Mimicking the Soil with Green Infrastructure
Green stormwater infrastructure refers to the network of green spaces in our cities that collectively provide stormwater management, recreation, and wildlife habitat. Central to this concept is the intentional practice of using natural or nature-inspired systems to manage stormwater on site.
This method is quickly being recognized as a low-cost alternative to traditional engineered systems, often termed grey infrastructure. Unlike these traditional solutions, green infrastructure increases the permeability of our cityscapes so that they behave more like natural soils—slowing, holding, and purifying stormwater. By managing stormwater where it falls, runoff to natural waters and the burden on wastewater treatment facilities is also reduced.
Types of green infrastructure
Green stormwater infrastructure includes permeable pavement, green roofs, urban forests, wetlands, and rain gardens and bioswales. Green infrastructure can be engineered, as with a bioswale, or nature-made, such as a preserved wetland. Preserved, natural systems have the advantage of not requiring upfront construction and design costs. Depending on their size, they may also provide more recreational opportunities and be capable of processing larger quantities of water.
Manmade systems, on the other hand, can be customized to meet a particular need and may require less land area to be effective.
Central to the success of any individual system is that it’s interconnected with others to create a more powerful whole. This requires strategic planning involving many community stakeholders. To learn how communities around the country are expanding their green infrastructure through highway projects, land preservation, and urban renovations, visit the Conservation Fund’s Green Infrastructure Project Profiles.
The Importance of soil
All these tools rely directly or indirectly on soil to achieve stormwater management strategies that are more affordable and effective than traditional grey infrastructure systems. While projects like urban forests rely on native soils, the soils used in green infrastructure systems, including rain gardens and bioswales, are specifically designed to infiltrate water quickly to reduce flooding. At the same time, these special soil mixtures must retain water long enough to remove contaminants and clean the water.
As the ratio of soil surface to rainwater is a lot smaller in cities than in natural systems, more is expected of these soils. For example, a bioswale will be designed to filter stormwater from a surrounding area much larger than the bioswale itself. Therefore these soils must be carefully designed both to manage stormwater quantity and quality.
When designing soil mixtures for bioswales, there are a few system requirements to keep in mind. Bioswale soils must:
- Infiltrate water quickly, but not too quickly. Water needs to travel slowly enough through the soil for filtering and adsorption of contaminants but must also infiltrate fast enough to prevent flooding.
- Resist compaction. Compaction of soils can greatly reduce infiltration rates, making a rain garden or bioswale ineffective.
- Hold enough water and nutrients to maintain healthy plant life. While soil alone can significantly reduce water pollution, no one will want a “rain garden” of bare soil or half dead plants in their front yard.
So what are these special soil mixtures composed of? Two specific soil ingredients are necessary to achieve the objectives above: sand and compost. Other ingredients might include topsoil and other ingredients such as water treatment residuals, wood chips, or perlite.
Many urban areas are adopting green infrastructure to avoid the costs of updating grey infrastructure systems and the negative environmental impacts of untreated stormwater. Beyond such obvious savings as reduced spending on water treatment, added benefits include energy savings and social benefits.
The Center for Neighborhood Technology addresses how to quantify these various savings in their report, The Value of Green Infrastructure. They also offer an interactive tool, the Green Values Calculator, which lets users specify local project parameters to determine the best stormwater reduction method based on cost and environmental impact.
Because so many approaches exist for doing economic assessments, the U.S. EPA wrote a report, Case Studies Analyzing the Economic Benefits of Low Impact Development and Green Infrastructure profiling how three communities assessed their cost savings.