Sustainable Land Development Today - October 2008 - (Page 14)

EROSION CONTROL Holding Your Ground A review of mulch options for erosion control on construction sites. By Deanna Babcock and Rich McLaughlin Erosion prevention is a major consideration on virtually every development site. The regulatory and enforcement implications can be substantial. While many are aware that erosion control techniques must be used near the final phase of a project to ensure that seeds and/or other plantings are not washed away or displaced, they are also used as temporary covers during different phases of grading. Like most segments of our industry, innovations and new products have resulted in numerous options in the area of erosion control. A variety of mulches are now used as a protective barrier that covers at least 75 percent of the soil surface, helps prevent seeds from washing away, provides a suitable environment for germination, and slows runoff on slopes. Mulches used for groundcover vary in cost, application procedures, and appropriate conditions of use. Three basic classes of mulch include loose mulch, erosion control blankets (ECBs), and hydraulically applied mulches (hydromulches). help reduce this problem. Common tackifiers applied on the straw include: I Emulsified asphalt, once a common tackifier because of its excellent adhesive properties, is toxic to both plants and animals. Prohibited in some states and all national parks, emulsified asphalt is usually applied at a rate of 12 ounces per square yard. I Wood and paper fiber hydromulches are natural tackifiers that are used at a rate of 125 to 750 pounds per acre. They are less toxic and more visually appealing than asphalt. I Guar- and starch-based tackifiers are two natural products usually applied at 100 to 200 pounds per acre. Starch-based tackifiers, unlike guar, contain seven to eight percent natural nitrogen and have a highmoisture retention capability, both of which help establish permanent vegetation. Although the use of tackifiers in conjunction with straw mulch is the most common practice to prevent displacement, using heavy equipment or hand tools to “crimp” or “punch” straw into the soil may be sufficient when the soil is not too sandy. Ideally, some straw is partially buried during crimping, which holds it in place and may increase infiltration rates. Straw has a relatively high rate of failure compared to other erosion control options. Main causes of failure include insufficient coverage during application, use on slopes that are excessively steep or long, or insufficient application of tackifier. However, when properly applied on acceptable slopes, straw can be an inexpensive and effective mulch for erosion control. The growth of vegeta- tion in strawed areas is similar to that of other mulch types. These pros and cons are summarized in table 1 below. Inexpensive Pros Quick and easy to May be displaced by apply by blower wind Good grass growth May introduce weed seeds Fines from blower drift long distances in wind Not as effective in preventing soil loss as more expensive options Cons No water needed for application Table 1 Loose Mulch Straw is usually the least expensive mulch available, with a cost as low as $0.09 per square yard. Straw mulch with seed, lime, and fertilizer ranges from $0.31 to $1.00 per square yard. Large areas can be covered with straw using commercial blowers. Applying straw by hand is also effective but is more expensive due to labor costs. Straw mulch is applied at a rate of about one to two tons per acre on slopes of less than 2:1 and lasts about three months, enough time to establish permanent vegetation in most areas of the country. Because straw is lightweight, it is easily blown away. Tackifiers or “crimping” Other loose materials used for groundcovers include compost and wood chips. Compost is made from a wide variety of organic materials, including animal manure, crop residue, municipal waste, biosolids, and yard waste. As a result, it can be highly variable in its properties. For example, the carbon to nitrogen ratio (C:N) of manure ranges from 3 to 56, while the C:N ratio of some crop residues may be more than 1,000. Materials with high C:N ratios can stunt or kill seedlings through microbially-mediated depletion of nitrogen and other nutrients from the root zone. Compost is generally applied as a one to two-inch layer on top of prepared soil with seed, fertilizer, and lime previously applied. On steep slopes (2:1 or steeper), up to four inches of compost may be applied. Some professional compost applicators include seed in the compost, which is recommended if more than two 14 October 2008 Sustainable Land Development Today

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Sustainable Land Development Today - October 2008 Contents Our Voice Editorial Board SLDT Resources Dynamic Duo Holding Your Ground Technology Low Impact Stormwater Management Seeking Your Feedback Preferred Providers Bookstore Back from the Brink Finance Transportation Building Smart Industry News Advertiser Index Products/Services Showcase Marketplace Last Word

Sustainable Land Development Today - October 2008

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