Post on 10-Jan-2016
description
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
2
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
5
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
7
Reservoir Erosion- Lake Powell Bank Failure
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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
Reservoir Erosion-Matilija Dam
Temporary Channel
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Reservoir Erosion-Matilija Dam
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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Reservoir Erosion Analysis: 1D models
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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Reservoir Erosion Analysis: 1D models
Comparison between GSTAR-1D and laboratory data of Cantelli et al. 2004 – Discharge
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Reservoir Erosion Analysis: 1D models
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
Downstream Transport: Sediment Wave Model
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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
Downstream Transport: Sediment Wave Model
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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
Downstream Transport: Sediment Wave Model
Experiments performed at St Anthony Falls Laboratory, Cui et al. (2004),
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Downstream Transport: Sediment Wave Model
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)
Downstream Transport: Sediment Wave Model
28
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
0.5
time (days)
Downstream Transport: Sediment Wave Model
29
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
ep
os
itio
n T
hic
kn
ess
(fe
et)
0
1
time (days)
Downstream Transport: Sediment Wave Model
30
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
ep
os
itio
n T
hic
kn
ess
(fe
et)
0
2
time (days)
Downstream Transport: Sediment Wave Model
31
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
4
time (days)
Downstream Transport: Sediment Wave Model
32
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 0.25 0.5 0.75 1 1.25 1.5
Distance From Dam (miles)
San
d D
ep
os
itio
n T
hic
kn
ess
(fe
et)
0
8
time (days)
Downstream Transport: Sediment Wave Model
33
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 0.25 0.5 0.75 1 1.25 1.5
Distance From Dam (miles)
San
d D
epo
siti
on
Th
ickn
ess
(fee
t)
0
16
time (days)
Downstream Transport: Sediment Wave Model
34
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 0.25 0.5 0.75 1 1.25 1.5
Distance From Dam (miles)
San
d D
epo
siti
on
Th
ickn
ess
(fee
t)
0
32
time (days)
Downstream Transport: Sediment Wave Model
35
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 0.25 0.5 0.75 1 1.25 1.5
Distance From Dam (miles)
San
d D
epo
siti
on
Th
ickn
ess
(fee
t)
0
32
time (days)
Downstream Transport: Sediment Wave Model
36
Downstream Transport: Sediment Wave Model
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
38
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?
39
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