Incorporating seepage processes into a streambank stability model

Authors: CHU-AGOR, M - Oklahoma State University; FOX, G - Oklahoma State University; Wilson, Glenn

Submitted to: Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE)
Publication Type: Proceedings
Publication Acceptance Date: 5/21/2009
Publication Date: 6/21/2009
Citation: Chu-Agor, M., Fox, G.A., Wilson, G.V. 2009. Incorporating seepage processes into a streambank stability model. Proceedings of the American Society of Agricultural and Biological Engineers International (ASABE). pp 1-10.

Interpretive Summary: Seepage processes are usually not included in the evaluation of a streambank’s stability even though seepage can be an important cause of bank failure. This study included the effects of seepage, such as the seepage gradient forces (the energy of the flowing water that is transferred to the soil particles) and the undercutting of streambanks by seepage induced erosion, into the Bank Stability and Toe Erosion Model (BSTEM). The importance of these seepage processes on bank stability were valuated using BSTEM. The effects of the seepage force were incorporated into BSTEM by modifying the force balance. Seepage erosion undercutting was simulated using a recently proposed sediment transport equation. The modified BSTEM was then used to evaluate the stability of a streambank along Little Topashaw Creek under different scenarios: (1) without seepage forces and undercutting, (2) with seepage forces only, (3) with seepage undercutting only, and (4) with both seepage force and undercutting. For a condition where the streambank was saturated due to a high water table, the Factor of Safety (FS) decreased by as much as 66% (i.e., FS decreased from 2.68 to 0.91) from that of a dry condition. A FS value below 1.0 indicates that the streambank is not stable and is expected to fail. This decrease in FS was due to the decrease in the strength of the soil as the soil-water pressure increased. Including the effects of the seepage force resulted in an average decrease in FS of approximately 30 to 50% for all water table depths. Undercutting of streambank due to seepage erosion reduced the FS by approximately 6% for an undercut that went 5 cm into the bank and 11% for a 10 cm deep undercut due to the loss of supporting material in the eroded layer. Seepage erosion required 15 to 20 cm of undercutting of the banks to become the dominant failure mechanism over seepage forces and pore-water pressure effects. The combined effects of all three seepage processes reduced the streambank’s FS by up to 63% when the water table reached the top of the bank height. The development of a bank stability model capable of simulating seepage processes was necessary in order to better understand site-specific failure mechanisms.

Technical Abstract: Seepage processes are usually neglected in bank stability analyses although they can become a prominent failure mechanism under certain field conditions. This study incorporated the effects of seepage (i.e., seepage gradient forces and seepage erosion undercutting) into the Bank Stability and Toe Erosion Model (BSTEM) and evaluated the importance of seepage mechanisms on bank stability. The effects of the seepage force were incorporated into BSTEM by modifying the force balance. Seepage erosion undercutting was simulated using a recently proposed sediment transport function. The modified BSTEM was then used to evaluate the stability of a streambank along Little Topashaw Creek under different scenarios: (1) without seepage forces and undercutting, (2) with seepage forces only, (3) with seepage undercutting only, and (4) with both seepage force and undercutting. For a condition where the bank was fully saturated, the Factor of Safety (FS) decreased by as much as 66% (i.e., FS decreased from 2.68 to 0.91) from that of a dry condition due to the decrease in the frictional strength of the soil as the pore-water pressure increased. Incorporating the effects of the seepage force resulted in an average decrease in FS of approximately 30 to 50% for all water table depths. Seepage erosion undercutting reduced the FS by approximately 6% for a 5 cm undercut (i.e., 2% of bank height) and 11% for a 10 cm undercut (i.e., 3.3% of bank height) due to the loss of supporting material in the conductive layer. Seepage erosion undercutting required 15 to 20 cm of seepage undercut to become the dominant failure mechanism over seepage forces and pore-water pressure effects. The cumulative effects of seepage reduced this streambank’s FS by up to 63% when the water table reached the entire bank height. The development of a bank stability model capable of simulating seepage processes was necessary in order to better understand site-specific failure mechanisms.