Water quantity and hydrology benefits of Green Stormwater Infrastructure

Download pdf

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).
• Infiltration practices remove pollutants by capturing runoff and infiltrating it vertically into underlying soil, the vadose zone, and groundwater. Attenuation occurs primarily through adsorption and filtering, though dilution in groundwater may also be a mechanism for reducing pollutant concentrations.

Practice	Water quality benefit	Notes
Bioretention	◕	Infiltration is most effective; potential phosphorus leaching in filtration practices
Tree trench and tree box	◕	Infiltration is most effective; potential phosphorus leaching in filtration practices
Green roof	◕	Potential phosphorus leaching
Vegetated swale	◔	Infiltration is most effective; less effective for dissolved pollutants
Vegetated filter strip	◔	Removes solids; less effective for dissolved pollutants
Permeable pavement	◕	Infiltration is most effective
Constructed wetland	◑	Removes solids; less effective for dissolved pollutants
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.

Bioinfiltration practices, which are bioretention practices with no underdrain and designed to infiltrate water, are effective at reducing runoff volume and peak rates for relatively small 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. On a watershed scale, bioinfiltration practices scattered throughout a watershed can reduce runoff volumes and peak runoff but not to predevelopment levels. Hunt et al (2009; 2012) discuss the importance of determining hydrologic goals prior to designing and constructing bioretention practices. The researchers state "A large cell media volume: drainage area ratio, and adjustments to the drainage configuration appear to improve the performance". In particular, the researchers advocate for deeper basins and thicker media depths.

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.

Additional recommend reading:

- Field Performance of Bioretention: Hydrology Impacts

- Bioretention Hydrologic Performance in an Urban Stormwater Network

- Meeting Hydrologic and Water Quality Goals through Targeted Bioretention Design

- Mitigation of Impervious Surface Hydrology Using Bioretention in North Carolina and Maryland

- Spatio-temporal effects of low impact development practices

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%.

Tree Trench

Trees can help reduce stormwater runoff by intercepting rainfall, promoting infiltration by increasing the presence of macrospores and the ability of soil to store water, transpiration, and evapotranspiration. Tree canopies also help reduce the impact raindrops have on barren surfaces. Through all of these mechanisms a tree canopy temporarily detain rainfall and gradually releases it, and regulates the flow of stormwater runoff downstream to storm sewer networks or other waterways. 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. That being said, it is difficult to estimate and quantify runoff reduction, thus results are likely to vary.

When selecting trees species, species appropriate for the local current and future water conditions should be selected in preference to production species, which typically combine high rates of biomass accumulation with high evapotranspiration. Additionally, fast-growing tree species such as production species are likely to reduce runoff more than slow-growing species, and may be more susceptible to drought and climate.

Additional recommended reading

-Balancing the environmental benefits of reforestation in agricultural regions

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.

Additional recommended reading

-Hydrologic and Water Quality Evaluation of a Permeable Pavement and Biofiltration Device in Series

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.

Additional recommended reading

-Water Recycling and Re-use: The Environmental Benefits

-Rainwater Harvesting

-Water Facts: Water Recycling

References

1. https://www.ncbi.nlm.nih.gov/pubmed/24569270
2. https://www.michigan.gov/documents/deq/Hydrologic_Performance_of_Vegetated_Roofs_-_Carpenter_457042_7.pdf
3. https://www.cwp.org/wp-content/uploads/2017/01/cwp_rr_jan17.7.pdf
4. http://chesapeakestormwater.net/wp-content/uploads/downloads/2012/01/JHE20Davios20Bioretention20Paper1.pdf
5. https://web.sbe.hw.ac.uk/staffprofiles/bdgsa/11th_International_Conference_on_Urban_Drainage_CD/ICUD08/pdfs/211.pdf
6. https://www.usgs.gov/science/evaluating-potential-benefits-permeable-pavement-quantity-and-quality-stormwater-runoff?qt-science_center_objects=0#qt-science_center_objects