Bed material transport
Recall: threshold for bed material entrainment
Entrainment occurs when flow achieves the critical Shields number for
the particle size in question. Dimensionless parameters makes it
generally applicable, but not intuitive…
Recall: threshold for bed material entrainment
Configuration of bed and distribution of transport mechanisms in flow is
determined by the extent to which Shields number exceed critical.
Re-dimensionalized entrainment threshold
Shields number can be dimensionalized to give critical shear velocity (or
alternatively shear stress) as a function of diameter for siliceous
particles.
RECALL: competence
Competence of a flow is the maximum particle size that a flow can
transport.
Capacity
If stream bed material consists of particles the flow is competent to carry,
HOW MUCH of that sediment can it transport?
Bedload transport
functions
Define dimensionless bed
material transport rate:
𝑞∗ =
𝑞𝑠
(𝛾𝑠 −𝛾)𝑑3
Typical form of bedload
equations:
𝑞 ∗ = 𝑐 𝜏 ∗ − 𝜏𝑐∗
𝑏
Similar to transport equations for
overland flow except with
dimensionless shear stress!
Bedload transport
capacity
Note that in:
Fully supplied
𝑞 ∗ = 𝑐 𝜏 ∗ − 𝜏𝑐∗
𝑏
the term 𝜏 ∗ − 𝜏𝑐∗ represents the
“excess Shields number”. This
fundamentally controls the
sediment transport capacity 𝑞 ∗ of
the flow.
The supply of transportable bed
material can be less than capacity.
undersupplied
Bed armor
Onion Creek, IA during drought, Aug.
2012.
Bed material vs. armor
“subpavement” is likely representative of bed material.
QUESTION:
How are velocity and bed
shear stress distributed
across the boundaries of
channels of different
shape?