University of Twente Student Theses


Modeling the influence of mussels and oysters on hydrodynamics and sediment transport

Alferink, N. (2016) Modeling the influence of mussels and oysters on hydrodynamics and sediment transport.

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Abstract:Mussels and oysters are known as ecosystem engineers; consequently, they have an influence on the hydrodynamics and sediment transport. At this moment, it is unknown to what extent these bivalves influence the hydro- and sediment-dynamics on a patch scale. By applying the process-based model Delft3D with the rigid 3D vegetation module, the influences of mussels and oysters on the hydrodynamics and sediment transport are determined on a tidal flat scale. Mussels and oysters are modeled as solid cylinders that affect the drag and turbulence; thereby these bivalves affect the local sediment dynamics. An essential parameter for the implementation of a bivalve bed in Delft3D is a variable bivalve shell height. A variable shell height is needed to simulate the flow velocity above the bivalve patch correctly, especially above oysters. Moreover, a variable shell height reduces the turbulence above the canopy of the bivalve bed and these turbulence levels corresponds with the two data sets. In contrast to oysters, mussels have an increased sediment flux towards the bed due to the production of heavy faeces. This process is implemented in Delft3D by adapting the transport equation; the settling velocity above the mussel bed is increased with an additional term, the filtration rate. The influence of mussels and oysters on the sediment dynamics have been tested on a current dominated tidal flat. The resistance forces of mussels and oysters result in strongly reduced near-bed flow velocities and a turbulent kinetic energy (TKE) peak at the top of the canopy. These model predictions showed good agreement with the velocity profiles of two flume experiments, while the TKE patterns above the bivalve beds correspond reasonably well with these data sets. The high resistance forces of mussels and oysters have also an effect on the sediment transport above and around the patch. The low near-bed flow velocities leads to high accumulation of sediment in the bivalve beds. Besides, the production of heavy faeces by mussels leads also to an increase of sediment deposition. The modeled net biodeposition is approximately 30% of the total deposition in a mussel patch in a cohesive environment and corresponds with field measurements. The physical structure of mussels and oysters also induces flow routing; the flow accelerate at the left and right side of the patch, relative to the flow direction, resulting in erosion; while calmer conditions occur at the lee side, relative to the flow direction, of the patch resulting in sedimentation. The effects of mussels and oysters reach beyond their patches and they might be helpful reducing hydrodynamic forces on the coast. Based on the sensitivity analysis, the bed shear stress is always lower than the critical bed shear stress. Consequently, forces induced by currents cannot erode the sediment between mussel or oyster shells and it appears that there is missing process. This process is probably waves; waves can presumably induce larger bed shear stresses than currents. The present models identify the need for more knowledge about bio-physical interaction, both from experiments as well as from models.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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