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The generation of stormflow on a glaciated hillslope in coastal British Columbia Utting, Mark Gregory

Abstract

An investigation into the mechanisms of stormflow generation on a glaciated hillslope in coastal British Columbia has been undertaken. The investigation included a controlled irrigation-runoff experiment on a 30 x 30 m hillslope plot in the U.B.C. Research Forest near Haney, B.C. Instrumentation included 12 rain gauges, 45 piezometers, and 2 outflow-tipping buckets. Piezometer slug tests to measure hydraulic conductivities and a geologic study to establish the representativeness of the experimental results were conducted to complement the irrigation experiment. The hydrogeologic units of the research plot consist of: A) 0.1 to 0.3 m of forest floor material consisting of organic material in various states of decay B) 0.3 to 0.8 m of heterogenous, red-brown B horizon containing many organic rich channels made up of live and decayed roots C) 0.5 to 2 m grey to grey-green Vashon till D) fractured to unfractured granodiorite bedrock The hydraulic conductivity of the till was approximately 10⁻⁷ m/s. A slightly higher value of 10⁻⁶ m/s was found for the lower B horizon matrix. A bulk conductivity for the lower B horizon was estimated at 10⁻⁴ m/s. The 2 to 3 order-of-magnitude difference is probably attributable to numerous, high conductivity root channels present throughout the lower B horizon. Stormflow was generated when the water table rose into the high conductivity B horizon. Outflow at the stream bank exited from the B horizon with most water flowing from high conductivity root channels. The rate of outflow was controlled by the position of the water table. Since the water table remained parallel to the overall hillslope, the hydraulic gradient remained approximately constant. Only the cross-sectional area available for flow varied. Once outflow had commenced, the rate of outflow was sensitive to variation in the rainfall rate. Input-outflow lag-times were as little as one hour. The time lag to initiation of outflow was 19 hours. Most of this lag was attributable to the filling of storage requirements after a two month period of no rain. The distribution of the hydrogeologic units in the research plot was found to be representative of the research area. Lag times were found to be in the range found in another similar B.C. mountain basin. It is concluded that the mechanism of stormflow generation operating in the research plot can be generalized to other similar basins.

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