How to size and calculate a simple storm water retention basin on an existing terrain with Civil 3D

Assuming you already know the Retention Volume Required (it's an Arizona thing), you need a fast way to design a retention basin if possible. Complex retention basins in tight areas have to be maximized according to the available room. Then Civil 3D volumes below a level HW surface have to be taken to determine the volume. And there is not much you can do to get more volume without spending money on retaining walls or underground retention. But some other basins are in wide open areas, and they can be relatively simple shapes. For those you need a fast way to get them drawn to the required volume.
Here are steps to quickly iterate to a satisfactory basin design on an existing surface such as a low-density rural home lot.

Watch the video playlist

Identify preferred location and shape

Make sure you are putting the basin where it fits the architect's or owner site plan and the drainage requirements and terrain of the site.

Find size and shape with polyline design

Offsets

Guess offsets and size from boundary lines to make an ideal basin bottom to fit the preference. Try to allow for slopes to fit all the grading to existing surface within the given boundaries.

Fillets and joins

Fillet the offsets and join them into closed polylines to make a top or bottom contour. See PJL routine below for quicker joining

Contour offset 

Offset the top or bottom to make the other contour.

Set fake elevation

Leave the bottom polyline contour at z=0. Use Properties to elevate the High Water contour to the ponding depth of the basin.

Volume calc

Use the Civil 3D ribbon Analyze, Design, Stage Storage (AeccStageStorage) tool calculate volume from the polyline contours.

Civil 3D Stage Storage contour volume calculator tool

Explode, erase, repeat

Roughly calculate how much you need to expand or shrink the basin to get closer to the Volume Required. Then erase the top contour, explode the bottom contour, and start over. Repeat until you are happy with the Volume Provided. Document it for your drainage report.

Create surface with Civil 3D design

I got tired of writing for now. But you can watch the video.