Have you seen our bioswales?
They look like normal tree pits, but they have some underlying secrets that make them a whole lot better, perhaps even 2,000 gallons better…
In November we hosted a lecture on Flow, Filter, and Foliage: Measuring Bioswale Performance. Suzanne Lipton of Columbia University Earth Institute both curated and moderated the lecture’s panel comprised of Walter Yerk from Drexel University, Sarah Bruner from Columbia University, and Nandan Shetty from Columbia University.
Bioswales are a type of green infrastructure that are designed to channel runoff from streets so that it can be absorbed by plant roots or percolated down through to groundwater thus decreasing the amount of water that enters into our stormwater drains. By absorbing rainwater and runoff, bioswales remove some of the untreated water that will enter directly into the harbor via CSOs in large wet weather events. During a 1 inch rain event (our average rainfall in NYC), one bioswale can prevent up to 2,000 gallons from enter our sewage system!
You can learn more about our combined sewage system and green infrastructure here.
The New York City Green Infrastructure plan has designated one billion dollars for green infrastructure, with a 2030 green infrastructure benchmark to reduce CSOs by 1.5 billion gallons a year.
The Gowanus Canal Conservancy does its part by working to create and maintain the eleven bioswales that are part of the 6th Street Green Corridor in Gowanus.
All three of our presenters spoke about their research that contributes to our understanding of bioswale success. Walter Yerk spoke about his study of water flux through shrubs. Shrub interception, i.e., the amount of water absorbed by shrubs and the evaporation of water off of a plant’s canopy, can be affected by a number of factors including canopy density, air density, and energy flux. Walter measures shrub interception by comparing throughfall (the water that ends up in the soil) and stem flow (the water funneled by a canopy along stems, leaves, and branches) with overall rainfall. His research suggested that some plants are better than others at interception and canopy density is not the only determinator of interception, as leaf type my play an important role in retention.
Sarah Bruner’s research measures the release of water by plant pores; the more water plants release into the atmosphere via their pores, the more water they collect from their roots—meaning that more water will enter the atmosphere instead of our sewage system. Sarah encouraged us to look at bioswales from the perspective of plants rather than engineers, and spoke about how different uses of water by different plant species contribute to the overall functionality of a bioswale. By measuring stomatal conductance, a proxy for the release of water by plant pores, Sarah found that plants use water differently throughout the day. For instance, New England Asters release a lot of water throughout the day, whereas Switchgrass is much more economical with its water use. She also brought to light a new way of measuring evaporation: thermal imaging. Species that demonstrate darker colors have cooler temperatures, meaning that they have higher levels of evaporation than those with warmer colors and temperatures. Sarah’s research informs the best practices for organizing plants in bioswales to maximize water retention.
Finally, Nandan Shetty presented on how bioswales impact the urban nitrogen cycle. Bioswales are set up to receive large influxes of water and the chemicals it brings along, including nutrients that plants typically need like phosphorus and nitrogen. Higher flow through bioswales allows for oxygen to be consistently replenished in soil creating an environment that promotes the conversion of Nitrogen to ammonium and nitrate by bacteria. These compounds are then absorbed by plants to help them grow. Although great for plants in soil that is nutrient deficient, these nutrients can enter our harbor via CSOs and create algal blooms, which negatively impact ocean critters. So theoretically, nitrogen and phosphorus from street runoff can be absorbed by plants in bioswales, and thus their potential to disrupt our harbor ecosystem is minimized. However, Nandan’s research contradicts this assumption to some extent. He found that while a bioswale reduces total nitrogen input from CSOs by 7 kg per year, it also leaches 2 kg per year. And though this is a net decrease in nitrogen, it is important to consider potential methods for diminishing the amount of leaching nitrogen. Nandan suggested reducing soil decomposition by removing soil nitrogen as a possible solution. How might we do that? Well, currently we add compost to our bioswales to help our plants grow, but this might be unnecessary because of the high nitrogen content in street runoff. If we try instead to plant our bioswales without using compost, they may grow just as well and leach a lot less.
Our lecture ended on the note that while bioswales are cool in their functions and helpful in combating CSO efflux, they are only one small piece of the puzzle. As concerned citizens and community members, we need to work together to help reduce water use in our homes and businesses to work towards reducing CSO output by 100%.
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