STREAM SEDIMENT

A. Introduction

• types of energy: potential, kinetic, thermal

• types of work

  • work expended as thermal energy: viscous shear & turbulence

  • mechanical work: erosion, transportation & deposition

• erosion threshold: under what conditions will a river do the mechanical work necessary to modify or maintain channel morphology?

B. Entrainment

1. Set of processes that initiate particle motion

2. Competence: size of the largest particle a stream can entrain under a given set of hydraulic conditions

C. Hydraulic Factors Affecting Entrainment

1. Critical bed velocity

a. sixth power law: r³ = kv⁶

b. Hjulstrom diagram

2. Critical bed shear stress

a. shear stress

• viscosity

• velocity gradient

b. bed shear stress

t_b = r x g x r x s

where: r = density of water

g = gravity

r = hydraulic radius

s = slope

OR

t_b = g r s

where: g = unit specific weight of water

3. Velocity gradient

a. rate of change in velocity with distance from bed or banks

b. for a given discharge and area:

• wide, shallow channels - steeper gradient along bed than along banks - promotes bed erosion

• narrow, deep channels - steeper gradient along banks than along bed - promotes bank erosion

c. lift force: velocity gradient creates a vertical pressure gradient, which results in an upwards force

4. Stream power: w

w = r g Q s / w

where: r = density of water

Q = discharge

s = slope

w = channel width

D. Sediment Characteristics Affecting Entrainment

1. Sorting of bed material

2. Layering or packing

3. Particle shape and orientation

E. Bank Erosion

1. Fluvial entrainment

2. Weakening and weathering processes

• soil moisture condition is most important control on weakening of bank material

F. Transportation

1. Suspended load

2. Bed load

3. Dissolved load

G. Deposition

H. Summary

• mechanical work: erosion, transportation & deposition

• amount & type of work function of:

  • hydraulic factors

  • sediment characteristics

  • bank erosion

• estimating when & how much geomorphic work is done

  • measure amount of sediment transported during any given flow

  • assess conditions under which rivers make adjustments to or maintain their channel morphologies

    • dominant discharge

    • recovery time (relaxation time)