Wes Marshall, P.E. University of Connecticut March 2007 CE 276 Site 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 facilities Porous pavements, constructed wetlands, etc.

Chapter 10 Soil 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 Control

Better Erosion & Sediment Control Hay Bales

Better Erosion & Sediment Control 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

Erosion & Sediment Control  All soils are subject to erosion, especially during construction This is due to the soils becoming unconsolidated during excavating & scraping Erosion is worse at long & steep grades

Erosion & Sediment Control The best way to minimize erosion is to minimize the amount or re-grading in terms of the area disturbed by construction

Erosion & Sediment Control  Best accomplished with good planning and proper installation / maintenance of controls  Good