An understanding of the interaction between flow, sediment and bed morphology is essential for dealing with engineering problems such as floods, river bank erosion and sedimentation in reservoirs. However, the morphodynamic processes in natural braided rivers are still not well understood due to difficulties in measurements in field. Numerical models provide a considerable assistance to investigate these complicated processes in natural rivers. In the present study, a physics-based two-dimensional model based on DIVAST with suspended load and bed load transport has been developed to simulate the braiding processes and morphodynamic changes in braided rivers. In this model, the hydrodynamic equations are solved using the ADI scheme and the advective-diffusion equations are solved using a modified ULTIMATE QUICKEST scheme. The TVD scheme has also been included to simulate trans-critical flows. Regarding sediment transport, a new module based on bed load transport theories has been developed. A module for suspended load transport based on energy theory has been improved. Secondary flow and slope effect are integrated into the model by altering the sediment transport rate. A multiple layer technique with a vertical sorting process has been applied including bank erosion. Graded sediment fractions are adopted to represent the coarsening and fining processes with sheltering effect. The model has been verified by solving a 2-D dam-break problem which worked well in predicting the water surface changes in trans-critical flow. It has also been tested by a sediment aggradation case and found to predict the flow and bed deformation effectively. The model has been applied to predict a laboratory river with bed load, with its prototype being the Sunwapta River, Canada. Braiding mechanisms and channel pattern responses to abruptly increased discharge have been investigated and compared with those of laboratory and natural rivers. Growth and relationship of active braiding intensity and total braiding intensity show similar trends to those of iii the laboratory river. The predicted river shows anisotropic scaling with periodical braiding morphology presented by sequential maximum scour depths. The model also simulates a large idealised braided river with suspended load transport. Its braiding mechanisms have been discussed and compared with the river with bed load and natural rivers. Important processes at bars and confluences have been investigated. Statistical characteristics of the river have been analysed with braiding indices, state-space plots and bar parameters. These findings have been compared with those from real rivers to assess the model simulating real braided rivers.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:590347 |
| Date | January 2013 |
| Creators | Yang, Haiyan |
| Publisher | Cardiff University |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://orca.cf.ac.uk/56986/ |
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