Elsevier

Geomorphology

Volume 35, Issues 3–4, November 2000, Pages 193-217
Geomorphology

Bank and near-bank processes in an incised channel

https://doi.org/10.1016/S0169-555X(00)00036-2Get rights and content

Abstract

Gravitational forces acting on in situ bank material act in concert with hydraulic forces at the bank toe to determine rates of bank erosion. The interaction of these forces control streambank mechanics. 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 at relatively frequent flows and to maintain steep lower-bank profiles. Seepage forces exerted on in situ bank material by groundwater, downward infiltration of rainwater and lateral seepage of streamflow into and out of the bank are critical in determining bank strength. Data from a study site on Goodwin Creek, MS, USA clearly show the temporal variability of seepage forces and the lag time inherent in reductions in shear strength due to losses of matric suction and generation of positive pore-water pressures. Negative pore-water pressures (matric suction) have also been shown to increase the resistance of failed cohesive blocks to entrainment by fluid shear. A stable bank can be transformed into an unstable bank during periods of prolonged rainfall through:

  • 1.

    increase in soil bulk unit (specific) weight,

  • 2.

    decrease or complete loss of matric suction, and, therefore, apparent cohesion,

  • 3.

    generation of positive pore-water pressures, and, therefore, reduction or loss of frictional strength,

  • 4.

    entrainment of in situ and failed material at the bank toe, and

  • 5.

    loss of confining pressure during recession of stormflow hydrographs.

Relatively small frequent flows during the winter have the ability to erode failed bank materials, maintain oversteepened, unstable bank surfaces and promote prolonged periods of bank retreat, channel migration and high yields of fine-grained sediment. Confining pressures provided by stormflow are not as significant in maintaining bank stability as the counteracting force of fluid shear on the bank toe, which steepens the bank. For example, more than 2 m of bank retreat occurred during the study period at the research site on Goodwin Creek, northern Mississippi. The loss of matric suction (negative pore pressures) due to infiltrating precipitation has been found to be as significant as the development of excess pore pressures in contributing to mass bank instability. Apparent cohesion, friction angle, soil bulk unit weight and moisture content were measured in situ. Matric suction was measured continuously, in situ with a series of five pressure-transducer tensiometers. A bank-failure algorithm, which combines the Mohr–Coulomb approach, for saturated conditions and the Fredlund modification for unsaturated conditions was developed for layered cohesive streambanks. The resulting equation has been used successfully to investigate the role of matric suction, positive pore-water pressures and confining pressure for layered streambanks composed of cohesive materials.

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)

  • Curini, A., 1998. Analisi dei processi di erosione di sponda nei corsi d'acqua, Universita degli Studi di Firenze,...
  • Darby, S.E., 1994. A physically based numerical model of river channel widening, PhD thesis, University of Nottingham,...
  • J.R.D. Francis

    Experiments on the motion of solitary grains along the bed of a water stream

    Proc. R. Soc. London, Ser. A

    (1973)
  • D.G. Fredlund

    Slope-stability analysis incorporating the effect of soil suction

  • D.G. Fredlund et al.

    The shear strength of unsaturated soils

    Can. Geotech. J.

    (1978)
  • D.G. Fredlund et al.

    Use of linear and nonlinear shear strength versus matric suction relations in slope stability analyses

  • D.G. Fredlund et al.
  • D.G. Fredlund et al.

    Nonlinearity of strength envelope for unsaturated soils

    Proc. 6th Int. Conf. Expansive Soils (New Delhi, India)

    (1987)
  • J.K.M. Gan et al.

    Multistage direct shear testing on unsaturated soils

    ASTM Geotech. Test. J.

    (1988)
  • E.H. Grissinger et al.

    Present channel stability and late Quaternary valley deposits in northern Mississippi

    Spec. Publ. Int. Assoc. Sedimentol.

    (1983)
  • E.H. Grissinger et al.

    Bank stability of Goodwin Creek channel, northern Mississippi, USA

  • E.H. Grissinger et al.

    Problems with Eocene stratigraphy in Panola County, northern Mississippi

    Southeast. Geol.

    (1981)
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