Green infrastructure stormwater management features differ in their ability to remove stormwater and/or from the system based on a look at the limited number of studies available.
My biggest take away from Green Infrastructure Lessons from Science and Practice is that green infrastructure is in its infancy. Studies are lacking, those that have been conducted a limited. Published by the Cary Institute of Ecosystem Studies with Syracuse University and Science Policy Exchange in June, authors pointed out that green water infrastructure is versatile.
Green water infrastructure can be installed and employed to manage water quantity (thus keeping stormwater out of the stormwater system), which is important for cities managing CSOs (combined sewer/stormwater systems). Green infrastructure is also important in managing water quality, keeping nutrients and pollutants out of water supplies, which is important for cities and other jurisdictions in managing their non-point source pollution loads into surface waters.
“Green infrastructure uses vegetation and natural processes to capture, store and slowly release water to the atmosphere, ground water or existing drainage systems,” authors wrote. Technology is generally small-scale and distributed through the urban landscape at the neighborhood and community scale such as planters, street trees, rain gardens, porous pavement, green roofs, bioswales, bioretention ponds, and constructed wetlands.
The report notes that based on the studies they reviewed, some green infrastructure (GI) practices (bioretention, green roofs and permeable pavement) retain more water, while others are less effective at water retention (swales, detention ponds, media filters, retention ponds, wetlands) although they allow additional storage during rain events. Research to date demonstrates that average GI performance differs more by site than within a given GI technology.
The effectiveness of GI in removing contaminants depends on the contaminant and the technology. All practices remove suspended solids. Some practices (bioretention, media filters, detention ponds, retention ponds) remove some nitrogen and phosphorus. Generally trace metals; lead and cadmium were effectively retained in all practices.
So far, there’s not enough data to predict the performance of GI effectiveness in diverse urban watersheds across climate and seasons.
Cold season performance may be a concern. Few studies have looked at how various technologies perform seasonally. Permeable pavement and bioretention cells performed equally well in summer and winter in reducing runoff. But the performance of green roofs seemed to be more closely tied to the size of the rain event and media depth, and likely less evapotranspiration.
Most green infrastructure removes sediment and trace metals, but N and P are mixed. The authors recommend that conservative performance estimates be used to manage public expectations and to meet regulatory requirements.–by Debbie Hamrick
For a copy of Green Infrastructure Lessons from Science and Practice:http://projects.iq.harvard.edu/science-policy/water
NewTerrain newsletter September 4, 2015.