The Building Blocks of Green Stormwater Infrastructure Design
Green stormwater infrastructure (GSI) has an important job: to reduce and treat stormwater at its source. To do that job, it must be designed with the right components to capture, filter and store stormwater properly. The design must also include features that protect its integrity and promote its overall functionality.
While each GSI design must be tailored to the site where it will be implemented, such solutions can share many common features. The nine primary components that should be considered on GSI projects include:
Inlets — GSI designers are often challenged with finding a way to bring stormwater from the street or other location into the GSI site. That often requires creating a stormwater collection point, which can be anything from an opening in a curbline to a traditional inlet added behind a curb. For roads without curbs and natural low points, stormwater might flow overland to collect in the GSI.
Permeable Pavement — Permeable pavement allows rainwater to pass through it into the ground below. While it can be a great alternative to traditional pavement because it reduces a site’s impervious area and functions as a stormwater collection point, designers need to consider site impacts, such as traffic loading and clogging potential from leaves or other foliage, when determining appropriate implementation.
Landscaping — As the most visible component of green infrastructure, GSI landscaping provides benefits at and below the surface. Because the public responds positively to the aesthetic qualities of trees, shrubs, grasses, perennials and native wildflower seedings and sodding, the selection of landscaping materials creates opportunities for public education and outreach that go beyond stormwater benefit.
Energy Dissipation and Pretreatment — Fast-moving stormwater can damage plantings and cause erosion. Energy dissipation and pretreatment features (filter strips, forebays and swales, for instance) are designed to slow stormwater velocity and collect sediment, trash and debris before it gets into the GSI. These components are key to protecting green infrastructure and supporting long-term maintenance.
Soil and Aggregated Media — A combination of soils and clean rock is used to filter and store stormwater in green infrastructure. Fine and coarse aggregate, in combination with soil media layers, allow stormwater to travel downward within a GSI facility. To create an environment that supports grasses, trees and other native plant growth, a growing media composed of sand, compost and topsoil or native soils is common.
Piping — It is often necessary to convey stormwater to and from a site. Piping can be used to transport stormwater to the surface or subsurface of GSI, or an underdrain pipe can be used to dewater the media over time. Piping also can provide access to observe the subsurface as well as to protect the surrounding utilities at the site.
Above-Grade Barriers — A physical or visual barrier is often required to define the edge of a GSI facility. Boulders, curbing, bollards and other natural and manmade structures can serve this purpose, protecting the infrastructure from vehicles or pedestrians that might otherwise cross over and damage it.
Outlets — Green infrastructure normally is designed for smaller storm events and requires some form of outlet to discharge excess stormwater volume once the storage capacity is exceeded. These outlets can be located either above or below grade and can be designed to control outflow to maximize the GSI’s storage and treatment capacity.
Media Liners — It is sometimes necessary to separate layers of rock, soil or other media in green infrastructure to limit mixing of media layers. In other cases, steps must be taken to keep stormwater from migrating toward streets, building basements or other structures. Permeable or impermeable synthetic fabric liners are typically used to serve these stabilization or separation functions.
Again, every GSI design is unique, and not all projects will require all of these components. By developing baseline criteria and standardizing the building blocks of GSI solutions, however, individual projects — and the industry as a whole — will benefit by improving how GSI is designed, constructed and maintained long-term.