THE HAZARD CYCLE OF SHALLOW LANDSLIDES

Following a shallow landslide, hydrogeomorphic processes in the initiation zone respond to the discontinuities of soil depth, topographic expression, and hydrologic and forest conditions to renew the landslide hazard in what can be a self-perpetuating fashion. At the headscarps of shallow landslides in forested terrain, episodic erosion events punctuate periods dominated by the deposition of hillslope material. Recovery of soil depth, topographic expression, and subsequently the failure hazard, occurs through small-scale processes that infill the failure area. Forests contribute structural elements to the headscarp soils that may act to trap smaller particles and further promote soil accumulation; they also contribute finer organic material that can have a defining influence on the hydrologic and geotechnical properties of the resulting soils. Recovery of the soil-hydrologic system within the accumulated soils may lag behind the recovery of soil depth and topographic expression and may therefore be a major factor in the case of recurring failures and the hazard cycle.

The rate of infilling of initiation zones determines the period of immunity from shallow landslide for that hillslope location; more rapid infilling reduces the immunity period and subsequent infilling increases the size of future failures and therefore the probability of inducing secondary hazards such as debris flow. Rates of redistribution of hillslope material decline with time and are attributable to both vegetation re-establishment and diminished surface topographic variability; however, stochastic elements of the surrounding forests strongly affect soil accumulation. The central tendency of soil accumulation was found to approximate a sigmoid curve with the majority of accumulation occurring within 100 years of failure. Soil depth on adjacent hillslopes positively influences soil accumulation in failure areas, but repeated shallow landslides can deplete hillslope materials from the contributing area. The sediment balance of weathering, storage, and evacuation strongly influences cycles of failure and recharge and the progression of landscape evolution.