Modeling Low Flow Hydraulics in Alluvial Channels (pools-Riffles, Open)
Miller, Barbara Ann
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https://hdl.handle.net/2142/69939
Description
Title
Modeling Low Flow Hydraulics in Alluvial Channels (pools-Riffles, Open)
Author(s)
Miller, Barbara Ann
Issue Date
1984
Department of Study
Civil Engineering
Discipline
Civil Engineering
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Mechanical
Abstract
The investigation of the flow condition has become important in recent years as a means of determining critical levels of water pollution, aquatic habitat and instream flow needs. To assess these critical conditions, it is necessary to predict the hydraulic response of a river system to a given low flow discharge. Conventional flow models, however, are often inaccurate at low discharges due to the geometric and hydraulic channel features peculiar to these flow conditions. The dominant channel feature at low stages is the pool-riffle sequence, where pools are distinguished by deep, slow moving water and riffles by shallow, relatively rapid moving flow. Conventional flow models have difficulty effectively representing the highly nonprismatic channel geometry, the rapidly changing flow hydraulics, and the dominating influence of flow resistance characteristic of pool-riffle sequences.
In this research, a mathematical model has been developed to simulate accurately channel characteristics under low flow conditions. The model is based on the iterative solution of four primary equations: the continuity equation; the one-dimensional energy equation; a formulation for the energy slope; and a relationship for the resistance coefficient. Energy losses are assumed to result from losses due to flow resistance, as well as from local losses generated by the contractions and expansions occurring through the pool-riffle sequence. For a given steady discharge, channel geometry and channel bed particle size distribution, the model predicts the flow depth, the mean velocity and the flow resistance. Laboratory data collected in conjunction with this research, as well as field data from two pool-riffle sequences, were used to verify the model.
Model development was based upon an in-depth analysis of the geometric and hydraulic characteristics associated with low discharges. As a result of the analysis, it was concluded that local losses contribute significantly to the total energy loss under low flow conditions. As the discharge increases, however, the significance of local losses decreases, and energy losses are more directly related to flow resistance. Furthermore, while the majority of flow parameters increase in magnitude with increasing discharge, the flow resistance coefficient shows a marked decrease in value with increasing flow rate. In general, the effects of pool-riffle sequences on changes in geometric, hydraulic and resistive properties increase as the flow depth decreases.
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