Changing a pond spillway

[jcummins]

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]

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]

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]

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]

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]

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]

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]

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]

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.

 

[Xfaxman]

Something like this might help: Conventional Pond Piping System

And: Pond Siphon Systems | Georgia Land Sales Blog

 

[ovrszd]

With method 2, are you considering creating a second earth spillway?

If so a transit should be used to get an accurate elevation.

If you are considering an overflow pipe, I think you better off installing it in the middle and dumping directly into the ditch below the dam.

I know it sounds risky, but actually it's not. An hour with a backhoe and it'll be done.

I'd use plastic dual wall pipe. Bell joint to prevent leaks at the splices. Comes in 20ft lengths. Should be $5 p/ft or less.

 

[jcummins]

With method 2, are you considering creating a second earth spillway?

If so a transit should be used to get an accurate elevation.

If you are considering an overflow pipe, I think you better off installing it in the middle and dumping directly into the ditch below the dam.

I know it sounds risky, but actually it's not. An hour with a backhoe and it'll be done.

I'd use plastic dual wall pipe. Bell joint to prevent leaks at the splices. Comes in 20ft lengths. Should be $5 p/ft or less.

I'd so no. I think there is too much 'regular' water that will erode it. BUT it occurs to me a wide spillway would handle more flow than a pipe.

I'd like to keep the water inside something...or a decent open culvert. A spillway other than dirt?

What size pipe with 8 acre pond? I'm would guess at least 10" perhaps larger.

Length of pipe is by far the shortest with method 1.

 

[Fossil Farm]

By the photos, yes, you have very little freeboard to work with. Typically the minimum freeboard is 12 to 18 inches from the spillway to the top of the dam, depending on watershed to pond ratio, of course. If the other pond is draining into this larger one, typically the larger one would be designed to hold the volume of the upper one in case it failed and the water rushed into the lower one.

I would still be hesitant to open up the dam and put in a pipe in the center. To put it in properly with anti-seep collars and compacting the fill around the pipe would be a lot of work. Just throwing in a pipe may work for a few years, but eventually it would fail.

Edit - Freeboard not only leaves room for storm water, but it also affects the saturation angle of the dam. With very little freeboard, the saturation angle of the dam is higher and in some cases, the dam may show some weep areas near the toe of the dam at the highest point of the dam as the pond ages.

The longer route would be best. Keeping the pipe away from the toe of the dam, so as not to affect it would be best. Again, after you get out of the toe dam area, you could switch to a cheaper type of pipe or have an open ditch/waterway scenario to reduce the area that remains wet after a storm. Ultimately it is your call. It does not look like there is anything immediately downstream to be worried about in the case of a failure. Get a couple people (contractors) to look at it onsite to get a better idea of the best plan. Photos are nice, but nothing is better than seeing it fully. Good luck, nice property you have there.

 

[ovrszd]

If seepage around the pipe is a fear, why would it be better to go around the end of the dam than thru the middle??

I've added pipes to at least a dozen dams. Longest ago was in the late 70s. None have failed. With plastic dual wall pipe I don't even use a seep collar. Corrugations are very deep.

In my world earth spillways are always wide and flat bottomed. They usually have terraced sides that keep hillside water from running across them and retain flood stage overflow inside the maintained, flat bottomed spillway. Seed them with Fescue and they last very well. There are hundreds of them in my County alone. Built over the past 40 years. Rarely are they a maintenance problem. Especially in your situation if you add a pipe to handle the every day dribble. Allows your spillway to get a solid sod base.

 

[Streetcar]

I do not like the wetland at the base of the dam. You appear to have seepage problem.
What s below the dam
I would be tempted to empty the pond and build proper dam and spillway

 

[jcummins]

I do not like the wetland at the base of the dam. You appear to have seepage problem.
What s below the dam
I would be tempted to empty the pond and build proper dam and spillway

I've lived here for 5 years,and it is unchanged. The person who built the pond wondered about it too...but he had it 10 years unchanged. There is a spring in the hill north of the pond dam. It comes out in the woods on one side of the hill and below the dam on the other side. There a 3 acre wet area that is coming off further from the north...well outside the dam. So...it perhaps is seeping but given the amount of the springs in the area....I live in Creal Springs, and it called that for a reason.... You would have to make the pond MUCH bigger to get the dam away from it.

Now...I would have built the pond different, but it's what I got, and looking to improve it.

 

[jcummins]

One of the reasons I think method 2 is probably better....the inside slope on the dam at method 1 is about the same as the outside of the dam. It's deep there. At the method 2 point, the pond is considerably shallower on the inside of the dam. Look at the outside of the pond in both areas.

 

[ovrszd]

One of the reasons I think method 2 is probably better....the inside slope on the dam at method 1 is about the same as the outside of the dam. It's deep there. At the method 2 point, the pond is considerably shallower on the inside of the dam. Look at the outside of the pond in both areas.

Yep, I can see that. I just don't understand why that matters??

 

[jcummins]

Yep, I can see that. I just don't understand why that matters??

Wouldn't there be less pressure from the water on method 2 point? The thickness of the earth holding back water is much greater at the method 2 point than method 1.

 

[ovrszd]

Wouldn't there be less pressure from the water on method 2 point? The thickness of the earth holding back water is much greater at the method 2 point than method 1.

Well, the undisturbed bank at the end of the dam would certainly be more stable than the "fill" out in the middle of the dam. But you are going to change all of that when you dig thru it to lay the pipe. After that, it's all "fill" dirt right??

The design of a dam transfers the pressure from the water into the core of the dam. You aren't going to disturb that with method 2. You aren't installing a vertical pipe to the bottom of the dam core. You aren't even going to dig below the surface of the water at the water's edge. The pipe is going to be straight and lay at whatever angle gets it to dump in the ditch at the bottom. So the only place you are digging completely thru the dam "fill" is at the very bottom of the backside of the dam.

I just installed one last Fall on a smaller pond than yours. I placed it 3ft above the previous earth spillway level. Also added 6' of dam height. I used a 12" pipe because I left no earth spillway. Only drains about 20 acres. I don't expect it to ever go over.