|Shields jr, Fletcher|
Submitted to: Hydrological Processes
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/21/2009
Publication Date: 3/31/2009
Citation: Shields Jr, F.D., Simon, A., Dabney, S.M. 2009. Streambank Dewatering for Increased Stability. Hydrological Processes, 23: 1537-1547. DOI 10.1002/hyp.7286. Interpretive Summary: Streambank erosion is a major source of sediment in disturbed stream systems. To reduce the amount of sediment being eroded from streambanks without relying on large-scale, expensive earth-moving operations to flatten bank slopes, methods to increase bank resistance by removing water were tested. Two techniques were employed and tested on Little Topashaw Creek, Mississippi: drains, and pumps. Both measures were installed in an eroding bank and monitored along with a control plot with digital instruments to determine (1) the amount of water that could be removed by each technique, (2) the increase in bank strength resulting from the removal of water, and (3) rates of streambank erosion. Results showed that dewatering of the bank generally reduced bank-retreat rates by a factor of two, indicating that these low-cost techniques may be appropriate as an in-channel conservation measures in eroding channels.
Technical Abstract: Streambank erosion is often the dominant source of sediment leaving watersheds disturbed by human activities. Collapse of high, steep banks is one of the most serious forms of streambank erosion. The risk of a given bank experiencing mass failure is a function of bank height, angle, and soil strength, which is governed by soil moisture. Bank stability may be improved by maintaining lower levels of soil moisture. Two methods for bank dewatering were tested in adjacent sections of streambank bordering a deeply incised channel in northern Mississippi: a low-cost submersible pump system and subsurface horizontal drains. Pore-water pressures (matric suction) were continuously monitored for two years at the pumped site, at an adjacent untreated control section, and for one year at the site stabilized with horizontal drains. Resulting data were used to calculate a time series of the factor of safety using a computer model. Over the course of two wet seasons, average bank retreat for the control, pumped and drained plots were 0.43, 0.21 and 0.23 m, respectively. At the pumped site, matric suction (negative pore-water pressure) was 3-4 kPa higher than at the control site during the most critical periods. Accordingly, computed factors of safety were above the failure threshold at the pumped site, but fell below unity at the control site on 11 occasions over the period of observation. Similarly, the drained site displayed generally higher values for matric suction and higher safety factors except for two events when drains were evidently overwhelmed with the volume of local surface and subsurface flows. Initial cost of the dewatering treatments were significantly less than orthodox bank stabilization measures, but operation and maintenance requirements may be greater.