Geography/Geology 352: Geomorphology

STREAM SEDIMENT

A. Introduction

1. Types of energy: potential, kinetic, thermal

• thermal energy: expended on internal friction & friction with channel perimeter

• potential & kinetic energy: mechanical work - erosion, transportation & deposition

2. Entrainment: set of processes that initiate particle motion

• competence: size of the largest particle a stream can entrain under a given set of hydraulic conditions

3. Thresholds: under what conditions will a river do the mechanical work necessary to modify or maintain channel morphology?

4. Agenda:

• explore hydraulic factors and sediment characteristics affecting entrainment

• examine differences between bed and bank erosion

• explore conditions for transportation and deposition of sediment
  

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B. 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

b. bed shear stress

t_b = r x g x r x s = g x r x s

where: r = density of water

g = gravity

r = hydraulic radius

s = slope

g = unit specific weight of water

c. Shields diagram

3. Velocity gradient

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

b. velocity gradient creates a vertical pressure gradient, which results in an upwards force

c. velocity gradient steeper for turbulent flow than for laminar flow

d. 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

4. Stream power: w

w = r g Q s / w

where: r = density of water

Q = discharge

s = slope

w = channel width

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C. Sediment Characteristics Affecting Entrainment

1. Size and density

2. Sorting of bed material

• equal mobility hypothesis

• downstream fining

3. Layering or packing

4. Particle shape and orientation

D. Bank Erosion

1. Fluvial entrainment: corrasion

2. Weakening and weathering processes

a. reduce strength of bank material

b. soil moisture condition is most important control

• positive pore pressure

• lubrication

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E. Transportation

1. Suspended load

2. Bed load

3. Dissolved load

F. Deposition

1. Ripples

2. Dunes

3. Antidunes

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G. Summary

1. mechanical work: erosion, transportation & deposition

2. type of work function of:

• hydraulic conditions

• sediment characteristics

• weakening and weathering of bank material

3. amount of work

• 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)