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Application of a rational model of stream equilibrium for predicting channel adjustments MacVicar, Bruce Jonathan

Abstract

In this thesis, channel geometry adjustments and possible restoration efforts were modeled and interpreted for three streams in British Columbia using the physical model of Millar and Quick (1993). The major challenge of the project was to overcome limitations in our ability to quantify important physical processes such as flow resistance, sediment transport, and bank stability in order to access the inherent predictive and modeling capabilities of a rational approach. In practical terms this meant the model had to be calibrated to past adjustments of channel geometry before it could be used to predict future changes. The success of calibration was found to be dependent on the type of disturbance a stream was adjusting to. Disturbances of bank stability were readily modeled. Calibration was facilitated by the sensitivity of modeled widths to changes in bank stability and the availability of air photographs to measure historical changes in channel width. Slesse Creek was found to have been disturbed by a reduction in bank stability due to forest harvesting in the riparian area of the creek. The creek adjusted by widening its channel and switching from a single to a multiple thread. Modeling results indicated that a moderate increase in bank stability could be used to reinstate a singlethread channel, reduce bank and floodplain erosion and allow vegetation to recover. Disturbances to water discharges were also readily modeled provided that flow records existed or that past flows could be estimated from a clearly defined bankfull flow condition. Shovelnose Creek was found to have been disturbed by an increase in the discharge conveyed by the channel. The creek adjusted to the increase by widening and deepening its channel, and breaks in the slope were introduced. These breaks were now control points as channel discharge has been reduced to pre-disturbance levels. Modeling indicated that the use of point deflectors to narrow and deepen the channel may be particularly suited to Shovelnose Creek due to the lateral and vertical stability of the channel. Disturbances to the sediment transport regime were not readily modeled. The example of Harris Creek highlighted the difficulty of assessing simultaneous changes of particle sizes, channel roughness, and sediment transport. The longer time scale of sediment supply waves and effect of bedrock control on slope were additional difficulties. The approach used to apply the rational model of Millar and Quick (1993) was advantageous because it focused on stream processes, produced exact numerical results and resulted in a stream response framework that was specific to each stream. The approach thus represents a step forward from other available approaches. Further research that could increase the applicability of the rational model is discussed.

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