When it Rains, it Pours
Managing stormwater and flooding with Green Infrastructure
Faced with compounding crises of COVID-19 and the related economic fallout, all against a backdrop of intensifying climate change and natural disasters, cities and nations worldwide are drafting green recovery plans aimed at putting people to work developing a more sustainable economy with lower impact on natural resources and ecosystems.
Driven by activists and advocates pushing for stimulus and relief that furthers both economic and climate goals, policymakers need investments and policies that can put people to work while improving the local environment and resiliency to climate change. Extreme weather, including both drought on the one hand and more severe rainfall and flash flooding events on the other, is a predictable and present outcome of our warmer, more volatile climate.
Green stormwater infrastructure reduces the harmful flooding or pollution impacts of more erratic rainfall by capturing stormwater and soaking it into the ground, which also generates wide ranging benefits for the local community and ecosystems. Storm resilience and other climate adaptive infrastructure are natural places to look for economic stimulus projects that can simultaneously repair natural resources.
The standard engineering approach to flood management generally looks to channel stormwater off each lot and onto the street as efficiently as possible, funnel it into storm sewers, and send it away to rivers or streams. In contrast to this strategy of getting water offsite, the sustainable alternative pursues the opposite goal of capturing and storing as much rain as possible close to where it falls, summarized with the motto “Slow it down, spread it out, soak it in.”
Green stormwater capture is valuable across climate zones, regardless of the location’s typical rainfall. In desert cities like Phoenix and Tucson, Arizona, on-site rain collection can cut reliance on expensive traditional stormwater infrastructure that is used only sporadically during the brief but rainy monsoon season. Harvested rain can also displace water used for lawns and outdoor landscaping with a renewable source; this potential for water conservation is especially significant in drier regions like the American West, where outdoor irrigation can account for half or more of a household’s total water use.
Other regions, like the Mediterranean climates along the US Pacific coast, also benefit from soaking more rainwater into the soil. The distinct wet and dry seasons in these places mean storing water in the ground for the hotter period is especially valuable. Banked rain can feed areas planted with hardy shade trees and native plants that are well-adapted to survive the long dry summers without additional irrigation, providing local cooling and habitat for birds and insects.
Depletion of streams and groundwater aquifers also contributes to stress on forest ecosystems that makes them more vulnerable to catastrophic fire and long-term disease or pest damage. Although management and protection of wilderness and undeveloped land is key to fully protecting our watersheds, green stormwater management enables cities and urbanized areas to contribute sustainable solutions to stream restoration.
Cities like Seattle already invest in Green Streets programs that integrate planted medians and other green space features to serve multiple functions and create a more user-friendly, multi-purpose street design. Bump-out rain gardens, mulched and runoff-collecting medians, landscaped roundabouts, and other bio-retention designs can also calm vehicle traffic and create a more comfortable environment to walk and bike. Replacing heat-absorbent pavement with trees, groundcover plants, and organic material also creates shade and mitigates the urban heat effect.
In the rainiest cities prone to especially large storm events, like New Orleans or Houston, naturally-absorbent surfaces and rain collection basins reduce runoff flow and the resultant flooding; heavy rain can quickly overwhelm traditional storm drains and release sewage overflow or road pollutants into streams and water supplies.
Policies to encourage deployment of green stormwater infrastructure can range from design guidelines to financial incentives to active mandates. (The right rule to reach a specific goal depends) on the local political mood and scope of policy makers’ authority, along with whether the policy aims to transform public assets like city streets or parks, or focuses on private city engineering like designs for new subdivisions or commercial properties.
Most cities that actively encourage green stormwater projects use a combination of requirements and incentives to enforce a baseline of sustainability goals and reward actors who go beyond the minimum. Local planning agencies also regularly include exactions or “impact fees” as a condition for approving new development, generally intended to mitigate possible negative impacts of the project.
One straightforward way to achieve policy outcomes, especially changes in the built environment, is to require them directly. In 2019, New Orleans passed a permeable street mandate requiring nontraditional pavement in many new road and parking lot projects. Rather than non-permeable asphalt, which lets rainwater collect and stagnate until it drains or gets pumped away, permeable pavement designs allow water to soak into the ground and organic matter in the soil.
This surface more closely performs with the water retention qualities of unpaved ground and reduces stormwater runoff, “creat[ing] more storage,” the New Orleans director of Public Works said. “It doesn’t solve the flooding problem in the city but it definitely helps.” By cutting the runoff that floods hardtop streets during major storms, absorbent parking and streets can help reduce strain on crucial stormwater infrastructure. In many flood-prone cities, the storm drains and pumps that keep the streets dry are often under-maintained and unprepared to deal with more frequent and more severe rain events resulting from climate change.
The fact that green storm improvements can drastically reduce localized flooding with a variety of ancillary benefits and relatively low costs and implementation time should be central to building a case for public support of these projects. In Kansas City, planners earned buy-in from neighborhood residents in support of new green stormwater projects by tying those improvements to sidewalk repairs, revealing another stacked benefit to the pedestrian environment of this infrastructure. On a larger scale, many flood retention basins double as open recreation space or park facilities during drier periods, and proper drainage design and use of native plants can compound the ecosystem and community benefits.
Standard requirements also help propagate best practices and engineering developments to ensure the best environmental result for a manageable cost. Using organic mulches to cover capture basins is one such key element of green stormwater management, because:
The use of organic mulch promotes healthy soils, the ability to process stormwater pollutants, cooler surface temperatures, enhanced soil moisture retention, and a reduction in germination of undesirable plants. The use of organic mulch also reduces maintenance and disposal costs since plant trimmings can be incorporated directly into surface mulch. (City of Avondale, AZ: Street Tree Master Plan Supplement for Integrating Green Stormwater Infrastructure)
Cities can also devote resources to subsidizing private improvements, like green roofs that capture and treat stormwater, to achieve public benefits and cost savings. For example, the Dutch city of ‘s-Hertogenbosch offers a green roof subsidy of up to €3,000 for individuals, or €15,000 for owners’ associations, based on the size and water retention capacity of the installation. Owners of smaller roofs that add up to the minimum size may also apply together, allowing further uptake and buy-in from a diversity of residents and businesses.
The Chinese city of Shenzhen is pursuing one of the more ambitious targets for green stormwater deployment with its “sponge city” plan. A large 2012 “rainstorm in Beijing led to flooding that caused 79 deaths and an estimated $1.7 billion in damage, galvaniz[ing] national leaders,” to the importance of localized rain absorption and a build-up of capacity to absorb extreme flooding. From scattered pilot projects to incorporation into the city’s master planning documents, the initiative has encouraged and mandated use of absorbent roofs, open pavement, garden spaces, and other designs aimed at preventing or mitigating future flood damage.
Few of the solutions to overwhelmed storm drains and runoff pollution are especially high-tech or require a breakthrough technology to become practical. Mostly, these problems require only a willingness to reconsider the status quo practices of engineering and design to deal with stormwater in a way more closely mimicking nature: roots and branches and soil intercept, treat, and store the rain in an ecosystem, and we can harness this power to create “spongier,” more comfortable, more sustainable cities.