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A. Introduction

Types of energy

• potential energy

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

• kinetic energy: expended on entrainment, transportation & deposition of sediment

• erosion and deposition modify or maintain channel morphology

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

Agenda

• explore driving and resisting forces affecting entrainment, transportation, & deposition of sediment

• compare and contrast bed and bank erosion

• discuss conditions of channel maintenance and modification

B. Driving Forces Affecting Entrainment

Velocity

• average velocity and bed velocity

• critical bed velocity: threshold for entrainment

• Hjulstrom diagram

• harder to initiate motion than to maintain motion

Shear stress

• force: a push or a pull in a particular direction

• stress: force/area

• shear stress: force per unit area parallel to the bed

  

• critical bed shear stress: threshold for entrainment

  Shields diagram

Velocity gradient

• rate of change in velocity with distance from bed or banks

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

• velocity gradient steeper for turbulent flow than for laminar flow

• 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

Stream power per unit bed area: w

w = r g Q s / w

where: r = density of water

Q = discharge

s = slope

w = channel width

C. Resisting Forces Affecting Entrainment

Grain size for coarser sediments

• affects mass

• diameter

• grain size distribution

  • d₅₀ (50% finer than) and d₈₄ (84% finer than)

Density (mass per unit volume)

• assumed to be a constant 2650 kg/m³ for sediment

• water: 1000 kg/m³

• specific gravity: density of a substance relative to density of water = 2.65 for sediment

Grain shape

• surface area exposed to flow

• ease with which particle can be rolled

• spherical particles entrained more readily than other shapes all other things being equal

Layering or packing

• important with poorly sorted sediment and irregularly shaped particles

Cohesion for finer particles

Sorting of bed material

• d₈₄/d₁₆ measure of sorting; degree of sorting increases as ratio decreases

• equal mobility hypothesis

• downstream fining; sorting & abrasion

D. Bank Erosion

Fluvial entrainment: corrasion

• Driving forces: flow velocity, shear stress, velocity gradient

• Resisting forces: shear strength of bank material

Weakening and weathering processes

• reduce strength of bank material

• strength function of:

  • cohesion

  • friction

  • effective normal stress, which depends on moisture content (pore water pressure)

  • particle size

  • presence of vegetation & roots

• upward fining of bank material; more cohesive at surface than below; may lead to undercutting

E. Transportation

Suspended load

• concentration decreases with distance from the bed

• estimated transport rate = depth-averaged sediment concentration x mean flow velocity x depth
  Qs = Cs x v x d

• main control over suspended sediment concentration: spatial & temporal variability in supply of sediment to streamm

Bed load: rolling, sliding, & saltation

Dissolved load

Bedforms

• function of flow intensity (indicated e.g. by Froude number or bed shear stress) & grain size

• ripples

  • form primarily in fine & medium sands (< 0.7 mm diameter)

  • height < ~3 cm; length <~40 cm

  • height & spacing increase as grain size increases

  • relatively smooth water surface

• dunes

  • form in medium & coarse sands; may form with gravel if flow conditions high enough

  • height up to several m

  • height & spacing increase as flow depth increases

  • small surface waves out of phase with dunes

• antidunes

  • surface waves in phase with dunes

  • occur when Froude number >0.84

  • not very common

F. Summary

Mechanical work: erosion, transportation & deposition

Erosion & deposition function of

• driving fources

  • velocity, shear stress, velocity gradient, stream power

• resisting forces

  • sediment characteristics (grain size, density, shape, packing, sorting, cohesion)

  • weakening and weathering of bank material

Transportation

• suspended, bed, & dissolved load

• bed forms function of grain size and flow intensity

Amount of work

• measure amount of sediment transported during any given flow (e.g. sediment discharge)

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

  • dominant discharge

  • recovery time (relaxation time)