Erosion prevention practices - temporary seeding and stabilization
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Contents
Temporary Seeding and Stabilization
Definition
The MPCA Construction General Permit defines stabilization, stabilized, and stabilize 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.”
Stabilization includes a wide range of erosion prevention practices that cover exposed soil such as the use of straw, mulch, erosion control blankets, plastic sheeting or tarpaulins. Temporary seeding is a soil stabilization practice involving the establishment of temporary vegetative cover to reduce erosion on construction sites that have disturbed areas that are temporarily idle (i.e., where no soil disturbing activities occur on that portion of the site for 14 or more consecutive days). Erosion prevention practices like stabilization are generally less costly and more effective than sediment control measures, which involve settling or filtering mobilized soil particles before they are transported by runoff to surface waters.
Purpose and Function
Temporary seeding and stabilization are intended to counteract the erosive influences of rainfall, rain and snowmelt runoff, and wind on bare soil. Stabilization prevents the mobilization and subsequent transport of soil particles by preventing erosion. Stabilization practices, which mostly include temporary vegetated cover and the application of a mulch, blanket, mat, or other cover on bare soil, is the easiest, cheapest, and most effective approach for addressing sediment loss (muddy runoff and dust) from construction sites. Since temporary seeding is only effective for erosion control once vegetation has established, mulch or other temporary cover is needed to protect seeded areas until vegetation emerges. A good stand of vegetation will protect soil from erosion by raindrop impact and help slow runoff to prevent rill erosion. The vegetation can also act as a filter, trapping coarse sediment particles carried by runoff.
Applicability
Temporary seeding and stabilization apply to areas of construction sites where soil-disturbing activities have temporarily ceased, and/or immediate measures are needed to prevent erosion and sediment runoff at its source during anticipated rainfall or snowmelt events. Note: Temporary seeding information is presented in this subsection; information on other approaches for temporary stabilization (e.g., blankets, mats, mulches, hydraulically applied products, etc.) can be found in other subsections.
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 under construction need some type of temporary cover to prevent or minimize erosion in the event of rainfall or snowmelt. Applicable areas include topsoil stockpiles, rough graded areas, sediment basin dikes, ditches, temporary earthen structures, and graded areas undergoing settlement.
Permit Applicability
The MPCA Construction General Permit has several requirements regarding temporary stabilization.
- Part IV.B.2 of the 2013 MPCA Construction Stormwater 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 that the Minnesota DNR has promulgated “work in water restrictions” during specified fish spawning time frames, per Part IV.B.2, “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 requirements exist during the construction of post-construction/permanent stormwater and temperature control BMPs discharging to special waters and impaired waters. In those cases, Appendix A Section C.1 of the 2013 MPCA Construction Stormwater 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.”
- Finally, it shall be noted that stabilization requires more than seed alone. Appendix B Section 26 of the 2013 MPCA Construction Stormwater General Permit defines stabilization as “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 seeding and stabilization is effective in reducing soil loss from construction sites once vegetation has become established (Table 1 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 at the time of seeding. Erosion prevention practices such as seeding and mulching are generally more effective and less expensive than sediment control practices.
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 |
The Expected performance for temporary seeding and stabilization practices table below summarizes expected performance for an array of typical water quantity and quality target constituents for temporary seeding and stabilization practices. Once established, a good stand of vegetation will protect soil from erosion by raindrop impact and help slow runoff to prevent rill erosion. The vegetation can also act as a filter, trapping coarse sediment particles (and associated pollutants, including nutrients and some heavy metals) carried by runoff.
Expected performance for temporary seeding and stabilization practices
Link to this table
| Water Quantity | |
|---|---|
| Flow attenuation | Little or no design benefit |
| Runoff volume reduction | Secondary design benefit |
| Pollution prevention | |
| Soil erosion | Primary design benefit |
| Sediment control | Secondary design benefit |
| Nutrient loading | Primary design benefit |
| Pollutant removal | |
| Total suspended solids | Primary design benefit |
| Total phosphorus | Primary design benefit |
| Heavy metals | Secondary design benefit |
| Floatables | Little or no design benefit |
| Oil and grease | Little or no design benefit |
Planning Considerations
Erosion prevention through soil stabilization is always preferred over sediment control, due to cost and overall effectiveness considerations. However, because of the nature of the work, erosion prevention is not always feasible on active construction sites. When a disturbed area will be inactive for a prolonged period, erosion prevention measures are typically required. Areas left un-disturbed over the winter should also be temporarily seeded and stabilized.
Temporary seeding is an effective erosion prevention practice that primarily uses the quick emergence of annual seedlings to stabilize bare soil surfaces. As such, proper seedbed preparation and the use of quality seed are important for good germination and growth. A poor stand (less than 50 percent cover) will not provide adequate erosion prevention.
A key planning objective involves the minimization of bare soil footprints at the site, followed by the immediate stabilization of inactive areas through temporary seeding or other measures until the next phase of construction begins. 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 disturbed areas can manageably be stabilized (either temporarily or permanently)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 – immediately seed and mulch graded building footprint(s) and proposed landscaped areas, then 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 idle areas.
Outside contractors are not typically required for temporary seeding and stabilization. Keeping the specified seed, straw mulch, rolled erosion control products, and other materials on hand ensures that temporary seeding is implemented timely and efficiently. High priority areas for immediate stabilization include areas within 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.
The current list of MnDOT certified/approved vendors for seed are available on the MnDOT website:
Design
Key design parameters for temporary seeding include: 1) the desired length of time for stabilization; 2) the appropriate type of seed or seed mix; 3) the rate of seed application; 4) the time of year in which the seed is planted; and 5) the type of mulch, blanket, or other material used to cover the site and support the emergent vegetation. In addition:
- Select a seed type or mix appropriate for the length of time that temporary stabilization is needed. Seeds labeled annual will die off at the end of the growing season and must be re-seeded to maintain vegetative cover afterwards. Perennial seeds will result in grass that persists for many growing seasons, and should be used where temporary cover is needed beyond the late fall and winter periods. Annual seed is often planted in combination with perennial seed to provide quick stabilization and help establish the longer-maturing plants.
- Select seeds that sprout quickly and are effective and economical in providing temporary cover (see Table 1 3 for MnDOT’s seed mixtures).
- Calibrate the seed application rate on the type of terrain to be seeded. For example, steep slopes, buffers next to surface waters, and flow channels require a denser stand of grass than flat areas.
- Straw mulch applied at two tons per acre is typically used to provide immediate cover and support emerging grass, although erosion control blankets and other options are available depending on site conditions.
Seed mixture selection and other information for various construction site stabilization periods