Retaining Stormwater on building roof. controls? 6

[dirkg8989]

I have to retain an arbitrary volume of water on the roof of a building. By the local regulation, the total volume of runoff from the site, for storms up to and including the 25 year storm, cannot exceed the volume of the 2 year storm. Infiltration is not an option, because the building covers the entire site. On other sites in this city (where the building did not cover the site) we have met this regulation with a combination of

1. A tank collecting parking/landscaped area runoff with a pump to allow use for irrigation and an overflow to the city drainage.
2. Permanent rooftop storage of an inch or two via a weir around the roof drains.
3.Infiltration.

I realize these will not function as intended all the time, mainly because of the variable timing of storms. i.e. the roof and/or the tank may already be full when the storm begins, therefore defeating the purpose, however the city has accepted the designs. The purpose of the regulation is to provide relief for the overloaded municipal drainage system.

Because we have no other space on this project, we are planning to store the water on the roof. However the required volume will be over 6" deep, and the architects are not comfortable with storing that much water indefinitely on the roof. There are 9 separate roof drains that do not all connect until they are in the basement, and a separate mechanical area on the roof with an unobstructed drain (which we are planning to use as an overflow, but which also connects to the others in the basement). The city will allow the water in excess of the 2-yr volume to be released from the site as long as it is held for at least 24 hours.

Questions: Has anyone else run into this sort of thing before? Is there an automated system that could release the water from the roof after the storm has ended? How fast would 6" of water evaporate from a roof at different times of the year (the site is near Boston, Massachusetts), I am doubting it could evaporate within 3 days, the average period between storms?

We have even contemplated creating an O&M Plan in which the building maintenance crew would have to open a valve the day after it rains. We could surely trust them to close it before the next storm, Ha.

Any ideas would be helpful. We have come up with the idea of some sort of siphon that would drain the roof when the water reached a certain level, but we have 9 roof drains, and I'm not sure how it would work. The Architect/Owner is willing to use some sort of electronic control system, But we don't know if there are any that can be adapted for our purpose.

 

[gbam]

Very interesting situation.

I personally have never encountered this. Have you considered additional underground storage and a pump or drywells/engineered filtration bed within the storage vault?

As to a timimg mechanism I would think that a standard irrigation valve on a timer would suffice. You could have it open every other day and provide the piping system to completely drain the roof in a day. Then some type of delay system to reset the timer during a preciptation event. I have a similar system for my lawn that shuts off the sprinklers during rain events.

 

[beej67]

Yeesh. 6 inches of water across an entire roof is a big load. Your architects are comfortable storing that? You're not worried about leakage? Maintenance access to the roof? Mosquitoes flying down your HVAC intake? Just seems like a nightmare to me. There's no opportunity to build a vault/cistern below the building to catch it?

In terms of a discharge system, it sounds to me like you want something that'll act like a toilet does. Right? Stage up until it reaches the 6 inch mark, then dump it?

What's the delta between the resting elevation of water in a toilet, and the elevation that activates it? May be something interesting to play with while you're trying to conceptualize your solution.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Ryb01]

I would assume that this building is some sort of a high-rise residential/commercial building? Parking underground?

Retaining 6" on the roof is a big load, however, could be done. However beej67 hit the key points regarding leakage, maintenance access, etc.

There are ways to calculate the evaporation rates, however, generally due to lack of information needed to produce accurate results in close proximity to the site, I would not rely on this as a way to remove storm water from the site.

Just throwing an idea out there but is there a way to re-use the stormwater in the building for re-use? (cooling systems, grey water reuse, etc)

Just out of curiousity, with a building that would cover the entire footprint of a site......how has the forum calculated runoff? More specifically, what I would be thinking is rainfall does not often fall vertically from the sky, does a side or two or three of the building come into play as an impervious surface in the hydrology? Again just curious, I would suspect no but interested to see what feedback the forum has.

 

[jgailla]

There is no way that 6" of water will evaporate in 3 days, maybe 0.6 inches is possible. A high evaporation rate in a hot, dry climate is about 1/2" per day. Except in the desert, evaporation will be about 1/8" per day.

http://www.grow.arizona.edu/Grow--GrowResources.php?ResourceId=208

 

[beej67]

***
"Just out of curiousity, with a building that would cover the entire footprint of a site......how has the forum calculated runoff?"
***

CN 98.


***
"Just out of curiousity, with a building that would cover the entire footprint of a site......how has the forum calculated runoff? More specifically, what I would be thinking is rainfall does not often fall vertically from the sky, does a side or two or three of the building come into play as an impervious surface in the hydrology?"
***

Never bothered with that before. Any extra rain you catch with the windward side of your building is going to create a rain shadow on the leeward side, so if your leeward side is impervious (likely in high density urban) then it's a wash. The only time it'd make a significant difference is if your leeward surface was high infiltration and not part of your watershed.

With small site hydrology, you're not building a watch. You realize this, I realize this, but the reviewers don't, so my opinion is always just jump through the hoops and give the reviewers something that meets their expectations. Tallest building I've worked with is ~15 stories though. If I got a project up in the 30+ range I'd probably write the report as if the rain fell vertically and do a check in-house for angled rain, to make sure nothing flooded.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[Ryb01]

beej67, as always, much appreciated. Thanks.

 

[beej67]

No problem. For the record, my only "downtown" sort of projects were in Atlanta (midtown, buckhead) and Miami. We did cast in place concrete vaults for detention in each case. Never did retention for a high rise, but did retention for some other highly urbanized watersheds, fishing for water reuse or LEED credits.

I want to go back to mosquitoes and HVAC for a second. I've heard more than one horror story of a roof that wasn't positively drained, and caught small pools of water near HVAC intakes, that caused them numerous problems. One big one was mosquitoes getting sucked into the intake. The other was mold, as all the intakes were 100% humidity, which increased condensate in the duct work.

Now I'm not an ME, and I've never done duct design, so its particulars are beyond me. But I would bring those two issues up to the MEP before I explored storing half a foot of water on a roof in permanent detention, even presuming you could figure out what to do with the water and how to keep it from leaking. I'd be *much* more comfortable storing it in a basement cistern and using it to flush toilets, even though you'd need booster pumps, because the potential failures aren't near as disastrous. I'd also document the conversation, so if anything litigious ever comes out of it, the MEP is on the hook for the decision to proceed with the rooftop detention system, not me.

I did think of an interesting way to solve this problem if your project is a hotel or high end condos. (not exactly likely in today's market) Put a pool on your roof, and slope the runoff into it.



Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com

 

[cvg]

use a tank at the basement level with a pump. Otherwise, hire a swimming pool designer to do the rooftop pool

 

[dirkg8989]

Thanks everybody. Cambridge, Massachusetts has interesting regulations and we have to come up with innovative ideas to meet them. The building occupies the entire site and there is a garage under, so they don't want a tank below that (in groundwater), and can't spare any spaces on the garage level. We have used roof detention before, but this new regulation (retain the volume) is difficult without infiltration or irrigation.

 

[beej67]

Got any plans for the groundwater yet?

Sounds to me like an ideal opportunity for gray water reuse. Put your tank below the garage, feed it with rainwater *and* with groundwater since it's below the water table, and flush all your toilets with it. If it's on a campus, use the gray water in adjacent buildings. Collect the LEED credit, get yourself a nice shiny plaque beside the front door, and save yourself two thirds of your water bill every month. Write an article for Green Scene magazine and spend the rest of your life doing seminars about how awesome your building is.

Just a thought.

Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -

http://www.campbellcivil.com