Pond Boss Forums POND CONSTRUCTION Building a dam pipe size for small ponds

OK, I'm new to posting here. Its been lots of fun cruizing around these forums picking up lots of great info while planning my pond project, but now that I'm in a position to get started with it, I have questions. And from what I've found, this is the place to ask them.

I've shown my site to a contractor that I know thru work. I've seen plenty of his work and its always very good. From talking to him, it is apparent that he has done lots of ponds. My pond won't be very big... 1/2 to 3/4 acres but thats with a 15 foot dam height. The contractor is proposing a 15" drop inlet pipe feeding a 12" pipe through the dam. He says I can expect that to run me about 5 grand. I'm blown away.

So I spoke to an engineer friend who's designed lots of ponds. He says unless the pond is for stormwater management, there is no need for anything but a 4 or 6" pipe. Let the trickle pipe handle the daily stuff and let the spillway take care of any large storms.

Does that sound reasonable? Somewhere during the last few years planning this project I've become paranoid about the idea of having to depend on the spillway for anything except for the largest of storms. How often should the auxiliary spillway be needed? Or if it is constructed properly, does it really matter?

Welcome Vamaz.

There are many things that you need to look at to determine what size pipe you need. You need to know how big the watershed is and what kinds of soil you have. A watershed with a lot of sand will have less runoff than one with all clay soils. You need to determine how much runoff you will get from a certain amount of rain. If you have a small watershed a 6" pipe would probably work well. If you have a large watershed even a 12" pipe may not be big enough.

This link will help you figure out how big the watershed is. http://acme.com/planimeter/

To find out what kind of soils you have do a search for web soil survey on google.

Thanks, Chris. I've just about worn out my copy of NRCS handbook 590 planning this thing for the past year and a half. The handbook suggests the pipe should be sized to handle runoff ranging somewhere between a 1 and a 10 yr frequency storm event. For a 5yr event, I'm looking at needing a barrel-to-riser ratio of something like 15:12". But then... the handbook goes on to suggest that with a drainage area less than 20ac a principal spillway may not even be needed unless there is constant flow. My drainage area will be about 15.5ac.

In an effort to cut down the cost of the project a bit, I'm planning on doing as much as I can myself - the initial clearing, for instance. I guess I'm just wondering if its a bad thing to let the auxillary spillway see some water once a year or so. Going to smaller pipe would save a bundle.

Well you are off to a good start by using the NRCS handbook.

I don't think there would be a problem with the spillway being used more than once a year. Before grass gets established you may have more erorion than if it was designed by the book but after that it should be ok.

Not sure that I understand the 5 grand. Is that for the whole job or just the pipe stuff?

Vamaz;
I have a pond just inder one acre with a fairly good size watershed. Pond was constructed by daming a stream, so it sits in a kind of "valley " between 2 pastures. I used 8" drain pipe and can say my spillway gets used a few times a year with good rains. Because of that, I'm having difficulty getting any grass on it and am dealing with some erosion issue's. My soil is predominantly clay.
Anyway, if hindsight was forsight I would have installed at least a 12" pipe. Not very technical, just an observation from my experience, as our ponds sounds simular.

Joe

Thats just for pipe. I showed the site to a second contractor this evening who basically gave me the same news. Both were talking Sch 40, 15" riser and 12" barrel. I guess I need to make some calls and find out for myself, but I've had professional relationships with both for several years and feel I can trust them.

I guess the more I think about it, the more leary I am of going with a small pipe. According to all the numbers, the handbook tells me I need at least a 15":12" ratio of riser to barrel. Between teh soils, slope, vegetation and all, I'm coming up with about 9 cubic ft/sec - thats for a 1yr, 24hr storm event. So my contractors are in the ballpark with their pipe recommendatons. And having lived here for the past 11 years, I can easily recall that channel rippin pretty good a couple different times. Even if it costs more, I hope I only have to build this pond once.

So it sounds like I'm answering my own queston. She hasn't said it yet, but I can tell my wife thinks I'm obsessing. Shes gotten to where she looks at me kinda sideways when I talk about it...

Hey, I appreciate all your help!

Even if it costs a bit more to go with the bigger pipe, it'll definately be cheaper in the long run rather than save a few $$, by using the smaller pipe and have to re-do it at a later date.

This link will give you the flow rate of different pipes.

http://www.sahra.arizona.edu/programs/water_cons/resources/pipeToFlow.htm

Using some of the information you provided 10" sch. 80 PVC may be enough. If the pipe is completely submerged with 12' of drop, one elbow, and 75 feet long it will handle 11 cfs.

Wow, thats a pretty handy calculator. I like it. I assume I still want a larger diameter for the riser to help load the barrel. Thanks, Chris.

Instead of a riser you could use a straight pipe that angles from the water level to the bottom of the dam. Another option is to have a flat pipe under the top of the dam with an elbow on the backside with the rest of the pipe angled down the back of the dam.

Here is what brettski did for his pond. Scroll down a little bit to look at his pictures.

http://www.pondboss.com/forums/ubbthreads.php?ubb=showflat&Number=5741&fpart=3