ARC developed a successful combination of Reef Cubes alongside our Dutch partners, to produce a stable and recoverable habitat enhancement structure. During the summer of 2020 hydrodynamic tests were undertaken at Waterproof Marine Consultancy and Services BV., in preparation for offshore trials of the Reef Cube structures in Dutch Waters. The testing was performed in collaboration with the Rich North Sea to ensure the positional stability of the Reef Cube structures, which were to be deployed for habitat enhancement at two sites: the North Sea Innovation Lab and Borssele Offshore Wind Farm. Through an iterative testing process in collaboration with Waterproof and the Rich North Sea we were able to produce a design deemed to be stable for the duration of the offshore trials. The aim of the structures is to enhance biodiversity and habitat complexity, with a particular focus on shark species, cephalopods (squids, octopus ad cuttlefish) and the blue mussel Mytilus edulis.
Flow flume experiments emulated the conditions at the test sites with the structures at a 1:17 scale. Waterproof also produced drag coefficients for single Reef Cubes at a 1:10 scale and were able to model a variety of scenarios to predict the stability of our structures over several time periods. Two major sources of stability failure were identified: sliding and tipping. To overcome these failure modes, we tweaked the structural design to increase the overall weight and the turning moment of the structure. Overall, the experimental approach to the stability design of the Reef Cube structures was a success and the full–scale product has now been deployed at the Offshore Innovation Lab, with the structures to be deployed at Borssele in 2021/ 2022. During the process we were able to demonstrate the ease of creativity with Reef Cubes that comes from the ease of combining different sized cube shaped units.
Our final design was a pyramid layout consisting of a layer of 0.75m sided Reef Cubes, with stacks of 0.5m sided Reef Cubes on top. To facilitate egg-laying of cephalopods and sharks we added additional natural fibre ropes, which we have witnessed having an effect in the UK seas. We also created more internal complexity by filling the upper 0.5m sided cubes with small 0.15m sided cubes. Increasing the internal complexity of artificial reefs with small refuge spaces like these increases the abundance of surrounding fish communities (Hylkema et al., 2020); and could create the perfect habitat for an octopus. Two treatments were created, one with a 0.5m sided Reef Cube at the pinnacle of the pyramid and another with an iron cage. Future monitoring by the Rich North Sea will investigate differences in the colonisation of these two structure types and will assess the overall ecological success of the designs.
Hylkema, A., Debrot, A.O., Osinga, R., Bron, P.S., Heesink, D.B., Izioka, A.K., Reid, C.B., Rippen, J.C., Treibitz, T., Yuval, M. and Murk, A.J., 2020. Fish assemblages of three common artificial reef designs during early colonization. Ecological Engineering: X, p.100021.