Sedimentation Issues Associated with Dam Removal

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Sedimentation Issues Associated with Dam Removal

• Steven J. Wright
• Civil and Environmental Engineering
• The University of Michigan Ann Arbor
• ASCE Dam Removal Workshop
• July 12-13, 2004

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Presentation Outline

• Overview of Fundamental Sediment Transport and
  Geomorphology Issues
• Implications for Dam Removal
• Results of Predictive Models
• Observations at Dam Removal Sites
• Summary

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Sediment Transport Issues

• Will sediment move under action of flowing water
  and in what amounts?
• Ability to transport depends primarily on applied
  shear stress (velocity) and sediment size
  (non-cohesive sediments)
• Within existing reservoirs, low shear stress
  environment promotes deposition of upstream
  sediments, usually all but finest material,
  coarser material often deposited upstream

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Sediment Transport Issues

• Some models predict transport rates as function
  of applied shear stress level above that required
  to initiate transport
• General form of relation
• Qs A Qn with n 2-3
• These relations are based on assumption of
  uniform flow at dam site, flow is definitely
  non-uniform but relations are qualitatively valid
  and may be applied

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Sediment Transport Issues

• Implications
• Flow downstream from dam will cause erosion in
  order to re-establish sediment sediment transport
  that was retained in reservoir
• Bulk of annual sediment transport may occur over
  relatively short time scales
• Amount of sediment retained in small reservoirs
  (run-of river) may not be large with respect to
  potential annual transport

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Geomorphological Principles

• Gets you beyond the basic concept that sediment
  transport occurs in a fixed channel cross-section
• Flow features such as sinuosity or braiding are
  an attempt of the river to adjust to a more
  stable configuration (often assumed to minimize
  some energy state) depending on flow rates,
  sediment supply, etc.
• River channel width and form are functions of
  bankfull discharge and slope downstream
  bankfull discharge changes from pre-to
  post-removal state this may influence downstream
  channel width

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Meandering Channel
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Braided Channel
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Geomorphological Principles

• Implications
• Since sediment supply has been interrupted by
  dam, removal may trigger downstream responses
  regardless of sediment releases in additional to
  flow changes
• If coarse material is deposited in upper reaches
  of reservoir, this may influence channel formed
• Channel adjustments may take considerable time
• One particular issue is channel incision in
  natural channel can be initiated by changes in
  groundwater level, in dam removal operations, due
  to reservoir drawdown

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Channel Incision
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Channel Incision
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Impact Within Reservoir

• Lowering reservoir level due to dam removal
  creates downcutting in deposited reservoir
  sediments
• If Sediments are unconsolidated (due to periodic
  drawdown to flush sediments, original drainage
  channel may be quickly re-established
• If deposits are consolidated, erosion may proceed
  as head-cut propagating upstream through
  reservoir.

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Example of Channel Re-establishment
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• Head-cut following dam removal in Evens Creek,
  Oregon

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Impact Within Reservoir

• Most sediment in reservoir will not be eroded
  pre-removal dredging may not be cost effective
  unless other objectives (water quality?) need to
  be met
• If headcut forms, erosion rates largely
  controlled by migration of headcut
• Channel incision may lead to bank failures and
  subsequent additional erosion

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Predictive Modeling of Sediment Transport
Processes

• Two-phase flow problem, requires formulation of
  continuity and momentum equations for both fluid
  and sediment
• Sediment momentum equation is some uniform
  flow sediment transport relation hard to apply
  accurately at discontinuity at dam. Need to
  assume channel shape within reservoir.

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Original Stratigraphy in reservoir behind Marmot
Dam, Oregon
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Predicted evolution following removal of Marmot
Dam, Oregon
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Predicted thickness of gravel deposition
following removal of Marmot Dam, Oregon
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Examples of Sediment Response at Monitored Dam
Removal Sites

• Few high head dams have been removed, little data
  available
• Two Low Head Dams in Wisconsin (Doyle, Stanley
  Harbor, WRR V39, 1, ESG-2
• Rockdale Dam, Koshkonong River
• Lavelle Dam, Baraboo River

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Rockdale Dam

• 1 m of fine sediment, underlain by gravel
• Dam breached 9/00 followed by structure removal
  2/01

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• Reservoir surface following breaching of the
  Rockdale Dam, Koshkonong Creek, Wisconsin. (Photo
  from Tom Hooyer, Wisconsin Geological and Natural
  History Survey.)

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• Establishment of vegetation after removal of the
  Rockdale Dam. (Photo from Mike Czechanski,
  Wisconsin Geological and Natural History Survey.)

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Lavelle Dam

• Periodically flushed every 5-10 years by opening
  gates
• Opened gates 7/00, structure removal 6/01
• 1.5 m fine sediment underlain by sand

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Observations Following Removal of Lavelle Dam,
Upstream

• Initial erosion increased channel width more or
  less uniformly along reservoir
• Erosion at upstream end of reservoir eroded sand
  that was subsequently deposited downstream in
  reservoir and below dam site
• Unconsolidated deposits suffered bank slumping as
  channel incision proceeded leading to subsequent
  erosion

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Observations Following Removal of Lavelle Dam,
Downstream

• High initial transport of fine-grained sediments,
  deposition along channel margins
• Once sand began to erode in reservoir, large sand
  bar formed immediately downstream of dam,
  dissipated after 3 months
• Sand deposition at various downstream locations

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Observations Following Removal of Rockdale Dam,
Upstream

• Head-cut developed in consolidated sediments
• Initial upstream migration rate of 10m/hour
• Average over first year 40 m/month
• No erosion upstream from head-cut
• Channel incision downstream of head-cut followed
  by mass wasting bank failures

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Observations Following Removal of Rockdale Dam,
Downstream

• Limited fine sediment deposited downstream from
  dam site

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Summary

• Only a small fraction of reservoir sediment may
  be mobilized
• Previous history of dam operation may have a
  significant impact on sediment processes
• If head-cutting occurs, then sediment erosion
  rates will be less, downstream deposition will
  also be reduced

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Summary

• Long-time operation of storage dams may have
  altered channel downstream of dam prior to
  removal
• Dam removal may result in channel alteration in
  attempt to adjust to new regime
• Downstream deposits may be temporary and isolated
  in location along channel