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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
14
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18
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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?