One of my favorite things about our profession is that we are constantly pushing the envelope to make things better – whether it be with more sustainable buildings, more efficient systems, or improved technology. Such is the case with stormwater management and the movement towards green infrastructure. Last month Austin, Texas became the latest city to officially endorse green stormwater infrastructure (GSI) when it unanimously passed a resolution calling for an increase in its usage. So what is GSI, and why the sudden focus on it?
In order to understand where we’re going with GSI, let’s start by understanding the basics of stormwater technology. Historically, the goal of stormwater management was to convey water away to another, hopefully downhill, location where the run-off could be dumped into a lake, wetland or stream. As cities and urban areas grew and became more developed, there were fewer pervious surfaces to absorb run-off. The resulting concentration of run-off into the receiving streams created massive erosion and scouring as higher volumes of water inundated watersheds that could not naturally handle those volumes.
As urban areas developed, we began to handle run-off underground through inlets and pipes that conveyed the water from one location to another. Water magically disappeared down a hole and was no longer a problem at that location. Unfortunately, it became a huge problem somewhere else.
The issue with this approach is that, while it solves a problem in one area, it exacerbates the problem in another area through the additional volume of water being dumped in one location. In addition to the challenge of accommodating the sheer volume of water, run-off typically brings with it a host of environmental contaminants – from roofing chemicals to motor oil and everything in between. These contaminants then make their way into our rivers and lakes.
The following graphic tutorial explains the negative impact of a gray stormwater system on a development site:
The first graphic shows a pre-development (or natural site condition) and what happens to the raindrops that fall on that site – how much is infiltrated, how much is soaked up through plant material, how much is transpired back into the atmosphere, and how little run-off happens from the site. The second graphic shows the traditional urban development and how those same raindrops are affected – the infiltration rate drops dramatically, the transpiration rate plummets, and the run-off rate increases exponentially (along with all the contaminants and soil that is carried along for the ride). My goal in using these graphics is to underscore that green infrastructure is all about emulating natural conditions.
I like to describe green infrastructure as the ‘big picture’ solution in that it is comprised of smaller, connected hydrologic features that provide larger environmental and community benefits. These are accomplished through the use of site-specific tools and technologies incorporated into existing land development practices collectively known as “Low Impact Development” (LID) tools.
High performance sites are those that accomplish multiple benefits. Often called “working landscapes,” they function as a green machine to improve water and air quality, incorporate infiltration and water harvesting, create habitat, and enhance downstream water quality by limiting the impact on the receiving stream. The goal is to use the natural rainfall to benefit a site in a natural manner. In other words, green infrastructure.
There are many methodologies that can be incorporated into projects to achieve these goals. Some of the more common are the use of bio-swales, bio-retention (also known as raingardens) permeable pavement, green roofs, water harvesting and re-use, and the use of native or naturalized landscaping. A common misconception is that these can only be used on new construction or greenfield development.
One has only to look at Portland, Oregon to see how these LID tools can be retro-fitted into an urban fabric to mitigate run-off. Here, green infrastructure is generally built into sidewalks and/or street medians to capture and infiltrate run-off. In many cases, they take the form of curb ‘bump-outs’ to collected and infiltrate water along the curb lines. There are also tree box filters that use the street run-off to water the trees and plant material. These practices do not replace the existing drop inlets and pipe systems, but they do limit the amount of run-off that runs into that system, decreasing the volume of water that is eventually conveyed to a downstream location.
We also have experience retrofitting raingardens into areas that are adjacent to buildings in order to collect the roof run-off. These raingardens are planted with materials that become foundation plantings for the building – and no one is any wiser that they are actually looking at a stormwater management facility! Taking this concept a step further, green infrastructure systems can also serve as demonstration gardens to help educate the public on the inner workings of the landscape. One such project I designed for The University of Georgia now functions as an educational demonstration of various green infrastructure methodologies, including a section of storm sewer that is “day-lighted” to expose its inner workings. The demonstration is not only fun to watch during a rain event, it also dramatically improves the water quality that ultimately outfalls the system.
While green infrastructure holds promise in our efforts to reduce the impact of humans on our environment, it has yet to become mainstream. Large-scale efforts like the ones seen in Portland and in Austin will help demystify the process and will likely serve as a model for other cities in the future. To read more about Austin’s recent push towards green infrastructure, check out this article from the Austin American Statesman.