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Erosion and Sedimentation

Processes, Factors and Impacts on the

EnvironmentLevel IB: Advanced Fundamentals Seminar

Education and Training Requirements for Individuals Involved in Land Disturbing Activities

Issued May 2009

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OVERVIEW

• Introduction of Erosion and Sedimentation (E&S)– Definitions and general stages of E&S– Basic processes and factors governing E&S– Natural erosion vs. accelerated erosion

• Impacts of Erosion and Sedimentation (E&S)– Environmental impacts– Economic impacts

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EROSION - The process by which the land surface is worn away by the action of water, wind, ice and gravity

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Predevelopment

Construction

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Photograph of erosion

Water Erosion

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Types of Water Erosion

1. Splash Erosion

2. Sheet Erosion

3. Rill Erosion

4. Gully Erosion

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RaindropRaindrop impactimpact on bare soil on bare soil

..Splash Erosion

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Sheet Erosion

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Photograph of rill erosion

RILL EROSION

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Photograph of gully erosion

Gully Erosion

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Insert picture hereSPLASH

SHEET

RILL & GULLY

STREAM & CHANNEL

TYPES OF WATER EROSION

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Sedimentation

The process by which the eroded material is transported and deposited by

• Water• Wind• Ice• Gravity

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SEDIMENT TRANSPORT

GA stream following1/2” of rain

June 26, 2001

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Photograph of sediment

SEDIMENTATION

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Stages in Erosion & Sedimentation

1. Detachment 2. Transport 3. Deposition

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Natural Erosion vs. Accelerated Erosion

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Natural (Normal) Erosion

• Erosion without the interference of

man’s activities.

• The rate is generally small except for

stream channel and shore erosion.

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Accelerated Erosion

Erosion intensified by human activities– Example: Farming– Example: Construction

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Estimation of Average Annual Soil LossRevised Universal Soil Loss Equation

(RUSLE)A = R K S L C P

A: Average Annual Soil Loss R: Rainfall Erosivity Factor

K: Soil Erodibility Factor S: Percent Slope

L: Length of Slope C: Cover Management

Factor

P: Conservation Practice Factor

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RUSLE Example A = RKLSCP

Given: Clarke County (R = 275) Table B-2.1

Cecil sandy loam (K = 0.28) Soil Survey

2:1 slope, 20 feet long (LS = 7.97) Table B-2.2

20% grass cover (C = 0.2) Table B-2.5

No other conservation practices (P = 1.00)

Find: Soil loss for the given conditions.

A = 122.7 tons per acre per year

*Information taken from the Manual for Erosion and

Sediment Control in GA

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RUSLE ExampleA = RKLSCP

If you reduce the slope steepness to 3:1 (LS = 4.93) …

A = 75.9 tons/acre/year, a 38% reduction in soil loss.

or

If you increase the vegetative cover to 80% (C = 0.013) …

A = 8.0 tons/acre/year, a 94% reduction in soil loss.

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Expected Erosion Rates(Tons/Acre/Year)

1. Forest Land 1 or greater

2. Farm Land 15 or greater

3. Construction Sites 100 or greater

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Impacts of Construction Activities

1. Removal of organic soil matter

2. Removal of vegetation

3. Reshaping of ground surface contours

4. Exposure of subsoil

5. Changing the pervious ground surface

to impervious

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Factors Affecting Erosion

1. Climatic (rainfall and runoff) Factors

2. Soil Factors

3. Topographic Factors

4. Vegetative Factors

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RAINDROP IMPACT & SPLASH EROSION

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The potential for soil erosion varies during the

year in GA

We have more thunderstormsin the summer

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Rainstorm EnergyAtlanta, GA

0

2

4

6

8

10

12

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Month

Pe

Per

cen

t E

ner

gy

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Photograph of river

VOLUME AND VELOCITY OF FLOW

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Topography

SLOPE GRADIENT

SLOPE LENGTH

Gradient is expressed as number of horizontal units per unit vertical such as 3 to 1 or 4 to 1 or in percentage as 33 percent or 25 percent.

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Slope Gradient

• 4 to 1 or 4:1 means that four feet of horizontal length are required for each foot of vertical change in elevation

1

4

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Slope Gradient

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Soil Factors

• Fine sandy & silty soils are more erodible than clay type soils & those with higher percentage of organic matter.

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Vegetative Factors

• Absorbs raindrop impact

• Reduces detachment

• Roots hold soil in place

• Slows water flow

• Adds organic material to the soil

• Reduces runoff

• Increases infiltration

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Photograph of vegetation holding soil in place

Vegetation holds soil in place

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Photograph of vegetation slowing water and trapping sediment

Vegetation slows the flow of water, reducing erosion and trapping

sediment

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Photograph rate and speed of runoff

Rate and speed of runoff

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Photograph

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Vegetation provides organic material

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Impacts of Erosion and Sedimentation1. Loss of soil productivity

2. Adverse effects on other water

resource facilities

3. Loss of reservoir storage capacity

4. Flood impacts

5. Recreational impacts

6. Deterioration of water quality

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Increased maintenance costs for stormwater

management systems

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Decreased Recreational Value

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Biological impacts

Screens Out Sunlight = Decline in Plant Growth

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ECOSYSTEM

Sedimentation Disrupts The Ecosystem

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SUMMARY

• Many processes and factors are involved in erosion and sedimentation

• Erosion is a natural process, but it can be accelerated by human activities. KEEP BARE SOIL TO AN ABSOLUTE MINIMUM!

• Impacts of erosion and sedimentation can be devastating to both the environment and the economy

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Georgia Soil and Water Conservation Commission

State Office706.542.3065Certification706.542.1840

www.gaswcc.georgia.gov

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QUESTIONS?