Hydrogeomorphic Relations Among Soil Pipes, Flow Pathways, and Soil Detachments Within A Permafrost Hillslope

Soil pipes have been cited as preferential flow features within permafrost hillslopes. However, there has been limited discussion on their mechanisms of formation and relation to other hillslope process such as slope stability and the development of preferential flow pathways. On an organic-covered permafrost-underlain subarctic slope in Yukon Territory, Canada, pipe formation and development processes were studied in an attempt to ascertain their larger role in the hillslope hydrogeomorphic framework. Pipes are widespread on the study slope, conveying water during the snowmelt period when the water table is at or near the surface. During snowmelt, excess pore pressures generated at the heterogeneous thaw front reduce soil strength, increasing the potential for thaw-zone detachment failures. Upon failure, seepage erosion at the interface between organic and mineral soils induces pipe formation. Pipes, which are closely associated with rills and other zones of preferential flow, then become integrated into the larger hillslope drainage network. Drainage within preferential flow pathways advects heat, leading to local increases in thaw and encouraging ground subsidence where the permafrost is ice-rich. In this way, a positive-feedback occurs whereby preferential flow pathways reinforce their influence on the drainage network through subsidence.