Changing a pond spillway

[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.