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Surface Water and ErosionThe amount of rain and how long it
rains.
Another factor for the amount of run off is steepness or slope
of the land, because gravity causes water to move down slopes more
rapidly.
Vegetation increases the amount of water soaked into the
ground.
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Types of ErosionSplash ErosionSheet Erosion (Overland Flow)Rill
ErosionGully ErosionChannel Erosion*
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Water ErosionWhen water travels down the same slope time after
time, it erodes a path into the slope. Rill erosion begins when a
small stream forms during heavy rain.
As water continues to move down this path, it creates a groove
on the slope called a channel. If this continues, rill erosion
turns into gully erosion, where the channel becomes broader and
deeper.
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Sheet and Stream ErosionWater often runs off in a thin broad
sheet before like water flowing over a car. Sheet erosion occurs
when water that is flowing as sheets, picks up and carries away
sediment.
Stream Erosion: As water in a stream moves along a channel and
picks sediment from the bottom and sides of it, which is called the
stream
The lighter part of this load is called suspended load.
The heavier part of the load is called the bed load.
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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 interestSediment
Yield
cont...
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Erosion PredictionEstimate of average soil loss, usually
calculated as an average loss over a siteLosses may differ greatly
from one area to anotherTypically calculated on an annual or
monthly basisModels are available to predict erosion rateUniversal
Soil Loss Equation (USLE) Revised Universal Soil Loss Equation
(RUSLE) and RUSLE2Most do not estimate sediment yieldRUSLE2
does
*
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Procedure for Estimating Gross ErosionGross Erosion = Sheet and
Rill Erosion + Other ErosionMay be calculated using the USLE,
RUSLE, or RUSLE2
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
*
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*There a 6 major factors influencing erosion: A = Average Annual
Soil Loss (tons/ac/yr)R = Rainfall FactorK = Soil Erodibility
FactorL/S = Slope Length and Steepness FactorsC = Soil Cover
FactorP = Practice Factor
Universal Soil Loss Equation (USLE)
*A = R x K x LS x C x PErosion Prediction
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Rainfall Erosivity (R) Factor*When 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)
R is the average annual sum of EI30 for storm events during a
rainfall record of at least 22 years
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Soil Erodibility (K) FactorFine-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 detachedMedium-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 ratesSoils 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*
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LS and C FactorAccounts for the effect of topography on erosionL
factor represents the slope lengthS factor represents the slope
steepness
C Factor: Reflect s the effect of plant cover and management
practices on erosion ratesThe factor used most often to compare the
relative impacts of management options on conservation plans
*
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*C Factors for Construction Sites
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
95
Ryegrass (annuals)
0.1
90
Small grain
0.05
95
Millet or sudan grass
0.05
95
Field 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
75
Hay
1.00
0.13
87
Hay
1.50
0.07
93
Hay
2.00
0.02
98
Small grain straw
2.00
0.02
98
Wood chips
6.00
0.06
94
Wood cellulose
1.75
0.10
90
l Percent soil loss reduction as compacted/with fallow
ground.
Source: USDA-NRCS, Connecticut Technical Guide.
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Practice (P) Factor
Ratio 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 SitesNote: P=0.48 for Track
Walking
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.