7 Dec 02 22:44
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