# Shared Flashcard Set

## Details

7GEOG landscapes and water
Sediment Transport
27
Geography
04/05/2010

Term
 why do we study sediment transport?
Definition
 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
Term
 how does flowing warter move sediment?
Definition
 the flow exerts a shear stress on the bed of the river. this stress produces a force on each sediment particle, called the  drag force. if the drag force on the sediment particle exceeds the force resisting motion (mostly weight) then the particle moves
Term
 what is shear stress?
Definition
 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)
Term
 what are the trhee phases to sediment transport by rivers?
Definition
 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
Term
 how does Entrainment begin to move a particle?
Definition
 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)
Term
 Where does the drag force arise from?
Definition
 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
Term
 what does the equation T=psDS tell us about shear stress?
Definition
 it increases with water depth (D) and with water surface slope (S)
Term
 why do we think of sediment transport as a "threshold" process?
Definition
 because it switches ON above a particular sheer stress
Term
 what is teh critical sheer stress?
Definition
 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
Term
 what is the 'no slip' condition?
Definition
 where the flow has no foreward velocity, occurs in water right at the bed of a stream
Term
 how does water velocity change based on hight in the flow?
Definition
 the higher in the flow, the more velocity the water has
Term
Definition
 the change in velocity with distance above the bed
Term
 what is the relationship between sheer stress at teh bed and the velocity gradient?
Definition
 they are proportional
Term
 Describe a Hjulstrom curve and how it explains the relationship between erosion, transportation, deposition, and the entrainment threshold
Definition
 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]
Term
 What does the distance of transport depend on?
Definition
 the grain size of the sediment and the caracteristics of teh flow in the river channel
Term
 at what velocities can sediment remain in transport?
Definition
 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
Term
 What are the three modes of transport by rivers?
Definition
Term
Definition
 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)
Term
Definition
 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
Term
Definition
 not solid material, but dissolved materials carried by river water that was produced in chemical weathering reactions
Term
 What is the total material load of a river?
Definition
Term
 how does sediment within a river vary?
Definition
 in both concentration (how much there is) and grain size with distance above the bed of river
Term
 How is sediment distributed in the water column?
Definition
 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]
Term
 Describe deposition
Definition
 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)
Term
 what are bedforms?
Definition
 ripples and mounds that are built and translate (migrate) along the river bed as a result of the non-uniform movement of sediment in the river
Term
 what are ripples?
Definition
 small repeated ridges of sediment, with wavelength (distance from crest to crest) of 0.6m or less, height less than 4cm
Term
 what are dunes?
Definition
 larger, and more variable than ripples. with a wavelength of 4-8 times teh flow depth, and a height up to 1/3 of the flow depth
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