Sediment Movement after Dam Removal Blair Greimann Ph.D. P.E.

Sediment Movement after Dam Removal

Blair Greimann Ph.D. P.E.

Technical Service Center, Sedimentation and River Hydraulics Group, Denver, Colorado

Prepared for EWRI Conference in Williamsburg, VA July 2005

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Outline

• Reservoir Erosion– Processes– Tools– Needs

• Downstream Deposition– Processes– Tools– Needs

Lake Powell

3

Predicting Physical Processes

• Reservoir Erosion– Current analysis methods– Needs

• Downstream Transport– Current analysis methods– Needs

4

Physical Processes -Reservoir Erosion

Natural Erosion Alternative:

Stage A. Reservoir sedimentation

Stage B. Dam removal

Stage C. Incision

Stage D. Widening

Stage E. Formation of floodplain

Stage F. Dynamic stability

From Doyle et al. 2003

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Physical Processes -Reservoir ErosionNatural Erosion Alternative:

Stage A. Reservoir sedimentation

Stage I. Incision

Stage II. Widening

Stage III. Secondary incision

Stage IV. Channel formation

From Wooster 2005

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Reservoir Erosion- Incision

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Reservoir Erosion- Lake Powell Bank Failure

8

Reservoir Erosion-Matilija Dam

•6 million yd3 of reservoir sediment

•Infrequent storms transport practically all the sediment

•Some of the largest sediment supplies in country

9


Temporary Channel

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Temporary Stabilization Structures: will be gradually removed starting at the dam and moving upstream

What material should be used for stabilization?

How fast should they be removed?

11

Reservoir Erosion Analysis Methods

• Conceptual Models • Laboratory studies

Example: Field scale model of Elwha dam at St. Anthony Falls

• Field scale drawdown tests Example: Glines Canyon Drawdown

• Empirical models• 1-D sediment models

Examples: HEC-6T, GSTAR-1D, DREAM

• 2-D sediment modelsBeing developed – plan to test on Elwha physical model experiments

12

Reservoir Erosion Analysis: Field Scale Test

Glines Canyon Dam

April 16 1994April 16 1994March 1994March 1994

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Reservoir Erosion Analysis: Physical

Models• Reclamation is using results

from physical model to design the incremental removal of Elwha and Glines Canyon Dam

• Reclamation Science and Technology Program is funding additional analysis of data in 2005

• Physical models of other removals are being proposed

Chris Bromley, St. Anthony Falls Laboratory

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Reservoir Erosion Analysis: Physical Models

Practical Questions:

• What is relationship between rate of drawdown and volume of sediment removed?

• What is the impact of armoring on rate of sediment erosion? How does this process scale to the field?

• How stable are the remaining sediments?

• Are the volume of sediments removed and stability of remaining sediments sensitive to initial channel position?

15

Reservoir Erosion Analysis:1D models

Most 1-D models require estimation of erosion width.

HEC-6T, GSTAR-1D : Erosion Width = aQb

DREAM: Erosion width is constant

Wong et al. 2005: Initial erosion width is specified, then calculated based upon an assumed shear stress distribution

CONCEPTS: Erosion width is uncertain, bank erosion is modeled but hydraulics are 1D

2455

2460

2465

2470

2475

2480

2485

0 200 400 600 800 1000

Station (ft)

Ele

vati

on

(ft

)

Initial

3 days

7 days

14 days

21 days

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Reservoir Erosion Analysis: 1D models

Comparison between GSTAR-1D and laboratory data of Cantelli et al. 2004

20

22

24

26

28

30

32

34

36

38

40

400 500 600 700 800 900 1000 1100

Channel Distance (m)

Bed

Ele

vatin

o (m

)

simulated t = 0

simulated t = 67 s

simulated t= 440 s

simulated t = 5400s

t = 0 s

t = 67 s

t = 460 s

t = 5000 s

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Comparison between GSTAR-1D and laboratory data of Cantelli et al. 2004 - Widths

0

5

10

15

20

25

30

35

40

0 50 100 150 200 250 300

Time (sec)

Top

Wid

th (

cm)

8.45 8.4 8.3 8.2 8.1

8.4 8.2 8 7.8

Distance in meters upstream of channel end

points are measured datasolid lines are simulated values

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Comparison between GSTAR-1D and laboratory data of Cantelli et al. 2004 – Discharge

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Conclusions for 1D models in reservoir:

1. 1D models can give reasonable predictions of initial incision process in non-cohesive sediment if the correct erosion width is specified

2. Bank failure and erosion processes are not well represented in 1D models.

3. Channel formation within reservoir is not modeled

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Reservoir Erosion Analysis:2D models

Under development: Need robust 2D model for Temporary Stabilization alternatives

Can temporary stabilization structures be made to fail at given storms?

What happens when structures are gradually removed?

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Downstream Transport Analysis Methods

• Analytical sediment wave model

• 1-D sediment models: HEC6, HEC-6T, GSTAR-1D, DREAM, CONCEPTS, others….

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Downstream Transport: Sediment Wave Model

sediment accumulation

original bed material

zb

S0

Need qualitative understanding of sediment movement before more complicated models are applied

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2

2

x

zK

x

zu

t

z bd

bd

b

1

*0

*

d

dd h

GGu

ud = sediment wave advection velocityGd

* = transport capacity of accumulationG0

* = transport capacity of bedhd = depth of accumulation = porosity

16 0

*00

*

S

GbGbK dd

d

Kd = Dispersion coefficientS0 = slope of downstream bedb = power of velocity in sediment

transport equation


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Key Assumptions:

• Uniform Flow

• Fraction of sediment accumulation in bed is proportional to the deposition thickness

d

bd h

zp

pd = fraction of sediment accumulation in bedzb = deposition thicknesshd = maximum depth of sediment accumulation


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Analytical model captures magnitude and timing of maximum deposition

-0.01

0

0.01

0.02

0.03

0.04

0 5 10 15 20 25 30 35 40

Distance Along Channel (m)

Ele

vati

on

ab

ove

Ori

gin

al B

ed (

m)

initial

predicted 15 min

measured 15 min

predicted 1 hr

measured 1 hr

Run 4b


Experiments performed at St Anthony Falls Laboratory, Cui et al. (2004),

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0

5

10

15

20

25

30

-7 -5 -3 -1 1 3 5

Channel Distance (m)

Bed

Ele

vatio

n (c

m) 3 mins20 mins115 mins

340 mins

760 mins

Experimental data from John Wooster

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Potential Sediment Release from Hemlock Reservoir During a Flow of 20 cfs

0.0

1.0

2.0

3.0

4.0

5.0

-0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50

Distance From Dam (miles)

San

d D

epo

siti

on

Th

ickn

ess

(fee

t)

0

time (days)


28


0.0

1.0

2.0

3.0

4.0

5.0

-0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50


San

d D

epo

siti

on

Th

ickn

ess

(fee

t)

0

0.5

time (days)


29


0.0

1.0

2.0

3.0

4.0

5.0

-0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50


San

d D

ep

os

itio

n T

hic

kn

ess

(fe

et)

0

1

time (days)


30


0.0

1.0

2.0

3.0

4.0

5.0

-0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50


San

d D

ep

os

itio

n T

hic

kn

ess

(fe

et)

0

2

time (days)


31


0.0

1.0

2.0

3.0

4.0

5.0

-0.25 0.00 0.25 0.50 0.75 1.00 1.25 1.50


San

d D

epo

siti

on

Th

ickn

ess

(fee

t)

0

4

time (days)


32


0.0

1.0

2.0

3.0

4.0

5.0

-0.25 0 0.25 0.5 0.75 1 1.25 1.5


San

d D

ep

os

itio

n T

hic

kn

ess

(fe

et)

0

8

time (days)


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0.0

1.0

2.0

3.0

4.0

5.0

-0.25 0 0.25 0.5 0.75 1 1.25 1.5


San

d D

epo

siti

on

Th

ickn

ess

(fee

t)

0

16

time (days)


34


0.0

1.0

2.0

3.0

4.0

5.0

-0.25 0 0.25 0.5 0.75 1 1.25 1.5


San

d D

epo

siti

on

Th

ickn

ess

(fee

t)

0

32

time (days)


35


0.0

1.0

2.0

3.0

4.0

5.0

-0.25 0 0.25 0.5 0.75 1 1.25 1.5


San

d D

epo

siti

on

Th

ickn

ess

(fee

t)

0

32

time (days)


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Practical Questions:

• Does sediment wave model apply at field scales?

• How does deposition peak affect flood peak? Are flood stages significantly affected?

37

Downstream Transport: 1-D models

GSTAR-1D was used to simulate movement of sediment accumulation downstream

-0.01

0

0.01

0.02

0.03

0.04

0.05

0 5 10 15 20 25 30 35 40

Distance Along Channel (m)

Ele

vatio

n ab

ove

Orig

inal

Bed

(m

)

Initial

Measured 1 hr

Analytical

GSTAR-1D

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Downstream Transport: 1-D models

Practical Questions:• How are pool-riffle sequences affected? How quickly

do they recover?• How do we model changes to morphology, such as

meandering to braided transitions?• Can the mixing of fines and coarse particles be

modeled accurately?• How does deposition peak affect flood peak? Are

flood stages significantly affected?• How is uncertainty in estimates calculated? How is

flood mitigation appropriated?

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Summary

• Dam Removal may or may not require accurate tools to predict sediment impacts

• Many areas of possible improvement:1. Quick assessment techniques

2. Multidimensional hydraulic and sediment transport models of bank erosion in reservoirs

3. Transport of fines in gravel bed rivers

4. Sediment transport through pools

Sediment Movement after Dam Removal

Blair Greimann Ph.D. P.E.

Technical Service Center, Sedimentation and River Hydraulics Group, Denver, Colorado

Prepared for National Center For Earth-Surface Dynamics, Minneapolis, MN, November 2004

Sediment Movement after Dam Removal Blair Greimann Ph.D. P.E.

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