Hey all, Ill upload pics soon. We built a 6 acre lake last summer and we just had a blowout, to be honest I'm not that heart broken about it because I had a feeling it wouldnt last - glad it went now and not 5 years from now.
The problem was that the water undermined the overflow culvert m it's possible that muskrats had something to do with it as well as there is visible boring at the washout and I trapped 2 there in the early winter. There is still about 3 feet of water in areas and I found no fish or even minnows downstream which I found odd. The lake flows into nothing - just a lower area and then seeps into the ground.
Old Dam specs:
length was 190 feet, it tapers from 6 feet high on one end and drops to about 8 or 10 feet at the far end... the 6 foot end being the easy access end for me and equipment.
Base was about 20 feet wide up to a driveable plateau and then a 4 foot wide higher dam face if that makes sense. Ill post a pic
Materials: Mostly clay, clay gravel, packs very well.
Dam face angle was about 1:1 clay (time and materials led to the hasty dam build with steep angle)
Overflow was an 18 inch plastic culvert in the clay about 20 inches below the top of the damn. No anti seep collar
Emergency spillway was set at the top of the culvert height appros 60 feet from the culvert - it did not overflow during the breech.
New dam ideas:
190 feet long
Average depth of the 400m long kidney shaped lake is 6 to 8 feet. I'm going to excavate an area about 20m from the dam down to about 12-15 feet deep and use the clay to rebuild the dam - so i get a deeper lake able to hold more reserve DO and colder temps
Dam face angle will be 1:3, so for every 1 foot high it will be 3 feet out. A 10 foot high dam will have a base 30 feet.
Top of the dam will be my flat road to drive on, roughly 8 feet wide - wide enough to drive equipment on to repair if needed.
Back of the dam will be the same slope.
Dam vegetation - I planted a mixture of Vetch and rye last fall and in the course of 2 weeks it was 4 inches high - 3 weeks 6 inches high and very thick so Ill plant the same this time. No erosion when raining.
Overflow:
I will not be placing a culvert in the top of the dam.
I will be using a catch basin type that we will pour ourselves, it will have removeable boards to adjust water height when needed.
It will be built at the shortest section in the dam for easy access and less materials needed ie. concrete.
Roughly 6 feet tall with 3 feet of removeable with oak boards 40 inches wide (board width)
30 inch culvert with anti seep collar (concrete poured). Will be approx 4 feet under the ground or more to prevent frost heave.
Because I have the clay available do i need a membrane on the face of the dam? Would it even make a difference?
Any other advice? I am a licensed trapper and I do the best to take care of the muskrats and beavers - beavers are easy but those muskrats are real buggers.
Sorry to hear about your blowout. But like you said, much better to have now, than five years from now!
Questions:
Was your entire dam scraped up and compacted in lifts, or was part of it an existing ridge of undisturbed material? If existing material, then that may have been part of your weak spot. Most existing deposits have layers visible when you excavate. It only takes one thin layer that is slightly permeable to water to cause a problem. As water slowly flows through such a layer, a few particles will tend to dislodge and go with the flow. That will then microscopically increase the flow rate - which then allows a few more particles to move. Eventually, you get a sudden and catastrophic failure when the leak reaches "critical mass".
Did you have any anti-seep collars on the old culvert? Two of those might be a good idea on your new outlet pipe. They are typically made just to stop any movement of water as I described above, so a tiny leak along the culvert does not worsen to failure.
As such, they are sometimes built with the thinnest stretched rubber membranes. However, sometimes they are also poured concrete. If muskrats were the culprits, then perhaps a concrete collar will end their tunnel and even though there is free water against the pipe, it would not then worsen into a leak and eventual dam blowout.
Dang, I'm sorry to hear that. I would make the top of the dam 12'-15' wide. Over time it will erode a bit and shrink in width. 10' wide could be 8' wide in a few years. You don't want to drop a wheel off an 8' wide dam. Dig a keyway in down to good soil and compact, compact, compact as you build up the keyway That's another reason for the width of the dam, it will allow equipment to pack the keyway.
No membrane needed. Extend the overflow pipe past the toe of the dam. OR make the outflow area concrete thick enough that it won't be impacted by freeze/thaw cycles. Use rebar AND fiber reinforced concrete - rebar for big cracks, fiber for small cracks. You don't want a crack opening up enough that water can get through it and start undermining the concrete.
Download this and read it. It has a ton of good information in it on building a dam. The clay for the dam can't be too wet or too dry for proper compaction.
Sorry to hear about your blowout. But like you said, much better to have now, than five years from now!
Questions:
Was your entire dam scraped up and compacted in lifts, or was part of it an existing ridge of undisturbed material? If existing material, then that may have been part of your weak spot. Most existing deposits have layers visible when you excavate. It only takes one thin layer that is slightly permeable to water to cause a problem. As water slowly flows through such a layer, a few particles will tend to dislodge and go with the flow. That will then microscopically increase the flow rate - which then allows a few more particles to move. Eventually, you get a sudden and catastrophic failure when the leak reaches "critical mass".
Answer: The part that blew out was all new material, clay from top to bottom, the smaller part of the dam is some old beaver dam material with about a meter or more of clay over it. This is marine clay.
Did you have any anti-seep collars on the old culvert? Two of those might be a good idea on your new outlet pipe. They are typically made just to stop any movement of water as I described above, so a tiny leak along the culvert does not worsen to failure.
Answer, no anti seep collar which i think was my main problem to be honest. Ill be poring two anti seep collars like what "Let's dig 18" on youtube does.
As such, they are sometimes built with the thinnest stretched rubber membranes. However, sometimes they are also poured concrete. If muskrats were the culprits, then perhaps a concrete collar will end their tunnel and even though there is free water against the pipe, it would not then worsen into a leak and eventual dam blowout.
Followup: the new culvert will be at the base of the dam so the muskrats won't bore that low.
Dang, I'm sorry to hear that. I would make the top of the dam 12'-15' wide. Over time it will erode a bit and shrink in width. 10' wide could be 8' wide in a few years. You don't want to drop a wheel off an 8' wide dam. Dig a keyway in down to good soil and compact, compact, compact as you build up the keyway That's another reason for the width of the dam, it will allow equipment to pack the keyway.
No membrane needed. Extend the overflow pipe past the toe of the dam. OR make the outflow area concrete thick enough that it won't be impacted by freeze/thaw cycles. Use rebar AND fiber reinforced concrete - rebar for big cracks, fiber for small cracks. You don't want a crack opening up enough that water can get through it and start undermining the concrete.
Download this and read it. It has a ton of good information in it on building a dam. The clay for the dam can't be too wet or too dry for proper compaction.
