Using Compost for Erosion Control & Stormwater Management
Raindrops hit the soil surface and displacement of the soil particles occurs (known as ‘splash’ erosion). On slopes, the soil particles tumble, causing ‘rills’ to form. Rills become more pronounced and they channel the water, increasing its velocity and ability to carry away soil particles. Pollutants such as heavy metals, nutrients and petroleum contaminants are carried off with the soil particles. The finer the soil particles (silt and clay) the more chemically reactive they are, so the greater the amount of contaminants they can bind up. Moreover, these finer soil particles often evade capture in well-known sediment control devices, such as silt/sediment fences, and water can carry these fine soil particles a long distance. This is the typical sequence of erosion on slopes where there is little or no vegetation and where no effective erosion control measures have been implemented.
In order to protect surface water quality, the United States Environmental Protection Agency (USEPA) and its delegated state environmental agencies implemented requirements that outline erosion and sediment control practices, as well as approved management techniques. To meet these requirements, each state implements its own erosion and sediment control policies and techniques, and often a set of best management practices (BMPs). Many agencies have amended the existing regulations and BMPs since the establishment of the National Pollutant Discharge Elimination System (NPDES) Phase II regulations. Among other things, these new requirements call for the establishment of erosion and sediment control plans, and appropriate measures for all construction projects of one acre in size or greater.
1 Often states, counties, and cities develop and implement their own set of regulations and BMPs. These BMPs or approved technologies and products vary based on their field application. Compost-based technologies for erosion and sediment control have been proven in the field and in research, and widely adopted over the past 15 to 20 years. The American Association of State Highway and Transportation Officials (AASHTO) has even approved a series of techniques using compost as a soil erosion control strategy. Many State Departments of Transportation, including PennDOT, have also adopted these techniques (and related specifications).
Compost blankets, berms, and socks are used for temporary and permanent erosion and sediment control (TESC) on construction sites, roadside and other projects. Unlike other erosion-control BMPs that are often expensive to remove and dispose of, compost can be left onsite permanently to enhance plant growth. In many situations, seed is applied along with the compost which acts as an enhanced soil. This provides for extensive rooting and a much better plant survival, long-term growth, and erosion prevention, even on difficult sites.2
It is important to note that a courser grade of compost (typically a ¾ or 1-inch screened compost) is typically used in these erosion control applications. When compared to the more finely screened composts (3/8 to ½ inch screened), typically used in landscape soil incorporation applications, these coarser composts are found to be more effective. You can find the AASHTO compost erosion control specifications at www.alexassoc.net (click on Library of Articles, then “erosion control” (within the compost specification section).
Probably the greatest advantages to using compost-based systems are that they:
1) Are cost competitive with existing methods and practices used today,
2) Provide immediate and effective control,
3) Improve the establishment of a vegetative cover,
4) Perform as a filter to capture sediment but let water flow through,
5) Bind and/or degrade many chemical contaminants, including heavy metals (e.g., Cadmium, Lead), nutrients, and hydrocarbons, and buffer alkaline pH (e.g. concrete wash-off) toward neutral, and
6) Improve the health of soils, creating a stronger base for long-term retention of vegetation and infiltration of water.3
Compost Blankets – are usually 1 to 2 inches thick. They are installed by pneumatically blowing compost onto up to 2:1 slopes, or on shallower slopes by spreading compost with conventional equipment. Compost blankets have also been used successfully on up to 1:1 slopes, with additional stabilization technologies used in conjunction. This technology is ideal for managing water running in ‘sheet’ flows, but not those running in concentrated flows (e.g., channeled water). As with any surface BMP, it is important to prevent “run on” of concentrated flows from the top of the grade that is being protected. In this application, the coarser compost particles (woody fraction) disperse the energy of the raindrops, while the fine particles absorb the water. Because compost is so absorbent, it can significantly reduce the volume of water leaving the treated slope. For this reason, compost blankets are also seen as a low cost method to manage excess stormwater.
Compost Berms – are a method of perimeter sediment control, as well as used in series running down a slope to capture sediment and reduce the velocity of storm water. Compost berms are increasingly preferred over silt fencing. Berm widths are typically twice their height, so a 1-foot high berm is 2 feet wide. Berms are typically installed using a pneumatic blowing unit, but can also be placed using a front-end loader. The compost used in berms (as well as compost filled socks) is coarser than the blanket compost and should also contain fewer ‘fine’ particles. In the case of both berms and socks, the coarse compost is used as a 3-dimensional water filter. Therefore, the compost is designed for the water to pass through it, leaving sediment on the slope side of the berm. Many university research studies have proven that compost filter berms and socks are many times better than silt fences at removing fine soil particles.
Compost Socks – are another perimeter control measure which are fast replacing silt fences and straw bales. They may be quicker and cheaper to install with a blower truck than compost berms, because they require about 1/3 the volume of compost. They control sediment and other pollutants well, because they possess a large filtration capacity and maintain excellent ground contact. You can chose biodegradable sock material for installations lasting up to 6 months, but if non-biodegradable socks are used, you still need only remove the light fabric, and can leave the compost on-site. This is much cheaper than silt fence removal. Socks can be filled in place by compost suppliers with a blower truck. Suppliers can also deliver filled socks on pallets.
Unlike many BMPs (wattles, silt fence), you do not have to trench socks in. Just walk along the top of them to ensure good soil contact, and stake through the sock on slopes. The great advantage of using compost-filled socks is that you can attach them to the soil surface using stakes, thereby allowing their use in concentrated water flow situations.
Much like any erosion and sediment control device, compost-based technologies should be used where appropriate and be installed properly. Further, the appropriate grade of compost should be used to obtain the best results. This should not be difficult for those located in Pennsylvania and Delaware, however, as many composters are now producing erosion control grade composts. But don’t just use this technology because it’s ‘green‘, use it because it’s extremely efficient and cost effective. Remember, these compost-based technologies have been well-tested and proven successful in difficult field conditions around the US, and internationally.
1 National Erosion/Sediment Control Specifications for Composted Products, The Recycled Materials Resource Center, University of New Hampshire. 2003.
2 Erosion Control with Compost. Seattle Public Utilities, www.Building Soil.org. 2008.
3 National Erosion/Sediment Control Specifications for Composted Products, The Recycled Materials Resource Center, University of New Hampshire. 2003.
Mr. Alexander has over 30 years of experience working with compost and other organic recycled products on large-scale construction projects. He began his career in Philadelphia, and is now the President of R. Alexander Associates, Inc., a consulting company specializing in product development for organic recycled products. He is the author of the ‘Field Guide to Compost Use’, American Association of State Highway and Transportation Officials’ Erosion Control Specifications for Compost, and ‘Landscape Architecture Specifications for Compost Utilization’.
He provides technical assistance to Laurel Valley Soils, a large composter in Avondale, PA (http://laurelvalleysoils.com/). Contact Ron Alexander at email@example.com or 919-367-8350 if you are interested in additional information on compost use or a related Lunch & Learn.