Location: Watershed Physical Processes Research Unit
Title: A seepage erosion sediment transport function and geometric headcut relationships for predicting seepage erosion undercutting
AuthorsChuAgor, M   
Fox, G   
Wilson, Glenn 
Submitted to: Proceedings of the World Environmental and Water Resources Congress Conference
Publication Type: Proceedings Publication Acceptance Date: February 1, 2009 Publication Date: May 17, 2009 Citation: ChuAgor, M.L., Fox, G.A., Wilson, G.V. 2009. A seepage erosion sediment transport function and geometric headcut relationships for predicting seepage erosion undercutting. Proceedings of the World Environmental and Water Resources Congress Conference. pp. 110. doi: 10.1061/41036(342)378. Interpretive Summary: Seepage erosion is an important factor in hillslope erosion. However, predicting erosion by subsurface flow or seepage and applying those effects into computer models that describe instability remains a challenge. Limitations exist with mathematical formulas that describe sediment movement, such as neglecting the geometry of the seepage undercut. The objective was to develop a model for sediment movement that can predict seepage erosion and undercutting of banks with time based on previously reported soil block experiments covering a wide range of hydraulic, soil type, and packing (i.e., slope and bulk density) combinations. The sediment movement formula was represented by an equation based upon the velocity of water flow wherein the rate of erosion was related to the difference between the velocity of flow and a "critical" velocity. The critical velocity was determined from laboratory measurements using soil blocks in which the water level imposed that results in seepage erosion being initiated. The relationship between the volume of sediment eroded per bank face area and the size of the undercut was also derived. Using a statistical function, the geometrical relationships between the lateral and vertical dimensions of the undercut were estimated. The relationship between the predicted and observed time at which a given amount of undercut developed was excellent. The velocity of flow out of hillslope can be used with the sediment movement formula derived in this study to predict the dimensions and shape of the undercut. This enables the prediction of the impact of seepage erosion undercutting on hillslope stability. Technical Abstract: Seepage erosion is an important factor in hillslope instability and failure. However, predicting erosion by subsurface flow or seepage and incorporating its effects into stability models remain a challenge. Limitations exist with all existing seepage erosion sediment transport functions, including neglecting the three dimensional geometry of the seepage undercut. The objective was to develop a sediment transport model that can predict sediment mobilization (i.e., seepage erosion and undercutting) with time based on previously reported threedimensional soil block experiments covering a wide range of hydraulic, soil type, and packing (i.e., slope and bulk density) combinations. The transport function was represented by an excess velocity equation wherein the rate of erosion was related to the difference between the steady state velocity and the critical velocity (R2 = 0.62). The critical velocity was derived from a critical head measured in the laboratory using the three dimensional soil block. The relationship between the eroded volume per bank face area and the amplitude of the headcut was also derived. Using a threedimensional Gaussian function, the geometric relationships between the lateral and vertical dimensions of the headcut were then estimated. Linear regression analysis between the predicted and observed time at which a given amount of headcut developed resulted in an R2 of 0.86. The ground water velocity exfiltrating a hillslope can be used with the derived sediment transport function to predict the dimensions of the headcut and the geometry of the undercut which enables the prediction of the impact of seepage erosion undercutting on hillslope stability.
