If you're using the SCS/NRCS runoff procedure with a 24-hour rainfall distribution, the peak rainfall occurs at 12 hours.  The peak runoff will occur sometime after that, depending on the Tc.  So a peak runoff at 13 hours sounds OK.

Peter Smart
HydroCAD Software
www.hydrocad.net

Using Hydraflow what is the best way to size a detention basin?

Use the pond wizard after you have the rainfall curve setup. It makes things pretty easy.

I almost never used the pond wizard because a regular geometry did not fit with the draws that my basins had to go into. If you have a flat site, sure go ahead and use it, but I suspect it'll take as much time trying to make the site grading fit the pond wizard design as it would've do it by hand. If you do use the pond wizard, make sure you have constrained it with the drainage regulations - pipe size, v-notch weirs, etc. and that you are designing something that will be practical to maintain.

Well, I agree with the constraints that you mentioned. However, I feel that it is good in estimating the required volume that you will need. After that you can design your pond as necessary and import the design contours into the pond function to get the exact design. Otherwise you have to guess and check on your required volume.

On a flat site it's easy to reproduce the stage/storage relationship of the pond wizard, but in a draw that can be tough. It really depends on the geometry of the site. I'm not belittling the tool, just pointing out that sometimes it is not a time-saver. You should still check your final pond geometry, regardless of how you got there.