Episode 4: Creativity and Constraints

(We’re bringing you this week’s episode of Remarkable Objects a little early, since Thursday is Thanksgiving here in the U.S. We’ll resume a normal publishing schedule next week.)

After a decade of working in urban forestry (and editing this blog), I’ve spent a lot of time thinking about the biggest challenges trees in cities face. There is no shortage of contenders. At times the goal of moving the needle, of really meaningfully increasing our urban tree canopy, can feel overwhelming.

This is one of the reasons I really wanted to talk to Randy Neprash. Randy is an engineer (stay with me here) at Stantec and also the sole staff member for the Minnesota Cities Stormwater Coalition. And we talked about a recent change that Minnesota made to their stormwater manual that could be a game changer for urban trees.

What is the change exactly? The manual formally recognizes street trees, and the soil they grow in, as a best management practice for stormwater regulations. And the state has created a crediting system to use street trees and soils to meet statewide regulatory stormwater standards.

In other words, Minnesota is using a stormwater regulation to drive changes in how we plant trees. They’ve combined two urgent urban design needs – one to keep rain where it falls, and another to increase tree canopy cover – into a single, elegant solution.

They’re the only state that we know of that has done this, and it could completely change not just how we manage water, but how well we sustain the urban forest.

Listen to the episode by clicking the player above, or by going to iTunesSoundCloud, or Google Play.

Here’s my favorite quote from the episode: “Instead of silver bullet we’re thinking in terms of silver buckshot to try to address these issues and improve water quality from urban stormwater in every way we can think of.”

Thanks for listening.

Leda Marritz | Creative Director, DeepRoot

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TRANSCRIPT: Creativity and constraints
GUEST: Randy Neprash, Stormwater engineer, Stantec and Minnesota Cities Stormwater Coalition


One of the wonderful and most exciting elements about all this is that what works well for stormwater management – uncompacted soil and significant amounts of soil volume – also turns out to be enormously beneficial if you want to grow healthy, long-lived, large trees. Even the engineers, and I’m speaking as an engineer, even the engineers recognize the value of being able to grow and support large, long-lived trees in the midst of these ultra-urbanized settings.

My name is Randy Neprash; I’m a civil engineer. I work for Stantec Consulting, and am the sole staff for an organization called the Minnesota Cities Stormwater Coalition, which is an organization of cities in Minnesota that are regulated for their stormwater discharges.

This Leda Marritz, and you’re listening to Remarkable Objects, a podcast from DeepRoot about the intersection of nature and the urban environment.

I wanted to talk to Randy because the state of Minnesota has done something very unusual. They’ve formally recognized street trees, and the soil they grow in, as a best management practice for stormwater regulations. And they’ve created a crediting system to use trees and soils to meet statewide standards. This is really significant, but before I go any further, a warning: the topic of stormwater management is complicated, and the discussion does get a little technical at times. I’ll do my best to explain everything as we go.

OK, back to what’s happening in Minnesota. As you may remember from earlier episodes, water that runs off paving causes major water quality and volume problems.  It can lead to damaged streams, diminished aquatic life, and high levels of pollution. In 2014, the state of Minnesota was revising their stormwater manual, and they wanted to minimize these types of environment impacts. So as part of the revisions, they laid out design guidelines for all kinds of best management practices for capturing stormwater where it falls.

Now, many states have documents like the Minnesota Stormwater Manual, and they mostly contain just a handful of accepted solutions. Things like rain gardens and stormwater trenches. But Minnesota included street trees and soils as a design solution comparable to those other tools. And they created a calculator for quantifying their benefits. This allows designers like engineers and landscape architects to show how this solution contributes to performance goals. It also entitles them to receive credit for meeting regulatory standards.

Minnesota is the only state that we know of that has done this, and it could completely change not just how they manage water, but how well they sustain another key form of green infrastructure – the urban forest.

There is a precedent that we can look to to understand the significance of including street trees and soils as a best management practice. And I want to start with this precedent to provide context and demonstrate just how powerfully stormwater regulations can affect sustainability and urban land use.

Back in the late ‘70s, U.S. EPA started to pay close attention to urban stormwater discharges. They started something called the National Urban Runoff Program, which was a national-scale research program to better understand how urban stormwater systems worked and how they might be managed. So, National Urban Runoff Program, the acronym becomes NURP, one of the results of the NURP program was the notion that building stormwater ponds was a very effective and beneficial way to address some of the challenges with urban stormwater.

What was the significance of that finding? Or, another way of asking that is maybe, how did the NURP program influence the way the landscape was developed?

So what the NURP program did was to work out the benefits of building ponds like these and quantified them very nicely; they also developed a set of good design standards so that we were able to build effective stormwater ponds everywhere. Using that information we could design a credit system that was built into regulations, and we were able to take all of that information and integrate it into land use regulation at the local, the county, and even the state level.

And the response by public entities – cities, counties, state departments of transportation, but also private development that was regulated and had to pass city review – the response to these regulatory requirements to build stormwater ponds was universal and the end result was that we build these ponds nationally by the hundreds of thousands. Just in Minnesota, since the early 1980s, we estimate that we’ve built about 30,000 constructed stormwater ponds. So that entire system becomes a regulatory structure and a crediting system that designers can work with, and regulatory reviewers can use to determine whether we’re doing things properly and protecting the environment in the way that we want things to happen.

So how has that thinking evolved since the 80s? Can you talk about what you’re trying to do now with the Minnesota Stormwater Manual?

What we’re trying to do now is build the new types of best management practices, that range of things that includes rain gardens and infiltration but also tree-based stormwmater management systems, build all those sorts of approaches into similar computer software modeling programs so that we can use all those new types of BMPs to meet the new regulatory requirements that we’re seeing in cities all over the country.

What’s so interesting about this is that Minnesota is using a stormwater regulation to drive changes in how we plant trees. They’ve combined two urgent design needs – to keep rain where it falls, and to increase tree canopy cover – into a single, elegant solution.

So this design solution is comprised of trees and soils, and that’s it. Can you tell me how that works? What processes do tree systems use to manage stormwater?

Particularly with some of these underground systems where we can build the equivalent of a rain garden to support the growth of a large, long-lived tree, and also route stormwater into the device to get water quality improvement as it moves through the root zone volume, but also water volume reduction because of infiltration out of the bottom device, and also evapotranspiration and canopy interception because of the significant large tree canopy that can be supported by these systems.

And we can do this in the absolute heart of the most densely developed, urbanized areas that we have without losing paved spaces, land that we need for parking and roads and sidewalks, and all the things that we do in the middle of big cities. There’s the potential, anyway, for accomplishing all of this at the same time, which is enormously exciting.

Can you explain how soil factors into this and how it impacts the overall function of the system?

That’s probably the single most important element, and certainly when you look at urban trees it’s been where we have failed most often. We tend plant trees, particularly in ultra-urbanized settings, we plant them in vaults that are far too small, at least that’s been the general pattern, and the result is that we end up with stunted and short-lived trees.

Tree systems like that don’t provide much in the way of stormwater controls and stormwater management. You’re just not going to be able to get much done with a vault of that size, or a tree of that size. And so probably the single most important foundational element to the entire process is to provide guidelines and design information to promote the planting of trees with larger soil volumes that are available to them.

So the credit that would be awarded to a tree-and-soil stormwater system would reflect the amount, or volume, of soil provided, and therefore the projected mature tree size, is that correct?

There are other variables in place, but your point that the soil volume available for the tree and the, that is the most important factor.

There are design guidelines where they call for certain amounts of appropriate soil volume to support tree growth. In the case of the Minnesota Storwmater Manual, that volume is in the range of 1,000 to 2,000 cubic feet, which is a pretty rare thing in design guidelines for tree planting. But those sorts of guidelines, those sorts of standards, make these installations appropriate and useful for stormwater management as well as good tree growth.

And we’ve made more progress particularly in the area of trees than just about every place else in the country I know of.

The benefits provided by these systems are dependent on the size of the tree and the volume of soil it’s planted it. This presented a challenge for creating the crediting system, because the credit amount is determined during the design phase of a project, before the trees are even planted. Yet as the trees grow over years and decades, they provide exponentially more benefits. To resolve this, the Stormwater Manual assigns credits based on the projected mature canopy size relative to the amount of soil provided at the time the tree is planted.  

For example, if you plant a tree that can have an 800 square foot canopy at maturity, but you’re only giving it enough soil to reach a 400 square foot canopy, you will only get credit for the smaller tree. This encourages practitioners to design systems that provide trees with the amount of soil they really need.

Trees and soils are both a little out of the purview of most engineers. What has been the reaction in the engineering community? How are people taking in this document?

It’s been a challenge. We’ve seen a good bit of very enthusiastic response.

The pollution control agency has surveyed stormwater practitioners – so those are engineers, people that run public works departments, or road departments in something like a department of transportation or a county – they’ve surveyed stormwater practitioners to see which types of these BMPs people are most interested in, that they most want to see research – and every time we’ve done these types of surveys, urban trees have come out as one of the most highly rated types of BMPs that stormwater practitioners want to see additional research and additional quantitative, analytic work. They want to see the investment on tree systems.

So is all this leading to changes in how engineers practice and, and design?

In a number of situations, what we’re seeing is the stormwater engineers working with other professionals. A good example for instance is landscape architects, so that now stormwater management has become a very sophisticated and complex multi-disciplinary exercise, where you have the potential to not just manage the stormwater, but also significantly change the sort of landscape that’s being created.

So it’s driving investment, but it’s also driving creativity. A good example, for instance, is we’re building rain gardens all over the place. Public works departments are now in the business of building and supporting gardens all over the urban landscape. It’s a little bit disconcerting and kind of scary for these public works folks. But at the same time they’re learning how to do it. They’re looking how to get homeowners and private property owners to help and come along in the effort. You know, we’re all trying to move forward and make this stuff happen. And these regulations are driving all sorts of investment and creativity.

What’s your take on how these regulations are shifting our collective attitude about the design of the urban environment relative to growing trees and managing stormwater?

Instead of silver bullet we’re thinking in terms of silver buckshot to try to address these issues and improve water quality from urban stormwater in every way we can think of.

Part of what I’ve tried to convey here is the threats and power of these water quality regulations, and the strength of the response to those water quality regulations in terms of changing how we build things. If we do as good a job with tree-based stormwater management BMPs, as we did with stormwater ponds, back in the ‘80s, we can see widespread implementation and construction of these systems which will yield long-lived, large trees in ultra-urban settings… with numbers and scale the likes of which we have never seen before. So these techniques, if they are properly integrated into the stormwater regulatory system, have enormous potential to change the American landscape in significant and very positive ways.

The stormwater manual is a product of the Minnesota Pollution Control Agency and is continually being updated. If you want to find out more, or try out their calculator, we’ve put a link in the description text of this episode. We’ll be sharing more resources on Twitter, too, so be sure to follow us @remarkableshow.

Remarkable Objects is produced and edited by me, Leda Marritz, with editorial assistance from Aylara Odekova. You can find us online at remarkableobjects.com.

This podcast is a production of DeepRoot Green Infrastructure. DeepRoot provides landscape solutions and technical support services to promote mature tree growth and sustainable stormwater management in the built environment. Find out more at deeproot.com.

If you like this podcast, help us reach a larger audience by sharing it with a friend, or leaving us a rating or a review in the iTunes store. We really appreciate it. Thanks!



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