Geomorphic Impacts of Dam Removal

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Geomorphic Impacts of Dam Removal

• Rollin H. Hotchkiss
• Director, Albrook Hydraulics Laboratory
• Washington State University, Pullman, WA
• rhh_at_wsu.edu

http//www.matilijadam.org/
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Defined

• Geomorphic impacts changes in hydrology and
  hydraulics which affect stream shape, planform,
  slope, and sediment transport

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Introduction

• Dams geomorphic disruptors!
• My purpose provide a primer on what happens
  when we tinker with a river by building or
  removing a dam
• Emphasize geomorphic processes and qualitative
  relationships
• What questions should you ask before you pull
  the plug

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Road Map

• Sediment Transport Processes
• Sediment Transport and Reservoirs
• Geomorphic Balance
• Regime Adjustment
• What do you need to Know?

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Sediment Transport Process

• Sediment weighs more than water!
• Forces required to move sediment
• Shear tangential dragging force downstream
• Lift upward-directed vertical velocity
  fluctuation

V
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Modes of Sediment Transport
Suspended Load
Bedload
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Yes, Its Nonlinear

• Shear stress designated as t
• t a S (S is slope)
• S a V2 (V is velocity)
• Therefore, t a V2
• Sediment Transport designated as Qs
• Qs a t 1.5
• Therefore, Qs a V3 or higher!

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To Further Complicate Matters
Armor layer coarser material on top, finer
beneath
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Road Map

• Sediment Transport Processes
• Sediment Transport and Reservoirs
• Geomorphic Balance
• Regime Adjustment
• What do you need to Know?

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Within the Reservoir
Delta Formation
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Delta Formation
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Reservoir a Sorting Machine

• Due to nature of reservoir, coarser material
  farthest upstream

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Vertical Sorting as Well
Dont just sample on the surface.
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Combining the Two Processes
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Upstream from the Reservoir
http//www.matilijadam.org/
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• Atbara River, Sudan 12 m deposition u.s. from
  reservoir pool

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Downstream from the Reservoir

• Sediment load decreased
• Smaller peak discharges
• Unfulfilled sediment transport capacity
• Excess energy erodes bed and banks
• Bed eventually becomes armored

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Bank Instability
Small dam Lower Elkhorn River, NE
Top view upstream from 5 ft-high dam Lower
view downstream from dam
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Downstream Impacts

• North Platte River, NE 7 u.s. dams for 50 years

Now
Before Dams
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Secondary Impacts

• All things biological adjust
• New upstream river type
• New downstream river type
• How long does that take?
• Eventually change reaches equilibrium.

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Road Map

• Sediment Transport Processes
• Sediment Transport and Reservoirs
• Geomorphic Balance
• Regime Adjustment
• What do you need to Know?

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Lane, E.W. 1955. The importance of fluvial
morphology in hydraulic engineering. ASCE
Proceedings, 81, paper 745 1 17.
Qs x ds
Q x So
From Applied River Morphology by Dave Rosgen,
Wildland Hydrology, Figure 2-1
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Implications of Balance

• Rivers constantly shifting geomorphology to
  adjust balance
• When in approximate balance
• in regime
• Dam in or Dam out is out of balance!

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Road Map

• Sediment Transport Processes
• Sediment Transport and Reservoirs
• Geomorphic Balance
• Regime Adjustment
• What do you need to Know?

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On the Day of the Ribbon Cutting

• These geomorphic shifts begin
• River system begins to change
• It all takes timetimetime..

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Downstream Adjustment Time

• For Dam Building OR Removal

River 1 and river 2 here
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Adjustments after Removal
Where on deposit curve are you now?
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Depends upon Future Runoff Sequence!

• Large flood lots goes out early
• No large flood wait and wait, and

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Dam Notching
http//www.matilijadam.org/
Matilija Dam helps control rate of downstream
transport
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Road Map

• Sediment Transport Processes
• Sediment Transport and Reservoirs
• Geomorphic Balance
• Regime Adjustment
• What do you need to Know?

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Hydrologic History

• Flooding sequence and history
• Change in downstream flood peaks, duration, and
  timing
• Changes in water temperature, turbidity, and
  quality downstream

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Sedimentation History

• Snapshots of bathymetry over time
• Qs coming in and going out
• Downstream streambed profiles over time
• Location of downstream tributaries

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Sediment Particle Size Distribution

• Within reservoir
• Spatial coverage
• Collect cores!!!!
• PSD inversion?
• Downstream
• Armored?

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Downstream Habitat

• Benthics
• Fish compared to pre-dam
• Condition of spawning gravels
• Riparian vegetation

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Summary

• Know how hydrology, sediment transport and size
  have changed
• Understand how downstream reaches have adjusted
• Need to predict (can we do that?) spatial and
  temporal scenarios of impact