Summary and Conclusions
The work presented in this dissertation consists of developing and applying 2D depth- averaged and 3D free surface models to study complex free surface flows under various boundary conditions. A 2D model has been developed by solving the depth-averaged or shallow water equations using first- and second-order-accurate finite volume meth- ods. A 3D model has been developed by incorporating a volume of fluid (VOF) module to an existing finite volume model developed by Huang et al. (2005). The VOF equation is solved using a high-resolution method, compressive interface captur- ing scheme for arbitrary meshes (CICSAM) (Ubbink and Issa, 1999). The numerical models have been applied to (i) study open channel flow in a contraction and re- solve the discrepancy between earlier measurements of Ippen and Dawson (1951) and depth-averaged model results obtained using finite difference method by others, (ii) conduct a comprehensive study of interaction between a salt wedge and freshwater discharge over fixed dune field and plane bed bottom boundaries, and (iii) the fine deposition partitioning over point bars in meander channels.
Experimental measurements in an open channel contraction have been done as well. The new measurements and 2D and 3D modeling have revealed the specific following conclusions:
• The original experimental data of Ippen and Dawson (1951) might have errors due to using a point gauge to measure shallow supercritical flow with a rough surface or the reported experimental condition was not correct;
• Shallow water models using different numerical modeling approaches create waveforms in an open channel contraction and the uniform downstream section that are too steep and inaccurate;
• Shallow water models are incapable of correctly predicting water surface profiles in an open channel contraction with straight sidewalls;
• A 3D model successfully reproduced new experimental data in the contraction and follow the trend of the waveforms in the uniform downstream section for the flow rate of 0.041 m3/s;
• At a higher flow rate the match between measurement and 3D simulation im- proved;
• The 3D numerical model can reproduce subcritical flow in an open channel contraction;
• A high fidelity turbulence model such as the LES model will likely improve the model prediction further as demonstrated by Larocque et al. (2013) for dam-break flows.
The 2D numerical study of the interaction between freshwater flow and slat wedge intrusions under the effect of tides over fixed bedforms showed the following:
• Both freshwater and salt wedge speed are affected by form drag caused by a dune field. A larger dune height may sometimes lead to propagation of the salt wedge farther inside the river than a smaller dune height during the high tide which means that the effect of the dunes on the freshwater flow is higher than its effect on the salt wedge.
• The speed of the salt wedge propagation over the dunes decrease linearly as the river flow increases while the opposite happens for an increase in tidal amplitude. The change in the speed under the effect of the dune height is different as it decreases rapidly with increase in dune height, but after a certain threshold value, the speed of the salt wedge starts to increase as the fresh water flow becomes much weaker.
• Higher fresh water flow and larger dune height result in larger Fri, as the salt wedge thickness in these cases is smaller.
• In the relative obstacle height phase diagram: the type of disturbance during the rising tide mostly fall in the ‘sb‘region expect for the case with the large dune height. In this cases, the dune acts as an obstacle for the salt wedge and, therefore, the disturbance type changes to ‘pb‘.
In the 3D numerical study on point bar fine deposits, five different factors are cov- ered; cross section, channel plan form, suspended load grain size, river flow variation, and the channel stream-wise slope. The results for these simulations have shown the following conclusions:
• The distribution of fine-grained deposits is found to be controlled by flow di- vergence. This study demonstrates that river cross section is a major control on the presence of flow divergence, and as a result it is a first order factor in the partitioning of the fines between different parts of the channel.
• Bend geometry also has a first order effect on the length scale of flow divergence. The share of the deposition on both bar tail and bar head increase as the bend angular amplitude increases until a certain value then the share starts to decrease again, but this conclusion should be taken with caution as all the tested cases are done using one wave length and changing this wave length might change the behavior.
• Runs made by varying the suspended load grain size showed that the bar tail would receive less coarse sediment compared to the bar head while the opposite is expected to happen with finer grain sizes.
• Runs made with different flow discharges showed that larger flows can change the share of bar tail fines more than the amount of change on the bar head.
• Channel gradient has a secondary effect on the fine deposits partitioning and quantities; streams with different slopes are expected to behave similarly in terms of fine deposit partitioning.
The present work is a first attempt to quantify the effects of a number of controls on fine sediment deposit pattern in meandering channels with special focus on bar head and bar tail deposits. Future studies should consider more variation in cross sections, bed load transport, tidal boundary conditions, and multiple channel bends.
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