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UBC Theses and Dissertations

Modelling channel morphodynamics : the effects of large wood and bed grain size distribution MacKenzie, Lucy


Within this thesis the results of a set of four stream table experiments are presented in order to examine the role that bed texture adjustments play in the development of a equilibrium channel form in the presence of large wood. Experiments were conducted using two physical models of Fishtrap Creek, an intermediate sized stream in the interior of British Columbia. While both flumes were Froude-scaled models with fixed banks and mobile beds, Model 1 contained a single grain size representative of the D₅₀ of the prototype stream while Model 2 contained a scaled grain size distribution (GSD) of Fishtrap Creek. Two treatments of wood load were run in each model: a moderate wood load (a scale equivalent of 160 m³/m²) and a high wood load (a scale equivalent of 220 m³/m²). Channel morphology was captured at five-hour intervals in order to create DEMs of the evolving bed surface. The results of this study show that bed grain size composition plays a dominant role in shaping channel morphology, even in the presence of large wood. The addition of large wood increased sediment storage which resulted in an increase in reach-averaged bed slope, the magnitude of which was proportional to the wood load added. Large wood also caused new areas of scour and deposition to be imposed onto the channel morphology that had been established prior to the addition of wood, causing an overall decrease in pool spacing and median pool area. The presence of a grain size distribution constrained the range of depth values in the flume as it allowed the bed to self-stabilize by limiting scour depth through the process of armouring. Regardless of the presence of large wood, maximum depths were approximately twice as deep in the single grain size flume and pools were deeper relative to their area. These results highlight the necessity of considering the full grain size distribution when modelling channel response to changes in the governing variables that influence channel morphology.

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