Replacing a dam, whats the best way do it...

[aczlan]

We have an old concrete dam in a small pond on my inlaws farm which has a pretty good leak around both sides.
Here is the dam from the pond side:

And the outlet side:
Looking West:

Looking East:

And one more from the top:


If you look at the two pictures from the outlet side, you can see where the water has been going around the dam.
Last year, I stacked bagged concrete around the east side and it worked well for the summer, but its leaking again.
This is built on what the conservation office calls "sandy loam" not a lot of "watertight" soil around the pond.

Along with this project, we would like to fill in the ditch that leads from the pond to a culvert (probbaly ~200'). Given how things drain (the ditch is a low spot), the pipe will be 12" perforated double walled plastic pipe so that the existing water flow can continue. Here is a picture of the area where the pipe will be put in and backfilled:

One priority is that when all is said and done, there must not be any place for grandkids to slip, fall and end up stuck in the pipe.

So far, I have had two contractors come out to give me estimates and they have had two very different ways of fixing the problem.

Contractor A: Contractor A is highly recommended by the facilities people at work, he has done some work there and is turned out good. He does a lot of drainage for the farms and orchards in the area.
His suggestion is to bury a 60" tall catchbasin (like: Filtered Images (ParkUSA - Design for Water)) at a slight angle (leaning toward the pond so that the side toward the pond is 1" lower than the outlet side), so the low side of the top is level with the top of the current dam and make a small berm (6-10"?)around the other three sides so that when the pond rises, it flows into the grate and out via the pipe.
He would fill in around the box and the first 10-15' of the pipe with clay to make a "plug" that will keep the water out.
At 2-3 points along the perforated pipe, he would backfill it with washed stone all the way to the surface to provide a place for surface water to drain to.[/php]

Contractor B: Contractor B has done work for us in the past and does good work, but he is very busy and he uses us for fill in work, so he will start, do a piece, then disappear for 2-3 weeks (often leaving his equipment parked here, one time his Bobcat 335 mini excavator sat in our back field for the better part of a month).
He came out today (while I was at work, so I just spoke to him over then phone).
He proposes pouring a 16-18' wide dam, with a spillway much like the current dam, but with a slot sized to put 4x4s across to adjust water level, then having that feed into the pipe (he wasn't very clear over the phone how the transition to pipe will be handled and I didn't press as he said he will have a drawing for me tomorrow).

I asked Contractor B about Contractor A's plan and he said that its a bad idea as it "will just get pushed out in a year or two"

Personally, I really like Contractor A's plan as it seems to be the least complicated way to get water from point A to point B without having someplace where kids could get stuck when playing, but I am not an expert.

Looking to get some opinions from some knowledgeable parties who don't have a financial stake in the matter...


Thanks

Aaron Z

 

[dave1949]

Do you really need to adjust the water level with those boards?

I've had good luck with an open rocked spillway. There is a U-shaped channel dug out. It has a layer of geotextile fabric in the channel that extends 2-3 feet down into the pond water. 6"-10" rocks are spread in the channel on top of the geotextile also going down into the water. The rocks provide a "hardened" surface that the water does not wash away. The geotextile keeps the soil below the rocks from washing out.

Ice is your enemy for any pipe or culvert on a pond. It tends to destroy them.

 

[Baby Grand]

My father in law did almost exactly what you describe as Contractor A's plan on what must be a 2 or 3 acre pond that he created in the 70's by excavating with a bulldozer and using the spoils to build his dam, which is about 150' long. He used a concrete manhole, almost identical to this one:
http://www.lbiw.com/documents/resources/Agency%20Standard%20Plan%20Drawings/COUNTY%20OF%20RIVERSIDE/607%20-%20PRECAST%20CONCRETE%20MANHOLE%20(CONCENTRIC).pdf

With a 24" concrete drain pipe at the bottom. According to him, it hadn't moved a bit since he set it, despite some heavy weather. He also built an emergency spillway, at the far end of the dam, so a week of heavy rain wouldn't overwhelm the pipe and cause the pond to overflow and wash out the dam. I like the basic design and it seems to be how most of the WPA era reservoirs around here were constructed.

 

[roadhunter]

Might consider burying a pipe that runs from the surface of the water down and turns 90 degrees and exits below the dam into your drainage pipe. This way the water is diverted around the dam while minimizing the erosion that water falling over a stop log can cause. I would think that something like a 6"-8" pipe would probably handle most situations for your application and be safe for the kids. Keep the inlet pipe off the shore where the kids can't get to it and put a screen on the top to keep debris from entering. Then rip out the existing dam and fill it in with packed dirt and install some short pieces of pipe to act as an overflow through the earthen dam just a few inches above the pipe inlet structure. Below the overflow install some geo fabric and the busted up pieces of concrete to act as rip rap. Fill in the rip rap with some gravel. would be a simple and safe solution.


Another thing you might consider for your application is sheet piling. You could probably push it into your soil a few feet with an excavator to form a simple dam.

 

[aczlan]

Do you really need to adjust the water level with those boards?
I've had good luck with an open rocked spillway. There is a U-shaped channel dug out. It has a layer of geotextile fabric in the channel that extends 2-3 feet down into the pond water. 6"-10" rocks are spread in the channel on top of the geotextile also going down into the water. The rocks provide a "hardened" surface that the water does not wash away. The geotextile keeps the soil below the rocks from washing out.
Ice is your enemy for any pipe or culvert on a pond. It tends to destroy them.

We do not need to regulate water level. If we need to drain the pond, we will pump or siphon it out.
We looked at a open spillway. The issue is that my mother in law has said that the ditch needs to go away and that necessitates a pipe running from the dam to the existing crossing. If we are going to do that, we may as well go with a catchbasin to start with.

My father in law did almost exactly what you describe as Contractor A's plan on what must be a 2 or 3 acre pond that he created in the 70's by excavating with a bulldozer and using the spoils to build his dam, which is about 150' long. He used a concrete manhole, almost identical to this one:
http://www.lbiw.com/documents/resources/Agency%20Standard%20Plan%20Drawings/COUNTY%20OF%20RIVERSIDE/607%20-%20PRECAST%20CONCRETE%20MANHOLE%20(CONCENTRIC).pdf
With a 24" concrete drain pipe at the bottom. According to him, it hadn't moved a bit since he set it, despite some heavy weather. He also built an emergency spillway, at the far end of the dam, so a week of heavy rain wouldn't overwhelm the pipe and cause the pond to overflow and wash out the dam. I like the basic design and it seems to be how most of the WPA era reservoirs around here were constructed.

Good to hear it has worked for him. We could put the box in the middle of the pond (that was an earlier idea), but then we would have to pump the whole pond down so they could have a square dryish place to set the catchbasin. As currently planned, we can make a cofferdam with a couple pieces of plywood and some 4x4 posts and not have to drain the pond down any more.

Might consider burying a pipe that runs from the surface of the water down and turns 90 degrees and exits below the dam into your drainage pipe. This way the water is diverted around the dam while minimizing the erosion that water falling over a stop log can cause. I would think that something like a 6"-8" pipe would probably handle most situations for your application and be safe for the kids. Keep the inlet pipe off the shore where the kids can't get to it and put a screen on the top to keep debris from entering. Then rip out the existing dam and fill it in with packed dirt and install some short pieces of pipe to act as an overflow through the earthen dam just a few inches above the pipe inlet structure. Below the overflow install some geo fabric and the busted up pieces of concrete to act as rip rap. Fill in the rip rap with some gravel. would be a simple and safe solution.

That was my first plan, but we worried about the ice pulling the standpipe out and we should never have to worry about the catchbasin.
We are going with 12" pipe to match the downstream culvert. Yes, it will cost more ($130 vs $70 from my looking online) but it will never be the most restrictive point in the drain system.

Aaron Z

 

[MossRoad]

If you never have to regulate the depth of the pond, plan A seems simplest. It has a cover to keep out debris and humans. You'll still need a spillway in case it gets plugged, though, or 100 year rains, etc...

 

[aczlan]

On the spillway, IIRC Contractor A was planning on having the "back" of the berm a little lower than the "sides" then having a washed stone "drain" just past his 10-15' of clay so that in the case of a major rain (or grate clog), it could overflow over the back, then drain right down.

Aaron Z

 

[Xfaxman]

Where we used to live the beavers started to dam the spillway. I put some flex pipe over the sticks and staked down the ends in the water. After a rain the water level would rise and start flowing through the pipes. It would continue to flow until the water level dropped enough to break the siphon. The beavers gave up and moved elsewhere.

You might be able to do something similar using a siphon pipe: Conventional Pond Piping System

 

[aczlan]

Where we used to live the beavers started to dam the spillway. I put some flex pipe over the sticks and staked down the ends in the water. After a rain the water level would rise and start flowing through the pipes. It would continue to flow until the water level dropped enough to break the siphon. The beavers gave up and moved elsewhere.
You might be able to do something similar using a siphon pipe: Conventional Pond Piping System

The problem is that we have to deal with snow and frost in the winter, so things need to be buried deep enough that they don't get moved by the frost. As such, a siphon wouldnt work (because it needs to be above the water line).

Thanks for the idea though.
Aaron Z

 

[Fossil Farm]

Before deciding on what replacement spillway to use, some basic engineering should be done.
1. What is the surface area of the pond (in acres)
2. How many acres drain into the pond to get the watershed to pond ratio. (The higher the ratio, the more water passes through the pond)
3. What is the land use of the drainage area above the pond.
4. How much freeboard is there on the dam. (Basically the height of the dam above the emergency spillway or above normal waterline)
5. What is below the dam in case of a failure.

These items will help you determine how much water you have to deal with in a rain event. A weir overflow like you currently have can
easily pass a greater amount of water than putting it in a pipe and getting head pressure to push water through the horizontal pipe, along with
other engineering details. So if you change from your current weir design, you really need more detail to size the pipe properly so as not
to waste money in an overdesign or under design it and compromise the dam.