Erosion Theory Module Rev1 Compressed

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Module 2Erosion Theory

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Erosion TheoryErosion and Sedimentation ProcessErosion Prediction

Presentation Agenda

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Erosion can be beautiful…

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But not on your project site!

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Soil erosion is the process by which soil particles become detached by water, wind, or gravity and are transported from their original location

What is erosion?

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Natural processCreated current featuresTempered by natural forcesCauses little damage

(unless assisted by human activity)

Geologic Erosion

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Accelerated Erosion = natural erosion x human

activities

Accelerated Erosion = natural erosion x human

activities

Erosion Process

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Removal of surface cover Increased imperviousness (i.e., paving) that increases

runoffExposure of more erodible soil

What can accelerate erosion problems?

Erosion Process

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Rain hitting the land surface can dislodge significant amounts of pollutants

Sheet flow overland can erode slopes

Unchecked erosion will commonly lead to formation of channels

The receiving water bears the impact of quantity and quality degradation

Erosion Process

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What is Sedimentation?Sedimentation is the deposition of the eroded

material

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Erosion

Sedimentation

Erosion and Sedimentation

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What is Turbidity? Turbidity is a measure of

the degree to which the water looses its transparency due to the presence of suspended particulates

The more total suspended solids in the water, the murkier it seems and the higher the turbidity

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What is Turbidity? Turbidity is measured in

Nephelometric Turbidity Units (NTUs)

The instrument used for measuring it is called nephelometer or turbidimeter, which measures the intensity of light scattered at 90 degrees as a beam of light passes through a water sample

Turbidity standards of 5, 50, and 500 NTU

Turbidity standards of 5, 50, and 500 NTU

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Splash Erosion

Sheet Erosion (Overland Flow)

Rill Erosion

Gully Erosion

Channel Erosion

Types of Erosion

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Splash ErosionRain drops striking bare soil

directly at 5-20 mphDetaches soil particlesParticles can then be transported

by the action of water and/or wind

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Raindrop Erosion• Primary source of

erosion

• Raindrop erosion is often imperceptible

• Indicators • Pedestals• Stains• Gravelling or

Lag

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Splash detachment carries away soil fines except where gravel

protects the soil

Splash detachment carries away soil fines except where gravel

protects the soil

Pedestal

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Sheet Erosion (Overland Flow)The removal of a uniform

thin layer of soil by raindrop splash or water run-off

Surface film of water 1/16” – 1/8” deep

This process may occur unnoticed on exposed soil even though raindrops are eroding large quantities of soil

This process eventually becomes more dramatic via the formation of rills and gullies

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Rill ErosionShallow surface flows that

become condensedWell-defined tiny channelsSmall enough to step

acrossOften end part way up

slope but can extend to crest by “headcutting”

Increased velocity and turbulence

The rate of rill erosion can be approximately 100 X greater than sheet erosion

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Rill Erosion• Rill Formation affected

by: • Distance traveled• Slope inclination• Surface roughness

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Accumulating runoff becomes concentrated and forms small rills throughout the soil

Several rills may form throughout a slope and eventually may join together to form Gullies

The rate of gully erosion can be approximately 100 X greater than rill erosion

Gully Erosion

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• Look for the following visual cues:• Large, deep cuts in soil• Single cuts• Branching cuts• Often too large to

step across• Often found in areas without

evidence of other erosion types

Key Point – Gully and Rill erosion are caused by concentrated flows. Always treat the “problem” first – not the symptom.

Key Point – Gully and Rill erosion are caused by concentrated flows. Always treat the “problem” first – not the symptom.

Gully Erosion

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Channel Erosion Results from increased volume, velocity and or duration of

flow, and concentration of flow - primarily from increased impervious surfaces

Channel erosion occurs in areas where tributaries, storm drains and or culverts flow into unprotected channels

Urbanization results in increases of impervious surfaces which is reflected in incised and degraded stream channels

Urbanization results in increases of impervious surfaces which is reflected in incised and degraded stream channels

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Wind Erosion Depending on wind velocity and particle size,

soil particles move by saltation, surface creep, and suspension.

May be estimated by:E = f (I x K x C x L x V)E = the potential average annual soil loss in tons per acre, f = a function of I = the soil erodibility index. It is related to the percentage of

non-erodible soil aggregates larger than 0.84 mm in diameter,

K = the surface roughness factor,C = the climatic factor. It is based on the average wind velocity

and surface soil moisture,L = the unsheltered distance across a field or strip along the

prevailing wind erosion direction, V = the vegetative cover factor.

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Wind Erosion - StockpilesE = 1.7 (s/1.5)(365-p)(f/15)

235whereE = Total suspended particulates, lb/day/acres = silt content, percentp = # days per year with > 0.01” rainfallf = percent time with wind speed > 12 mph at

mean pile height

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Wind Erosion ControlControl system for wind erosion work in one

of two ways:Reduce wind speed on the soil surfaceForm a new, less erodible soil surface

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Reducing Wind Speed at Soil SurfaceCovering the pile with a wind-impervious

fabric or other materialErecting a windscreenChanging the pile orientation and shape

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Forming a New Less Erodible SurfaceSpraying water to compact and weight the

soil particlesApplying a chemical dust suppressant or soil

binder to form a crust or bind the surface soil particles together

Establishing vegetation. Roots bind the soil together; stems and leaves reduce wind speed at soil surface

29Play Jr. Raindrop video clip

It is often instructive to look at things from another perspective

Let’s get another perspective from “Junior Raindrop”, “Papa Cloud”, and “Mother Earth”

1948, U.S. Department of Agriculture, Forest

Service

1948, U.S. Department of Agriculture, Forest

Service

Junior Raindrop

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What did we learn from video? How raindrop impact

can infuriate Jr. or slowly let him infiltrate

How Jr. will start to ‘run’ if “nobody cares”

How rills and gullies form when Jr. starts to run as a “Gang”

• Key remedies:• Mulch or Cover for raindrop

impact• Increase infiltration• Decreased surface

compaction - organic matter incorporation as feasible

• Slope breaks, surface roughness, fiber rolls etc. to slow Jr. down

• Runoff reduction techniques to prevent concentrated flows and “gangster action”

• Key remedies:• Mulch or Cover for raindrop

impact• Increase infiltration• Decreased surface

compaction - organic matter incorporation as feasible

• Slope breaks, surface roughness, fiber rolls etc. to slow Jr. down

• Runoff reduction techniques to prevent concentrated flows and “gangster action”

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What Did We Learn From “Junior”?

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Pop Quiz – What kind of erosion is it?

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Question #1A - Raindrop

erosion

B - Sheet erosion

C - Rill erosion

D - Gully erosion

E - Channel erosion

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Question #2A - Raindrop

erosion

B - Sheet erosion

C - Rill erosion

D - Gully erosion

E - Channel erosion

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A - Raindrop erosion

B - Sheet erosion

C - Rill erosion

D - Gully erosion

E - Channel erosion

Question #3

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B - Sheet erosion

C - Rill erosion

D - Gully erosion

E - Channel erosion

Question #4

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B - Sheet erosion

C - Rill erosion

D - Gully erosion

E - Channel erosion

Question #5

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B - Sheet erosion

C - Rill erosion

D - Gully erosion

E - Channel erosion

Extra credit: How would you fix it ?Extra credit: How would you fix it ?

Question #6

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Estimate of average soil loss, expressed as “A” Usually calculated as an average loss over a

site Losses at various parts of the site may differ

greatly from one area to another Typically calculated on an annual basis but can

also be calculated on a less frequent basis My be calculated on a storm basis

Erosion Prediction

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Should not be confused with erosion; the terms are not interchangeable

Amount of eroded soil delivered to a point in the watershed that is remote from the origin of the detached soil particles

Includes erosion from slopes, channels, and mass wasting, minus sediment deposited before it reaches the point of interest

Sediment Yield

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Models are available to predict erosion rateUniversal Soil Loss Equation (USLE) Revised Universal Soil Loss Equation

(RUSLE) and RUSLE2Most do not estimate sediment yield…

RUSLE2 does

Erosion Prediction

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There a 5 major factors influencing erosion: A = Average Annual Soil Loss (tons/ac/yr) R = Rainfall Factor K = Soil Erodability Factor L/S = Slope Length and Steepness Factors C = Soil Cover Factor P = Practice Factor

A = R x K x LS x C x PA = R x K x LS x C x P

Universal Soil Loss Equation (USLE)Erosion Prediction

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Rainfall Erosivity (R) FactorWhen factors other than rainfall are held

constant, soil loss is directly proportional to a rainfall factor composed of total storm kinetic energy (E) times the maximum 30-min intensity (I30) (Wischmeier and Smith, 1958)

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R is the average annual sum of EI30 for storm events during a rainfall record of at least 22 years

"Isoerodent" maps developed for R values

R can also be obtained from http://ei.tamu.edu/

Rainfall Erosivity (R) Factor

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Soil Erodibility (K) FactorEase with which soil is detached by splash during rainfall or

by surface flow, or bothFine-textured soils with clay have low K values (about 0.05

to 0.15)…particles are resistant to detachmentCoarse-textured soils (e.g., sandy soils) have low K values

(about 0.05 to 0.2)…high infiltration resulting in low runoff even though these particles are easily detached

Medium-textured soils (e.g., silt loam) have moderate K values (about 0.25 to 0.45)…moderately susceptible to particle detachment and they produce runoff at moderate rates

Soils having a high silt content are especially susceptible to erosion and have high K values (can exceed 0.45) and can be as large as 0.65. NRCS soil data

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Soil Erodibility (K) FactorK can be obtained from

http://websoilsurvey.nrcs.usda.gov/app/

Site-specific data

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LS FactorAccounts for the effect of topography on

erosionL factor represents the slope lengthS factor represents the slope steepness

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RUSLE Slope Schematic

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LS FactorsLS FactorsAverage Watershed Slope (%)

Sheet Flow Length (ft) 0.2 0.5 1.0 2.0 3.0 4.0 5.0 6.0 8.0

<3 0.05 0.07 0.09 0.13 0.17 0.20 0.23 0.26 0.326 0.05 0.07 0.09 0.13 0.17 0.20 0.23 0.26 0.329 0.05 0.07 0.09 0.13 0.17 0.20 0.23 0.26 0.32

12 0.05 0.07 0.09 0.13 0.17 0.20 0.23 0.26 0.3215 0.05 0.07 0.09 0.13 0.17 0.20 0.23 0.26 0.3225 0.05 0.07 0.10 0.16 0.21 0.26 0.31 0.36 0.4550 0.05 0.08 0.13 0.21 0.30 0.38 0.46 0.54 0.7075 0.05 0.08 0.14 0.25 0.36 0.47 0.58 0.69 0.91

100 0.05 0.09 0.15 0.28 0.41 0.55 0.68 0.82 1.10150 0.05 0.09 0.17 0.33 0.50 0.68 0.86 1.05 1.43200 0.06 0.10 0.18 0.37 0.57 0.79 1.02 1.25 1.72250 0.06 0.10 0.19 0.40 0.64 0.89 1.16 1.43 1.99300 0.06 0.10 0.20 0.43 0.69 0.98 1.28 1.60 2.24400 0.06 0.11 0.22 0.48 0.80 1.14 1.51 1.90 2.70600 0.06 0.12 0.24 0.56 0.96 1.42 1.91 2.43 3.52800 0.06 0.12 0.26 0.63 1.10 1.65 2.25 2.89 4.24

1000 0.06 0.13 0.27 0.69 1.23 1.86 2.55 3.30 4.91

Average Watershed Slope (%)Sheet Flow Length (ft) 10.0 12.0 14.0 16.0 20.0 25.0 30.0 40.0 50.0 60.0

<3 0.35 0.36 0.38 0.39 0.41 0.45 0.48 0.53 0.58 0.636 0.37 0.41 0.45 0.49 0.56 0.64 0.72 0.85 0.97 1.079 0.38 0.45 0.51 0.56 0.67 0.80 0.91 1.13 1.31 1.47

12 0.39 0.47 0.55 0.62 0.76 0.93 1.08 1.37 1.62 1.8415 0.40 0.49 0.58 0.67 0.84 1.04 1.24 1.59 1.91 2.1925 0.57 0.71 0.85 0.98 1.24 1.56 1.86 2.41 2.91 3.3650 0.91 1.15 1.40 1.64 2.10 2.67 3.22 4.24 5.16 5.9775 1.20 1.54 1.87 2.21 2.86 3.67 4.44 5.89 7.20 8.37

100 1.46 1.88 2.31 2.73 3.57 4.59 5.58 7.44 9.13 10.63150 1.92 2.51 3.09 3.68 4.85 6.30 7.70 10.35 12.75 14.89200 2.34 3.07 3.81 4.56 6.04 7.88 9.67 13.07 16.16 18.92250 2.72 3.60 4.48 5.37 7.16 9.38 11.55 15.67 19.42 22.78300 3.09 4.09 5.11 6.15 8.23 10.81 13.35 18.17 22.57 26.51400 3.75 5.01 6.30 7.60 10.24 13.53 16.77 22.95 28.60 33.67600 4.95 6.67 8.45 10.26 13.94 18.57 23.14 31.89 39.95 47.18800 6.03 8.17 10.40 12.69 17.35 23.24 29.07 40.29 50.63 59.93

1000 7.02 9.57 12.23 14.96 20.57 27.66 34.71 48.29 60.84 72.15

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Cover (C) FactorReflect the effect of plant cover and

management practices on erosion ratesIs the factor used most often to compare the

relative impacts of management options on conservation plans

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Table 3-4

COVER INDEX FACTOR C -- CONSTRUCTION SITES

Type of Cover Factor C Percent1

None (fallow ground) 1.0 0.0

Temporary Seedings (90 percent stand):

Ryegrass (perennial type) 0.05 95Ryegrass (annuals) 0.1 90Small grain 0.05 95Millet or sudan grass 0.05 95Field bromegrass 0.03 97

Permanent Seedings (90 percent stand): 0.01 99

Sod (laid immediately): 0.01 99

Application RateTons Per Acre

Mulch:

Hay .50 0.25 75Hay 1.00 0.13 87Hay 1.50 0.07 93Hay 2.00 0.02 98Small grain straw 2.00 0.02 98Wood chips 6.00 0.06 94Wood cellulose 1.75 0.10 90

l Percent soil loss reduction as compacted/with fallow ground.

Source: USDA-NRCS, Connecticut Technical Guide.

C Factors for Construction

Sites


Sites

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Practice (P) FactorRatio of soil loss with a specific support

practice to the corresponding soil loss with upslope and downslope disturbance

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P Factors for Construction

Sites

P Factors for Construction

Sites

Note: P=0.48 for Track Walking(Testing performed at San Diego State Erosion Control Laboratory)


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RUSLE2 is a computer-aided method for predicting erosion

Helps document site data needed for analyses RUSLE2 can help the designer justify an

erosion control strategy Selecting BMPs in RUSLE2 is an Iterative Process RUSLE2 does not provide BMP specifications, cost,

or absolute effectiveness

Erosion Prediction: RUSLE2

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RUSLE2 is interactive (i.e., when input values are changed the soil loss and sediment delivery (yield) are re-calculated)

RUSLE2 is interactive (i.e., when input values are changed the soil loss and sediment delivery (yield) are re-calculated)

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Gross Erosion = Sheet and Rill Erosion + Other

Erosion

Procedure for Estimating Gross Erosion

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May be calculated using the USLE, RUSLE, or RUSLE2

Sheet and Rill Erosion

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Soil loss from gullies, channels, other concentrated flow may be determined by calculating the annual volume of soil removed from the eroded area

Annual tons of soil loss can be determined by multiplying the volume by the weight of the soil

Other Erosion

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Table 3-6

ESTIMATED WEIGHT OF SOILS1

Soil Textural Class Dry Density (lbs./ft3)clay 70-95silty clay, silty clay loam 75-100sandy clay, loam, sandy loam 80-105clay loam, silt loam 85-100sandy clay loam, loamy sands, sands 95-110

1 Data and estimates from published soil surveys, laboratory data and soil interpretationrecords are to be used where available. Parent materials, soil consistency, soil structure, porespace, soil texture, content of coarse fragments all have an influence on unit weight.

Estimated Weight of Soils

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Review Problems

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A new school development project will be constructed in San Diego

The Project will disturb 19.4 acres Project will have a duration of 1 year The average slope is ~25% The average slope length is 100 feet Site will be compacted smooth and scraped

with a bulldozer

RUSLE Problem 1

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Determine the estimated soil loss from this site if it remains unprotected.

RUSLE Problem 1

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A = Average Annual Soil Loss (tons/ac/yr) R = Rainfall Factor K = Soil Erodability Factor L/S = Slope Length and Steepness Factors C = Soil Cover Factor P = Practice Factor

A = R x K x LS x C x PA = R x K x LS x C x P

RUSLE Problem 1

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Obtain R from http://ei.tamu.edu/ Result: R=51

Project locationProject location

RUSLE Problem 1

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Obtain K from http://websoilsurvey.nrcs.usda.gov/app/ Result: K=0.20

RUSLE Problem 1

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Average Watershed Slope (%)Sheet Flow Length (ft) 10.0 12.0 14.0 16.0 20.0 25.0 30.0 40.0 50.0 60.0

<3 0.35 0.36 0.38 0.39 0.41 0.45 0.48 0.53 0.58 0.636 0.37 0.41 0.45 0.49 0.56 0.64 0.72 0.85 0.97 1.079 0.38 0.45 0.51 0.56 0.67 0.80 0.91 1.13 1.31 1.47

12 0.39 0.47 0.55 0.62 0.76 0.93 1.08 1.37 1.62 1.8415 0.40 0.49 0.58 0.67 0.84 1.04 1.24 1.59 1.91 2.1925 0.57 0.71 0.85 0.98 1.24 1.56 1.86 2.41 2.91 3.3650 0.91 1.15 1.40 1.64 2.10 2.67 3.22 4.24 5.16 5.9775 1.20 1.54 1.87 2.21 2.86 3.67 4.44 5.89 7.20 8.37

100 1.46 1.88 2.31 2.73 3.57 4.59 5.58 7.44 9.13 10.63150 1.92 2.51 3.09 3.68 4.85 6.30 7.70 10.35 12.75 14.89200 2.34 3.07 3.81 4.56 6.04 7.88 9.67 13.07 16.16 18.92250 2.72 3.60 4.48 5.37 7.16 9.38 11.55 15.67 19.42 22.78300 3.09 4.09 5.11 6.15 8.23 10.81 13.35 18.17 22.57 26.51400 3.75 5.01 6.30 7.60 10.24 13.53 16.77 22.95 28.60 33.67600 4.95 6.67 8.45 10.26 13.94 18.57 23.14 31.89 39.95 47.18800 6.03 8.17 10.40 12.69 17.35 23.24 29.07 40.29 50.63 59.93

1000 7.02 9.57 12.23 14.96 20.57 27.66 34.71 48.29 60.84 72.15

Obtain LS from Table Result: LS=4.59

RUSLE Problem 1

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Table 3-5

PRACTICE FACTOR P SURFACE CONDITION FOR CONSTRUCTION SITES

Surface Condition with No Cover Factor P1

Compact and smooth, scraped with bulldozer or scraper up and downhill.

1.3

Same condition, except raked with bulldozer root rake up and downhill.

1.2

Compact and smooth, scraped with bulldozer or scraper across the slope.

1.2

Same condition, except raked with bulldozer root rake across the slope.

0.9

Loose as a disked plow layer. 1.0

Rough, irregular surface equipment tracks in all directions.

0.9

Loose with rough surface greater than 12” depth.

0.8

Loose with smooth surface greater than 12” depth.

0.9

1 Values based on estimates. Source: USDA-NRCS, Connecticut Technical Guide.

Obtain P from Table

Result: P=1.3

RUSLE Problem 1

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A = (51)(0.20)(4.59)(1)(1.3) =61 tons/acre/year

x 19.4 acres =1,183 tons per year

RUSLE Problem 1 - Solution

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What if we track walk the slope and spread straw mulch at 2 tons/acre?

RUSLE Problem 2

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Table 3-4

COVER INDEX FACTOR C -- CONSTRUCTION SITES

Type of Cover Factor C Percent1

None (fallow ground) 1.0 0.0

Temporary Seedings (90 percent stand):

Ryegrass (perennial type) 0.05 95Ryegrass (annuals) 0.1 90Small grain 0.05 95Millet or sudan grass 0.05 95Field bromegrass 0.03 97

Permanent Seedings (90 percent stand): 0.01 99

Sod (laid immediately): 0.01 99

Application RateTons Per Acre

Mulch:

Hay .50 0.25 75Hay 1.00 0.13 87Hay 1.50 0.07 93Hay 2.00 0.02 98Small grain straw 2.00 0.02 98Wood chips 6.00 0.06 94Wood cellulose 1.75 0.10 90

l Percent soil loss reduction as compacted/with fallow ground.

Source: USDA-NRCS, Connecticut Technical Guide.C Factors for Construction

Sites


Sites

RUSLE Problem 2

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Table 3-5

PRACTICE FACTOR P SURFACE CONDITION FOR CONSTRUCTION SITES

Surface Condition with No Cover Factor P1

Compact and smooth, scraped with bulldozer or scraper up and downhill.

1.3

Same condition, except raked with bulldozer root rake up and downhill.

1.2

Compact and smooth, scraped with bulldozer or scraper across the slope.

1.2

Same condition, except raked with bulldozer root rake across the slope.

0.9

Loose as a disked plow layer. 1.0

Rough, irregular surface equipment tracks in all directions.

0.9

Loose with rough surface greater than 12” depth.

0.8

Loose with smooth surface greater than 12” depth.

0.9

1 Values based on estimates. Source: USDA-NRCS, Connecticut Technical Guide.



RUSLE Problem 2

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A = (51)(0.20)(4.59)(0.02)(0.48) =0.4 tons/acre/year

x 19.4 acres =~8 tons per year!!

RUSLE Problem 2 - Solution

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Erosion Theory Module Rev1 Compressed