Download Bank stability - Paper Genna Slape CE 598 Mark Stone DEFINING

Bank stability - Paper
Genna Slape
CE 598
Mark Stone
DEFINING BANK 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 the 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”. The mentioned mass-wasting and related processes refer
to the erosion, sediment transport, and deposition processes that constantly and
simultaneously occur in any and every fluvial system.
Streambank stability cannot be compared to the structural stability of a building; it
is much more dynamic and therefore complex, because it was designed by nature to
be responsive to the fluvial dynamics. To put simply, streambank stability is
dependent on the balance between the resistive forces of the streambank and the
driving forces acting on the streambank
Streambank stability is often the main driver for river restoration. Streambank
stability has ultimately become a strong driver in most river restoration efforts. To
put simply, streambank stability is dependent on the balance between the resistive
forces of the streambank and the driving forces acting on the streambank.
Acknowledging rivers as dynamic, complex systems, in which streambanks naturally
change location and shape, makes it difficult to precisely and definitively classify the
streambank stability for a particular system. Every natural river has erosion,
sediment transport, and deposition processes occurring simultaneously. These
processes are ultimately responsible for the formation of terrace and floodplain
features that are actually beneficial to a fluvial system. Acknowledging the dynamic
nature of rivers is the first step in understanding streambank stability.
In an attempt to understand bank stability, river experts have defined three
classifications of stream stability: completely stable, dynamically stable, and
unstable. This classification scheme ultimately depends on the 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, which is confined by
limiting thresholds that operate within the extent of a standard range of erosion and
sediment deposition for a system. Channel Equilibrium is disrupted when
aggragation and degradation which extends beyond the 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 current 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 may be subject to change,
but the general feature relations remain constant over time.
A channel is considered unstable when the system (aggradataion and degradation)
thresholds have been exceeded and the system is then put into a transition stage
where a new equilibrium is being established. In this unstable transition stage,
major changes occur in channel width, depth and sinuosity occur in a relatively
short period of time consisting of anywhere from days to years. Figures () illustrate
the range of ,
Fluvial systems and fluvial geomorphology are responsive to driving influences.
Their level of response to these driving influences, ultimately depends on the
characteristics distinct to each system.
Streambank stability is essentially influenced by the interaction of fluvial and
geotechnical processes that are occurring within and at the streambank boundaries.
These interacting processes include:
Things that are considered to be driving or resisting forces on streambank stability
are hydraulic and conteracting hydrostatic forces, pore pressure defined by
moisture content within the streambank, and root reinforcement within the bank
matrix (A.Simon, et al).
Streambank and toe-bank process:
When analyzing streambank and near bank processestreambank mechanics. The
interaction of the gravitational forces acting on the in situ bank material and the
hydraulic forces acting on the bank toe control streambank mechanics. For this
reason incised, alluvial streambanks are prone to structural instability, also known
as erosion. Incised streambanks consist of the in situ bank material and the banktoe. Figure ( ) shows a simplified cross-sectional view of an incised streambank.
As can be seen in the figure the bank toe acts as a protective base for the in situ bank
material.
Figure () -
The various mechanisms of streambank erosion can be 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 shear stresses exerted by the streamflow on the channel boundaries. The
wetted perimeter for a conceptual channel cross-section is illustrated in Figure ().
Figure 1 – (http://www.ess.co.at/MANUALS/WATERWARE/webreacheditor.html)
As scouring causes undercutting of the bank-toe to occur, the overlying in situ
bank material will be subject to the increase in driving stresses. Mass bank failure
occurs when the scouring of the bank-toe and the adjacent channel bed steepens
(increases the height and angle) of the bank to the point that the shear strength of
the bank material can no longer resist the increased gravitational forces exceed the
resistive shear strength of the bank material (Simon et al). The shear strength of the
bank is dependent on two main factors:
First the nature of the bank material; specifically the bank stratification and the
corresponding particle distributions, distinct to each stratum. The Mohr-Coulomb
failure criterion can be used to quantify the shear strength of the bank material
(Simon et al). The particle distribution of each layer ultimately determines the
values of the input variables applied in the Mohr-Coulomb equation
Second the saturated conditions within the bank material. The polar attraction of
water and mineral surfaces causes the water to spread and itself across these
mineral surfaces The saturation within the bank matrix affects the shear strength of
the bank material in two, conflicting ways. The degree of saturation within the bank
mass dictates the confining pore water pressures. The first scenario, involves
negative porewater pressures in unsaturated bank conditions, and contributes to
the bank’s shear strength. Above the water table the pores are filled with water and
air, creating an effect known as matric suction; this effect increases the shear
strength of the bank. Oppositely positive pore water pressure,
The saturation conditions also contribute to the effect of gravitational forces
acting on the bank material. Amplified saturation of streambank soils not only
increases pore-water pressures, it also increases the soil unit weight. Weight is
simply the measurement of the pull of gravitational forces acting on an object,
therefore the more the bank weighs, the more likely mass failure will occur.
Mass failure is evident at points of a channel reach where large fragments of
bank material are released from the channel bank formation. 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 2. An observable, in-stream
example of a cantilever bank is provided in Figure 3.
Figure 2 – Selection of observable mass bank failure types. (Simon et al. BSTEM)
Geotechnical Forces:
The balance of the internal resisting forces to the driving forces determines
the stability of a streambank. Therefore to analyze bank stability it is necessary to
understand the nature of shearing resistance, as well as to quantify it. The same
understanding and calculation of the driving gravitational forces acting on the bank
soil mass is necessary when analyzing bank stability. The shear strength of a soil
mass is defined as the internal resistance per unit area that the soil mass can offer to
resist failure and sliding along a plane inside it (Das), and is quantified by the Mohr-
Coulomb failure criterion (Equation 1). Oppositely, Equation 2 quantifies the driving
gravitational force.
The resistant shear strength described in Equation 1 accounts for the effects
of pore-water pressures in the soil mass. Above the water table, pores are filled
with water and air. The negative pore-water pressures that develop as the result of
this unsaturated effect is known as matrix suction, which is represented in as the
soil has a hasThe The effects of matrix suction are described in
𝜏𝑟 = 𝑐 ′ + (𝜎 − 𝑢𝑎 )𝑡𝑎𝑛ϕ′ + (𝑢𝑎 − 𝑢𝑤 )𝑡𝑎𝑛ϕ𝑏
(Equation 1)
𝜎 = 𝑊 ∗ 𝑐𝑜𝑠𝛽
(Equation 1a)
𝜓 = 𝑢𝑎 − 𝑢𝑤
(Equation 1b)
𝜏𝑑 = 𝜎 ∗ 𝑐𝑜𝑠𝛽
(Equation 2)
Where:
𝜏𝑟 = shear strength(kPa)
𝜏𝑑 = driving (gravitational) force (kPa)
𝜎 = normal stress on failure plane (kPa)
𝜑 = matric suction (kPa)
𝑐 ′ = effective cohesion (kPa)
𝑢𝑎 = air pressure (kPa)
𝑢𝑤 = pore water pressure within the soil mass (kPa)
ϕ′ = effective friction angle (°)
ϕb = angle describing increase in τr due to increase in ψ (°)
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 3) is defined as the factor
of safety; the streambank is considered unstable when Fs is less than one.
𝐹𝑠 =
𝜏𝑟
𝜏𝑑
Hydraulic Forces:
(Equation 3)
In an incised channel 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. In a
storm event, during which channel flows are increasing, bank toe erosion generally
occurs prior to the mass failure of the in-situ bank. Therefore the quantification of
the hydraulic forces acting on the bank-toe, channel bed, and lower in-situ bank, is
equally as important as calculating the geotechnical forces acting on the overlying in
situ bank material and in understanding the bank failure processes.
The erodibility of the bank-toe region is dependent on the balance between the
shear resistance of the bank-toe and the erosion driving, hydraulic forces applied to
the channel boundary by the flow. The shear resistance of the bank-toe materials is
called the critical shear stress. The applied hydraulic stresses vary with stream flow
velocity, therefore they are not constant along a channel boundary at a particular
cross-section. Equation 4 represents the erosive capability of the channel flow as
the mean boundary shear stress:
𝜏0 = 𝛾𝑤 𝑅𝑆𝑤
The shear resistance of the bank-toe material is called the critical shear stress
Vegetation:
Riparian vegetation has typically been seen as having a positive effect on
streambank stability. However more studies are beginning to show that the
presence of riparian vegetation induces hydrologic and hydraulic effects that have
negative correlations on streambank stability. The effect of mechanical rootreinforcement on soil stability can be considerable
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 regions protects
these regions from hydraulic scouring by impeding channel flows; which reduce
flow velocities, divert flows and ultimately reduce boundary drag forces. The effect
of mechanical root-reinforcement on soil stability can be considerable. Streambank
vegetation also provides root networks that provide scouring and mass bank failure
resistance. Vegetated foliage varies temporally due to a number of 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.
Soil is strong in compression but weak in tension. Reversely, the roots of
trees and other herbaceous species that may be present in a stream bank possess
tensile resistance but lack compressive strength. Therefore roots provide the soil
matrix the lacking tensile strength, and ultimately enhanced stability of streambank.
This enhanced tensile resistance provides resistance in the bank material against
gravitational forces that increase as the bank-toe is eroded.
More recently the Streambank vegetation affects the hydrologic conditions
within a streambank which can have a negative effect on streambank stability
Stabilizing Methods
Conclusion