Victoria-The proceedure used in evaluating detention basin is storage Indication Method (reservoir routing). It consists of stage, storage and discharge relationship through which an inflow hydrograph is routed resulting in a maximum stage (elevation), discharge and storage volume. Any hydrologic analysis text should assist in this. A good reference source is "Stormwater Collection Systems Design Handbook", by Larry W. Mays. You may be able to find the hand calculation steps through a web search. HEC-1 or HECHMS manuals may heve some information as well. Hope that helps.
For HEC-1, you need to code the reservoir routing. In the HEC-1 manual look at Appendix A under storage routing. You will need the following cards: SA or SV, SE and SQ which represent the storage area or volume, stage (Elevation) and Discharge. These are typically entered in a rating table and coded below your hydrologic cards. Example problem #9 provides example coding for your reference. If you do not have the manual, it can be downloaded from USACOE website.
1. Do you know which type of hydrologic method you will be using? Will it be a unit hydrograph? Modified Rational? This is something you need to know ahead of time and may be dictated by your local municipality or reviewing body.
2. Do you know what your target release rate is? This may also be governed by the above-referenced entities.
Once you determine your methodology, you will need to find a program to perform your reservoir routing (or do it by hand/spreadsheet).
Some free programs that will do reservoir analysis include HEC1, HEC-HMS, and EPA-SWMM (but I believe these only do unit hydrograph methods). Several licensed software including PondPack and XP-SWMM will also do detention computations.
For modified rational detention routing, there may be spreadsheets available online (or it would be relatively easy to come up with one.) Good modified rational detention examples can be found in "Stormwater Conveyance Modeling and Design," which is a manual published by Haestad (now owned by Bentley). Some time ago, Haestad had published PDFs of this for free download. I don't know what the current availability of this document is.
Regardless, the concept as described by the users above is still true. You determine your post-developed runoff and route that through your proposed pond volume and your selected outlet structure. This will then tell you your max pond volume and max 100-year WSEL.
If you just want to do it with a spreadsheet, download TR-55 and follow the procedure.
I hate to be a sourpuss, but this is something basic that you should have learned in school, and its important enough that you should probably hire someone more familiar with it to get it right if you are unsure about how to do it. If you do it wrong, and something fails, it's your butt.
Hydrology, Drainage Analysis, Flood Studies, and Complex Stormwater Litigation for Atlanta and the South East -
Maybe it's a smaller catchment and she doesn't need to do all that pond routing. She should check and make sure she is following the local criteria. There may be very simple 100 year required volume equations for her to use, or she could be allowed to use the Modified Rational Method.
Once you have the required 100 storage volume, you can set up your own pond to accommodate the volume and interpolate the 100 yr WSEL from the stage-storage curve for it.
