UBC Theses and Dissertations
Groundwater hydrology and slope movement at Pavilion, B.C. Nadler, Denise Eileen
The rate of earthflow motion at Pavilion, British Columbia varies seasonally with maximum displacements occurring from early winter (December) to mid- July, and minimum movement occurs during the period September to December. Earthflow motion is a moisture-dependent process; therefore increased movement rates in spring and summer may be attributed to high pore water pressures in the failure zone as a result of seasonally high groundwater recharge from snowmelt and rainfall. The objectives of this thesis are to examine the causal relationship between the climate, moisture regime and subsurface hydrology, and to correlate cumulative movement with piezometric observations from several sites on the earthflow. The hydrologic budget for the drainage basin from August 1981 to June 1982 was estimated from precipitation records and potential evapo-transpiration values. The climate record indicated that more than 95% of the annual groundwater recharge occurred from snowmelt and rainfall in April and May 1982. Two notable increases of groundwater levels were observed in 1981, and one rise of groundwater elevation was seen in 1982. The first rise in 1981 and the rise noted in 1982 occurred between May and July and were correlated with low-elevation snowmelt from March 1 to April 1. An estimated one-month phase lag between recharge and piezometer response suggests that recharge from early snowmelt occurs relatively quickly. Tension cracks and fractures at shallow depths (10-20m) of the earthflow and its surrounding areas probably provide conduits for efficient groundwater flow and rapid piezometric response. The second groundwater level rise in 1981 did not occur until the period from October to November and was substantially greater than the earlier rise. This was attributed to snowmelt higher in the basin during May and June, and rainfall in May and July 1981. Thus a four- to five-month time lag between regional groundwater recharge and piezometer response is indicated. This is accounted for, in part, by travel time from Mt. Cole, south of the earthflow, to the earthflow itself and by slow upward movement of groundwater from the basal failure zone into the standpipes in the earthflow. Earthflow acceleration commenced in December 1981 as a result of high pore water pressures from the impulse of recharge from Mt. Cole observed in November 1981, and the rate of movement remained relatively high at least until July 1982. The rate of earthflow motion was also high from March to July 1981, which correlates very well with high groundwater levels observed in spring 1981. These observations indicate that the earthflow is sensitive to changes of the effective normal stress according to the seasonal hydraulic head fluctuations on the basal slip surface of the earthflow.
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