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A part of transport : testing sediment models under partial transport conditions

Janssen, Stephan R. (2010) A part of transport : testing sediment models under partial transport conditions.

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Abstract:River managers rely on sediment transport models, for example when predicting morphological behavior. In rivers, under certain conditions, part of the sediment will move and part will remain immobile. This is known as partial transport. Partial transport influences the transport rate, bed forms, bed roughness and transport composition. In this study three transport models, Meyer-Peter and Muller (1948), van Rijn (2007c) and Wilcock and Crowe (2003), are tested on their performance under partial transport conditions. The first two transport models are alluvial models, whereas the third model was specifically developed for partial transport. The models are tested on flume experiments available from the literature where aspects of partial transport were studied or partial transport was observed in the measured transport rates. Testing the models in various ways demonstrates their performance on different aspects of partial transport. In this study �rst a bulk grain size distribution is used with both a uniform and fractional transport calculation and secondly a surface-based grain sizedistribution is used with a f actional transport calculation. The surface-based grain size distribution is determined from photographs of the bed surface and bed samples. As the transport depends largely on mobile and immobile sediment present at the surface, the surface-based grain size distribution is preferable. Two data sets, with trimodal and natural sediment are predicted well by the original transport models. However, three data sets with strong bimodal sediments cannot be predicted accurately with the transport models. Under supply limited conditions the bimodal sediment sorts into two parts, mobile and immobile sediment. This influences the functionality of the transport models. The hiding/exposure process that occurs with the strong bimodal sediment cannot be represented with the functions of the transport model. The process differs between parts where the immobile sediment is present at the surface and where the mobile sediment is present at the surface. Hiding exposure occurs mainly in the mobile sediment, but the model calculates the hiding exposure correction for the entire sediment. Therefore the calculation should be split for the mobile and immobile sediment. This should also be applied for several other represented processes in the transport models. In the last part of this study a different approach is tested. The transport rates of only the mobile sediment is predicted and reduced with a reduction function. With different reduction functions the resulting predictions improve for the data sets with bimodal sediment.
Item Type:Essay (Master)
Faculty:ET: Engineering Technology
Subject:56 civil engineering
Programme:Civil Engineering and Management MSc (60026)
Link to this item:http://purl.utwente.nl/essays/59701
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