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: tpapanic@utk.edu

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.

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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.

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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: