EROSION

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EROSION SEDIMENT CONTROL

• Historical Perspective
• Erosion and Sediment Modeling

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GEOLOGIC EROSION vs ACCELERATED EROSION
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EARLY WORK
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What is SCS?

• Soil Erosion Service
• Soil Conservation Service
• Natural Resource Conservation Service

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The Universal Soil Loss EquationARKLSCP

• Massive data base assembled (1950s 1970)
• Standard plots established throughout the US
• 72.6 ft long
• 13.3 ft wide
• 9 slope
• Fallow for two years
• Up and downhill tillage

(Wischmeier and Smith, 1965 and 1978)
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The Coshocton Wheel Samplerand H-flume
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PREDICTING EROSION USLE/RUSLE
A statistically-based model

• A R K LS C P

Some things we can control Some things we cannot
control
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Rainfall Energy
R-factor

• We cannot control R factor
• Mobile, AL R 650
• Oklahoma R 80 300
• OKC R 200

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Soil Erodibility K-factor

• We have little control of K factor
• Silt loam soil K .32
• Loamy fine sand K .17
• Affected by soil amendments

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Length-Slope
LS-factor

• We have limited control on LS factor
• 9 slope, 72.6 ft LS 1.0
• 10 slope, 200 ft LS 2.41
• 15 slope, 200 ft LS 4.3
• 20 slope, 200 ft LS 11.31

Terraces, diversions, grade control
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Cover Factor - C
0.5 tons/ac straw C0.3 2.0 tons/ac
straw C0.09 4.0 tons/ac woodchips C0.42
We can have a big impact on C-factor
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Cover Factor
Geotextiles C ???
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Cover Management

• Location of cover relative to receiving stream is
  important

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EXAMPLE 1

• Typical R 200
• Darnell K 0.32
• Slope Length 300 ft
• Slope 5
• Mature Forest C 0.001
• Practice Factor P 1

gt LS 1.0
A 0.064 tons/ac-yr
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EXAMPLE 2

• Typical R 200
• Darnell K 0.32
• Slope Length 300 ft
• Slope 5
• Bare Soil (bulldozed) C 1
• Practice Factor P 1

gt LS 1.0
A 64 tons/ac-yr
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Ratio of Sediment ProductionChanged Land
Use/Undisturbed Forest
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EROSION vs. SEDIMENT YIELD
GULLY
CHANNEL
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Event-based erosion estimationModified
USLE(MUSLE)

• Q Storm runoff
• qp Peak discharge
• KLSCP USLE parameters

WILLIAMS (1975)
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Statistically Based Erosion Models

• Not process based
• Strong interactions between parameters,
  particularly with steep slopes characteristic of
  many urban sites
• Requires a lot of data to project from one
  situation to another without massive data base

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RECENT WORK

• Extensive use of rainfall simulators and
  instrumented plots
• Process based models
• Incorporation into watershed models of hydrology
  and sedimentology
• Include the impact of BMPs and channel erosion

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RAINFALL SIMULATORS

• Earliest version of rainfall simulator
• Simulation became a powerful tool in erosion
  research

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MODERN SIMULATORS
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MODERN EROSION PLOTS
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Characteristics of Recent Models 1990s

• Process based
• Divided into rill and inter-rill erosion
• Consider concentrated flow erosion separate from
  rill and inter-rill
• Deposition and detachment based on transport
  capacity and detachment potential.

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Rill Interrill Erosion

• Interrill erosion due to rainfall impact

• Rill erosion due to shearing forces of
  runoff

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RILL EROSION POTENTIALWEPP Model

• Drc Rill detachment potential
• Kr Rill erodibility
• Channel shear gRS
• tc Critical tractive force
• Fa Correction factor for soil
• type, cover, and freeze thaw

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ACTUAL RILL EROSIONWEPP Model

• Drc Rill detachment potential
• Dr Actual detachment
• qs Sediment load
• Tc Transport capacity

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TYPES OF EROSION Concentrated Flow or Ephemeral
Gully

• Location is due to macrorelief
• Shearing forces of runoff causes erosion

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Concentrated Flow Erosion Foster-Lane Model
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Concentrated Flow Erosion Foster-Lane
ModelAssumes Constant Flow Rate
Included in CREAMS and WEPP Models
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Concentrated Flow Erosion KY DYRT Model

• Allows varying flow rates
• Allows layers of varying density
• Allows deposition
• Included in SEDIMOT III

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DETACHMENT LIMITED AND TRANSPORT LIMITED FLOW
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SEDIMENT SIZE DISTRIBUTION

• Eroded size distribution needed to predict
  transport and BMP effectiveness
• Methods available
• CREAMS based model
• Empirical study

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OTHER WATERSHED MODELSWith Erosion Components

• CREAMS
• SEDIMOT II
• SEDCAD
• SEDIMOT III
• AGNPS
• SWAT