(3) Sediment Movement

(3) Sediment Movement
•  Classes of sediment transported
–  Dissolved load
–  Suspended (and “wash load”)
•  Important for scouring algae
–  Bedload (5-10% total load
•  Moves along bed during floods
•  Source of crushing for benthic
organisms, fish eggs, etc.
Big Thompson Flood (1978)
“Graded” stream -- current sediment supply and
transport) regimes are “in balance”
–  Aggradation
•  Sediment supply > transport
–  Channel “aggrades”
–  Degradation
•  Sediment transport > supply
–  Channel “degrades”
velocity (shear stress) that is sufficient to move the particles on the bed (varies
with particle size and mass)
Ability of streamflow to move particle as bedload (for a given Q)
Q x Slope: How does competence change with slope for same Q?
Hjulstrom curve (Fig.3.8, Allan text) – Erosion, Transport and
Sedimentation for individual particles
Erosion
à When critical erosion velocity exceeded (compare
sand to gravel)
–  Why is sand the most erodible?
Small size, low cohesion
–  Why is clay least erodible?
Very high cohesion
suspended
Bed load (larger particles)
Transport
à  Continued movement once eroded if current
velocity > fall velocity for particle
–  Why is clay most transportable?
Small size … once suspended …
Sedimentation
à Current elocity drops below fall velocity
Thought question:
How can some particles continue to be transported when average velocity < fall velocity?
•  Streambeds are well sorted
–  Distribution of particle sizes
reflects hydraulic conditions
•  coarse grains in faster
flowing areas (erosional)
•  finer grains in slow-flowing
areas (depositional)
–  Sorting occurs in different
spatial dimensions:
•  longitudinally (e.g., pool-riffle)
•  laterally (thalweg to shore)
•  Sorting reflects a erosional and
depositional processes across a
range of flows (Hjulstrom curve)
–  Coarse sediments sorted by flood flows
–  Finer sediments sorted by less than
flood flows
•  Movement of streambed during high flows
maintains channel form
•  What determines bed mobility?
–  Sediment size (Hjulstrom curve)
–  Shear stress acting on streambed particles
•  Recall for a particle on the streambed:
-- Shear stress, τ ~ ΔU/Δy
•  The critical shear stress (τcr) is that force that
initiates incipient motion of a particle(s) on the
streambed
τcr
= critical shear stress Function of:
- grain packing
- gravity constant
- particle density
- fluid density
- particle diameter
- drag and lift forces
Virtually impossible to apply to a
single particle in a mixed particle
streambed!
What’s alternative?
Use average mobility of a larger
area of the streambed.
- often calculated for the REACH
scale.
Knighton 1998
Usually we’re interested in whole streambed mobility
•  Focus on the population of particles in a reach, and estimate
the average shear stress acting on the bed and see whether
it is sufficient to move the average particle size (median or
D50):
τ = ρgRS
Above equation simplifies to τ ∼ DS, so, shear stress acting on
streambed is Directly proportional to
–  Mean Depth
–  Channel Slope (or gradient)
• 
• 
As shear stress increases, what happens to stream “competence”?
What determines how depth and slope change with increasing Q?
–  ΔDepth: depends on channel “constraint”
–  ΔSlope: depends on channel bedform
•  ΔDepth: depends on channel
“constraint” (Fig. 4.15 Knighton)
–  Constrained channel
•  Depth increases rapidly with Q
•  energy doesn’t dissipate laterally
–  Unconstrained channel (e.g.,
with floodplain)
•  Depth increases to fill channel
•  Higher flows spill onto floodplain
(energy dissipated laterally)
•  For same Q and slope, which
channel type has greatest
competence?
•  ΔSlope: depends on channel bedform (e.g., riffle v.
pool)
•  Which has greater shear stress at LOW FLOW? (Fig.
1.12) Hint: Think about depth and slope and velocity profile!
–  Riffle at low flow has
shallow depth but high
velocity and steep slope.
–  Pool has greater depth, but
slope is very low.
•  ΔSlope: depends on channel bedform (e.g., riffle v.
pool)
•  Which has greater shear stress at LOW FLOW? (Fig.
1.12) Hint: Think about depth and slope and velocity profile!
–  Riffle at low flow has high velocity, high slope, thus greater shear stress
–  Pool has greater depth, but slope is flat
•  What about during
HIGH FLOW?
–  Depth?
–  Velocity?
–  Slope?
–  Shear Stress?
•  Sediment transport in riffles vs.
pools
–  At low flow, pools are depositional
–  At flood flow, pools are erosional
•  coarse grains move through
pools, deposit at head of riffles
as flood recedes
•  Next question: When does
sediment move? What
flow levels are responsible
for transporting sediment
and maintaining the
channel form that we
observe?
•  Bankfull discharge (Qbkf ) – (important!)
– Fills the channel and does work on
boundaries. The Qbkf maintains the
channel form we observe.
The "Dominant discharge” (Qd ) is the flow level that
moves the greatest total volume of sediment. There
is a Qd for both suspended load and bedload.
Qbkf and Qd are often the same for suspended load,
but not generally for bedload.
* The Qd is determined by how large the magnitude
of the flow AND the frequency with which it occurs.
Frequency-Magnitude Concept
•  Qbkf a “channel
forming flow”
• Bankfull discharge
occurs on average
once in 2 - 3 years and
moves most
suspended sediment,
maintaining channel.
• The discharge level
that moves most
bedload occurs every
1.5-10 years,
depending on channel
type
• Applies best to
gravel-cobble rivers
with floodplains
Suspended
sediment load
Bedload
Freq. of Q
Occurs every
1.5 – 10 yr
Otter Creek in Vermont
Record of annual peak flows
Last few years
not high
annual peaks
2 easy ways to calculate Qbkf (1 in 2 year peak):
Annual Maximum Discharge (ft3/
s)
1)  Graph observed annual peak flows on
probability plot and find flow with p = 0.5
2) Assume peak flows have a
logarithmic distribution. Convert
values to log scale and find
average value, which has a 50%
chance of occurring in a given
year.
Mean of Loge peak flows = 8.56
14000
12000
10000
8000
6000
5570
4000
2000
0
1
0.1
Probability of Occurence
0.01
e8.56 = 5,251 cfs = BFQ