TIME-EVOLUTION AND STABILITY OF THE BED IN SINE-GENERATED MEANDERING STREAMS: AN EXPERIMENTAL STUDY

Alluvial meandering streams are very dynamic systems, which strive to achieve an equilibrium (or stable) state. Any time the flow and the stream are, either naturally or through human intervention, brought into a non-equilibrium state, substantial morphological changes occur. This research investigates the bed morphological adjustments of equilibrium undertaken by alluvial meandering streams. In agreement with the prevailing approach, it is assumed that the stream centreline follows a sine-generated curve. The flow is turbulent and sub-critical, and the width-to-depth ratio is "large". The movable bed is flat at time t = 0; at t = T, the bed reaches its equilibrium or developed state. The transport is by bed-load only.

The specific objectives of this research are: 1) to investigate whether or not the location of the deposition "hills" and the erosion "deeps" moves (migrates) along the longitudinal direction with the passage of time; and 2) to gain insight into the time-scale of the bed deformation process.

Three experimental runs were carried out in a 70-degree sine-generated laboratory channel, formed by two consecutive meander cycles. The runs were conducted by varying the longitudinal channel slope and thus the sediment transport capacity, while maintaining (approximately) the same flow geometry and relative bed roughness. The evolution of the bed from time t = 0 to t = T was monitored by periodically stopping the flow in order to measure changes in bed surface elevation.

It is found that the location of deposition "hills" and erosion "deeps" remains approximately the same throughout the duration of bed development, and thus that the location of these bed features at t ³ T is representative of their location at any time during the development process. With the aid of dimensional analysis, an expression is proposed for the duration of bed development, T. According to this expression, T is proportional to the square of the flow width B and inversely proportional to the channel averaged bed-load rate. The proportionality factor is found to be a function of the initial deflection angle alone. The data resulting from this work, as well as data available in the literature, are used to gain insight into the form of this function.