Nature and time-scale of bed morphological adjustments towards equilibrium in meandering streams: an experimental study
Laboratory tests , Meandering streams , Time factors , River beds , Topography , Equilibrium
This thesis concerns the nature and time-scale of bed morphological adjustments towards equilibrium in alluvial meandering streams. Following the prevailing approach, the stream centrelines are assumed to follow sine-generated curves, the banks are fixed, and the flow is turbulent and sub-critical. The movable bed is flat at time t = 0; at t = Tb, the bed reaches its equilibrium or developed state. The specific objectives of this thesis are: 1) to develop a predictive equation for the bed development time Tb; 2) to determine the rate of growth of pool-bar complexes in meandering streams; and 3) to determine whether or not the large-scale, curvature-induced erosion-deposition zones of developing beds migrate upstream or downstream throughout their development. This thesis builds on the preliminary work and experimental tests by Binns (2006). The duration of bed development is found to be proportional to the square of the flow width B and inversely proportional to the channel-averaged bed-load rate (qsb)av, the proportionality factor being a function of the initial deflection angle (i.e., stream sinuosity). The form of this function is revealed on the basis of an extensive series of experimental runs carried out in meandering channels of varying values of the initial deflection angle (i.e., 20, 45, 70 and 95 degrees). In the present tests, the temporal development of the bed from time t = 0 to t = Tb was monitored by periodically stopping the flow in order to measure changes in bed surface elevation. In all runs the bed was observed to deform rapidly during the early stages of the run and slow down considerably as the bed approached equilibrium conditions. Once formed, the location of the erosion-deposition zones remained invariant in flow plan with the passage of time. Results from the runs are used to provide insight into the nature of the deformed bed in meandering streams. An expression describing the temporal rate of growth of pool-bar complexes in meandering streams of varying sinuosity is also proposed. The practical application of the equation for Tb is illustrated with available field data.