1. 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 Downstream Deposition Lake Powell Processes
Tools
Needs
Downstream Deposition

3. Predicting Physical Processes
Reservoir Erosion
Current analysis methods
Needs
Downstream Transport

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 Erosion
Natural Erosion Alternative:
Stage A. Reservoir sedimentation
Stage I. Incision
Stage II. Widening
Stage III. Secondary incision
Stage IV. Channel formation
From Wooster 2005

6. Reservoir Erosion - Incision

7. 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. Reservoir Erosion -Matilija Dam
Temporary Channel

10. 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 models
Being developed – plan to test on Elwha physical model experiments

12. Reservoir Erosion Analysis: Field Scale Test
Glines Canyon Dam
March 1994
April

13. 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

14. 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

16. Reservoir Erosion Analysis: 1D models
Comparison between GSTAR-1D and laboratory data of Cantelli et al. 2004

17. Reservoir Erosion Analysis: 1D models
Comparison between GSTAR-1D and laboratory data of Cantelli et al Widths

18. Reservoir Erosion Analysis: 1D models
Comparison between GSTAR-1D and laboratory data of Cantelli et al – Discharge

19. Reservoir Erosion Analysis: 1D models
Conclusions for 1D models in reservoir:
1D models can give reasonable predictions of initial incision process in non-cohesive sediment if the correct erosion width is specified
Bank failure and erosion processes are not well represented in 1D models.
Channel formation within reservoir is not modeled

20. 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?

21. Downstream Transport Analysis Methods
Analytical sediment wave model
1-D sediment models: HEC6, HEC-6T, GSTAR-1D, DREAM, CONCEPTS, others….

22. Downstream Transport: Sediment Wave Model
Need qualitative understanding of sediment movement before more complicated models are applied

23. Downstream Transport: Sediment Wave Model
ud = sediment wave advection velocity
Gd* = transport capacity of accumulation
G0* = transport capacity of bed
hd = depth of accumulation
l = porosity
Kd = Dispersion coefficient
S0 = slope of downstream bed
b = power of velocity in sediment
transport equation

24. Downstream Transport: Sediment Wave Model
Key Assumptions:
Uniform Flow
Fraction of sediment accumulation in bed is proportional to the deposition thickness
pd = fraction of sediment accumulation in bed
zb = deposition thickness
hd = maximum depth of sediment accumulation

25. Downstream Transport: Sediment Wave Model
Experiments performed at St Anthony Falls Laboratory, Cui et al. (2004),
Analytical model captures magnitude and timing of maximum deposition

26. Downstream Transport: Sediment Wave Model
Experimental data from John Wooster

27. Downstream Transport: Sediment Wave Model

28. Downstream Transport: Sediment Wave Model

29. Downstream Transport: Sediment Wave Model

30. Downstream Transport: Sediment Wave Model

31. Downstream Transport: Sediment Wave Model

32. Downstream Transport: Sediment Wave Model

33. Downstream Transport: Sediment Wave Model

34. Downstream Transport: Sediment Wave Model

35. 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

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:
Quick assessment techniques
Multidimensional hydraulic and sediment transport models of bank erosion in reservoirs
Transport of fines in gravel bed rivers
Sediment transport through pools

40. 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