design of stormwater holding tank in nj

[peinnj123]

what is the best approach to designing a stormwater holding tank below a proposed parking lot for a proposed office building in nj? in other words, what are the steps to take in designing the stormwater holding tank... and where in the nj design codes can this info be found?

 

[psmart]

Why are you holding the water? For reuse, runoff management, or something else?

Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[cvg]

hydrology study first, geotechnical investigation, structural analysis, what type of information are you unclear on? You should ask your building department what codes to follow

 

[peinnj123]

For reuse as greywater routed back to the proposed office building. I am unclear on the technical design of the stormwater holding tank. I have calculated the stormwater runoff volumes, and need some guidance on sizing of the holding tank.

 

[gbam]

It would depend on the agency approval for bypass. What percentage are you looking to store? What type of treatment do you need to provide in order to reuse the water?

 

[beej67]

Unlike peak storage for event storms, sizing tanks for greywater reuse typically involves doing some sort of water balance analysis.

So you're less interested in what happens to the "10 year storm" going into the tanks or whatever, and you're more focused on making sure the amount of water you accumulate in your tanks during the wet season carries over to match your demand in the dry season. Most times I've done it, I've used monthly rainfall averages, or if available I've used higher percentage confidence interval rainfall numbers.

So, for instance, say your owner wants to make sure that his tanks only go dry once every ten years on average. Then you go and drum up some numbers on what the 10% driest month is (if that makes sense) and you use those numbers in your water balance analysis.

Then the water balance analysis for your system will be something like this:

assumed rainfall in a given month produces
calculated runoff in a given month plus
carryover from previous month equals
total available that month, minus
whatever you use that month (irrigation demand varies, toilets typically don't) equals
carryover to the next month

Then you size your system so it can adequately carry over the stuff from the wet months to the dry months.

Then your client tells you that's too big, and actually we're just going to install whatever we can afford, regardless of whether it'll work or not, so you shrug and move on.



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

http://www.campbellcivil.com

 

[peinnj123]

Thank you beej67, I will give it a try.

 

[beej67]

That's just a very high level overview of the concept. Fish around for some better instructions than that. I know there's some simplified design procedures in the Georgia Stormwater Management Manual, and the concept is covered quite extensively in some collegiate water resources classes. Water balance and reservoir optimization is a field where you can make it as simple or as complex as you like. In my graduate program, we developed iterative nonlinear optimization tools to maximize hydropower output from the Lake Victoria Dam, as a class project. Not what you need obviously, but pretty neat.

The lesson you're going to learn when you run this exercise, is you want to drain as much watershed to your tanks as you can. I prefer trying to limit it to impervious runoff because it's cleaner and less trash to intercept. I typically like to put some sort of pretreatment in the system as well, like an oil-grit separator or other hydrodynamic water quality device, to keep dixie cups out of your cistern.

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

http://www.campbellcivil.com

 

[peinnj123]

How would I size the oil-grit separator?

The total volume to be stored in the holding tank area is 13,369 cu.ft. From uphill, there are 2 bio-retention basins that, when combined, are a 10,700' area; and the outflow pipe is located 2' above the bottom of the basins. So therefore, there is some TSS removal, and I am not sure how to quantify the amount of total sitewide TSS, nor the amount of TSS that would be held in the 2 basins.

I've designed the holding tank system with StormTech's SC-740 system (24" dia) 60'x95' area. How would I size the width of the oil grit separator prior to flow into the StormTech holding tank area? I've attached a typical cross section.

 

[beej67]

If I was using a StormTech system, I'd probably skip the oil-grit separator and just go with that "isolator row" thing they talk about in their literature.

http://www.ads-pipe.com/en/documentlisting.asp?documenttypeID=118

Sounds like an interesting project. If you're going to be using storage in a stone reservoir as your cistern, make sure to spec washed stone. Wouldn't want sand in your toilets. I would also recommend wrapping the whole thing (stone and all) in impermeable geotextile or similar so you don't lose any water to groundwater infiltration, unless that's part of your intent.



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

http://www.campbellcivil.com