wes marshall, p.e. university of connecticut march 2007 ce 276 site design chapter 10 – soil...
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Wes Marshall, P.E. University of ConnecticutMarch 2007
CE 276Site Design
Chapter 10 – Soil Erosion & Sediment Control
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What did we talk about last week?
The Hydrologic Cycle Impact of Site Development Storm Water Management
Philosophical evolution of practice State of the art Best Management Practices (BMPs)
Retention, detention, and infiltration facilitiesPorous pavements, constructed wetlands, etc.
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Chapter 10Soil Erosion &
Sediment Control
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Erosion & Sediment
What is erosion? The detachment and movement of soil or
rock fragments by water, wind, ice, or gravity
What is sediment? Solid material in suspension, being
transported, or having been moved from its original site by water, wind, ice, or gravity
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Erosion & Sedimentation
In terms of storm water management, these are critical issues…
Especially During Construction!
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Erosion & Sedimentation
The rate of erosion on a construction site can be… 5 times that of agricultural land 10 times that of pasture land 250 times that of forested land
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Regulations
Most towns require an “Erosion & Sediment Control” plan indicating: The temporary control measures to be
taken during construction The permanent measures that will
remain in place post-construction
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Soil Erosion Factors
4 Primary Factors:
1. Soil
2. Vegetative cover
3. Topography
4. Precipitation
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Erosion & Sedimentation Processes
Preparing a good plan requires the designer have a good understanding of
the erosion potential of a site
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Soil Erosion Factor Components
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Soil
High erodibility Poorly drained soils High silt content Fine sands
Low erodibility Well-drained soils Gravel-sand mixtures Increased clay content (although once clay is in
suspension, it does not settle easily) Increased organic content
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Vegetative Cover
Vegetation prevents soil erosion by: Shielding soil from direct rain Increasing surface friction reduces runoff
velocity Roots hold soil in place & increase water
absorption capacity
Try to retain as much existing vegetation as possible during construction
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Topography
Erosion increases with: Steepness of slope Length of slope
This is due the increase in rate and velocity of runoff
Avoid long and steep slopes if possible When disturbance is necessary, protect
slope as quickly as possible
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Precipitation
Runoff determined by rainstorm: Frequency Duration Intensity
When possible, site construction should be scheduled: During months with low anticipated
precipitation To coincide with optimum seeding periods
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Erosion & Sedimentation Processes
2 Strategies:1. Erosion control
Focus on keeping the soil in place Evaluate the runoff patterns and erosion potential Develop a plan to minimize disturbed area erosion
2. Sediment control Focus on minimizing off-site transport of sediment Greater emphasis on capturing sediment generated
from disturbed areas Usually less erosion control measures required
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Erosion & Sedimentation Processes
The most effective plans incorporate a combination of both erosion control
and sediment control features
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Runoff Considerations
Managing runoff helps control erosion
This is achieved by:1. Reviewing pre-development runoff patterns
2. Determining post-development runoff patterns
3. Determining how to manage runoff during construction
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Managing Runoff
Basic principles of managing runoff
It is important to…
Divert runoff from newly graded areas Capture and treat runoff from disturbed areas
to reduce off-site sediment transport
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Construction Sequencing
Planning when & how site development takes place can help erosion & sediment control In addition to avoiding the rainy seasons
in drier climates, construction can be sequenced and staged in smaller phases to minimize impact
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Measures
Soil Stabilization Vegetative soil cover Non-vegetative soil cover Diversions
Runoff Control Diversions Waterways Outlet stabilization Slope Protection
Sediment Control Sediment Basins Sediment Filters & Barriers Mud & Dust Control
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Runoff Control
Diversions Channels that intercept and redirect runoff
Waterways Natural or constructed channels that provide for the
safe disposal of excess water Outlet protection
Allows for energy dissipation and stable transition to receiving channel (i.e. reducing flow velocity with a series of low drop structures)
Slope protection Helps move concentrated runoff down steep slopes
without as much erosion (i.e. riprap)
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Soil Stabilization
Concerns protecting the soil surface from rain and erosion caused by sheet flow runoff Not effective against concentrated flow runoff
Vegetative measures Non-vegetative measures
Mulches, gravel, crushed stone, & geotextiles
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Sediment Control
Capturing sediment on-site before it can be transported downstream Typically temporary
Methods include: Silt fences Storm drain inlet protection (i.e. hay bales, filter
fabrics) Vegetated filter strips Sediment basins (used to treat concentrated
flows)
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Poor Erosion & Sediment Controlhttp://www.grandriverpartners.org/Construction_site_erosion.jpg
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Better Erosion & Sediment Control
http://www.metrokc.gov/exec/esa/images/haybales.jpg
Hay Bales
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Better Erosion & Sediment Control
http://www.remfilters.com/UserFiles/Image/Silt_Fence.jpg
Silt Fence
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Maintenance
Erosion & sediment control measures, both temporary and permanent, require maintenance
Erosion control Vegetative measures must be closely monitored during
the first growing season Structural measures must be inspected periodically,
especially after large storms Sediment control
Trapped sediment must be periodically removed to prevent clogging & maintain effectiveness
Some towns require filing a maintenance plan
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General Rules of Thumb
Minimize the amount of disturbed area Place silt fences:
At locations where construction area storm water runoff is leaving the site
At locations where runoff is entering the site Place hay bales:
Around existing drainage structures (along with a layer of filter fabric helping keep sediment out of the storm sewer system)
If necessary, use sediment basins for concentrated flows Restore vegetation to disturbed areas as quickly as
possible