University of Twente Student Theses


Impact of Benthic Species on River Morphology

Elizondo Garcia, F.A. (2011) Impact of Benthic Species on River Morphology.

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Abstract:The invasion of benthic exotic species in aquatic systems has increased in the last couple of decades due to the increase of human activities in waterways (e.g. ballast water transport, attachment to boat hulls). Many studies have been done on the interaction between benthic organisms, sediment dynamics and thus morphology of aquatic systems. Nevertheless, these studies have been mainly conducted in marine and estuarine systems, involving different sediment and hydrodynamic conditions. Until now no studies are known on the impact benthic species on river morphology. The main objective of this Master Thesis is to investigate, by using the Delft 3D model, the influence of benthic species on sediment dynamics, and the possible changes on river bed morphology due to the presence of benthic species. Delft 3D is a numerical modelling system developed by Delft Hydraulics, fully applicable for 3-dimensional computations of hydrodynamics and morphodynamic simulations of coastal, river and estuarine areas. The study considers three exotic bivalve species in the River Waal: the Corbicula fluminea, Dreissena polymorpha and the Dreissena bugensis, where through the alteration of the erosion threshold, the C. fluminea living in the groyne field are the only species exerting a direct impact on sediment. Literature shows that the effect of the benthic activity on sediment dynamics may result in an increase/decrease of the erosion threshold, merely depending on two mechanisms: the bioturbation caused by benthic communities and the formation of biofilms (EPS) that “sticks” the sediment together. In order to model the possible benthic effects, three scenarios were investigated: One scenario subjected to sediment biostabilization (high erosion threshold), another one to biodestabilization (low erosion threshold) and finally a scenario that accounts for the anticipated maximum possible biostabilization in rivers. When using the transport formulation developed by Van Rijn (84), the erosion threshold is proportional to the median grain size D50, and therefore, in this study this value is locally increased/decreased inside the groyne fields in order to introduce the benthic effects into the Delft 3D model. The results show that under steady hydrodynamic conditions, the benthic effects do not exert significant changes in morphology. Contrary to this, the impact of changing hydrodynamic conditions and navigation are expected to alter the sediment processes within the groyne fields and therefore the benthic impact. These were not quantitatively considered in this study, however, are highly recommended for future research due to the importance of groyne fields for the sediment exchange in a river.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
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