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

Outline Reservoir Erosion Downstream Deposition Lake Powell Processes Tools Needs Downstream Deposition

Predicting Physical Processes Reservoir Erosion Current analysis methods Needs Downstream Transport

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

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

Reservoir Erosion - Incision

Reservoir Erosion - Lake Powell Bank Failure

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

Reservoir Erosion -Matilija Dam Temporary Channel

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?

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

Reservoir Erosion Analysis: Field Scale Test Glines Canyon Dam March 1994 April 16 1994

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

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?

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

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

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

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

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

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?

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

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

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

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

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

Downstream Transport: Sediment Wave Model Experimental data from John Wooster

Downstream Transport: Sediment Wave Model

Downstream Transport: Sediment Wave Model

Downstream Transport: Sediment Wave Model

Downstream Transport: Sediment Wave Model

Downstream Transport: Sediment Wave Model

Downstream Transport: Sediment Wave Model

Downstream Transport: Sediment Wave Model

Downstream Transport: Sediment Wave Model

Downstream Transport: Sediment Wave Model

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?

Downstream Transport: 1-D models GSTAR-1D was used to simulate movement of sediment accumulation downstream

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?

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

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