Introduction

Runoff from construction sites can contribute significant sediment loads to receiving water. Thus, effective erosion and sediment control at construction sites are crucial in stormwater management. This module focuses on the development of erosion and sediment control plans at construction sites. Good planning is the first step in preventing sediments from damaging the receiving water ecosystem. However, it is equally important to ensure erosion and sediment control measures are correctly installed and maintained on site.

Water Erosion

Water-induced soil erosion is caused primarily by falling raindrops which dissipate their energy and the shearing force of surface runoff. The whole process involves detachment of soil materials, transport of soil materials, and deposition of eroded materials.

There are four main erosion types as indicated in the following diagram.

• Raindrop erosion is caused by the direct impact of falling rain drops on soil particles. This impact dislodges soil particles and splashes them into the air. The dislodged soil particles can then be easily transported by the flow of surface runoff.
• Sheet erosion is referred to the removal of a layer of exposed surface soil by the action of raindrop splash and runoff. The water moves in broad sheets over the land and is not confined in small depressions.
• Rill and gully erosion is caused by concentrated runoff in rivulets, cutting several inches deep into the soil surface. These grooves are called rills. Gullies may developed in unrepaired rills or in other areas where a concentrated flow of water moves over the soil.
• Stream and channel erosion is caused by increases in the volume and velocity of runoff
  

ESC Planning

The principles of ESC are prevention of erosion and control of sediments from leaving the construction site. Erosion prevention should be the primary objective of ESC planning. However, conventional approach in ESC planning is still focused primarily on sediment control unless regulatory agencies enact strict regulation on erosion prevention.

The typical steps of ESC planning are identification of problems areas, selection of erosion and sediment control measures, and preparation of document and drawings.

Problem Areas

The following pictures show the typical problem areas at construction sites where erosion and sediment are likely to occur.

1. Unprotected steep slopes are prone to erosion as runoff velocity is high.
   
   
   
2. Any construction works near or at streams or waterways are caused dislodged sediments to enter water directly.
   
   
   
3. Unprotected drainageways such as ditches are a source of sediments as runoff concentrates and moves quickly.
   
   
   
4. Storm inlets should be protected from sediment laden runoff which may clog underground storm sewers. High discharge velocity at a storm outfall may cause significant erosion downstream.
   
   
   
5. Large flat exposed areas are prone to sheet erosion and should be protected.
   
   
   
6. Borrow and stockpile locations are exposed areas which are disturbed continuously over the construction period.
   

Principles of ESC

1. Plan the development to fit the site characteristics.
   Site characteristics such as topography, soils, drainage patterns, and covers should be considered when developing a site. Areas which are prone to erosion should be left undisturbed and undeveloped if possible. Entrance and exits points for runoff should be protected from erosion and equipped with sediment control devices.
   
   
   
2. Minimize the extent of the disturbed area and the duration of exposure and stabilize disturbed areas as soon as possible
   Conventional land development practices favour grading of the whole site in the beginning of the project. Sometimes, a development may take years to complete. Thus, we have these disturbed areas which may last a long time and subject to erosion. The key of ESC is to minimize the extent of disturbed areas by phasing. Grading of development sites should be consistent with the development plan. By staging construction and preserving existing vegetation, erosion can be reduced significantly. Once a land surface is disturbed, we should minimize the duration of exposure by protecting it from erosion if possible. Typically, if an area is not going be worked on in more than 45 days, it should be protected by erosion control mats. The State of Maryland has demonstrated the effectiveness of stage construction by enacting strict regulation on grading practices.
   
   
   
3. Keep runoff velocity low.
   Runoff velocity should be kept as low as possible. For drainageways such as ditches, high velocity can be reduced by a series of rock check dams which break the flow velocity. Overland flow velocity can be reduced by minimizing slope length and steepness.
   
   
   
4. Direct runoff away from problems areas
   Concentrated flows if possible should be diverted away from problems areas as discussed in the last section.
   
   
   
5. Retain sediments within the site area.
   Sediment control devices such as sediment control ponds should be used to retain sediments from leaving the site.
   
   
   
6. Implement a thorough maintenance and follow-up program.
   Poorly maintained ESC devices are not going to work effectively. Budgets should be allocated for inspection and maintenance of ESC devices over the construction period.
   
   
   
7. Planning should be focused on pre-grading, after grading, during construction, and after construction phases. Different techniques may be required for each phase of development.

Erosion Control Devices

Erosion control devices include:

• Temporary seeding
• Temporary mulching
• Permanent sodding
• Temporary or permanent erosion control blankets
• Permanent vegetative buffer strips

Sediment control devices include:

• Site fencing
• Straw bales
• Sediment basins
• Sediment traps
• Storm inlet traps
• Rock check dams
• Interception berms/swales

ESC Plan

An ESC plan is:

• A written descriptive portion and a visual component of maps and plans
• Detailed depend upon the construction site and the surrounding streams and lands
• It is often integrated with stormwater management or drainage reports
• Location maps and property lines
• Limits of disturbance
• Existing site information
• Proposed grading
• Control measure details
• Construction schedule
• Stabilization details

The reasons for a failed ESC plan may be:

• The E&S plan did not address all stages of construction
• Changes occurred on adjacent sites
• On-site changes were made
• Devices were improperly installed
• Maintenance activities were not conducted
• Excessive rainfall occurred
  

An Example

Problem areas:

• The steep slopes in the southern portion of the site
• The watercourse along the south-west and southern portion of the site
• The railway ditch along the east side of the development

Principles:

• The development is a 17 lots rural subdivision. It is designed to maximize the development areas and may not fit the site characteristics. This is typical in most situations as designers and planners rarely pay attention to ESC in the draft plan.
• The railway ditch along the east boundary should be protected from excessive runoff which may cause erosion
• Stockpile areas should be located as far as possible from the watercourse
• Construction of houses should start from the lots farther away from the watercourse
• Sediment control devices should be located at the low point of the development (i.e. near Lot 5 and 7)
• Sediments from overland flow should be controlled around the perimeter of the site

ESC Devices:

• Silt fences along the perimeter of the site
• A sediment control pond at Lot 7
• Rock check dams along the railway ditch in the east
• The stockpile area is located in Lot 18 and 19 at the north-east corner of the site