Several site challenges can compromise the effectiveness of green stormwater infrastructure. Here are strategies for overcoming them.
LET'S SAY YOU WANT TO DESIGN a rain garden, but the heavy clay soil won’t allow water to infiltrate effectively. Or you want to direct roof runoff into a swale, but you’re dealing with a tiny lot.
“Green infrastructure” refers to the host of techniques and strategies that slow stormwater runoff, clean and filter stormwater and enhance groundwater recharge. These include rain gardens, bioswales, permeable pavement, “green” parking areas, trees and disconnected downspouts. Designers must always choose from this menu of practices, selecting those best suited to the site. However, sometimes restrictive site conditions can be addressed—and the options widened. Here are the most common issues, along with the solutions that will help you design around them.
The roots of native prairie plants penetrate five to 15 feet into the soil. In contrast, shallow-rooted turfgrasses don’t penetrate beyond a few inches.
1. Slow Soils
Soils that are dominated by clay and/or glacial till often have low infiltration rates, and stormwater may run off before it can be captured on the site. But it is still possible to design infiltration-based stormwater controls, such as rain gardens and swales, for sites with these soils. Take these steps to ensure your plan succeeds:
Measure infiltration rates. This should be done before designing stormwater controls. A generally accepted guideline is that the infiltration rate of native soils beneath swales and rain gardens should be greater than 0.25 to 0.5 inches/hour.
Amend soils. Adding compost or other organic matter can increase soil infiltration rates, while improving the soil’s fertility and its ability to remove pollutants
Go deep. Enhance soil infiltration rates by planting deep-rooted vegetation. The roots create small conduits for water to infiltrate and increase biological activity in the soil. The U.S. Geologic Survey found that the median infiltration rate of a clay soil planted with prairie species was more than three times the rate of a clay soil planted in turf (0.88 inches/hour compared to 0.28 inches/hour).
Expand storage. Including a larger storage layer with an underdrain can significantly slow peak flows and increase infiltration, protecting stream banks and potentially reducing combined sewer overflows.
Use alternative practices. For sites for which mitigating clay soils is too difficult or expensive, choose features that do not require infiltration. These include rainwater harvesting, green roofs and vegetated swales.
2. Poor Urban Soils
It’s especially important to manage stormwater on urban lots, since urban areas are often dominated by impervious surfaces. But the soils on these lots are often severely compacted, lack sufficient organic matter and/or contain construction debris. (This is, of course, also a problem if you want to incorporate other types of landscaping and/or grow food.) Fortunately, there are a number of methods for transforming these soils.
Before doing anything, the soil should be evaluated for its structure, percentage of organic matter and possible presence of contaminants.
Some landscape rakes, such as this model from Wicko, even have hoppers that collect the stones and other debris.
Physical reconditioning. These methods physically change the characteristics of the soil to improve its structure.
Raking can remove construction debris, stones, loose root pieces and other objects as small as ¾”. The jobs can be done by hand if the area is small; otherwise a landscape rake and a tractor can make quick work of it.
Tilling requires turning over and/or mixing the top 12 to 24 inches of soil, and is a time-honored method for loosening and aerating soil. For more extreme cases, “subsoiling” may be required. This involves fracturing the soil at deeper levels with a machine consisting of metal shanks and either pointed or winged tips.
You can improve drainage through grading or by altering surface drainage routes, or by amending the soil. Proper plant selection can mitigate areas where changing drainage and infiltration rates is difficult (for example, in naturally low, wet spots).
As a last resort, you can remove the soil from the site. This extreme (and extremely expensive) measure should only be taken when no other options are available.
Soil amendments and additives. Physical, organic, biological, mineral and chemical amendments can improve soil structure and fertility.
Physical amendments include structures such as geowebs and turf cells, which stabilize soils within the root zone.
Organic amendments include compost. One of the best all-around amendments, compost can improve both soil structure and fertility; it also stimulates biological activity and promotes healthier, more disease-resistant plants. The quality of compost can vary, so make sure you know the source. If making it yourself, make sure the organic materials are thoroughly broken down before using.
Mineral amendments include perlite, hadite and pumice. These commonly used minerals help create large pores in the soil through which both air and water can move; they are also sterile and dimensionally and chemically stable. Sand and gravel improve infiltration and are good materials to layer under rain gardens and other stormwater infrastructure, but by themselves they will not correct the drainage issues with clay soils.
When adding earthworms to soil, they need to be “planted” several inches below the surface. They also need organic matter for a food source.
Biological amendments improve the diversity and number of soil organisms. Typical amendments include mycorrhiza, a symbiotic fungus that colonizes plant roots, and compost teas, which inoculate soil with microorganisms. “Macrofauna” such as earthworms provide a number of benefits, including improved pore structure and detoxification of contaminants.
Chemical amendments, also known as fertilizers, are used to change soil pH or add missing nutrients, but in general, they are the least effective type of amendment and can have unintended consequences. They can also run off the landscape and end up in waterways, especially if over applied. Chemical amendments should be used only after other methods have been tried.
Mulch. Anything added on top of the soil layer can be called mulch, and common mulching materials include peat moss, leaves, wood chips, bark, compost, rice hulls and straw, and synthetic materials such as sheet plastic and geotextiles. Mulch protects soil, keeping it cool and moist and stimulating biological activity. A layer of mulch can also help soils recover from mild compaction or prevent further compaction.
Cover crops. Planting a cover crop such as oats, clover or alfalfa is a good strategy for improving the soil before installing a rain garden or other stormwater feature. Cover crops can add organic matter, stimulate biological activity, inhibit weeds and protect the soil from erosion and moisture loss. Cover crops can be sown in the fall or summer.
Bioremediation. Taking advantage of organisms’ natural process to repair soil is called bioremediation. Plants, microorganisms and/or soil amendments such as compost can all be used to digest harmful chemicals and transform them into nontoxic byproducts. Phytoremediation refers to the practice of using plants to take up harmful chemicals from the soil and groundwater and storing them in their roots, stems or leaves. The plants are usually harvested once they have done their job.
3. Brownfield Sites
If your project involves redeveloping a brownfield site, the possible presence of contaminants could affect stormwater infrastructure, especially if the features rely on infiltration. These features must be designed carefully, so contaminants in the soil are not mobilized, which could increase the risk of groundwater contamination.
Lead Removal Best Practices
- Remove soils and replace.
- Add clean soil on top of the lead-contaminated soil.
- Maintain soil pH levels above 6.5 by adding organic matter. Lead binds to organic matter, making it less available to plants.
Any lot that’s being redeveloped could house contaminants in the soil, so it’s a good idea to take the following steps before designing green infrastructure or other landscaping.
Learn the history of the lot. Contact previous property owners and take advantage of public records available at county offices, planning departments and historical societies. In general, lots that were previously used for housing are less likely to have soil contaminants than those used for industrial purposes.
Conduct a field survey. Observe drainage patterns, identify foundations of previous structures and look for evidence of compaction.
Sample the soil. At a minimum, the soil test should include pH, percent organic matter, nutrients, micronutrients and metals, including lead. Soil samples can be sent to USDA Cooperative Extension System offices, land grant universities or private local laboratories.
In 2013, EPA released Implementing Stormwater Infiltration Practices at Vacant Parcels and Brownfield Sites (http://bit.ly/1RqP0aV). This document guides decision-makers through a series of questions to determine whether infiltration or other stormwater management approaches are appropriate for a specific brownfield property.
4. Sediment-Laden Stormwater
In arid regions, bare soils are common and rates of erosion and sedimentation are relatively high. Stormwater flows can deliver fine sediments to infiltration features, clogging and degrading their performance. The following strategies are recommended for infrastructure in areas with high sediment loads.
Mulch it. Mulch acts as a filter for sediments carried in stormwater. Add a mulch layer above infiltration practices and replace the layer when it is filled to protect the soil and gravel layers below from sedimentation.
Trap it. A “sediment trap” is a small depression bordered by a small berm that captures and collects sediment at the entrance to a bioretention area, such as a rain garden. Use traps at the inflow of green infrastructure features to facilitate the removal of accumulated sediment and prevent the feature from becoming clogged.
Maintain it. If you include a mulch layer or sediment trap in your stormwater management practice, regularly remove the accumulated sediment to maintain its function.
5. Limited Irrigation Supply
Plants chosen for green roofs in arid and semi-arid regions should be able to tolerate wind and temperature extremes, as well as periodic drought.
Limited water resources can be a barrier to green infrastructure in arid and semiarid regions. Follow the principles of xeriscaping to conserve water, and create a plan that balances water supply and demand. You will first need to determine the annual water budget (assuming the feature will use native plants at native densities).
Use low-water use plants. You can drastically reduce, if not eliminate the irrigation requirements of green infrastructure by using native and drought-tolerant plants. These include drought-tolerant shrubs and trees. Rain garden plants should be able to tolerate occasional inundation.
Use efficient irrigation. Make your irrigation systems most efficient by grouping plants according to their water needs, and by adjusting the frequency and depth of irrigation according to plant type, plant maturity and season.
Amend it. Healthy soils are essential to retaining soil moisture, sustaining vegetation, and treating stormwater runoff. If your site’s soils are poor, amend them with organic material.
Mulch it. Organic mulch can increase water retention and pollutant removal while building soil structure and suppressing weeds. Note, however, that many desert trees and shrubs react poorly when their trunks come in contact with mulch.
Maintain it. All landscapes require maintenance; xeriscaping is no exception.
6. Space Constraints
Many green infrastructure features require land area to allow stormwater to infiltrate into the soil. This can pose a challenge when space is limited (for example, in a retrofit project or tight urban lot). Designers have developed strategies for overcoming this challenge:
Features that serve multiple purposes. Integrate swales and bioretention areas into landscaped areas, medians or parking strips. Permeable pavements provide volume reduction and water quality treatment without requiring additional space.
Features for small spaces. Planter boxes, tree pits and other green infrastructure features can be custom designed to fit into small spaces.
Subsurface storage. Underground storage or infiltration tanks can serve as an alternative when space is too limited for any surface features.
7 Steps to Healthier Soils
Working in compost can improve drainage or moisture retention, encourage biological activity and grow healthier plants. A good rule of thumb is to add two inches of compost over the entire area and work it in to a minimum depth of six inches, adjusting this depth to accommodate tree roots and other obstacles.
Here’s a simple formula for estimating the volume needed:
One inch compost spread over 1,000 square feet = three cubic yards.
Here are some general guidelines for application:
- Rototill to a depth of six to eight inches. If the soil is too dense for a rototiller, the soil should first be broken up into large aggregates using a soil ripper.
- Clear obstructions. The soil surface should be reasonably free of large clods, roots and stones greater than two inches.
- Distribute compost evenly to a depth of two inches over the soil surface. For small areas, compost may be spread by hand with a shovel and raked evenly over the soil. For larger areas, use a tractor-mounted spreader or similar device.
- Spread lime and nutrients, if indicated by soil testing.
- Rototill several times in perpendicular directions to incorporate compost and other soil amendments.
- Water thoroughly. Allow soil to settle for one week; if compost is immature, extend settling period to two to five weeks.
- Finely grade soil to desired evenness.
REPRINTED IN PART FROM EPA PUBLICATIONS, EDITED BY JULIET GRABLE