By Sara Heger
Erosion control measures prevent erosion caused by runoff, which is water flowing over land surfaces that wears away soil and rock. Runoff can cause problems by choking vegetation, contribute to water pollution, and erode onto adjoining properties and create neighbor problems.
Federally effective March 10, 2003, any land-disturbing activity that will “disturb” an area of one or more acres is required to have an NPDES permit for its stormwater discharge. Erosion and sediment control requirements exist at the federal, state and local levels of government.
Some local governments (city and county) have adopted site development or sediment control ordinances or regulations, and it is recommended that onsite system installers check with local units of government to determine whether local ordinances may affect their proposed activities. Key issues for installers to consider include construction entrances, sediment control and site stabilization.
The purpose of a gravel construction entrance is to minimize the amount of mud, dirt, rocks, etc., transported onto roads via motor vehicles or stormwater runoff, by constructing a stabilized pad of gravel at entrances and exits to construction sites. Any construction site where traffic will be leaving the site and moving directly onto public roads, other paved areas or other approved access points is recommended to have a gravel construction entrance. Gravel is typically composed of large 2- to 3-inch stones and the gravel entrance should be maintained throughout the life of the project.
To control sediment, work in dry weather and preserve existing vegetation. Bare soil should be covered with mulch, plastic or rock during construction. Grading on the site should be done to slow the water down with benches and berms, slope roughening and ditch blocks. Sediment control devices are constructed in order to slow runoff and trap sediment within the construction site. Examples include a silt fence, filter sock and straw bale barriers.
- A silt fence is constructed of stakes and plastic sheeting that gets buried a minimum of 6 inches below the soil line.
- A filter sock is a tubular mesh sock filled with a specified filter material that normally is a blend of composted materials or similar organic products, used to slow flow velocity, capture and degrade chemical pollutants, and trap sediment. Compost logs and rock and mulch barriers can also be used to filter the water.
- A straw bale barrier is a series of straw bales placed on a level contour to intercept sheet flows and allow sediment to settle out.
Site stabilization occurs near or at the end of construction when final grade has been established. Straw mulch and seed or an erosion blanket are the most common methods used. A uniform layer of straw and seed over a disturbed area of soil will help stabilize the soil.
An erosion blanket is a reformed protective blanket of straw or other plant residue, or plastic fibers formed into a mat, usually with a plastic mesh on one or both sides and placed over an area already prepared with topsoil, seed and fertilizer. Mulch can be chosen if the soil treatment area is not too steep, but an erosion blanket is recommended for steeply sloped sites. Blankets are staked in place, covering the entire surface of the system. Holes can be cut through the layers to create spots for the plants. The blanket is biodegradable, so it can be left in place. Systems installed late in the year can have an erosion control blanket laid down for the winter and plant establishment may begin in the spring. The material can be purchased at landscape supply stores.
For more information on erosion refer to the publication Designing for Effective Sediment and Erosion Control on Construction Sites by Jerald Fifield, published by Forester Communications, 2004, Santa Barbara, California.