2 ~ fluid flow + sediment transport

what is hydrodynamics?

study of flow + behaviour of sediment particles

what is fluid flow?

entrainment of sediment particles by fluids (water, air, water/sediment mix)

gravity of pressure driven flows

what is a fluid?

substance that continually flows under applied shear stress

what is fluid viscosity?

measure of ability of fluid flow

low visc - flow readily

high visc - flow sluggishly

temp dependent - flow faster with inc T

flow by internal shear

what is newtons law of viscosity?

relationship between shear stress and fluid flow

what is the shear box experiment?

viscous fluid sheared btwn 2 plates

lower plate immobile, upper plate moves to the right

laminar shear transmitted through fluid

velocity increases upward

SKIPPED A BUNCH OF EQ

SKIPPED A BUNCH OF EQ

what is reynolds experiment?

inject dye streak int fluid moving and constant velocity through tube

varied fluid density, tube diameter and velocity of flow

what were the results of reynolds experiment?

low discharge - dye follows straight path

med discharge - dye follows wavy path

high discharge - dye rapidly mixed with fluid

what are the 3 types of flow behaviours?

laminar (low) - fluid molecule follows straight path parallel to boundaries of the tube

transitional (med) - fluid molecules follow wavy parallel path

turbulent (high) - fluid molecules follow very complex path that mixes

what is reynolds number?

predicts transition from laminar to turbulent flow

what is the turbulent flow structure?

velocity increases upward

outer layer - fully turbulent, not affected by bed friction

transition layer - intermediate between laminar and turbulent

viscous sub layer - thin layer, laminar flow

what is hydraulic roughness?

smooth bed - particles withing VSL, low flow rate, behaves laminar

rough bed - particles above VSL, VSL thins, particles subject to eddies

what are the 3 forces on the particle?

lift force (upwards)

drag force (parallel to bed)

gravity force (downwards)

how does the lift force act on a grain?

flow lines converge over grain

inc in velocity + dec in pressure at top

how does gravity act on the grain?

opposes lift force

acts to keep grain on bed

what is fluid force?

result of lift force and drag force

is diagonal

what is a bed load?

sediment that resides on bed but goes into transport during high flow events

what are the types of bed loads?

contant - sliding/rolling

saltation - jump/hop along

suspensive saltation - longer trajectory of hopping

intermittent suspension

what is a suspended load?

sediment suspended in upper region of turbulent flow

continuous - eddies keep particles aloft

intermittent - grains periodically drop back to bed

what is a wash load?

silt and clay size material remains in suspension even during low flow events

what is stokes law?

falling particles reach terminal fall velocity where drag and buoyancy forces are balanced by gravity

what are the complications of stokes law?

natural grains are not perfect spheres

grains are in contact as they settle - clump and collide

grains larger than fine sand fall fast enough to cause turbulent eddies

what did hjulstrom determine?

determined critical velocity for entrainment/transport experimentally

what is cohesion?

clay + fine silts are cohesive sediments

held together by electrostatic forces

• must break bonds to erode clay

what is shields diagram?

determine critical boundary shear stress required to initiate entrainment of spherical grains of varying diameters n density

what is middletons diagram?

particles will remain in suspension when the shear velocity exceeds settling velocity