MIDUSS - remove volume from flow?
MIDUSS - remove volume from flow?
(OP)
Good day!
I'm trying to model rainwater harvesting in a development; we'd like to have the first 20mm of rainfall that hits the roofs be captured and reused. Since we're assuming 50% of the lots are roofs (employment uses), this is a major component of minor storm events (e.g. the 2-year storm is 36mm).
I can't model this as a diversion, because diversions deal in flow rates - this 20mm over the roof area is a volume.
Does anyone have an idea of how I can model this correctly? Thanks in advance!
Chris from Ontario Canada
I'm trying to model rainwater harvesting in a development; we'd like to have the first 20mm of rainfall that hits the roofs be captured and reused. Since we're assuming 50% of the lots are roofs (employment uses), this is a major component of minor storm events (e.g. the 2-year storm is 36mm).
I can't model this as a diversion, because diversions deal in flow rates - this 20mm over the roof area is a volume.
Does anyone have an idea of how I can model this correctly? Thanks in advance!
Chris from Ontario Canada





RE: MIDUSS - remove volume from flow?
Richard A. Cornelius, P.E.
WWW.amlinereast.com
RE: MIDUSS - remove volume from flow?
RE: MIDUSS - remove volume from flow?
The first 20mm that lands on the (flat) roof will be captured; the 21st mm that lands will find that the cisterns or infiltration galleries are full and will be either sent to grade or else diverted to the storm sewers (as long as the infiltration galleries are lower than the storm sewers this should work).
You raise an interesting point about dirty water from the roofs .. I'll look into that. Typically though we assume that roofwater is 'clean' and can be infiltrated immediately.
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cvg: you are a legend among men. I'll simply modify my Ia to 11.1mm (50% of my site is roof, 40% is parking lot, so 20mm over the roofs is equivalent to 11.1mm over all the impervious surfaces) and the runoff hydrograph is modified just how I want it to be. Thanks!!
RE: MIDUSS - remove volume from flow?
RE: MIDUSS - remove volume from flow?
RE: MIDUSS - remove volume from flow?
Peter Smart
HydroCAD Software
www.hydrocad.net
RE: MIDUSS - remove volume from flow?
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Chris
RE: MIDUSS - remove volume from flow?
What's your intent with this modeling? There's two things you could be modeling here -
1) Water Balance - where you show that you'll always be able to flush your toilets or water your plants even in the dry season, because of how much water you've stored up from the wet season, or
2) Detention - where you model what a rare event storm (e.g. 25 year or something) does when routed through your system.
For Water Balance, you typically pick monthly rainfalls, abstract those to volumes, subtract your monthly usage, and calculate a running volume of water in your cistern. Usually you pick rainfall numbers that show a dryer season than usual. This sort of thing is typically just done in Excel for a development application.
For Detention, you usually don't take credit for volume in the cistern, because to do so wouldn't be conservative. What happens if there's a slow drizzle for a week before your 25 year storm? What happens if the 25 year storm hits during Christmas and everyone's been on vacation, so nobody's flushed the toilets in a week? If the cisterns are full before the storm starts, then the volume you may have assumed for detention isn't there, which can cause downstream flooding. This is especially true for irrigation cisterns, because you don't irrigate during the wet season, and the wet season is when you get your peak storms.
If you're going to use any dead storage in your cistern for detention volume, talk the assumption over with your reviewer, explain your assumption to your client, and document those conversations, at a minimum. Whether or not you're technically allowed to do so will vary by municipal and state code. If you do take volumetric credit for them, at least do a "cisterns full" run of your stormwater detention model to check for flooding conditions, for your own edification.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East - http://www.campbellcivil.com
RE: MIDUSS - remove volume from flow?
RE: MIDUSS - remove volume from flow?
But I certainly agree with others that it's problematic to assume that the cisterns will be empty at the start of the event. A conservative approach would assume they're full and that all the runoff will appear downstream. Talk to your reviewer.
Peter Smart
HydroCAD Software
www.hydrocad.net