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Erosion and Sediment Concepts

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Title: PowerPoint Presentation Author: Denyse Lemaire Last modified by: Susan M Bolton Created Date: 2/20/2003 2:17:38 PM Document presentation format – PowerPoint PPT presentation

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Title: Erosion and Sediment Concepts


1
Erosion and Sediment Concepts
Slides with this background are From Wiley
Science
2
Streams as Geological Agents
  • Streams are vital geologic agents.
  • Streams carry most of the water that goes from
    land to sea (essential part of the hydrologic
    cycle).
  • Streams transport billions of tons of sediment to
    the oceans each year.
  • The load is the sediment and dissolved matter the
    stream transports
  • Load (expressed in kilograms per cubic meter).
  • Dissolved matter generally does not affect
    stream behavior.

3
EROSION
  • Erosion is a process of detachment and transport
    of soil particles by erosive agents.
  • Ellison, 1944
  • Erosive Agents
  • Raindrop impact
  • Overland flow surface runoff from rainfall
  • Bed and bank turbulence in streams

4
TYPES OF EROSION
  • Interrill and rill (sheet-rill)
  • Ephemeral gully
  • Permanent, incised (classical) gully
  • Stream channel
  • Mass movement
  • Geologic

5
EROSION IS A CONCERN
  • Degrades soil resource
  • Reduces soil productivity
  • Reduces soil organic matter
  • Removes plant nutrients
  • Causes downstream sedimentation
  • Produces sediment which is a pollutant
  • Produces sediment that carries pollutants

6
Physical factors leading to high soil erosion
  • Intense storms
  • May occur anywhere
  • Likelihood may increase with climate change
  • Sparse vegetation and ground cover
  • Particularly in association with intense storms
  • Silt-rich soils
  • Tendency to crusting, giving high runoff when
    bare
  • Mechanically weak and so highly erodible
  • Steep slope gradients
  • But steepest slopes may be on resistant soil or
    rock
  • Combination of steep slopes and weak soils most
    common on marginal lands
  • Large collecting area for runoff
  • Often related to flow convergence rather than
    slope length

www.geog.leeds.ac.uk/people/m.kirkby/Presentations
...
7
Erosion by Running Water (1)
  • Erosion by water before a distinct channel has
    formed occurs in two ways
  • By impact as raindrops hit the ground.
  • By overland flow during heavy rains, a process
    known as sheet erosion.
  • The effectiveness of raindrops and overland flows
    in eroding the land is greatly diminished by a
    protective cover of vegetation.

8
Erosion mechanics
Factoid large raindrops fall at 30 km/hr !
  1. Detachment
  2. Transport
  3. Deposition

Most hillslope erosion is initiated by the impact
of raindrops, NOT by the flow of running water
9
Erosion by Running Water (2)
  • The ability of streams to erode is influenced by
    the way water moves through a stream channel.
  • If the velocity is very slow the water particles
    travel in parallel layers, a motion called
    laminar flow. Seldom occurs in natural streams
    except in the boundary layer
  • With increasing velocity, the movement becomes
    turbulent flow.
  • The ability of a stream to pick up particles of
    sediment from its channel and move them along
    depends largely on
  • Turbulence.
  • Velocity of the water.

10
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11
The Streams Load
  • The solid portion of a streams load consists of
    two parts
  • The bed load the coarse particles that move
    along the stream bed.
  • The suspended load fine particles that are
    suspended in the water.
  • Wherever they are dropped, these solid particles
    constitute alluvium.
  • Streams also carry dissolved substances called
    the dissolved load.
  • These are chiefly a product of chemical
    weathering.

12
Stream Load
Fluvial propcesses
Strahler, A. and Strahler, A., 2004. Physical
Geography. Wiley, NY.
13
Bed Load
  • The bed load generally constitutes between 5 and
    50 percent of the total load of a stream.
  • Particles move discontinuously by rolling or
    sliding at a slower velocity than the stream
    water.
  • The bed load may move short distances by
    saltation (series of short intermittent jumps).
  • Coarse-grained sediment is concentrated where the
    velocity is high.
  • Finer-grained sediment is found in zones of
    progressively lower velocity.

14
Lane (1955) diagram showing relationship between
Sediment load, water discharge, sediment size
and channel slope
15
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16
Sediment Yield (1)
  • Sediment yield is a function of
  • Rock type and structure.
  • Local climate.
  • Relief and slope.
  • Because of storage differences, smaller basins
    tend to show higher yields than larger basins on
    unit area basis.
  • The greater the precipitation the greater the
    potential erosion.
  • In moist regions, plant roots tend to anchor the
    soil, thereby curtailing erosion.
  • In temperate eastern North America and Western
    Europe, vegetation cover is more or less
    continuous and erosion rates are low.

17
Sediment Yield (2)
  • In drier regions, reduced precipitation limits
    vegetation, making the land vulnerable to
    erosion.
  • Areas receiving abundant precipitation may
    actually experience less erosion than some
    relatively dry regions,
  • Fields measurements suggest that some of the
    greatest local sediment yields are from desert
    landscapes.
  • Some of the highest measured sediment yields are
    from basins that drain steep mountains along
    plate boundaries.
  • Monsoon regions of southeastern Asia receive
    abundant precipitation that generates high
    runoff.

18
Sediment Yield (3)
  • In southern Alaska and the southern Andes, large
    active glaciers contribute to high sediment
    yields.
  • Rocks that are highly jointed or fractured are
    more susceptible to erosion than massive rocks,.
  • The clearing of forests, cultivation of lands,
    damming of streams, construction of cities, and
    numerous other human activities also affect
    erosion rates and sediment yields.

19
Langbein and Schumm Model
Slopeeroson
From Ritter et al., 1995 After Lanbein and
Schumm, 1958
20
Universal Soil Loss Equation
  • Most widely used model
  • Empirically based
  • Watch your units!
  • USLE still hidden in some more complex models
  • Various more complex models now available
  • WEPP Watershed Erosion Prediction Project

21
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22
USLE - Universal Soil Loss Equation
A RKLSCP Annual soil loss rate
Hydrologic cycle factor
  • R rainfall erosivity
  • K soil erodibility
  • L slope length
  • S slope steepness
  • C cover and management
  • P erosion-control practices

Soil/topography-related factors
Land management factors
Revised (RUSLE) and Modified (MUSLE) versions
also exist
http//cropandsoil.oregonstate.edu/classes/css305/
lectures/Chpt13_erosion_short.pdf
23
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24
http//snr.osu.edu/current/courses/NR675/transport
_soil.pdf
25
http//snr.osu.edu/current/courses/NR675/transport
_soil.pdf
26
R-factor Rainfall erosivity
  • R EI for a given storm
  • E is the kinetic energy of the storm
  • I is the maximum 30-minute intensity of the storm
  • EI is calculated for each storm and then summed
    to get the annual erosivity index
  • Figure 9.6, p 260 in your book show R for the U.S.

27
K soil erodibility factor
A RKLSCP
  • Infiltration capacity
  • Structural stability
  • Particle cohesion
  • cementation by Organic Matter and clays
  • Particle mass (2.0 g/cm3 for bauxite, 5.3 for
    hematite, 7.6 for galena)

http//cropandsoil.oregonstate.edu/classes/css305/
lectures/Chpt13_erosion_short.pdf
28
Soil properties resulting in low K values (less
erosion)
  • High organic matter content
  • Non-expansive clays
  • Strong granular structure
  • stoniness macropores
  • Nomograph in figure 9.8, p. 264 in your book to
    get K value
  • Nomgraph uses values of silt and very fine
    sand, sand, organic matter, soil structure,
    and soil permeablity

29
Soil erodibility factor for various soil textures
30
Topographic factor - LS
  • More erosion from steep, longer slopes
  • Everything compared to a 9 slope of 72.6 feet
  • Figure 9.10, p 265 in your book

31
Cover Management Factor, C
  • Vegetation cover
  • Crop sequence
  • Productivity level
  • Length of growing season
  • Tillage practices
  • Residue management
  • Temporal distribution of erosion events

32
  • Ag systems C based on crop rotation and tillage
    practices
  • Forest and range systems C based on vegetation
    density, ( ground cover) and litter/duff
    condition
  • Table 9.1, p 260 in your book shows C factors for
    non-ag systems
  • Table 9.2, p 261 shows an example of C factors
    for Ohio crops

33
1. Mulch as a means to reduce erosion
A RKLSCP
C cover and management
http//cropandsoil.oregonstate.edu/classes/css305/
lectures/Chpt13_erosion_short.pdf
34
A few practices to reduce soil loss caused by
timber production
  • Tree removal cable not skidder
  • Scheduling when dry or frozen. snow great.
  • Road design 99 of soil loss avoided by gravel,
    planting grasses on road cuts
  • Buffer strips 1.5 times the height of the
    tallest trees

http//cropandsoil.oregonstate.edu/classes/css305/
lectures/Chpt13_erosion_short.pdf
35
Erosion Control Practice Factor- P
  • Often this factor 1, no practice in place
  • Contouring
  • Strip cropping
  • Terracing
  • Table 9.3, p 262 in your book show P factors
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