ABSTRACT: Modeling the influence of large wood on the morphodynamics of an intermediate gravel bed stream.

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The objective of this research is to investigate the effects of varying amounts of in-stream large wood on channel hydraulics and morphology, using several experiments on two separate stream tables with irregular, fixed banks and an erodible bed. Previous field studies have shown that the presence of large wood increases sediment storage, alters the grain size distribution of the channel bed, and decreases pool spacing. These morphologic changes aid in the creation and maintenance of aquatic habitat. Due to the inherent difficulty of manipulating large volumes of wood in the field, it remains necessary to study how different quantities of in-stream wood influence stream channel hydraulics and morphology in the laboratory. The experimental apparatus represents a 1:30 Froude-scaled model of Fishtrap Creek, an intermediate-sized stream near Kamloops, British Columbia. The first set of experiments was performed using a relatively wide grain size distribution that represents prototype grain sizes ranging from about 8 to 181 mm. The second set was performed using a narrow grain size distribution that represents the median of the bed material (about 16 to 32 mm in the prototype) to eliminate the possibility of surface texture adjustment. The first set of experiments was designed to study the magnitude, direction and timing of the changes in bed material transport and storage that might be expected in the prototype following a wildfire-related increase in the large wood load. The second set of experiments was designed to elucidate the potential effect of large wood without the confounding effect of bed surface texture adjustments. Preliminary results indicate that the addition of wood decreases water velocity, while increasing channel gradient. Wood addition also appears to be associated with prolonged sediment storage, changes in surface grain size, and increased pool formation and variability in depth. The effects of large wood addition on channel hydraulics and bed morphology were rapidly reversed following the removal of large wood. When compared with the slow changes associated with wood addition, the rapidity of the response to wood removal suggests that field studies involving the removal of large wood from natural systems do not accurately portray the temporal scale over which morphologic change develops following wood addition. This result is particularly relevant to stream restoration attempts, which now increasingly involve the addition of large wood to depleted reaches: the hysteresis in the response to wood addition and removal implies that it will require longer to restore aquatic habitat than is suggested by previous field studies of wood removal, and that long-term monitoring of restoration projects is necessary.

Bibliography of Canadian Geomorphology