Look's great from up here at street level... but what's beneath?

What’s Missing From NYC’s Enhanced Bioswale Tree Pits

New York has an ambitious Green Infrastructure plan, and lately their enhanced bioswales have gotten additional attention. These five foot by twenty foot planters look great from street level, so why do I feel uneasy about them?

For one reason, the original plans for these pits had vast majority of the excavation area being occupied by a holding tank system. Overflow pipes are often used in situations like these to act as emergency water conveyance systems, but this thing is huge:

I don’t know what exact proportion of the volume the tank occupies, but it appears to displace quite a bit. This is space that might otherwise be filled with a high-quality loam or bioretention soil mix that could nourish the tree while also delivering both water quality and water quantity/rate-control benefits.

We know from the New York City Department of Environmental Protection that these enhanced bioswale trees are receiving around 200 cubic feet of soil each. That’s only a fraction of what urban trees and soils expert James Urban recommends that trees receive in order to reach maturity. Limited soil volumes vastly reduce the stormwater management capacity of the soil and the tree – which isn’t going to get too big on such small soil reserves.

Perhaps more importantly, 200 cubic foot of soil per tree is not compliant with New York’s own High Performance Landscape Guidelines, which make the following recommendation:

As a minimum, provide individual shades trees with at least 800 cubic feet, 3 feet deep, and cluster trees in a common rooting volume at least 600 cubic feet of soil volume, 3 feet deep. This is the absolute minimum in area, and more total volume is always better.

The performance guidelines go on to say:

Soil volume is critical in that  it provides sufficient water storage and nutrient availability for  plants… Adequate soil volumes and depths are also important to stormwater management. Sufficient volumes and depths are required to provide sufficient infiltration, filtration, detention, and retention. Research over the past two decades has  revealed that, industrywide, the provision of soil volumes has been poorly understood and greatly underestimated.

The recommendation of the perfomance guidelines is that “In urban stormwater management, the simplest approach to reducing stormwater runoff and pollutants is to maintain or restore the ability of soils to absorb rainfall.”

If these are the city’s own recommendations, then what exactly is going on with these bioswale tree pits?

Judging by this graph, which I pulled directly from their “Greenprint,” the city is projecting huge long-term benefits from green infrastructure efforts like these bioswales. But as far as the trees are concerned, these kinds of results simply won’t be possible without significantly larger soil volumes. Trees need way more than 200 cubic feet to grow to environmentally significant sizes.

We recently learned that the designs that used the orange tank have since been modified. Instead, the bioswales will use 2′ of aggregate along with a sandy bioretention soil mix that has about 30% water holding capacity. Our feeling is that this design would be improved by less aggregate, and more soil. Nette Compton with the Department of Parks does point out, however, that they are putting these bioswales in areas where roots will also have access to adjacent soil volumes, and have been placed thoughtfully in sites where the trees are expected to succeed.

This design isn’t suitable for high traffic areas because of how much sidewalk space it occupies, but permeable pavements could be a way around that. Using them, or other water conveyance mechanisms, you can design tree wells that act as underground raingardens, but have regular-sized surface openings.

Here are some of the numbers that New York has used for the development of their Green Infrastructure plan:

  • Two-thirds of New York City is served by combined sewers
  • 216,000 acres of Combined Sewer Overflow (CSO) drainage area
  • 7,400 miles of sewers
  • 422 CSO outfalls
  • 30 billion gallons of CSO per year

We believe that a simple number that we use a lot could make a big difference in this effort: 1,000 cubic feet of soil per tree.

According to Nette Compton, some pilots are still being installed as they try out new improvements and techniques. Starting in 2012, however, they will be rolling out large installations at the neighborhood scale.

Images: A Bold “Greenprint” for the City’s Future – Spotlight on New York City

2 comments

  1. Just think of how we could enhance the benefit of the street trees with pervious asphalt over engineered soil at 24-36″ depth. everytime i see con ed or the city tearing up the street i keep hoping for that to happen. so far my prayers go unanswered……….

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