By Jack Rosen, TTF Summer Intern
Green stormwater infrastructure (GSI) management restores or mimics natural processes provided by vegetation that existed before land disturbance, but preserving this vegetation actually is a more cost-effective approach to stormwater management than using pipes, pumps, storage chambers, etc. This is because trees capture and store rainfall in the canopy, release water into the atmosphere by evapotranspiration, and transform nutrients and pollutants from soil into less harmful substances. Their roots and leaf litter create soil conditions that help infiltrate rainfall into it, which replenishes groundwater supply and maintains streamflow. Benefits of these processes include improved air quality, reduced summer air temperatures, reduced heating and cooling costs, increased property values, and recreation and aesthetic value.
Developers have to meet stormwater management requirements in exchange for conserving forests or using site design techniques that reduce paved surfaces, and they have no incentive to use trees unless they can get credit for it, and most municipalities don’t provide these credits. Stormwater credit systems reduce the size of stormwater management practices that must be constructed.
Site designers usually are required to capture and remove pollutants from a specific runoff volume; this helps them control the maximum peak rate of runoff from the site for certain size storm events. By this system, water quality treatment is defined solely by the pollutant removal functions of the SWM practices—it doesn’t account for their ability to reduce the overall volume of runoff.
However, many states and communities are realizing that there are benefits to focusing on reducing the volume of runoff leaving a site, not on the peak rate of runoff. In other words, an approach that involves not removing pollutants from runoff that has already occurred, but instead on preventing the runoff (and the pollutants) from going where it shouldn’t. This approach would work best for small storms, matching up well with the benefits provided by trees.
In addition to removing pollutants, this volume reduction method recharges groundwater, reduces the size and cost of hard infrastructure, and provides better protection of sensitive aquatic resources.
We can minimize the impacts of stormwater runoff by limiting paved surfaces and disconnecting them so they don’t drain the storm drainage system, increasing tree canopy over paved surfaces to increase interception of rainfall, and preserving natural topography and vegetation. As per that least suggestion, the National Agroforestry Center (NAC) recently proposed a program called “Working Trees”, which refers to putting the right trees in the right places and in the correct design to do specific jobs, thus helping to reduce and slow down runoff and trap pollutants. Such troublesome pollutants include fertilizers, herbicides, nutrients and bacteria from livestock operations, sediment, oils, antifreeze, and salts—most of which are primarily used by humans. This program advocates an “integrated watershed approach,” which could involve making use of silvopasture, windbreaks, alley cropping, green infrastructure (which refers to the vegetative system of stormwater management previously described), wastewater treatment, and riparian forest buffers.
The NAC provides a more detailed explanation of how trees reduce flooding and flood damage, for those who are interested:
The leaves and branches of trees intercept rainfall and prevent erosion; root growth and plant litter improve soil structure, enhancing infiltration of rainfall; the stems of trees and shrubs resist and slow out-of-bank streamflow; plant debris protects exposed soil; and roots bind soil particles to resist erosion and stabilize slopes.
Aquatic habitats are important natural managers of stormwater runoff too. The shade of trees reduces water temperature and prevents large fluctuations. Trees supply debris to streams, creating habitat structure and detritus which contributes to the aquatic food chain; woody roots promote stream channel stability and permanence of habitat structure; and improved infiltration of runoff results in contaminant filtering and the gradual release of groundwater into streams.
As vegetation and plant debris slow down surface runoff, sediment and sediment-bound contaminants can settle before entering surface water. Root growth and plant residue improve soil structure, enhancing infiltration of dissolved contaminants. Once in the soil, they can be immobilized and transformed by soil microbes or taken up by vegetation. These processes can also filter groundwater in the root zone, and trees can trap windblown dust before it enters streams and lakes.
The functions performed by Working Trees would depend on buffer design and the choice of plant materials—when managing soluble nutrients (e.g. nitrogen), you would need designs that detain and infiltrate water into the soil, whereas insoluble nutrients (e.g. phosphorus) are commonly bound to soil particles. If you decide to install Working Trees, you should determine which locations will produce the greatest benefit for water quality. In deciding where to place buffers in watersheds, focus on maximizing water protection while optimizing for other benefits such as wildlife habitat and carbon sequestration.
Existing forests can be protected during construction and managed as conservation lands. In places where there are no existing forests, reforestation can offset the impacts of stormwater runoff. We can redirect runoff from rooftops or parking lots to vegetated areas or GSI practices, and allow it to soak into the ground.
GSI practices usually rely on herbaceous vegetation because trees often die when planted in GSI practices as a result of conditions or maintenance crews. New designs for GSI practices have been developed to identify limitations to planting trees in GSI practices in terms of engineering and tree health, through design modifications, species selection, etc.; these designs aim to harness the benefits of trees to increase the effectiveness of GSI practices and provide other benefits to the community.