beause this is the suite of processes that form river channels and adjacent floodplains, because this is what controls fluvial (river-associated) erosion and deposition, and to understand how sediment moves through rivers to the sea

there are practical questions to answer like how dam construction or dam removal will alter a channel or how long before a reservoir will fill with sediment

there are also questions of intrinsic interest like how dunes migrate on river beds, what controls the details of the stratigraphy of fluvial sedimentary rocks, or what can be learned about ancient fluvial environments from the sediments left behind

the force that is exerted parallel to a surface, rather than normal (prependicular) to a sruface. a brick sitting on a table exerts a normal stress (or forc) down on the table due to its weight. if you push the brick across the table, it also exerts a shear stress on the table that can do work on the table (like scratch it)

water in a river exerts a shear stress on the bed because of the difference in velocity between water right at the bed (velocity - 0) and the velocity at some point above the bed (greater than 0)

1. entrainment - getting a particle to begin moving

2. transport - the movement of a particle once it is entrained

3. deposition - the particle comes to rest

water exerts a drag force (Fd) on a particle on the river bed

the particle will be dislodged from the bed when Fd is large enough to pivod (torque) the particle out of its pocket on the bed (exeeding the torque pulling it back into the pocket)

the restoring torque that acts to rotate the particle back into the pocket is from the weight of the particle (Fg)

it arises from (and is proportional to) the shear stress exerted by the flowing water

sheer stress is usually represented as T (tau)

at the bed of the river is easy to compute as

T =pgDS

where T= tau, p=density of water, g=acceleration due to gravity, D=depth of water, S=water surface slope

because it switches ON above a particular sheer stress

the shear stress at which a particle will move

it dpends on the particle grain size - the bigger the particle the greater the sheer stress to be entrained

the line for the entrainment threshold is fuzzy because the velocity at which entrainment occurs depends on the geometry of the pocket where a particle sits and the sheer stress exerted by the water

at velocities below the line, the grain will not be entrained by the flow, at velocities above the line, the grain will be entrained by the flow

the entrainment threshold flow does not uniformly increase with grain size

there is high shear stress for intrainment in silt and clay b/c they are cohesive, so both weight and cohesion have to be hovercome by the shear stress

REMEMBER: entrainment does not occur for any velocity/grain size combination that falls below the entrainment threshold curve

[image]

many velocities including those lower than that required to entrain it

fine grain sized sediment (silt, clay) does not deposit until the water is nearly motionless - even though it took higher velocities to get it wafted up into the channel to begin with

in contrast, pebbles or larger material only move in very short steps, the particles are entrained and deposited over and over again in their trip down stream

1. bedload

2. suspended load

3. dissolved load

frequent contact with river bed

move more slowly than water itself

includes grains that roll, slide, saltate

(saltation =a process where grains hop along the bed. the grain ejects from the bed and then has an arcing trajectory back to the bed)

grains in suspension, moving with the water

stay suspended in flow b/c turbulent eddies keep them afloat against the tendency to settle back to the bed

carried at nearly the speed of water itself

what makes rivers look muddy

suspended sediment = generally fine grained (silts and clay) but in estreme flows, gravel and larger sizes can be suspended

 bedload+suspended load+dissolved load

in some rivers the dissolved load (the mass in solution) is greater than the sediment load. in most rivers the suspended load is greater than the bedload. (bedload is hard to measure so we often assume that bedload is 1/10 of the suspended load)

bedload and suspended load are found close to the bed of the river; the bedload portion = coarse grained

higher in the water column only the suspended material is found, which is finer than the material at the bed

material in true suspension is found at uniform concentrations throughout the water column, so a plot of sediment concentration as a function of depth in the flow tends to show a strong decline with height above the bed down to a constant concentration[image]

bedload is generally just barely moving (hopping, sliding, rolling along bed) so it is deposited whenever the sediment moves into a place where shear stress at the bed is reduced (dune, bar, big rock, shallower water, stream less steep, flow stage declines after rainstorm)

coarse bedload material deposited into flowing water, but in places where flow is too small to get particle to be entrained again

suspended load is carried with the water, deposited only where water slows down significantly or becomes still (as in a lake)