The effect of seasonal variations in the Red River and upper carbonate aquifer on riverbank stability in Winnipeg

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Tutkaluk, Jeffrey M.
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A transient Finite Element seepage model has been developed which incorporates a confined aquifer, river, and groundwater within a lacustrine riverbank. The transient seepage modeling is performed over a period when the piezometric elevations of the unconfined aquifer are increasing and river levels are decreasing. The transient groundwater regime computed, within an idealized riverbank section, are similar to those observed through piezometer monitoring of a site on which the model is based. The seepage results are then incorporated into slope stability analysis and the influence of seasonal fluctuations in piezometric elevation of the aquifer and river are examined. Parallel slope stability analysis is also performed using assumed static groundwater elevations. The results of the slope stability modeling of the two different methods of determining piezometric elevations within a riverbank are compared and contrasted using a range of effective shear strength parameters from c' = 3 kPa, [straight phi] ' = 8 to c' = 5 kPa, [straight phi] ' = 17. Safety factors computed using FEM generated porewater pressures are typically higher than those using assumed static groundwater levels for a given set of effective shear strength parameters. However, the reduction in safety factor over the modeling duration is greater when using FEM porewater pressures compared to assumed groundwater levels. The difference in computed safety factors is attributed to the transient model incorporating the combined destabilizing influence of the recharging unconfined aquifer and decreasing river levels (FE computed piezometric elevations) compared with only the destabilizing influence of decreased river level (assumed groundwater elevations) in the static analysis.