Wes Marshall, P.E. University of ConnecticutMarch 2007

CE 276Site Design

Chapter 10 – Soil Erosion & Sediment Control

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.

Chapter 10Soil Erosion &

Sediment Control

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

Erosion & Sedimentation

In terms of storm water management, these are critical issues…

Especially During Construction!

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

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

Soil Erosion Factors

4 Primary Factors:

1. Soil

2. Vegetative cover

3. Topography

4. Precipitation

Erosion & Sedimentation Processes

Preparing a good plan requires the designer have a good understanding of

the erosion potential of a site

Soil Erosion Factor Components

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

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

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

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

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

Erosion & Sedimentation Processes

The most effective plans incorporate a combination of both erosion control

and sediment control features

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

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

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

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

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)

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

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)

Poor Erosion & Sediment Controlhttp://www.grandriverpartners.org/Construction_site_erosion.jpg

Better Erosion & Sediment Control

http://www.metrokc.gov/exec/esa/images/haybales.jpg

Hay Bales

Better Erosion & Sediment Control

http://www.remfilters.com/UserFiles/Image/Silt_Fence.jpg

Silt Fence

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

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