The role of subsurface flow in hillslope and streambank erosion: A review of status and research needs

Authors: FOX, G. - Northwestern Oklahoma State University; Wilson, Glenn

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2009
Publication Date: 3/5/2010
Citation: Fox, G.A., Wilson, G.V. 2010. The role of subsurface flow in hillslope and streambank erosion: A review of status and research needs. Soil Science Society of America Journal. 74:717-733.

Interpretive Summary: Sediment is the most common cause of stream impairment. Great progress has been made in understanding processes of soil erosion due to surface runoff and incorporating these in prediction tools. However, in many locations the dominant source of sediment comes from mass slumping of hillslopes and streambanks and the main cause can be subsurface flow. This paper highlights the mechanisms and importance of subsurface flow processes in erosion associated with hillslopes and streambanks. Subsurface flow effects erosion indirectly by the relationship of soil properties with soil water pressure and directly by seepage and flow through large pores called soil pipes. Seepage contributes to erosion through several related mechanisms, e.g. forces related to the gradient in soil water pressure which affect the resistance of soil particles to dislodgement from the soil matrix, and particle movement when soil particles become suspended in the seepage. Preferential flow through large soil pores results in erosion of the insides of these soil-pipes, which can cause landslides and debris flows when pore water pressures build up inside soil pipes. This process can also cause embankment failures, and gully initiation by tunnel collapse or reestablishment of ephemeral gullies by sudden soil mass failures. Research in the past decade has advanced our understanding of these processes and mathematical relationships for incorporating these processes into mechanistic, process-based models. However, further research advances are necessary for soil characterization of the controlling parameters, especially considering the complexity that arises due to the interactive effects of surface flow, seepage, pipe flow and vegetation on soil erosion properties. The authors believe that multidisciplinary efforts between soil scientists, engineers, and hydrologists are necessary to fully understand and integrate subsurface flow and soil erosion processes.

Technical Abstract: Sediment is the most common cause of stream impairment. Great progress has been made in understanding processes of soil erosion due to surface runoff and incorporating these in prediction technologies. However, in many landscapes the dominant source of sediment is derived from mass wasting of hillslopes and streambanks and the driving mechanism can be subsurface flow. This paper highlights the mechanisms and importance of subsurface flow processes in erosion associated with hillslopes and streambanks. Subsurface flow effects erosion indirectly by the relationship of soil properties with soil water pressure and directly by seepage and pipe flow processes. Seepage contributes to erosion through interrelated mechanisms, e.g. hydraulic gradient forces which reduce the resistance of the particle to dislodgement from the soil matrix and particle mobilization when soil particles become entrained in exfiltrating water. Preferential flow through soil pipes result in internal erosion of the pipe, which can cause landslides and debris flows when pore water pressures build up inside soil pipes, embankment failures, and gully initiation by tunnel collapse or reestablishment of ephemeral gullies by pop-out failures. Research in the past decade has advanced our understanding of these processes and mathematical relationships for incorporating these processes into mechanistic, process-based models. However, further research advances are necessary for soil characterization of the controlling parameters, especially considering the complexity that arises due to the interactive effects of surface flow, seepage, pipe flow and vegetation on soil erosion properties. The authors believe that multidisciplinary efforts between soil scientists, geotechnical engineers, hydraulic engineers, and hydrologists are necessary to fully understand and integrate subsurface flow and soil erosion processes.