appendix 1 sediment transport by water

8/2/2019 Appendix 1 Sediment Transport by Water

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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

appendix 1 sediment transport by water

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