Green roof: Drainage, Irrigation, and Water Quality all in one

Landscape Architecture for Landscape Architects Forums SUSTAINABILITY & DESIGN Green roof: Drainage, Irrigation, and Water Quality all in one

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    Sarah Rivenburgh

    Hi fellow LA’s,


    I need some precedent help. I am working with a team to use a green roof as a water quality event. Here is how it will work:

    1. The green roof is located within an architectural setback on level 2.
    2. We want to collect the runoff from level 4 (top roof) in area drains. These drains connect internally via the plumbing system and day light to the green roof via multiple lamb tongues.
    3. The lamb tongues empty into steel catch basins.
    4. The catch basins are connected to steel U-channels that span the width of the green roof.
    5. The steel channels sit atop a rock bed, and are perforated.
    6. The idea is the perforated channels will act as a flood irrigation system dispersing the water from the upper roofs across the green roof (simple sedum beds).
    7. There will be the typical area drains (double drains from top of planting medium and connected to the structural deck) located amongst the sedum beds to catch the runoff from the green roof and send it back internally through the plumbing and ultimately to the detention at grade.

    Can anyone offer me some precedents for this? The design team is confident the system will be fine, however our ownership is a bit skeptical and nervous. We’d like to be able to present them with similar systems (whether on the ground or on architecture) to help boost their confidence and encourage innovation.


    Any and all help is greatly appreciated.

    Let me know if you need more details.




    Rob Halpern

    My first thought is about the sedum:
    Depending on the species, they will not likely survive repeated flooding. Depends of course on how much water they are expected to put up with.

    Secondly, sedum – unlike reeds and other wetland species – are not likely to do much to clean the water. You refer to “water quality” so I’m assuming this is the intent. Is it?

    Thirdly, located in a lower level architectural set-back, there is possibility of this being somewhat shaded. So what sedum grows in shade in occasionally flooded ground and adapts equally well to drought?

    Finally, you don’t state the depth of planting mix (“soil”) on this roof. A typical intensive green roof of a couple of inches depth will not hold the water for any length of time. Studies have been done on these roofs and slowing storm run-off. They failed.

    I get the impression this plan is attempting to mix rain garden-with-artificial wetland-with-green roof and has assumed sedum. If that is the case, I am skeptical of your success. The approach is sound, just incomplete. Perhaps you have these issues all figured out already?
    I’d enjoy hearing more.

    Chris Whitis

    Are you providing an irrigation system, or are the perforated channels intended to provide the irrigation for the entire roof? If you’re going with a traditional extensive (4″) green roof media, I’m afraid you’re not going to provide any irrigation other than immediately adjacent the channel. Green Roof media typically conducts water pretty quickly and doesn’t generally flow over the surface.

    I’m interested in seeing some of the research you’re referencing, Rob. The research we’ve seen typically does a pretty good job at water quality and volume reduction. For WQ, most places are providing credit at 80% TSS and research is there that supports that. The plants aren’t doing the work with filtering, it’s mostly the media/ filter fabric that’s trapping fines (think sand filter).

    Volume is trickier and dependent upon the size of the storms. For smaller storms, 1″ or smaller, green roofs have the ability to capture upwards or 90% or more of the rainfall. Granted, this is counting rainfall directly to the roof and not additional flow directed to it. Chicago had been studying volume reduction through test plots and their annual averages were around 65% total reduction. NC State was showing closer to 55%. Even when events overwhelmed the green roof, they’re shown to significantly delay runoff. Don’t know that I’d quantify that as a failure and I’d be interested in seeing those studies.

    There was a system being tossed around a couple of years ago that was a flooded green roof system where irrigation was provided by upward capillary action. I can’t recall the name, but basically it maintained a set level of water in a deeper soil media. Of course, that adds a ton of weight to the structure by holding water continuously.

    Rob Halpern
    Chris Whitis

    His beef appears to be more of biodiversity of what’s going on in the soil, not related to stormwater. Or at least I’m not gleaning that green roofs have failed in that aspect.

    Sarah Rivenburgh

    Thanks Chris and Rob for your initial thoughts.

    Some additional information based on your questions:

    1. We are in Denver. Mass amounts of rain are rather infrequent, so flooding the sedum beds with the occasional rocky mountain downpour shouldn’t be an issue as far as excess water content is concerned. We are more worried about it drying out than remaining too wet.
    2. The deck is south and west facing. Sun exposure for the sedum won’t be an issue either.
    3. We are planning a typical 4″ soil installation with an added moisture mat for additional moisture retention. We are counting more on the complete green roof system for water quality as opposed to just the vegetation, before the water hits the inlet that will take it down to grade.
    4. We are not counting on a high amount of detention to happen on the green roof just that first flush filter and a little added slowdown. The outlets at grade will be ultimately connected to a subsurface vault detention system.
    5. There will be supplemental drip irrigation with a moisture sensor in the soil. The hope is that that moisture mat and cup system will aid in giving us the ability to irrigate on a less frequent basis.
    6. This is a new building so we are working side by side with our structural guys to make sure everything is properly supported.

    Chris, are you refering to the “Basal” irrigation? It was in the June ’08 LAM.

    Rob Halpern

    “Brenneisen also found that many sedum roofs have minimal positive effects on storm water retention, energy use, or the urban climate — core arguments that have been used to justify the implementation of green roofs. ”

    He is well known for promoting biodiversity aspects of green roofs. But it is thios part of his report that is troubling.

    Chris Whitis

    Yeah, the basal system is the one I’m remembering. Haven’t heard any more about it or looked into it further, but am guessing the added weight costs would be of concern with owners. Plus, having a full-time bath tub overhead might scare a few people as well.

    Chris Whitis

    Do you have his report? I can’t seem to find it. I’d love to see his numbers to back up his statement. Curious if he’s trying to discredit other systems in order to push his.

    Here’s some additional info:

    MSU –
    “Over the 14-month period the study was conducted, the vegetated roof treatments retained 60.6% of rainfall from 83 measured rain compared to 50.4% and 27.2% for the media-only and conventional gravel ballast roofs, respectively.”

    I’m having trouble tracking down the CNT report on Chicago for some reason and can pull the NC State info when I have some time.

    Rob Halpern

    Yeah, I’d love to see it too.
    Of course, he seems to be accusing certain applications of failing to meet goals… not all. It does speak to the necessity to design systems specifically to do these “green” jobs rather than simply throwing up a green roof.
    And then there’s the issue of performance over time.
    I really would like to see more long term, wide ranging research!

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