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.

Types of ErosionSplash ErosionSheet Erosion (Overland Flow)Rill ErosionGully ErosionChannel Erosion*

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.

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.

*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...

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

*

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

*

*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

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

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*

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

*

*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.

Practice (P) Factor

Ratio of soil loss with a specific support practice to the corresponding soil loss with upslope and downslope disturbance*

*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.