Pond Boss Forums POND CONSTRUCTION Building a dam Question about overflow pipe size

Sorry in advance if I sound like the noob that I am. Still trying to figure all of this out before we look for contractor to help move dirt.....


Recently we found a piece of land that looks like it will nicely support a 1/2 - 3/4 acre pond in western Illinois. After meeting with our local nrcs guy I walked away understanding that our biggest challenge will be getting all the extra water that won't fit in the pond out of the area out and down the creek bed without screwing up the dam.

Couple of vital stats he gave me:
Watershed area: 234 acres (rule of thumb here is 20 acre watershed = 1 acre pond)
Hydraulic length = 5400 ft

10 year storm data
24 hrs precipitation (p) = 4.63"
Runoff = 2.15"
Peak discharge (cfs) = 288.9
Ia/p = 0.14
Volume of runoff (Ac-Ft) = 42.0

25 year storm data
24 hrs precipitation (p) = 5.60"
Runoff = 2.94"
Peak discharge (cfs) = 407.2
Ia/p = 0.12
Volume of runoff (Ac-Ft) = 57.4

So given all of this, how do I begin to calculate the size of overflow pipe(s) I will need to move this water. Additionally does everyone size these so that the overflow will take care of a 10 year storm and you rely on spillway to make up the difference between a 10 and 25 year Storm? Or do you just build a lager freeboard into the damn so that it can hold more to absorb the 25 year rain?

You need a monster emergency spillway or diversion. No pipe will handle that watershed.

Just so I understand diversion = rerouting creek bed around pond joining back up around the other end of the dam back into the original creek? Is this a common thing to do?

in addition to the problem of handling this much water, you probably will get an unmanageable amount of silt. in my old and new pond, i installed a rock barrier right above the pond. it is unreal how fast the area will accumulate a large amount silt.

An emergency spillway here means a section of dam which is several feet lower than the rest of the dam, usually on one or both sides and a specified width such that a high water event can discharge water in a low slope area and divert it away from the back toe of the dam before it reenters the creek below.

I think you need to look elsewhere for a pond location.

A neighbor of mine has three ponds in a chain on a small mountain creek. The first pond is about 1.4 acres, and has about 350 acres or so of watershed. The second pond is 4 acres and has nearly 400 acres total watershed. The last pond is about 7 acres and has a total of about 425 acres of watershed. We get about 47 inches of rainfall a year. He doesn't have problems with silt in the first pond because the creek carries relatively little silt even in floods. The other ponds have some silt problems due to the disturbance of the land in the upper parts and between them when building them. He has big overflow pipes, and enormous emergency spillways. In a big flood of 2011, he said the emergency spillway on the big (last) pond was running about 4 feet deep and 90 feet wide at peak flow. He usually drops the level of the middle pond about three feet in late winter in anticipation of spring flooding. Large, heavily fortified emergency spillways are the keys in a larger than needed watershed.

His watershed could support a 40 acre lake if he could build taller dams, but it's not practical in that steep valley as they already are near maximum height allowed without a lot of engineering, permitting, and concrete pouring.

I think you need to either look elsewhere, or build a 10-12 acre lake.

If you can divert a lot of the water around the pond, it could work. The challenge is to engineer things so that flood water goes around your pond, but normal flow keeps your pond full. In your situation, that is probably the reasonable thing to do if the land will allow it at a cost you are willing to spend. Flowing water is a powerful force, so it will need to be engineered properly. Perhaps the NRCS can help?

i would keep looking unless your budget is unlimited. I was in a similar situation as far as water amounts in high rain events. It was just not worth all those problems that come with it. so now my pond is where the watershed is much smaller and that also causes problems. Water wells are not cheap either.

I'll throw another variable in here having lived your situation for the last 10 years - also in West IL - it's not worth the stress.

I've got a decent size pond (6-7acres) with a similar watershed to the one you describe. Every time we get spring rains or a big water event I basically stop sleeping. My emergency overflow is about 20 ft wide and during big rain events it'll run 3 feet deep.

I'm adding a larger culvert and also a siphon drain so I can lower the pond level in anticipation for a big rain - but if I had it to do over again I'd rethink the pond location.

Skullet - can you post up an image of the topo map including an outline of the property line? that may help chose an alternate site with less watershed - like the side of a hill off the main creek with a smaller valley.

I will have to get back to the NRCS office to get the 2m and 4m contour maps of the the property as I am not finding them online.

I have taken a couple of screen shots from overhead from google however. The first one is the raw overhead. In the second image you can see the the property lines in purple and a thought I had about rerouting the creek on the property in yellow. This would provided the same entry point and exit point for the water on the property. That would open up the area in the grey outline as a potential area to dig a pond rather than damning a pond. According to google this could potentially yield a little bit bigger than 1/2 acre pond.

Obviously digging is more $ but might be worth it to sleep at night trying to deal with that watershed.