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 ∗ = 𝑞 𝑠 (𝛾 𝑠 −𝛾) 𝑑 3 Typical form of bedload equations: 𝑞 ∗ =𝑐 𝜏 ∗ − 𝜏 𝑐 ∗ 𝑏 Similar to transport equations for overland flow except with dimensionless shear stress!

Bedload transport capacity Note that in: 𝑞 ∗ =𝑐 𝜏 ∗ − 𝜏 𝑐 ∗ 𝑏 the term 𝜏 ∗ − 𝜏 𝑐 ∗ 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. Fully supplied 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?