The reason we hire the civil engineer on this task is not only to draft up a sensible plan, but to do the math to show the City that what we are planning is not completely bogus.  I have taken a snapshot of the actual calculation page in this, and it shows the real black box type work that goes on in these jobs.

What we need to know from this is mostly contained at the end. First, we want to prove a storage volume, which we do at 12.63 m3.  This is a lot of storage, and is a combination of the manholes, surface pond and the interior of the pipes that would be filled with water during a flood event where water backs up from the outflow restrictor all the way to the surface.  We also want to know the post development flow rate off site, that is 32.33L/s/ha, note that we have far less than a hectare of property, so our flow is a fraction of 32 L/s.    The rest of the material, like the Manning's formula, coefficient of slop and absorption of the landscaping is stuff I have not researched, but does play a role in the final calculation (or at least the engineer stamps this drawing and he knows best).

 Another key piece of information to gain from this is we also need to put on a flow restriction device.  While we may have a six or eight inch outlet pipe, we constrict that down using a special valve at our property line.  I will feature this device in an upcoming post.  Constricting an 8 inch pipe down to a fraction of that, just to have it open back up to 8 inch capacity has always been a confounding issue to me. Why not just use a 2-3 inch outflow diameter?  In the event of some catastrophe the outflow restrictor will just exacerbate the problem of moving water downstream.  During the devastating flood of 2013, did anyone demand nature somehow restrict its flow?  Despite this and other storm water storage dilemmas, we do our best to comply and cut cost where possible (these days hardly anywhere).  I plan to install this system this summer at the threeplex, and further break down the cost of this project once we have selected the contractor