
Designing and Constructing Multi-Purpose Artificial Reefs
The design and deployment of artificial reefs for coastal protection is a complex process that requires careful study and consideration of various factors. The complexities of designing artificial reefs stem from the need to balance stability, hydrodynamic processes, morphological response, and the interaction with local marine ecosystems.
Stability: The stability of an artificial reef depends on the materials used (e.g., rock armouring, geotextile containers or others), the structure's shape, and the forces exerted by waves and currents. Careful engineering analysis is required by coastal engineering specialists.
Hydrodynamic Processes: Understanding the impact of an artificial reef on local wave patterns and currents is crucial. The reef's design affects wave transmission, wave breaking, and the creation of circulation patterns that can significantly influence sediment transport and deposition around the reef. Estimating wave transmission over submerged structures, considering the permeability of the structure, the crest width, and the structure's position relative to the shore is a highly curated process requiring an extensive knowledge base with the latest in numerical and physical modelling capabilities.
Morphological Response: The shoreline response to the construction of an artificial reef can vary widely, with potential outcomes including beach accretion, erosion, or no significant change. Factors influencing these outcomes, such as the reef's distance from the shore, its submergence depth, and the prevailing wave conditions can have significant impacts. Designing a reef that enhances coastal protection without causing unintended negative impacts requires a nuanced understanding of these morphodynamic processes.
Environmental Considerations: Beyond their physical and engineering aspects, artificial reefs also interact with the marine environment. They can create new habitats for marine life, alter local ecosystems, and impact marine biodiversity. The design process must consider these environmental impacts, aiming to create structures that provide coastal protection while also supporting or enhancing the local marine environment as a nature based solution.
Safety and Usability: A Multi Purpose Artificial Reef will be designed to allow for user interaction which creates a significant safety factor consideration that comes into the design process. Typically there are some 'trade-offs' in efficiency versus safety that need to be balanced specifically for the site and local conditions relating to the reef crest height and width. This will impact the depth over the reef at various tides as well as rip currents around the reef during different wave conditions.
In summary, the design of artificial reefs for coastal protection is a multifaceted process that demands a thorough and well-researched approach. It involves not just engineering and physical considerations but also a deep understanding of the local marine environment.
This complexity underscores the necessity of engaging multidisciplinary teams in the design and implementation phases, ensuring that the reefs not only protect the coast but also preserve or enhance the marine ecosystem.
Multi Purpose Artificial Reefs. One Piece of the Solution
While the study has shown that after 20 years there are significant positive impacts of the Narrowneck Reef on the local conditions (beach volume, marine habitat and surf amenity in reef vicinity), it is part of a larger coastal resilience design approach.
In order to create a "healthy beach profile" and "living shoreline", both the top and bottom of the beach need to be addressed in conjunction with short and long term sand management strategies. This includes activities like nearshore nourishment (an ICM developed approach), as well as dune vegetation and management.
For over 30 years International Coastal Management has been at the forefront of coastal resilience design and implementation, specifically in multi purpose artificial reef design. Through the years our highly specialised team has developed new materials, construction and monitoring methods which are considered worlds best practice.
As we move forward and encounter new locations and changing climate conditions we are continually developing on successful reef projects to ensure ongoing longevity and knowledge hub development for the improvement of eco-engineered reefs as a means for coastal resilience.