Soil Pipe Flow and Internal Erosion: What We Don't Know Can Kill Us.

Flow through macropores can be sufficiently rapid that the shear forces exceed the critical shear strength of the macropore walls causing particle detachment (internal erosion). Development of a soil pipe from enlargement of the pore results in more rapid flow and thus greater internal erosion, particularly detachment of aggregates and mass failure of pipe walls and roofs. When the sediment transport capacity of the soil pipe is exceeded by detachment, the soil pipe can clog causing rapid pressure increases and hillslope instability. Many of the worst-case scenarios of soil erosion, e.g., dam and levee failures, landslides and debris flows, streambank failures, and gully erosion are attributed to internal erosion from flow through soil pipes. Considering the large number of landscape processes that may be influenced by soil piping, especially in the future considering land use changes and climate change projections of enhanced extreme events, considerable research advances are needed in this field. While soil physicist among other disciples have made considerable advances in understanding preferential flow and mathematically describing soil processes, very little work has been done on the processes involved in this related field. Questions exist as to the conditions under which preferential flow through soil pipes result in internal erosion, pipe collapse and/or clogging, and hillslope instability by increased pore-water pressure. Also, how do we model the turbulent flow and sediment transport processes? How well do laboratory measurements and numerical models translate to field observations? This paper will present laboratory and field observations to highlight gaps in our understanding of the processes associated with soil piping as well as analytical and numerical representations of individual processes. The paper will highlight future research needs to address these complex soil processes.