|Practices to consider
|Cities or other local units of government may have ordinances or regulations that promote or hinder implementation of practices that improve climate resilience. For example, tree preservation and replacement ordinances promote resilience, while restrictions on infiltration may hinder resilience.|
Trends indicate Minnesota’s climate is becoming warmer and wetter. Over the last 25 years, annual average precipitation increased by 3.4 inches and temperatures warmed by 3.0°F (MN DNR, n.d.). As a result, the frequency and severity of local flooding is increasing (MPCA, n.d.). Summer droughts are also projected to worsen, partly due to increases in temperature (States at Risk, 2015) and longer times between rain events. These climate trends present unique challenges for managing urban stormwater systems. Green stormwater infrastructure (GSI) helps bolster a community’s climate resilience. Standard design practices for GSI can be modified or enhanced to better mitigate and adapt to climate change. These design mechanisms and practices include, but are not limited to
These are discussed below.
The previous sections show how GSI can be designed to improve resilience in the face of climate change. Effective GSI climate change design strategies begin during the planning stage of GSI. Local climate change trends and hazards should be identified early in the planning stage to effectively design GSI that can withstand and mitigate the added stress brought about by climate change. The Planning GSI Projects and Practices section of the MN Stormwater Manual includes useful guidance for planners and practitioners who want to implement GSI. Some key questions to ask include:
Planning for GSI systems can be informed by an understanding of future climate change projections as well as an understanding of past severe weather events. The Minnesota Department of Natural Resources has developed a Minnesota Climate Trends tool that compiles and displays historic precipitation, temperature, and drought severity index data to understand temporal trends in each of the Minnesota watersheds. This tool is helpful in identifying past severe events and understanding current trends in the data. The University of Minnesota used statistical downscaling to project changes to Minnesota’s climate at the end of the century (2100) and published a variety of maps showing projected changes to precipitation and temperature. The NOAA Atlas 14 Intensity-Duration-Frequency Curves, which are often used in engineering guidance, are currently being updated to incorporate climate change and are expected to be published in December of 2023. Other guidance is also available to update these curves based on climate change projections to update these curves (Batalini de Macedo et al., 2022).
Hydrologic and hydraulic (H&H) modeling of GSI can also help plan and design GSI for future climate change conditions. Most GSI model applications can effectively simulate conditions under different climate change precipitation and temperature projections. Precipitation and temperature projections are readily available from Global Climate Models, but these are often too coarse to apply at the local level. However, Maimone et al. (2019) provides guidance on scaling the Global Climate Model precipitation data to hourly and sub-hourly timeseries for local H&H model input. Note that temperature projections are sensitive to emission scenarios, so consideration should be given to which (and how many) emission scenarios to include in a modeling exercise. Several studies (Batalini de Macedo et al., .2022; De Paola et al., 2015; Madsen et al., 2014; Wang et al., 2013) recommend updating Intensity-Duration-Frequency precipitation curves to develop design storms under future climate change conditions. Additionally, it may also be appropriate to use historic extreme events in a model since these provide insight into the response of the local landscape and GSI to actual rainfall. When modeling future scenarios under climate change, also consider land use changes such as urbanization, since these often go hand-in-hand with climate change. Accounting for land use changes will increase peak flows and runoff volumes and add additional stress to GSI beyond that caused by climate change.
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Infiltration based GSI such as bioretention 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. Infiltration based GSI can also help mitigate the impacts of drought by enhancing groundwater recharge as well as help improve local water quality through natural processes associated with infiltration such as filtration, settling, and biological uptake of some pollutants. Enhancing the infiltration potential of GSI can be an important design criteria when designing climate resilient infiltration based GSI. The primary mechanisms to enhance infiltration include improving the soil/media of the GSI, decompacting the GSI soil, establishing deep-root vegetation, and increasing the storage volume of the GSI. These techniques can increase the rate of infiltration (i.e., how fast stormwater can infiltrate into the GSI) and improve the overall capacity for infiltration (i.e., how much stormwater can infiltrate into the GSI). Enhancing a GSI’s detention time can also help mitigate climate change impacts such as downstream localized flooding by storing more water upstream for a longer period of time and, in the case of infiltration based GSI, by providing more time for water to infiltrate. The primary mechanisms to enhance detention include increasing the GSI storage volume and increasing the residence time of the system, both of which can also enhance infiltration. The various mechanisms for enhancing infiltration and detention are explained in more detail below.
Soil and media improvements can be made through amendment with organic matter which promotes a healthy soil matrix and enhances infiltration (USDA, 2008). This is particularly helpful for soils that are compacted or consist largely of clay soil. Amendments should be added to the entire GSI rather than just the planting holes. Amendments can be incorporated throughout the GSI media using tillage equipment (USDA FS, 2020). If nutrients are a local water quality concern, avoid amendments with high phosphorus or nitrogen content as these can leach from the soil to surface water causing undesirable impacts like algal growth. Note that the benefits obtained from the amendment will diminish over time as the organic matter is broken down and consumed. It is, therefore, advised to incorporate other infiltration enhancement techniques such as decompaction and diverse deep-root vegetation to create lasting infiltration benefits.
Additional resources and guidance:
Soil compaction is common in urban environments and can reduce infiltration rates by as much as 70 to 99 percent (Gregory, 2006). Decompaction techniques can be used to break up dense, compacted soil. Both shallow (<18”) and deep soil decompaction (>18”) techniques exist. Deep soil decompaction will allow for roots, microbes, fungi, etc. to spread deeper into the soil which enhances the soil structure and increases infiltration. Accompanying decompaction with soil/media amendments also allows for more rapid vegetation establishment and better infiltration (USDA FS, 2020; Schwartz, 2021). Note that the effectiveness of decompaction depends on the soil moisture. Soil that is too dry is at risk of pulverization by decompaction which could create fine sediment that is likely to recompact during rain events (USDA FS, 2020), while decompaction on wet soils will not break up the soil effectively (Natural Resources Canada, 2018).
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Deep-rooted vegetation in GSI is a key component to enhancing infiltration and detention in GSI. Infiltration can be enhanced by the active roots creating macropores through which water can infiltrate and by dead roots providing organic matter to the soil and improving the integrity of the soil matrix. Additionally, leafy vegetation can shelter the soil beneath it by dissipating some of the energy of rain drops before they reach the soil surface and preventing crust formation (i.e., the formation of a compacted, poorly infiltrative layer at the top of the soil surface) on bare soil (Alizadehtazi et al., 2019). Vegetation can also enhance the detention capacity by removing water from the GSI through uptake and evapotranspiration, thereby emptying the GSI and regenerating the storage volume between rain events. Incorporating vegetation with high evapotranspiration rates into the GSI increases the removal rate and detention of runoff (GSA, 2011). In the case of retention practices, which are functionally similar to detention practices with the exception that the water is stored permanently, performance is driven more strongly by evapotranspiration rather than the GSI’s overall storage capacity, further emphasizing the importance of incorporating vegetation with high evapotranspiration rates into the GSI design (Johannessen et al., 2018). See the “Selecting Climate Resilient Vegetation” section below for more vegetation guidance.
Both infiltration and detention can be enhanced by increasing the storage capacity of the GSI. Increasing the storage capacity (i.e., making the GSI bigger) allows for more water to enter and be stored in the GSI, and depending on the surface area of the GSI, can also promote more diffuse infiltration into the surrounding soil. Historical and future projections of rainfall data can help determine how much bigger to make GSI. For example, if GSI are designed to retain the 90th percentile of rainfall, future rainfall projections may show that the 90th percentile of rain may be more in the future compared to twenty years ago. For GSI with limited area, as is common in urban landscapes, vertical walls provide greater surface storage capacity than graded edges.
Subsurface storage can be increased by increasing the dimensions (depth, width, length) of the GSI or by selecting media with higher void space. In areas with poor infiltrating native soils, underdrains are often required. While underdrains themselves do not add additional storage to GSI, they can be used to convey water from the GSI to the storm sewer system, reducing the risk of GSI flooding and allowing for more rapid draining of the GSI.
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Increasing the hydraulic residence time slows the rate of export of stormwater from the GSI and increases the detention time of water in the GSI. Residence time can be increased by lengthening the flow path between where water enters and leaves the GSI; by reducing the slope along the flow path; and, if using an underdrain, by reducing the size of the underdrain orifice. In tightly packed urban areas where there is not room to lengthen the flow path or expand the area of the GSI, increasing the GSI depth can be a meaningful way of increasing both the storage capacity and increasing the residence time. Residence time can also be increased by incorporating media with mixed particle size distribution that are packed more tightly, which will slow the rate of transport through the soil; however, this will also lower the infiltration rate, which may be undesirable depending on the design criteria for the GSI (GSA, 2011).
Enhancing interception and evapotranspiration through targeted plant and tree selection can allow more stormwater to be treated by the GSI by intercepting rainwater before it reaches the GSI or by uptake of the water in the GSI by the vegetation, thereby draining the GSI faster than through infiltration or underdrains alone.
Plants and trees with larger leaf surface areas provide greater opportunity for interception. Further, trees with more rigid and rough surfaced leaves have greater interception potential compared to flexible and smooth-surfaced leaves (Xiao and McPherson, 2016). This applies to bark as well: stemflow, or rainfall reaching the ground by conveyance down tree bark, decreases with increasing bark roughness (Barbier et al., 2009). This results in lower throughfall rates for rough-barked trees compared to smooth-barked trees (Beidokhti & Moore, 2021). Table 1 below shows interception rates for numerous mature coniferous and deciduous tree species. Interception by trees can also be estimated using models (Gómez et al., 2001 and Xiao et al., 2000)
In addition to enhanced interception, tree selection can also target species that have enhanced evapotranspiration capabilities. Tree xylem structure is one feature that can influence evapotranspiration since water is transported through the tree by xylem. Trees with deep sapwood and diffuse-porous xylem (e.g., birch and maple trees) have been shown to evapotranspire greater volumes of water than shallow sapwood species with ring-porous, semi-ring-porous, or tracheid xylem such as oak and conifers (USDA FS, 2020). Understory plants (i.e., plants under the tree canopy) can also support interception and evapotranspiration. Consider selection of shade-, heat-, and salt- tolerant vegetation to better prepare for climate change.
See the Selecting Climate Resilient Vegetation section below for more vegetation guidance.
Interception rate for various tree species
Link to this table
|Tree type||Tree name||Percent interception||Reference|
|Coniferous||Red cedar||76||Huang et al (2017)|
|Douglas fir||69||Huang et al (2017)|
|Red cedar||61||Asadian & Weiler (2009)|
|Douglas fir||49||Asadian & Weiler (2009)|
|Black pine||47||Zabret & Srag (2019)|
|White pine||43||Hathaway (2019)|
|Deciduous||White peppermint||44||Livesley et al (2013)|
|Gingko||38||Xiao & McPherson (2011)|
|Red maple||29||Hathaway (2019)|
|Willow oak||29||Hathaway (2019)|
|Blue gum||29||Livesley et al (2013)|
|Silver birch||24||Zabret & Sraj (2019)|
|Sweetgum||14||Xiao & McPherson (2011)|
|While native plants are the best choice for a GSI plant palette, it is important to choose the natives that are appropriate for the site’s particular conditions. Considerations include:
Vegetation helps enhance GSI functions like infiltration, interception, and evapotranspiration and can make GSI more resilient to climate change impacts such as increased rain, longer dry spells, or warmer temperatures. Vegetation that is native, climate change tolerant, and, if needed, salt tolerant, can help GSI function better and longer in the face of climate change impacts.
Native plants typically are better adapted to local conditions and have a natural resiliency to the variations in climate of the region where they are native. This makes them naturally more resilient to long term changes in climate change too. Additionally, native vegetation provides ecological benefits for local wildlife (ASLA, n.d.). The Minnesota Department of Natural Resources’ Native Plant Encyclopedia includes detailed information on native plants in Minnesota and the US Department of Agriculture Agricultural Research Service’s Plant Hardiness Zone Map identifies the plant types that are likely to thrive in different regions across Minnesota. Selection of native vegetation for GSI should include consideration for the local site conditions (e.g., soil type, amount of sunlight received, periods of drought, or standing water at the site). For instance, if the GSI will be holding water for an extended time, it is essential that the native plants can tolerate wet conditions while also potentially withstanding periods of drought. It is also important to incorporate a diverse palette (e.g., grouping of plants appropriate for certain site or environmental conditions) to reduce the risk of vegetation loss from plant diseases or insect infestations.
Native plants often produce extensive root structures that assist in creating a complex and diverse soil structure which promotes infiltration and a healthy soil structure. If selecting vegetation to increase infiltration, design a plant palette with a diversity in plant roots. Shallow-rooted vegetation typically establish quickly but deep-rooted and slower-growing vegetation such as trees will over time develop extensive root networks and should also be considered for selection. In fact, trees have been shown to increase infiltration at a young age (Bartens et al., 2008).
To select vegetation for climate resilience, consider future rainfall, temperature, and drought potential. See the Climate Adapted Urban Trees List from the MN Department of Natural Resources for a breakdown of trees suitable to several climate change conditions (e.g., increased temperatures, more frequent drought). Incorporate drought-tolerant plants that will survive long dry spells once established (typically after the first growing season). Additionally, consider salt-tolerance when selecting vegetation for GSI particularly when designing GSI near roads and paths that are likely to receive salt application in the winter. Salt reduces a root’s ability to absorb water, and the sodium in road salt can attach to the soil particle and increase soil density and compaction which has negative infiltrative results (Gould, 2013). Refer to the Minnesota stormwater manual “Salt Tolerant Plants” page for lists of salt tolerant vegetation (including trees) as well as recommended salt tolerant mixes.
Additional Resources and Guidance:
Protecting and preserving natural areas from development is one of the most effective ways to mitigate future climate change. Preserved land can protect nearby neighborhoods from heavy rain and potential flooding, protect water quality, sequester carbon, reduce surrounding air temperature, and provide ecological habitat. The benefits of preserving natural areas are unrivaled compared to the benefits of any other GSI system. Communities can be selective in preserving natural areas, prioritizing, for example, natural areas around water bodies or creating continuous corridors of natural areas rather than stand-alone pockets. Preserving existing trees is an important component of preserving natural areas. Mature trees have a more expansive canopy cover and root network than saplings or young trees, so they are capable of providing greater shade, interception, evapotranspiration, and infiltration benefits and do not require the potentially lengthy maturation period that new plantings would require. Natural areas can also be used as means of connecting GSI through continuous corridors to design an interconnected, treatment train approach to stormwater treatment as well as to allow for connectivity of species and ecological processes through the GSI system (Snäll et al., 2016).
Rainwater harvest and reuse systems capture and store rainfall on-site and reduce the volume of stormwater going into the storm sewer network. Rainwater harvest and reuse systems can be designed to mitigate climate change trends such as increased rainfall and increased periods of drought by increasing the capacity of the storage tank. This allows more rainwater to be captured and stored on-site for future use during dry periods, and for less rainwater to flow to, and potentially overwhelm, the storm sewer system.
To determine the size of the rainwater storage tank, consider future precipitation projections as well as historic extreme storm events. The Minnesota Department of Natural Resources has developed a Minnesota Climate Trends tool that compiles and displays historic precipitation data to understand temporal trends Minnesota watersheds. This tool is helpful in identifying past severe events and understanding current trends in the data. The University of Minnesota used statistical downscaling to project changes to Minnesota’s climate at the end of the century (2100) and published a variety of maps showing projected changes to precipitation and temperature. The NOAA Atlas 14 Intensity-Duration-Frequency Curves, which are often used in engineering guidance, are also currently being updated to incorporate climate change and are expected to be published in December of 2023.
When including rainwater harvesting and reuse systems in a building’s design, also consider using smoother roofing material (e.g., standing seam metal or corrugated metal) to reduce water losses and capture a greater portion of the rainwater. This provides more reserve water for use during dry spells. Additionally, dynamic design with real-time controls can prevent losses that occur from overtopping the storage tank. Real-time controls optimize water reuse and draw down the stored water prior to storm events to increase storage capacity (Traver & Ebrahimian, 2017; Xu et al., 2018). Incorporating real-time controls increases the long-term storage capacity without increasing the physical footprint of the storage tank. For more information on real-time controls, see the “Implementing Real-Time Controls (Dynamic Design)” section below.
Combining grey and green practices can be a powerful approach to climate resilient stormwater management in urban MS4 communities. Gray stormwater infrastructure was traditionally used to whisk stormwater away as quickly as possible from public property as a flood mitigation technique. Examples of grey infrastructure include stormwater pipes, ditches, culverts, and traditional detention ponds. Grey infrastructure is or was typically designed for a certain design storm, and are costly to retrofit to changing rainfall patterns. However, grey practices can be combined with green infrastructure to provide more capacity for stormwater mitigation, particularly in light of climate change, and to provide more ecological benefits. For example, some communities have programs to disconnect roof leaders from stormwater sewer and install rain gardens between the roof leaders and the stormwater sewers to promote stormwater infiltration, provide local ecological benefits, and restore some of the natural hydrologic cycle that had been previously lost.
Prior to designing a combined grey-green system, a hazard analysis should be performed to identify weak points in the existing grey infrastructure that could be ameliorated with GSI. For example, if flood control capacity is a weak point, the additional required upstream flood capacity should be determined and used to size the GSI. If contamination or poor water quality is an issue, the required water quality improvements should be incorporated in the GSI selection and design process. Additional considerations include the timeline for the GSI to mature and reach full effectiveness. Any anticipated water quality or water quantity benefits should be assessed in the context of this timeline, as well as ecosystem and public health benefits. Gray and green infrastructure can be used in series as part of a treatment train. See the section “Sequencing GSI in Treatment Trains” below for more information.
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A GSI treatment train is the application of multiple GSI in sequence. One example of this is the sequence of a green roof, rain barrel, permeable pavement, bioretention, and sewer (Figure 4). Rain and runoff will be channeled from one GSI practice into the next, allowing for greater runoff storage, infiltration, and treatment than a single GSI could achieve on its own. Treatment trains provide stormwater capture and treatment redundancy, thereby improving the ability to capture increased rainfall anticipated under climate change.
To determine the size of the GSI included in treatment trains, consider future precipitation projections as well as historic extreme storm events (refer to section “Enhancing Harvest and Reuse” for additional guidance on future precipitation projections). Treatment trains are easiest to design as part of an initial GSI design during the planning stages of development, but they can also be included as part of a retrofit of existing GSI to adapt to climate change.
Additional Resources and Guidance:
Dynamic design uses real-time rainfall data and weather forecasting to automatically open or close GSI controls such as valves or pumps to control the timing and rate of stormwater flows through the GSI. This allows for GSI to, for example, release stored water to increase storage capacity before a storm event. Incorporating real-time controls increases the long-term storage capacity without increasing the physical footprint of the GSI. Dynamic design can enhance detention and infiltration and can be used as part of a grey-green or treatment train approach (Lewellyn et al., 2017).
Continuous monitoring and real-time control can be used to release water between and prior to storm events. This increases the available storage capacity of the system, promotes greater infiltration and evapotranspiration if the water is released to a vegetated infiltration based GSI (Traver & Ebrahimian, 2017) and also has the potential to prevent downstream flooding by strategically capturing and releasing water. Additional Resources and Guidance: