Sediment Transport

Dry mass wasting

Dry mass wasting

• gravitational collapse of slope

• no water/little water

• avalanche

Sediment entrainment

• sediments enter the fluid

• more energy needed to entrain than keep moving

parameters for how fluids flow:

• density

• viscosity/resistance to flow

laminar flow

uniformly in parallel/subparallel sheets

• low velocity water

• lower flow velocity near the bed

turbulent flow

unpredictable flow that changes in local pressure and velocity ie eddies (more mixing)

• high velocity water flow

• higher flow velocity near the bed

What determines if a flow is laminar or turbulent

Reynolds number

• <500 = laminar

• 500<R<2000 transitional

• 2000 = turbulent

reynolds number is dependant on:

• velocity

• viscosity

• flow boundary roughness

• confinement of flow

froude number

measures how gravity affects fluid flow (shallow water)

• inertia vs shallow wave

subcritical flow

Fr<1

some direction towards downstream

critical flow

Fr = 1

velocity = wave speed

supercritical flow

Fr>1

most downstream flow

hydraulic jump

sudden change from supercritical to subcritical flow

bernoullis principle

flow entering and leaving at equal marks must be the same magnitude of pressure and speed

• fluid must be incompressible

• friction by viscous forces = negligible

• this keeps planes up

traction

sediment is dragged and rolled along bed surface

• bedload

saltation

sediments jump along bed

• bedload

suspension

sediments float along with flow

• suspended load

hjulstrom curve

both larger grain and smaller grain sizes need higher velocity to be transported

why does the hjulstrom curve include small grain sizes

• flocculation due to charges of clay platelets to increase diameter

• viscous sublayer at the bed make it difficult to erode

stokes law

predicts settling velocity of sediment for spherical grains, laminar flow, and dilute suspensions

• if density of particle > density of fluid, it will settle

Types of Gravity Flows

• turbidity current

• liquefied flow

• grain flow

• mud/debris flow

turbidity currents

gravity flows where sediment is supported by upward turbulence of fluid

• flows down slope due to denser than water

• fining upwards/normal grading due to decrease in energy

liquefied flow

gravity flow via concentrated grains supported by pore space fluids

• flow when force is encountered and lose cohesion = liquefaction

• leads to pipes/fluid escape structures

grain flow

gravity flow via cohesionless sediment moves downslope via gravity (30 degrees under air)

• water/air is present btwn grains but dont propel

• need high slope

debris flows

gravity flows through masses of liquified mud that move via gravity with BOULDER-sized clasts

• 10 degree after heavy rains

• inverse graded