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

[aczlan]

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.

1. The pond might be 1/2 acre, not very big.
2. Drainage into the pond? No idea, there are at least 20 acres "uphill" from the pond between us and the neighbors, very little surface runoff into the pond though.
3. Pasture and hayfields
4. Currently, the weir overflow is the only exit, unless the pond goes up a foot, then it could go around the dam
5. Nothing important. The outflow from the dam goes through the ditch (which will be filled), then through a 12" culvert which has never been more than 1/4 full in the 6 years since we had it put in, and it rarely filled the 8" half crushed galvanized culvert that it replaced. After that, it has a bunch of ditch (1000+ feet?), then it picks up more flow (from the neighbors 20-30 acres) and then goes through a 500' long 12" culvert and out back.

The 12" culvert should be plenty to handle the outflow.

Aaron Z

 

[Fossil Farm]

If the conditions are as you describe, a 60 inch catch basin (riser pipe) would be much larger than is needed. There would never be enough
head pressure to fill the horizontal pipe (which would be the 12 inch, I presume) anywhere near full. It would take several feet of water over
the riser to fill the horizontal pipe. The catch basin riser (which becomes the pond overflow) should be level on all sides if you go this route
so water goes in evenly on all sides. Water does tend to swirl just as water going down a sink drain. If one side really needs to be lower,
cut that side down an inch or two instead of tipping the entire structure. Additional earthen fill would be required to cover the pipe and
the area where the old weir was located.

From the photos, there is quite a bit of dam erosion around the spillway. If you go with "Plan B" then the sides need to have
riprap placed on them to prevent the dam from eroding and compromising the weir spillway. That would be an additional cost.

Looks like you will have to figure the cost of the riser pipe and additional fill material (A) versus concrete and forms and riprap (B) and what option
would be more pleasing visually. Good luck. Hope it turns out very well for you.

 

[MossRoad]

Looking at the pictures again, have you considered driven steel interlocking sheets? You could drive the center one down farther than the others and bolt some angle irons to the edges of the ones on either side of the center for your boards to control height if you want. See if there's any sea-wall contractors in your area. That's what they use on most of the lakes around here to control shore erosion on speed boat lakes. Its cheap and fast. One excavator with a vibrating clamp could do it in half a day. Heck, it would probably take them longer to unload, set up, tear down and load than it would to do the actual job. It could probably be driven in right inside of the old one. Just a thought. :confused3:

 

[aczlan]

Well, I heard back from Contractor B, he says that option B would be $4800 for the dam itself (plus the pipe in the ditch and filling in the ditch). I talked to Contractor A and he has been too busy to put together a quote, but he should have a number for option A this weekend.

Aaron Z

 

[aczlan]

Looking at the pictures again, have you considered driven steel interlocking sheets? You could drive the center one down farther than the others and bolt some angle irons to the edges of the ones on either side of the center for your boards to control height if you want. See if there's any sea-wall contractors in your area. That's what they use on most of the lakes around here to control shore erosion on speed boat lakes. Its cheap and fast. One excavator with a vibrating clamp could do it in half a day. Heck, it would probably take them longer to unload, set up, tear down and load than it would to do the actual job. It could probably be driven in right inside of the old one. Just a thought. :confused3:

Might have to look into that, would be hard to have a kid-safe transition to the culvert in the ditch, but I might explore that if option A looks to expensive.

Aaron Z

 

[MossRoad]

Might have to look into that, would be hard to have a kid-safe transition to the culvert in the ditch, but I might explore that if option A looks to expensive.

Aaron Z

DOH! Forgot about the ditch-to-culvert!!! :mur:

 

[aczlan]

DOH! Forgot about the ditch-to-culvert!!! :mur:

The kid safe transition seems to be the hardest part to manage without either a standpipe or the catch basin...

Aaron Z

 

[MossRoad]

You could put in that steel driven option and put the catch basin with child excluder grid lid on the outside down at ground level, then run the pipe to the culvert and bury the pipe, couldn't you? Some cement work would be required around the catch basin, I suppose. Then you'd still have the option to put in the boards to control water level if you wanted. The upside to that would be if there was a major flood, the catch basin would overflow and you could make a swale in the grass over the buried pipe to act as an emergency spillway.