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Mulching

Minimum Measure: Construction Site Stormwater Runoff Control

Subcategory: Erosion Control

Photo Description:  Grass mulching is applied to stabilize exposed soils and to reduce stormwater runoff velocity

Description

Mulching is an erosion control practice that uses materials such as grass, hay, wood chips, wood fibers, straw, or gravel to stabilize exposed or recently planted soil surfaces. Mulching is highly recommended and is most effective when used in conjunction with vegetation. In addition to stabilizing soils, mulching can reduce stormwater velocity and improve the infiltration of runoff. Mulching can also aid plant growth by holding seeds, fertilizers, and topsoil in place, preventing birds from eating seeds, retaining moisture, and insulating plant roots against extreme temperatures.

Mulch matting is made from materials such as jute or other wood fibers that are formed into sheets and are more stable than loose mulch. Use jute and other wood fibers, plastic, paper, or cotton individually or combine them into mats to hold mulch to the ground. Use netting to stabilize soils while plants are growing; although, netting does not retain moisture or insulate against extreme temperatures. Mulch tackifiers made of asphalt or synthetic materials are sometimes used instead of netting to bind loose mulches.

Applicability

Mulching is often used in areas where vegetation cannot be established. Mulching can provide immediate and inexpensive erosion control. On steep slopes and critical areas, such as those near waterways, use mulch matting with netting or anchoring to hold it in place. Use mulches on seeded and planted areas where slopes are steeper than 2:1 or where sensitive seedlings require insulation from extreme temperatures or moisture retention.

Siting and Design Considerations

When possible, natural mulches should be used for erosion control and plant material establishment. Suggested materials include loose straw, netting, wood cellulose, or agricultural silage. All materials should be free of seed. Anchor loose hay or straw by applying tackifier, stapling netting over the top, or crimping with a mulch crimping tool. Materials that are heavy enough to stay in place (for example, gravel or bark or wood chips on flat slopes) do not need anchoring. Other examples of organic mulches include hydraulic mulch products with 100 percent post-consumer paper content, yard trimming composts, and wood mulch from recycled stumps and tree parts. Use inorganic mulches such as pea gravel or crushed granite in unvegetated areas.

Mulches may or may not require a binder, netting, or tacking. To ensure effective use of netting and matting material, keep firm, continuous contact between the materials and the soil. If there is no contact, the material will not hold the soil and erosion will occur underneath the material. Grading is not necessary before mulching. Use biodegradable netting, if possible.

There must be adequate coverage to prevent erosion, washout, and poor plant establishment. If an appropriate tacking agent is not applied, or is applied in insufficient amounts, mulch will be lost to wind and runoff. The channel grade and liner must be appropriate for the amount of runoff, or the channel bottom will erode. Also, apply hydromulch in spring, summer, or fall to prevent deterioration of mulch before plants can become established. Table 1 presents guidelines for installing mulches.

Table 1. Typical mulching materials and application rates

Material

Rate per acre

Requirements

Notes

Organic Mulches

Straw

1 - 2 tons

Dry, unchopped, unweathered; avoid weeds

Spread by hand or machine; must be tacked or tied down

Wood fiber or wood cellulose

½ - 1 ton

 

Use with hydroseeder; may be used to tack straw; do not use in hot, dry weather

Wood chips

5 - 6 tons

Air dry; add fertilizer N, 12 lb/ton

Apply with blower, chip handler, or by hand; not for fine turf areas

Bark

35 yd3

Air dry, shredded, or hammermilled, or chips

Apply with mulch blower, chip handler, or by hand; do not use asphalt tack

Nets and mats

Jute net

Cover area

Heavy, uniform; woven of single jute yarn; use with organic mulch

Withstands water flow

Excelsior (wood fiber) mat

Cover area

   

Fiberglass roving

½ - 1 ton

Continuous fibers of drawn glass bound together with a non-toxic agent

Apply with compressed air ejector; tack with emulsified asphalt at a rate of 25 - 35 gal/1000 ft2

Limitations

Mulching, matting, and netting might delay seed germination because the cover changes soil surface temperatures. The mulches themselves are subject to erosion and may be washed away in a large storm. Maintenance is necessary to ensure that mulches provide effective erosion control.

Maintenance Considerations

Anchor mulches to resist wind displacement. When protection is no longer needed, remove netting and compost it or dispose of it in a landfill. Inspect mulched areas frequently to identify areas where it has loosened or been removed, especially after rainstorms. Reseed these areas, if necessary, and replace the mulch cover immediately. Apply mulch binders at rates recommended by the manufacturer. If washout, breakage, or erosion occurs, repair, reseed and remulch surfaces, and install new netting. Continue inspections until vegetation is firmly established.

Effectiveness

Mulching effectiveness varies according to the type of mulch used. Soil loss reduction for different mulches ranges from 53 to 99.8 percent. Water velocity reductions range from 24 to 78 percent. Table 2 shows soil loss and water velocity reductions for different mulch treatments.

Table 2. Measured reductions in soil loss for different mulch treatments (Source: Harding, 1990, as cited in USEPA, 1993)

Mulch characteristics

Soil loss reduction (%)

Water velocity reduction
(% relative to bare soil)

100% wheat straw/top net

97.5

73

100% wheat straw/two nets

98.6

56

70% wheat straw/30% coconut fiber

98.7

71

70% wheat straw/30% coconut fiber

99.5

78

100% coconut fiber

98.4

77

Nylon monofilament/two nets

99.8

74

Nylon monofilament/rigid/bonded

53.0

24

Vinyl monofilament/flexible/bonded

89.6

32

Curled wood fibers/top net

90.4

47

Curled wood fibers/two nets

93.5

59

Antiwash netting(jute)

91.8

59

Interwoven paper and thread

93.0

53

Uncrimped wheat straw, 2,242 kg/ha

84.0

45

Uncrimped wheat straw, 4,484 kg/ha

89.3

59

Cost Considerations

Costs of seed and mulch average $1,500 per acre and range from $800 to $3,500 per acre (USEPA, 1993).

References

Harding, M.V. 1990. Erosion Control Effectiveness: Comparative Studies of Alternative Mulching Techniques. Environmental Restoration, pp. 149-156, as cited in USEPA. 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA 840-B-92-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC.

Smolen, M.D., D.W. Miller, L.C. Wyatt, J. Lichthardt, and A.L. Lanier. 1988. Erosion and Sediment Control Planning and Design Manual. North Carolina Sedimentation Control Commission; North Carolina Department of Environment, Health, and Natural Resources; and Division of Land Resources Land Quality Section, Raleigh, NC.

USEPA (U.S. Environmental Protection Agency). 1993. Guidance Specifying Management Measures for Sources of Nonpoint Pollution in Coastal Waters. EPA 840-B-92-002. U.S. Environmental Protection Agency, Office of Water, Washington, DC.

USEPA (U.S. Environmental Protection Agency). 1992. Stormwater Management for Industrial Activities: Developing Pollution Prevention Plans and Best Management Practices. U.S. Environmental Protection Agency, Office of Water, Washington, DC.

 

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