Webinar Q&A: How does green stormwater infrastructure really work? Improving design through monitoring

In April, Stormwater Solutions hosted a green infrastructure webinar, “How does green stormwater infrastructure really work? Improving design through monitoring” featuring Dr. Andrew Earles, P.E., P.H., BC.WRE, executive vice president of Wright Water Engineers, Inc.; Stacy Wilson, water resources engineer at Wright Water Engineers, Inc., and; Matthew Howard, project engineer at Wright Water Engineers, Inc.

This webinar starts with an overview of GSI practices and performance data in the International Stormwater BMP Database. While the BMP Database contains hundreds of studies on how SCMs perform, there is wide variability in performance data and in many cases, the data providers do not provide sufficient design information to tie performance to specific design criteria. The webinar then presents two case studies of GSI monitoring that provide insights into how SCMs function and how adjustments to design criteria can potentially enhance effectiveness of GSI. The case studies will include a study completed in early 2024 of infiltration benefits of receiving pervious areas including grass swales with underdrains, vegetated buffers, and infiltration basins and a study from 2022 related to interception of rainfall by urban tree canopies. The presenters discuss how the results of these case studies can help shape GSI design criteria.

The webinar is now available to view on-demand here, and below, you will find responses to some of the questions posed by the audience in the Q&A session at the end of the live webinar.

Audience: What would you need to do if providing outfall for the bioretention drain is not possible near the property?

Speakers: You would need to design a "full infiltration" section that infiltrates the water quality design event. If your underlying soils are of poor quality, some over-excavation and import of material can help, as well as avoiding compaction of the soil beneath the SCM.

Audience: Why reduce runoff from pervious areas? Is there a requirement from your permit?

Speakers: We are using pervious areas to reduce runoff from impervious areas adjacent to the pervious area. We define receiving pervious areas as those receiving runoff from impervious areas. All our sites had impervious areas draining to the pervious areas we monitored.

Audience: For the tree leaf study, was ambient wind velocity tracked which might affect gross fall-through?

Speakers: We did not monitor wind velocity or temperature in our study. These are major components of the interception process that should be incorporated into interception monitoring, when possible, to explain variability in interception loss between storm events and throughout the year. Wind can impact interception and throughfall values by agitating water stored on the canopy surface, causing it to fall as throughfall. Wind can also cause precipitation outside of the tree canopy to blow laterally and increase the total volume of water reaching the ground under the tree.

Audience: How does increased precipitation increase the amount of storage in a tree?

Speakers: Tree canopies theoretically store a finite volume of precipitation before throughfall initiates. This storage can be temporarily (or instantaneously) increased as precipitation intensity increases. This is analogous to a bowl or a container with a finite volume. If you fill the bowl at a greater rate than water can spill over the edges of the bowl, then temporarily the volume of water stored in the bowl is increased above the "static" volume of the bowl. It is a little more complex than that when it comes to tree canopies, but the result is similar. As for the total interception loss during a storm event, interception is not only a function of canopy storage but a function of evaporation, rainfall intensity and event duration. Regardless of a given tree canopy's storage capacity, interception loss cannot occur unless the rainfall captured on the tree surface evaporates back into the atmosphere. Evaporation occurs during wetting, saturation, and drying of the tree canopy. Evaporation occurring during the wetting and saturation phases is responsible for the linear trend that is apparent between interception and gross precipitation.

Audience: In the BMP Database, what do you think is the percentage of sites that are working properly?

Speakers: I (Earles) think most of the BMPs work as intended, some more effectively than others. The ones that do not work tend to be the outliers, but for most BMPs, even those working well, there are usually small things that can be done to improve performance. This could be an improved outlet trash rack for a pond to minimize clogging of the outlet and excess standing water, implementation of forebays (or HDSs to act as forebays in space limited situations), or other design measures that can improve performance or facilitate better maintenance. Maintenance is key to performance; sites that are not well maintained will experience degraded performance.