Skogs:
If you don’t know that MikeH is pullin your leg just a little bit, you might have to reveal the “true source of your wisdom” and more than that, your personal lack thereof and mine too, on this particular matter. Whatever will happen when/if the sharp edges on those holes rust a little and become chamfers? But, he’s pointing you in the right direction. No offense Mike, I didn’t check your calcs. for size and number of holes. And, Skogs you oughta know all this stuff, isn’t their some analogy btwn. flow rate, pressure, and pipe size; and amps, volts, and wire size? Get out your electric meter out and figure this out, does that darn thing just read in units of amps and volts, switch it to l/s and pressure head.
There won’t be more water falling on the site from a given design rain storm, but its runoff rate from the shed roof will be quicker than exists now. So, your retention pond is the right idea and must be large enough to hold this difference in runoff/time to the sewer. Will the local soils absorb the water over a few days so that you can argue that you are actually reducing the flow to the sewer? Maybe install a few crushed rock filled drainage trenches across the flow lines on the site, to distribute and retain the water to a larger soil area, as in a septic drainage field. Is it a sanitary sewer where they dislike adding storm water to total treatment volume, or is it a storm sewer where they might be a bit more lenient and negotiate a bit? Can you run the water over a longer route to the sewer and through higher grass, or some such, which slows the flow. Can you do some regrading of the site to change the flow rate to sewer or change the direction of flow away from the sewer? Otherwise, a properly sized pipe set a foot above the bottom of the retention pond, or some such, will drain 3 l/s, with some increase in flow as the water level goes above that one foot elev. Is the little shed and the parking area all new impervious area? Or, is the little shed just replacing some impervious parking area, in which case the runoff rate wouldn’t change much. Talk with one of your local Civil Engineering friends, they should be able to help you resolve this with the city.
