Interconnected Pond Modeling Question

[VAEOhio]

Hi there,

I am currently working on an interconnected pond design that is giving me some trouble. Here is the basis of the design...

I have two ponds, both of which will be wet with a constant water surface elevation. The two ponds (northern & southern) are seperated by an existing drainage ditch.

The idea is to connect the two ponds with a large 60" pipe laid at a relatively flat slope, that will go under the existing ditch. This pipe will be approximately 6' below the normal pool of each pond. By connecting the ponds in this manner we will allow the existing ditch to function the way it currently does and provide a "free flow" condition between the retention ponds.

The outlet for the southern pond will function as a typical stage storage outlet structure and discharge into an open ditch.

I am currently using Haested Pondpack to try to model this design but its giving me some headaches. The problem I am having is verifying if the 60" culvert between the two ponds is adequate size to provide a "free flow" condition between the two.

Does anyone have any experience with this situation? How would you verify the size of the connecting pipe without a program like Pondpack?

Thanks in advance for any help.

 

[psmart]

Depending on the pipe and flow rate, there will be some difference in WSE elevation between the two storage volumes. In order to size your equalizing pipe you'll need to know the maximum flow rate and the allowable difference in the WSE. The more difference you can tolerate, the smaller the pipe can be.


Peter Smart
HydroCAD Software

http://www.hydrocad.net

 

[VAEOhio]

So in this case the maximum flow rate into the ponds is around 850 cfs. The water surface elevations for the two storage areas need to remain relatively equal.

How do I confirm the size of the equalizing pipe?

 

[jgailla]

VAEOhio,
I am not very familiar with Haested Pondpack, but some engineers I know who use it tell me it has trouble with interconnecting ponds.

I use ICPR by Streamline, and it models the interconnected ponds well.

One hand method you can use to size the pipe is to look at the Pondpack output and take the largest difference between the inflows into the separate ponds. This would be your design cfs for the interconnecting pipes.

Assume a 0.1' elevation difference between the ponds and divide by the length of the interconnecting pipe to get the HGL slope. Then use Manning's equation to size the pipe for the design flow.

I'm sure there is a better way to do this, but this method should be conservative. This method is what I use to backcheck the computer output to make sure the computer output is reasonably correct.

 

[beej67]

I don't know Pondpack very well, but if you want a good answer to this you'll need to use something that does time series routing. ICPR is widely used in Florida and other flat areas. I use XP-SWMM. I've heard of people using EPA-SWMM for it, since it's free, but you'll need to spend a little time working through it since it's more of a 'from scratch' kind of modeling than you may be used to.

I generally agree with jgailla's approach, but the rub is what Q to use.

Are your two ponds of approximately equal live storage volume? If one is the same size as another, and your peak flow into your system is 850 cfs, then you could imagine that half the water is going into one pond and half into the other, and size the equalizer pipe for 425 cfs.

A slightly more detailed way to do it, would be to determine the ratio of the storage volume of each pond, determine the release rate of the downstream pond, and think about where the water is going during your peak event.

Say your peak inflow is 850, your peak discharge is 100. Then 750 of it is going "into ponds" and 100 of it is going "downstream." Say your upper pond has twice the volume your lower pond does. Then 500 is headed to the upper pond, 250 is headed to the lower pond, and 100 is headed downstream. You size your pipe for 350 cfs, which is the 250 headed to the lower pond plus the 100 passing through the system. Then pick an acceptable head differential between your ponds (a tenth of a foot sounds good to me) and use any culvert calculator to size the pipe.

Again, that's all back of the envelope stuff. If you want to do this "right" use software that can do time series flow calculations of connected ponds.

 

[VAEOhio]

Thanks everyone for the input. The "back of the envelope" type stuff is what I was looking for. Something to be able to check the output from the software.

I think I have this figured out now.

Thanks.