Find Storage-outflow function for a culvert detention pond

[ADaniela]

I'm trying to follow an example from Ven Te Chow's Applied Hydrology (p247) to determine the inflow/outflow vs time of a detention pond in order to size the pipe culvert.
In the book, the storage-outflow graph its a given, and I don't know how to model that for my pond.

The pond is (aprox) a triangular prism in a rural area, with a single CMP culvert (submerged inlet). I have the storm info and I've been able to calculate peak discharge for the culvert based on "Design of Small Dams" (US Department of the Interior publication). But I need to model the progress of a storm.

I've found a Q=CA(2gH)^(1/2) formula somewhere but I don't think it's appropriate.

I'm very new doing this (and a bit out of my field) so I don't have any software, so if you also happen to know of some free ones for helping me with this, I'd be grateful.

thanks in advanced

 

[froude]

So is this an existing pond that you are modifying? Or a new pond that is being constructed? Typically you would want to determine the existing flow reaching the outlet. While you have done this, you need to be sure it is by a method acceptable by a reviewing agency (if there is one; some methods include Modified Rational Method, SCS Unit Hydrograph, etc.) Different methods can give you different flows and you may want to consult a hydrologist if you haven't done this much.

Then design a pond volume and outlet structure to attenuate the flows down to some desired point (perhaps a known outflow, etc.)

HEC-HMS and EPA-SWMM are free programs that will allow you to "design" and analyze a pond/reservoir and it's outlet works. However, each one can get somewhat complex, so be careful.

 

[cvg]

you can do much of this with free software available online. HEC-1 (I prefer this over HEC-HMS) can handle the routing through the reservoir and can also be used to generate the inflow and outflow hydrographs. HY8 can be used to calculate the culvert rating curve. Stage-storage can be calculated using a planimeter and a spreadsheet. You will need stage-storage and stage-discharge relationships for the reservoir. You will also need the inflow hydrograph.

 

[gbam]

Typically you size an outlet culvert based on a specific flow rate and stage (volume for your basin). Cvg mentioned tools for your use (HY8 & HEC1). I prefer Visual Urban for smaller basins since it works in cubic feet rather than acre feet as HEC1 does. Visual urban is a free download from FHWA website. To determine your volume of basin you can use the straight line method; you graphically draw a straight line on your inflow hydrograph from t=0 to your desired outflow rate. the area between the inflow hydrograph and your straight line is a starting point volume you need for your basin. Hope this helps.