This past Saturday I participated in a tour of Green Line “Green Infrastructure” as part of Public Art Saint Paul’s City Art Collaboratory Program. We are a group of multidisciplinary artists and STEM (Science, Technology, Engineering and Mathematics) professionals who embark on monthly field trips together, exploring the living systems of the City of Saint Paul. One of our cohort is Matt Kumka, a landscape architect at Barr Engineering. He and his coworker, green engineer Nathan Campeau, guided us on this three-stop tour looking primarily at the stormwater management features.
Though I had read Sam Geer’s excellent introduction to street-side rain gardens, this was my first time looking intently at stormwater infrastructure in person and I want to share a smattering of my favorite FYI’s from our tour.
Our guides worked on two types of plantings that filter street runoff along the Green Line; stormwater planters and rain gardens. The map below shows where these features are distributed along the line.
The stormwater planters have their own storm drains to bring in water from the street. They are planted with “native cultivars” – native species subtly bred to have particular traits. In this case, allowing prairie species to thrive in the hotter, drier urban space of the Green Line.
These planters will collect a top layer of impermeable fine sediment every few years, slowing down the infiltration rate, though it is fairly easy to dig out that top layer and restore the function. Further, the plants and their root systems will break up the impermeable sediment and promote infiltration.
The Rain Gardens are built with small, cascading ramps to bring the street water rather than storm drains. These plantings are trench-shaped with water-loving plants situated at the bottom and dry grasses along the edges. The architects and engineers must design for the practical water-infiltration function of the gardens first and foremost but there is also attention paid to their aesthetic impact and how they will be received and treated by the community they inhabit.
Our guides explained that within the last few years the general public is getting more and more used to seeing plantings of native species. Some of the grasses can be perceived as weeds but if you have multiple similar plantings in a small area, this can help show that they are intentionally planted and cared-for.
A Very Big Tree Trench
A Tree Trench is a stormwater management feature that connects the storm drains on the side of the road with the trees planted within the sidewalks. The Green Line Trench stretches 5-miles, the longest in Minnesota!
I was surprised to learn that most trees planted in the boulevard in urban areas tend to live less than 15 years. Tree roots tend to stay close to the surface making them vulnerable when nearby sidewalks slabs are replaced and roots get cut. If more than 50% of a tree’s root system is disturbed it will likely fall over and die. The trees planted along the Green Line will have been given extra room for the roots and water storage with all the porous rock and soil in the trench, which should promote healthy trees.
The most eye-opening discussion was around the complexity of using “permeable pavers.” I’d heard the phrase in passing many times and had assumed they developed some magic brick that could allow water through while remaining structurally sound with no drawbacks whatsoever!
Well, it turns out “permeable pavers” are regular bricks that are spaced out further from each other than conventional brick paths and the space in between is filled with a permeable gravel. That is, until fine sediment from the environment fills the crevices, blocking water flow and holding pockets of soil for weeds to take root.
In the case of the Green Line, the tree trench was topped with permeable pavers, though the majority of the water comes into the trenches through the storm drains and when the permeable pavers become plugged, water will enter the tree trench through the storm drains in the street exclusively.
All and all it was a tremendously uplifting tour of some really interesting, complex , and mostly successful projects. If you are more of a facts and figures type, check out this page by the Capitol Region Watershed District for more details.
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