Roughness and bedforms under partial mobility conditions
Relevance
The modelling of large scale aggradation / degradation processes of river beds strongly relies on adequate sub-models for the river flow, bed roughness and sediment transport. Especially when the river bed is composed of graded sediments, e.g. sand-gravel mixtures, the existing prediction methods for bed roughness still contain large uncertainties. Sorting processes of sand and gravel strongly interact with sediment transport, bedform formation and the bed roughness. In case of sand-gravel mixtures, layers of coarse material can develop in the river bed, protecting the underlying sediment from erosion. The currently applied roughness models have a limited validity under these conditions as processes associated with formation of bedforms and transport of sediment over these coarse layers are not represented. The transport and bedform development predictions are still very uncertain for these situations. These so-called ’partial transport’ and ’supply limited’ conditions are typical for rivers with sand-gravel mixtures such as the River Meuse and parts of the Rhine branches.
Goal
This project focuses on the modelling of the interaction of sediment transport, bedform formation, and the resulting roughness, under supply limited conditions. The main objective is to develop an integrated model for the small-scale interaction processes which can be incorporated in large-scale morphodynamic models. Since the scale of bedform formation is much smaller than the large-scale morphological adaptations, a modelling approach will be followed using parameterised empirical knowledge from lab or field data and/or detailed process-based models.
Role of experiments in the project
In collaboration with the Leichtweiss Institute (University of Braunschweig) flume experiments have been conducted to improve the understanding of the processes of bedform formation and sediment transport under partial transport conditions. More specifically the main goals of the experiments are I) to gain insight into bedform types and characteristics for different sediment compositions and different hydraulic conditions and II) to identify and quantify the processes controlling the roughness in the suply limited regime.
Model formulation
The new data from these experiments have been used to formulate a method for bedform dimensions prediction in the supply limited regime (Tuijnder et al., 2009). This method takes the availability of sediment on top of an immobile layer into account. The new insight into bedform dimensions under supply-limited conditions allows a better roughness estimation under these conditions (Tuijnder et al., in prep 1). Bedform dimensions, and therefore the roughness, are overestimated if the limited sediment availability is not taken into account in the model concepts. The improved roughness en bedform dimension model concepts have been applied to improve the prediction of sediment transport rates over immobile layers (Tuijnder et al., in prep 2).
Promoter
Prof.dr.ir. S. Hulscher (Universiteit Twente)
References
Tuijnder, A.P., Ribberink, J.S., and Hulscher, S.J.M.H. (2009) An experimental study into the geometry of supply-limited dunes. Sedimentology Online Early doi:10.1111/j.1365-3091.2009.01054.x
Tuijnder, A.P., Ribberink, J.S., and Hulscher, S.J.M.H. (In Prep. 1) Riverbed roughness prediction under supply limited conditions.
Tuijnder, A.P., Ribberink, J.S., and Hulscher, S.J.M.H. (In Prep. 2) Morphodynamic modelling of transport over immobile layers.
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