by Genna Slape

Introduction to Streambank Stability

Streambanks are principal features of a fluvial system that are popularly recognized as the regular, non-flooding, boundaries of a stream channel. Along with providing an identifiable fluvial boundary, steambanks deliver critical functions for fluvial systems. Amongst these critical functions, lies the responsibility of energy response and dissipation, through adjustment processes. This concept is articulated by Andrew Simon in his journal paper: Bank and Near Bank Processes in an Incised Channel: “The adjustment of channel width by mass-wasting and related processes represents an important mechanism of channel response and energy dissipation in incised alluvial channels”.
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This wiki focuses on streambank stability, it therefore important to discern the positioning of a streambank within a riparian corridor. Figure 1 defines the fundamental riparian zones; the toe and bank zones form the streambank. Mass-wasting and related processes significantly contribute to the dynamic and complex nature of a fluvial system. Mass-wasting and related processes refer to the erosion, sediment transport, and deposition processes that are constantly and simultaneously occurring in every fluvial system. These processes are ultimately responsible for the formation of terrace and floodplain features that are actually beneficial to a fluvial system. Acknowledging rivers as dynamic, complex systems in which streambank and other channel features naturally change location and shape is critical in understanding streambank stability. However this same acknowledgement makes it difficult to precisely and definitively classify streambank stability for a system. A common misconception is that streambank erosion is a sure indicator of streambank instability, and ultimately stream health deterioration. Consequently, streambank instability is often a primary motivator for river restoration efforts.

Streambank stability cannot simply be compared to the structural stability of a building; streambank features are more dynamic and therefore more complex, as they have been designed by nature to respond to fluvial dynamics. Streambank stability ultimately depends on the balance between the driving hydraulic and gravitational forces and the resistive forces of the streambank (Simon et. al.).

To ensure the application of the best river management practices for a fluvial system it is important to understand the fundamental concepts and mechanics of streambank stability, which will be presented in this wiki. The following topics will be discussed in this wiki: Streambank stability classification, the in situ bank and bank toe processes, the resisting and driving forces acting on a streambank, the effects of vegetation on streambank stability, common destabilizing causes, and streambank stabilization methods.


Broad Classification of Streambank Stability

In an attempt to define bank stability, river experts have defined three classifications of stream stability: completely stable, dynamically stable, and unstable. This classification scheme ultimately depends on streambank stability as the main cataloging indicator. Channelized, concrete lined streams are considered completely stable. In these types of systems, channel features such as streambanks, channel bed, and meanders are completely fixed in space and time. Streams that run through bedrock are naturally occurring examples of completely stable streams.

Every fluvial system strives for an equilibrium state. A channel is considered to be in equilibrium when it is operating within an extent of a standard range of erosion and sediment deposition for a system. Channel Equilibrium is disrupted when aggregation and degradation extends beyond this standard range of erosion and sediment deposition. Channel equilibrium is dependent on the balance of specific in-channel variables: sediment load, sediment size, channel flow, and channel slope. If one variable is subject to change the remaining variables must respond accordingly to re-establish the desired equilibrium state. If the in-channel variables are subjected to extensive changes that exceed the allowable thresholds, the fluvial system may seek a new equilibrium state, confined by a new threshold. A system functioning within a specific threshold range is considered dynamically stable. This means that in a dynamically stable system, the locations of channel features such as streambanks, channel bed, and sand bars may be subject to change, but the general feature relations remain constant over time. For more information on channel equilibrium refer to the Fluvial Geomorphology and Channel Evolution Model pages.

A channel is considered unstable when the system’s aggradation and degradation thresholds have been exceeded, triggering a transition phase where a new equilibrium is sought and established. In this unstable transition phase, excessive streambank erosion, also known as mass bank failure, occurs causing major changes in channel width, depth and sinuosity in a relatively short period of time consisting of anywhere from days to years. Figures 2 through 4 illustrate completely stable, dynamically stable and unstable systems.
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stable, dynamically stable and unstable are broad classifications ofstreambank stability. In addition to understanding the large-scale geomorphic processes it is also important to understand the basic physical forces acting on the streambank, which ultimately control streambank mechanics. This requires looking more closely at the in situ bank and bank toe processes.


Gens_BSTEM_SB.pngStreambank and toe-bank processes

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Streambank and near bank processes are driven by the application of the gravitational forces acting on the in situ bank material and the hydraulic forces acting on the bank toe. For this reason incised, alluvial streambanks are prone to structural instability, also known as erosion. This wiki will focus on the bank stability of incised alluvial channels. Incisedstreambanks consist of two distinctregions: the in situ bank material and the bank-toe. Figure 5 shows a simplified cross-sectional view of an incised streambank. Figure 6 showsan incised streambank found along New Mexico’s Rio Puerco. The figures show that the bank toe acts as a protective base for the in situ bank material.Figure 5 - Cross-sectional diagram of a typical incised streambank Figure 6 - Incised Rio Puerco streambank.

Erosion is a natural process, necessary for the healthy evolution of a channel, however excessive bank erosion may indicate streambank instability. The various mechanisms of streambank erosion can generally be classified as either scour or mass bank failure. Scouring involves the direct removal of the materials forming the channel bottom, bank-toe, and lower portion of the in situ bank that contributes to the wetted perimeter. Scouring is the result of the hydraulic forces exerted by the stream flow on the wetted channel boundaries.

As scouring causes undercutting of the bank-toe to occur, the overlying in situ bank material will be subject to mounting gravitational forces acting on the bank. Mass bank failure occurs when the scouring of the bank-toe and the adjacent channel bed steepens (increases the height and angle) the bank to the point that the shear strength of the bank material can no longer resist the driving gravitational forces (Simon et al). Figure 7 illustrates these bank failure processes. Equation 2 quantifies the driving gravitational force acting on a streambank.

Gens_ErosionProcess.pngMass failure is evident at points of a channel reach where large fragments of bank material are released from the channel bank formation, and is often perceived as an indicator for streambank instability. There are various types of mass bank failure mechanisms; the type failure that occurs reflects the degree of undercutting due to fluvial scouring, as well as the characteristics of the in situ bank material (Simon et al). Rotational, planar, and cantilever are the simplest forms of mass bank failure, and are illustrated in Figure 8.
The resistive strength of a bank is dependent on two factors: The properties (geotechnical strength) of the bank material and the hydrologic and hydraulic processes acting on the streambank. The influence of these two physical factors on streambank stability is discussed below. Streambank vegetation also contributes to bank stability, which will also be discussed later in this wiki.

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The geotechnical strength of a streambank’s material is a key component to the overall resistive strength. Different materials provide various ranges of cohesion to a streambank. For example,clay
particles are more cohesive than sand or gravel particles, therefore the properties of bank materials is an important consideration in streambank stability. Determining the geotechnical strength of a streambank is complicated as there is a high level of material heterogeneity found in any streambank formation. The heterogeneity of materials provides a major source of uncertainty in estimating the shear strength of the soils, and subsequently the prediction of bank erosion rates. The standard Mohr-Coulomb failure criterion (Equation 2) can be used to quantify the shear strength of the in situ bank material (Simon et al). The input variables are dependent on the particle distribution of the bank material.

In addition to geotechnical conditions, the hydrologic and hydraulic effects associated with infiltration and channel flow play a significant role in streambank stability. Studies have shown that streambank stability is most compromised after prolonged periods of rainfall through four destabilizing mechanisms. First, the gravitation forces acting on a streambank are amplified as infiltration increases the soil bulk unit weight. Second, infiltration results in the loss of negative pore water pressures or matric suction in a streambank. In a streambank, above the water table, pores are filled with water and air. The negative pore-water pressures that develop in this unsaturated zone provide additional cohesion to the streambank, called matric suction, which can be defined with Equation 4. Matric suction is lost as the bank becomes more saturated. Third, lateral seepage from excessive channel flows and surface infiltration generate positive pore water pressures within the bank. Generation of positive pore-water pressures reduces the frictional resistance of the bank material. Lastly, the rapid-drawdown of channel flows results in the loss of the confining hydrostatic pressures acting on the bank. The effect of pore-water pressure on a bank’s shear strength is incorporated into the standard Mohr-Coulomb Equation, as shown in Equation 5.





Geotechnical Forces

Streambank stability is controlled by the balance between the gravitational forces acting on a steepened bank and the resisting forces of the streambank. Equations 1-5 describe the contrasting driving and resisting forces acting on the overlying in situ bank material in an incised streambank.

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The values of the effective cohesion, and effective friction angle are dependent on the particle distribution of the soil mass in question. The ratio between the resisting forces and driving forces, describes the stability of a soil mass (streambank). This ratio (Equation 6) is defined as the factor of safety; the streambank is considered unstable when Fs is less than one.
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Hydraulic Forces

In incised channels bank heights are typically taller than the extending length of stabilizing Riparian roots and are highly vulnerable to erosion. Bank-toe erosion causes steepening of the bank profile and the potential for mass bank failure. During high channel flows, bank toe erosion generally occurs prior to the mass failure of the in-situ bank. Therefore understanding the processes occurring at the bank toe is critical in evaluating streambank stability.

Bank toe erosion occurs when the hydraulic forces applied by the channel flow exceeds the critical shear stress of the bank toe material. The applied hydraulic forces vary with stream flow velocity, therefore they are not constant along a cross-sectional channel boundary. Equation 7 estimates the average hydraulic force acting on the bank toe.

The critical shear stress represents the resisting forces of the bank toe material and is defined as the hydraulic stress required to mobilize sediment particles. This essentially defines the maximum allowable hydraulic stress that can be applied the toe boundary and before erosion occurs. Various empirical calculation methods have been developed to estimate the critical shear stress while accurately representing the assorted physical processes that can occur at the bank toe. Equation 8 can be applied to estimate the critical shear stress of a bank toe in an incised channel where packing, matric suction and cementation results in the side slope angles being steeper than the friction angle of the comprising material.

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Streambank Vegetation

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Riparian vegetation influences streambank stability through various mechanisms. Accurately quantifying the stabilizing effects of riparian vegetation is difficult due to the complexities of the root networks and the influences of vegetation foliage on surface infiltration and groundwater extraction.

Riparian vegetation provides streambank stability through the mechanical effects of the plant foliage cover and the root networks found within the bank soil matrix. Vegetation growing along the bank toe and lower bank boundaries protects these regions from hydraulic scouring by: diverting channel flows, reducing flow velocities, and ultimately reducing the applied shear-boundary forces.

Streambank vegetation also provides root networks that provide scouring and mass bank failure resistance. Vegetated foliage varies temporally due to fluctuating environmental factors including temperature and available moisture. Times in which plant foliage does not cover streambanks, the extended root networks are still able to slow and divert channel flows; thus providing scouring protection for the lower bank-toe region. In incised channels with excessive bank heights, root networks are not able to reach the lower bank regions, exposing the bank-toe to scouring effects. This ultimately causes deeper channel incision and higher potential for streambank failure.

Root networks provide mechanical reinforcement for bank material, which can contribute significantly to bank stability. Soil is strong in compression but weak in tension. The roots of riparian trees and other herbaceous species provide tensile reinforcement for a streambank. This tensile reinforcement significantly enhances the gravitational resistance of a streambank.

The removal of the invasive salt cedar along New Mexico’s Rio Puerco demonstrates the stabilizing effects of riparian vegetation. In 2006, herbicide spray was applied along a stretch of the Rio Puerco’s riparian corridor, resulting in the massive eradication of the invasive salt cedar and nearly all other reinforcing species. Figures 9-10 show the incised Rio Puerco in 2006, prior to the application of the herbicide spray. Compare these to Figure 11, which displays the dramatically altered channel observed in December 2011. Between 2006 and 2011, the bare streambanks were completely defenseless to erosion driving, gravitational and hydraulic forces; triggering the transitional geomorphic evolution that resulted in the non-incised, widened channel shown in Figure 11.



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Recent studies are beginning to show that the presence of riparian vegetation induces hydrologic and hydraulic effects that can actually reduce streambank stability. For more information on this refer to Andrew Simon's Hydrologic and hydraulic effects of riparian root networks on streambank stability: Is mechanical root-reinforcement the whole story?


Bank Stability and Toe Erosion Model

Dr. Andrew Simon and Dr. Natasha Pollen-Bankhead applied the previously described streambank mechanics to develop the Bank Stability and Toe Erosion Model (BSTEM 5.2). BSTEM 5.2 is a macros model that estimates the stability (factor of safety) of the in situ bank and bank toe. The BSTEM 5.2 considers the mechanical effects of the following factors on streambank stability: streambank geometry and materials, the hydraulic and hydrologic conditions within the channel and bank, and the vegetative cover and root reinforcement. BSTEM 5.2 is available for downloading on the USDA website: http://www.ars.usda.gov/Research/docs.htm?docid=5044.


Destabilizing Causes

Evaluation of streambank stability requires an understanding of channel evolution processes. This requires identifying the nature and spatial extent of the destabilizing factors. Destabilization factors are not limited to anthropogenic causes; natural processes that conflict with the riparian or watershed land use or disrupt channel equilibrium may also cause bank instability. In addition to identifying the nature of the cause, spatially pinpointing the trigger is critical in analyzing streambank stability. Locating the destabilizing source is not easy as it may be detectable at the erosion site, or it may extend to activities that occur in other reaches of the stream or watershed. Destabilizing sources can be broadly categorized as upstream, downstream, watershed, and/or local factors.

Watershed factors result from land use activities such as urbanization, agriculture, mining, and deforestation. These activities usually dramatically alter the watershed cover and slope, resulting in consequential increases in the watershed runoff that is converted to channel flows. Altering the volumes and durations of a channel’s peak flows is consequential for bank stability as it affects the hydraulic processes acting on the bank.

Channel modifications will also most certainly induce streambank instability either upstream, downstream, or/and at the modified site. Typical channel modifications include: the construction of dams and levees, channel straightening, channel narrowing, and channel lining. Like the watershed factors, these types of channel alterations affect the flow conditions and the subsequent hydraulic processes occurring at the bank-toe. In addition to impacting the hydraulic stresses, urbanization, channel lining and dam construction reduces the available sediment in a system that would otherwise be deposited to form the bank toe and is necessary for fluvial geomorphic processes.

Once the existing or predicted destabilization factors are identified the appropriate stabilization methods can be employed.

Stabilizing Methods

Soil bioengineering combines traditional engineering practices with ecological principles to establish living vegetative systems that will provide erosion protection and slope stability. The overall goal of soil bioengineering is to restore the natural health and function of a stream and/or watershed and is favored for its holistic approach to slope stability. This method can be applied for streambank stabilization, as a mitigative or preventative approach for streambank erosion. The popularity of this stabilization method has grown in conjunction with ecological awareness. In addition to bank stabilization, the benefits of soil bioengineering practices also include: improved water quality, restored physical habitat for fish and wildlife, aesthetic improvements, and project sustainability. It is important to acknowledge that although this is a natural approach, soil bioengineering is not the best bank stabilization method for every scenario. If soil bioengineering is not applied properly it can impose negative effects on the health and function of a fluvial system and/or watershed. The establishment of salt cedar in southwest riparian zones is an example of the misapplication of soil bioengineering practices that has had ecological consequences. To achieve the overall goal of soil bioengineering and to avoid generating more problems, a thorough assessment of the ecological and fluvial conditions specific to the system in question is necessary. If applied properly, soil bioengineering is a favorable yet intricate approach to streambank stability. To learn more about this
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stabilizing method refer to the technical supplement (TS14I Streambank Soil Bioengineering), provided in the USDA’s Part 654 - Stream Restoration Design Manual
(http://policy.nrcs.usda.gov/viewerFS.aspx?id=3491)

The use of rock typically referred to as Riprap is a common streambank stabilization technique. Riprap is known for being one of the most effective bank-toe protection methods. This a more traditional engineering streambank stabilization approach than soil bioengineering, as the selection of the riprap size, type and placement method is dependent on established design methods.
Riprap provides a bank with stability and erosion protection by dissipating energy associated with channel flow and providing resistance to the hydraulic shearing forces. Consequently, riprap selection and placement is dependent on the existing or predicted stream conditions. If immediate bank stabilization is necessary, riprap is advantageous over vegetative protection measures because it does not require a development period. Figure 12 shows a riprapped streambank. Refer to TS14K-Streambank Armor Protection Stone Structures (http://policy.nrcs.usda.gov/viewerFS.aspx?id=3491) for more information on riprap applications and design. Concrete lining is more extreme lining method used to achieve completely stable banks.

Other streambank stabilization methods utilize man-made, in-stream structures. Some of the more common in-stream stabilization structures include: weirs, gabbions, drop structures, and levees (Figures 13-15). The design options for each of these structures are extremely diverse; ranging from the use of typical specs to more unique, site-specific designs. The scale of these structures also ranges dramatically, from minor streams to large-scale navigable rivers. While the design and construction of these man-made structures may differ, they all apply the same fundamental flow and energy diversion and dissipation mechanics to prevent channel incision and bank toe scouring.Gens_Levee.png



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Gens_weir.pngConclusion

Bank Stability is a significant concern for land management efforts worldwide. Bank retreat is a physical response to the driving forces applied to the bank formation. Understanding the dynamics and the sources of the resisting and driving forces that govern the stability of a streambank, and which have been discussed in this wiki, is critical in determining the appropriate stream restoration tactics for a system.

















References:
Charlton, Ro. Fundamentals of Fluvial Geomorphology. New York: Routledge 2008.

Julien, Pierre Y. Erosion and Sedimentation. New York: Cambridge University Press 1995.

Pollen-Bankhead, Natasha. Hydrologic and hydraulic effects of riparian root networks on streambank stability: Is mechanical root-reinforcement the whole story? Geomorphology 116 (2010): 353-362.

Simon, Andrew. Bank and near-bank processes in an incised channel. Geomorphology 35 (2000): 193-217.

Parker, Chris. The effects of variability in bank material properties on riverbank stability: Goodwin Creek, Mississippi. Geomorphology 101 (2008): 533-543.

Williams, David T. “Fluvial Geomorphology and Its Use in River Stabilization.” DNRC Water Resources Division. 3-4, December 2009.