With the heavy snowfall this year, Utahns are in for greater springtime snowmelt runoff. A better way to deal with the extra water is green infrastructure, an alternative to traditional stormwater management, which focuses on large, centralized networks to handle nuisance water.
The term nuisance is used since some urbanized areas combine their sewer networks (i.e. both wastewater and stormwater), while others separate them. Unfortunately, combined sewer systems are notorious for overflowing during large precipitation events. In 2008, overflows caused contamination of waterways in over 750 communities, affecting nearly 40 million people nationwide (see map).
Stormwater runoff alone carries increased potential for pollution to enter the water system. How many times have you seen trash, leaves, and dirt running down the gutters? Where do you think all of that goes? Therefore, in an effort to lessen the strain upon the nation’s sewer networks, an alternative was proposed – green infrastructure.
According to the Environmental Protection Agency (EPA), green infrastructure “uses vegetation, soils, and natural processes to manage water and create healthier urban environments…[via a] patchwork of natural areas that provides habitat, flood protection, cleaner air, and cleaner water.” More detailed information on these practices, including fact sheets, benefits, and case studies, can be found here. However, we’ll examine one example within each broad category of green infrastructure: vegetative, subsurface, and surface storage.
Using plants and soils to tame runoff
Vegetative green infrastructure uses plants and soils to slow, delay, and treat stormwater runoff. This is in response to the effect that impervious surfaces have on runoff, including causing faster rates and larger quantities of runoff to be transported to streams. Rain gardens are on-site, vegetated areas that promote filtration and evaporation of detained stormwater runoff. They are a great ultra-urban solution since they can manage high pollutant concentrations and be placed near sensitive receiving bodies. These can be retrofitted into existing projects or integrated into the design of a new site.
Regional characteristics should be considered since they affect design and implementation. For instance, even in arid Utah, it’s possible to choose plants that require little to no irrigation. In the Salt Lake City area, 95% of storm events yield approximately 0.8 inches of precipitation. Stats like this should be taken into consideration when designing rain gardens, so that it doesn’t flood.
Other examples of vegetative methods include planter boxes, bioswales, green alleys and streets, green roofs, urban tree canopy, and land conservation practices.
Capturing stormwater underground
Subsurface storage captures, stores and treats stormwater below the ground. Permeable pavement is a mode of subsurface storage that comes in many different styles, including pervious concrete, porous asphalt, and pavers. Physically, permeable pavement may not appear to be different than a typical street or parking lot, but looks can be deceiving. Within a hard surface that you can stand upon are void spaces (i.e. empty pockets). Think of a fluffy pancake. The fluffier the pancake, the more syrup it can store and the tastier the breakfast, yet I digress. By increasing the “fluffiness” factor of a road surface, water that falls on or runs over the area soaks through to the other side.
Beneath the pavement layer is a vault, like an earthen storage tank, filled with either natural soil or an engineered soil mix. This vault temporarily stores the infiltrated water and allows for it to enter the groundwater. For great examples, check out the west side of the Frederick Albert Sutton building (U of U campus) and the Wasatch Touring (702 East, 100 South) parking spaces.
Subsurface storage can be particularly effective in Utah, where flashy precipitation can overwhelm surface storage methods. Event flashiness is the amount of time in which precipitation falls. Faster storms with larger amounts produce greater magnitudes of runoff and, therefore, are flashier.
Other examples of subsurface storage include rain gardens, green streets and alleys, green parking, and green roofs.
Capturing stormwater aboveground
Surface storage intercepts stormwater runoff and provides storage and potential for treatment. Rainwater harvesting is often the first example that comes to mind, which directs a household’s rooftop runoff to a barrel for future uses (e.g. watering lawns). This reduces the amount of runoff entering the storm sewer system, reintroduces water into the natural hydrologic cycle (i.e. water soaking into the ground rather than being directed to the sewer), and has the potential to reduce demands on potable water. However, increasing treatment is required with increasing human contact.
Rainwater harvesting legislation in Utah (Senate Bill 32), passed in 2010, allows individuals to use either cisterns (i.e. larger volume) or barrels (i.e. smaller volume) to collect rainwater. The bill limits users of a land parcel, registered with the Utah Division of Water Rights, to either one (1) underground 2,500 gallon cistern or two (2) aboveground 100 gallon barrels.
Other examples of surface storage include rain gardens and bioswales.
Green infrastructure can be useful to communities, neighborhoods, and individuals. It is important, as citizens of watersheds, to be mindful of the opportunities available and to understand one’s connection to the greater hydrologic system.
Further examples and expertise on such practices can be found at the University of Utah, with active research carried out by the Urban Water Group. If you’re keen to Twitter, then follow us @UrbanWaterUtah and @UofUTheFlood
Map photo courtesy of EPA, other photos courtesy of Thomas Walsh