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Natural and Synthetic Mulches

Definition

Natural and synthetic mulches include a wide range of practices used to cover seed and exposed soil. Mulch products are intended to reduce raindrop (splash) erosion, decrease sheet erosion, promote rain/snowmelt infiltration, increase soil moisture retention, regulate soil temperature, and in most cases, improve soil texture and increase organic matter. Mulch products include natural materials such as straw and other grasses, coconut fiber, and bark. Synthetic mulches combine a variety of chemical bonding agents with wood fibers, cellulose, or synthetic fibers (e.g., bonded fiber matrix). The mulch material may be disc-anchored into the soil, hydraulically bonded, or covered with netting and stapled. The choice of materials and anchoring of mulches should be based on slope steepness and length, soil conditions, season, type of vegetation, and size of the area.

Purpose and Function

Soil stabilization with mulch is intended to counteract the erosive influences of rainfall, snowmelt, and wind on bare soil. Other benefits include soil moisture retention and improved soil texture. Mulches can be used for areas of the construction site that will be idle for 14 or more consecutive days to prevent erosion during lag times in grading operations, or they can be applied with seed or other vegetation to establish final, permanent cover for bare soil.

Applicability

Stabilization with mulch applies to 1) areas of construction sites where soil disturbing activities have temporarily ceased, and measures are needed to prevent erosion and sediment runoff during rainfall or snowmelt; and 2) areas of the site that require permanent stabilization.

Site Applicability

Construction sites often have areas where soil disturbing activities such as clearing, grading, or cut/fill work has stopped for a period of time. Bare areas that are not actively being worked need some type of cover to prevent or minimize erosion in the event of rainfall or snowmelt. Applicable areas include topsoil stockpiles, rough graded areas, sediment basin dikes, temporary earthen structures and graded areas.

In addition, all areas of the site require permanent stabilization prior to project close out and termination of permit coverage. Mulch can be applied by hand, installed with mechanical spreaders/blowers, or sprayed hydraulically, depending on the product and site conditions. Mulch is often used instead of rolled erosion control products to support seed germination and early growth.

Permit Applicability

The MPCA Construction General Permit has several requirements regarding temporary stabilization with seed, mulch, or other methods.

  • Part IV.B.2 of the MPCA Construction General Permit states that the permittee(s) “must stabilize all exposed soil areas (including stockpiles). Stabilization must be initiated immediately to limit soil erosion whenever any construction activity has permanently or temporarily ceased on any portion of the site and will not resume for a period exceeding 14 calendar days.” In addition, “(s)tabilization must be completed no later than 14 calendar days after the construction activity in that portion of the site has temporarily or permanently ceased.”
  • Near public waters for which the Minnesota DNR has promulgated “work in water restrictions” during specified fish spawning time frames, all exposed soil areas that are within 200 feet of the water’s edge, and drain to these waters must complete the stabilization activities within 24 hours during the restriction period. Temporary stockpiles without significant silt, clay or organic components (e.g., clean aggregate stockpiles, demolition concrete stockpiles, sand stockpiles) and the constructed base components of roads, parking lots and similar surfaces are exempt from this requirement.
  • Other permit stabilization requirements relate to permanent stabilization and stormwater controls. For sites discharging to special or impaired waters, Appendix A Section C.1 of the MPCA Construction General Permit requires that (s)tabilization of all exposed soil areas must be initiated immediately to limit soil erosion but in no case completed later than seven (7) days after the construction activity in that portion of the site has temporarily or permanently ceased.”
  • Appendix B Section 26 of the MPCA Construction General Permit defines stabilization as meaning that “the exposed ground surface has been covered by appropriate materials such as mulch, staked sod, riprap, erosion control blanket, mats or other material that prevents erosion from occurring. Grass, agricultural crop or other seeding alone is not stabilization. Mulch materials must achieve approximately 90 percent ground coverage (typically 2 ton/acre).”

Effectiveness

Temporary and permanent stabilization with mulch or other products is highly effective in reducing soil loss from construction sites (see Table 2 1). Vegetative cover can reduce erosion by up to 99 percent, with the application of mulch at the MPCA recommended rate of two tons per acre achieving similar results. Because seeding is only effective after plants have emerged, the application of straw mulch or other cover is required to stabilize exposed surfaces and help establish vegetation growth. Table 2 2 summarizes expected performance for an array of typical water quantity and quality target constituents for natural and synthetic mulches. Refer to Reference Materials for additional links to reported soil loss reduction values among various mulch types.

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