# Sediment Transport by Water

Μηχανική

21 Φεβ 2014 (πριν από 4 χρόνια και 2 μήνες)

86 εμφανίσεις

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
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

momentum by large scale eddies

Velocity more evenly distributed with
depth

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
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

-
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

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

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)
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

events

Different in semi
-
arid channels:

stress
-
strain rel
n

more complicated

large spatial variation

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