Submitted to: Journal of the American Water Resources Association
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/25/2003
Publication Date: N/A
Interpretive Summary: Riverbanks collapse when they become saturated with water, increasing the forces that drive banks to slump and reducing their strength. As an alternative to conventional methods of bank stabilization such as using rock armor or shaving the bank back to a gentler angle, we have experimented with pumps located below the water table in an eroding streambank. The pumps lower the water table, drying out the slope. Our experiment involved two identical plots on a streambank. In the first we actively pumped water out of the bank. In the second, we had no treatment (the control site). We measured the water table depth at both plots, and used the information to run a computer model that predicts when banks will slump. The site with the pumps is consistently drier than the control site. The model shows the pumped site to have a greater degree of bank stability, and shows that during the two year study period it should never have collapsed. The control site had two periods when the model predicts it should have collapsed. Our measurements of bank edge position show that the pumped bank eroded half as fast as the control plot, and that the movements agree with the model. The work suggests that pumping banks dry may be an effective method of increasing their strength.
Technical Abstract: Bank retreat due to mass failure causes the loss of thousands of hectares of land every year in the US and elsewhere, and is a major source of sediment and contaminants in rivers and streams. A self-contained and low-cost submersible pump system has been tested in a section of incised streambank in Northern Mississippi, to assess the potential of de-watering as an innovative bank stabilizing method. Pore-water pressures have been continuously monitored for two years, and the data have been applied to the ARS Bank Stability Model to calculate Factor of Safety (Fs). Compared with an identical control plot, the pumps reduced pore-water pressures in the de-watered streambank by 5-10 kPa during the critical winter and spring wet period, increasing Fs by 15-20% and most importantly, maintaining values above the failure threshold throughout the two year study. By contrast Fs on the control plot fell below the failure threshold on several occasions in both years. During the first winter period average bank retreat was 21 cm for the de-watered plot, compared with 43 cm for the control plot. No mass failures were recorded at either portion of the de-watered plot, in contrast to the control plot. The cost of the system is significantly less than alternative bank stabilization measures.