The stability of slopes before and after removal of forest cover was investigated. Porewater pressures and shear strengths were measured and the soil properties were determined by laboratory and it1 situ tests. A model of the soil-root system was developed to evaluate the contribution of tree roots to shear strength. The computed safety factors are in general agreement with observed behaviors of the slopes. Decay of tree roots subsequent to logging was found to cause a reduction in the shear strength of the soil-root system. La stabilite des pentes avant et aprks dkboisement a Ct C CtudiCe. Les pressions interstitielles et les rCsistances au cisaillement ont Ct C mesurCes et les propriCtCs gCotechniques ont Ct C determinies par essais en laboratoire et in situ.On a dCveloppC un modkle du systkme sol-racines pour Cvaluer la contribution des racines d'arbres B la rCsistance au cisaillement. Les facteurs de sCcuritC calculCs sont gCnCralement en accord avec le comportement observC des pentes. On a trouvC que la dCcomposition des racines aprks le diboisement produit une reduction de la rksistance au cisaillement du systkme sol-racines.

More than 300 landslides and debris flows were triggered by an October 1993 storm on Prince of Wales Island, southeast Alaska. Initiation, runout, and deposition patterns of landslides that occurred within clearcuts, second-growth, and oldgrowth forests were examined. Blowdown and snags, associated with cedar decline and "normal" rates of mortality, were found adjacent to at least 75 percent of all failures regardless of land use.Nearly 50 percent of the landslides within clearcuts occurred within one year following timber harvest; more than 70 percent of these sites had hydrophytic vegetation directly above failures. In following the runout paths of failures, significantly more erosion per unit area occurred within clearcuts than in old-growth forests on slopes with gradients from 9 to 28 (16 to 54 percent). Runout length, controlled by hillslope position within deglaciated valleys, was typically longer in old-growth forests than in second growth and clearcuts (median values were 334, 201, and 153 m, respectively). Most landslides and debris flows deposited in first-and second-order channels before reaching the main stem channels used by anadromous fish. Slide deposits in old-growth forests were composed of a higher proportion of woody debris than deposits derived from slides in second growth or clearcuts. (KEY TERMS: landslides; debris flows; land use planning; erosion and deposition; woody debris; deglaciated valleys; Alaska; anadromous fish.)1Paper No.

On October 1, 1980, a moderately intense storm at Trap Bay, Alaska, triggered a small debris slide in a forested midslope depression that was instrumented to record the maximum piezometric rise. Although the return period for the storm was <2 years, 54% (3.68 cm) of the total rain fell during the final 3 h causing the soil mantle to become nearly saturated. This resulted in an average maximum pore-water pressure of 2.2 kPa at the sliding surface, causing failure. Analysis of the mechanics of the failure showed that an apparent cohesion of 2.0 kPa, probably due to rooting strength, was acting on the 43° hillslope. Calculated factors of safety for such thin, partially saturated soils are very sensitive to small changes in apparent cohesion and should be used with caution on such sites.

Abstract:Hydrogemorphic linkages related to sediment transport in headwater streams following basin wide clear-cut logging on Prince of Wales Island, southeast Alaska, were investigated. Landslides and debris flows transported sediment and woody debris in headwater tributaries in 1961 , 1979 , and 1993 . Widespread landsliding in 1961 and 1993 was triggered by rainstorms with recurrence intervals (24 h precipitation) of 7Ð0 years and 4Ð2 years respectively. Occurrence, distribution, and downstream effects of these mass movements were controlled by landform characteristics such as channel gradient and valley configuration. Landslides and channelized debris flows created exposed bedrock reaches, log jams, fans, and abandoned channels. The terminus of the deposits did not enter main channels because debris flows spread and thinned on the unconfined bottom of the U-shaped glaciated valley. Chronic sediment input to channels included surface erosion of exposed till (rain splash, sheet erosion, and freeze-thaw action) and bank failures. Bedload sediment transport in a channel impacted by 1993 landslides and debris flows was two to ten times greater and relatively finer compared with bedload transport in a young alder riparian channel that had last experienced a landslide and debris flow in 1961. Sediment transport and storage were influenced by regeneration of riparian vegetation, storage behind recruited woody debris, development of a streambed armour layer, and the decoupling of hillslopes and channels. Both spatial and temporal variations of sediment movement and riparian condition are important factors in understanding material transport within headwaters and through channel networks.