appendix 1 sediment transport by water
TRANSCRIPT
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Sediment Transport by Water
Theory
Processes
Rainsplashoverland flow transport
Rilling and gullying
Mass movementsWeathering limited versus supply limited
The extreme event
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Theory
Mechanics of flow
Stream energy
Entrainment
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Mechanics of flow
Water is subject to two forces:
gravity (Wa = g sin )
friction
Defines ability of water to erode andtransport sediment
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Types of flow in open
channels
Type of flow Criterion
Uniform/non-uniform Velocity is constant/variable
with position
Steady/unsteady Velocity is constant/variable
with time
Laminar/turbulent Reynolds number is < 500/
>2500Tranquil/rapid Froude number is 1
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Laminar Flow
Each fluid element moves along a specificpath with no significant mixing between
layersBoundary layer in contact with the bed
has no forward velocity
Each layer can slip past each other
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Turbulent flow
At a critical velocity or depth laminar flowbecomes unstable and the parallel
streamlines are destroyedAdjacent layers mix, transferring
momentum by large scale eddies
Velocity more evenly distributed withdepth
Steeper near bed velocity gradient
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Reynolds Number (Re)
Re = h u/
where = fluid density
h = flow depthu = fluid viscosity
= viscosity
larger values, larger turbulence
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Entrainment
Movement of material depends on itsphysical properties;
grain size shapedensity structual arrangement
Basic distinction;
cohesive (silt-clay size)
non-cohesive
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Shear stress
Causes initial movement
Shear stress = estimate of force exerted
on the bed by the fluid
slopes
radiushydraulicR
waterofweightspecific
stressshearboundarymeanwhere
Rs
0
0
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cr D
but doesnt include lift forces
Lift due to:eddies
difference in velocity at top and bottom of
grain
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Critical shear stress
reposeofangledensityentse
densityfluid
packingofreediametergrainDwhere
Dg
s
scr
dim
deg
tan6
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Shields (1936)
Dimensionless critical shear stress
Plot against particle Reynolds no. (ratio of
grain size to thickness of laminar sublayer)
Dg scr
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Factors producing scatter
use of average or
spatial variability over
bedchannel size
irregularity of eddies
degree of exposure
pivot angles
imbrication degree ofpacking
grain shape
microtopography
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Erosion
Entrainment/detatchment
Transport
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Detatchment vs Transport
Rainsplash
Weathering
TillageTrampling
Runoff
Rainsplash
Overland flow
Rill flowGully flow
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Rainsplash
varies with rainfall intensity
varies with land cover
varies with slope
varies with % of area which is rilled
varies with lithology
crusting?
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Surface Wash
particles detatched and transported bysurface flowing water
force = velocity x mass (i.e. Q)controls relate to character of materials,
especially ability to produce rainfall excess
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Resistance to detatchment
non-uniform
varies with particle size
cyclic variation with season
sand/silt clay ratio
stoniness
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Rills
Impermanent channels
vary in lateral position year to year
develop once threshold exceeded in asingle event
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Gullies
permanent incised X-sectional form
develop once threshold exceeded over
longer term average conditionsmay be discontinuous
gully / arroyo / donga
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Mass Movement
possibly only important in extreme events
directly contribute to load or rills/gullies
4 main typesshallow slides
slab failure
rockfalls
deep seated slides
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Soil Erosion
Soil loss = R K L S P C
R = rainfall erosivity
K = erodibility of soil
L = slope length
s = slope angle
P = coefficient of cultivation methods
C = crop management factor
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Weathering Limited
When unlimited capacity for
transport occurs, removal of material islimited by the rate at which material is
detatched.
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Transport Limited
When there is an abundant
supply of material and erosion
depends on the efficiency of
forces transporting the
material away.
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Equilibrium condition
Removal of material = supply of material
Contionuous range between extremes
Occurs over different timescales:
Cyclic
Graded
Steady-state
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Transport vs Weathering
0
2
4
6
8
10
12
0 2 4 6 8 10 12
Rate of Production
RateofRemoval
Accumulation
Erosion
Limit set by availabilty of material
Limit set by rate of removal
X Z
Y
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Timescales
Cyclic
period over which an effective change in basinelevation can be measured
Graded (equilibrium)a change in any factor will cause a displacement of
the equilibrium in a direction which will absorb theeffect of change
Steady statea measurement can be taken and the system
assumed to be in a constant condition
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Timescales
0
120
0 20 40 60 80 100 120 140 160 180 200
Time (years)
Elevation(m)
Steady State
Graded
Cyclic
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Magnitude-Frequency concept
Wolman and Miller, 1960
majority of work carried out by events
which occur on average 1 or 2 times peryear
basin characteristics adjusted to these
events
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Different in semi-arid channels:
stress-strain reln more complicated
large spatial variation
morphology adjusted to extreme events
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Extreme events
Do majority of work because:
larger particle size
transmission losses
poor sorting
vegetation
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Themes of dryland floods
Get scour and fill in times of extremefloods but channels restore themselves
afterwardsAverage sediment yields before a flood
are exceeded for sometime afterwards
Work done during a flood is poorly relatedto flow volume or total ppt