erosion & transport

Fundamentals of fluid flow

1) density: mass/volume

2) viscosity: the ability of fluids to flow

Type of fluid flow: laminar

straight, parallel sheets

flow in ice or mud; debris flow

turbulent flow

chaotic, unsteady fluid motion

• common in air or water (e.g. fluvial, aeolian)

• more effective for sedimentary environments

Reynolds number

• dimensionless number that describes the conditions of flow turbulence

• measures the resistance to the momentum of a fluid

• high Re = turbulent Re>2000

• low Re = laminar Re<500

Froude number

as a way to measure the “mood” of a river or stream. it compares how fast the water is moving versus how deep it is.

Fr>1= supercritical (rapid flow)

Fr<1= sub critical (tranquil: water is moving slow)

Suspension: fluid transport

particles spend most time in fluid (fine grained)

Traction: fluid transport

continuous contact with bed

Saltation: fluid transport

intermittent contact with bed (bouncing)

Sediment transport mechanisms

1. wind: low density & viscosity; eolian transport; fines

2. glacial ice: high viscosity; huge volume of sediments; variety of grains sizes

3. gravity flows

Example of mass transport deposits

i. slides: detaches, slides long distance

ii. rockfall: broken falling pieces

iii. slump: detach, rotate, stays in place

iv. debris(mass) flows: debris, mud, grain, laminar

v. fluvial flows: turbidity

Flow mechanisms

i. addition of inter granular fluid (liquid or gas occupying the space between crystal grains in rocks)

ii. grain-grain collision

iii. turbulent flow

iv. support of cohesive matrix- adding mud/finest fluid

turbidity current

an underwater avalanche of sediment and water

• driven by gravity (dense sediment-water mix vs clear seawater)

• occurs on continental slopes and creates deep sea fans

• triggers: earthquakes, slope failure storms, or storms

• travels 1000 of km at rate of 70 km/h

• two types:low density: <20-30% grains ; high density: >30% grains