The role of soil pipe and pipeflow in headcut migration processes in loessic soils

Authors: XU, XIMENG - Northwest A&f University; Wilson, Glenn; ZHENG, FENLI - Northwest A&f University; TANG, QIUHONG - Northwest A&f University

Submitted to: Earth Surface Processes and Landforms
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/19/2020
Publication Date: 3/14/2020
Citation: Xu, X., Wilson, G.V., Zheng, F., Tang, Q. 2020. The role of soil pipe and pipeflow in headcut migration processes in loessic soils. Earth Surface Processes and Landforms. 45. pp. 1749-1763. https://onlinelibrary.wiley.com/share/2AJQFAGBGRMB4GM2XCQT?target=10.1002/esp.4843.

Interpretive Summary: When surface runoff is concentrated into channels, sediment losses will increases due to formations of gullies. The head of the gully will form a scour hole or headcut that migrates upslope. However, headcut formation is sometimes mistaken as the result of surface runoff when instead it was formed by and its movement upslope was controlled by flow through soil pipes below the channel. The objectives were to determine the effects of the presence of a soil pipe and flow through a soil pipe on headcut movement. Experiments were conducted in the laboratory on headcut movement under freely drained conditions and conditions of a shallow water table. Soil beds with a 3-cm deep initial headcut were formed in a flume with a 1.5-cm diameter soil pipe 15 cm below the bed surface. Runoff was applied at a constant rate of 68 L/min and flow into the soil pipe was applied at a constant rate of 1 L/min both at the upper end of the flume. The typical response to runoff was the formation of a headcut that extended in depth until a scour hole was established at which time the headcut migrated upslope. Flow through the soil pipe caused erosion of the inside of the soil pipe at the same time that runoff was causing a scour hole to deepen and migrate. When the headcut extended to the depth of the soil pipe, surface runoff entering the scour hole interacted with flow from the soil pipe also entering the scour hole. The interaction of these two sources resulted in sudden increase in erosion of the soil pipe downslope and both sources of flows were quickly diverted through the soil pipe without any surface runoff below the headcut. This dramatically altered the headcut processes and greatly accelerated the headcut migration rates and sediment concentrations. Conditions in which a perched water table provided seepage into the soil pipe in addition to upslope inflow into the soil pipe increased the sediment concentration by 42% and the headcut migration rate by 47% compared with flow into the soil pipe under free-drainage conditions. The time that overland flow converged with subsurface flow was advanced under seepage conditions by 2.3 and 5.0 minutes compared with free drainage condition. This study confirmed that pipeflow dramatically accelerates headcut migration especially under conditions of shallow perched water tables and highlights the importance of understanding these processes in headcut migration processes.

Technical Abstract: Headcut migration by concentrated overland flow significantly increases sediment yield. However, headcut formation and migration is sometimes mistaken as the result of overland flow without realizing that the headcut was formed by and being influenced by flow through soil pipes into the headcut. The objectives were to determine the effects of the presence of a soil pipe and pipeflow on headcut migration by conducting controlled laboratory experiments on headcut migration under free drainage condition and seepage conditions. Soil beds with a 3-cm step were formed in a flume with a 1.5-cm wide soil pipe 15 cm below the bed surface. Steady overland flow at 68 L/min was applied simultaneously with inflow into the soil pipe under a constant rate of 1 L/min. The typical response was the formation of a headcut by overland flow that extended in depth until an equilibrium plunge pool depth was established at which point the headcut migrated upslope. Simultaneously, internal erosion of the soil pipe enlarged the pipe diameter and increased the pipeflow rate as two independent sources, overland flow and pipeflow. When the headcut extended to the depth of the soil pipe, pipeflow then entered into and interacted with the headcut plunge pool which resulted in both flows converging into the exiting soil pipe. This dramatically altered the headcut processes and greatly accelerated the migration rates and sediment concentrations. Conditions in which a perched water table provided seepage into the soil pipe in addition to flow into the soil pipe from an upslope contributing area increased the sediment concentration by 42% and headcut migration rate by 47% compared with flow into the soil pipe under free-drainage conditions. The time that overland flow converged with subsurface flow was advanced under seepage conditions by 2.3 and 5.0 minutes compared with free drainage condition. This study confirmed that pipeflow dramatically accelerates headcut migration especially under conditions of shallow perched water tables and highlights the importance of understanding these processes in headcut migration processes.