abstract id #: 271 corresponding author: tommy sutarto...
TRANSCRIPT
ABSTRACT ID #: 271
ABSTRACT TITLE: Bank Stability Analysis for Fluvial Erosion and Mass Failure
CORRESPONDING AUTHOR: Tommy Sutarto
ADDITIONAL AUTHOR(S): Thanos Papanicolaou, Tommy Sutarto, Christopher Wilson, Eddy Langendoen
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1
Bank Stability Analysis for Fluvial Erosion and Mass Failure
A.N. Thanos Papanicolaou1,*
, Tommy Sutarto2, Christopher G. Wilson
2, Eddy J.
Langendoen3
1Department of Civil & Environmental Engineering, University of Tennessee, 325
J.D. Tickle Bldg., Knoxville, TN 37996; 2IIHR - Hydroscience & Engineering, Dept. of Civil and Environmental Engineering,
University of Iowa, Iowa City, IA 52242-1585; 3U.S. Department of Agriculture, Agricultural Research Service, National
Sedimentation Laboratory, Oxford, MS 38655. *Corresponding Author: PH (865) 974-7836; e-mail: [email protected]
ABSTRACT
The central objective of this study was to highlight the differences in
magnitude between the mechanical and fluvial streambank erosional strength with the
purpose of developing a more comprehensive bank stability analysis. Mechanical
erosion and ultimately failure signifies the general movement or collapse of large soil
blocks due to geotechnical instability and is the upper limit of streambank erosion.
Conversely, fluvial erosion is the detachment of individual particles or aggregates due
to the shearing action of flow and is the lower limit of streambank erosion. A total of
24 streambank samples from a representative stream in the U.S. Midwest (i.e., Clear
Creek, IA) with semi-cohesive soils were analyzed in terms of both mechanical and
fluvial erosional strength. Mechanical strength was measured using a direct shear
device and ranged from 400 to 6,600 Pa. Fluvial erosional strength was measured
using a conduit flume, which applied a shearing force to the sample, and had values
between 1.28 and 2.37 Pa. Thus, mechanical strength was 2 to 3 orders of magnitude
larger than fluvial erosional strength, which suggests that identifying the different
modes of streambank erosion (e.g., mechanical or fluvial) during a hydrograph is
needed to provide better design specifications for bank stabilization practices.
INTRODUCTION
Streambank erosion is a key process that challenges restoration efforts in
riverine systems. To assess the severity of streambank erosion along a channel reach
in hopes of designing appropriate bank stabilization practices, a bank stability
analysis is often conducted for identifying the key mechanisms and conditions that
lead to streambank failure and evaluating the effectiveness of different stabilization
techniques. However, a standardized methodology for assessing streambank stability
is still under-developed. More importantly, a key mechanism of streambank erosion
that is often overlooked is the mechanism of fluvial erosion.
In a bank stability analysis, it is essential to identify both the upper and lower
limits of bank erosion, namely mass failure and fluvial erosion, respectively. Yet in
2
many cases, only the mechanical erosional strength is determined (e.g., ASCE Task
Committee, 1998; Barrett et al., 2006). The misconception of the sole importance of
mechanical strength towards bank erosion may lead to the improper design of
stabilization practices that fail due to the more persistent and prevalent fluvial
erosion. To shed some light on the mechanisms affecting bank erosion, both
mechanical and fluvial erosional strengths were measured for a semi-cohesive
streambank in a representative watershed of the U.S. Midwest with the ultimate goal
of developing a more comprehensive bank erosion and stability analysis.
BACKGROUND
Mass failure is the slumping or collapse of large soil blocks from the bank
face due to geotechnical instability that typically occurs along a slip surface of the
bank profile either as planar or rotational failure. As the process that represents the
upper limit of bank erosion, the onset of mass failure is mostly quantified by the soil
shear strength, , (e.g., Fredlund and Rahardjo, 1993) defined as:
(1)
where is the effective cohesion (Pa), is the normal stress produced by the soil
block weight (Pa), is the soil pore-water pressure (Pa), refers to the effective
friction angle (deg.) and is the angle (deg.) expressing the rate of increase in shear
strength relative to the matric suction. The effective cohesion, , is also known in
the literature as the soil mechanical strength and is a macroscale quantity describing
its yield strength (Millar and Quick, 1998), while the term is known as inter-
particle frictional strength. When the bank is saturated, matric suction diminishes and
.
The stability of a streambank against mass failure can be represented with a
Factor of Safety, , which is essentially the ratio between resisting forces and
forces promoting failure. The potential failure block is divided into a number of
vertical slices to account for differing failure block geometry, soil layering, and
external loads, such as trees (Langendoen et al., 2009). The driving and resisting
forces are then calculated for each slice and integrated over the entire profile. Thus, a
Factor of Safety can be determined as (Langendoen et al., 2009):
∑
∑
(2)
where is slice number, is the number of slices, is the length of the slice base (m),
is the weight of the slice (N), is the confining hydrostatic force from the water
in the channel (N), and is the angle of planar failure surface (deg.). The shear
strength along the slip surface, , is given by Eq. 1. The bank is stable if > 1,
while conversely, the bank is unstable if < 1.
At the lower end of the bank erosion spectrum, fluvial erosion is the removal
of soil particles or aggregates from the bank surface by the action of a shearing flow
3
(ASCE Task Committee, 1998; Millar and Quick, 1998). The rate of fluvial erosion
can be determined with an excess shear stress formula similar to the one introduced
by Kandiah (1974) as follows:
(
)
(3)
where is the erodibility coefficient (kg/m2/s), is the shear stress (Pa) exerted by
the flow on the bank surface (i.e., the near-bank or side-wall shear stress), is the
critical erosional strength (Pa) of the bank surface soil, and is an experimental
coefficient assumed to be 1 for most cohesive soils that are consolidated and aged for
more than 24 days, such as those found in most banks (e.g., Vermeyen, 1995).
Fluvial erosion is governed by the low-magnitude, particle-to-particle cohesion, ,
that is provided by inter-particle forces of attraction or repulsion acting at the
microscopic level, including electrostatic, van der Waals, hydration, and biological
forces (e.g., Zreik et al., 1998; Papanicolaou et al., 2007).
The stability of a streambank against fluvial erosion is applicable at the grain
scale and can also be expressed with a Factor of Safety, , defined as the ratio
between as the resisting component and as the driving component (e.g., Millar
and Quick, 1998):
(4)
The streambank soils are likely to erode by particle-to-particle dislodgement if <
1. In contrast, the bank is resistant to particle entrainment if > 1.
As mentioned previously, streambank stability is more often assessed solely
by the value of , with only a few studies recommending the use of both and
(e.g., ASCE Task Committee, 1998). It is assumed that bank stability is a
geotechnical problem, which is governed by the slumping of soil block along a failure
plane. However, several observations (e.g., Pizzuto, 2009), numerical model results
(e.g., Rinaldi and Darby, 2008), and conceptual studies (e.g., Lawler, 1992) is also a
particle-to-particle dislodgement problem. These studies have highlighted the
dominance of fluvial erosion in terms of frequency of occurrence, as well as in
creating favorable conditions for catastrophic mass failures by bank toe undercutting.
Thus, the misconceptions surrounding bank stability analyses regarding the
use of only the mechanical strength, , to determine the onset of bank erosion can
lead to the eventual failure of the bank stabilization structure. It is critical to use the
fluvial erosional strength, , since it corresponds to the lower limit of streambank
erosion. Before a standardized methodology for bank stability analysis can be
developed, these misconceptions regarding mechanical and fluvial erosional strength
must be addressed.
OBJECTIVES
The central objective of this study was to quantify the differences in
magnitude between mechanical and fluvial erosional strengths, as represented by
4
and , for a streambank in a representative system of the U.S. Midwest in hopes of
developing a more comprehensive bank stability analysis. Additionally, a
methodology is provided herein that measures fluvial erosional strength by a conduit
flume where a shear force is applied to the sample contrasting the more often used
method of an impinging jet where a normal force is considered (e.g., Hanson and
Simon, 2001).
STUDY AREA
This study was focused near the mouth of Clear Creek, IA (Figure 1a and b).
Clear Creek, a tributary of the Iowa River, has an average slope of 0.001 and a
sinuosity between 1.27 and 1.49, since the channel was straightened significantly to
facilitate water movement through the system. The sampled streambank had an
average bank height of 3.2 m and an average bank angle of 34o, based on a geodetic
survey of 6 cross-sections.
Clear Creek is an ideal location for studying streambank erosion since the
soils are comprised of highly erodible loess and both mass failure and fluvial erosion
are present. A geotechnical analysis of the streambank soils (Table 1) classifies them
as sandy loam and loam, based on the Unified Soil Classification System. The
Plasticity Index (PI) ranges from 12.36 to 14.19 and the clay minerals are most likely
illite, based on an average clay activity of 1.37.
Figure 1. (a) Clear Creek Watershed. (b) Study location.
Study location
a
b
5
Table 1. Properties of streambank soils.
METHODOLOGY
Soil Sample Extraction. Soil samples were extracted for quantifying, in the
laboratory, the mechanical strength, , and critical erosional strength, . The
samples were collected in October 2011 to prevent freeze-thaw cycles and soil
desiccation affecting the soil strength.
For the purpose of mechanical strength measurement using direct shear
device, 3 soil samples were extracted respectively from the crests, midbanks, and toes
of both the left and right banks by inserting 40 cm long Shelby tubes (ID = 7.62 cm)
perpendicularly into the bank face.
To test the erosional strength of the streambank soils, an additional 18 soil
samples were extracted from the crest, midbank, and toe of the left and right bank
faces at the study site (three from each location). Samples were collected (Figure 2)
by initially cutting the grass to the soil surface, thereby keeping the roots intact and
avoiding any damage to the soil structure. Soil blocks (35 cm long x 20 cm wide x
15 cm deep) were then carefully excavated from the bank face with two long soil
knives and a wire saw (Figure 2a). To minimize soil water loss or expansion, the soil
blocks were carefully wrapped in cheese cloth (Figure 2b), covered in wax (Figure
2c), placed within plastic boxes, and stored at room temperature (20oC) before
testing.
Mechanical and Erosional Strengths Determination. A standard direct shear
device was used to determine the mechanical strength of the streambank samples.
Following ASTM D 3080-98, the samples were consolidated and saturated before
being placed in the shearing box. The top of the shear box was moved horizontally at
a rate of 0.5 mm/s to induce the shear. At least three samples from each location
were tested under different normal loads to develop specific shear stress - normal
stress relationships, which were fit with linear regression lines. The slopes of these
regression lines were equivalent to and the y-intercept was considered to be
the effective cohesion, . A water and sediment-recirculating, straight conduit, flume with a rectangular
cross-section, which was designed and built in-house at IIHR - Hydroscience &
Sampling Location
Sand
%
Silt
%
Clay
%
D50
(mm) PI Ac
ø'
(deg.)
c'
(Pa)
(1) (2) (3) (4) (5) (6) (7) (8)
Right bank
Crest 43.00 49.91 7.09 0.040 12.36 1.22 31.32 500
Midbank 45.00 47.17 7.83 0.058 14.97 1.71 36.06 3,700
Toe 58.74 30.95 10.31 0.100 NA NA 35.02 6,600
Left bank
Crest 65.00 30.54 4.46 0.130 NA NA 26.33 400
Midbank 40.00 49.59 10.41 0.056 14.19 1.18 37.03 3,000
Toe 65.00 28.46 6.54 0.100 NA NA 34.88 6,000
Average 52.79 39.44 7.77 0.08 13.84 1.37 33.44 3,367
6
Engineering, was used to measure (Figure 3). This flume was designed to deliver
a shear force over the sample (e.g., Papanicolaou, 2001). Each soil sample was
removed from the wax - cheese cloth coating and carefully cut with a razor or wire
saw to fit a 30 cm long x 10 cm width x 5 cm height sample box (Figure 3). The
sample box was then placed in the conduit so that the soil surface was even with the
flume bottom. Every effort was made to avoid disturbing the sample face so as to
maintain the original surface roughness and microstructure.
The conduit was then filled slowly with water to avoid disturbing the sample.
Once filled, the flow discharge was incrementally increased every 10 minutes by
adjusting a variable frequency control. A 10-minute time step was considered
sufficient to allow the flow to stabilize after a sudden change and obtain a constant
reading on the flow meter. The wall shear stress, , was determined using Darcy-
Weisbach equation:
(5)
where is wall shear stress (Pa), is Dracy’s friction factor, is bulk velocity (m/s)
and is water density (998.2 kg/m3). An explicit formula provided by Haaland
(1983) was used to quantify the friction factor, . Future studies performed by this
group will provide more accurate methods of estimating Two 1-L water samples were collected approximately 9 and 10 minutes after
each discharge increase. The sediment concentration (mg/L) for each sample was
determined afterwards by filtration. The concentrations for the two samples collected
at the same discharge were then averaged (Cav). The experiment was terminated if
localized scour was observed at the edges of the soil sample and at its interface with
the tray as continuation of the run could result to an experimental error in terms of
erosion measurements due to edge effects. In this case, the soil sample was removed
and patched up along its edges to remove any localized irregularities. Before starting
a new run, the flume was thoroughly cleaned and flushed of soil deposited in the
conduit.
a b
c
Figure 2. (a) Soil sample extraction. (b) Soil block
wrapped in cheese cloth. (c) Soil block covered in wax.
7
RESULTS AND DISCUSSION
The mechanical strength, , of the streambank soils increased moving from
the bank crest to the toe (Table 1, column 8) as the soils at the bank toe were more
compacted from the weight of the overlying soils. Additionally, the compression of
the soils from the bank crest could have been impeded by the cyclic process of
erosion and deposition from overbank flows during flood events. Thus, compaction
of the banks was a major factor influencing the magnitude of . Regarding the fluvial erosional strength, Figure 4 provides the average
concentration, , for each applied discharge of a representative sample tested in the
conduit flume. The concentration increased, as expected, in response to the increased
discharges showing that erosion progressed during the tests. The erosion rate, , was
calculated as follows:
(6)
where is erosion rate (kg/m2/s), is the difference in average concentrations
(kg/m3) between two consecutive time intervals, is the corresponding flow
discharge (m3), and is the area of the soil surface (0.03m
2).
The fluvial erosional strength, , of the streambank soils was obtained by
plotting the corresponding pairs of and for the flume runs performed on each of
the 18 streambank samples and fitting linear regression lines to the plots. The value
of corresponded to the point at which the regression line intercepted the shear
stress axis. A summary of fluvial erosional strengths for the 18 soil samples were
found in Table 2.
a
b c
e
j
i
f
d
g h
Figure 3. Recirculating erosion flume. (a) conical tank, (b)
electrical pump, (c) 7.6-cm galvanized pipe, (d) flow meter, (e) flow
control valve, (f) air release, (g) diffuser, (h) plexiglass conduit (W
x H x L = 10 cm x 5 cm x 305 cm), (i) sample box, (j) outlet valve.
Inset: Removable sample box with a soil sample.
8
Table 2. Fluvial erosional strengths.
The mechanical strength, , was 2 to 3 orders of magnitude higher than the
erosional strength, . The ranged from 400 to 6,600 Pa (Table 1, column 8) while
the values were between 1.28 and 2.37 Pa (Table 2, column 2 and 5). The large
difference between the two measures of erosional strength suggested that they
reflected different underlying processes in nature. The stress needed for the onset of
mass failure is much larger than the stress required for the onset of fluvial erosion and
is probably one reason why mass failure occurs far less frequent (i.e., more episodic)
than fluvial erosion being a more quasi-continuous process.
Determination of and . To demonstrate how both and are used in a
bank stability analysis, Factors of Safety for both mass failure and fluvial erosion
(i.e., and , respectively) were determined for a 129 m long channel reach
where the streambank samples were collected. Six cross sections were geodetically
Sampling
location
Left Bank
Right Bank
Sample
ID
τc
(Pa)
τc avg
(Pa)
Sample
ID
τc
(Pa)
τc avg
(Pa)
(1) (2) (3) (4) (5) (6)
Crest
CC-L-C1 1.67
1.57
CC-R-C3 1.30
1.46 CC-L-C2 1.47 CC-R-C4 1.28
CC-L-C3 NAa CC-R-C5 1.80
Midbank
CC-L-M1 1.75
1.53
CC-R-M1 1.49
1.47 CC-L-M2 1.31 CC-R-M2 1.59
CC-L-M3 NAa CC-R-M3 1.33
Toe
CC-L-T1 1.75
1.92
CC-R-T1 1.60
1.83 CC-L-T2 1.90 CC-R-T2 2.37
CC-L-T4 2.12 CC-R-T5 1.50 aSample was disrupted when it was cut for fitted in the tray.
Figure 4. A typical result of conduit flume test.
9
surveyed along the reach three weeks prior to a flash flood event that passed through
the study reach on June 19, 2009 and produced a long period of high flow rates,
which provided favorable conditions for both mass failure and fluvial erosion.
The was quantified using Eq. 1 and Eq. 2, which are incorporated into an
established 1D, channel evolution model, namely the Conservational Channel
Evolution and Pollutant Transport System (CONCEPTS version 1.0). This model
was developed in 2000 at the U.S. Department of Agriculture - Agricultural Research
Service National Sedimentation Lab in Oxford, MS. The model is capable of
simulating open-channel hydraulics, sediment transport, stream bed evolution, and
bank retreat associated with bank erosion. For more information about CONCEPTS,
the reader is directed to Langendoen and Alonso (2008).
The 129 m long channel reach was modeled in CONCEPTS. The soil channel
geometry data surveyed on May 28, 2009 were available for 6 cross sections along
the model reach. The flow hydraulics and streambank stability were simulated for the
period of May 18 to June 22, 2009. A hydrograph, developed from water elevation-
discharge relationship (Abaci and Papanicolaou, 2009) and time series of water
elevation (Denn, 2010), was imposed at the upstream boundary. Downstream
boundary condition was time series of water elevation obtained from a previous study
conducted by Denn (2010). The left and right banks at each cross-section were
divided into 3 layers (e.g., crest, midbank, and toe) and the different soil properties of
each layer (Table 1and 2) were introduced in the model. Other processes, namely
positive pore-water pressures, matric suction, confining pressures, groundwater table
dynamics were simulated in streambank stability analysis.
The factor of safety for fluvial erosion, , was determined using Eq. 4. The
value for each layer (i = crest, midbank, and toe) was obtained from the conduit
flume tests (Table 2). In the model, the near bank shear stress, , exerted on each
layer along the bank profile was quantified as:
(7)
where (kg/m3) is the mass density of water; (m/s
2) is the gravitational
acceleration; is the hydraulic radius corresponding to each layer , and
denotes the friction slope.
Figure 5a demonstrates the change in for all cross sections during the
June 19, 2009 event as determined by CONCEPTS. The of both the right and
left streambanks never fell below unity indicating that the streambanks were
geotechnically stable during the event. The of a cohesive bank could drops
below 1 during the recession of the hydrograph as the weight of the saturated
streambank can no longer be supported by the confining hydrostatic force from the
volume of water in the stream channel. This was not the case during the simulated
event, as the shear strength, , of the streambank soils remained higher than the
difference between the streambank weight and the confining hydrostatic force.
In contrast, the values of , more specifically for the toe layer were lower
than 1 (Figure 6b) for all the cross sections throughout the event signifying that
fluvial erosion was a continuous process during the event. This is important since
persistent fluvial erosion can cause undercutting of the bank toe, thereby increasing
10
the bank height and angle and leading to the slumping of soil blocks from the bank
face. Thus, the geotechnical stability criterion alone is not sufficient to assess the
stability of a streambank and both and must be used in a bank stability
analysis.
CONCLUSIONS
Streambank erosion can occur by two different mechanisms, namely mass
failure and fluvial erosion. Mass failure represents the upper limit of streambank
erosion and it occurs when a streambank is geotechnically unstable. Fluvial erosion
is the lower limit of bank erosion and it occurs when the flow shear stress is larger
than erosional strength of the streambank soils.
Streambank soils from Clear Creek, IA, were tested in the laboratory to
determine their mechanical and fluvial erosional strengths using a standard direct
shear device and a conduit flow designed to administer a shearing force to the
streambank samples. It was found that the mechanical strength, , was 2 to 3 orders
of magnitude larger than the fluvial erosional strength, . The large difference
between the two measures of erosional strength suggested that they reflected different
underlying processes in nature. The mechanical strength, , is a macroscale quantity
describing soil yield strength, while is a microscale quantity describing the
strength provided by interparticle forces of attraction. Since is a much smaller
value, it corresponds to the lower limit of bank erosion, and therefore it should be
used to determine the onset of bank erosion. On the other hand, and should be
used to determine the onset of streambank collapse, the upper limit of bank erosion.
A geotechnical stability criterion alone (i.e., using only ) is not sufficient
to test the stability of streambanks at the study site since fluvial erosion can be a
precursor to the collapse of streambank soils or mass failure. This study
demonstrated that even, though, the factor of safety in term of mass failure, , was
larger than 1, the factor of safety in term of fluvial erosion , , could be lower than
1. This implied that fluvial erosion occurred with implications on the streambank
geometry and ultimately leading to failure due to geotechnical instability. With this in
Figure 5. Factors of safety for (a) mass failure and (b) fluvial erosion at
the toe layer during the June 19, 2009 event.
b a
11
mind, it is imperative to consider both the upper and the lower limits of the bank
erosion (in terms of and ) in a bank stability assessment or in the evaluation
of stabilization practices. Future work should be conducted at the headwaters of
Clear Creek as well, where the stream narrow in order to offer a comparison with the
mechanical and fluvial erosional strength values attained near the mouth of Clear
Creek.
REFERENCES
Abaci, O., and A. N. T. Papanicolaou. (2009). “Long-term effects of management
practices on water-driven soil erosion in an intense agricultural sub-
watershed: monitoring and modeling.” Hydrological Processes, 23(19), 2818-
2837.
American Society of Civil Engineers Task Committee (ASCE). (1998). “River width
adjustment II: Modeling.” Journal of Hydraulic Engineering, 124, 903-917.
Barrett, K., W. Goldsmith, and M.Silva. (2006). “Integrated bioengineering
and geotechnical treatments for streambank restoration and stabilization
along a landfill.” Journal of Soil and Water Conservation, 61(3), 144-153.
Denn, K.D. (2010). “Sediment budget closure during runoff-generated high flow
events in the South Amana sub-watershed, Ia.” M.S. Thesis. University of
Iowa.
Fredlund, D.G., and H. Rahardjo. (1993). Soil Mechanics for Unsaturated Soils. John
Wiley and Sons, Inc., New York. 517 p.
Guo, J., and P.Y. Julien. (2005). “Shear stress in smooth rectangular open-channel
flows.” Journal of Hydraulic Engineering, 131(1), 30-37.
Haaland, S.E. (1983). “Simple and explicit formulas for the friction factor in
turbulent pipe flow.” Journal of Fluids Engineering, 105, 89-90.
Hanson, G. J. and A. Simon. (2001). “Erodibility of cohesive streambeds in the loess
area of the Midwestern USA.” Hydrological Processes, 15, 23-38.
Kandiah, A. (1974). “Fundamental aspects of surface erosion of cohesive soils.”
Ph.D. Dissertation. University of California – Davis.
Langendoen, E. J., and Alonso, C. V. (2008). “Modeling the evolution of incised
streams. I: Model formulation and validation of flow and streambed evolution
components.” Journal of Hydraulic Engineering, 134(6), 749–762.
Langendoen, E.J., R.R. Wells, R.E. Thomas, A. Simon, and R.L. Bingner. (2009).
“Modeling the evolution of incised streams. III: model application.” Journal
of Hydraulic Engineering, 135(6), 476- 486.
Lawler, D. M. (1992). “Process dominance in bank erosion systems”, in Carling, P.
A. and Petts, G. E. (Eds), Lowland Floodplain Rivers: Geomorphological
Perspectives, Wiley, Chichester, 117-143.
Millar, R.G., and M.C. Quick. (1998). “Stable width and depth of gravel-bed rivers
with cohesive banks.” Journal of Hydraulic Engineering, 124(10),1005-1013.
Papanicolaou, A. N. (2001). ”Erosion of cohesive streambeds and banks.” State
Wash. Water Res. Cent. Rep. WRR-08, Wash. State Univ., Pullman,Wash.
12
Papanicolaou, A.N., M. Elhakeem, and R. Hilldale. (2007). “Secondary current
effects on cohesive river bank erosion.” Water Resources Research.
43(W12418), doi: 10.1029/ 2006WR005763.
Pizzuto, J. (2009). “An empirical model of event scale cohesive bank profile
evolution.” Earth Surface Process Landforms, 34, 1234–1244.
doi: 10.1002/esp.1808.
Rinaldi, M., and S.E. Darby. (2008). “Modeling river-bank-erosion processes and
mass failure mechanisms: Progress towards fully coupled simulations.” In:
Habersack, H., H. Piegay, and M. Rinaldi (eds.). Gravel-bed Rivers VI: From
Process Understanding to River Restoration. pp. 213-239.
Vermeyen, T. (1995). “Erosional and depositional characteristics of cohesive
sediments found in Elephant Butte Reservoir, New Mexico.” Technical
Report R-95-15. Water Resources Services, Technical Service Center, Bureau
of Reclamation, Denver, CO.
Zreik, D.A., B.G. Krishnappan, J.T. Germaine, O.S. Madsen, and C.C. Ladd. (1998).
“Erosional and mechanical strengths of deposited cohesive sediments.”
Journal of Hydraulic Engineering, 124(11), 1076-1085.
A.N. Papanicolaou1, T. E. Sutarto2, C.G. Wilson1, E. J. Langendoen3
1Department of Civil & Environmental Engineering, University of Tennessee, Knoxville, TN 37996.2Department of Civil Engineering, Samarinda State Polytechnic, Samarinda, Indonesia 75131.
3U.S. Department of Agriculture, Agricultural Research Service, National SedimentationLaboratory, Oxford, MS 38655.
Bank Stability Analysis for Fluvial Erosion andMass Failure
World Environmental & Water Resources CongressJune 1-5, 2014
Portland, Oregon USA
Different Modes of Bank Erosions
Fluvial Erosion(grain to grain removal)
Mass Failure(slump of soil blocks)
Bank Erosion
Flow direction
= , − 1Near bank or side wall shear stress (Pa)Erodibility coefficient for
fluvial erosion (kg/m2/s)
Critical shear stress or fluvialerosional strength (Pa)
m = 1 for consolidated andaged, cohesive soils.
Rate of fluvial erosion(kg/m2/s)
The rate of fluvial erosion, Ef , in kg/m2/s, can be determined by an excessshear stress formula from Kandiah (1974):
Fluvial Erosion
= + ∅ − ∅Mechanical
strength (Pa)Frictional
component (Pa)Pore water
pressurecomponent (Pa)
Soil shearstrength (Pa)
According to the Mohr-Coulomb theory, the shearing strength of a soilblock, Sr, is dependent on mainly the internal friction angle, Ø’, andmechanical strength, c’.
Where:c’ = mechanical strength (Pa)σ = normal stress produced by the weight of the soil block (Pa)Ø’ = internal friction angle (degrees)u = soil pore water pressure (Pa)Øb = matric suction angle (degrees)
Mass Failure
1. To quantify the differences in magnitude betweenmechanical and fluvial erosional strengths, for a streambankin a representative system of the U.S. Midwest.
2. To develop a combined field-laboratory methodology forquantifying fluvial erosional strength.
3. To improve our fundamental understanding of theinterlinkage between fluvial erosion and mass failure as wellas refine current approaches for bank stability analyses.
Study Objectives:
Historical planform changes in Clear Creek from 1937 to present.
Site Selection: Clear Creek Watershed
Modified from Langel (1996).
Study location
Site Selection: Camp Cardinal, Clear Creek Watershed
Study location: Camp Cardinal
Sampling time: July – October, 2011
Soil Sample Extractions for Laboratory Analyses
A total of 30 bank soil sampleswere extracted from the left andright bank .
Bank Soil Index PropertiesSampling Location
Sand%
Silt%
Clay%
D50
(mm)ρbulk
(kg/m3)LL%
PL%
PI%
Ac
(1) (2) (3) (4) (5) (6) (7) (8) (9)
Camp Cardinal,left bank
Crest 65.00 30.54 4.46 0.130 1,553 NA NA NA NA
Midbank 40.00 49.59 10.41 0.056 1,794 27.50 13.31 14.19 1.18
Toe 65.00 28.46 6.54 0.100 2,014 NA NA NA NA
Camp Cardinal,right bank
Crest 43.00 49.91 7.09 0.040 1,299 31.49 19.12 12.36 1.22
Midbank 45.00 47.17 7.83 0.058 1,618 30.21 15.24 14.97 1.71
Toe 58.74 30.95 10.31 0.100 1,880 NA NA NA NA
Note: Percent of sand, silt and clay were determined from sieving and hydrometer tests; liquid limit LL,and plastic limit PL, were measured by fall cone technique; plasticity index PI = LL- PL; clay activity Ac =PI/clay percentage. The numbers in the parentheses are the column numbers.
Ilite
L=17.5 cm
H =15 cm
Selected soil samples from thestudied streambank. Gamma scanning test.
Bulk Density Heterogeneity
1000 1200 1400 1600 1800 2000
ρbulk (kg/m3)
0 1 2
3 4 5
6 7 8
9 10 11
12 13 14
15 16 17
18 19 20
w
Toe
Width (cm)1000 1200 1400 1600 1800 2000
ρbulk (kg/m3)
Midbank
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
1000 1200 1400 1600 1800 2000
Hei
ght
(cm
)
ρbulk (kg/m3)
Crest
Bulk Density Heterogeneity
Increasing , and
Stream bank soils are heterogeneous due to erosional and depositionalactivity at the flood plain.
R² = 0,9846
0
20
40
60
80
100
120
140
0 20 40 60 80 100 120
SHE
AR
ST
RE
SS (
kPa)
NORMAL STRESS (kPa)
ø' = 42.35o
c' = 33.5 kPa
A saturated sample from odometer was thensubmerged in the shear box, loaded with a normalstress and sheared with displacement rate of 0.5mm/min.
Shear box movingdirection.
Soil specimen submerged andloaded in odometer.
Load gradually increased.
Soil sample was consolidated to obtain saturated sample before being shearedin direct shear device.
Mechanical Strength , c’
Operational Range:Q = 40 to 186 GPM.U = 0.5 to 2.3 m/s.τw = 1 to 19 Pa.Re = 3.4 - 1.6 x 105
A conduit flume device was used to estimate the fluvial erosional parameters (τc,f and Mf)
Downstreamsamplingtubes
Sample tray/box
Direction of flow
Plexi-glassconduit
Fluvial Erosional Strength, τc,f
0
200
400
600
800
1000
1200
1400
30
40
50
60
70
80
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70
Conc
entr
atio
n (m
g/L)
Flow
rate
(GPM
)
Elapsed Time (min)
Flow Rate Q (GPM)
Concentration C (mg/L)
A typical result of conduit flume testFluvial Erosional Strength, τc,f
Fluvial Erosional Strength, τc,fThe average % deviation at all bank profile positions was less than 15.9%,which suggests that the flume does provide repeatable values for theerosional strength of a sample.
1,47 1,53
1,94
1,46 1,47
1,83
0,0
0,5
1,0
1,5
2,0
2,5
3,0
Crest Midbank Toe
τ c,f
(Pa)
Left Bank
Right Bank
Magnitude Difference between τc,f and c’
τc,f and c’ increase over the downslope of the bank profile.
The differences between mechanical and fluvial erosional strengths are2 to 3 orders of magnitude.
Fluvial Erosional Strength τc,fMechanical Strength c’
400
3.000
6.000
500
3.700
6.600
0
2.000
4.000
6.000
8.000
Crest Midbank Toe
c' (P
a)
Left Bank
Right Bank
Actual stream corridor Stream corridor representation inCONCEPTS
CC2 CC3 CC4 CC5CC1 CC6
A reach is a stream segmenttransferring info between two crosssections
A cross section is a node holding crosssectional geometry and hydraulic data.
Simulated reach: Clear Creek at Camp Cardinal, L =129 m.
Simulation period: October 2007 – March 2013.
CONCEPTS Simulation
1. Bank geometry and layering.2. Soil composition and bulk density.3. Soil fluvial erosional strength.4. Soil mechanical strength.5. Bank roughness “n”.
From laboratoryanalyses.
Results of laboratoryanalyses areimplemented inCONCEPTS model.
CONCEPTS SimulationModel inputs:
= ,Critical shear stress orfluvial erosionalstrength (Pa)
Near bank or sidewall shear stress (Pa)
Factor ofsafety forfluvial erosion
Stability Analysis for Fluvial Erosion
CONCEPTS Simulation
If FSf < 1, fluvial erosion occurs.
If FSf > 1, the bank resistant tofluvial action.
Stability Analysis for Mass Failure
If FSmf < 1, bank collapse.
If FSmf > 1, bank stable.
Processes included in bank stabilityanalysis: Positive pore-water pressures Matric suction Confining pressures Groundwater table dynamics
Failure surface and bankprofile after mass failure.
Bank retreat dueto fluvial erosion.
CONCEPTS Simulation Results
Factor of safety for mass failure.
The banks are stable with respect to mass failure for all cross sectionsduring the June 19, 2009 flood event. However, fluvial erosionoccurred at the toe for all cross sections during the flood event.
Factor of safety for fluvial erosionat the toe layer.
Bank retreat at cross section CC6 due to the interaction between fluvialerosion and mass failure during the period from October 2007 toMarch 2013.
CONCEPTS Simulation Results
1. The mechanical strength, c’, was 2 to 3 orders of magnitude largerthan the fluvial erosional strength, τc,f. This implies that thoseparameters reflect different underlying processes in nature. The c’ is amacro-scale quantity describing soil yield strength, while τc,f is a micro-scale quantity describing the strength provided by interparticle forcesof attraction.
2. This study has demonstrated that the combined field-laboratorytechnique using conduit flume can provide repeatable results inmeasuring fluvial erosion of semi-cohesive and highly compacted banksoil.
3. The estimate of FSmf must be completed with the estimates of FSf toavoid underestimating mass failure. Otherwise, using mass failurealone in a bank stability analysis ignores the potential forinterconnection between bank toe undercutting and mass failure overan interval by as much as 30-40% of the eroded mass.
Conclusions: