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Underground Stormwater Detention Design

Brad Ott
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Please share educational resources to this greenhorn engineer, who barely even knows how to spell engineer.

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Impervious surfaces will be in the mix to be sure.... 😎 

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I have only designed SWM things in Ontario, Canada.  New Jersey has a lot of info that may be more relevant to you.  I don't know your state so I can't direct beyond that.

Note--this is not professional advice and should only be read for its entertainment value.


Ontario SWM Manual:


Example of LID info:


General overview in Ontario:


Personal experience:

If you require underground storage, you will most likely want to suggest "super pipes" if the municipality will allow it.  This avoids costly underground storage tanks or other plastic structures.  


You need X litres of storage.  You have filled out your storm design sheets and find that a 300mm will convey the necessary amount, but provides no storage.  

Potential solution:

You run the calcs and find that two runs of 750mm concrete pipe have the storage capacity.  You can either place an outlet restrictor on the last run or, if they allow, decrease the slope so that the pipe backs up.  Run a hydraulic head analysis to see what the maximum height of the water will be.  If it crests the lid, either apply the maximum ponding elevation municipally given or calc what it actually will be (it will likely be less as once you start "stacking" the water on your outlet it creates head and pukes out).  Adjust your grading and OFR to suit that elevation.  If you decide to go with an outlet restrictor, have them run all those calcs to deflect liability and they usually do it anyway depending on the firm.  I do not suggest an outlet smaller than 4" as it is apt to get clogged (think plastic trash bags or old pop bottles).

Pros: no increased cost due to specialized structures, no special installations, and your sewer crew just has to lay pipe...........which, hopefully, a sewer crew is familiar with doing.


Cons: concrete pipe isn't sexy unless you work for CCPPA 😉


As with all critical engineering on the civil side, I imagine it will involve C3D, some sort of SWM program, and Excel.  


Always Excel.


Chris Bouffard
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Step 1, have soil borings done in the area of UG detention and figure out your percolation rate.  Step 2, consider your drainage area.  Step 3, calculate your volume (Q=CIA), Step 4, size your perforated pipes to handle your volume in a 10 year storm event.  Step 5, size your UG basin and set the bottom to assure adequate pipe coverage.  Step six, find out one of the above doesn't work and start allover.  LOL!