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*[[Construction stormwater photo gallery]]
*[[Construction stormwater photo gallery]]
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. See Section 8.
Temporary and permanent stabilization with mulch or other products is highly effective in reducing soil loss from construction sites. 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. The following tables summarize the effectiveness and 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.
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|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|
|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
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|Flow attenuation||Little or no design benefit|
|Runoff volume reduction||Little or no design benefit|
|Soil erosion||Primary design benefit|
|Sediment control||Little or no design benefit|
|Nutrient loading||Primary design benefit|
|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|
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 the following.
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 to 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 the following.
The current list of MnDOT certified/approved vendors for mulch are available on the MnDOT website:
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 following tables list various mulch types, recommended application method, application rates, and pros/cons of different products. Note that rolled erosion prevention/control products (temporary erosion control blankets or permanent turf reinforcement mats) are often used instead of mulch.
Types of mulch products typically used on construction sites
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|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|
Mulch types application rates benefits, and limitations
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|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.
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 below.
Applied at a rate of 5 to 8 tons per acre, this mulch material should also be evenly distributed across the surface to a depth of about 2 inches. If soil building and revegetation are desired, increase the application rate of nitrogen fertilizer by 20 pounds of nitrogen per acre. This compensates for the temporary loss of available nitrogen to soil microbes as they break down the carbon-rich mulch.
Rock may be useful for stabilizing long slopes that will not support thickly seeded grass. Install non-woven geotextile on graded slopes and place rock of mixed sizes on the geotextile, starting at the bottom and working uphill. Generally, rock is not suitable for residential or other areas where aesthetics are a design consideration.
Hydraulic mulch (also known as hydromulch) can be used to rapidly stabilize critical areas that are difficult to reach or are located on slopes greater than 3:1. The specific composition of the mulch and application rates vary. In general, a hydraulic mulch is a processed material that can be applied in a continuous stream when mixed with water, and can vary in type, composition, additive materials, and durability (i.e., from light-duty to heavy-duty).
When applied, hydraulic mulches form a thick crust or mat-like barrier that controls water and wind induced erosion.
Hydraulic mulches can be made of recycled newsprint, magazines, wood, or other wood/paper waste sources. Many proprietary products feature a mix of natural and synthetic fibers and cellulose. This type of mulch is typically mixed in a hydraulic application machine (hydroseeder) and applied via sprayer as a liquid slurry at a minimum rate of 1.5 tons per acre. The slurry usually contains a dye to aid in visual metering during application, although the dye must be biodegradable and not inhibit plant growth. Hydraulic mulches can also contain the recommended rates of seed and fertilizer for the site, and be specified with or without a tackifier. Hydraulic mulches from wood and paper fiber are combination mulches generally composed of 70 percent wood fiber and 30 percent paper fiber, and manufactured from lumber mill waste, virgin wood chips, recycled newsprint, office paper, and/or other waste paper.
One or two application rates are generally specified for hydraulic mulch. The first is the blanket equivalent rate required for erosion prevention (usually between 3,000 and 4,000 pounds per acre). The other, typically half the erosion blanket control rate, is useful for enhancing seed germination and soil stabilization where slopes are 6:1 or flatter. The following table summarizes MnDOT approved products for hydraulic mulch.
MnDOT approved - qualified hydraulic mulch products
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|Brand/Model Name||Manufacturer||Date Approved|
|Hydraulic Compost Matrix|
|TerraVita HGM||Organic Earth Industries||4-2014|
|Biotic Earth Black||ErosionControlBlanket.com||11-2014|
|Hydraulic Mulch Matrix|
|Verdyol Virgin Plus||ErosionControlBlanket.com||10-2015|
|Excel Fibermulch II Bindex Wood WT||American Excelsior Co||5-2011|
|Second Nature Wood Fiber Plus Enviro-Gold Plus Second Nature
Wood Fiber Blend Plus Enviro Mix Plus (40-60%)
|Central Fiber Corp.||3-2014|
|HydroStraw Guar Plus HydroStraw Straw Lock||HydroStraw||9-2014|
|Mat-Blend Plus, Mat Fiber Plus||Mat Inc.||4-2014|
|Stabilized Fiber Matrix|
|Spray Guard||Mat Inc.||4-2014|
|Terra-Matrix SMM||Profile Products||9-2014|
|Hydra CM||Tensar/No. Am Green||9-2014|
|Bonded Fiber Matrix|
|Verdyol Virgin BFM||ErosionControlBlanket.com||10-2015|
|Bindex BFM||American Excelsior Co.||5-2011|
|Spray Matt, Enviro Matt||Central Fiber Corp.||3-2014|
|Soil Guard||Mat Inc.||4-2014|
Hydro Blanket BFM
|Fiber Reinforced Matrix|
|Central Fiber Corp.||3-2014|
|Flex Guard||Mat Inc.||4-2014|
|Hydra CX||Tensar/No. Am Green||11-2014|
|Earth Guard Fiber Matrix||Terra Nova||3-2015|
|Flexterra HP||Profile Products||4-2014|
Important considerations for hydraulic mulch application include the following.
MnDOT Specification 2575.3 part C (page 504) provides guidance for mulch application, including application methods, MnDOT mulch types, and winter considerations. Part D (page 504) covers disk anchoring of mulch (applicable to Types 1, 3, and 8). Part K.4 (page 508) covers maintenance of mulch. Specification 2575.4.C (page 510) prescribes requirements for measurement of mulch, and Specification 2575.4.E (page 510) covers measurement of disk anchoring. Mulch should meet Specification 3882 (“Mulch Material”; pages 675-676).
Inspect mulched areas weekly and after rainstorms to check for rill erosion, dislocation, or failure. Repair or replace any bare areas promptly. If properly applied and anchored, little additional maintenance is required for mulch during the first few months. After high winds or significant rainstorms, mulched areas should be checked for adequate cover and re-mulched if necessary. For permanent stabilization, mulch needs to last until vegetation is well established to provide permanent erosion resistant cover. Straw mulch can last from 6 months to 3 years, depending on local conditions. For permanent stabilization, maintain downgradient sediment controls (e.g., silt fence, fiber rolls) until dense vegetation with fairly uniform coverage is established. Irrigate seeded areas if dry conditions hinder germination or early growth. Cover bare or sparse areas with mulch until they can be reseeded or inter-seeded, which should be done at the earliest opportunity.
MnDOT’s workmanship and rework schedule in the following table (2016; version under development at the time of manual update) identifies common deficiencies for various types of stabilization BMPs – including mulch – and corrective actions for these deficiencies. Once complete, the full, final version of this table will replace Table 2575-4 in MnDOT Standard Specifications for Construction (2016 edition).
Excerpt from Table 2575-4 - Required Corrective Action
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|Item||Corrective Action Required if:||Corrective action|
|Mulch material, hydraulic erosion control products||
The following table summarizes estimated BMP costs based on MnDOT data summarizing average bid prices for awarded projects in 2014.
Average mulch bid prices for spec year 2014
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|Bid Item||Item Description||Units||Average Price|
|2575.511/00010||Mulch Material Type 1||ton||💲178.58|
|2575.511/00030||Mulch Material Type 3||ton||💲411.45|
|2575.513/00050||Mulch Material Type 5||CY||💲13.63|
|2575.513/00060||Mulch Material Type 6||CY||💲65.27|
|2575.513/00090||Mulch Material Type 9||CY||💲64.92|
|2575.562/00020||Hydraulic Matrix Type Mulch||lb||💲0.65|
|2575.562/00030||Hydraulic Matrix Type Bonded Fiber||lb||💲11.00|
|2575.562/00040||Hydraulic Matrix Type Fiber Reinforced||lb||💲1.46|
CY = cubic yard
Except where more stringent requirements are presented in this guidance, BMPs shall comply with MnDOT and other state requirements. Primary design references include.
The following is a list of additional resources that are not specific to Minnesota.