University of Twente Student Theses


Modelling bio-physical interactions by tube building worms.

Schellingerhout, J. (2012) Modelling bio-physical interactions by tube building worms.

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Abstract:The objectives of this study read: (1) defining the state-of-the-art knowledge on bio-physical interactions by tube building worms on hydrodynamics, sediment dynamics and the ecological environment, (2) determining the most important processes and input parameters included in the hydrodynamic model Delft-3D, (3) calibrating the model by the recently executed flume experiments and (4) determining the sensitivity in outcome of the model for a given range in input parameters for a typical North Sea situation. The first objective has been addressed by former studies (Bouma et al., 2007; Friedrichs and Graf; 2009; Peine et al., 2009; Friedrichs et al., 2009), which confirmed that tube building worms such as the polychaete Lanice conchilega can both act as stabilizers and destabilizers of bed material. The patches of tube building worms have a direct effect on the near bottom water velocities and consequently on the sediment dynamics. In extreme situations, tube building worms could cause skimming flow behaviour already at 5% area coverage (Eckman et al., 1981; Friedrichs et al., 2000). As result of the biological activity of these bioengineers, the sediment fluxes could be modified by a factor 2 and more, compared to the solely physical case (Graf and Rosenberg, 1997). Indirectly, these biotic patches could have a strong (positive) environmental impact on the structure, configuration and functioning (e.g. biodiversity) of marine ecology (Callaway, 2006; Rabaut et al., 2007; Godet et al., 2008). The latter three objectives are addressed by measuring the hydrodynamic effects in detail in the flume as result of patches of artificial structures (thin piles) and by simulating the flume set-up and five typical North-Sea scenarios with a three-dimensional hydrodynamic model. The sensitivity analysis and the calibration of the model on the flume experiments showed that this hydrodynamic model is able to provide comparable flow patterns with the flume data: (i) deceleration within the patch, (ii) acceleration above the patch, (iii) uplift in front of the patch and (iv) acceleration in front of the patch. Both the patch density and the flow velocity increase these effects of the patch on the flow dynamics. The margin of error in the velocity profiles is realistic compared to the model discrepancies of Bouma et al. (2007). Implementing the final calibration parameter set in the model facilitated up-scaling of flume to field conditions. Using of five different typical North-Sea scenarios provided a rough estimation about what levels of bed shear stress could be found in the field as result of the patches of tube building worms. In the most extreme situation, the bed shear stresses increases with almost 60% in front of the patch and reduced with at least 80%, compared to the case with no biological activity. Concluding, as the model performs reasonably accurate, and given that the computing time should be minimized as much as possible, it is believed that the appropriate k-ε model should be used for modelling flow through elements. However, the quality and quantity of the data should be increased in order to get more reliable results for up-scaling flume settings to field conditions. Furthermore, more processes (e.g. wave-flow interaction and flow-sediment interaction) should be included, more scenarios tested and the model should be ran in 3D.
Item Type:Essay (Bachelor)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering BSc (56952)
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