There are many bright and competent engineers who say "retention" when they should say "detention".
Please pay attention to this matter.
In over 40 years of practice (or should I say career?), I have designed many detention areas and only 1 retention area.
The retention area had no outlet, but had a very porous bottom deep in the ground. It worked very well. Yes, it was an effort to convince the government approving agency that it would work correctly. There was no surface outlet for storm water.
Buy a dictionary, keep it nearby and USE it. Webster's New World Dictionary of American English is recommended, and Webster's Collegiate Dictionary.
The retention pond requires knowledge of the soil permeability expected (long-term) in the basin. Leaf debris, ect. can greatly reduce perms over the years, so be conservative in your choice. I've written some computer modeling programs in excel to provide a stepwise simulation for various storms and runoff scenarios.
Having the nickname "chicopee" sounds likes you're maybe 10 mins. away from my office in Granby.
Just a note on backfill, I tested compaction on detention pond walls with moderate clay content which decreased 5-10% during winter weather (and turn moderately sloppy!). We weren't aware of this phenomenon, so I thought I'd mention it here.
Sound that if you're in the North, frost action may be causing this de-compaction effect.
The agronomy people appreciate the effect - it naturally loosens and aerefies the soils. Of course, it also pushes rocks upward - thats a phenomenon in itself.
Jimbo is right. Terminology is important. It reflects your credibility.
Down here in the land of few hills and lots of water(Florida), we often have requirements for both retention and detention on the same site. Retention is used primarily for pollution abatement of the first runoff, while detention is used to mitigate downstream peaking affects from the post-development condition.
Land of few hills and not much water - Phoenix. We only get about 7 inches of rain per year (in a normal year). But, it generally comes only during the summer monsoon and during the winter.
We also use retention and detention. Typically, all development is required to provide 100-year, 2-hour retention. This has a huge effect on reduction of the 100-year discharges downstream. Detention basins are also constructed to further reduce flooding or to handle flows from older developments or undeveloped land which doesn't have the retention basins.
