Sediment Transport in Channels

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Sediment Transport in Channels

• Tom Dunne
• Winter 2008

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What sediment issues confront a Watershed Analyst?

• Sediment sources
• see earlier on hillslope erosion and sediment
  budgets
• Sediment transport in channels
• Sedimentation (accumulation for varying periods
  of time) in channels, floodplains, reservoirs,
  estuaries, and downstream water bodies

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Sediment sources
J.P. Syvitski, U. of Colorado

• Identify (easy to misinterpret extensive vs.
  point sources)
• Measure size/extent, yield, texture, chemistry(?)
• Identify controls
• Assess potential controls, need, cost,
  probability of success bang/buck
• Anticipate consequences of reducing sediment
  supply to channel system

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Sediment BudgetRapid Evaluation of Sediment
Budgets (Reid and Dunne, 1996)

• Sediment budget definition (from ESM 203)
  accounting of the sources, transport and
  deposition of sediments (processes, storage
  areas, and weathering)
• USDA Forest Service approved
• Designed to make a complex process more of a
  reasonable calculation
• Requires less than 2 months of field work and
  analysis
• Step required carefully define the problem,
  acquire background information, subdivide the
  area, interpret aerial photographs, conduct
  fieldwork, and analyze the data

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Information Required

• Type and location of major natural and
  management-related sources of sediment
• Approximate amount of sediment contributed by
  each type of source (land use)
• Grain-size distribution of sediment contributed
  from each source (pebble count)
• Approximate volumes and grain sizes of sediment
  in storage along streams and
• Approximate transport rate of sediment through
  stream channels and valley floors.

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Validity of the Calculation

• How wisely parameters are measured, analyzed and
  the method performed.
• Requires a sound understanding of erosion and
  sedimentation processes
• Field experience for mapping and measuring
• Analytical skills
• An insufficient number of sediment budget studies
  exists to allow statistical evaluation of the
  accuracy and reproducibility of the general
  approach. Because many sediment budget
  applications require only approximate estimates,
  this level of accuracy is thought to be adequate.
• Construction of sediment budgets is more
  difficult in some areas than others rates must
  be evaluated using as long a period of record as
  possible.
• The most difficult aspects of a sediment budget
  quantifying transport and storage of sediment in
  channels.

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Need for Bren Students in Sediment Management
(among other environmental problems)

• It can cost a lot of money to manage sediment at
  its source, in transit, or at the deposition site
• Somewhat like groundwater, sediment processes are
  not well understood by the general public. That
  certainty can get in the way of rational
  decision-making, as it can be exploited by
  specialists with a non-public-spirited agenda.
• Therefore, clear conceptual models of what is
  happening at basin scale are very influential in
  guiding analysis, monitoring, study, and
  management
• Understanding at least the basics of
  sedimentation in a quantitative manner is useful
  for managing problems

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Sediment transport in channels

• What is the sediment concentration in the flowing
  water (mg/L)? Turbidity (JTUs)?
• Relationship varies with sediment mineralogy and
  carbon content
• Usually calibrate locally
• What is the transport rate (t/day) or basin yield
  (t/km2/yr or kg/ha-yr) for design calculations ?
• What bioactive chemicals are attached to the
  sediment? (C, N, P, metals, pesticides,
  radionuclides)
• Sediment is basis of many (most?) TMDL analyses,
  urban runoff contamination problems, etc.

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Assessment of In-Stream Processes in the
Development of Sediment TMDLs for Urban Streams
Technical Report GWRI Terry W. Sturm (Georgia
Tech) Abstract Sediment loads and water quality
are inextricably linked in Georgia streams,
particularly in urban areas in the piedmont
region where fine-grained sediments contribute
turbidity in the water column and deposition in
downstream areas. Urbanization results in
increased washload to the stream due to runoff
from construction sites that are inadequately
protected by erosion control measures. In
addition, the runoff volume and peak discharge
increase due to an increase in impervious area on
the watershed. The result is a loss of
equilibrium in the sediment regime of the stream.
The consequences include bank erosion,
degradation, loss of aquatic habitat and spawning
areas, inhibition of photosynthesis due to
turbidity in the water column, increased water
treatment costs, loss of reservoir storage
capacity, and transport of contaminants
associated with fine sediments. The resulting
impairment of water quality has to be addressed
with respect to compliance with section 303(d) of
the Clean Water Act. In particular, where excess
sediment loads threaten the biological integrity
of streams, TMDLs (total maximum daily loads)
must be established to quantify allowable
sediment loads for the purpose of controlling the
sources of water quality impairment. The
development of TMDLs for sediment is complex
because of various in-stream processes that
contribute to the problem as well as watershed
sources of sediment. The objectives of the
proposed research are to develop a procedure for
(1) measuring sediment loads in streams due to
both watershed sediment yield and in-stream
processes and (2) evaluating the contribution of
in-stream processes such as bed and bank erosion
to the sediment budget of stream reaches selected
for establishment of TMDLs. The objectives will
be achieved through a combination of field
measurements on an urban stream and numerical
modeling of sediment loads and stream stability.
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Sediment accumulation (short- or long-term)

• Fine sediment trapped in spawning gravels
• Sediment deposited in pools (fish-rearing
  habitat)
• Coarse sediment deposited as in-channel bars
  forces channel to shift around
• Coarse sediment accumulating on channel bed
  reduces flood conveyance capacity (dredging
  conflict)
• Fine sediment deposited in floodplains with As,
  Cu, Hg attached from mining operations Central
  Valley of CA Butte, MT)
• Reduction of reservoir capacity
• Trapping of sediment behind dams starving beaches
  (S. Cal). Largest impediment to dam removal is
  sediment mangement.

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Faced with an unpredictable multitude of problems

• Take a process view of the activity of sediment
  in watersheds and channels
• Sediment originates somewhere
• Therefore it has certain characteristics (size,
  sorting, chemistry, durability)
• It is transported in a number of ways
• It comes to rest somewhere (for short of long
  periods)
• While in motion or at rest it affects water
  quality and habitat quality
• The controls on its rate of production,
  transport, and accumulation are subject to
  natural variability and management controls

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Need for Bren-ism in Sediment Management (among
other environmental problems)

• It can cost a lot of money to manage sediment at
  source, in transit, or at the deposition site
• Somewhat like ground water, sediment processes
  are not well understood by the general public.
  That uncertainty not only gets in the way of
  rational decision-making, but it can be exploited
  by specialists with a non-public-spirited agenda.
• Therefore, clear conceptual models of what is
  happening at basin scale are very influential in
  guiding analysis, monitoring, study, and
  management
• Understanding at least the basics of
  sedimentation in a quantitative manner is useful
  for managing problems

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Focus of rest of course
Watershed Sediment Budget (all the problems
referred to above)
Sediment processes in the channel, especially
affecting form/habitat
Issues covered earlier
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General principle of fluvial geomorphologyRiver
channel morphology and behavior are controlled by

• The probability distribution of flows (often said
  to be represented by a dominant discharge),
  which determine the sediment transport capacity
  of the channel
• The magnitude and texture of sediment supply
  (including organic debris). Linkage to the basin
  sediment budget (see earlier lecture notes)
• Nature of bank materials (sediment in transit
  reinforced by riparian vegetation)
• All are subject to management (conscious or
  inadvertent)

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Factors that control channel morphology and its
response to environmental change (incl. land
management)
D.R. Montgomery and J.M.Buffington, Channel
processes, classification, and response. InRiver
Ecology and Management (Eds. R.J. Naiman and R.E.
Bilby), Springer Verlag, 1998.
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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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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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Identification of differential transport behavior
of grain size classes
Bed material (sediment on channel bed/bars)
Form channel features, aquatic habitat, reduce
flood capacity, require dredging, etc.
Total sediment supply
Washload (grain sizes present in sediment supply
--- e.g. soil but not in bed material)
Cause turbidity, transport contaminants, deposit
downstream
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Example of a first-cut analysis of a
sedimentation problem a habitat manager faces

• Landslide (volume 106 m3) of sediments of
  various sizes (silt to gravel) enters Deer Creek,
  WA, a prime steelhead stream, and continues to
  fail
• Bed material in reach of landslide is cobble
  steelhead spawning habitat downstream is habitat
  of various kinds for other salmon spp.
• Progressive reduction in bed material size
  downstream, but all gravel
• Nearest sandy bed material is 50 km downstream
• Off-shore oyster beds in Puget Sound
• Examine failing materials, estimate of each
  grain size in failure
• Observe and calculate where each grain size is
  likely to be deposited in short term to decide
  where/when/whether you are likely to have a
  habitat management problem

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After sediment enters a river channel

• Is it mobile?
• By what transport process?
• How far will it travel?
• How fast will it travel?
• Will it travel continuously or episodically?
• Will the sediment change on the way downstream?
• If it comes to rest, what form(s) will it take?

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Whats a transport process?

• Modes of transport
• Bedload rolls, slides and saltates along the bed
  (within 1-100 grain diameters of the bed)
• Suspended load held up in the flow by eddies,
  which counteract the tendency for the particle to
  settle under gravity

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Measurement of bedload Helley-Smith sampler

• Directly measures sediment flux per unit time per
  unit width of channel bed

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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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Sediment concentration varies with depth in the
flow and particle size
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Measuring Suspended Sediment Discharge (actually
concentration)

• Grab sample (best for dissolved or extremely fine
  particulate load and not near bank where velocity
  slow)
• Vertically integrated concentration
• Isokinetic pumping sampler (usually at river edge)

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Measurement of suspended load by averaging the
vertical profile of 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)
• Automatic monitoring of turbidity at a single
  depth in the flow, correlation with a small
  number of concentrations

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Measurement of suspended load by sampling
sediment concentration
Tana R., Kenya
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Prediction of sediment behavior

• Shields criterion for mobility as bedload
• D50 is the median grain size of the bed material
• Field checking
• Used to estimate discharge (and therefore depth)
  required for disturbing the bed and flushing
  fines
• So

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Criterion for suspendibility

• Balance settling rate against rate of uplift by
  eddies

Particle settling velocity (measured in lab and
listed in handbooks) Shear velocity of flow v
t/?w
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Suspendibility

• The lower the value, the higher the particle
  will ride in the water column
• Affects access to the floodplain and of-channel
  water bodies
• Affects the length of a hop flood event
• Approximate criterion for washload
• Check by examining the bed material (what sizes
  are not included)

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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
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Bed material transport rate

• Many equations. See Reid and Dunne (1996) Rapid
  Evaluation of Sediment Budgets, Catena Verlag,
  169 pp for review
• qb bed material flux per unit width of channel
  (e.g. kg/day-meter)
• B decreases as D50 increases
• b 1.5 for bedload 2 for suspended bed
  material load

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Particle abrasion during bedload transport

• Converts coarse bedload sediment (habitat
  forming) into suspendible bed material or
  washload
• Suspendible bed material reduces quality of
  substrate
• Washload causes turbidity
• Abrasion
• Dinitial and k both depend on rock type and
  contributing erosion process. Soft rocks
  generate a lot of fine sediment.

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Large Woody Debris Sediment Production,
Recruitment, Transport, Retention, Function
(Physical and Biological)
http//www.fsl.orst.edu/cfer/research/resproj/lrgw
d/lrgwd.html http//www.fisheries.nsw.gov.au/aquat
ic_habitats/aquatic_habitats/snags www.aslo.org
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Physical effects

• Slows flow and traps sediment that would not
  otherwise be stored
• Transition from plane-bed to low-gradient
  pool-riffle channels, under a particular
  sediment-supply and flow regime, can be
  controlled by the presence of LWD
• Protects banks from scour
• Forces scour of pools and creates bars/spawning
  beds
• Beechie and Sibley 1997, Nelson 1998

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Effect of LWD loading on frequency of
poolsGreater loading ? closer spaced
pools(i.e. more pool habitat)Montgomery et
al., (1995) Water Resources Research
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Biological Effects

• Creates flow complexity for energy-saving growth
  of fish
• Shelter of fish from predators
• Invertebrate food substrates
• Algae and bacterial retention of nutrients (e.g.
  N)

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Loading (pieces or m3 per km) depends on

• Rate of riparian tree production
• Size and durability of riparian trees
• Channel width
• wider channels trap larger pieces mainly in pools
• smaller channels more thoroughly disturbed
• Channel gradient
• Recent watershed history (fire, landsliding,
  flooding, logging/salvage LWD removal)

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Unwelcome effectsNot always real, but need
evaluation

• Increase hydraulic roughness and reduce the flood
  conveyance capacity of rivers --- only if gt10 of
  channel cross section, but
• Can wash downstream and block bridges, etc.
• Increase local bank erosion
• Endanger watercraft and swimmers
• Esthetics --- thought to be 'messy'.