Green Infrastructure Practices

Part of Nashville's ability to meet the challenge of balancing growth and environmental protection is through incorporating Green Infrastructure into both new and redevelopment efforts. Investing in Green Infrastructure offers the opportunity to enhance the existing infrastructure and protect the environment while simultaneously creating new green jobs, creating demand for green technologies, and revitalizing local neighborhoods. Twelve of the most common Green Infrastructure practices are listed below.

1. Green Roofs

Benefits can include: reduced runoff volume, reduced peak discharge rate, reduced pollutant loading, reduced runoff temperature, enhanced site aesthetics, reduced building energy use, and reduced urban heat index.

Green roofs, also known as vegetated roofs, ecoroofs, or nature roofs, are structural components that capture, filter, and detain rainfall. Vegetated roofs can be constructed over any type of roofing material, providing that the roof itself can handle the weight of the vegetation. The system consists of waterproofing material, growing medium, and vegetation.

Most green roofs can be considered “intensive” or “extensive,” depending on the type of vegetation used and the roof area's planned usage. Intensive vegetated roofs employ a wide variety of plant species that require deep layers of media and continuous maintenance. This type is generally limited to flat roofs, and often serves as a parklike area accessible to the public. Extensive vegetated roofs have shallow vegetation, usually 4 inches or less, consisting of succulents, grasses, herbs, or mosses. This type of roof requires minimal maintenance, and is generally not open for public access.

A major benefit of green roofs is their ability to absorb stormwater and release it slowly over a period of several hours. They help reduce the volume of runoff as well as the amount of pollution entering local drainage systems and, ultimately, receiving waters. In addition, adding vegetation to a roof will provide protection from ultraviolet radiation and extreme temperature fluctuations, two elements that cause standard roof membranes to deteriorate, and reduce urban heat island effects.

2. Downspout Disconnection

Benefits can include: reduced runoff volume, reduced peak flow rate, and improved water quality.

Downspout disconnection is the process of separating roof downspouts from the sewer system and redirecting roof runoff onto pervious surfaces, most commonly a lawn. This gives the roof runoff the opportunity to infiltrate, reducing the volume of runoff that must be captured by additional GI practices, or runoff into the public stormwater system.

3. Filter Strips

Benefits can include: reduced peak discharge rate, reduced TSS, reduced pollutant loading, and enhanced site aesthetics.

Filter strips are uniformly sloped areas of dense vegetation that act as a buffer often used to provide providing water quality pretreatment to runoff flowing from its source to another GI Practice. Water quality improvements are accomplished through infiltration and vegetative filtering of sediments, organic matter, nutrients, and pesticides.

Treatment effectiveness depends on dense vegetation and sufficient contact time. Filter strips are not designed to provide storage of large runoff volumes, but can significantly reduce the volume of runoff from small, frequently-occurring storms if the soils are sufficiently pervious. Filter strips increase surface roughness, reducing runoff velocity and thereby decreasing peak discharge rates.

4. Infiltration Practices

Benefits can include: reduced stormwater volume, reduced pollutant loading, and increased groundwater recharge.

Infiltration practices are GI practices, such as infiltration trenches and dry wells, that enhance water percolation through a media matrix that slows and partially holds stormwater runoff, facilitating pollutant removal.

Infiltration trenches are excavated 3 to 12 feet deep, lined with filter fabric, and filled with stone, allowing stormwater runoff to infiltrate into surrounding soils through the trench's bottom and sides. Dry wells are pits filled with gravel or stone aggregate and are designed to catch stormwater from roof downspouts or paved areas. Water quality is improved through filtering by the media and surrounding soils, allowing these infiltration techniques to remove a variety of pollutants.

5. Pocket Wetlands

Benefits can include: reduced peak discharge rate, reduced TSS, reduced pollutant loading, and enhanced site aesthetics.

Pocket wetlands are shallow marsh-like systems constructed to control stormwater volume and remove pollutants for drainage areas of 5 to 10 acres. Because they are engineered structures, pocket wetlands have less biodiversity than natural wetlands yet still provide robust pollutant removal and habitat value.

Pollutant removal in these systems occurs through settling, microbial biodegradation, and uptake by vegetation. By increasing the duration of discharge and controlling stormwater volume, pocket wetlands are able to significantly reduce peak discharge.

6. Permeable Pavement

Benefits can include: reduced runoff volume, reduced peak discharge rate, reduced TSS, reduced pollutant loading, reduced runoff temperature, groundwater recharge, reduced heat island effect, and dual purpose.

Permeable pavement allows stormwater to pass through voids in the paved surface and infiltrate into the sub-base. Permeable pavements may be constructed of four basic material types: porous asphalt; porous concrete; interlocking paver blocks; and plastic grid. Permeable pavements may be designed to exfiltrate captured runoff into the subsoil, discharge stored runoff into existing storm drains, or store runoff for use in irrigation.

The amount of exfiltration depends on the permeability of the existing soil. Permeable pavements simultaneously serve as hardscape and as stormwater infrastructure, and are therefore especially practicable where space constraints preclude the use of other water quality GI practices.

7. Rainwater Barrels/Cisterns

Benefits can include: reduced runoff volume, reduced peak discharge rate, reduced TSS, reduced pollutant loading, and reduced potable water demand.

Rain barrels and cisterns store rooftop runoff. The stored water is a source of untreated ‘soft water', free of most sediment and dissolved salts and ideal for later reuse in lawn and garden watering. Rain barrels are most often used for private residences while cisterns are typically larger, can be stored above or below ground, and have both residential and commercial applications.

Rain barrels and cisterns can effectively reduce runoff volumes for very small storms. An initial runoff volume is retained by the storage devices, ranging from approximately 50 gallons to many thousand for each device, prior to the remaining runoff bypassing the systems. Modest water quality improvements will be gained by using rain barrels and cisterns to reduce the volume of stormwater available to convey pollutants.

8. Bioinfiltration / Rain Gardens / Bioretention

Benefits can include: reduced runoff volume, reduced peak discharge rate, reduced pollutant loading, groundwater recharge, habitat creation, enhanced site aesthetics, and reduced heat island effect.

Bioinfiltration cells, also known as bioretention or rain gardens, are vegetated depressions that store and infiltrate runoff from impervious surfaces, such as roofs and pavement. Bioretention and rain gardens generally work through filtration of the runoff into a collection system. An engineered soil medium maximizes infiltration and pollutant removal. Uptake by plants reduces runoff volume and pollutant concentrations.

Bioinfiltration is typically designed to drain within 24-48 hours. Bioinfiltration is suitable for use in a wide range of land uses, from residential to commercial, industrial, and ultra-urban settings. Use of Bioinfiltration for stormwater management is ideal for median strips, parking lot islands, and swales. Bioinfiltration provides storage and exfiltration capacity to surrounding soils, as well as plant uptake and evapotranspiration. Bioinfiltration is among the best GI practices for stormwater quality control, taking advantage of both physical and biological removal pathways.

9. Soil Amendments

Benefits can include: reduced runoff volume, reduced peak discharge rate, reduced pollutant loading, reduced runoff temperature, groundwater recharge, habitat creation, enhanced site aesthetics, and reduced heat island effect.

Soil amendments are any materials, organic or inorganic, that are added to a soil to increase its physical properties and enhance plant growth. Soil amendments increase a soil's infiltration and water retention capacity and thereby add storage volume to a site.

The maximum stormwater flow rate is reduced by the enhanced infiltration capability of the site and the additional storage volume that is realized in the amended soils. Amended soils have the ability to remove pollutants through sorption, precipitation, filtering, and bacterial and chemical degradation.

10. Street Trees and Afforestation

Benefits can include: reduced runoff volume, reduced peak discharge rate, and reduced phosphorous loads.

Planting of individual street trees and afforestation of larger disturbed areas can have a significant impact on stormwater runoff. Trees reduce runoff volume through evapotranspiration and interception (capture and storage of rainfall on leaf surfaces). They also improve the infiltration capacity of the soil, reducing runoff potential.

Planting individual trees will reduce volume from small, frequent storm events, but will not appreciably affect large storms. Afforestation of an entire area, however, can have a dramatic effect on soil permeability. An area replanted and allowed to grow into a mature stand of trees with little or no clearing of undergrowth can have a much higher infiltration rate than other land uses. Trees may be planted as seeds, seedlings, or semi-mature trees.

11. Tree Box Filters

Benefits can include: reduced TSS, reduced pollutant loading, and reduced urban heat island effect.

Tree box filters are mini filtration areas beneath trees in in-ground containers, typically installed in urban areas. An efficient use of land, the surface appearance of a tree box is a tree or shrub in a tree grate along a curb. The vegetation sits in a concrete box of bioretention media through which street or parking lot runoff is filtered prior to discharge into the storm drain system.

For low to moderate flows, stormwater enters through the tree box's inlet, filters through the soil, and exits through an underdrain into the storm drain. For high flows, stormwater bypasses the tree box filter if it is full and flows directly to the downstream curb inlet. A single tree box can store 100-300 gallons of stormwater; therefore the use of multiple devices is required to achieve significant reductions in the volume or peak flow rate of large storms. Tree box filters are based on bioretention technology, with improvements in performance, reliability, pollutant removal, ease of construction, aesthetics, and maintenance costs.

12. Vegetated Swales

Benefits can include: reduced runoff volume, reduced peak discharge rate, reduced pollutant loading, reduced runoff temperature, and enhanced site aesthetics.

Vegetated swales are broad, shallow channels designed to convey and infiltrate stormwater runoff. The swales are vegetated along the bottom and sides of the channel, with side vegetation at a height greater than the maximum design stormwater volume.

Vegetated swales reduce stormwater volume through infiltration, improve water quality through infiltration and vegetative filtering, and reduce runoff velocity by increasing flow path lengths and channel roughness. Reductions in discharge volume will be most apparent in moderate to small storms. Channel vegetation removes large and coarse particulate matter from stormwater.