April 2010 – Urban Trees & Stormwater

Webcast Resources:

“Integrating Urban Trees into Stormwater Management“
Tuesday, April 28, 2010, 11:00 AM (Eastern)
Susan Downing Day, Ph.D., Virginia Tech

» Watch the video
» Find supporting material
» Discuss this webcast

Archived Video:

You can view this month’s webcast below using the embedded video viewer. Just click on Play and the video will start momentarily. Alternatively, you may download a copy to your own PC. Simply right-click on the provided link and save the presentation to your desktop or other appropriate directory. You can then open the file once it is downloaded and view it by using the default Windows Media Player software.

Look here for Video Soon

Supporting Material:

Web Links:

Documents and Files:

Environmental Literacy:

Other Resources:

  • Slmartin3161

    Hello,

    I have enjoyed reading the comments, but as a late comer to this web series, I am wondering if the video is going to be posted on the site.

  • Steve Findley Question To: Also, the CU structural soils have been cricized (by Jim Urban) as not providing sufficient organic matreial and water to support the growth. Have you tested other amended soils?

    A: Yes, we tested the Carolina Stalite structural soil. This one has the advantage of not needing a hydrogel to prevent segregation and the disadvantage of higher cost if you are not reasonably near the company. Also, some are concerned with embedded energy–although I have not seen any real analysis of the energy costs associated with any of these. They all work roughly the same way. The key to success is to have the correct mix and correct compaction so that it works as a moisture-holding matrix. We find all the mixes are droughtier than our native soils (which are clay loams, loams, and silt loams). There is a new paper out (in Arboriculture and Urban Forestry) that calculates moisture to be like a loamy sand, which fits fairly well with our experience. As far as organic matter, I haven't heard that as being an issue or a concern. In any sealed system (under pavement) whatever OM is in there to start will likely decompose (unless it is within stable soil aggregates), so input over time is important. The most significant input source would likely be root turnover–i.e. roots are born, grow, and die. Fine roots often liver for very short periods of time and this can provide significant OM input. So if roots grow in it, there will be organic matter input.

    • Nathalie Shanstrom

      Jim Urban’s view that CU structural soils are not comparable to a topsoil or loam for plant growth are supported by a study which compared plants (Benjamin Fig, Ficus benjamina) grown in loam soil vs an equal volume of CU structural soil. The plants grown in a loam soil yielded significantly greater leaf counts, height, shoot dry weight, chlorophyll concentration, root length, root dry weight, and root volume than those grown in CU structural soil. For example, those in the loam soil were almost twice as tall as those in CU structural soil and had more than three times as many leaves at the last sampling, which was 500 days after the beginning of the study. The study concludes that “Plants grown in skeletal soil material [aka CU structural soil] exhibit morphological features markedly similar to root restricted plants. The media effect suggested that on an absolute volume basis, skeletal soil material is disadvantaged given its lower inherent soil content and corresponding nutrient pool (Loh, Grabowsky, and Bassuk . 2003. Growth response of Ficus benjamina to limited soil volumes and soil dilution in a skeletal soil container study. Urban Forest, Urban Green, 2: 053-062).

      If trees in an urban area respond the same way as the ones in the greenhouse study, it would be expected that any techniques that can provide uncompacted loam type soil under pavements would grow larger trees than those in an equal volume of CU structural soil.

      • I have to say I tend to take claims from the manufacturers of products with a grain of salt, especially when they are comparing their product to one they do not produce or have financial interest in :) However, my expectation would be that increased growth is probably not so much related to the nutrient pool as it is to the root exploration and thus access to a more resilient pool of resources (water). I have done quite a lot of research with compacted soils and the interaction with roots is fairly well understood. Any soil compaction, even slight compaction, decreases tree growth, although the magnitude of the response depends upon the exact species/soil combination. So I think you would certainly expect that an uncompacted soil would maximize tree growth. On the other hand, that is not generally the question at hand in situations where you would use structural soil, i.e. you are not making a choice between good uncompacted soil and structural soil–at least one hopes not! There is also some interesting research going on with pervious pavements and various pavement sections that is quite interesting and I think we will have a better understanding of the interaction of pavements and water supply to roots in the near future.

      • Amilton

        Based in James Urban material, It is a little strange the comparisons, considering the formal scientific methods.
        Bassuk and Gravowsky producing material to condemn their work?
        Soil must have structure to well support root growth, and with loam soil, something must do it fast, or the small particles tends to aggregate and increase density without roots, fungi, earthworm, etc.
        Is the medium inside Silva’s Cell, considering macro and micro structure, life, chemical and physical activity compared to natural soil?
        Using F. benjamina as a parameter, we can’t deduce anything, because it is not a normal tree, since it can start the life as an epiphytic until finding the soil many meters below.
        In pots, roots find easily small cracks over the pavement, and it is said, they can cross an soccer stadium searching for nutrients. If roots can’t find any place to escape, plants can decay as quickly as they grow in the beginning.
        I don't put it in the street tree category, unless you conduce as Topiary.
        Just to remember the study: … “INTRODUCTION” : … “SHORT TERM STUDY” … “HYPOTHESIS” … “CONSTRUCTED” …
        “ABSTRACT” …… “THE STUDY ALLOWED A METHOD” …
        I can’t see both methods being used for the same purposes, and don’t expect street trees growing like in nature.
        Trees in Tropical and Subtropical areas can turn into big troubles, since some of them effectively need no soil or need them in small amounts, producing by itself their own substrate.
        I know it is not only a regional problem, because others countries like Japan controls the size pruning the trees, and, others, with growth regulators.
        I'd like to see more opinions, since I'm a beginner in this, before recommend this or that in my country (and forgive me, I’m not fluent in English).

  • Jackson Bird Asked: Thoughts on Silva Cells?

    Hi Jack! It's good to hear from you. Still plugging away at answering these questions, but not sure there are any readers, so may quit soon. I don't have any experience with Silva Cells, but my understanding is that they are essentially modular vaulted sidewalks. I don't know how they compare cost-wise, but presumably they have been evaluated to be load bearing. I would think they would function like a vaulted sidewalk. In terms of stormwater, if you had a one-to-one reservoir to mitigated surface, as was tested in our work, I would think you would have some similarities and some of the evaluations of tree survival might be similar. The main differences would be the amount of storage–that would depend on the storage capacity of the soil used in the cells. You might also have a perched water table and so a wetter zone near the bottom–this would depend on the soil again. In addition, you would not have rapid lateral movement, so the methods you would use to direct water below the pavement would have to be altered.

    • Nathalie Shanstrom

      Thank you for the thorough answers to these questions Susan. Yikes there are a lot of them! I enjoyed your webcast and was just looking up the additional resources when I saw all the questions. Since my office frequently works with Silva Cells, I wanted to chime in on this one. Silva Cells are a fiberglass re-inforced polypropylene modular underground framework that can support pavement with loads up to AASHTO H-20 standards while still preserving uncompacted rooting space and stormwater bioretention volume under the pavement. The modular design enables flexibility to size the rooting/bioretention volume as needed for each site and also enables the system to easily accommodate most utilities. Stormwater can be directed to the cells in a number of ways. More information about Silva Cells, standard details and specifications, installation and design guidelines, and supporting documents are available at http://www.deeproot.com/products/silva-cell/silva…

      A study by Tom Smiley has been comparing trees in an urban plaza setting in 7 different treatments: compacted soil, gravel/soil mix, Stalite, Stalite/soil mix, and suspended pavement over non-compacted soil. Suspended pavement over non-compacted soils (analogous to Silva Cells) are providing the greatest amount of tree growth and health (Smiley et al, Comparison of Structural and Non-Compacted Soils for Trees Surrounded by Pavement, 2006 International Society of Arboriculture; Tom Smiley personal communication).

      Another study was just started in Minneapolis and St Paul comparing trees grown in compacted soils, vs CU structural soils, vs. Silva Cells, vs Swedish Structural soils. Stay tuned for results at http://www.deeproot.com/ in the next few years!

  • Stan Kamys Question: Are there calculations to determine what size of cu area will treat the otherwise pervious parking lot for 25yr/100yr storwater calculations? Has the data been used to address stormwater pond configurations with a local water management district and the EPA/DEP rules.

    A. Yes, the reservoir conservatively holds 30% (typically closer to 35%) by volume of water. Therefore the recommended 24" bed of structural soil would handle a 4.5 to 5" rain -so whatever storm that is for you. This allows an overflow drain to be placed at 14" up from the subgrade. Note that the system is NOT designed to handle a lot of off-site water, just to be zero runoff.

  • Shelley Allen-Czepiel Question: What makes the tree roots from growing up into the pavement?

    SD replies: I am assuming this question is in regard to porous pavement? I am not an expert in porous pavement. However, porous pavement is rigid just like other pavement and I had never really considered a root growing through it. Nonetheless, it probably is not a consideration unless the pavement is buried. Roots follow the path of least resistance and proliferate where there is water and air and nutrients. In a conventional pavement section, this area is often the base course under the pavement. With structural soils, or uncompacted base soils (as are allowed for some uses in some countries), this path of least resistance can move lower in the profile (away from pavement). I hope that is the question you were asking.

  • Dion Doepker Asked: Regarding Structural Soil: Is there a requirement to the type and shape of the rock used for the aggregate

    SD replies: Yes, the size and angularity of the rock are quite important, as is the distribution of sizes. Here in Virginia we used #357 (I think, if I am remembering correctly). What is critical is that the stone be large enough (at least 3/4 of an inch and up to about 2") and that there not be a lot of smaller stones mixed in. An angular stone will make better gaps. I have seem some installations of supposed "structural soil" that were more akin to crusher run than structural soil. When constructed according to specifications, the percolation rate and other measurements are made. Water should enter structural soil extremely rapidly. The best way to ensure quality control is to employ someone licensed by Amereq to install CUSoil, or work directly with the manufacturer of something like Stalite on establishing specs and working with the contractor. There are some contractors who say they are licensed to install CUSoil where we are, but after calling Amereq, I learned that they were not licensed. So make sure specs are met. It's not that difficult to meet the specs, but just mixing gravel and soil does not make a structural soil.

  • Karen Engel Question To: I was disconnected & may have missed explanation, but….why do you need to use the geotextile fabric at all? What happens if you don't?

    SD replies: see answer to Phil Rodbell above. Most structural soil installations do not use geotextile. However, if the subgrade may become saturated it is an added precaution to insure stability.

  • Mike Fleischhauer Question To: Has consideration been given to the possible issues created by tree roots continuously penetrating the geotextile (eventually the structural soil settling as the geotextile is destroyed, causing settling of pavement)? Also, has there been study of the longevity of the porous asphalt surfaces under normal use considerations (Ex. Pores pluging as vehichles on surface deposit dirt and oils)?

    SD replies: I don't know that the tree roots damage the geotextile too much. Tree roots will penetrate it, but it still creates a barrier of sorts. Our main interest was to see if tree roots would penetrate it enough to puncture the compacted zone and allow greater infiltration. There have been a number of studies about the longevity of porous asphalt that others have done. I think the book by Ferguson might describe some of them, but there are many case studies as well. There has been a lot of interest in this at Villanova university (Robert Travers I think?).

  • Phillip Rodbell Asked: So roots do grow through the geotextile? Only value is separation of soil medium from gravel?

    SD answers: Yes, they grow through it, but it will restrict large roots unless it is torn/damaged. But some tearing is normal–and in the case of trees, I suppose desirable. In this case it is a "separation geotextile" so that is its only function. If it were not being used as a stormwater reservoir, then the geotextile would be unnecessary as the stones would not likely imbed in the subgrade. However, it is a precautionary measure.

  • Johanna Bell Question To: Could you describe the stabilizing "hydro-gel" used to stabilize the structural soils? What is it made from? How does it bind the organics, and allow the organics to be accessed by the plants at the same time

    follow up… It is a polyacrylamide. As mentioned above, it doesn't stabilize the mix and doesn't bind organics.

  • ________________________________________
    Johanna Bell Asked: Could you describe the stabilizing "hydro-gel" used to stabilize the structural soils?
    ______________________
    I think my reply to this got deleted… the hydro gel doesn't stabilize the structural soil. It really just facilitates mixing and installation. It makes the stone portion sticky to prevent the soil fines from segregating during mixing. Over time, the hydrogel degrades, but it is no longer needed after installation and compaction. I recommend visiting the Urban Horticulture Institute websitehttp://www.hort.cornell.edu/UHI if you want to learn more about that type of structural soil. The stalite soil doesn't use hydrogel because the roughness of it serves the same purpose.

  • ________________________________________

  • avery HARDEN Asked: As I understand the new SWM regs for Baltimore County, the places where we have traditionally planted trees around parking lots will now be taken over with SWM structures that preclude having tree roots intruding.

    Susan Day: See the question from Johanna Bell above. For me this is a fundamental of how we think about stormwater. Now we have SWMF that are retention or detention ponds. Just making them smaller and more numerous doesn't change the way we are handling stormwater. In my view, this is a problem we need to address and a reason urban foresters need to have a voice in stormwater regs.

  • MMacario

    I come from the West (California). My assumption is that the trees that you have used are indigenous to your area/locality. Would you know what trees from our region would be functionally equivalent to the trees that you have employed?

    • susan day

      Hello. Yes they are. We used green ash, a tree that is tolerant of flooding and drought and also tolerates a high soil pH. If using a limestone based soil mix such as we did, the tolerance of high pH is important. The Carolina Stalite and other stones would have less of an issue with this. I think bottomland hardwoods would be the right type of plant–but I am not familiar with the western species. Hope that at least gives some direction.

  • Will answer more later…. -SD

  • Johanna Bell Asked: Did you look at the use of geotextiles or other barriers to root growth in order to protect underground pipes and utilities?

    No–and this question comes up a lot. My personal experience suggest that even the nonwoven geotextiles are NOT going to keep roots out of a desirable rooting area. So the design needs to take that into account. This is one of the conflicts of engineered structures and plants. However, you can make these pipes unnecessary and rely on the roots instead, so instead of detaining water only briefly and draining it away via a pipe, you can allow the roots to work and improve infiltration. An overflow pipe, higher in the profile, is less likely to create a desirable rooting environment as it should virtually never have water in it except for extreme events. Again, the degree of concentration is critical. If stormwater management is maximized throughout the landscape–then you won't be concentrating as large an amount of water.

  • Gary Belan Asked: Maybe I missed it, but how do you avoid compaction from the cars on the pavement?

    Normally you don't–and in fact the soil has to be compacted to support the pavement at all. In the structural soil system–the structural soil is compacted to support the pavement, so cars don't compact it any more. But there are pockets of uncompacted soil within it. A vaulted sidewalk works the same way but on a much larger scale–the pavement is supported by structures and the "vault" holds the uncompacted soil.

  • There are a lot of questions, so I'll try to answer some. Concerning Ian Hanou's comment. The way I look at it, the canopy interception is PRIOR to or perhaps IN ADDITION to the capture potential of whatever is on the ground. The potential for layering (for example, arching over a street) is a characteristic special to trees. I think this harkens back to how we are looking at stormwater, whether we are trying to concentrate it and manage it, or mitigate it across the landscape. So a tree could have grass or pervious surface below it–or impervious, but these would receive water after passing through the canopy. So I don't really look at it as a question of should we have a tree vs. something else. In addition, the other environmental benefits of trees are considerable.

  • Part 3 of 3 pages of Questions for Dr. Susan Downing Day —

    Mike Fleischhauer Question To: Has consideration been given to the possible issues created by tree roots continuously penetrating the geotextile (eventually the structural soil settling as the geotextile is destroyed, causing settling of pavement)? Also, has there been study of the longevity of the porous asphalt surfaces under normal use considerations (Ex. Pores pluging as vehichles on surface deposit dirt and oils)?
    ________________________________________
    Sally Thigpen Question To: Advice from one urban forester to another! When installing underground systems make sure you get your sequencing right with your contractor. Make sure the structural soil or silva cells or root pathways etc. go in AT THE SAME TIME as the utilities and other below ground infrastructure. Contractors think of these systems as "landscaping" that typically goes in last. When they get to it there is no room or it's difficult to maneuver to install
    ________________________________________
    Dion Doepker Asked: Regarding Structural Soil: Is there a requirement to the type and shape of the rock used for the aggregate?
    ________________________________________
    Shelley Allen-Czepiel Question: What makes the tree roots from growing up into the pavement?
    ________________________________________
    Karen Engel Asked: Do you absolutely need to use the geotextile fabric?
    ________________________________________
    Nandan Shetty Question: NY state stormwater Design Manual requires the use of non-woven filter fabric for bioretention / rain gardens. But tree roots cannot penetrate it. Would you recommend woven fabric for biortention?
    ________________________________________
    Mike Fleischhauer Question: Has consideration been given to the possible issues created by tree roots continuously penetrating the geotextile (eventually the structural soil settling as the geotextile is destroyed, causing settling of pavement)?
    ________________________________________
    Stan Kamys Question: Are there calculations to determine what size of cu area will treat the otherwise pervious parking lot for 25yr/100yr storwater calculations? Has the data been used to address stormwater pond configurations with a local water management district and the EPA/DEP rules.

  • Part 2 of 3 pages of Questions for Dr. Susan Downing Day —

    ________________________________________
    Travis Thiel Asked: Obviously, most of us see the importance of trees, especially in an urban setting. One thing that hasn't been mentioned or asked about urban forestry is what about the potential impact from leaf litter and nutrient loading from increased tree establishment along or within boulevards and near other impervious surfaces? Little volume would be needed to mobilize nutrient residue and street cleaning processes can't take care of all leaf litter at a reasonable cost. Education efforts to clean up leaves do help, but don't solve the entire problem. We've seen in some of the urban, heavily impervious watersheds here that large fluxes of nutrients are seen in WQ results from storm sewer systems and have to wonder about what this negative aspect and other potential negative aspects may have on establishing new trees or more trees in the urban landscape. That being said, we don't want to work on one problem and introduce others at the same time while dealing with water quality and quantity issues. Any thoughts on this matter?
    ________________________________________
    Stan Kamys Asked: Are there calculations to determine what size of cu area will treat the otherwise pervious parking lot for 25yr/100yr storwater calculations?
    ________________________________________
    Jackson Bird Asked: Thoughts on Silva Cells?
    ________________________________________
    Sally Thigpen Question To: How do you prevent fines from washing out of the pore space when water can move in and out of the constructural soil?
    ________________________________________
    avery HARDEN Asked: As I understand the new SWM regs for Baltimore County, the places where we have traditionally planted trees around parking lots will now be taken over with SWM structures that preclude having tree roots intruding.
    ________________________________________
    Steve Findley Asked: What is your experience with road salts and other pollutants running off impervious surfaces affectiing tree growth and health?
    ________________________________________
    Shelley Allen-Czepiel Asked: What makes the tree roots from growing up into the pavement?
    ________________________________________
    Emily King Asked: You mentioned that the geotextile is meant to prevent the structural soil from sinking into the subsoil. Would tears in the fabric from tree root penetration affect this function?
    ________________________________________Mike Fleischhauer Asked: Has consideration been given to the possible issues created by tree roots continuously penetrating the geotextile (eventually the structural soil settling as the geotextile is destroyed, causing settling of pavement)?
    ________________________________________
    Jennifer Fais Question To: What prevents the pavement over structural soil from heaving/buckling due to root growth?
    ________________________________________
    Steve Findley Question To: What is your experience with road salts and other pollutants running off impervious surfaces affectiing tree growth and health?
    ________________________________________
    Deb Powers Asked: Could you repeat: What was the size (dbh) of the large Platinus in the rainfall event comparison?
    ________________________________________

    Steve Findley Question To: Also, the CU structural soils have been cricized (by Jim Urban) as not providing sufficient organic matreial and water to support the growth. Have you tested other amended soils?
    ________________________________________

  • (Page 1 of 3 pages)

    The followng 3 Comment Pages show the questions that were raised following Dr. Downing Day's presentation. Please take a look for her responses, which will be coming soon. Thanks, Dave Bloniarz, UNRI Webcast Coordinator.

    Meeting Questions and Answers:
    ________________________________________
    gregory smith Asked: In the forest, after a rainfall, water drips off of a cliff face for several to many days. I understand that water is a bipolar molecule, and attracted to charges in soil and organic matter. Is this how water is held in soil? What is mechanism of water being slowly released and dripping for many days after rain ends?
    ________________________________________
    Kent Holm Asked: Will the webcast be available in archive form?
    ________________________________________
    Ian Hanou Asked: For the Jacaranda vs. sycamore example and 1" rainfall, how does the rainfall interception differ with the sycamore being there vs. any other pervious surface, e.g. grass or bioretention? That would better describe the value of trees specifically, not just pervious. Thx.
    ________________________________________
    Frank Rodgers Asked: Wow! What's with those tree guards? Just how bad do they drive in NY? That must have cost a bundle and will ultimately be an obstruction in the care (and ultimate removal) of the tree.
    ________________________________________
    Amy Lindsey Asked: One of the concerns with using trees in stormwater management is the effect of pollutants on tree health. What is your take on this issue and what do you think of the possibilitites of using trees for urban water phytoremediation?
    ________________________________________
    Gary Belan Asked: Maybe I missed it, but how do you avoid compaction from the cars on the pavement?
    ________________________________________
    Brian Koch Asked: What about Road Salt and Chemicals applied to this permeable pavement?? How does this effect the plants and trees?? Also, what happens when permeable pavement clogs from grass clippings, leaves decaying, etc.?? What happens to runoff??
    ________________________________________
    Johanna Bell Asked: Did you look at the use of geotextiles or other barriers to root growth in order to protect underground pipes and utilities?
    ________________________________________
    Johanna Bell Asked: Could you describe the stabilizing "hydro-gel" used to stabilize the structural soils?
    ________________________________________
    Johanna Bell Question To: Could you describe the stabilizing "hydro-gel" used to stabilize the structural soils? What is it made from? How does it bind the organics, and allow the organics to be accessed by the plants at the same time?
    ________________________________________
    Phillip Rodbell Asked: So roots do grow through the geotextile? Only value is separation of soil medium from gravel?
    ________________________________________
    Karen Engel Question To: I was disconnected & may have missed explanation, but….why do you need to use the geotextile fabric at all? What happens if you don't?

  • Here are the questions from Dr. Downing Day's presentation. Please take a look for her responses, which will be coming soon. Thanks, Dave Bloniarz, UNRI Webcast Coordinator.

    Meeting Questions and Answers:
    ________________________________________
    gregory smith Asked: In the forest, after a rainfall, water drips off of a cliff face for several to many days. I understand that water is a bipolar molecule, and attracted to charges in soil and organic matter. Is this how water is held in soil? What is mechanism of water being slowly released and dripping for many days after rain ends?
    ________________________________________
    Kent Holm Asked: Will the webcast be available in archive form?
    ________________________________________
    Ian Hanou Asked: For the Jacaranda vs. sycamore example and 1" rainfall, how does the rainfall interception differ with the sycamore being there vs. any other pervious surface, e.g. grass or bioretention? That would better describe the value of trees specifically, not just pervious. Thx.
    ________________________________________
    Frank Rodgers Asked: Wow! What's with those tree guards? Just how bad do they drive in NY? That must have cost a bundle and will ultimately be an obstruction in the care (and ultimate removal) of the tree.
    ________________________________________
    Amy Lindsey Asked: One of the concerns with using trees in stormwater management is the effect of pollutants on tree health. What is your take on this issue and what do you think of the possibilitites of using trees for urban water phytoremediation?
    ________________________________________
    Gary Belan Asked: Maybe I missed it, but how do you avoid compaction from the cars on the pavement?
    ________________________________________
    Brian Koch Asked: What about Road Salt and Chemicals applied to this permeable pavement?? How does this effect the plants and trees?? Also, what happens when permeable pavement clogs from grass clippings, leaves decaying, etc.?? What happens to runoff??
    ________________________________________
    Johanna Bell Asked: Did you look at the use of geotextiles or other barriers to root growth in order to protect underground pipes and utilities?
    ________________________________________
    Johanna Bell Asked: Could you describe the stabilizing "hydro-gel" used to stabilize the structural soils?
    ________________________________________
    Johanna Bell Question To: Could you describe the stabilizing "hydro-gel" used to stabilize the structural soils? What is it made from? How does it bind the organics, and allow the organics to be accessed by the plants at the same time?
    ________________________________________
    Phillip Rodbell Asked: So roots do grow through the geotextile? Only value is separation of soil medium from gravel?
    ________________________________________
    Karen Engel Question To: I was disconnected & may have missed explanation, but….why do you need to use the geotextile fabric at all? What happens if you don't?

  • the questions from today's session will be posted here, soon.

Northern Research Station
United States Department of Agriculture
USDA Forest Service

Webcasts

Watch our latest webcast:

Watch our latest webcast

New Tools Showcase

More...

News Watch

Here's what we are reading:

(News from enn.com)