A THEORETICAL-MODEL OF THE EFFECTS OF TIMBER HARVESTING ON SLOPE STABILITY

An infinite slope stability model is proposed which incorporates changes in root cohesion and vegetation surcharge through several timber management cycles along with the stochastic influence of rainfall on pore water pressure. Recovery of rooting strength and tree surcharge following timber harvest are simulated by a sigmoid relationship, while root deterioration of harvested vegetation is described by an exponential decay function. The effects of long-term timber management on probability of failure are simulated by overlaying the impacts of a prior vegetation removal on a more recent removal. For each year the critical pore water pressure (u(crit)) needed to trigger slope failure is computed. An empirical function relating piezometric level to antecedent rainfall, storm intensity, and total precipitation is presented to assess the probability of occurrence of u(crit), based on historical rainfall records for a site in coastal Alaska. Simulations of probability of failure indicate that alternate thinnings and clear-cuts and clear-cuts alone produce less stable conditions than shelterwood harvesting systems and partial cuts. Repeated harvestign cycles with progressively shorter rotations, reduced regeneration potential on new vegetation, and destruction of understory vegetation during logging or site preparation can all increase the probability of failure. These analyses can provide land managers with options for acceptable vegetation management strategies on potentially unstable hillslopes.