Erosion control for green building
By Kevin Caldwell on 02/01/2008
Soil is an amazing resource that is often overlooked as “dirt.” Soils are dynamic, living systems, the health of which dictate the health of forests, farms, wetlands and wildlife. But they are a nonrenewable resource, at least as far as our lifetimes are concerned. Consequently, our impacts upon them are long-term.
On mountain slopes, one inch of topsoil forms naturally in about 1,000 years. In new developments that lack proper erosion controls, a few thousand years of topsoil are lost in a single rainfall. In North Carolina, up to 100 tons of topsoil per acre have been documented as eroding off construction sites. By volume, soil erosion is the single greatest water pollutant in the state. It comes with a very high price: death of fish, smothered fish eggs, destroyed aquatic food webs and increased water-filtration and hydro-electric power-generation costs.
And yet, erosion control can be performed easily and affordably. Creating and following an erosion-control plan is the key. The N.C. Sediment Pollution Control Act requires a sediment-control plan for soil disturbance on one or more acres, submitted and approved by the city or county planning board prior to construction. The law mandates slope stabilization, groundcover establishment, stream buffering and adherence to the plan. Failure to submit or follow the plan can result in fines up to $5,000, stop-work orders and restoration fees. For more information, contact the N.C. Division of Land Quality for a copy of their Erosion Control Manual, or subscribe to Sediments, a newsletter of the N.C. Sedimentation Control Commission.
Despite these regulations, most home sites impact less than one acre, and thus require no sediment-control plan approval. But why not make one anyway? A good plan will address square footage, aspect, slope, seed quantity and type, materials, timing, expense and maintenance. Typical materials include staked silt fencing, wire fence (for backing silt fence on steep slopes), straw mulch, crank-seeders and seed. For ditches and roadsides, materials may include seeded fiber-mats, coir logs and/or straw wattles or bales for sediment traps.
Before breaking ground, consider some of the following ecological-planning guidelines:
- Photo document existing conditions in excavation zones, noting native trees, shrubs and herbs. Strive to return cleared areas to natural conditions.
- Remove exotic-invasive plants near clearings prior to excavation. These invaders will dramatically increase after excavation due to increased light on fresh soils.
- Bank topsoil and restore it to exposed subsoils prior to seeding. Store topsoil away from streams, drives and roads. Cover it with plastic, or seed with annual rye.
- Preserve existing soils and vegetation where possible. Remember that covering tree bases with soil usually kills them.
- Seed and mulch exposed soils within 24 hours of grading, especially on steep slopes. Plan to finish up before moderate to light rain.
Once you’ve broken ground, there are a few important steps to take. First, install silt fencing at the base of exposed slopes, trenching the fence base with six to eight inches of soil. In steeper areas, consider backing silt fencing with hard-wire fence and metal stakes for support. Additional rows of fencing should be placed in coves and mid- to upper-slopes, depending on the steepness and width of the cut. Monitor fences, as they often require maintenance until vegetation is established.
Hydro-seeding is best for large areas and steep slopes. Smaller sites can be controlled with hand-seeding much more affordably, with similar results. Try to use native sources when possible. Ernst Conservation Seed (http://www.ernstseed.com) and Roundstone Native Seed (http://www.roundstoneseed.com) offer numerous native-plant mixes tailored to all moisture and light gradients. As for nonnatives, weeping lovegrass is an excellent, noninvasive, mat-forming perennial that is hydro-seeded for best results. Other good nonnative seed mixes include brome grass, ryes, purpletop, creeping fescues and mixed clovers for nitrogen fixation. You’ll need from 20 to 50 pounds of seed per acre, but keep some aside for touch-up seeding later.
Seeding is best performed immediately following grading while soils are compact but textured, especially if rain is expected in the next day or two. For steep areas, begin seeding at the slope base, seeding a width of 8 to 12 feet, then mulch. Repeat in strips moving upward, allowing gravity to carry seed and straw downward. For less steep areas, follow parallel lines, watching carefully for good seed coverage. Return on a parallel path, overlapping slightly with the previous batch of seed.
Ditches are a major source of sediment, and difficult to control. Ditches should be lined and staked with seeded fiber mats, upon which coir-log or straw-bale or wattle sediment traps are placed, in lieu of gravel or rock dams. Traps are located so the bottom of the upper trap is at the same elevation as the top of the lower trap. Recent DOT studies have shown these traps remove 10 to 800 times the sediment of rock dams.
Ultimately, proper erosion controls maintain the health of our rivers and streams. Leaders in green building and environmental awareness have a duty to be regional models for keeping soil on the land and our waters free of sediment.