Two different kinds of ponds are often used for flood control and stormwater runoff treatment:
wet ponds and dry ponds. Both systems function to settle suspended sediments and other solids
typically present in stormwater runoff.
Dry detention ponds generally use a very small slope to divert water. The inlet needs to be not more than 15% higher than the outlet to ensure the correct amount of water flow through the system. The system works by allowing a large collection area, or basin, for the water. The water then slowly drains out through the outlet at the bottom of the structure.
Wet retention ponds are a stormwater control structure that provides retention and treatment of contaminated stormwater runoff. By capturing and retaining stormwater runoff, wet retention ponds control stormwater quantity and quality. The ponds natural processes then work to remove pollutants.
I agree that their location on the site does determine the effectiveness. I think the real question is what is the site trying to do: Hold the water permanently for stormwater treatment or provide an area for stormwater to collect and drain naturally during a storm surge.
I think your question is really about how effective is a wet retention pond on a single site. If at a higher elevation on the site, the water will have a great site area to drain across/into the earth. if it is lower on the same site, less area for the water to go.
I hope this helps. Further reading can be found via google searching. I found this document from a municipality that is helpful for further understanding.