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United States Department of Agriculture

Agricultural Research Service

Research Project: Technologies for Managing Water and Sediment Movement in Agricultural Watersheds

Location: Watershed Physical Processes Research Unit

Title: Pore-water pressures associated with clogging of soil pipes: Numerical analysis of laboratory experiments

Authors
item Wilson, Glenn
item Fox, Garey -

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: April 7, 2012
Publication Date: July 1, 2013
Citation: Wilson, G.V., Fox, G. 2013. Pore-water pressures associated with clogging of soil pipes: Numerical analysis of laboratory experiments. Soil Science Society of America Journal. 77(4):1168-1181.

Interpretive Summary: Water flows so fast through large soil pores, called soil pipes, that the inside of the pipe walls erode. When inside erosion is high, the pipe can clog which can cause a buildup or water pressures inside the pipe. Many extreme erosion events such as landslides, debris flows, and gullies have been thought to be caused by clogging of soil pipes but proof has been lacking. Laboratory and field measurements have failed to measure water pressures within pipes and models of pipe flow have not included erosion inside pipes or pipe clogging. The objectives of this study were to model laboratory experiments of pipe flow in which clogging was observed. The soil pipe was treated like a layer of highly conductive soil in the model. The model used two different conditions for water entry into the soil pipe: constant flow into the pipe (CF) and constant water level at the pipe entry (CH). This was done to understand the pressure buildups due to pipe clogging and differences between the two conditions for flow into a pipe. Unlike past work. these simulations were the first to include enlargement of the pipe due to erosion of the inside walls, pipes that were only partially filled with water, and clogging of pipes. Both CF and CH conditions proved that water pressures within soil pipes will buildup rapidly as a result of pipe clogging. Water pressures within the pipe jumped around 60 m for CF and 20 cm for CH in less than 0.1 s while water pressures in the soil just 4 cm from the pipe had not changed. These findings demonstrate the need to measure pressures within soil pipes due to the differences between the pipe and the soil. Water pressures within the pipe below the clog drained to unsaturated conditions in less than 0.25 s indicating the ability of soil pipes to rapidly recover when clogs are flushed from the soil pipe. These processes need to be included into erosion prediction models in order to properly model hillslopes.

Technical Abstract: Clogging of soil pipes due to excessive internal erosion has been hypothesized to cause extreme erosion events such as landslides, debris flows, and gullies, but confirmation of this phenomenon has been lacking. Laboratory and field measurements have failed to measure pore water pressures within pipes and models of pipeflow have not addressed internal erosion or pipe clogging. The objectives of this study were to model laboratory experiments of pipeflow in which clogging was observed. Richards' equation was used to model pipeflow with the soil pipe represented as a highly conductive, low retention porous media. The modeling used two contrasting boundary conditions: constant flux (CF) and constant head (CH), in order to quantify pressure buildups due to pipe clogging and differences in simulated pressures between the two imposed boundary conditions. Unique to these simulations was inclusion of pipe enlargement with time due to internal erosion, representation of partially-full flow conditions, and inclusion of pipe clogging. Both CF and CH boundary conditions confirmed the concept of pressure buildups as a result of pipe clogging. Pressure jumps of around 60 m for CF and 20 cm for CH occurred in less than 0.1 s while soil water pressures 4 cm from the pipe had not responded. These findings demonstrate the need to measure pressures within soil pipes due to hydraulic nonequilibrium between the pipe and soil matrix. Pore water pressures within the pipe below the clog rapidly (less than 0.25 s) drained to unsaturated conditions indicating the ability of soil pipes to drain hillslopes and rapidly recover when clogs are flushed from the soil pipe. These dynamic processes need to be incorporated into stability models in order to properly model hillslope processes.

Last Modified: 10/24/2014
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