Sediment Transport by Water

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21 Φεβ 2014 (πριν από 3 χρόνια και 7 μήνες)

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Sediment Transport by Water


Theory


Processes


Rainsplash


overland flow transport


Rilling and gullying


Mass movements


Weathering limited versus supply limited


The extreme event

Theory


Mechanics of flow


Stream energy


Entrainment

Mechanics of flow


Water is subject to two forces:


gravity (W
a

= g sin

)


friction


Defines ability of water to erode and
transport sediment

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/
>2500
Tranquil/rapid
Froude number is <1/>1
Laminar Flow


Each fluid element moves along a specific
path with no significant mixing between
layers


Boundary layer in contact with the bed
has no forward velocity


Each layer can slip past each other

Turbulent flow


At a critical velocity or depth laminar flow
becomes unstable and the parallel
streamlines are destroyed


Adjacent layers mix, transferring
momentum by large scale eddies


Velocity more evenly distributed with
depth


Steeper near bed velocity gradient

Reynolds Number (Re)


Re =


h u/



where


= fluid density


h = flow depth


u = fluid viscosity




= viscosity


larger values, larger turbulence

Entrainment


Movement of material depends on its
physical properties;

grain size


shape

density



structual arrangement


Basic distinction;


cohesive (silt
-
clay size)


non
-
cohesive

Shear stress


Causes initial movement


Shear stress = estimate of force exerted
on the bed by the fluid


slope
s
radius
hydraulic
R
water
of
weight
specific
stress
shear
boundary
mean
where
Rs









0
0


cr



D


but doesn’t include lift forces


Lift due to:


eddies


difference in velocity at top and bottom of
grain

Critical shear stress



repose
of
angle
density
ent
se
density
fluid
packing
of
ree
diameter
grain
D
where
D
g
s
s
cr

















dim
deg
tan
6
Shields (1936)


Dimensionless critical shear stress


Plot against particle Reynolds no. (ratio of
grain size to thickness of laminar sublayer)




D
g
s
cr






Factors producing scatter


use of average


or



spatial variability over
bed


channel size


irregularity of eddies



degree of exposure


pivot angles


imbrication

degree of
packing


grain shape


microtopography


Erosion


Entrainment/detatchment


Transport

Detatchment vs Transport


Rainsplash


Weathering


Tillage


Trampling


Runoff


Rainsplash


Overland flow


Rill flow


Gully flow

Rainsplash


varies with rainfall intensity


varies with land cover


varies with slope


varies with % of area which is rilled


varies with lithology


crusting?

Surface Wash


particles detatched and transported by
surface flowing water


force = velocity x mass (i.e. Q)


controls relate to character of materials,
especially ability to produce rainfall excess

Resistance to detatchment


non
-
uniform


varies with particle size


cyclic variation with season


sand/silt clay ratio


stoniness

Rills


Impermanent channels


vary in lateral position year to year


develop once threshold exceeded in a
single event


Gullies


permanent incised X
-
sectional form


develop once threshold exceeded over
longer term average conditions


may be discontinuous


gully / arroyo / donga

Mass Movement


possibly only important in extreme events


directly contribute to load or rills/gullies


4 main types


shallow slides


slab failure


rockfalls


deep seated slides

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

Weathering Limited


When unlimited capacity for

transport occurs, removal of material is
limited by the rate at which material is
detatched.

Transport Limited

When there is an abundant

supply of material and erosion

depends on the efficiency of

forces transporting the

material away.

Equilibrium condition

Removal of material = supply of material



Contionuous range between extremes


Occurs over different timescales:


Cyclic


Graded


Steady
-
state



Transport vs Weathering

0
2
4
6
8
10
12
0
2
4
6
8
10
12
Rate of Production
Rate of Removal
Accumulation
Erosion
Limit set by availabilty of material
Limit set by rate of removal
X
Z
Y
Timescales


Cyclic


period over which an effective change in basin
elevation can be measured


Graded (equilibrium)


a change in any factor will cause a displacement of
the equilibrium in a direction which will absorb the
effect of change


Steady state


a measurement can be taken and the system
assumed to be in a constant condition

Timescales

0
120
0
20
40
60
80
100
120
140
160
180
200
Time (years)
Elevation (m)
Steady State
Graded
Cyclic
Magnitude
-
Frequency concept


Wolman and Miller, 1960


majority of ‘work’ carried out by events
which occur on average 1 or 2 times per
year


basin characteristics adjusted to these
events


Different in semi
-
arid channels:


stress
-
strain rel
n

more complicated


large spatial variation


morphology adjusted to extreme events

Extreme events

Do majority of work because:


larger particle size


transmission losses


poor sorting


vegetation

Themes of dryland floods


Get scour and fill in times of extreme
floods but channels restore themselves
afterwards


Average sediment yields before a flood
are exceeded for sometime afterwards


Work done during a flood is poorly related
to flow volume or total ppt