well, for free - search for the USDA TR-55 report "Urban Hydrology for Small Watersheds"

This will really walk you through it, although it's a bit technical... There are many computer programs, but I don't know if any are free. Here (at my new job) we use Hydraflow by Intelisolve (www.intelisolve.com) At my old job we had a dos program called "Quick TR-55" by - i think - Pondpack.

The calcs for runoff are easy to do, but the routing through the basin can be iterative, so I would really recommend a computer program for that.

Hope that helps - let me know if you would like additional info.

Kate
kthomson@ekmail.com

Kate,
USDA TR-55 report "Urban Hydrology for Small Watersheds" I have found on the net is not free and it would take time (I do not have) to get it. It would be great if you can email a copy to m63@canada.com

Thank you

Try HydroCad, it a good program for newcomers to detention pond design.  I think you can download the software from their website, www.hydrocad.net. I don't work for them, I've just used their product before and it works well, plus the learning curve is short.  There is also a Forum on this website related to the software... AMS: Hydrocad Stormwater Modeling that you can check out.  good luck

hmmm... i got it for free.. try here:

http://www.wcc.nrcs.usda.gov/water/quality/common/tr55/tr55.html

if that does not work, i would have to get back to you - i don't have it on my computer at work.

kate

m63, what part of Canada are you working in ?  If the Municipality or City that you doing the work in does not have SWM design guidelines and criteria, then I would imagine that your Provincial Ministry of Environment would have something (or should).

I am working on PEI. Nobody has regulated SWM yet re detention ponds.
When traveling to NB, NS or ON one would say 'Im going to Canada'

A few suggestions;
1a) You use your local municipal requirements or if vague, use the rational method instead of a synthetic hydrograph method (TR55).  For example, if your municipality just says that the post developed 25 year peak runoff shall not exceede the predeveloped rate, then use the rational method, it is much easier or a modified puls method - both can be a simple spread sheet.
1b) Please review the following concepts - Time of concentration, hydraulic length, "C" factor for rational, (TR55 - CN, Initial abstraction, soils type, antecendant soil moisture, directly connected impervious area), rainfall distribution (SCS) or Intensity-duration-frequency curves (rational).  
1c) The above answer assumes an "open basin" or a basin that has a positive outfall for your rainfall event, if not, then you have the additional restriction of total runoff volume limited to the existing volume.
2) Assuming an "open basin", you also need to design a pond outfall control structure that will limit your pond outfall at your peak stage.  Please consider external tail water and you need to size the conveyance (usually an outfall pipe) from your outfall control structure to your outfall.  If you use a Wier or an orifice, make sure that the outfall pipe will not cause tailwater.  A drop pipe inlet might be easier for you.  Please consider baffling the inlet to keep grease and oils on site.  If you use a baffle, remember to size it so that it does not obstruct the wier/orifice.
3) Next you might consider the sizing of the internal conveyances to bring the stormwater into the pond.  This includes the master culvert system and the curb/gutter/inlets for the roads at a minimum.  PLease consider the spread of flow in the streets, the depth of flow and the lnlet capacities.
4) I almost forgot, you need to grade the site and account for groundwater elevations and flood elevations.  Your pond can not store water below the seasonal high water level and your level of service will determine the roadway height above the  desing flood elevation.  PLease remember that the flood wil back up the culverts and if the pond peak stage is at 100', then the upppermost inlet may be at 101'.

Clifford H Laubstein
FL Certified PE #58662

I will plug this since I have developed the software over a period of 20+ years doing hydrology/hydraulics.

My WINhydro(c)program available from McTrans at the University of Florida.
It will use TR-55 and WINTR-55 output as input. Can compute basin staging and/or sediment basin sizing and will do split flows (first flush to sediment basin) to each, if desired.

Outlet configurations can be up to 9 openings, weir, pipe, vertical orifice or horizontal orifice and can produce outflow hydrographs of each outlet.

Don Grisham, PE/RPLS/CPESC

McTrans web page
http://mctrans.ce.ufl.edu/

I might suggest that in terms of cost, it may be more efficient for you to retain another firm to provide solutions to this problem.  The reason is that you indicated you do not have experience in this field.  I contract out frequently, particularly drafting and soils designs, because there are many individuals much more experienced than I am who can very quickly determine a cost effective design.  Calculating a detention pond size based on the flood and hydrology is the easy part.  Overcoming land issues, outfalls, storage issues, discharge issues and construction is another matter.  I mentioned cost because I have found that the learning curve for new areas of design (software, design research, estimating) is quite sharp and in many cases my time spent on a problem would be triple that of a more experienced individual.

KRS Services
www.krs-services.com

for computer software,
try this site

http://www.kesco.sphosting.com