University of Twente Student Theses


Modeling the two-way coupling between Lanice conchilega and sand waves on the bottom of the North Sea

Maris, H.L. (2018) Modeling the two-way coupling between Lanice conchilega and sand waves on the bottom of the North Sea.

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Abstract:The tube-building worm Lanice conchilega is an ecosystem engineer, which affects its environment by changing the local hydrodynamics and therewith the sediment dynamics. The worm occurs in subtidal and intertidal areas and it lives in patches of hundreds per m2. They form mounds of 10-80 cm height in soft-bottom sediments. The goal of this study is to determine the effects of a two-way coupling between dynamic patches of Lanice conchilega and dynamically active sand waves on the bottom of the North Sea. The effects of Lanice conchilega onto sand waves are studies as well as the effects of the sand waves on the tubes. The numerical process-based model Delft3D has been used to the study the two-way coupling. The tubes are modeled as thin solid piles that affect drag and turbulence, thereby they affect the local sediment dynamics. The effects of Lanice conchilega on the local hydrodynamics and morphodynamics are investigated by modeling static patches with tubes on a flat bottom. The patches are also modeled in sand waves, to study the two-way coupling. The bathymetries consisted of a sinusoidal and a self-organizational bottom. The patches were located where the bed shear stress was lower than the mean tide-averaged bed shear stress for the model domain. The density of the patch was updated every season by a growth curve and the available suspended sediment. Furthermore, the patches disappeared every year, every five years, and never. The protruding tubes from the sediment cause more bottom roughness, therefore, within the patch, the near-bottom flow is decreased and the turbulence is increasing to its maximum value which occurs at the top of the tubes. Due to the decrease of near bottom flow, sediment was deposited between the tubes and this forms mounds. At the leading edge (zero cm in front of the patch) erosion holes were formed due to an increased flow velocity. Because of tidal symmetry, the leading edge switches and erosion holes are formed at both sides of the patch. Patches of Lanice conchilega were also implemented in sand waves. After one year of morphological development, both for the sinusoidal and self-organizational bottom, the bed level was only locally changed at the locations of the patches. The degradation rates of the mounds were different in troughs, half-way the flanks, and at local crests (only in self-organizational bottom), because of differences in bed shear stresses. After 20 years of morphological development the case where the patches disappeared every year was the most realistic case, because the mound heights were in agreement with field studies. Furthermore, for the self-organizational bottom, a smaller sand wave growth was shown for the bottom with patches compared to the bottom without patches.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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