Water quantity and hydrology benefits of Green Stormwater Infrastructure
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Water quantity and hydrology benefits of Green Stormwater Infrastructure

This page provides information on the water quantity and hydrology benefits of green stormwater infrastructure (GSI) practices ( best management practices). The water quantity benefit of a practice is defined by its ability to retain runoff or detain and slowly release runoff. These benefits include the following.

Decreased downstream flooding as water is either captured or slowly released to reduce peak runoff volumes (rate control). These reductions in downstream runoff volume also reduce impacts to receiving waters by reducing erosion and protecting habitat.
Increased groundwater recharge, which can lead to improved baseflow and deep aquifer recharge
Reductions in downstream runoff volumes also provide additional benefits,

Water quantity benefits vary between each practice, primarily as a result of the mechanism by which stormwater runoff is captured and by its ultimate fate.

Constructed ponds ( wet ponds) and wetlands ( stormwater wetlands) temporarily capture and store water, releasing it slowly. This decreases peak discharges downstream. There is some water loss through seepage and evapotranspiration, but these losses are considered negligible. See Calculating credits for stormwater ponds. For more information on sedimentation processes, link here.
Filtration practices include bmps that have an underdrain ( biofiltration, permeable pavement, tree trench, swales, green roofs, and media filters) or bmps that provide some water retention in soil or engineered media ( vegetated filter strips, swales, green roofs). In practices with underdrains, there is always some infiltration below the underdrain, provided the practice does not have an impermeable liner. The annual volume lost depends on the position of the underdrain and the infiltration characteristics of the underlying soil. In vegetated practices with underdrains, there is also water loss through evapotranspiration. Annual losses in these practices are typically in the 5 to 20 percent range. Green roofs are effective at retaining water, while volume reduction in swales and filter strips is generally small.
Infiltration practices retain all water captured by the practice. This captured water is infiltrated through the soil, vadose zone, and into groundwater.

Practice	Water quality benefit	Notes
Bioretention	◕	Benefit is high for infiltration, low for filtration
Tree trench and tree box	◕	Benefit is high for infiltration, low to moderate for filtration
Green roof	◕	Benefit is associated with rate control; effectiveness increases with media thickness
Vegetated swale	◔	Benefit is high for infiltration, low for filtration
Vegetated filter strip	◔
Permeable pavement	◕	Benefit is high for infiltration, low for filtration
Constructed wetland	◑	Benefit is associated with rate control
Rainwater harvesting	◕	Can be used on low permeability soils
Level of benefit: ◯ - none; ◔; - small; ◑ - moderate; ◕ - large; ● - very high

Green infrastructure and multiple benefits

Green infrastructure (GI) encompasses a wide array of practices, including stormwater management. Green stormwater infrastructure (GSI) encompasses a variety of practices primarily designed for managing stormwater runoff but that provide additional benefits such as habitat or aesthetic value.

There is no universal definition of GI or GSI (link here for more information). Consequently, the terms are often interchanged, leading to confusion and misinterpretation. GSI practices are designed to function as stormwater practices first (e.g. flood control, treatment of runoff, volume control), but they can provide additional benefits. Though designed for stormwater function, GSI practices, where appropriate, should be designed to deliver multiple benefits (often termed "multiple stacked benefits". For more information on green infrastructure, ecosystem services, and sustainability, link to Multiple benefits of green infrastructure and role of green infrastructure in sustainability and ecosystem services.

Bioretention

Bioretention practices can be incorporated into street landscapes. Image Courtesy of Emmons & Olivier Resources, Inc.

Bioretention can be designed as an effective infiltration / recharge practice (bioinfiltration) when parent soils have high permeability. For lower permeability soils an underdrain is typically used and some infiltration and rate control can be achieved.

Bioinfiltration practices, which are bioretention practices with no underdrain and designed to infiltrate water, are effective at reducing runoff volume and peak rates for small and medium intensity storms. A bioinfiltration practice designed to capture 1 inch of runoff from a 2 acre site consisting of 1 acre of impermeable surface and 1 acre of turf on B soils, for example, will capture about 90 percent of annual runoff.

Effects of bioinfiltration on groundwater hydrology are poorly understood. It is often assumed increased recharge will result in increased aquifer recharge and/or increased baseflow to surface waters, but limited data exists to support these assumptions.

The following design considerations can increase the water quantity/hydrologic benefits of bioretention practices.

Maximize infiltration
Utilize internal water storage
Maximize water storage in media
Bioinfiltration practices scattered throughout a watershed can reduce runoff volumes and peak runoff

See Calculating credits for bioretention and Hunt et al (2009; 2012).

Tree Trench

Completed tree system, Marquette and 2nd Avenue Busways project, Minneapolis, MN. Image Courtesy of The Kestrel Design Group.

Tree trenches are a type of bioretention practice. However, their potential benefits for water quantity and hydrology are greater than for traditional bioretention practices due to their ability to treat runoff from larger areas, hence more runoff volume, because of the enhanced evapotranspiration provided by trees, and due to interception of rainfall by tree canopies. On higher permeability soils they can be designed as infiltration practices, while on lower permeability soils they typically have an underdrain.

Tree trenches with no underdrain are effective at reducing runoff volume and peak rates for small and medium intensity storms. A bioinfiltration practice designed to capture 1 inch of runoff from a 2 acre site consisting of 1 acre of impermeable surface and 1 acre of turf on B soils, for example, will capture about 90 percent of annual runoff.

Reduction in runoff due to trees is highly variable and depends on many factors such as, but not limited to, tree characteristics, planting configuration, soil conditions, and precipitation event characteristics. For examples, studies show that annual rainfall interception by urban trees and forests can range from 6% to 66%, while urban trees can transpire from 0.2 to 46.7 gallons per tree per day³.

The following design considerations may increase the water quantity and hydrology benefits of tree-based practices.

Maximize infiltration by designing larger practices, if feasible, and identifying the most permeable soils on a site
Utilize internal water storage
Maximize water storage in media
Select species appropriate for the local current and future water conditions, which typically combine high rates of biomass accumulation with high evapotranspiration.

See Calculating credits for tree trenches and tree boxes

Green Roofs

Green roofs have the ability to store significant amounts of water in their growing media. Water can evaporate from the soil and be transpired by the vegetation and plans growing in the media. As a result, runoff from the roof is reduced which decreases flow to storm sewer systems and can ultimately reduce overflow events and flooding events. This can be beneficial especially in highly developed urban areas where stormwater is conveyed to storm sewer systems that can frequently experience combined sewer overflows. In some cases both laboratory and field measurements show a reduction in stormwater runoff volume by 30% to 86%, a reduction in peak flow rate by 22% to 93% and a delay in peal flow by 0 to 30 minutes using green roofs¹. The selection of growth medium, plant species, and roof slope of green roof can have a significant effect on the effectiveness and performance of a green roof for hydrologic purposes. Green roofs can generally be categorized into two categories; intensive and extensive. Intensive roofs have a soil depth of 6 inches or greater, whereas extensive roofs have less than 6 inches of soil depth². One experimental case study that implemented an extensive roof in Michigan showed a peak discharge reduction of 54% to 99%.

Bioretention

Bioretention practices are able to store and infiltrate stormwater, which helps mitigate flood impacts and prevents stormwater from polluting local waterways. A study analyzing 2 bioretention cells subject to over 49 rainfall events showed flow peak reductions of 44% to 63%⁴. The study summarized that “from a hydrological perspective, the bioretention facilities were successful in minimizing the hydrologic impact of the impervious surface and major reductions can be expected for about 1/3 to 1/2 of the rainfall events. A separate study that analyzed 5 bio-retention cells found that “bioretention cells were able to mitigate peak flows because of their infiltration rales, potential to store water in soil pores, and slow drawdown time.”⁵

Permeable Pavement

Permeable pavement reduces surface runoff volumes by allowing stormwater to infiltrate into underlying soils as opposed to allowing stormwater to flow into storm drains and out to receiving water as effluent. Additionally, permeable pavement helps reduce peak flow rates and decreases the risk of flooding through this same process by preventing large, fast pulses of precipitation through stormwater collection systems⁶. A study conducted by the Wisconsin Department of Natural Resources using a Permeable Interlocking Concrete Pavement (PICP) found that a permeable pavement system facilitate a volume reduction of 56%. It should be noted that the deteriorating upland drainage area is thought to have clogged the porous pavement, allowing a greater percentage of surface runoff to bypass the system than originally hypothesized.

Water Re-use and Harvesting

Water re-use and harvesting systems help capture rainfall and minimize stormwater runoff volumes and rates to receiving stormwater sewer systems and conveyances. Re-use and Harvesting systems capture water and increase a user’s water supply that can be stored and used on site, immediately or in the future, in lieu of the local water supply. This can be particularly advantageous in times of droughts when water may not be readily available. These systems also help alleviate pressure on the local water supply and allows communities to allocate water to other users. Reusing rainwater for irrigation purposes can also help increase groundwater recharge.

Contents

Green infrastructure and multiple benefits

Bioretention

Tree Trench

Green Roofs

Bioretention

Permeable Pavement

Water Re-use and Harvesting

Additional recommended reading

References