Sediment Transport

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

*

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

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