Streambank dewatering for increased stability

Abstract

Abstract:Streambank erosion is often the dominant source of sediment leaving modified watersheds. Mass failure 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. Two methods for bank dewatering were tested in adjacent sections of streambank bordering a deeply incised channel in northern Mississippi: a low-cost pump system and subsurface horizontal drains. Pore water pressures (both positive and negative pressures, or matric suction) were continuously monitored for 2 years at the pumped site, at an adjacent untreated control section, and for 1 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 retreats for the control and pumped plots were 0Ð43 and 0Ð21 m, respectively. More limited monitoring revealed that the site with passive drains retreated about 0Ð23 m. At the pumped site pore water pressure was 3-4 kPa lower 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 lower pore water pressure and higher safety factors except for two events when drains were evidently overwhelmed with the volume of local surface and subsurface flows. These results suggest, but do not prove, that bank dewatering promoted lower rates of bank retreat and higher levels of stability since the three sites had slight differences in soils, geometry and boundary conditions. Initial cost of the dewatering treatments were significantly less than orthodox bank stabilization measures, but operation and maintenance requirements may be greater.

Burrowing Invasive Species: An Unquantified Erosion Risk at the Aquatic‐Terrestrial Interface

“…Under baseflow conditions, however, increased drainage of the bank associated with burrow presence may reduce antecedent near‐bank face pore water pressures, reducing susceptibility to erosion during lower magnitude flow events. A similar effect has been observed in artificially drained high, steep river banks (Shields et al, ). Further research is needed to quantify the hydrological significance of burrows and their implications for bank stability.…”

Interactions between stream channel incision, soil water levels and soil morphology in a wetland in the Hogsback area, South Africa

“…Both these factors are controlled by the permeability of the soil and difference in hydraulic head. The relationship between depth of water tables and depth of erosion gully is in line with the understanding (see Shields et al 2009Shields et al , 2010 of changing hydraulic head by lowering the outlet (Figure 4). However, the response of the observations in group 9 does not relate to the degree of drainage, implying localised hillslope aquifers supply water at a high rate, balancing the drainage.…”

“…With the lowering and widening of the streambed, wetlands are often dewatered as groundwater will flow towards incised streams as opposed to parallel to unincised streams (Shields et al 2009(Shields et al , 2010. Water flow through incised wetlands is concentrated to the stream channel and high turbidity flow with high erosive energy are often recorded (Shields et al 2009). Increased sediment loss and reduced base flows results in increase in pollution levels, degradation of the physical habitat and consequently a reduction in biodiversity (Shields et al 2010).…”

A low-cost subsurface drainage technique to enhance gully bank stability in the sub-humid highlands of Ethiopia

“…These shear strength and weight changes destabilize the bank and cause failures along incised channels (Langendoen et al, 2013;. Thus, preventing water table rise and maintaining unsaturated conditions (negative pore water pressure) could provide greater bank stability and reduced mass failure frequency (Shields et al, 2009). Gullies in the Ethiopia highlands formed after the deforestation in the late 1980s have been advancing uphill (Tebebu et al, 2010;Zegeye et al, 2016).…”