From Tech Transfer Newsletter, Summer 2004
This document is found at www.techtransfer.berkeley.edu/newsletter/04-3/ero-ctrl.php

Temporary Erosion Control, Or How to Keep a Dirty Job Clean

(Summer 2004 Tech Transfer Newsletter)

Without erosion control, the run-off from a construction site can be heavily laden with sediment. Sediment is a pollutant which can also carry other pollutants with it from a construction site. Storm runoff transports the sediment, eventually depositing it in streams, storm drains, or low-lying areas. When the water flow slows, gravity settles the suspended particles resulting in sedimentation. Sedimentation upsets the natural structure and balance of waterways, contaminating streams, rivers, lakes, reservoirs, estuaries, coastal bays, and, ultimately, the ocean. Sedimentation clogs storm drains and can necessitate additional maintenance and costly repairs to roadways and structures. In low-lying areas, sedimentation deteriorates the environment for vegetation and destroys habitats.

Physical and Environment Factors

While erosion can be caused by water, wind or gravity, water-generated erosion is generally the most damaging. The extent of damage caused by erosion depends on characteristics of the soil, topography of the ground, and the volume and velocity of water running over the soil.

Soils with very high runoff potential, such as clay, are less permeable to water, have slower infiltration rates, and are more susceptible to erosion. Soils with low runoff potential, such as sands and gravels, are highly permeable with fast infiltration, and are less susceptible to erosion. Generally, the permeability and water holding capacity of a soil, and it's ability to resist erosion, increases with its organic content, soil structure, and fertility, and decreases with its level of compaction.

Topography also plays a significant role. The flow of water down slopes forms rills and gullies in exposed soil areas, then concentrates in those rills and gullies, increasing in speed and volume, scouring soil from the slope, and carrying sediment in the runoff. Erosion increases with the length and angle of the slope face, and with the volume of flowing water.

Erosion can be reduced by slowing the flow of water, dividing the flow of water into smaller quantities, increasing water infiltration into the soil, or by providing mechanical or structural retention.

Temporary Erosion Control

Adequate vegetative cover is the best prevention for erosion. Re-establishment of vegetation, when done properly, can reduce erosion potential back to that of the natural geologic condition prior to the disturbance caused by construction activities. However, re-establishment of vegetation must usually wait until construction activities have concluded.

Inevitably, soil is left bare, drainage channels and ditches are dug and left open, fills are placed, and cuts are made on construction sites. These disturbed soil areas (DSAs), when left unprotected, accelerate erosion. To keep as much soil in place as possible and to prevent sediment from leaving the construction site, temporary erosion control measures, or construction site best management practices (BMPs), are needed.

Soil stabilization practices are intended to keep soil in place and prevent sediment runoff. Caltrans approved temporary soil stabilization BMPs include: hydraulic mulching, hydroseeding, use of soil binders, straw mulching, use of geotextiles, plastic covers, erosion control blankets/mats, wood mulching, and streambank stabilization. Earth dikes, drainage swales, lined ditches, outlet protection, velocity dissipation devices, and slope drains may be approved for use on a project-by-project basis.

Sediment control practices intercept and slow or detain runoff allowing sediment to settle and be trapped. Sediment control practices are the "last line of defense" -- designed to remove sediment from runoff before it leaves the construction site. Silt fences and street sweeping/vacuuming are Caltrans-approved temporary sediment control BMPs. Other BMPs that can be approved on a project-by-project basis include: sediment/desilting basins, sediment traps, check dams, fiber rolls, gravel bag berms, sandbag barriers, straw bale barriers, and storm drain inlet protection.

To control erosion on construction sites effectively, thoughtful planning, appropriate selection and proper implementation of both soil stabilization and sediment control best management practices are usually required. Refer to Caltrans' Construction Site BMPs Manual for more information on each construction site BMP, including appropriate applications, limitations, standards and specifications, and maintenance and inspection requirements.

Erosion Control for Stockpiles

Stockpiled soil and other materials are common on construction sites. Soil stockpiles, like disturbed soil areas, are susceptible to erosion. While requirements vary by type (material in stockpile), status (active or non-active) and season (rainy or non-rainy), the best ways to minimize or prevent runoff pollution from stockpiles are:

Locate stockpiles away from drainage courses, storm drain inlets or concentrated flows of storm water.
Cover stockpiles, whether covering is required or not. Covering stockpiles is more effective than application of water, dust palliatives or soil stabilization measures; these measures should only be used when covering the stockpile is neither required nor feasible.
Use temporary perimeter sediment barriers, such as berms, dikes, silt fences, or sandbags when stockpiles are not active; use temporary linear sediment barriers at all times.

Refer to Caltrans' Construction Site BMPs Manual (pdf, 7.6M), WM-3 Stockpile Management, for details on managing stockpiles by type, status and season.

Inspection, Repair and Evaluation

Once implemented, BMPs require continued attention. Inspections before and after each storm and at 24-hour intervals during extended storms are required year-round. During the rainy season, inspections are required at least every two weeks, even if there is no storm activity. Built up sediments should be removed from barriers; ineffective BMPs should be replaced with more effective methods; and, damaged or improperly installed BMPs should be repaired or replaced. Corrective actions should be made as soon as possible, and must be completed prior to the onset of subsequent storms.

Plan for Success

Planning is critical to the successful control of erosion at construction sites. Evaluating the construction site, selecting, obtaining and implementing the appropriate BMPs, scheduling construction activities and tracking weather are just some of the issues. Addressing these issues with foresight and attention to detail prior to the initiation of construction is essential for successful soil stabilization and sediment control.

Best Practices for Reducing or Controlling Erosion During Construction

Minimize the exposed area of disturbed soil and the duration of the exposure. Clear vegetation only as needed, rather than clearing the entire site at once. Develop a staging plan that specifies temporary or perhaps even final seeding as areas of construction are completed.
Try to keep soil in place as much as possible. Erosion control is generally more cost-effective to purchase, install, maintain and repair than sediment control.
Ensure that sufficient supplies of soil stabilization and sediment control materials are on hand to protect the site in the event of unexpected rain.
Install soil stabilization and sediment control measures correctly the first time, and keep up with maintenance and repair. Follow the recommendations from the manufacturer; deviations often lead to failure, subsequent rework and costly replacement.
Implement the required rainy season BMPs in a timely manner. Plan ahead so installations fit within the work schedule.
Keep up with local weather predictions, and enforce inspections before, during and after storms.

Definitions

Best Management Practice (BMP). A measure using the best conventional or available technology to reduce the potential for pollution from construction site runoff.
Disturbed Soil Area (DSA). Exposed, erodible soil resulting from construction activities, such as clearing, grading or excavating, and including stockpiles. DSAs can be active or non-active.
Active Areas. Construction areas where soil-disturbing activities have already occurred (DSAs), and soil-disturbing activities will continue to occur or will occur during the ensuing 21 days. Active areas must be protected with both soil stabilization and sediment control at the end of each working day, unless fair weather is predicted.
Non-Active Areas. Formerly active construction areas (DSAs) that will be idle for at least 21 days. Non-active areas should be protected as soon as possible, and must be protected within 14 days from the end of soil-disturbing activities, or one day prior to predicted rainfall, whichever occurs first.
Soil Stabilization. Practices designed to control erosion by holding soil in place and preventing sediment runoff.
Sediment Control. Practices designed to remove sediment from runoff before the runoff leaves the construction site.

Suggested Coursework

Design and Implementation of Erosion and Sediment Control will be offered June 15-16, 2005 in Pismo Beach.

References

Construction Site Best Management Practices (BMPs) Manual.
California Department of Transportation, March 2003, 257 pages.
www.dot.ca.gov/hq/construc/stormwater/CSBMPM_303_Final.pdf (7.6 M)

Guidance for Temporary Soil Stabilization.
California Department of Transportation, July 2003, 184 pages.
www.dot.ca.gov/hq/construc/stormwater/tempsoilstabilizationguide.pdf (6.9M)

Construction Storm Water Pollution Prevention Bulletin.
California Department of Transportation, April 1999 - February 2002.
www.dot.ca.gov/hq/env/stormwater/publicat/const/index.htm

Erosion Control Handbook for Local Roads.
Minnesota Local Road Research Board and Federal Highway Administration, 2003, 122 pages.
www.lrrb.gen.mn.us/PDF/200308.pdf (2.4M)

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