Sediment Transport

Outline

1. Incipient motion criteria for unisize and mixed-size sediments

2. Modes of sediment transport

3. Bedload transport

4. Suspended load

5. Bedforms

Incipient Motion

(Middleton and Southard, 1984)

Forces Acting on Stationary Grain

(Middleton and Southard, 1984)

gDgD

D

F

F

G

D

0

3

20

Threshold of Motion

(Shields,1936; Julien, 1998)

(Miller et al., 1977)

gDc

c

Motion

Smooth Transitional Rough

045.0c

No Motion

Sample Calculation

What is c for D = 0.005 mm quartz-density particle?

Pa 6.3005.081.910002650045.0

gDcc

gDc

c

Entrainment of mixed-size sediment

Due to:1. Relative Protrusion2. Pivoting angle

Relative Protrusion

Pivoting Angle

Threshold of Motion for a Stationary Grain (Unisize or Graded Sediment)

Wiberg and Smith (1987), Bridge and Bennett (1992), + many others

, :Mixtures Dy

DLDG

DG

G

D

FFll

ll

F

F

tan1

sincostan

4.06.050045.0 ici DDg

Entrainment of mixed-size sediment

Sample Calculation

What is c for 0.001 and 0.010 m quartz-density particles in a mixture with D50 = 0.005 m?

Pa 8.401.0005.081.910002650045.0

m 0.01For

Pa 9.1001.0005.081.910002650045.0

m 0.001For

4.06.0

4.06.0

4.06.050045.0 ici DDg

Using Shields for unisize sediment

0.7 Pa

7.3 Pa

Sediment Transport

(Leeder, 1999)

Modes of sediment transport

Criteria for Sediment Transport Modes

• Bedload:

• Suspended bed material:

• Washload: D 0.063 mm

c 0

suau *

(Bridge, 2003)

Modes of sediment transport

c 0

suau *

Washload:D 0.063 mm

Bedload Transport EquationsMeyer-Peter and Muller (1948)

Bagnold (1966)

323 18 gDg

gq cb

ccb uua

q

0**tan

Bedload traps (K. Bunte)

Helley-Smith sampler

Measuring bedload transport

Bedload Transport Observations

Gravel-bed stream (Cudden & Hoey, 2003)

Gravel-bed streams (Bunte et al., 2004)

fib Qfib

trap

HS

HS

Bedload Transport EquationsWilcock & Crowe (2003)

Reference threshold condition

Hiding function

Reference dimensionless shear stress for median size base don fraction of sand

Transport rate based on t/tri

Bedload Transport Equations

Meyer-Peter and Muller (1948)

Bagnold (1966)

323 18 gDg

gq cb

ccb uua

q

0**tan

Barry et al. (2004)

Abrahams and Gao (2006;following Bagnold, 1966, 1973)

sss ddQ

qb

fq

QAq

50502,,*

257 56.3*45.241.3

TTG

U

Gi

g

b

2

4.3

Barry et al. (2004)

Abrahams and Gao (2006)following Bagnold (1966, 1973)

Predicting bedload transport

(c) Ackers and White [1973] equation by di

(a) Meyer-Peter and Müller [1948] equation by d50ss

(b) Meyer-Peter and Müller equation by di

(d) Bagnold equation by dmss

(e) Bagnold equation by dmqb

(e) Bagnold equation by dmqb

(g) Parker et al. [1982] equation by di (Parker et al. hiding function)

(h) Parker et al. [1982] equation by di (Andrews [1983] hiding function)

(Barry et al., 2004)

Predicting bedload transport

Suspended Sediment

• Simple criterion for suspension: suau *

(van Rijn, 1993)

DH48 – Wading Sampler

DH59 – Hand line Sampler

D74 – Hand line Sampler

Others: Super-critical flumes, ISCO, OBS, Acoustics

Measuring suspended load transport

Suspended Sediment

• Sediment-diffusion balance (equilibrium):

downward settling + upward diffusion Total suspended load

• Rouse equation:

01 y

CCCu ss

C

C

d y

y

a

d aa

z

*u

uz s

h

adyuCqs

(van Rijn, 1993)

Suspended sediment profiles and Rouse equation

Z

Ripples

Dunes

Upper-stage plane beds Bedload sheet

Bedform Stability

Suspended Load Observations

Mobile river dunes with acoustic probe, Wren et al. (2007)

Stochastic simulation, Man (2007)

Mobile orbital ripples with acoustic probes, P. Thorne

Sediment Transport and Stream Restoration

• Deficient or excessive sediment transport based on design discharge will result in erosion or deposition, which can redirect flow and threaten infrastructure and ecologic indices

• Sediment transport prediction depends on grain size, gradation, and bed topography

• Uncertainty can be large• Excludes bank erosion and wash load• Use multiple relationships

Sediment Transport

Conclusions• Threshold conditions defined by Shields

criterion• Modes of sediment transport depend on

Shields criterion and grain size• Bedload and suspended load transport

treated separately• Load is modulated by bedforms