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Experimental investigations of step-pool channel formation and stability Zimmermann, Andre Eric

Abstract

The stability of steep streams with step-pool and cascade morphologies cannot be assessed using knowledge developed from lowland streams due to the structured nature of headwater streams. Thus there is a need for experimental studies examining the stability of such channels. The structuring of these channels occurs as a result of boulders and cobbles jamming across the width of the channel and the more typical pattern of armouring and imbrication, which results from a relatively low rate of sediment supply. To conduct such a study, new experimental techniques were developed and an artificial stream channel (flume) was designed and built. Channel width, bed grain size and channel gradient were varied and step-pool bedforms were created and subsequently destroyed. The variables governing the dimensions, frequency and form of step-pools were observed to be channel slope, bed grain size and channel width. Video records show that on occasion groups of larger stones moved together as a coherent group, forming a line of stones that locked together across the width of the channel. The failure of a step most frequently occurred when the downstream scour pool undermined the step-forming stones and was often associated with headward migrating instabilities. With smaller jamming ratios (channel width/D84steps —the diameter at which 84% of the step stones are smaller than) stable beds persisted at larger Shields numbers (ratio of shear stress to grain size), confirming that such channels do gain stability by having grains jam across their width. The failure of the bed was shown to be a stochastic process with nearly half of the failures occurring within the first minute following an increase in discharge, while 26% of the failures did not start to occur until tens of minutes or more after the flow was increased. Detailed bed morphology, channel grain size and flow velocity measurements suggest that a dimensionless hydraulic geometry approach is the best method of predicting flow velocities in headwater channels. The bed stability criterion, in combination with a dimensionless hydraulic geometry approach, provides a means of assessing the stability of mountain stream channels.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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