Climate benefits of Green Stormwater Infrastructure

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Climate adaptation

Many of today’s stormwater treatment practices (permeable pavement, tree trenches, bio-retention, swales with check dams, green roofs, sand filters, re-use, constructed ponds) can be designed, constructed and maintained to adapt to climate change. Certain adjustments will need to be made during design and maintenance in order to account for the fluctuation in climate, most notably vegetation selection in vegetated BMPs and materials and design specifications in permeable pavements. Since climate change is largely believed to occur slowly over a long period of time many key adaptation issues related to specific BMPs will depend not on how to deal with potential impacts but instead on when to modify design and maintenance procedures.

Accounting for climate change and resilience

Precipitation trend for the Twin Cities Metro Area (MN DNR)

In Minnesota, the annual rainfall depth and frequency of large storms has been increasing in recent years (MNDNR, 2021). GSI can be a valuable tool to help offset runoff generated from more precipitation, but planners may want to design these practices to manage the higher flow rates and volumes associated with larger storms. With increasing rainfall and runoff, as well as with the general trend of increasing imperviousness, the target stormwater quality treatment volume may need to be increased as well. For example, in Austin, TX a site with 50% impervious cover is required to treat the first 1-inch of precipitation, but sites with 100% impervious cover are required to treat the first 1.5-inches of precipitation (Franke, 2016).

Minnesota’s winter temperatures also have increasing (MNDNR, 2021), consequently more frequent “mid-winter thaws” can be expected, which can impact the proper functioning of infiltration-based practices in particular. For example, freezing and thawing events will impact infiltration in aboveground retention systems with surface conveyance differently than underground retention systems with subsurface pipe conveyance. Properly designed infiltration-based practices should function during winter; however, the efficiency of saturated, frozen infiltration practices can be greatly diminished. Mid-winter thaws may increase the likelihood that infiltration practices are saturated in near-freezing weather, which may render them less effective during the spring runoff events. If the goal of a GSI practice is to infiltrate the spring melt water, then increasing the stormwater retention capacity of the practice may be warranted.

A diverse and preferably native plant palette should also be considered in order to make the overall planting bed more resilient to a broader range of climate and hydrologic scenarios. Installing GSI can help reduce local temperatures associated with climate change, and drought resilient species can help maintain effective ecosystem function during drier periods. More planting guidance can be found in the Minnesota Plant List page of the Minnesota Stormwater Manual.

Permeable Pavement

Climate change impacts are projected to occur slowly over a long period of time, including changes in precipitation patterns and average temperatures. As a result, there are no immediate changes to the current design, construction, or maintenance of permeable pavement. However, over time as climate patterns change, changes to materials and design specifications may need to be made to adapt to a new climate regime such that pavement does not fail under the expected vehicle/pedestrian load. The Federal Highway Administration (FHWA) recently outlined a study by Washington State that “shows that for a majority of well-constructed, high-volume asphalt pavements, rehabilitation is eventually triggered by rutting distresses, while for most low-volume, pavements, rehabilitation is eventually triggered by cracking distresses¹. In general, changes in these rends over time may be influenced by climate change and may instigate a strategic change to more cut resistant materials, such as stone matrix asphalt (SMA) and polymer-modified binders in surface courses.” The FHWA has compiled a table outlining climate change adaptation and pavement design-temperature items that can be useful for future design considerations.

Permeable pavement can also be used to reduce stormwater runoff volumes as it is estimated to reduce storm-runoff volume by 70-90%². Anticipated climate change conditions project more frequent or intense rains, leading to larger runoff-volumes over a shorter period of time. Permeable pavement can help manage and mitigate the change of rainfall events by slowing runoff leading to flood prevention and reduction in municipal pumping demand and energy costs.

Tree Trench

Trees help intercept and filter stormwater runoff in urban settings and help prevent flooding, provide wind breaks, and help mitigate the heat island effect through shading and evaporation. A typical medium-sized tree can intercept as much as 2,380 gallons of rainfall per year². Trees also intercept rainfall during precipitation events that can reduce runoff rates. A study in Sacramento, CA showed that evergreens and conifers intercepted over 35% of rainfall and reduced runoff by 7%. In Oakland, the tree canopy intercepts approximately 4 inches of rain, of 108,000 gallons of water, per acre in a typical year. In certain areas, trees can reduce runoff up to 17% in urban areas.

Trees allow for shaded cover and can reduce surface temperatures considerably. Trees absorb between 70-90% of sunlight in summer and 20-90% in winter. A study showed trees are able to reduce surface temperatures of roods and walls of buildings between 50-70 degrees Fahrenheit. Trees planted around houses can result in an annual cooling energy savings around 1% per tree while annual heating energy use decreased by almost 2% per tree².

Bioretention

Studies show that bio-retention cells reduce peak flow by at least 96.5% for small to medium-sized storm events. Denser vegetation in bio-retention cells can increase rainwater retention while greater biomass and plan productivity are associated with greater evapotranspiration losses³.

The University of New Hampshire Stormwater Center 2007 Annual Report documented an 82% and 85% removal of average peak flow in bio-retention cells with a 30 inch and 48” soil base, respectively².

Bio-retention basins also reduce the heat island effect and when situated adjacent to buildings can help reduce energy demand for cooling. However, more research must be done to quantify the reduction of energy created by the construction of bio-retention basins.

Swale with Check Dam

Vegetated Swales with check dams help reduce stormwater runoff volumes and aid in heat reduction and energy conservation. The University of New Hampshire Stormwater Center 2007 Annual Report documented a 48% removal of average peak flow in a vegetated swale².

In Portland, Oregon, vegetated swales were installed on street project and resulted in a reduced peak flow from a 25-year storm event (2 inches in 6 hours) by 88%, protecting local residents from flooding and a reduction of flow into local sewers by 85%.

Swales also reduce the heat island effect areas and help reduce energy demand for cooling when situated adjacent sidewalks and streets in urban. However, more research must be done to quantify the reduction of energy created by the construction of vegetated swales.

Green Roofs

Green roofs can help reduce energy savings and electricity costs from air conditioning demands and reduce stormwater runoff from buildings. Several studies show that energy savings from green roofs range between 15-45% of annual energy consumptions – mostly from lowering cooling costs². It is estimated that green roofs can reduce annual stormwater runoff by 50-60% on average, including peak runoff. Vegetated roofs control between 30-90% of the volume and rate of stormwater runoff, while detaining 90% of volume for storms less than one inch and at least 30% for larger storms.

Constructed Wetlands

Wetlands have been proven valuable as a first line of defense against large storm events and rising sea levels along coastal areas. Following Hurricane Katrina in 2005, the State of Louisiana and the City of New Orleans implemented a wetland restoration project to help protect populated coastal areas from rises in sea levels, hurricanes and river flooding. Wetlands are considered a buffer between the Gulf of Mexico and New Orleans and increase the City’s resiliency to natural disasters⁴. Restoration project such as New Orleans’ involves a combination of restoration of natural delta building, marsh creation from use of dredged material, water control structures, and hard structures. Upon completion, the restored wetlands will offer storm surge protection to densely populated areas. Wetlands also are effective in reducing peak flow rates and flooding in inland urban settings.

Carbon sequestration

Vegetated stormwater practices (bioretention, tree trenches/boxes, green roofs, vegetates swales, constructed ponds and wetlands) can be designed, constructed, and maintained to capture and store atmospheric carbon dioxide. The sequestration of atmospheric carbon can be used to mitigate or defer climate change by slowing the atmospheric accumulation of greenhouse gases. Carbon sequestration is impacted mainly by the age, type, and density of vegetation⁵.

Bioretention

Bioretention basins (infiltration and filtration) are often constructed with a variety of native and non-native species plantings (shrubs, grasses, sedges, etc.). Due to their extensive and deep rooting characteristics, native prairie plantings are able to store more carbon in soil than non-native plantings⁶. More specifically, studies shows that grasses and shrubs are likely to accumulate higher concentrations of Carbon then other planting types⁷. Grasslands that are predominantly populated with native species accumulate more carbon in soil than those grasslands that are predominantly accumulated by non-native species. The amount of carbon accumulated in the plants is affected by the plant species. Different species of grasses and shrubs have varying potential to accumulate and sequester carbon and species selection should be taken into consideration during the plant design and selection process. It is important to ensure consistent plant cover and to avoid soil exposure/disturbance as soil organic carbon can be released by oxidation into the atmosphere in the form of carbon dioxide⁶.

Tree Trench

Nowak (2002) states that “urban forests both sequester CO2, and affect the emission of CO2 from urban areas,” and that “urban forests can play a critical role in helping combat increasing levels of atmospheric carbon dioxide. Nowak (2013) states that “Trees act as sink for carbon dioxide (CO2) by fixing carbon during photosynthesis and storing carbon as biomass. He outlines that in urban areas trees can both sequester carbon and emit carbon depending on that life cycle stage of the tree. As a tree grows it will sequester carbon, and when a tree dies carbon can be emitted back into the atmosphere. Additionally, urban trees can have an influence on local climate as they can help reduce air temperatures. With the increase in urban land growth, trees within these areas can help sequester considerable amounts of carbon⁸.

Green Roofs

Green roofs can contribute to reducing atmospheric CO2 by two means. One, the plant structures on green roofs are largely comprised of carbon, which is naturally sequestered in the plant tissues and into the green roof soil layer through plant litter and root exudate. Two, a building with a green roof will reduce the building’s heat island effect and energy needs and corresponding regional electricity demand as the roof will function as an insulator⁹. Major components to green roof effectiveness that should be taken into consideration are plant species selections, substrate (soil) depth, substrate composition and management practices. Rowe found that “above-ground sequestration ranged from 64 g C m-2 to 239 g C m-2 for S. acre and S. album, respectively.” Some ways to increase carbon sequestration effectiveness would be to implement the following design strategies.

• Increase depth of soil slayer – helps provide more volume for carbon storage and allow for deeper rooted vegetation
• Soil mix – can maximize carbon sequestration
• Plant species selection – vegetation with deeper roots have higher potential to store more carbon
• Operation and maintenance practices that address irrigation/watering and landscape care

A properly designed green roof cam help reduce the need for power from regional power plants. The reduction in energy demand is a result of the green roof’s ability to insulate the building it serves and also reducing the heat island effect. In the U.S., buildings are responsible for 38% of carbon dioxide emissions⁹. By reducing the need to use energy provided by regional power plants, green roofs act as a natural insulator and decrease the amount of carbon released into the atmosphere.

Vegetated Swales

Vegetated swales can sequester carbon within the vegetation and soil that grows in the swales. A study performed alongside North Carolina highway right-of-ways (ROWs) examined carbon sequestration potential in swales. Results showed that carbon sequestration was more significant in wetland swales than dry swales and that in order to promote carbon sequestration in the vegetated ROW, wetland swales appear to be preferable over dry swales¹⁰.

Constructed Ponds and Wetlands

Constructed stormwater ponds and wetlands provide carbon sequestration benefits, albeit more research must be done to have a greater understanding of carbon sequestration in these stormwater BMPs. It appears that emergent vegetation is one of the most important contributors to the accumulation of carbon in stormwater pond and wetland soils¹¹. Again, more research must be done to determine the effectiveness of stormwater ponds and wetlands and their ability to sequester carbon, though it is believed that stormwater ponds/wetlands could play an important role in global carbon cycles¹².

References

1. https://www.fhwa.dot.gov/pavement/sustainability/hif15015.pdf
2. http://dev.cakex.org/sites/default/files/Green_Infrastructure_FINAL.pdf
3. https://ac.els-cdn.com/S0301479714003740/1-s2.0-S0301479714003740-main.pdf?_tid=695db2ac-1292-11e8-ae4e-00000aab0f6c&acdnat=1518728248_d3dc6ee2fd233f691e979637da4ad228
4. http://orca.cf.ac.uk/64906/1/Database_Final_no_hyperlinks.pdf
5. https://www.epa.gov/sites/production/files/2015-12/documents/may_gi_webcast_-_getting_more_green_-_combined_slidespptx.pdf
6. http://www.bwsr.state.mn.us/practices/carbon_sequestration-grasslands.pdf
7. http://www.conference.ifas.ufl.edu/intecol/presentations/125/0240%20M%20Greenway.pdf
8. https://ac.els-cdn.com/S0269749113001383/1-s2.0-S0269749113001383-main.pdf?_tid=spdf-03ab9ba1-270d-4161-8715-5c27e6485e34&acdnat=1518726476_881847fa2957e87608a349912724abbc
9. https://ac.els-cdn.com/S0269749110004859/1-s2.0-S0269749110004859-main.pdf?_tid=b0fa2c66-128e-11e8-9e17-00000aacb35d&acdnat=1518726658_a9b674611558c7c1a05a2ca4d61f1a37
10. https://ac.els-cdn.com/S0925857413000335/1-s2.0-S0925857413000335-main.pdf?_tid=dd6e6046-128e-11e8-bd0e-00000aacb361&acdnat=1518726725_d7334f15a6668820283b8095b5333274
11. https://ac.els-cdn.com/S004313541100710X/1-s2.0-S004313541100710X-main.pdf?_tid=61c4d1d4-1291-11e8-8b80-00000aab0f02&acdnat=1518727805_e21ec77a180104c32852074fabcd54d8
12. http://onlinelibrary.wiley.com/doi/10.1890/12-0825.1/full