UBC Theses and Dissertations
The effects of tributary sediment pulses on mainstem channel dynamics in mountain streams Lowe, Emma Kathryn
In the mountainous regions of British Columbia, debris flows are common, resulting in masses of coarse sediment entering larger streams in upland valleys. Relatively wide valleys form distinctive debris flow fans that partially fill the valley bottom. Where valleys are narrower and the streams in the valley bottom are confined, debris flows interact with the main stem stream in more complicated ways. This thesis presents eight experiments documenting the interactions between steep tributaries capable of generating debris flows and the main stem streams into which the debris flows enter. Using imagery that provides a 2D comparison model of the channel bed and video photography, experiments reveal the role of tributary sediment inputs (TSI) in controlling the behavior of mainstem stream profiles and defining features. The first set of experiments focuses on the natural channel and morphology changes with no influence of tributary sediment pulse events. The second set observes the change in morphological behavior as a function of the tributary confluence angle. A third set focuses on how debris flow sediment texture influences the effects of these pulses. The investigations consider the effect of changing the angle of tributary and effects on geomorphology of the main channel. The experiment iterations considered entrance angles of 90°, 75°, 60° and 45°. The second experiment involved adding coarse sediment to the bed material used for TSI in the first set, representing boulders found in step pool streams. Boulders are common in most debris flow materials but most mainstem streams are typically incompetent in their transportation; this experiment investigates the degree to which sediment alters behaviour at the confluence. The 90° and 75° junction angles formed less stable stream blockages which eroded rapidly and had lower elevation. The 60° and 45° angles posed a greater threat to up and downstream areas. These angles caused blockages with higher elevation and persistence. Their stability within streams meant their elevation grew following consistent debris flow activity creating greater geohazard threats downstream. Boulders (large grains) in the debris flows caused a larger magnitude of risk because of higher dam elevations and stream blockages posing potential outburst flood occurrence.
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