Bank and near-bank processes in an incised channel
Introduction
The adjustment of channel width by mass-wasting and related processes represents an important mechanism of channel response and energy dissipation in incised alluvial streams. In the loess area of the Midwest United States, for example, bank material contributes as much as 80% of the total sediment eroded from incised channels (Simon et al., 1996). In unstable streams, rates of width adjustment by mass-wasting processes can occur over several orders of magnitude: 1.5 m/year in the Obion–Forked Deer River System, West TN (Simon, 1989a); 14 m/year in the Cimarron River, KS (Schumm and Lichty, 1963); about 50 m/year in the Gila River, AZ; and more than 100 m/year in some reaches of the Toutle River System, WA (Simon, 1992). The Brahmaputra River commonly adjusts its width from 50–1100 m/year (Thorne, 1999, personal communication). The range of rates reflects a diversity of channel-disturbance characteristics, environmental settings and boundary materials.
Conceptual models of bank retreat and the delivery of bank sediments to the flow emphasize the importance of interactions between hydraulic forces acting at the bed and bank toe, and gravitational forces acting on in situ bank Carson and Kirkby, 1972, Thorne, 1982, Simon et al., 1991. Failure occurs when erosion of the bank toe and the channel bed adjacent to the bank have increased the height and angle of the bank to the point that gravitational forces exceed the shear strength of the bank material. Failed bank materials may be delivered directly to the flow and deposited as bed material, dispersed as wash load, deposited along the toe of the bank as intact blocks, or as smaller dispersed aggregates (Simon et al., 1991). If deposited at the bank toe, failed bank material may temporarily increase bank stability by buttressing the bank and protecting in situ bank material from attack and entrainment by the flow. The properties of the failed bank material, in tandem with the hydraulic forces acting at the bank toe, control the residence time of failed bank material (Thorne, 1978).
Recently, attempts have been made to apply mass-wasting analyses of in situ bank materials (e.g., Little, 1982, Osman and Thorne, 1988, Lohnes, 1991, Simon et al., 1991, Casagli, 1994, Darby, 1994) in conjunction with hydraulic and sediment transport models to simulate interactions between bed (hydraulic) and bank (gravitational) processes and hence channel adjustment and evolution Simon et al., 1991, Darby, 1994, Simon and Darby, 1997a. However, these attempts are limited by a lack of understanding of the way in which hydraulic and gravitational processes interact to control long-term rates of bank retreat, channel migration and the development of equilibrium channel morphology.
Section snippets
Purpose
This paper addresses many of the fundamental issues related to the interaction of fluvial and geotechnical processes affecting streambanks and the near-bank zone. In particular, the research described here evaluates specific forces and processes controlling bank failures and channel widening in incised channels. These include:
- 1.
the role of negative pore pressures (matric suction) in the unsaturated zone in increasing bank strength, contrasted with the role of positive pore-water pressures in
The nature of streambank failures
Bank failures can be characterized by the shape of the failure surface (planar and rotational) or by the mode of failure. Rotational failures, although more damaging in terms of loss of land and generally less common along streambanks than other failure types, occur along the highest banks Simon, 1989a, Thorne, 1990 because shear stress increases quicker with depth than does shear strength (Terzaghi and Peck, 1948). As incised channels evolve with time, planar failures often occur earlier in
Forces controlling bank failures in incised alluvial channels
To better understand the complex interaction of hydraulic and geotechnical forces and processes affecting streambank mechanics, it is helpful to conceptualize these interactions in terms of typical driving forces and resisting forces during the course of incised channel evolution. Channel evolution refers to systematic temporal and spatial adjustments to morphology.
The equilibrium channel represents the initial predisturbed stage (I) of channel evolution and the disrupted channel as an
Evaluation of geotechnical forces
Apparent cohesion (ca) and effective friction angle (φ′) were measured in situ using an Iowa Borehole Shear Tester (BST; Luttenegger and Hallberg, 1981). Continuous measurement of pore-water pressures at five depths (0.3, 1.48, 2.0, 2.7 and 4.3 m) was undertaken with 5 pressure-transducer tensiometers along a 4.7-m high unstable streambank starting in November 1996. The tensiometers are subsequently referred to as T-30, T-148, T-200, T-270 and T-433, respectively. Data from the tensiometers are
Conclusions
The interaction of gravitational forces working on in situ bank material with hydraulic forces acting at the bank toe and channel bed determine rates and styles of bank erosion and, therefore, bank morphology. Hydraulic forces exerted by flowing water on in situ bank-toe material and failed cohesive material at the bank toe are often sufficient to entrain materials and to maintain steep low-bank profiles. Seepage forces exerted on in situ bank material by groundwater, downward infiltration of
Acknowledgements
The following individuals are greatly appreciated for their assistance in this study. Students from the Department of Geography, University of Nottingham: Anna Wood for photographic analysis of particle-size distributions of bank-toe material, John Bromley for collection of matric-suction data at the interface between failed cohesive blocks and in situ bank-toe materials; from the USDA, Agricultural Research Service, National Sedimentation Laboratory, Brain Bell and Mark Griffith are recognized
References (45)
A method for estimating land loss associated with stream channel degradation
Eng. Geol.
(1991)Energy, time, and channel evolution in catastrophically disturbed fluvial systems
- et al.
Process-form interactions in unstable sand-bed river channels: a numerical modeling approach
Geomorphology
(1997) Experiments on a gravity free dispersion of large solid spheres in a Newtonian fluid under shear
Proc. R. Soc. London, Ser. A
(1953)An approach to the sediment transport problem from general physics
U.S. Geol. Surv. Prof. Pap.
(1966)- et al.
Erosional development of valley-bottom gullies in the upper midwestern United States
- et al.
Hillslope Form and Process
(1972) Determinazione del fattore di sicurezza per scivolamenti planari nelle sponde fluviali
Studi Geol. Appl. Geol. Ambiente
(1994)- et al.
Effects of pore pressure on the stability of streambanks: preliminary results from the Sieve River, Italy
- Casagli, N., Curini, A., Gargini, A., Rinaldi, M. Pore water pressure and streambank stability: results from a...
Experiments on the motion of solitary grains along the bed of a water stream
Proc. R. Soc. London, Ser. A
Slope-stability analysis incorporating the effect of soil suction
The shear strength of unsaturated soils
Can. Geotech. J.
Use of linear and nonlinear shear strength versus matric suction relations in slope stability analyses
Nonlinearity of strength envelope for unsaturated soils
Proc. 6th Int. Conf. Expansive Soils (New Delhi, India)
Multistage direct shear testing on unsaturated soils
ASTM Geotech. Test. J.
Present channel stability and late Quaternary valley deposits in northern Mississippi
Spec. Publ. Int. Assoc. Sedimentol.
Bank stability of Goodwin Creek channel, northern Mississippi, USA
Problems with Eocene stratigraphy in Panola County, northern Mississippi
Southeast. Geol.
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