Graduation of Steven Haarbosch

The influence of bivalve shells, of different shapes and sizes, on current-driven sediment transport

• Professor of graduation: Dr. ir. B.C. van Prooijen

• Supervisors: Dr. ir. S.G. Pearson, Dr. ir. S. de Vries, MSc. T.J. Kooistra

In the Netherlands, the coastlands act as the principal protection against the sea. Without human interventions, the Dutch coast would be eroding due to an imbalance in the sediment budget of the coastal zone. The preferred coastal management strategy is sand nourishment, which have become larger in scale and more complex over the last 70 years. Additionally, besides sand, large quantities of mollusc shells can also be found on the ocean floor. This study aims to gain insight into the current-driven bed load transport of a sediment-shell bed composition using bivalve shells of different shapes and sizes.

Flume experiments were conducted to measure the influence of bivalve shells on the threshold of motion of sand grains and current-driven bed load transport. The sediment-shell bed composition was altered in both experiments by varying the volumetric percentage of shell content. The bed compositions consisted of two distinct bivalve shell species: Ensis leei (elongated) and Spisula subtruncata (rounded) bivalve shell species.

It is proposed that the presence of shells initially disrupts the flow, leading to an increase in turbulence intensities. As shell content increases, a second effect becomes increasingly prevalent, which is sediment stabilisation. The smaller sediment grains are hidden from the flow by larger, more exposed shell valves and shell fragments. Consequently, a higher bed shear stress is needed to mobilise the sand grains. The relative importance of these processes varies depending on shell content, shell species and the potential of the shell to enhance turbulence intensities. The elongated shape of the E. leei bivalve shell enhanced turbulence intensities significantly and thereby influencing the threshold for sand grain motion. Bed load transport rates are reduced with increasing shell content. Initially, at a shell content of 10%, the rates only decreased marginally compared to a situation without shells, and no clear difference between shell species was observed. At a shell content of 20%, bed load transport rates decreased further. Thus, when predicting sediment transport, considering the presence of shells alone is insufficient, and consideration of shell shape and size is crucial.