Erosion prevention practices - natural and synthetic mulches
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Contents
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.
Bare soil cover types and percent reduction of soil loss. The C factor, used to determine the relative effectiveness of soil and crop management systems in terms of preventing soil loss, is a ratio comparing the soil loss from land under a specific crop and management system to the corresponding loss from continuously fallow and tilled land. Source: Northwest California Resource Conservation and Development District 2016.
Link to this table
| Vegetative Cover | C factor | Percent reduction of soil loss |
|---|---|---|
| None (fallow ground) | 1.0 | 0 |
| Native vegetation (undisturbed) | 0.01 | 99 |
| Temporary tyegrass, 99% (perennial) | 0.05 | 95 |
| Temporary ryegrass, 90% (annuals) | 0.1 | 90 |
| Permanent seedings (90%) | 0.01 | 99 |
| Sod (laid immediately) | 0.01 | 99 |
| Mulching (for slopes 2:1 or less) | ||
| Hay (0.5 tons/acre) | 0.25 | 75 |
| Hay (1.0 tons/acre) | 0.13 | 87 |
| Hay (1.5 tons/acre) | 0.07 | 93 |
| Hay (2.0 tons/acre) | 0.02 | 98 |
| Wood chips (6 tons/acre) | 0.06 | 94 |
| Wood cellulose (1.75 tons per acre)X | 0.10 | 90 |
| Other | ||
| Competent gravel layer | 0.05 | 95 |
| Rolled erosion control fabrics | (for slope greater than 2:1) | Variable C value by type |
{{:Expected performance for natural and synthetic mulches]]
Planning Considerations
A key stormwater planning objective should be to keep the bare soil footprint at the site as small as possible by stabilizing inactive areas with mulch or other means until construction resumes in those portions of the site, or until temporary or permanent cover has been established. Planning and staging projects in a manner that minimizes the extent and duration of soil disturbance helps to reduce both erosion and sediment loss. In practice, this often means scheduling operations to complete clearing, grading, and cut/fill operations in a phased manner, so that manageably sized cleared and graded areas can be temporarily – or permanently – stabilized as the project proceeds. Planning approach examples include:
- Subdivision development – complete clearing, grading, and primary road layout, then temporarily seed and mulch home site and secondary road locations until construction begins in those areas.
- Commercial projects – grade building footprint(s) and proposed landscaped areas, then seed and mulch them and focus construction and material storage on areas immediately adjacent to the building pad(s). Parking lots can be graveled and used for material storage, staging, and parking.
- Institutional and manufacturing facilities – same as commercial projects. Keep activities close to the vertical construction site, minimize disturbed areas, and temporarily seed and mulch idle areas.
Site personnel with minimal training can install most mulch materials (e.g., straw, rolled products). However, outside contractors are often needed to apply synthetic, hydraulically applied products (e.g., bonded fiber matrix). Keeping a supply of straw or other temporary cover (e.g., rolled erosion control products) on hand can help to ensure that temporary seeding is implemented both regularly and quickly. High priority areas for immediate stabilization include areas within 50-100 feet of a lake, river, stream, or wetland; slopes steeper than 4H:1V; and ditches and channels within 200 feet of a waterbody or property line.
Additional important planning considerations include:
- Mulch temporary or permanent seed installations immediately.
- Areas where vegetation cannot be established because of the season should be mulched to temporarily protect the soil surface.
- Mulching is especially important when conditions for germination are not optimum, such as midsummer and early winter, and on difficult areas, such as cut slopes and slopes with southern exposures.
- After mulching, seed the area as soon as conditions are favorable for germination and seedling growth.
- Do not use materials that may contain competing weed and grass seeds.
- Mulch may be spread by hand or with a mulch blower. Straw may be lost to wind and must be chemically or mechanically anchored to the soil immediately after it is spread.
- Additional methods, including erosion control blankets and turf-reinforcement mats, may be needed in critical areas such as waterways and channels and slopes steeper than 3:1.
- Tackifiers, or chemical soil stabilizers and soil binders, are useful for tacking organic mulches (see next subsection).
- Various types of netting materials are also available to anchor organic mulches.
The current list of MnDOT certified/approved vendors for mulch are available on the MnDOT website:
Design
Key design parameters for mulch application are 1) the length of time stabilization is needed (i.e., temporarily or permanently); 2) whether the mulch will be used as a stand-alone cover or with seed; 3) site conditions, such as size, slope steepness, slope length, and accessibility; 4) available labor and equipment; and 5) cost. Mulch provides temporary and/or permanent stabilization of soil during and at the completion of construction, and aids in seed germination for vegetation establishment. Before mulching, install any needed erosion and sediment control practices such as diversions, grade stabilization structures, berms, dikes, grass-lined channels and sediment basins. The Types of mulch products typically used on construction sites table lists various mulch types, including their recommended application method.
Types of mulch products typically used on construction sites
Link to this table
| Mulch type | Description | Application Method |
|---|---|---|
| Straw, hay, or other grasses | Wheat, oat, or pine straw; rolled or baled pasture grasses also used in some cases | Hand scattering for small areas; chopper/blower used for larger areas, sometimes with co-applied tackifying agent to promote adhesion |
| Wood chips, bark, sawdust | Waste product from sawmills and other timber harvest and processing operations | Hand scattering or mechanized spreader |
| Rock | Can include all classes of aggregate, riprap, and large stone; used for permanent erosion protection | Placement by hand or equipment (e.g., track-hoe, skidder, front-end loader) |
| Hydraulically applied mulches | Bonded fiber matrix products, including those manufactured with natural and/or synthetic fibers, cellulose, or other materials | Spray application via high-pressure pumping from the mixing tank, through a hose and nozzle apparatus |
The Mulch types, application rates, benefits, and limitations table lists various mulch types, application rates, and pros/cons. Note that rolled erosion prevention/control products – temporary erosion control blankets or permanent turf reinforcement mats – are often used instead of mulch.
Mulch types application rates benefits, and limitations
Link to this table
| Mulch Type | Application Rate | Benefits | Limitations |
|---|---|---|---|
| Straw, hay, or other grasses | 1.5 to 2.5 tons per acre | Readily available and inexpensive; very effective in controlling erosion; can be applied on large sites via blower | Can carry unwanted seeds; might need tackifier or anchoring,
especially on steep slopes |
| Wood chips, bark, sawdust | 5 to 8 tons per acre | Very low cost in some locations; chips can be effective on slopes up to 30% | High nitrogen demand when decomposing; can float away or blow away during rain storms |
| Rock | 200 to 500 tons per acre | Could be inexpensive and readily available in some locales; might be suitable for smaller sites | Inhibits plant growth; adds no nutrients to the soil; can be costly to apply on slopes and large sites; adds hardened look to slopes |
| Hydraulically applied mulches | 1.5 to 2.5 tons per acre | Easily and rapidly applied with sprayer equipment; can include seed, fertilizer, flexible/fibrous mulches, and soil binders | Could be too expensive for small or very remote sites; after application, must dry for at least 24 hours before rainfall |
Where mulches are used in conjunction with vegetation establishment, they should be selected to last as long as it takes to establish effective vegetative erosion prevention. On slopes greater than 2.5H:1V, or where the selected product mulch is susceptible to movement by wind or water (e.g., straw), the mulch material should be hydraulically applied or appropriately anchored. Bonded fiber matrix mulches and tackifying agents are used effectively to bind mulch materials and prevent displacement by wind or rain. Straw mulch can also be covered by degradable netting or secured by crimping into the soil. Other mulch application considerations are listed below.
Straw and other grasses
Wheat, oat, barley, and rice straw make excellent mulch. Because of its length and bulk, straw is highly effective in reducing the impact of raindrops and in moderating the microclimate of the soil surface. Straw mulch can be applied by hand on small sites and blown on by machine on large sites. Straw blowers have a range of about 50 feet. Some commercial models advertise a range up to 85 feet and a capacity of 15 tons per hour. Straw mulch should not be applied more than 2 inches deep on seeded sites, unless it is incorporated into the soil by tracking, disking (crimping), or other ground-securing techniques. If the straw is applied at rates higher than 3 tons per acre, the mulch could be too dense for the sunlight and seedlings to penetrate. Look for clean straw to prevent the spread of noxious weeds. Avoid moldy, compacted straw because it tends to clump and is not distributed evenly.
The straw must be evenly distributed by hand or machine to the desired depth (about 2 inches maximum), and should cover the exposed area to a uniform depth. One bale (approximately 80 lbs) of straw covers about 1,000 square feet adequately. The soil surface should be barely visible through the straw mulch. On steep or high-wind sites, straw must be anchored to keep it from blowing away. Straw mulch is commonly anchored by crimping, tracking, disking, punching into the soil, covering with a net, spraying with asphaltic or organic tackifier, or tacking with cellulose or other product. These various straw mulch anchoring techniques are described as follows:
- Hand Punching - On small sites where straw has been distributed by hand, it can be anchored by hand punching it into the soil every 1–2 feet with a dull, round-nosed shovel. A sharp shovel will merely cut the straw and not anchor it.