Sorry I'm late to this thread, been too busy in the last week designing ...
...stormwater management ponds for new development.
What you need to do for stormwater management varies widely, WIDELY on where you are. East coast / west cost, north / south of the mason dixon line, everybody does it differently. Before accepting any advice here you must must must check your local regulations. They're typically driven either by your local municipality, your state, or some state subdivision such as a "water management district," sometimes broken out by basins.
You've typically got several different factors to consider with these things:
1) Detention of an event storm or event storms to a specified level, which could be to predevelopment, or to some idealized version of predevelopment (as if the site were all trees) or to some specified discharge rate (so many cfs per square mile of drainage area) because of a flooding history in your basin. Sometimes you have to match more than one event. In Georgia, you often have to match the 2 year, 5 year, 10, 25, 50, and 100 year storms as a matter of courtesy to your reviewer, even though the regulations don't often state that anymore.
2) Water Quality treatment of runoff is often required by municipalities as a function of their permit process, so they can turn around and hand that off to the state EPD or DNR as evidence of non-point-source Clean Water Act compliance. These sorts of regulations are often framed in terms of a TSS reduction, or a maximum TSS load rate, or a mass load rate of other pollutants such as nitrogen or phosphorous. Sometimes this varies by basin or by which watercourse you're discharging to, if it's got a TMDL. Water Quality treatment is typically provided with either a proprietary stormwater cleansing device, or a retention/infiltration volume, or incorporating elements such as littoral shelves into your stormwater management pond.
3) Other criteria - there always seems to be a third wheel in these things. "Channel Protection" is a big thing in some areas, and is handled by a drawdown of a specified volume over a specified time. Water Quality is sometimes handled with a drawdown as well. Sometimes you'll have land-use specific criteria that kicks in for hot spots, such as the 0.5 inch of required retention volume for commercial development in SFWMD. I've seen some areas make you do not only a peak discharge match, but a volume match as well, which means you must retain and infiltrate (or reuse) a certain amount of water, which involves geotechnical investigations and water balance math.
4) LEED credits are there if you want them for this stuff, one for detention and one for water quality, and you might be able to get them for free just by meeting local code, depending on the code.
5) Reuse (cisterns/etc) also get you LEED credits, and may get you water quality credit too if you can show you're capturing the first flush and running it through your toilets.
7) Out West things get weirder, because your water rights usually aren't riparian, so something as simple as a rain barrel could be outright illegal. You don't own the rain that falls on your head, some user downstream does.
There are a lot of elements to this, and you should speak to someone who knows about them when you're in the land planning phase to get a good, solid understanding of how you're going to handle it. An engineer should be able to investigate the details, help you choose a stormwater strategy, and give you a ballpark estimate on the size of your system for a pretty small fee. (one to a few thousand depending on the size of the project and the complexity of the regulations)
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -
