Alternate Bars Under Steady State Flows: Time of Development and Geometric Characteristics

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Date
2014-03-31
Authors
Boraey, Ahmed
Keyword
Development time , Alternate bars , Geometric characteristics , Predictive equation , River process
Abstract
This thesis concerns the development of alternate bars under steady state flows. The movable bed is flat at the beginning of the experiment; the bars reach their equilibrium or developed state at the time Td. The thesis has two objectives. The first is to introduce new equations for the geometric characteristics, namely height and length, of alternate bars at the fully developed stage, and to evaluate them against the existing equations. The second objective is to present the results of two series of experiments carried out to characterize the process of development of alternate bars and obtain estimates of their time of development. The data resulting from these experiments are intended as a foundation for future work towards the establishment of a predictive equation for the development time of alternate bars. The new equations for bar height and length rest on dimensional considerations and all the available data. Bars produced under rough turbulent and transitional flows are treated separately. The proposed equations are found to consistently give more accurate estimates of alternate bar dimensions than existing equations. The experiments to quantify the time of development of alternate bars are carried out in the 21 m long, 0.76 m wide sediment transport flume of the Queen’s Coastal Engineering Laboratory. In addition to providing estimates of the time of development of alternate bars, these experiments reveal aspects of the process of development of alternate bars that had not been reported previously. In particular, they show that, all other conditions being the same (including the sediment transport capacity of the initial flow), the more pronounced alternate bars formed under shallower flows develop faster than less pronounced bars formed under deeper flows. The findings of this study highlight the fact that the previously unexplained wide variation in alternate bar dimensions is related to the plotting position of the data point in the alternate bar existence region of Ahmari and da Silva (2011). This study also sheds light on the evolution and development of alternate bars, which establishes a strong foundation for future studies on the topic.
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