Changing a pond spillway

[jcummins]

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.

Method 1 is going to have a trench on the back side of the dam...not deep and not into the core...to bury the down pipe...UNLESS I can use some type of open culvert. The total digging somewhere on the dam, is straight across and down the back side....I'm guessing it would be 120 ft or so of trenching....solely on the dam. Method 2 would cut 20 ft on an area that is at best a 1 ft high dam. The rest of the trenching would be completely off the dam.

I can see both ways, but method 2 seems safer...but more expensive. How much do you think it would cost to have someone put it in...method 1...method 2? I don't have a rear blade...but know where I could probably borrow a really heavy one....with TNT on my tractor, I've even though of digging a trench (method 2 only...not 1) and putting a pipe in that way.

I just went out and measured the freeboard at the very lowest point on the dam...which happens to be right in the middle....13". On average the dam freeboard is 16" or so. I have given thought of adding dirt to parts of that dam also.

 

[jcummins]

Giving that a bit more thought.....I put in a septic system some 10 years back. Rented a mini excavator for a few days....worked great. perhaps that's the route I might go, if I can ever convince myself that it would be the best thing to do.

 

[ovrszd]

I don't recall any distance numbers. Like I mentioned, pipe should be around $5 p/ft. 2 hours max with a backhoe. You aren't digging deep.

 

[rekees4300]

Opinions on either method?

I'm facing a similar situation. My 1 acre pond has a 6" PVC pipe going thru the middle of the dam. ASAP I (actually a contractor) is going to remove it and install a 8" double wall at the end of the dam. In January, a large area by the PVC pipe broke loose and slide down the back of the dam. I think it was caused by water leaking around the outside of the pipe? Anyway, IMO, it makes more sense to place an overflow pipe in the stable ground at the end of the dam instead of just putting the new pipe in the same unstable location as the old pipe.

 

[ovrszd]

Never use slick smooth plastic pipe or smooth steel pipe without a seep collar. Thats why I suggest corragated dual wall plastic.

If you move to the end of the dam and install PVC you will simply move the failure.

 

[jcummins]

Never use slick smooth plastic pipe or smooth steel pipe without a seep collar. Thats why I suggest corragated dual wall plastic.

If you move to the end of the dam and install PVC you will simply move the failure.

I know on the other two ponds that he built, he did put in pipes....smooth pvc pipes. Have no idea of anything he might have put around them to avoid leaking around the pipe. They both look to be in good shape. The biggest pond of the two has that pvc pipe smack dab in the middle of the dam.

 

[ovrszd]

Sometimes it's just luck.

I understand your concerns and respect them. It's simply most logical and practical to put the pipe inline with the drainage ditch.

 

[rekees4300]

Never use slick smooth plastic pipe or smooth steel pipe without a seep collar. Thats why I suggest corragated dual wall plastic.

If you move to the end of the dam and install PVC you will simply move the failure.

I agree. Not only was the PVC pipe inappropriate, it was installed half-assed by the contractor in 2002. He used 2 DIY anti-seep collars made from 1/4" hard plastic that didn't seal. Also when he back-filled, at least 2 of the pipe joints separated causing sinkholes! What a mess! This time have hired a more experienced contractor to install the dual-wall corrugated at the end of the dam.

 

[joefromga]

Can someone explain why a pond needs a spillway instead of it bei dug deeper?

Also, won't they go stagnant with no fresh water source? How do you keep it from being a green swamp?

I'd like a pond in the future. It's on bottom of the to-do list. One day.

 

[Fossil Farm]

Can someone explain why a pond needs a spillway instead of it bei dug deeper?

Also, won't they go stagnant with no fresh water source? How do you keep it from being a green swamp?

I'd like a pond in the future. It's on bottom of the to-do list. One day.

Once a pond fills up, the excess water has to go somewhere. Preferably through a primary spillway, not over the top of the dam.
Hard to dig deeper once the pond is full.

No. A properly built pond (1/4 acre or larger) maintains itself pretty well. There is always some maintenance.
Go to a pond school if your extension service or soil and water conservation district has one, usually during the spring.
Several books and pamphlets available from them also.

 

[joefromga]

Pond school? Wow, you learn something new every day.

I know a guy that works in pond restoration and management. He says he can take a green pond and make it swimmable.

I figured if it was big enough it wouldn't overfill. . I guess it also depends on a lot of things such as the surrounding areas being able to absorb the rain.

All the ponds near my property are low to full. I never see them topped out but our soil is very absorbent. It rains all the time.

 

[Creamer]

I figured if it was big enough it wouldn't overfill. . I guess it also depends on a lot of things such as the surrounding areas being able to absorb the rain.

Now that's a new one! If one keeps pour water in a container at some time it overflows. Where I grew up in the Dakotas we did not really have drainage, i.e. water ran to the bottom of the hill and stopped and then it was dry enough that the water evaporated to lower the level over time until it rained again. Then in the 90's they started getting more rain and soon one state highway in SD was under 17 feet of standing water in August and they had built a new road out around the pond that had developed. Then a few years later the water subsided and they rebuilt the road straight through the middle where it had been. Most of the rest of us have drainage though, i.e. it fills up and it overflows.

A real interesting story is Devils Lake, ND where a very large lake rose nearly 30' engulfing huge amounts of surrounding area. Devils Lake (North Dakota) - Wikipedia, the free encyclopedia

 

[Creamer]

Here's a better account of Devils Lake. Where the Roads End in Water: The Lake That Won't Stop Rising - The Atlantic

 

[rekees4300]

I'm facing a similar situation. My 1 acre pond has a 6" PVC pipe going thru the middle of the dam. ASAP I (actually a contractor) is going to remove it and install a 8" double wall at the end of the dam. In January, a large area by the PVC pipe broke loose and slide down the back of the dam. I think it was caused by water leaking around the outside of the pipe? Anyway, IMO, it makes more sense to place an overflow pipe in the stable ground at the end of the dam instead of just putting the new pipe in the same unstable location as the old pipe.

Yesterday the contractor did the repair. He verified the reason for dam slippage was the 6" PVC pipe leaking around the outside and said he has seen many other ponds with same PVC pipe problem. He removed the PVC and installed the 8" double wall (corrugated on outside, smooth on inside) at the end of the dam. Hopefully this change will solve the slippage problem permanently.

 

[ovrszd]

I've installed a couple hundred dual wall corrugated plastic pipes of sizes ranging from 12" to 54". I've never had one seep around the outside. That problem will be solved with this change you are planning. Whether the new 8" dual wall can handle the flow is yet another question.

 

[rekees4300]

I've installed a couple hundred dual wall corrugated plastic pipes of sizes ranging from 12" to 54". I've never had one seep around the outside. That problem will be solved with this change you are planning. Whether the new 8" dual wall can handle the flow is yet another question.

Good to learn that from someone who is experienced, thanks. :thumbsup: I'm not worried about the new 8" handling the flow since the old 6" was adequate 99% of the time. The capacity of an 8" pipe is almost double a 6" pipe. Even with a 1,000 year flood like they had in West Virginia, I'm confident the new 8" pipe plus emergency spillway (20 feet wide) would be adequate to prevent water from topping the dam.

Pipes - Water Content

 

[ovrszd]

I agree. Your earthen spillway will pick up the slack.