Sediment Management for River Restoration

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Sediment Management for River Restoration

• Tom Dunne
• Fall 2008

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Readings for week 4

• Slaymaker, O., The sediment budget as conceptual
  framework and management tool, Hydrobiologia, v
  494, pp 71-82.
• Walling, D. E., The role of overbank floodplain
  sedimentation in catchment contaminant budgets,
  Hydrobiologia, v 494, pp. 83-91
• Brooks, A.P. and G. Brierley, Framing realistic
  river rehabilitation targets in light of altered
  sediment supply and transport relationships
  lessons from East Gippsland, Australia,
  Geomorphology, v 58, pp 107123, 2004.
• Nelson, E.J. and D. B. Booth, Sediment sources in
  an urbanizing, mixed land-use watershed, Journal
  of Hydrology, 264, 5168, 2002.

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Managing the role of sediment supply in river
restoration is more difficult managing the
effects of flow
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Sediment supply is more difficult to predict than
flow

• Total amounts
• Timing strongly stochastic

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Sediment Management

• Extent and nature of sediment management depends
  on
• the problem to be solved
• whether the project is conceived as river
  channel restoration or watershed restoration.
• May involve
• Stabilization of sediment sources on hillslopes,
  etc.
• Removal of dams and disposition of sediment
  (release slurry pipe dredge, truck and
  stabilize)
• Gravel augmentation (addition)
• Dredging/gravel harvest from channel
• Channel bank stabilization that reduces the
  supply to the channel

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Examples of analyses required for sediment
management

• Stabilization of sediment sources on hillslopes,
  etc.
• Requires sediment budget analysis for watershed
  and design of erosion controls.
• May require judgments to be made about
  controversial methods of sediment stabilization
  e.g. hydromulching on Goleta fire
• Removal of dams and disposition of sediment
  (release, slurry pipe, dredge and truck)
• Requires extensive sediment transport modeling
  Matilija dam studies
• Gravel augmentation
• Should require transport calculation of mobile
  volumes and particle sizes, but rarely done.
  Often determined by available for purchasing
  gravel and activism of interest groups.
• Dredging/gravel harvest from channel
• Modeling of sediment transport and storage ---
  difficult because of highly disturbed conditions,
  but estimation of bed-material supply is
  necessary
• Channel bank stabilization
• Geotechnical analysis of slope stability
  hydraulic analysis of scour velocities, and
  empirical analysis of channel shifting record

7
Importance of the watershed sediment transport
system

• Sustainable functioning of the habitats in a
  river channel-floodplain depends on the
  functioning of the sediment transport system from
  the watershed through the channel reach(es) of
  interest
• Planning and designing a restoration scheme
  depends on understanding and adjusting the design
  to this sediment transport system
• Anticipation/prediction of responses of the
  sediment transport system is one of the most
  sensitive and difficult components of restoration
  design. May be others, such as
• triggering of some biogeochemical process ---
  methylation of Hg or acidification of S- or
  As-bearing sediments
• or a social process ---- failing to meet social
  expectations.
• But sediment management is the hardest one that I
  know anything about.

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Importance of the watershed sediment transport
system

• Arises because the morphology and functioning of
  the channel/floodplain result from erosion,
  deposition, and transport conditions (e.g.
  turbidity, bed texture and mobility).
• Our prediction skill is limited.
• Best evidence of future states is
  empirical/historical, augmented by judicious use
  of theory and calculation.
• Users must be made aware of the uncertainties in
  these predictions.
• Made more difficult by the fact that the
  transport system may need to be analyzed on a
  range of temporal and spatial scales, e.g.
• channel or dike integrity on reach length/flood
  event
• watershed-scale and timber-harvest cycles,
  colonization waves, etc.
• watershed scale and deglaciation history

9
Upland zone High sedimentsupply and low
storage.Alluvial transport zone sediment
transport rate sediment supply rate.
Significant transient sediment storage in valley
floors and tributary fans. Multi-threaded
channels in upper, steeper reaches
single-thread, meandering channels on lower
gradients. Free alluvial landforms.Alluvial
accumulation zone sediment transport capacity
decreasing downstream floodplain
aggrading.Outlet fans deltas, estuaries.
Length scale Amazon to Atascadero. Depends on
plate tectonics,. Again and always! ESM 203
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Sediment means particles that enter the channel
system in a wide range of sizes (clay to
boulders)Identification of differential
transport behavior of grain size classes
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Sediment Transport Mechanisms in River Channels
Bed material ( bedload) and washload
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Sediment Transport Mechanisms in River Channels
Bed material bedload suspendible bed
material
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Selective sediment transport

• Particles traced by distinctive lithology,
  painting, radio transmitters, magnets
• Average annual transport rates
• Gravel 50 500 m/yr
• Sand 100-10,000 m/yr
• Silt-clay many km, or into floodplain, where it
  stays for a long time

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Grain-size-dependent Transport Mechanisms
Washload ?s/ u lt 0.1
Suspended load (DH-48)
Suspended bed-material load 0.1lt ?s/ u lt 1.0
Bed-material load ?s/ u gt 0.1
Bedload (Helley-Smith)
Traction bedload ?s/ u gt1.0
?s particle settling velocity u
flow shear velocity v gds
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Evaluation of sediment supply from a watershedA
river restoration plan must be designed to handle
the sediment supply to which it will be subjected

• Direct sampling of flow sediment concentrations
  or fluxes
• Suspended sediment sampling
• Bed load sampling

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Measurement of suspended load by sampling
sediment concentration

• Depth-integrating sampler
• Passed up and down through flow at constant speed
• Design to average the flow correctly over the
  velocity and concentration variations
• Automated samplers pump samples from streamflow
  on a regular basis, but average not so
  representative (OK for very fine particles of
  interest in water quality)

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Measurement of suspended load by sampling
sediment concentration
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Measurement of bedload Helley-Smith sampler
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Measurement of bedload transport rate rating
curves

• Construct bedload rating curve by sampling
• Combine rating curve with flow record to
  calculate instantaneous transport rates
• Add them for a time period of interest (day
  flood year)
• For gravel-bed streams, bedload transport begins
  at flows near bankfull
• For sand-bed streams, bedload transport more
  frequent

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Evaluation of sediment supply from a watershed

• Direct sampling of flow sediment concentrations
  or fluxes
• Suspended sediment sampling
• Bed load sampling
• Transfer of measurements of sediment yields per
  unit area from comparable watersheds in the
  region (Important spatial and temporal
  uncertainties ---needs great care)

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Evaluation of sediment supply from a watershed

• Direct sampling of flow sediment concentrations
  or fluxes
• Suspended sediment sampling
• Bed load sampling
• Transfer of measurements of sediment yields per
  unit area from comparable watersheds in the
  region (Important spatial and temporal
  uncertainties ---needs great care)
• Sediment budget construction
• A sediment budget is an accounting of the
  sources and disposition of sediment of each grain
  size as it travels from its point of origin to
  its eventual exit from a drainage basin
• Rapid evaluation of Sediment Budgets by L. M.
  Reid and T. Dunne, Geoecology Texts, Catena
  Verlag, 1996.

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Mixed-grain-size sediment sourcesMass wasting
supplies all sizes in soil (clay-gravel)
sheetwash supplies finer sediment only
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Universal Soil Loss Equation prediction of soil
erosion rates, Kenya (Could be applied to
individual catchments)
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Evaluation of sediment supply from a watershed

• Direct sampling of flow sediment concentrations
  or fluxes
• Suspended sediment sampling
• Bed load sampling
• Transfer of measurements of sediment yields per
  unit area from comparable watersheds in the
  region (Important spatial and temporal
  uncertainties ---needs great care)
• Sediment budget construction
• A sediment budget is an accounting of the
  sources and disposition of sediment as it travels
  from its point of origin to its eventual exit
  from a drainage basin
• Rapid evaluation of Sediment Budgets by L. M.
  Reid and T. Dunne, Geoecology Texts, Catena
  Verlag, 1996.
• Stochastic analysis of sediment supply
• L. Benda and others, Dynamic Landscape Systems,
  In Ecology and Management of Streams and Rivers
  in the Pacific Northwest Ecoregion, (eds. R.
  Naiman and R. Bilby), pp. 261-288, Springer, New
  York, 705 pp., 1998.,
• E. J. Gabet and T. Dunne, A stochastic sediment
  delivery model for a steep, Mediterranean
  landscape, Water Resources Research, 39,, 2003.
• L. Benda and others, Network dynamics hypothesis
  spatial and temporal organization of physical
  heterogeneity in rivers, Bioscience, 55( 4), 413-
  427, 2004.

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Stochastic nature of basin-scale sediment
mobilizationThe range of morphology and
behavior of a river each depends on the power
spectrum of sediment supply (texture and rates)
and the sediment transport capacity of the reach
(f(Q,s).
Benda et al., Bioscience, 2004
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Supply of sediment from watershed and its storage
in channel reaches are stochastic processes,
modulated by drainage area for a given set of
environmental conditions.Modeling of this kind
is useful for making qualitative
interpretations of sedimentation risk, spatial
variability of relative risk in various parts of
a catchment, but not for channel design.
Benda et al., Bioscience, 2004
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Some tributary junctions are preferred sites of
sediment accumulation and spatial and temporal
habitat complexity
Benda et al., Bioscience, 2004