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