Mulch Options for Erosion Control on Construction Sites

8/12/2019 Mulch Options for Erosion Control on Construction Sites

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Soil FactsMulch Options for Erosion Mulch Options for Erosion

ontrol on onstruction SitesControl on Construction Sites Mulch Options for ErosionControl on Construction Sites

Contractors are required to use mulch on construction sites to help reduce erosion andallow newly planted seed to become established. A protective mulch application that coversat least 75 percent of the soil surface helps prevent seeds from washing away, provides abetter environment for germination, and slows runoff on slopes. Mulches used for groundcover vary in cost, application procedures, and appropriate conditions of use. Three basicclasses of mulch include loose mulch, erosion control blankets (ECBs), and hydraulically-applied mulches (hydromulches).

Groundcover Types

Loose MulchStraw is usually the least expensive mulch avail-able, with a cost as low as $0.09 per square yard.Straw mulch with seed, lime, and fertilizer rangesfrom $0.31 to $1.00 per square yard. Large areascan be covered with straw using commercial

blowers, which break up straw bales and blow thestraw onto the soil (Figure 1). Applying straw byhand is also effective but is more expensive dueto labor costs. Straw mulch is applied at a rate of

Figure 1. Applying straw with a blower pulled by a straw supply truck.

about 1 to 2 tons per acre on slopes of less than2:1 and lasts about three months, usually enoughtime to establish permanent, stabilizing vegeta-tion.

Because straw is lightweight, it is easily blownaway. Tacki ers or crimping help reduce this

problem. Common tacki ers applied on the strawinclude:

Emulsi ed asphalt, once a common tacki

er because of its excellent adhesive properties,is toxic to both plants and animals. Prohib-ited in some states and all national parks,


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emulsi ed asphalt is currently al-lowed in North Carolina and is usu-ally applied at a rate of 12 ounces

per square yard.

Wood and paper ber hydromulch-

es are natural tacki

ers that areused at a rate of 125 to 750 pounds per acre. They are less toxic thanasphalt and are more visually ap-

pealing when applied.

Guar- and starch-based tacki ersare two natural products usuallyapplied at 100 to 200 pounds peracre. Starch-based tacki ers, unlikeguar, contain 7 to 8 percent naturalnitrogen and have a high moistureretention capability, both of whichhelp establish permanent vegeta-tion.

Although the use of tacki ers in conjunc-tion with straw mulch is the most com-mon practice to prevent displacement,using heavy equipment or hand tools tocrimp or punch straw into the soilmay be suf cient when the soil is not toosandy. Ideally, some straw is partially

buried during crimping, which holds it in place and may increase in ltration rates.

Straw has a relatively high rate of failurecompared to other erosion control op-tions. Main causes of failure includeinsuf cient coverage during application,use on slopes that are excessively steepor long, or insuf cient application oftacki er. However, when properly ap-

plied on acceptable slopes, st raw can bean inexpensive and effective mulch forerosion control. The growth of vegetationin strawed areas is similar to that of othermulch types. These pros and cons aresummarized in Table 1 below.

Other loose materials used for ground

covers include compost and wood chips.Compost is made from a wide variety

of organic materials, including animalmanure, crop residue, municipal waste,

biosolids, and yard waste. As a result, itcan be highly variable in its properties.For example, the carbon to nitrogen ratio

(C:N) of manure ranges from 3 to 56,while the C:N ratio of some crop resi-dues may be more than 1,000. Materialswith high C:N ratios can stunt or killseedlings through microbially-mediateddepletion of nitrogen and other nutrientsfrom the root zone.

Compost is generally applied as a 1- to2-inch layer on top of prepared soil withseed, fertilizer, and lime previously ap-

plied. On steep slopes (2:1 or steeper), upto 4 inches of compost may be applied.

Some professional compost applicatorsinclude seed in the compost, which isrecommended if more than 2 inches ofcompost is applied. Studies have shownthat compost can be an appropriateground cover for establishing grass onlow to moderate slopes, but compost isoften more expensive than other erosioncontrol options.

Wood chips, or shredded woody ma-terials, are often generated during theclearing stage of construction when trees

are shredded using large tub grinders.Such materials are ideal for landscap-ing around established vegetation orlarge plants, but they should be appliedonly where no permanent vegetation is

planned. Similarly, wood chips shouldnot be incorporated into the soil where

permanent vegetation is planned. Typicalground covers such as grass have dif- culty growing through woody material

because as this high-carbon materialdecomposes, it removes plant nutrients

from the soil (through microbial process-es), resulting in low soil fertility.

Erosion Control BlanketsErosion control blankets (ECBs) varywidely in composition, price, and cir-cumstances of appropriate use. In gener-al, ECBs are more expensive to purchase

and more time- and labor-intensive toinstall compared to straw mulch, but arecomparable to hydromulching.

When selecting an appropriate ECB, important considerations are the slope ratio,duration of use, cost, and effectivenesscompared to other viable options. Themain types of ECBs, and guidelines foruse, are summarized in Table 2.

Despite the variety of ECB selections,the general method of installation is sim-ilar. The rst step is proper slope prepa-ration, including application of lime, fer-tilizer, and seed. The soil surface should

be uniformly graded and without rocks,large roots, or other materials that could

prevent good soil-blanket contact. Afterslope preparation, the top edge of a roll istrenched in or staked into the ground 6inches deep and buried, then rolled overitself down-slope (Figure 2).

When laying out the ECB after trench-ing, adjacent rolls should overlap by3 inches on the sides and 18 inchesdown-slope. Rolls are secured to the

ground with lawn staples at a density of1.5 staples per square yard as de ned bythe Erosion Control Technology Council.Staples are generally put a foot acrossthe top and every 3 feet along the sideson both overlapping and non-overlapping

portions. Staples should also be installeddown the middle of the roll in a diamondor square pattern.

When installing ECBs, good contactwith the soil surface is important toreduce soil bridging and washouts under

the ECB. Correct installationmakingsure the blanket is ush with the soilsurfaceis important for long-term ero-sion control and the vegetative establish-ment.

HydromulchesLike ECBs, there are many types of hy-dromulches available for erosion control.The selection of a hydromulch for a par-

Pros ConsInexpensive Not as effective in preventing soil loss as more

expensive erosion control options

Quick and easy to apply by blower May be displaced by wind

Good grass growth May introduce weed seeds

No water needed for application Fines from blower drift long distances in wind

Table 1. Pros and cons of using straw mulch for erosion control.


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Mulch Options for Erosion Control on Construction Sites

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ticular purpose depends on the slope andsoil type where the mulch is being ap-

plied, the price of mulch application, and

the effectiveness of a particular mulchtype under a unique set of circumstances.The wide variety of hydromulches allowsthe applicator to select the optimummulch for a job, although the number ofchoices can also make it dif cult to selectthe best mulch.

Unlike straw and ECBs, hydromulchingrequires specialized equipment, includ-

ing a water tank with a mixer and ahigh-powered pump to apply the mixtureto a slope. A water source near the site

where the mulch is being applied is alsoneeded. A pond or large stream may

be suitable for tank lling, or a permitcan be obtained to ll the tank from rehydrants. The amount of water needed is

based on the manufacturers recommend-ed amount of material in the tank and therecommended application rate. A samplecalculation follows, as well as a table of

water needs for different combinations(Table 3).

Unlike erosion control blankets, hy-dromulch application does not requireadvance site preparation or direct accessto the slope, making it suitable for largeslopes (Figure 3). In addition, hydro-mulch does not contain netting that couldentangle wildlife or degrade slowly.

Lime, seed, and fertilizer are sometimesmixed with the mulch, but avoid this mix

SlopeRatio

Material Longevity Notes

3:1 sod > 36 months Most expensive to purchase and install, but quickly increases propertyvalue and does not introduce weeds. Poor survival in sandy soils andsteep slopes. Must select appropriate turfgrass species. Irrigation required.

1 :1 synthetic turf reinforcementmat / geotextile ber

> 36 months Expensive but effective for high-velocity ows in channels. Good for steepslopes. Heavy.

1:1 double-net coconut blanket 36 months Good moisture absorption but poor moisture retention. Can handle highow.

1:1 double-net straw/coconutblanket

24 months Good for steep areas with medium runoff. May import weed seed.

1:1 double- or triple-net excelsior 36 months No introduction of weed seeds. Netting may entangle wildlife and lastyears.

2:1 double-net straw blanket 12 months Lightweight and fast to degrade. May import weed seed. 3:1 single-net excelsior 12-24 months No introduction of weed seeds. Netting may entangle wildlife or may be

removed.

3:1 net-free excelsior 12-24 months Does not entangle wildlife or introduce weed seeds. 100% biodegradable. 3:1 single-net straw blanket 12 months Lightweight and fast to degrade. May import weed seed.

Table 2. Types and circumstances for using sod and erosion control blankets, arranged from most expensive (top) to leastexpensive (bottom). Adapted from Erosion Control Technology Council, 2002. Longevity estimates from Erosion ControlTechnology Council, 2002. Ranking based on cost estimates from Daillaire (2002) and Missoula Technology DevelopmentCenter (2006).

Figure 2. Trenching in an ECB to avoid undercutting near the top of a slope. Adapted from L. Honnigford, Erosion ControlTechnology Council, 2007.


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if possible. Lime and fertilizer can harmthe seed when left in the tank for toolong and are actually much more effec-tive when incorporated into the soil priorto seeding and mulching. Seed applied

with the mulch may land on the mulchsurface, where it is unlikely to survive.Hydromulch is best applied after liming,fertilization, and seeding.

When applying hydromulch, followthe manufacturers directions for rateand method of application. The rate ofhydromulch depends on the slope whereit is being applied. Mulch can be appliedat a known rate by calibrating the sprayhose, but many applicators prefer toapply the mulch by eyeballing it until

coverage is suf

cient. Regardless of theapplication method, the mulch should be applied from both the top and bot tomof a slope to ensure uniform coverage.Several basic types of hydromulches arediscussed in Table 4, along with notesconcerning application and appropriatecircumstances of use.

Hydromulching cost varies with thetype of mulch selected, the applicationequipment, water availability, and areasize. The least expensive products are

paper based, whi le the most expensiveare bonded ber matr ix products. Ap-

plication costs can range from $0.41 to$1.15 per square yard, not including seed,

Amount of hydromulch to mix in tank: 40 pounds of mulch per 100 gallons ofwaterApplication rate: 3,000 pounds of mulch per acreWater needed: 3,000 pounds of mulch per acre * (100 gallons of water / 40pounds of mulch) = 7,500 gallons per acre

Mulch Mix Rate(lb per 100 gallons)

Appl ication Rate(lb per acre)

Water Needed(gallons per acre)

40 3,000 7,500

40 4,000 10,000

50 3,000 6,000

50 4,000 8,000

60 3,000 5,000

60 4,000 6,700

Table 3. Volume of water needed for various mixing rates and application ratesof hydromulch.

Figure 3. Hydromulching a large slope. This process should be repeated fromthe bottom of the slope. Note the berm at the top of the slope to prevent runofffrom the upper areas from running onto the slope.

Table 4. Various types of hydromulches, adapted from Erosion Control Technology Center descriptions.

SlopeRatio

Material Rate(lbs per ac)

Description

2:1 Stabilized mulch matrix 1,500-2,500 Organic bers with soil occulants or cross-linked hydro-colloidal polymersor tacki ers. Used to provide erosion control and facilitate vegetative es-tablishment on moderate slopes. Designed to be functional for a minimumof 3 months.

2:1 Bonded ber matrix 3,000-4,000 Organic bers and cross-linked insoluble hydro-colloidal tacki ers. Usedto provide erosion control and facilitate vegetative establishment on steepslopes. Designed to be functional for a minimum of 6 months. May need 24h cure time.

2.5:1 Fiber reinforced matrix 3,000-4,500 Organic de brated bers, cross-linked insoluble hydro-colloidal tacki ers,and reinforcing natural or synthetic bers. Used to provide erosion controland facilitate vegetative establishment on very steep slopes. Designed tobe functional for a minimum of 12 months.

6:1 Hydraulic mulch 1,500 Paper, wood or natural bers that may or may not contain tacki ers. Usedto facilitate vegetative establishment on mild slopes. Designed to be func-tional for up to 3 months.


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Mulch Options for Erosion Control on Construction Sites

fertilizer, or lime. As a general rule, themore expensive hydromulches, such as

bonded ber matrices, tend to offer bet-ter protection against erosion, but actualresults are site-speci c.

SummaryRegardless of the type of erosion controlmeasure used, the long-term ef cacyof the ground cover depends on properslope preparation and correct installa-tion. Considerations for proper slope

preparation include lling any existingrills or gullies and applying the appropri-ate amounts of lime, fertilizer, and thecorrect seed mixture. Seeding blendsand guidelines depend on the geographiclocation, slope, soil condition, and time

planting. For North Carolina, goodresources to nd this information are the

North Carolina Erosion and SedimentControl Planning and Design Manual(http://www.dlr.enr.state.nc.us/pages/

publications.html#eslinks) and the NorthCarolina Department of TransportationWeb site (http://www.ncdot.org/doh/operations/dp%5Fchief%5Feng/roadside/soil%5Fwater/special_provisions/).

SourcesCalifornia Stormwater Quality Association. 2003. California stormwater BMP hand-

book. .

Carlson, B. 2003. Tacki ers put to the test: Erosion and sediment control comes to a

natural conclusion. Land and Water 47( 4): 10.Dallaire, G. 2002. Cost-effective erosion control with blankets and mats. Erosion Con-

trol 2 (May/June). .

Erosion Control Technology Council. 2008. Erosion control product categories. .

Erosion Control Technology Council. 2002. Slope gradient selection chart. .

Honnigford, L. 2007. Guidelines for installing rolled erosion control products in slopeand channel applications. Erosion Control Technology Council. .

Honnigford, L. 2002. Stabilize soil and increase pro ts: Guidelines for installing

rolled erosion control products. Erosion Control Technology Council. .

Missoula Technology Development Center. 2006. Wilderness and backcountry siterestoration guide: Tools of the trade and other resources. .

Pack, J. 2007. Hydromulch passes test for steep slope stabilization with the North Caro-lina DOT. Land and Water 51 (6): 15-17.

Propex Geosynthetics. 2007. Installation guidelines for Landlok ECBs. .

Reinco Inc. 2000. Hydrograssing and power mulching: A guide to methods and costanalysis. .

State of California Department of Transportation. 2003. Guidance for temporary soilstabilization. .

Distributed in furtherance of the acts of Congress of May 8 and June 30, 1914. North Carolina State University and North Carolina A&T State University commit th emselvesto posit ive action to secure equal opportunity regardless of race, color, creed, national origin, religio n, sex, age, veteran status or dis ability. In addition, the two Universitieswelcome all persons w ithout regard to sexual orientation. North Carolina State University, North Carolina A&T State University, U.S. Department of Agr iculture, and localgovernments cooperating.

Prepared byDeanna Babcock,

Extension Associate and

Richard A. McLaughlin,Associate Professor and Soil Extension Specialist

Department of Soil Science, North Carolina State University

The Copyright 2008 by North Carolina State University

Published by North Carolina Cooperative Extensionhttp://www.soil.ncsu.edu/publications/Soilfacts/AG-439-67W/

AG-439-67W E09-50336VBRM-H

Mulch Options for Erosion Control on Construction Sites

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