Landslides are caused by a variety of conditions, many of which are present in the PSN neighborhood.
Heavy rainfall leading to ground saturation (at times combined with wind)
Legacy logging roads and reduced root reinforcement due to timber harvest
Steep topography
Rain on snow events
Thick accumulation of soils or sediments
Past landslide events across SE Alaska suggest that intense rainfall over a few hours is needed to saturate slopes and increase the likelihood of landslides.
Runout: Once initiated, the path and the downslope area impacted by landslides is determined by two key factors:
· Volume of the slide material
· Degree of saturation
Highly saturated landslides often transition into debris flows which can travel long distances due to their fluid-like behavior. These flows can entrain large debris such as boulder and wood as they travel rapidly downslope which makes them increasingly destructive. These flows tend to follow stream channels although larger ones can sometimes form their new pathways and travel further and inundate extensively.
Debris flows are common on Prince of Wales and particularly along the PSN area. These events tend to initiate on steep, rocky slopes along Rainbow Ridge, which has been subject to timber harvest activities and road construction in the mid to late 20th century. Slides generate visible scars on the landscape which can be subsequently colonized by red alder trees (and other species), which contrast with predominate coniferous forest that characterizes PoW. The pattern and age of alder tracks around PSN and PoW reflect the timing and extent of landslide activity. These tracks are often (but not guaranteed) to be the location of future landslides.
Debris fans: Over long time periods, numerous debris flow deposits accumulate at the base of steep slopes and build cone-like landforms called debris or alluvial fans. These depositional landforms are readily discernible with topographic data and thus they serve as key indicators of where debris flows are likely to occur in the future.