Sustainable Land Development Today - October 2008 - (Page 15)

inches of compost is applied. Studies have shown that compost can be an appropriate groundcover for establishing grass on low to moderate slopes, but compost is often more expensive than other erosion control options. Wood chips, or shredded woody materials, are often generated during the clearing stage of construction when trees are shredded using large tub grinders. Such materials are ideal for landscaping around established vegetation or large plants, but they should not be applied or incorporated into soil where permanent vegetation is planned. Typical groundcovers such as grass can have difficulty growing through woody material, and as this high-carbon material decomposes, it removes plant nutrients from the soil (through microbial processes), resulting in low soil fertility. Figure 1. Trenching in an ECB to avoid undercutting near the top of a slope. Adapted from L. Honnigford, Erosion Control Technology Council, 2007. Erosion Control Blankets Erosion control blankets (ECBs) vary widely in composition, price, and circumstances of appropriate use. In general, ECBs are more expensive to purchase and more time- and labor-intensive to install compared to straw mulch, but are comparable to hydromulching. When selecting an appropriate ECB, important considerations are the slope ratio, duration of use, cost, and effectiveness compared to other viable options. The main types of ECBs, and guidelines for use, are summarized in table 2. Despite the variety of ECB materials, the general method of installation is sim- ilar. The first step is proper slope preparation, including application of lime, fertilizer, and seed. The soil surface should be uniformly graded and without rocks, large roots, or other materials that could prevent good soil-blanket contact. After slope preparation, the top edge of a roll is “trenched in,” or staked into the ground six inches deep and buried, then rolled over itself down-slope (figure 1). When laying out the ECB after trenching, adjacent rolls should overlap by 3 Slope Ratio ≤ 3:1 ≤ 1:1 sod Material > 36 months > 36 months ≤ 36 months ≤ 24 months Longevity ≤ 1:1 ≤ 1:1 ≤ 2:1 ≤ 3:1 ≤ 3:1 ≤ 3:1 ≤ 1:1 synthetic turf reinforcement mat / geotextile fiber double-net coconut blanket double-net straw/coconut blanket double-net straw blanket single-net excelsior net-free excelsior Most expensive to purchase and install, but quickly increases property value and does not introduce weeds. Poor survival in sandy soils and steep slopes. Must select appropriate turf grass species. Irrigation required. Good moisture absorption but poor moisture retention. Can handle high flow. Good for steep areas with medium runoff. May import weed seed. Lightweight and fast to degrade. May import weed seed. No introduction of weed seeds. Netting may entangle wildlife and last years. No introduction of weed seeds. Netting may entangle wildlife or may be removed. Does not entangle wildlife or introduce weed seeds. 100% biodegradable. Lightweight and fast to degrade. May import weed seed. Expensive but effective for high-velocity flows in channels. Good for steep slopes. Heavy. Notes double- or triple-net excelsior ≤ 12 months ≤ 36 months 12-24 months 12-24 months ≤ 12 months single-net straw blanket Table 2. Types and circumstances for using sod and erosion control blankets, arranged from most expensive (top) to least expensive (bottom). Adapted from Erosion Control Technology Council, 2002. Longevity estimates from Erosion Control Technology Council, 2002. Ranking based on cost estimates from Daillaire (2002) and Missoula Technology Development Center (2006). www.SLDTonline.com 15 http://www.SLDTonline.com

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Sustainable Land Development Today - October 2008

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