Green infrastructure for stormwater management

Green infrastructure is defined in the United States by section 502 of the Clean Water Act as the range of measures that use plant or soil systems, permeable surfaces, stormwater harvest and reuse, infiltrate or evapotranspirate stormwater and reduce flows to sewer systems or to surface waters. Green infrastructure encompasses various water management practices such as vegetated rooftops, absorbent gardens and other measures to capture, filter, and reduce stormwater. Green infrastructure prevents or reduces the amount of water that flows into storm drains and is proved as an important tool for cities with combined sewer overflows and nutrient problems. It provides many environmental, social, and economic benefits such as improved surface water quality, water conservation, and community safety. Green Infrastructure is a cost effective and resilient approach to managing stormwater. While gray stormwater infrastructure is designed to move stormwater away from the built environment, green infrastructure treats the water at the source.
History.
The National Research Council's definitive report on urban stormwater management described that urban drainage systems began in the United States after World War II. These structures were based on simple catch basins and pipes to transfer the water outside of the cities. Urban stormwater management started to evolve more in the 1970s when landscape architects focused more on low-impact development and began using practices such as infiltration channels. Parallel to this time, scientists started becoming concerned with other stormwater hazards surrounding pollution. Studies such as the Nationwide Urban Runoff Program showed that urban runoff contained pollutants like heavy metals, sediments, and pathogens, all of which water can pick up as it flows off of impermeable surfaces. It was at the beginning of the 21st century where stormwater infrastructure to allow runoff to infiltrate close to the source became popular. This was around the same time that the term green infrastructure was coined.
Types of green infrastructure.
Green and blue roofs.
Green roofs improve water quality while also reducing energy cost. By providing an extra layer of insulation, green roofs help minimize cooling and heating energy costs and usage. Green and blue roofs also help reducing city runoff by retaining rainfall providing a potential solution for the stormwater management in highly concentrated urban areas.
Green roofs also sequester rain and carbon pollution. Forty to eighty percent of the total volume of rain that falls on green roofs are able to be reserved. The water released from the roofs flow at a slow pace, reducing the amount of runoff entering the watershed at once.
Blue roofs, not technically being green infrastructure, collect and store rainfall, reducing the inrush of runoff water into sewer systems. Blue roofs use detention ponds, or detention basins, for collecting the rainfall before it gets drained into waterways and sewers at a controlled rate. As well as saving energy by reducing cooling expenses, blue roofs reduce the urban heat island effect when coupled with reflective roofing material.
Rain gardens.
Rain gardens are a form of stormwater management using water capture. Rain gardens are shallow depressed areas in the landscape, planted with shrubs and plants that are used to collect rainwater from roofs or pavement and allows for the stormwater to slowly infiltrate into the ground . Rain gardens mimic natural landscape functions by capturing stormwater, filtering out pollutants, and recharging groundwater. A study done in 2008 explains how rain gardens and stormwater planters are easy to incorporate into urban areas where they will improve the streets by minimizing the effects of drought and helping out with stormwater runoff. Stormwater planters can easily fit between other street landscapes and ideal in areas where spacing is tight.
Downspout disconnection.
Downspout disconnection is a form of green infrastructure that separates roof downspouts from the sewer system and redirects roof water runoff into permeable surfaces. It can be used for storing stormwater or allowing the water to penetrate the ground. Downspout disconnection is especially beneficial in cities with combined sewer systems. With high volumes of rain, downspouts on buildings can send 12 gallons of water a minute into the sewer system, which increases the risk of basement backups and sewer overflows. In attempts to reduce the amount of rainwater that enters the combined sewer systems, agencies such as the Milwaukee Metropolitan Sewerage District amended regulations that require downspout disconnection at residential areas.
Bioswales.
Bioswales are stormwater runoff systems providing an alternative to traditional storm sewers. Much like rain gardens, bioswales are vegetated or mulched channels commonly placed in long narrow spaces in urban areas. They absorb flows or carry stormwater runoff from heavy rains into sewer channels or directly to surface waters. Vegetated bioswales infiltrate, slow down, and filter stormwater flows that are most beneficial along streets and parking lots.
Permeable surfaces.
Cities all around are filled with streets that obstruct the filtration of water into the ground. Permeable surfaces let water infiltrate into the soil where it the soil then can filter out different pollutants as well as recharge the water table underground. Impermeable surfaces cause an array of problems such as water pollution and the flooding of surface water. This practice could be particularly cost effective where land values are high and flooding or icing is a problem. It can cost as much as fifty percent less than conventional pavement systems and can be cheaper in the long run to maintain.
Benefits for stormwater management.
Green infrastructure keeps waterways clean and healthy in two primary ways; water retention and water quality. Different green infrastructure strategies prevents runoff by capturing the rain where it lies, allowing it to filter into the ground to recharge groundwater, return to the atmosphere through evapotranspiration, or be reused for another purpose like landscaping. Water quality is also improved by decreasing the amount of stormwater that reachers other waterways and removing contaminants. Vegetation and soil help capture and remove pollutants from stormwater in many ways like adsorption, filtration, and plant uptake. These processes break down or capture many of the common pollutants found in runoff.
Reduced flooding.
With climate change intensifying, heavy storms are becoming more frequent and so is the increasing risk of flooding and sewer system overflows. According to the EPA, the average size of a 100 year floodplain is likely to increase by 45% in the next ten years. Another growing problem is urban flooding being caused by too much rain on impervious surfaces, urban floods can destroy neighborhoods. They particularly affect minority and low-income neighborhoods and can leave behind health problems like asthma and illness caused by mold. Green infrastructure reduces flood risks and bolsters the climate resiliency of communities by keeping rain out of sewers and waterways, capturing it where it falls.
Increased water supply.
More than half of the rain that falls in urban areas covered mostly by impervious surfaces ends up as runoff. Green infrastructure practices reduce runoff by capturing stormwater and allowing it to recharge groundwater supplies or be harvested for purposes like landscaping. Green infrastructure promotes rainfall conservation through the use of capture methods and infiltration techniques, for instance bioswales. As much as 75 percent of the rainfall that lands on a rooftop can be captured and used for other purposes.
Heat management.
A city with miles of dark hot pavement absorbs and radiates heat into the surrounding atmosphere at a greater rate than a natural landscapes do. This is urban heat island effect causing an increase in air temperatures. The EPA estimates that the average air temperature of a city with one million people or more can be 1.8 degrees to 5.4 degrees F warmer than surrounding areas. Higher temperatures reduce air quality by increasing smog. In Los Angeles, a 1 degree temperature increase makes the air roughly 3 percent more smog. Green roofs and other forms of green infrastructure help improve air quality and reduce smog through their use of vegetation. Plants not only provide shade for cooling, but also absorb pollutants like carbon dioxide and help reduce air temperatures through evaporation and evapotranspiration.
Health benefits.
By improving water quality, reducing air temperatures and pollution, green infrastructure provides many public health benefits. Cooler and cleaner air can help reduce heat related illnesses like exhaustion and heatstroke, as well as respiratory problems like asthma. Cleaner and healthier waterways also means less illness from contaminated waters and seafood. Greener areas also promote physical activity and can boost mental health.
Reduced costs.
Green infrastructure is often cheaper than more conventional water management strategies. Philadelphia found that its new green infrastructure plan will cost $1.2 billion over 25 years, compared with the $6 billion a gray infrastructure would have cost. The expenses for implementing green infrastructure are often smaller, planting a rain garden to deal with drainage costs less than digging tunnels and installing pipes. But even when it isn’t cheaper, green infrastructure still has a good long term effect. A green roof lasts twice as long as a regular roof, and low maintenance costs of permeable pavement can make for a good long term investment. The Iowa town of West Union determined it could save $2.5 million over the lifespan of a single parking lot by using permeable pavement instead of traditional asphalt. Green infrastructure also improves the quality of water drawn from rivers and lakes for drinking, which reduces the costs associated with purification and treatment, in some cases by more than 25 percent. And green roofs can reduce heating and cooling costs, leading to energy savings of as much as 15 percent.
The future of green infrastructure.
Communities are starting to understand that green infrastructure can deliver numerous economic and environmental benefits. For example, in the United States, cities across the country are taking major green infrastructure initiatives. Such as Philadelphia where they have one of America’s oldest sewer systems. The city is investing $2.4 billion to resurface 10,000 acres of impermeable surfaces to manage stormwater runoff. One aspect of this program is its grant program to help along the voluntary development of green infrastructure on private property. There are plenty of ways green infrastructure can be used on a smaller scale as well, including in our own homes. Simple strategies range from the use of disconnected downspouts and permeable surfaces for outdoor spaces.