Pond Boss Forums POND CONSTRUCTION Building a dam New pond built overflow question

I just got my pond complete it's about 13 foot deep and was cut into a slow running stream. It is always full and I just got my 12 inch drainpipe in. Now the problem I'm having is, I am getting more water running over my emergency overflow than in the drain pipe that is still under construction. My question is, should I put a larger pipe for my emergency overflow because now it's just running over the shallow side of th my question is, should I put a larger pipe for my emergency overflow because e dam. i'm thinking if I put a larger pipe at that corner of the dam and covered it up that could be my emergency overflow .

It sounds like your primary overflow may be undersized? How did your builder calculate the size pipe needed?

I know that after 5 years and tons of research (after building, duh) my #1 regret about my big pond is going with the smaller culvert. NRCS sized my pond as needing a 20" diameter, based on the 100 year volume calculation they used. I could either have gotten an 18 or a 24" diameter...I went with the 18 and have pretty much regretted it ever since. Biggest mistake I made on my pond construction...not the only one by far, but the largest.

As far as the emergency overflow, most folks I know just have a very wide spot cut out away from the levee to allow the really heavy rains a path to overflow that won't cause any cutting of the levee itself. Someone smarter than I will need to address whether there's any reason you can't just use another culvert as the emergency.

As soon as I figure out how to put a vid on here I'll share.

Ok here is the YouTube link. On the far side you can see my overflow pipe. Up front you can see my E-overflow.

https://youtu.be/tQ4rbiJLENw

Should I have that much coming over the overflow area? I'm considering building the levy up 2 more feet and pulling the 12" pipe and going with a 24" pipe. Thoughts??? Or put a pipe where the E-overflow Is and covering it up. Of course I'll put a seap coller on it for sure!!! I'm kinda lost. I hate the E-overflow being that close to the house and don't want a bunch of riprock for the E-overflow.

Hi cblack, what part of Indian are you from? I'm in southern Indiana. It looks to me like you already have too much water going over the dam for the pipe you have installed to carry. I think you should raise the dam 3 feet and install a 24 inch pipe, also build a emergency over flow 10-15 feet wide in case of heavy rains.

Ditto

cblack, the "Emergency Spillway" is for, emergencies. Such as massive rain events like much of Texas and the rest of the Midwest are getting. A pipe has a maximum flow and if a heavy rain causes your dam to be topped, chances are high the erosion caused will wip out your dam quickly. Pips are for the normal flow, plus 100 year rain events. The E-spillway should be able to let 500+ year rain events pass over it. Also, an e-spillway should divert water around and away from any excavated berm.

What about non-pipe, earthen spillways? We have two ponds that utilize this system as their only means to discharge water. No pipes to plug?

Tony, with ponds that don't have a lot of water inflow, or the possibility of a lot of water inflow, those types of ponds work fine.

But where a pond will see a considerable amount of water over an extended amount of time, I would be concerned that over time the spillway would erode and wash out the whole dam.

I think some may depend upon the type of pond also. Ours are dammed up ravines, and the earthen spillways are located at the junction of the dam and the sides of the ravine. Most importantly, in my eyes anyway, is the fact that whoever designed and built our ponds didn't simply leave the deep water at the dam.....in the areas of the overflow, there is a considerable "shelf" of shallow water that extends back into the pond.

This means that we don't have exceptional depth at the overflow section, which would tend to allow fast moving water to cut "deep" into the dam. As it stands, we have approx 30' of shallow shelf, 20' wide.

Since I've been here, I have personally experience two 500 year rain events, and what they are calling a 1000 year event. Never saw the water get deeper than 2" over the spillway. Pretty slick.

What type of material is in the bottom of your emergency spillway? The speed of the water moving through your spillway is what causes erosion and that is determined by the water that the primary spillway does not handle (in your case all the water), and the width and slope of the emergency spillway, rather than the slope of the interior of the pond inside the spillway.

I'm a little confused on all of this. How can you have water going over the E-spillway if the overflow pipe is still under construction. The overflow pipe should be lower than the E-spillway. Is it?

From the video, I can't see the overflow pipe, nor can I see water movement in any area that it may be. Is the water going thru the overflow pipe? Perhaps the elevation grades are wrong? If there is water going thru that overflow pipe, it should be a huge amount. I would think that pipe should be under water before the E-spillway starts being used.

Perhaps what I am seeing is that the overflow pipe is way under the water level? If that's the case, somebody REALLY got the water shed volume wrong when deciding size of pipe to put in. Is this rain you have been getting normal, as in it happens once a year?

Ok here is another vid. This time I pushed some more dirt up higher than the top of the drain pipe. I plan on adding a few feet to the top of the levee as soon as it drys up enough.
http://youtu.be/S69W8zhmQvE

There's a great article in this months Pond Boss Magazine from Mike Otto about spillways....

CB,

No expert here, but I can give some practical advice as I also constructed most of my pond this time around and have learned from mistakes made along the way.

You need as much freeboard above your overflow pipe as possible. Freeboard is the space from the top of your overflow pipe to the top of your dam. I would go 2 feet minimum, and more if you've got the dirt. Your emergency overflow should not come into play until the water is at least to the top of your pipe, perhaps a bit above assuming 2-3 feet of freeboard. That said, make sure to keep in mind any structures or buildings that could be affected by raising your emergency overflow level. I'm in the flatlands of Illinois, and a few feet increase in pond level could put the pond at your back door. Sometimes lowering your overflow pipe, thus pond level is the better option. It was in my case.

My original contractor thought that 1 foot was enough. $10,000.00 and a whole lot of my time spent on a dozer says he was wrong. Spend the time and money now to do it right and you can enjoy those 2-3 inch rain events inside listening to the rain patter on the roof instead of outside watching the waves lap up against the top of your dam. I've been there. Not fun.

I would say that the minimum freeboard for the dam would be a couple feet above the bottom of the emergency spillway, and that the emergency spillway should be a couple feet above the top of the primary spillway pipe. If the pipe has much drop, it will carry a lot more water that way.

I completely agree with RAH. He says it in about the simplest way possible.

Since that overflow pipe seems undersized, gives two choices. Tear out that one and put a bigger one in, or put in a E-Spillway that can handle a lot of water on a regular basis (kind of not the point of the E-spillway). Which ever way, better build it right. Sure wouldn't take long for the water to be pouring over that dam , like you had, before you lose the whole pond and cost you a lot like Deerman said.

Well thanks for the helpful suggestions. I'm gonna build the levee up.

Most of the small cattle watering ponds built around here have nothing more than a wide spot at the end of the dam where water overflows out over grass into the pasture. Ponds are usually a ravine or wash that was dug out deeper using the dirt to build the dam. It is a quite common sight to see 20-30 year old ponds that are a couple feet low on water because over time some large rains have started an erosion path and cut a wash where the grassed overflow area used to be.

As long as the ponds have been maintained (such as putting rock riprap or some other erosion protection) in the overflow area they are ok. But if left to wash out over time, they end up lowering the pond. It usually happens over time because the overflow area is in original sodded area at the end of the dam instead of in the middle of the dam (where it would wash the whole dam out if erosion started), but it still happens.

In my old pond, I spread about ten ton of 3-4" lateral rock (for septic system drain fields) over the 12' wide overflow area. It is also now grassed within the rocks, and so far no erosion problems. So I think if done right and with the correct sizing, they work fine for intermittent water discharges. Probably would not be good at all for extended overflow periods like in a spring fed pond.

I do agree that a lot depends upon the type of pond, and the skill of the builder. We have both types of ponds, primary pipe/emergency overflow, and simply a primary, earthen overflow. From what I have observed over the years, I wouldn't hesitate to skip the pipe and go with the earthen overflow. Provided it was built the way I directed, and the geography suited it.

I only have an overflow, and who would have thought so much water could run down our hill like this year?!? I have been putting a lot of rocks in the overflow areas to break up the flow and reduce erosion, but I am starting to get concerned as some new little waterfalls are making their way upstream to the overflow.

I wish I had a pipe in there, but nobody thought I would need one. The overflow runs 6 months out of the year.