sediment transport part 2: transport rate
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Sediment transport Part 2: transport rate. GEOL/CE/EEB 8601 Intro to Stream Restoration. Steps in analyzing sediment mobility. Determination of bed sediment characteristics: grain size distribution and texture Will it move? Apply the Shields criterion (Shields diagram) - PowerPoint PPT PresentationTRANSCRIPT
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Sediment transportPart 2: transport rate
GEOL/CE/EEB 8601 Intro to Stream Restoration
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Steps in analyzing sediment mobility
1. Determination of bed sediment characteristics: grain size distribution and texture
2. Will it move? Apply the Shields criterion (Shields diagram)
3. Estimation of bed-material transport rate4. Understanding channel change:
watershed, natural history and effects of imposed changes
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Bedload: the movie!
John Gaffney movie
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Measuring bedload
Helley-Smith sampler
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Estimating bedload: Meyer-Peter & Müller2/3)( cskMPMs Kq
qs =
qs
(s–1)gD3 1 21 2
qs =volumetricsedimentdischarge
perunitwidth.............[L2T–1]
s=sedimentspecificgravity.............[0]
g =gravitationalacceleration.............[LT–2]
=
u 2
( s – 1 ) gD
u =frictionvelocity=
0
1212
0=boundaryshearstress.............[ML–1T–2]
=fluiddensity.............[ML–3]
c= forinitiationofsedimentmotion
KMPM = 8 in the original formulation; corrected to 4 by Wong & Parker (2005)
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Form drag: Smith & McLeanIndirect estimate of skin friction component of total stress
28.00 ))/(1.0ln(105.0
skbf
sk
skT zh
0sk 50
spatially averaged skin friction stress
total stress
z skin roughness length = / 30
0.4
sk
T
D
hbf
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Surface layer thickness La [1-2]Ds90
fraction of material in the surface (active) layer in the ith size range = Fi
unit bedload transport rate in the ith size range = qbi
Mixed sizes: definitions
Lasurface
subsurface
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Bedload vs suspension
Fw ~ w’ (turbulence)
Fw ~ bed (collisions, contact)
suspension
bedload
Turbulent fluctuations w’ ~ u* suspension if u* ~ settling vel ws
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Bedload vs suspension
Turbulent fluctuations w’ ~ u* suspension if u* ~ settling vel ws
u
ws
Ro
Low Ro: turbulence > settling, suspension > bedload; sediment column is well mixed
High Ro: turbulence < settling, bedload only; sediment transport confined to bed region
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Bedforms
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Bedform types: unidirectional flow
ripples
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Bedform types: unidirectional flow
dunes
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Bedform types: unidirectional flow
plane bed
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Bedform types: unidirectional flow
antidunes
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Bedform stability
Southard (1991)
0.2 0.4 0.6 1.0 2.00.2 0.4 0.6 1.0 2.0
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Bedform stability
Southard (1991)
0.2 0.4 0.6 1.0 2.0 0.2 0.4 0.6 1.0 2.0
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Bedform stability
Southard (1991)
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Bedform stability
Southard (1991)
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Bedform stability
Southard (1991)
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Bedforms - summaryRipples
• D < 0.5 mm
< 0.3 m
/hbf ~15
• u ~ uc
Dunes
• D > 0.2 mm
• hbf ~ 0.3 h
/hbf >15
• ws> u >> uc
Upper plane bed
• D = any
• u > ws
Antidunes
• D = any
• Fr = U/gh ~ 1
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Can you explain this?
Pigeon Roost Creek, Mississippi
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Total load formulasEngelund-Hansen (1967)
05.0 2/5
f
s cq
2
2
U
uc f
Brownlie (1981)3301.0
50
6601.0978.10
r )(7115
DSFFKqq ggws
50)1( Dsg
UFg
r = hydraulic radius (Area/wetted perimeter
K = 1 for lab data
= 1.268 for field data
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Bedload, suspended load, gravel and sand: adding fines to a gravel channel
• Establish steady-state slope profile with constant sediment feed Qs_coarse of a fixed coarse grain size, fixed water supply Qw
• Double Qs by adding an equal supply rate of a second finer grain size
• Measure new steady-state slope with the additive Qs_coarse + Qs_fine
John Gaffney, Sarah Baumgardner, Kimberly Hill, Peter Wilcock
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Adding fines to gravel channel
John Gaffney, Sarah Baumgardner, Kimberly Hill, Peter Wilcock
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D50 coarse/D50 fine
Adding fines to gravel channel
John Gaffney, Sarah Baumgardner, Kimberly Hill, Peter Wilcock
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Adding fines to gravel channel
John Gaffney, Sarah Baumgardner, Kimberly Hill, Peter Wilcock
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Adding fines to gravel channel