Quantifying Reductions of Mass- Failure Frequency and Sediment

Loadings from Streambanks using Toe Protection and Other Means

Andrew Simon, Natasha Pollen-Bankhead,

Virginia Mahacek and Eddy Langendoenandrew.simon@ars.usda.gov

National Sedimentation Laboratory

Lake Tahoe Basin

Problem and Objectives

• Trend of declining lake clarity for more than 30 years• This has been attributed to the delivery of fine sediment• An estimated 25% of this fine sediment comes from

streambank erosion• About 90% of this emanates from three watersheds

What kind of fine-load reductions can be expected using mitigation measures?

Fine-Sediment Delivery

Annual fine load contributions

0.00 - 0.40

0.41 - 4.00

4.01 - 40.00

40.01 - 400.00

400.01 - 4000.00

©

Ward Creek

Upper Truckee River

Blackwood Creek

General Approach

1. Select critical erosion sites within watersheds known to produce substantial quantities of fine-sediment from streambank-erosion processes.

2. Quantify annual loadings from streambank erosion for existing conditions by simulating toe-erosion and bank-stability processes over the course of an annual hydrograph.

3. Quantify annual loadings from streambank erosion for mitigated conditions at these sites by simulating toe-erosion and bank-stability processes over the course of the same annual hydrograph.

4. Compare loadings reductions for the modeled sites and extrapolate results to the remainder of the channel system.

Bank-Stability and Toe-Erosion Model

• 2-D wedge-failure and cantilever model

• Tension cracks

• Hydraulic toe erosion

• Incorporates both positive and negative pore-water pressures

• Simulates confining pressures from stage

• Incorporates layers of different strength and characteristics

• Inputs: s, c’, ’, b , h, uw,

k, c

Confining pressure

Tensiometers(pore pressure)

shear surface

12/29/97 01/05/98 01/12/98 01/19/98 01/26/98 02/02/98

FAC

TO

R O

F SA

FETY

0.9

1.0

1.1

1.2

1.3

1.4

1.5

1.6

RIV

ER

STA

GE, I

N M

ETER

S A

BO

VE

SEA

LEV

EL

80

81

82

83

84

85

BFactor of safety

Effect of confining pressure

Bank failures

Stage WA

TE

R L

EV

EL

, M

Required Field Data

Table 3. Summary of input requirements and associated symbols for BSTEM simulations. Sub-Model

Toe erosion (hydraulic) Bank stability (geotechnical) Driving Resisting Driving Resisting

Flow depth (y) Critical shear stress (c)

Bank height (H) Effective cohesion (c’)

Channel gradient (S) Erodibility coefficient (k) Bank slope (a)

Effective friction angle ()

Flow duration (h) Bulk unit weight () Bulk unit weight () Vegetation (cr) Pore-water pressure

() Matric suction ()

Site Characteristics

StreamLocation

(km)

Bank height(m) Special characteristics

Blackwood Creek

1.94 3.0 No top-bank vegetation

2.39 2.4 Lemmon’s willow (moderate)

Upper Truckee River

4.51 2.6 Meadow vegetation

8.45 1.9 Mixed meadow and woody vegetation

13.1 2.7 Golf course with lodgepole pine

Ward Creek2.48 14.9

14.9 m steep, terrace slope adjacent to channel; coarse material at toe; Mature conifers

3.60 1.3 Meadow vegetation

Quantified Vegetation CharacteristicsTable 5. Root reinforcement and surcharge values for Upper Truckee Creek, Ward Creek and Blackwood Creek sites.

Site Species Rooting depth

(m)

Root-reinforcement

(kPa) Surcharge

Upper Truckee 4.51

Wet meadow sedges and grasses

0.5 16.5 0.0

Upper Truckee 8.45

Wet meadow sedges and grasses with 5-10 year old Lemmon’s willow,

Coyote willow, X willow

0.5 9.15 0.0

Upper Truckee 13.1

5-10 year old Lemmon’s willow, Coyote willow,

X willow 1.0 3.02 0.0

Ward 2.45

30 year old Lodgepole Pine

1.0 23.4 1.2

Blackwood 1.94

No bank top vegetation - - -

Blackwood 2.39

5 year old Lemmon’s willow

0.63 3.02 0.0

Selection of Annual Hydrograph

0.0

0.5

1.0

1.5

2.0

1/1 1/31 3/2 4/1 5/1 5/31 6/30 7/30 8/29 9/28 10/28 11/27 12/27

DATE

ST

AG

E, I

N M

ET

ER

S

1990

1991

1992

1993

1994

1996

1997

1998

1999

1995

Discretized Hydrographs

Bank-Toe Model

By comparing applied shear stress with critical shear stress and erodibility, actual erosion is calculated for each facet, and the profile is redrawn. The new and old profiles can be assessed for bank stability.Layer 1

Layer 2

Layer 3 Toe material

Toe Erosion

 

Click this button to export eroded profile to Option A in Input Geometry worksheet

-1.00

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0.00 1.00 2.00 3.00 4.00 5.00 6.00 7.00STATION (M)

ELE

VA

TIO

N (

M)

Base of layer 1

Base of layer 2

Base of layer 3

Base of layer 4

Base of layer 5

Eroded Profile

Water Surface

Initial Profile

‘Toe Erosion Step 2’ worksheet

Results

Toe Erosion for Initial Flow EventInput bank materialsSpecify the erodibility of the different materials. Use the drop down boxes to select material type or select "Enter own data" and add valuesin the 'Bank Model Data' worksheet. If you select a material, the values shown in the 'Toe Model Data' worksheet will be used. Once youare satisfied that you have completed all necessary inputs, hit the "Run Shear Stress Macro" button (Center Right of this page).

Bank Material Bank Toe Material Bed material

Layer 1 Layer 2 Layer 3 Layer 4 Layer 5

0.36 0.36 0.36 0.28 0.28 0.28 248.83

0.167 0.167 0.167 0.189 0.189 0.189 0.006

Bank Protection

Input bank protection

Bank Toe Protection

Input toe protection

Average applied boundary shear stress 2.61 Pa

Maximum Lateral Retreat 61.65 cm

Mean Eroded Area - Bank 0.14 m2

Mean Eroded Area - Bank Toe 0.12 m2

Mean Eroded Area - Bed 0.00 m2

Mean Eroded Area - Total 0.255 m2

Enter own data Fixed bed

No protection

No protection

Enter own data Enter own data Enter own data Enter own data Enter own data

1905.50

1906.00

1906.50

1907.00

1907.50

1908.00

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00STATION (M)

ELE

VA

TIO

N (

M)

Base of layer 1

Base of layer 2

Base of layer 3

Base of layer 4

Base of layer 5

Eroded Profile

Water Surface

Initial Profile

Export Coordinates back into model

Export new geometry

Stability Analysis for First EventSelect material types, vegetation cover and water table depth below bank top(or select "own data" and add values in 'Bank Model Data' worksheet)

Bank top Reach LengthLayer 1 Layer 2 Layer 3 Layer 4 Layer 5 vegetation cover (age) (m)

100Constituent

Vegetation safety margin concentration (kg/kg)

50 0.001

Water table depth (m) below bank top1.06

Own Pore Pressures kPa

Pore Pressure From Water Table

-6.79 Layer 1 -7.95

-12.71 Layer 2 -3.04

-12.71 Layer 3 0.78

1.56 Layer 4 3.53

3.52 Layer 5 6.28

Factor of Safety

2.65 Stable

29.5 Shear surface angle used Failure width - mFailure volume - m3

Sediment loading - kgConstituent load - kg

Rounded sand Silt Stiff clay Soft clay Own data

Rounded sand Silt Stiff clay Soft clay Own data

Rounded sand Silt Stiff clay Soft clay Own data

Rounded sand Silt Stiff clay Soft clay Own data

Rounded sand Silt Stiff clay Soft clay Own data

1905.50

1906.00

1906.50

1907.00

1907.50

1908.00

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00 16.00

STATION (M)

ELE

VA

TIO

N (

M)

bank profile

base of layer 1

base of layer 2

base of layer 3

base of layer 4

failure plane

water surface

water table

Use water table

Input own pore pressures (kPa)

None

Export Coordinates back into model

Stability Analysis after Second Flow EventSelect material types, vegetation cover and water table depth below bank top(or select "own data" and add values in 'Bank Model Data' worksheet)

Bank top Reach LengthLayer 1 Layer 2 Layer 3 Layer 4 Layer 5 vegetation cover (age) (m)

100Constituent

Vegetation safety margin concentration (kg/kg)

50 0.001

Water table depth (m) below bank top0.45

Own Pore Pressures kPa

Pore Pressure From Water Table

-6.79 Layer 1 -1.96

-12.71 Layer 2 2.94

-12.71 Layer 3 6.77

1.56 Layer 4 9.52

3.52 Layer 5 12.31

Factor of Safety

0.78 Unstable

69.0 Shear surface angle used Failure width 0.80 mFailure volume 90 m3

Sediment loading 164125 kgConstituent load 164 kg

Rounded sand Silt Stiff clay Soft clay Own data

Rounded sand Silt Stiff clay Soft clay Own data

Rounded sand Silt Stiff clay Soft clay Own data

Rounded sand Silt Stiff clay Soft clay Own data

Rounded sand Silt Stiff clay Soft clay Own data

-0.50

0.00

0.50

1.00

1.50

2.00

0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00

STATION (M)

EL

EV

AT

ION

(M

)

bank profile

base of layer 1

base of layer 2

base of layer 3

base of layer 4

failure plane

water surface

water table

Use water table

Input own pore pressures (kPa)

None

Export Coordinates back into model

Iterative Modeling Scenarios

1. Existing bank and vegetative conditions

2. Toe protection (rock) modeled by simulating 256mm clasts 1.0 m up the bank toe.

3. Addition of top-bank vegetation in some cases

Example of Iterative ResultsExisting Conditions (assuming 100m reach)

Event # Toe

erosion Shear stress Amount

FS SW=GW Failure Amount

FS Drawdown Failure Amount

Shear emergence

Failure Angle

Total Erosion

Total fines

1995 Pa m3 m3 m3 m degrees m3 m3 1 Yes 6.57 0.7 1.22 No 0 1.21 No 0 1912.03 40 0.70 0.13 2 Yes 6.32 8.5 0.95 Yes 362 - - 0 1911.88 40 371 67.4 3 Yes 8.12 1.4 1.56 No 0 1.49 No 0 1911.91 34 1.40 0.25 4 Yes 5.34 0.3 1.47 No 0 1.45 No 0 1911.88 34 0.30 0.05 5 Yes 2.53 0.2 1.29 No 0 - No 0 1911.88 34 0.20 0.04 6 Yes 7.08 3.5 0.99 Yes 194 1.37 No 0 1911.88 44/32 198 35.9 7 Yes 6.55 0.5 1.48 No 0 - - 0 1911.98 32 0.50 0.09

8a Yes 7.89 64 0.91 Yes 194 - - 0 1911.88 46 258 47.0 8b Yes 7.89 8.7 0.97 Yes 185 1.29 1911.88 44.5/32 194 35.3

9 Yes 6.46 1.1 1.41 No 0 1.35 No 0 1911.94 34.5 1 0.20 10 No 3.04 0 1.51 No 0 1.49 No 0 1911.94 34.5 0 0.0 11 No 3.13 0 1.50 No 0 1.47 No 0 1911.94 34.5 0 0.0 12 Yes 5.18 0 1.35 No 0 1.28 No 0 1911.91 34.5 0 0.0

1/1/1997 Yes 13.8 1.6 1.03 No 0 0.35 Yes 262 1911.88 34.5 264 48.0 TOTALS 12 90.5 3 935 1 262 1288 234

Toe Protection (assuming 100 m reach)

Event # Toe

erosion Shear stress Amount

FS SW=GW Failure Amount

FS Drawdown Failure Amount

Shear emergence

Failure Angle

Total Erosion

Total fines

1995 Pa m3 m3 m3 m degrees m3 m3 1 No 6.57 0 1.41 No 0 1.40 No 0 1912.10 40 0 0 2 No 6.32 0 1.44 No 0 - - 0 1912.10 40 0 0 3 No 8.12 0 1.31 No 0 1.25 No 0 1912.10 40 0 0 4 No 5.34 0 1.36 No 0 1.34 No 0 1912.10 40 0 0 5 No 2.53 0 1.38 No 0 - - 0 1912.10 40 0 0 6 No 7.08 0 1.27 No 0 1.19 No 0 1912.10 40 0 0 7 No 6.55 0 1.33 No 0 - - 0 1912.10 40 0 0 8 No 7.89 0 1.26 No 0 1.13 No 0 1912.10 40 0 0 9 No 6.46 0 1.34 No 0 1.30 No 0 1912.10 40 0 0

10 No 3.04 0 1.45 No 0 - - 0 1912.10 40 0 0 11 No 3.13 0 1.44 No 0 1.43 No 0 1912.10 40 0 0 12 No 5.18 0 1.36 No 0 1.32 No 0 1912.10 40 0 0

1/1/1997 Yes 13.8 0.1 1.19 No 0 0.28 Yes 137 1912.10 40 137 25.0 TOTALS 1 0.1 0 0 1 137 137 25.0

River Station Rkm Condition

Blackwood 1.94 1.94 Existing

Blackwood 1.94 1.94 Toe Protection

Blackwood 2.39 2.39 Existing

Blackwood 2.39 2.39 Toe Protection

Upper Truckee 50+34 4.51 Existing, No Vegetation

Upper Truckee 50+34 4.51 Toe Protection

Upper Truckee Hole 6 13.1 Existing

Upper Truckee Hole 6 13.1 Toe Protection

Upper Truckee 50+34 4.51 Existing-Veg

Upper Truckee 50+34 4.51 Toe Protection-Veg

Upper Truckee 179+74 8.45 Existing

Upper Truckee 179+74 8.45 Toe Protection

Ward 22+35 2.48 Existing-No Toe Slope

Ward 22+35 2.48 Toe Protection-No Toe Slope

Ward

Ward

26+44

26+44

3.6

3.6

Existing

Toe Protection

Failure events

at peak drawdown Total

4 3 7

0 1 1

2 1 3

1 0 1

3 3 6

0 2 2

3 1 4

0 1 1

2 1 3

0 1 1

7 3 10

0 0 0

1 0 1

0 0 0

5 0 5

0 1 1

Results: Failure Frequency

River Station Rkm Condition

Blackwood 1.94 1.94 Existing

Blackwood 1.94 1.94 Toe Protection

Blackwood 2.39 2.39 Existing

Blackwood 2.39 2.39 Toe Protection

Upper Truckee 50+34 4.51 Existing, No Vegetation

Upper Truckee 50+34 4.51 Toe Protection

Upper Truckee Hole 6 13.1 Existing

Upper Truckee Hole 6 13.1 Toe Protection

Upper Truckee 50+34 4.51 Existing-Veg

Upper Truckee 50+34 4.51 Toe Protection-Veg

Upper Truckee 179+74 8.45 Existing

Upper Truckee 179+74 8.45 Toe Protection

Ward 22+35 2.48 Existing-No Toe Slope

Ward 22+35 2.48 Toe Protection-No Toe Slope

Ward

Ward

26+44

26+44

3.6

3.6

Existing

Toe Protection

Eroded Fines Load reduction

m3 m3/km m3 %

897 89703373 93.3

61 610

79.7 797398 84.3

12.5 125

147 1470733 70.7

43 430

234 23401151 89.4

25 250

70 700336 68.6

22 220

535 53503792 99.9

0.310 3.102

336 33605256.2 100.0

0 0

35.0 350502 83.1

5.9 59.2

Load Reduction25th percentile = 80.0

Median = 86.8%

75th percentile = 94.9

Effects of Toe Protection

SITE

ST

RE

AM

BA

NK

ER

OS

ION

IN

CU

BIC

ME

TE

RS

0

1000

2000

3000

4000

5000

6000

Hydraulic toe erosion Geotechnical erosion

7

13

12 1 1

3

4

10

6

0 0

1

5

1

Load Reduction: Other Treatments

• Toe protection = 86% (average)• Top-bank vegetation = 53%• Bed-slope reduction (meandering) = 42-54%

Extrapolation of Results

Toe Erosion Failure Erosion Toe Erosion Failure events Total Erosionm3 m3

% # m3 m3%

Blackwood 1.94 Existing 418 3199 11.6 7 3617Blackwood 1.94 Toe Protection 0 244 0.0 1 244Blackwood 2.39 Existing 26.7 445 5.7 3 472Blackwood 2.39 Toe Protection 0 74.0 0.0 1 74.0

River Condition

3373

Rkm

733

Load reduction

398

1. Assessments of longitudinal extent of recent failures, and

2. Loadings rates: High, Medium and Low

(values for 100m-long reach)

High rate = 36,170 m3/km

Medium rate = 4720 m3/km

Low rate= 472 m3/km

Distance Extent of failures

(km)(%)

Left Right Average

8.290 0-10% 0-10% 5

8.190 0-10% 26-50% 21.5

7.690 11-25% 11-25% 18

7.180 11-25% 11-25% 18

7.170 11-25% 76-100% 53

6.840 0-10% 11-25% 11.5

6.510 0-10% 51-75% 34

6.030 0-10% 26-50% 21.5

5.550 0-10% 26-50% 21.5

5.080 0-10% 51-75% 34

4.150 26-50% 11-25% 25.5

3.950 0-10% 76-100% 46.5

2.800 51-75% 0-10% 34

1.970 26-50% 11-25% 25.5

1.770 11-25% 51-75% 40.5

0.320 51-75% 0-10% 34

Extrapolation Based on %

Reach Failing

(Blackwood Creek)

Distance (km)

Extent of failures(%)

Reach Length (km)

ReachFailing

(%)

Weighting factor Total

volume(m3)

Fraction <0.063 mm

(%)

Fines volume

(m3)Left Right Mean 1 2 (1)*(2)/100

8.29 0-10 0-10 5.0 - - - - - -

8.19 0-10 26-50 21.5 0.10 13.25 0.0133 62.5 5.8 3.6

7.69 11-25 11-25 18.0 0.50 19.75 0.0987 46.6 0.00 0.00

7.18 11-25 11-25 18.0 0.51 18 0.0918 43.3 26.0 11.3

7.17 11-25 76-100 53.0 0.01 35.5 0.0035 128 26.0 33.4

6.84 0-10 11-25 11.5 0.33 32.25 0.1064 50.2 26.6 13.4

6.51 0-10 51-75 34.0 0.33 22.75 0.0751 354 22.1 78.3

6.03 0-10 26-50 21.5 0.48 27.75 0.1332 629 20.0 125.7

5.55 0-10 26-50 21.5 0.48 21.5 0.1032 487 7.9 38.5

5.08 0-10 51-75 34.0 0.47 27.75 0.1304 616 23.5 144.7

4.15 26-50 11-25 25.5 0.93 29.75 0.2767 1306 3.6 47.0

3.95 0-10 76-100 46.5 0.20 36 0.0720 2604 21.4 557.3

2.80 51-75 0-10 34.0 1.15 40.25 0.4629 2185 12.3 268.7

1.97 26-50 11-25 25.5 0.83 29.75 0.2469 1165 24.8 289

1.77 11-25 51-75 40.5 0.20 33 0.0660 2387 16.6 396.3

0.32 51-75 0-10 34.0 1.45 37.25 0.5401 2549 16.3 415.6

0.00 26-50 26-50 38.0 0.32 36 0.1152 544 16.3 88.6

Fine-Sediment Loading: 2,511 m3 or 4,432 T

Simulated vs. Measured Loadings

Stream Simulated Contribution

(T) (%)

1995 Jan 1-2, 1997 Sum 1995 Jan 1-2, 1997 Sum

Blackwood Creek 5002 17610 22612 1927 8223 10150 4432 43.7

Upper Truckee River 8652 3129 11781 3500 1958 5458 5691 104

Ward Creek 2413 19112 21525 1083 5189 6272 2956 47.1

General 403 1111 1514 100 160 260 1171 451

Third 5973 29.3 6002 1329 5.2 1334 1331 101

(T) (T)

Measured Load Measured Fine Load

Cost Basis

ExistingAll High only H + M All High only H + M All High only H + M

Blackwood Creek 4432 585 2920 623 8,159,449$ 403,543$ 6,840,551$ 2,121$ 267$ 1,796$ 86.8% 34.1% 85.9% 86.8% 34.1% 85.9%

Upper Truckee River 5691 751 3789 914 20,911,417$ 2,601,378$ 10,735,138$ 4,233$ 1,368$ 2,247$ 86.8% 33.4% 83.9% 86.8% 33.4% 83.9%

Ward Creek 2956 390 910 451 6,358,661$ 1,731,594$ 3,120,669$ 2,478$ 846$ 1,246$ 86.8% 69.2% 84.7% 86.8% 69.2% 84.7%

Totals 13079 35,429,528$ 4,736,516$ 20,696,358$

Unit Cost($/T of Load Reduction)

StreamTotal CostLoads (T)

Toe Protection Toe Protection

Summary and Conclusions

1. Used iteratively, BSTEM is an effective tool to quantify potential load reductions by bank treatments

2. Toe erosion is a small component (about 13%) of total streambank loadings

3. However, by reducing toe erosion, mass-failure frequency and associated sediment loadings can be drastically reduced.

4. Other treatments can be effectively simulated with BSTEM and show significant load reductions.