Green stormwater infrastructure - climate resiliency and climate adaptation Revision as of 14:35, 26 January 2023 by Mtrojan (talk | contribs)(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff) Green stormwater infrastructure - climate resiliency and climate adaptation

Climate change is an ongoing and evolving issue impacting the environment, infrastructure, ecology, and people. Climate change impacts include increased flooding, more intense rainstorms, more frequent droughts, longer periods of no rain (drought), and warmer temperatures.

Climate trends in Minnesota show the state is generally getting warmer and wetter, with more frequent damaging rains, and warmer winters (Minnesota Department of Natural Resources, 2022). The frequency and severity of nuisance and severe flooding in Minnesota are likely to increase as larger and more intense storms occur (Minnesota Pollution Control Agency). Heavy and more frequent rainfall has the potential to mobilize and transport large pollutant and sediment loads from our built environment to nearby streams, rivers, and lakes. This in turn can erode stream banks and shorelines, reduce water quality, and harm aquatic ecosystems. Summer droughts are also projected to get worse, partly due to increases in temperature (States at Risk, 2013). Increases in temperature can intensify the urban heat island effect, putting people at risk for heat exhaustion and heat stroke (USEPA, 2022a). Climate change also affects air quality and exposure to air pollutants such as particulate matter, ozone, pollen, and mold (Minnesota Dept. of Health, 2022).

While the stresses brought about by climate change can be severe, green stormwater infrastructure (GSI) is one tool communities can use to help mitigate the negative impacts of climate change, bolster climate resilience, and adapt to a changing environment. This page describes how GSI has been used to mitigate specific climate change impacts. The table below also shows the relative effectiveness of different types of GSI in mitigating specific climate change hazards.

To read more about the role of specific GI practices on reducing climate change impacts, visit the Climate Benefits of Green Stormwater Infrastructure page of the Minnesota Stormwater Manual.

Reduce Flooding Impacts

Climate trends in Minnesota show that the state is generally getting wetter, with more frequent and intense damaging rains, and warmer winters. These changes impact stormwater runoff, streamflow, and timing of snowmelt (US EPA, 2022f), and can cause larger and/or more frequent flooding.

Infiltration-based GSI, can reduce the impact of localized inland flooding by providing storage and infiltration of rainfall and stormwater that would otherwise overwhelm sewer and drainage systems, including small streams. Bioinfiltration, bioswales with impermeable check dams, permeable pavement without underdrains, and tree trenches without underdrains are examples of GSI practices that provide capture and infiltration of rainfall (US EPA, 2022c). Green roofs, urban forestry (Foster et al., 2011), tree trenches/boxes with underdrains, biofiltration, swales with permeable check dams, permeable pavement with underdrains, preservation of natural landscapes, and wetlands (Flynn, 2022) are examples of GSI practices that provide storage (temporary or permanent) of rainfall.

Incorporating these GSI practices at scale into the built environment can reduce stormwater runoff volume and flow rate and reduce stress on existing stormwater infrastructure such as storm sewers and drainage ditches. For example, green roofs have been shown to reduce annual stormwater runoff by 50-60% and can reduce up to 90% of stormwater runoff in events with rainfall less than 1 inch (Gateway Team, 2007). These green roof stormwater reduction values are dependent on seasonality (e.g.: less efficient during winter freeze), as well as soil type, volume of soil media, depth to groundwater table, and vegetation makeup (Minnesota Pollution Control Agency, n.d.).

The City of La Crosse, Wisconsin, is an example of how GSI can be used as a means of flood mitigation. The city has a drainage infrastructure that is very sensitive to rain and flooding due to its flat topography and long pipe network. The city conducted a study in 2014 showing that GSI had the capacity to nearly eliminate flooding from the 3-month, 24-hour rainfall event, and that bioretention can reduce localized manhole flooding by nearly 88% (US EPA, 2014). The study also showed that permeable pavement was the most effective for reducing the impacts of flooding with larger storms. Upon completion of the study, the city embarked on a “green streets” program to install different types of GSI – including permeable pavement, bioretention, trees, landscaped medians, and vegetated swales – on either side of impervious pavement to capture runoff from the road and reduce flooding impacts (Askew-Merwin, 2020; City of Lacrosse).

Staten Island’s Bluebelt is another example of how GSI can be used as flood mitigation strategy. During the rapid expansion of Staten Island in the late 1900s, the sewer systems were frequently overwhelmed, and flooding was a common occurrence in areas of new development due to the disturbance of the natural flow paths. The solution was to preserve and restore natural stream corridors which have intrinsic flood reduction characteristics. The natural stream corridors meander, slowing the rate of flow through the system and allowing for the settling of sediment within the water. Not only did this natural stormwater conveyance system reduce flooding and improve water quality, but it also restored nearly 400 acres of wetlands and riparian stream habitat while coming in at only a fraction of the cost that a more traditional stormwater infrastructure solution would have cost. To this day, nearly ⅓ of the area of Staten Island drains to the Bluebelt (NYC H2O, n.d.; “The Staten Island Bluebelt,” n.d.). A similar approach is being applied in Duluth, MN which has faced numerous severe storms in recent years. Duluth’s Strategic Lands Realignment Project includes the purchase of nearly 3,000 acres of land, dominated by wetlands, with the goal of protecting and restoring the land to improve the community’s resilience to intense flood events (Bunch & Chiasson, 2021). Duluth is able to purchase the land for a modest price under Minnesota Statute § 282.01 Subd. 1(a)(h) and Subd. 1(e)(a) which allow for a county to sell tax forfeited land for less than market value if the land is to be used for environmental protection and for a county to transfer tax forfeited land at no cost if the land is to be used for parks, respectively.

Mitigate Drought Impacts

Under climate change, some communities will experience longer and more severe droughts due to higher temperatures and changes in precipitation patterns. In Minnesota, summer droughts are projected to get worse due to increases in temperature (States at Risk, 2017).

Some GSI practices can help mitigate the impacts of drought. One example is rainwater harvesting systems. Rainwater harvesting systems capture rainfall in cisterns or rain barrels where it is stored for future use. Under Minnesota Administrative Code Section R 4714.1702, rainwater can be harvested for non-potable uses including toilet flushing, water features, vehicle washing facilities, and cooling tower makeup. Using rainwater for non-potable applications alleviates the stress on the potable water supply by reducing the water demand during droughts and by preserving potable water for critical needs such as bathing and drinking (US EPA, 2022d). The impact of rainwater harvesting systems is dependent on the volume of rainwater stored and the ubiquity of their use within a community.

The strategic landscaping of green infrastructure can also mitigate drought impacts. Incorporating drought resistant plants in green infrastructure such as bioretention systems and wetlands allows for less water use as well as provide habitat and ecological benefits when other less tolerant plants die off during drought periods. In addition to drought resistant plants, consideration should be given to include a diverse selection of plants, including native plants, which improves the likelihood of survival during drier periods and supports the GSI’s integrity. For more information on GSI plant selection, refer to the Minnesota Plants List page of the MN Stormwater Manual.

Infiltration based GSI such as rain gardens or bioretention, bioswales, permeable pavement (US EPA, 2022c), urban forestry (Foster et al., 2011), trees and tree trenches/boxes, preservation of natural landscapes, and wetlands (Flynn, 2022) also help mitigate the impacts of drought by enhancing groundwater recharge, as explained in the next section.

Enhance Groundwater Recharge

Climate trends in Minnesota show that warmer temperatures will lead to increased evapotranspiration, longer periods of potential summer drought conditions, and shorter periods of snowmelt. At the same time, increased urban development and imperviousness across the state reduces the potential for rainfall and runoff to infiltrate into the ground. This combination of impacts results in overall slower rates of groundwater recharge. In addition, groundwater supplies 75% of Minnesota’s drinking water and 90% of agricultural irrigation (Minnesota Department of Natural Resources, n.d) . With warmer temperatures, this demand is likely to go up , further stressing groundwater supplies (Wu et al., 2020).

Infiltration-based GSI practices such as rain gardens, bioswales, permeable pavement (US EPA, 2022c), urban forestry (Foster et al., 2011), trees and tree trenches/boxes, and wetlands (Flynn, 2022) can help reduce the stress on the groundwater supply by enhancing groundwater recharge through slow infiltration of rainfall and runoff. Additionally, harvested rainfall can be stored in deep groundwater reserves, reducing the need for pumping from natural aquifers during drought events. For example, the city of Los Angeles Department of Water and Power (LADWP) and its partners actively capture and recharge 29,000 acre-feet per year of stormwater through stormwater infiltration basins and other GSI practices (Mansell et al., 2016). Increase Shoreline and Streambank Resiliency Climate impacts are being increasingly felt where coastal and lake shorelines, beaches and dune systems, as well as streambanks and streambeds are subject to erosion by the action of wind, water, and human activity such as development and recreation.

Changes in snow and ice melt as well as precipitation are increasingly impacting shorelines and streambanks through erosion. Erosion is catastrophic for the natural riparian habitats as well as for the homes and businesses situated in erosion-prone areas. Armoring shorelines and streambanks can result in more harmful than beneficial impacts due to scouring and destruction of vegetation in near shore habitat, or due to downstream displacement of erosion (Norton et al., 2017).

Some GSI practices can bolster shoreline resiliency without destroying habitat. Examples of this include living shorelines, riparian forest buffers, and regenerative stormwater conveyance (RSC also known as step pool conveyance system (SPCS)). Living shorelines are stabilized shoreline edges made of natural materials like plants, sand, and rock that reduce erosion, improve water quality, and provide valuable habitat (Center for Coaster Resources Management, n.d.; NOAA – Fisheries, n.d.). Riparian forest buffer areas adjacent to a stream, lake, or wetland that contains a combination of trees, shrubs, or other vegetation, and provides benefits such as erosion control, water quality improvements, and ecologically rich habitat. Regenerative stormwater conveyance, or step pool conveyance, are open-channel stormwater conveyance structures that slow down and infiltrate stormwater runoff, particularly in the sensitive area between a stormwater outfall and a stream. Other, more urban GSI practices such as bioretention, green roofs, and permeable pavement can also be used as upland strategies to protect downstream erosion.

Staten Island’s Bluebelt (NYC H2O, n.d.; “The Staten Island Bluebelt,” n.d.), Duluth’s proposed extensive wetland system (Bunch & Chiasson, 2021), and Washington DC’s RSC projects (Younts Design Inc. a-b, n.d.) are examples of GSI strategies that successfully limit erosion and increase shoreline and stream bank resilience. Reduce Urban Heat Island Effect Urban heat island is a term used to describe the elevated temperatures observed in urban areas due to the presence and density of impervious areas such as buildings, roads, and sidewalks which emit more heat back to the surrounding setting than natural systems, particularly those that are vegetated (US EPA, 2022h). This urban heat island effect (UHIE) is predicted to intensify as urbanization continues and as temperatures rise with climate change (US EPA, 2022g).

GSI can be used to reduce the urban heat island effect through three primary mechanisms 1) cooling via evaporation or evapotranspiration, 2) providing shade, and 3) increasing solar reflectance. Examples of GSI practice that can reduce UHIE include rain gardens or bioretention, bioswales, green roofs, urban forestry, trees and tree trenches/boxes, and constructed wetlands.

GSI practices such as rain gardens or bioretention primarily reduce the urban heat island effect through evapotranspiration, particularly compared to impermeable surfaces. Even GSI practices that only hold water but are not vegetated, such as permeable pavement, can reduce near-surface temperatures through evaporative cooling. Studies have shown that permeable pavement has a lower thermal impact on near-surface air than impermeable pavement due to its ability to hold and evaporate water from its pores, cooling the surface above (US EPA, 2012).

GSI such as trees provide shade in addition to evaporative benefits. A tree’s canopy cover can provide extensive shade and absorb 70-90% of sunlight before it reaches the surface, thereby reducing the UHIE (US EPA, 2008). The impacts of trees vary seasonally, with deciduous trees reducing UHIE primarily during the growing season and evergreen trees reducing UHIE year-round.

Green roofs provide cooling due to evapotranspiration, shade, and increasing solar reflectance compared to traditional black roofs and have been shown to reduce ambient temperatures by 5 °C (Foster et al., 2011).

Use of GSI to reduce the UHIE can also lead to lower energy demands, as explained in the next section. Lower Energy Demands Increased energy demand is an indirect impact of climate change in response to variable rainfall and increased temperatures. Particularly in urban communities where the urban heat island effect exists, greater energy will be expended to cool buildings. A potential co-benefit of GSI is its capacity to reduce energy demands by lowering the air temperature and buffering extreme temperature swings at or adjacent to buildings.

For example, Green roofs have a higher reflectivity than many common roofing materials, help intercept direct solar UV radiation, cooling benefit from evapotranspiration, and provide natural insulation to buildings through soil and vegetation. The combination of mechanisms can significantly reduce surface temperatures of roofs (Foster et al., 2011). This in turn results in a reduced need to heat or cool the building below and, therefore, lower energy demands. Blue roofs, or controlled flow roof drain systems (“Stormwater Management,” n.d.), act primarily to detain water and provide temporary storage and slow release of rainwater from a rooftop. However, they can also provide thermal buffering similar to vegetated roofs through higher reflectivity and evaporative cooling.

To estimate the energy saving benefits of a green roof on your building, visit: https://greenroofs.org/green-roof-energy-calculator/

Trees are another GSI option that can similarly provide localized cooling by shading a building, reducing ambient air temperatures and thus reducing energy demands. Trees can be coupled with vegetated facade systems as part of a building-integrated vegetation strategy, allowing buildings to work even more efficiently. Other GSI systems such as rain gardens or bioretention, bioswales, or constructed wetlands also have the capacity to reduce localized temperatures near buildings, again lowering energy demand. Improve Carbon Sequestration Greenhouse gases from human activities are the most significant driver of observed climate change since the mid-20th century. EPA’s Inventory of US Greenhouse Gas Emissions and Sinks reports that the majority of the US’s greenhouse gas emissions are carbon dioxide (US EPA, 2022e).

Vegetated GSI, such as trees, green roofs, rain gardens or bioretention, and wetlands, have tremendous collective capacity for sequestering carbon in place while also managing stormwater. Through photosynthesis and carbon fixation, trees and other vegetation remove carbon from the atmosphere by assimilating it into their vegetation and serve as a sink for carbon. The Arbor Day Foundation reports that a mature tree has the potential to remove 48 pounds of carbon from the atmosphere annually (Mounce Stancil, 2019). Undisturbed forested areas are particularly effective at sequestering carbon into their vegetation (Ecological Society of America, 2000). Grasslands, meadows, and well-managed pastureland sequester most of their carbon underground in root systems and the soil itself (CLEAR Center, 2019). Soils are a critical site for carbon storage. Soil organic matter (SOM), a complex mixture of carbon compounds consisting of decomposing plant and animal tissue, microbes, and carbon associated with soil minerals, is the primary means of locking carbon in.

To the extent possible, undisturbed and natural areas should be preserved during development to maintain the natural hydrology and minimize loss of carbon sequestration. Introducing well-designed GSI as part of the development process will help mitigate stormwater impacts and sequester carbon through strategic combinations of plant types and soil media (Leslie, n.d.; CLEAR Center, 2019). If a primary goal of the GSI is carbon sequestration, consideration should be given to GSI design to reduce the carbon footprint of both the construction and the maintenance activities. The carbon footprint of GSI can be reduced by sourcing local materials for GSI construction to reduce transportation of materials or by implementing low-carbon construction methods to reduce emissions during the construction of the project (Casal-Campos et al., 2013).

Improve Air Quality

Activities related to climate change are also known to reduce air quality through increased ground-level ozone and atmospheric particulate matter. Climate change impacts such as elevated temperatures, less frequent rain, and wildfires intensify these air quality issues (US Dept. of HHS, n.d.).

Reduction of particulate matter is of high importance as it has a greater effect on human health than other atmospheric pollutants, according to the World Health Organization (WHO, 2022). Vegetation is one effective means of capturing and reducing atmospheric particulate matter. Vegetated GSI such as bioretention, trees, constructed wetlands, and green roofs can contribute to the reduction of particulate matter and improve air quality through thoughtful plant and species selection.

Trees in particular have been shown to reduce a number of atmospheric pollutants including ozone, particulate matter, and carbon monoxide (Arshad et al., 2019). Trees can provide a significant air quality benefit at low installation and maintenance costs. As always, protection of existing trees and natural land should be maximized as possible.

Green roofs have also been shown to significantly remove atmospheric particulate matter, especially intensive green roofs that have a deeper growing substrate (typically greater than 6-inches) and can support a wider variety of vegetation (Yang et al., 2008). One study noted that a 1,000 square foot green roof is able to remove the equivalent of particulate matter produced by 15 cars in a year (“The Staten Island Bluebelt”, n.d.).

Improve Water Quality

Climate change negatively impacts water quality in several ways. Heavy and more frequent rainfall has the potential to mobilize and transport pollutants such as pesticides, nutrients, sediments, and oil and grease from our built environment to nearby streams, rivers, and lakes. Heavy rainfall can also erode stream banks and shorelines, reducing the water clarity and harming aquatic ecosystems. Additionally, warmer temperatures also provide an environment more conducive to harmful algal bloom formation in ponds and lakes.

GSI uses natural processes such as retention, infiltration, filtration, settling, and biological uptake to improve stormwater quality before discharging to streams or lakes. Constructed wetlands, bioretention, and trees are among the GSI that provide the strongest water quality benefits.

According to the EPA “wetlands are among the most productive ecosystems in the world” (US EPA, 2022b). Constructed wetlands, as well as natural wetlands, improve water quality through physical, biological, and chemical mechanisms. Physical mechanisms include infiltration, settling, and trapping of pollutants (particularly sediment and pollutants that bind to sediment). Biological mechanisms include uptake of pollutants (particularly sediment and nutrients) by plants, microbes, and benthic invertebrates. And chemical mechanisms include transformations of nutrient forms and chemical precipitation

Similarly to constructed wetlands, bioretention or rain gardens capture runoff and remove sediment, nutrients, and other pollutants through physical, biological, and chemical mechanisms. Soils in bioretention systems can be amended with materials such as iron filings and steel wool to improve phosphorus adsorption and provide greater water quality benefits. More information on Soil Amendments to Enhance Phosphorus Sorption can be found in the Minnesota Stormwater Manual.

Trees can enhance the infiltration of runoff allowing for greater contact time with soil microbes for enhanced immobilization, transformation, and uptake. The large surface area of trees also provides contact areas for deposition of atmospheric pollutants before they are able to enter and pollute surface waters (USDA NAC, 2012). Similarly to bioretention systems, soils in tree trenches and tree boxes can be amended to achieve enhanced phosphorus removal.

For more information on pollutant removal performance of green stormwater infrastructure, visit the Information on Pollutant Removal by BMPs page of the Minnesota Stormwater Manual.  

References

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