Pond volume vs. Peak discharge

[SCEngr1]

I used the HydroCAD sizing feature for ponds to get an initial estimate of the pond volume required (using the pre-developed 50-year runoff flowrate). Then I set up my model to reflect the post-developed flow (Subcatchment) routed to the pond and then routed to a Reach (the receiving basin / stream).

The bottom of my pond is at Elev. 186.00 and top is at 192.00. I modeled the outlet structure with 2 - 12" orifices/holes at 186.25; then a V-notch weir starting at elev. 189.00 that is 3.0' wide and 1.5' tall; then a horizontal 4'x4' orifice for outlet structure overflow. The outlet pipe from the riser structure leaving the pond is an 18" RCP starting at elevation 186.00 at 2.56% slope. My current pond volume is 43,029 CF.

I have tried various hole sizes for the orifices and various elevations for the orifices and V-notch weir, but I can't get the 2-year and 25-year events to discharge below the pre-developed rate. Do I just need to keep increasing the pond volume until it works? I am alredy much larger than my initial pond size of 32,000 cf. Is there any tips/tricks for configuration of the outlet structure to get the discharge flowrates less than the pre for the 2, 5, 25 and 50 year events?

"You're only as old as you want to feel."

 

[SCEngr1]

Here is the attached file of the HydroCAD model.

"You're only as old as you want to feel."

 

[psmart]

What is your target outflow for the 2-year and 25-year events?

Even for the 2-year event, I see that the pond is discharging through the trapezoidal weir. Reducing the orifice won't help much, since it will just create more head over the weir. So you probably need to raise the weir and/or increase the storage below that point.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[SCEngr1]

The target outflow rates are:

For 2-year: 11.46 cfs

For 25-year: 29.40 cfs

So, since the 2-year event is discharging through the emergency spillway, I need more pond volume, correct?

"You're only as old as you want to feel."

 

[psmart]

Yes, you may need to add more storage for the 2-year event. But you should also consider other outlet configurations that would provide better utilization of the avaialble storage, such as lowering the outlet to the bottom of the pond (if permitted), using a different orifice shape, or using a vortex valve. There are many options, which is why it's difficult to automate the pond design process.

BTW, I would model the 18" RCP as a culvert, rather than an orifice. The culvert will consider a wider range of flow conditions.



Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[SCEngr1]

Thanks for the help! I will try those various options.

("You're only as old as you want to feel." - Satchel Paige)

 

[SCEngr1]

How do you model a vortex valve in HydroCAD?

Are they very expensive? Are they very common?





("You're only as old as you want to feel." - Satchel Paige)

 

[psmart]

Vortex valves are certainly much more expensive than a standard orifice, so it's a matter of how much you can save in storage costs. Also check your regulations to see if this in an accepted option. For modeling details see

http://www.hydrocad.net/vortex.htm

Peter Smart
HydroCAD Software

http://www.hydrocad.net