Shellfish as a protection of revetments

Abstract

The main objective of this thesis is to increase the understanding of the effect of shellfish presence on the stability of loose rock revetments and to investigate the possibilities in the design. Pacific oyster (Crassostrea gigas) and blue mussel (Mytilus edulis) presence on loose rock revetments was quantified. C. gigas and M. edulis are common shellfish species in the port area and act as ecosystem
engineers. They require a certain salinity level, water temperature, phytoplankton ratio, and sufficient submersion time for daily feeding and respiration. The current presence of shellfish in the port area was predicted based on salinity levels generated by an OSR model. Outcomes were compared to qualitative measurements of C. gigas and M. edulis presence performed in the port area. C. gigas is present on revetments where salinity levels exceed 16‰, which is the case at Maasvlakte 2, Beerkanaal, and Calandkanaal. M. edulis were hardly observed in the port area on revetments. M. edulis can move relatively easily and detach from the surface after mortality occurs in contrast to C. gigas. They are, therefore, not considered a reliable structural addition, so only the effect from C. gigas was studied. C. gigas presence leads to binding of stones in revetments. This binding of stones will increase the stability of stones in the revetment. The relationship between the presence of oysters and stability upgrading is described using a connectivity model. The relationship between the presence of oysters and binding of stones is measured in the port area, these measurements are used to validate the model. Measurements showed that the cumulative coverage ratio of dead and living C. gigas decreases as one moves up to the intertidal zone from the mean low water level (MLW). The binding of stones increases when the coverage ratio of C. gigas is larger. This shows that there is a relationship between the coverage ratio of oysters and the binding of stones, and this assumes that there is a relationship between the coverage ratio of oysters and stability upgrading. The presence of oysters comes with structural, environmental, and ecological impacts. The conditions in the Port of Rotterdam as a system itself can pose various opportunities and threats for oyster coverage, such as global warming, pollution, and future port plans. Complementary measures can be implemented to mitigate the limitations and threats. The maintenance demand, and associated costs, decreases when oysters are present. Oysters are usually present naturally; their presence can be stimulated by enhancing C. gigas populations at locations with conditions that would allow potential oyster growth. Enhancement strategies depend on local conditions. Application is most promising when bow thruster or ship wave impact is normative because oysters are mainly found up to NAP +0.5 m. Additional enhancement costs are approximately equal to the expected reduction in maintenance costs. Possible ways of application in the Port of Rotterdam are summarized in a flowchart.