Risk of Landslides in Shallow Soils and Its Relation to Clearcutting in Southeastern Alaska

Abstract

A significant increase in the frequency of landslides in shallow soils on hillside slopes of southeastern Alaska following timber harvest by clearcutting has been observed. This phenomenon relates to the loss of root strength and evapotranspiration stress that follows the cutting of the trees. A method for evaluating the landslide risk is described in this paper. A hillside with a nearly uniform slope is represented by an infinite slope and the piezometric level required for shear failure is computed. A one-dimensional infiltration-seepage model is used to calculate the response of the piezometric level to rainfall. Weather data are used to calculate the probability of the piezometric level exceeding the value required for slope failure. Uncertainties in soil strength and slope angle may also be accounted for in the calculation of failure probability. Field data obtained from a site near Hollis, Alaska, are used to illustrate the method of risk evaluation and cost analysis. FORESX SCI. 26:495-510. TIMBER HARVEST BY CLEARCUTTING is a common practice in the coniferous for- ests of the Cordilleran area. Among the many ecological problems caused by clearcutting in this area are the effects on the stability of steep hillside slopes. Data have been presented to show that clearcutting increases the creep rate of slopes (Barr and Swanston 1970, Gray 1970). Empirical evidence gathered in southeastern Alaska (Bishop and Stevens 1964, Swanston 1969) indicates that the frequency of landslides classified as debris avalanches increases significantly a few years after clearcutting. The debris avalanches in southeastern Alaska usually occur during periods of heavy autumn rain and begin with a small slide located near the top of the slope. Because of the high water content, the disturbed soil flows rapidly downhill as a viscous fluid and erodes much of the soil along its path. This results in a scar that extends from the initial slip to the bottom of the slope (Fig. 1). The probable occurrence of debris avalanches and its consequences are clearly important factors to be considered in the planning of logging opera- tions. Because the various factors that contribute to the development of a debris avalanche cannot be predicted with precision, a probabilistic approach to risk evaluation is appropriate. This approach is comparatively simple when applied to slides on approximately uniform or planar slopes with a shallow soil cover underlain by impervious bedrock such as those near Hollis, Alaska. Hence, so- lutions for these special conditions are readily obtained. The solutions presented