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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 Presentation
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Sediment transportPart 2: transport rate
GEOL/CE/EEB 8601 Intro to Stream Restoration
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
Bedload: the movie!
John Gaffney movie
Measuring bedload
Helley-Smith sampler
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)
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
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
Bedload vs suspension
Fw ~ w’ (turbulence)
Fw ~ bed (collisions, contact)
suspension
bedload
Turbulent fluctuations w’ ~ u* suspension if u* ~ settling vel ws
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
Bedforms
Bedform types: unidirectional flow
ripples
Bedform types: unidirectional flow
dunes
Bedform types: unidirectional flow
plane bed
Bedform types: unidirectional flow
antidunes
Bedform stability
Southard (1991)
0.2 0.4 0.6 1.0 2.00.2 0.4 0.6 1.0 2.0
Bedform stability
Southard (1991)
0.2 0.4 0.6 1.0 2.0 0.2 0.4 0.6 1.0 2.0
Bedform stability
Southard (1991)
Bedform stability
Southard (1991)
Bedform stability
Southard (1991)
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
Can you explain this?
Pigeon Roost Creek, Mississippi
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
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
Adding fines to gravel channel
John Gaffney, Sarah Baumgardner, Kimberly Hill, Peter Wilcock
D50 coarse/D50 fine
Adding fines to gravel channel
John Gaffney, Sarah Baumgardner, Kimberly Hill, Peter Wilcock
Adding fines to gravel channel
John Gaffney, Sarah Baumgardner, Kimberly Hill, Peter Wilcock
Adding fines to gravel channel
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