Green infrastructure in the context of stormwater management is any structure, created environment, or management policy that improved water permeability to the soil and/or reduces runoff. Throughout this series we have talked about how runoff is the ultimate problem with stormwater, and so if we can reduce runoff we also reduce harmful pollutant loading and reduce peak flows. While most green infrastructure projects include the use of vegetation to meet this goal, hence why it is called “green”, some infrastructure, such as permeable pavement discussed below, do not. Given the objective of green infrastructure is so broad, and that it doesn’t even have to be “green” to qualify, one can imagine that there are a limitless number of ways green infrastructure can look in practice. The number only increases when one looks to the city-scale, where it is common for a combination of several practices and techniques to be used simultaneously.
The purpose of this post is not to provide a comprehensive list of all the techniques and practices available to a municipality in their stormwater management plans, but rather to provide a handful of commonly used examples that have been successfully employed in cities across America to reduce runoff, improve water quality, and reduce costs.
Bioswales
Bioswales are aboveground channels with gently sloped edges meant to move stormwater above the surface, similar to how gutters and channels underground, but slower (NRC, 2017). Due to their linear nature, they are perfect techniques for implementing alongside roads and parking lots and can replace traditional curb-and-gutter systems. The important difference is that bioswales allow for infiltration of water to the ground, thus reducing runoff, and provide several other water quality and volume benefits.
Bioswales are typically lined with vegetation, mulch, and/or riprap, a material designed to prevent erosion. Vegetated bioswales also reduce stormwater volume through the process of evapotranspiration, where plants absorb the water and use it for photosynthesis. Vegetation also helps filter stormwater, reducing the nutrient load, toxicity, and amount of suspended solids, like debris and sediment, of water that does not infiltrate the soil and makes its way to either waterbodies or the sewer system (Anderson et al. 2016). Depending on the type of vegetation used, some plants, such as certain mustard plants and ragweed (Rascio & Navari-Izzo, 2011), can also help reduce the amount of heavy metal pollutants in runoff.
Permeable Pavement
Permeable pavement is an interesting example of green infrastructure, as there is no necessary “green” or vegetative component to them. Remember that green infrastructure is a broad category when it comes to stormwater management, and that it encompasses anything that improves water infiltration in the soil. Permeable pavement looks an awful lot like gray infrastructure, except it does not use impervious materials. Instead of paving roads, sidewalks, or parking lots with impervious asphalt, this technique uses materials such as porous concrete that allow water to pass through (Scholz & Grabowiecki, 2007).
After water passes through the permeable materials a number of things can happen depending on the project’s design. The simplest form would essentially place the pavement directly on soil, with certain modification to prevent too much soil compaction and still allow structural support (Scholz & Grabowiecki, 2007). This way, water flowing through the pavement can directly infiltrate the ground. Another more indirect method would be to direct water under the pavement to a retention tank (Fassman & Blackbourn, 2010), which can then be allowed to slowly infiltrate the ground or be used as a source for watering lawns and landscaping; however, this water should not be used for vegetation meant for human consumption, as it can have an increased concentration of certain toxins (Harvey et al. 2013).
City Greenspace
The last example is greenspace in and around cities. While the topic of “greenspace” is wide, we typically do not think of things like parks and forest patches as “infrastructure;” however, in terms of stormwater management the existence of such places reduces the total amount of impervious surface in a city, thus reducing runoff and we can think of it as green infrastructure. Greenspace in cities can range from lines of street trees, to community gardens, to vegetated vacant lots, and designated parks. The important part is that they serve to break up the concrete jungle of impervious surface many cities have and replace it with vegetation. As we said earlier about bioswales, vegetation serves to reduce runoff, reduce nutrients, and filter pollutants in our stormwater before it reaches sewers and waterways. Of course, greenspaces like parks and gardens can be much larger than bioswales, and accommodate larger and denser vegetation, thus compounding these positive benefits.
In addition, greenspaces also serve many other functions for the community. Street trees provide an aesthetic beauty to neighborhoods and can improve property values and help form a community identity (Westphal, 2003). Parks provide recreational opportunities, offering opportunities for healthier lifestyles (Westphal, 2003), as well as valuable habitat for wildlife. City trees in general provide cleaner air and lower a city’s carbon footprint through carbon sequestration (Morancho, 2003). Homes shaded by trees experience lower cooling costs during summer months (Morancho, 2003). The list goes on, and it is clear that incorporating greenspace into stormwater management yields much higher benefits to society than traditional gray infrastructure techniques when considering all the different ways greenspace is good for people and the planet.
Moving Forward
These are just a few examples of the techniques available for city planners to incorporate green infrastructure into their stormwater management plans to improve water quality and decrease runoff issues. In practice, techniques are rarely used in isolation and good designs have a well rounded portfolio of green infrastructure. We can see that these techniques not only address stormwater but have many other benefits to people and the environment. In the final post of this series, we will investigate how green infrastructure fits into the concept of urban sustainability.
References:
Anderson, B., Phillips, B., Voorhees, J., Siegler, K., & Tjeerdema, R. 2016. Bioswales reduce contaminants associated with toxicity in urban storm water. Environmental Toxicology and Chemistry, 35(12), 3124-3134.
Fassman, E. & Blackbourn S. 2010. Urban runoff mitigation by a permeable pavement system over impermeable soils. Journal of Hydrological Engineering, 15(6), 54-62.
Li, H., Harvey, J., Holland, T., & Kayhenian, M. The use of reflective and permeable pavements as a potential practice for heat island mitigation and stormwater management.
Environmental Research Letters, 8(1), 1-14.
Morancho, A. 2003. A hedonic valuation of urban green areas. Landscape and Urban Planning, 66(1), 35-41.
Naturally Resilient Communities (NRC). 2017. Bioswales. Retrieved from: http://nrcsolutions.org/bioswales/ on 04/16/2019.
Rascio, N. & Navari-Izzo, F. 2011. Heavy metal hyperaccumulating plants: How and why do they do it? And what makes them so interesting? Plant Science, 180(2), 169-181.
Scholz, M. & Grabowiecki, P. 2007. Review of permeable pavement systems. Building and Environment, 42(11), 3830-3836.
Westphal, L. 2003. Urban greening and social benefits: A study of empowerment outcomes. Journal of Arboriculture, 29(3), 137-147.