Introduction
Why is sediment transport important? Very few U.S./North American streams are
in their natural state, unaffected by dams or diversions (find percentage?) “When
nature’s balance is modified at one location, changes will migrate both up and down
the basin. ” (USAC, 1989) Sediment can have impacts on receiving waters in the
form of water quality and reduced reservoir volume, and can cause changes to the
channel by eroding the banks and aggrading or degrading the channel bed. (Wilcock
(lecture slides) 2004) All surface water resource projects impose some changes on
water yield from basin; water velocity and depth; the concentration and size of
sediment particles moving with the water; and the width, depth, slope, hydraulic
roughness, planform and lateral movement of the stream channel. (USAC, 1989)
Sediment transport models are an important, but often overlooked, key to
understanding the significance of these changes.
The goal of this paper is to provide an understanding of the fundamentals of
sediment transport modeling, and to (hopefully) evaluate several restoration case
studies based upon guidelines from “Guidance for Modeling of Sedimentation in
Stream Restoration.”
Basics
For a better understanding of the Sediment transport process, see here (Link to A.
Gregory’s page.) The range of particle sizes in stream beds is often pretty large and
often heterogeneous, though this distribution is not static since the grain size and
distribution varies due to changes in flow. (Grain size/distribution important….how
much detail is needed here? Type of movement also sort of important (saltation,
bedload, suspended  related to shear)
Shear Stress Calculations
Shear stress relates flow strength to sediment transport and mobilization. The
critical shear stress is the stress, or force per sediment area, that must be overcome
for grains to begin moving. Shear is converted to a dimensionless number (tau star)
which represents the ratio of shear stress (flow force per area) acting on the bed to
the grain weight per area. (Wilcock (lecture), 2004) This is known as the Shield’s
number. (How much detail to go into?, also different methods used for different
models)
τ= dimensional shear stress;
ρs = density of the sediment;
ρ= density of the fluid;
g= acceleration due to gravity;
D’= characteristic particle diameter of the sediment.
Shield’s Diagram (Incipient Motion)12
By examining the forces on a particle we see that it is influenced by gravity (g),
stress (tau), the particle diameter (D), particle density (rho p), fluid density (rho f),
and the velocity (v). Using dimensional analysis on these quantities, we eventually
come to two dimensionless numbers: Shield’s stress (tau star) and the Reynolds
number (Re). (Talk more about the Theory/derivation of this, why they must be
related) Shields was the first to propose that some function must exist to relate the
particle Reynolds number to the Shields stress. Based on experimental data, the
Shields Curve or Diagram was created to relate the two dimensionless numbers in
order to estimate the critical shear stress. This holds for sand and gravel but not for
clays and fines that clump together because fine sediment tends to be more poorly
sorted, electrostatic forces become important, and turbulance regulates movement
more.3
http://myweb.unomaha.edu/~junkeguo/Cao_Zhixian.pdf - Explicit Formulation of the Shields
Diagram for Incipient Motion of Sediment, Journal of Hydraulic Engineering
2 Sediment Transport Primer: Estimating Bed-Material Transport in Gravel-bed Rivers,
http://www.stream.fs.fed.us/publications/PDFs/rmrs_gtr226.pdf
3 http://www.ocean.washington.edu/people/faculty/parsons/OCEAN549B/transport-lect.pdf
1
Bimodal Models – (Wilcock lecture and Sediment Transport Primer)
When does this occur
What are the assumptions here, how does this affect the stress calculations.
Sediment Transport Capacity
Sediment transport capacity – maximum amount of sediment a flow can carry.4,5
(may want to know max velocity before bank erosion/scour OR velocity needed to
remove certain fines)
“Changes in sediment transport rate along a channel are balanced by bed
aggradation /degradation and bank erosion. Anticipating these changes and
designing channels that will successfully convey the supplied sediment load with the
available water is the goal of channel design.” 6
the available water is the goal of stable channel design
Brief Background and equations for:
a) Bedload formulas (Principles of sediment transport in
rivers estuaries and coastal seas, Ch. 7.2)
b) Suspended load formulas (Principles of sediment
transport in rivers estuaries and coastal seas, Ch. 7.3)
Numerical Models – include fundamental equations and differences (1D or 3D,
what are the assumptions, strengths, weaknesses, etc. Found in Technical Manuals)
SRH - http://www.usbr.gov/pmts/sediment/model/srh2d/
CCHE - http://www.ncche.olemiss.edu/sw_collaboration
HEC-6 - http://www.hec.usace.army.mil/software/legacysoftware/hec6/hec6.htm
SAM - http://chl.erdc.usace.army.mil/chl.aspx?p=s&a=ARTICLES;67
SIAM - http://www.usbr.gov/pmts/sediment/model/srhsiam/
4
Sediment transport capacity and erosion processes: model concepts and reality, 1999
Estimating sediment transport capacity in watershed modeling, 1981
6 Sediment Transport Primer: Estimating Bed-Material Transport in Gravel-bed Rivers,
http://www.stream.fs.fed.us/publications/PDFs/rmrs_gtr226.pdf
5
Applications to Restoration Projects (Guidance for Modeling of Sedimentation
in Stream Restoration Projects, Stone, 2007)
Project Planning
Site Analysis
Selection of Design Procedure
Alternative Selection
Implementation
Monitoring
Case Studies
Elwha River in Washington (Simulating the recovery of suspended sediment
transport and river-bed stability in response to dam removal on the Elwha River,
Washington, 2009)
Chesapeake Bay