Lesson 4:
Erosion and Sedimentation Control Devices

 

Objective

In this lesson, we will answer the following question:

 

 

Reading Assignment

There is no reading assignment for this lesson.

 

 

Lecture

Introduction

Erosion and Sedimentation

In a previous lesson, we showed how erosion can result in the formation of a gully.  Erosion is problematic in other ways as well.  By removing the topsoil from an area, erosion reduces the soil productivity.  Erosion also causes a problem called sedimentation.

Sedimentation occurs when the sediments removed from the ground during erosion are deposited elsewhere.  Although sedimentation helped reclaim the gully in the previous lesson, sedimentation in natural waterways reduces the quality of these waters.  

When the ground is disturbed during construction of roads, buildings, etc., erosion is greatly accelerated.  This lesson will be concerned with methods used to minimize the erosion and sedimentation at construction sites.  

 

Erosion and Sedimentation Control Devices

Many erosion and sedimentation control devices exist.  In this lesson, we will consider the six devices which are regularly used by the Department of Transportation when widening and improving secondary roads.  When used correctly, these six devices provide good protection of nearby waterways during and after the construction process.

 

 

Types of Control Devices

Temporary Silt Fence

The temporary silt fence is the erosion control device most frequently used.  Its purpose is to prevent sedimentation in off-site waters caused by erosion on the construction site.  


Temporary Silt Fence

 

The temporary silt fence is simply a band of fabric running along the ground which holds back the sediments and allows the water to pass through.  

Silt fences are often installed incorrectly.  The fabric should be buried at least 8 inches deep, but most people do not take the time to bury the fabric.  If the bottom is not buried in the ground, the silt fence is rendered less effective.


Location of the Sedimentation Fence


A silt fence is place at the toe of a fill site, a location where loose dirt and rocks have been placed during a grading operation.  The fill site is very prone to erosion since the dirt has no vegetation covering it.

 

The temporary silt fence filters sediment out of the runoff coming out of the fill site, preventing it from contaminating off-site waters.  The silt fence should remain in place until the disturbed area has been stabilized, meaning that grass has established on the loose soil.  

 

Special Sediment Control Fence



Special Sediment Control Fence

 

A special sediment control fence works in essentially the same way as the fabric silt fence described above.  But it is more effective at filtering out sediment and is often used in areas of increased environmental sensitivity, such as near a spring.  

The special sediment control fence is constructed of hardware cloth on metal posts with number 5 or number 57 crushed stone a minimum of 1 foot high at the base of the fence.  The stone slows the velocity of the runoff and retains sediments.  

The hardware cloth should be ¼ wire mesh and should be at least 2 feet high.  The posts should be 5 feet high and should be spaced 3 feet apart.  

 


Type C Rock Inlet Sediment Trap

 

Special sediment control fences are also often used around stormwater drop inlets on the shoulder or median of a highway, as shown above.  A drop inlet is a structure which allows water to descend in elevation through a series of pipes without eroding the soil as it would if flowing over such a steep incline.  The special sediment control fences prevent sediment from entering the drop inlet.  In this case, the fences are known as Type C rock inlet sediment traps.

 

 

Silt Basin Type B


Silt Basin Type B

 

A silt basin is a simple excavation in the earth located so that it intercepts drainage.  Runoff is collected in the basin, where silt and fines settle out.  (Fines are very small particles, including silt and clay.)  Once the silt basin becomes full of sediments, it should be cleaned out.  

The width of a silt basin depends on the width of the channel draining into it.  Its length should be twice its width, and it should be at least 2 feet deep.  The total storage capacity of the silt basin should be 1,800 cubic feet per acre of disturbed ground.  

 

 

Temporary Rock Silt Check Type B


Temporary Rock Silt Check Type B


A temporary rock silt check type B, also known as a check dam, is simply a pile of 8-inch rip-rap placed in a channel to act as a dam.  Rip-rap is broken rock, cobbles or boulders placed on earth surfaces such as the face of a dam or the bank of a stream for the protection against erosive forces such as flow velocity and waves.  

The check dam slows the velocity of the stormwater in the channel so that it is below the suspension velocity of silt, allowing the silt to settle out.  The middle of a check dam is slightly lower than the sides, creating a weir effect.

A channel will typically have a series of check dams along its length, with the number of dams depending on the slope of the channel.  Steeper channels will require more check dams to slow the velocity of the stormwater sufficiently to remove the silt.  

To determine how far apart to space check dams, use the following formula:

 

The following chart summarizes this formula:

Slope of Channel
(Percent)
Spacing of Rock Silt Checks
(Feet)
3
100
6
50
9
33
12
25
15
20

 

 

Pipe Inlet Sediment Trap Type A

 


Pipe Inlet Sediment Trap Type A


The pipe inlet sediment trap type A is designed to filter all water entering a drain pipe.  The sediment trap is simply a horseshoe-shaped, 8-inch structure around the inlet of a pipe.  This structure should be at least 3 feet high.  The outside of the trap is lined with #5 or #57 stone to capture sediment.  

An excavated storage basin should surround the outside of the device.  As with a silt basin, this excavation should have a volume of 1,800 cubic feet per disturbed acre.



Rock Silt Screen


Rock Silt Screen

 

The final type of erosion and sedimentation control device which we will consider in this lesson is the rock silt screen.  A rock silt screen is a 8-inch rip-rap dam placed in a live natural stream below a construction area, such as a pipe installation or stream bank restoration.  The silt screen causes the water to ripple, which helps suspended fines settle to the bottom of the stream.  The silt screen will also filter sediments out of the water.  


Cross-section Through a Rock Silt Screen

 

The top cross-section of the dam should be about 1.5 feet wide, and the side slopes should be 2:1.  The upstream side should be lined with #5 or #57 stone to a thickness of 1 foot.  The top of the silt screen should be near the water surface, but no more than 1 foot above it.  

Before the silt screen is removed from the water, it should be carefully cleaned.

 

 

Symbols

Most of the devices considered in this lesson have specific symbols which are used on stormwater control plans.  These symbols are shown in the chart below:


Device

Symbol
Temporary Silt Fence

Special Sediment Control Fence

Silt Basin Type B

Temporary Rock Silt Check Type B



Reference

North Carolina Department of Transportation, Division of Highways, Roadside Environmental Unit. "Erosion and Sediment Control Measures."

 

 

Review

Six devices are commonly used by the Department of Transportation to prevent erosion and sedimentation at construction sites.  These devices are: temporary silt fence, special sediment control fence, silt basin type B, temporary rock silt check type B, pipe inlet sediment trap type A, and rock silt screen.  The devices remove sediments from runoff by slowing the water's velocity so that the sediments settle out.  

 

 

Assignment

  1. In nature, sediments settle out of streams as the streams enter large bodies of water such as ponds.  Which type of erosion and sedimentation control device works most like a pond?

  2. All of the devices in this lesson prevent sedimentation in nearby streams by removing the sediments from runoff before the runoff leaves the construction site.  What is another way of preventing sedimentation in nearby streams?  

 

 

Quiz

There is no quiz for this lesson.