Changing a pond spillway

jcummins · Apr 27, 2016

I've lived here 5 years. Have a pretty nice 7-8 acre pond, but the spillway is built in the wrong place. The former owner says he was 'talked' into it, and think he knows it was a wrong decision. It is a simple earthen spillway, and has to flow 200+ yards to get to a creek. It's creating a ditch halfway there, and it's not going to get any better.

There should have been a pipe put into the dam. I've thought of two methods to fix.

1. Put a pipe where it should have been in the dam, and thus the shortest distance to the creek below. But obviously it could not be install as it should have been when the pond was built. Just have the pipe in the very top portion of the dam, to exit on the none pond side close to the top of the dam. This would dump into an open culvert of some kind that I haven't figured out, making a straight run some 25-30 yards to the creek. What I don't like about this....is your working on the dam, not deep into it...but your doing some disturbance to it.

2. At sort of the corner of the dam, do the same thing, then empty the pipe into the same type of open culvert. The flow would have to do a 40 degree or so swing, and the open culvert would run along the base of the **** into a wet spring area that is outside the pond. The open culvert would be 3 times the length of method one....but you are not really digging into the dam.

Opinions on either method? On the open culvert....what could be used for it? The now existing spillway would be raise a little and serve as an emergency spillway.

Creamer · Apr 27, 2016

I can't really understand the "not digging into it" comment in #2. Either way you are digging into the retaining earth to the same level.

I am not sure what to recommend but I am sure somebody will add something and I want to read it.

How big of pipe are you thinking?

jcummins · Apr 27, 2016

Creamer said:
I can't really understand the "not digging into it" comment in #2. Either way you are digging into the retaining earth to the same level.

I am not sure what to recommend but I am sure somebody will add something and I want to read it.

How big of pipe are you thinking?

By not digging into it...I mean the dam itself. Your digging at the edge of it. If weather clears, I'll try to take some pictures.

As to size of pipe....that's another question that needs an answer. I know since I've been here, we've had two rains that was so fast and hard, no pipe regardless of the size would handle it. That's when the emergency spillway comes into play. But for an 8 acre pond....it needs to be a decent size.

Fossil Farm · Apr 28, 2016

If the current primary spillway is earthen, and it has held up for this long, then the watershed to pond ratio must be pretty good. Typically, the minimum ratio is 6 acres watershed to 1 acre pond, and if you get above 40 to 1, the pipe size can get pretty large depending on the fill height above the pipe. Lots of engineering goes into the pond when the ratios start to get large.

The principal spillway is designed for a certain size storm, depending on what is below the dam in case of failure. No principal spillway is designed to handle a large storm, such as a 100 year rain, that is what the emergency spillway is installed and designed to do. To put in a pipe spillway now in the center of the dam would create a lot of engineering problems, such as compaction around the pipe, installing anti-seep collars, anti-swirl plate, etc.

Pipe principal spillways are usually no less than a 6 inch pipe. Less chance of it plugging and if any debris would get into the pipe, cleaning a pipe any smaller can be challenging. Since the current principal is earthen you may want to leave the major portion of the dam alone and install a pipe spillway near the existing earthen one. Use solid wall SDR 35 or 40 for 40 or 50 feet, then you could switch to non-perforated corrugated to the swale. Just really depends on many on-site features. You might want to have someone look it over before doing any work.

If there is no emergency spillway now, raising the current earthen one would reduce the freeboard to the top of the dam and might cause overtopping in a large rain event. Place the pipe spillway a few inches lower than the existing earthen one. Then just let the pipe spillway and the current earthen one work together in a large rain event. Good luck.

Oh - the freeboard allows for a buffer in holding flood water so it does not over top the dam and can go out the emergency spillway in a normal pond design.

jcummins · Apr 28, 2016

Fossil Farm....there a reason it's held up this long....it's a longer story but needs to be told I guess.

I suspect the pond is 15 years old or so. At the time built, the person built three ponds. The other two both have pipes through the dam. The owner found the big pond did not have a big enough water shed and was not filling. So he put in ...guessing about a 10" pipe from the middle pond to the big pond for more water. He plugged the middle pond pipe...and that's where it went wrong. He plugged it poorly. After time that pipe started leaking.

After living here a bit, I was noticing the big pond was not filling as full as the other ponds. Some checking, I discovered the middle pond pipe leaking, diverting water from the bigger pond. So I fixed that. Also around the same time, a new driveway was built over the top of the middle pond dam, and at the time, more dirt was put over the pond pipe, assuring the water would flow to the big pond, as intended.

Subsequent to that started realizing that earthen spillway was now handling the water for both the middle pond and the big pond. It was not realized that the leaking pipe was there all along acting like a throttle to the amount of water going there....which occurred by the poorly plugged pipe. So all of this was regulated by how hard and fast rains came.

Now over time, the earthen spillway is starting to erode....and in particular out in the pasture ditch is forming.

jcummins · Apr 28, 2016

Pictures to help explain.

Existing spillway. It's 200+ yards through the pasture, with problems all the way from the pond to the woods.

Method 1 with the pipe/culvert in the middle of the dam. The shortest route to the creek. You can see the wet area from a spring that is below the pond.

Method 2. The water ends up in the same place as method 1, taking a longer route....but no disturbance to the dam at all.

ovrszd · Apr 28, 2016

I've added pipe overflows to ponds built with only earth spillway. How it's done is dependent on dam design.

Most times the dam has a lot of freeboard to an emergency spillway. Water very rarely running over it. After large rain it's common for the pipe to be under water for a day or so until it catches up. Pipe size is usually 8"-10".

It's easy to install one to an existing dam with little risk to the dam. Pipe is installed at a vertical pitch to match the dam. Pond end should stick out of dam at least 18" to minimize shoreline trash from plugging it. Usually a seep collar is used to prevent seepage along the pipe but isn't always necessary. Depends on dam soil type.

If you don't have sufficient freeboard you may have to lower pond level by lowering pipe as you install it.

Pictures would be great. Stand at upper end of pond shooting entire dam and spillway. This gives us a complete view. Then as we ask questions we can get details concerning freeboard height etc.

ovrszd · Apr 28, 2016

You posted pics while I was typing. Sadly, and typically of cosmetic structures, you have NO feeeboard. Without lowering pond level, or raising dam, all a pipe is going to handle is dribble from light rain. This will not help your problem at all.

Maybe modifications to the spillway drainage area to control runoff such as terraces or "dry ponds"?

jcummins · Apr 28, 2016

Here is another look at the dam from the back side. The yellow is method 1. The red is method 2.

Ovrszd I think method 2 is 'safer' because of the freeboard you mention. In this pictures, the pond is full...right at the point water would flow out. I would envision the emergency spillway still coming into play with a 2"+ rain.