I plan to build a 2.5 acre pond in an area that will very likely need to have a plastic liner. The site is in a relatively low lying area on my property near San Angelo Tx. It has a theoretical 460 acre watershed, but area has very subtle slope.
Will I still need a core trench, and clay for the dam, or can it be built up from the abundance of caliche that we will very likely find just beneath the soil? NRCS folks are coming out next week to survey the site, and tell me if it will be considered a pit or dam pond. It will actually be a combination due to the topography, but needs to be classified as one or the other according to NRCS.
Randy Cooper
Cactus Blum Ranch
That will be one big liner.
Sorry no help on other stuff.
Randy, I doubt that you will need much more than a hole in the ground. Why is the NRCS having to classify it?
Since this is going in an area that water flows through when it rains hard, It has to do with how much water would be impounded with a dam. They said if a dam would impound more than three feet depth of water (I believe relative to the current depth of the draw) we would need a designed spillway, and the dam will need to be compacted a certain way. If less than 3 ft of water would be impounded, it would be considered a pit pond and I only need to "designate" an area for the emergency spillway, and the dam can just be fill dirt extending into the lower portion of the draw.
If I set the water level where I intend by constructing a dam, I will be impounding about 4 1/2 ft of water before any excavation.
Randy
Not sure if this works the way I think it does, but here is an attempted link to the image of my plan uploaded to image gallery. I think a found the problem and uploaded an updated image of the project too.
This might belong in Building a Dam sub-forum...is there a way to easily change where it is posted?
I can move it.
No matter how tall the dam is, I'd make sure it was properly compacted as if it was a dammed pond. Better to be safe than have something happen and go through all the expense to have to fix it.
The NCRS released their calculations for my pond site, and show that I have a 649 acre watershed and need to accomodate 712 CFS of flow in the case of a 1% chance storm. That is going to take a pretty good size auxilliary spillway, which needs to pass under or over my dam road to be. Anybody have any formulas to share on calculating the size of culvert that could handle that.
I would plan for it to pass over the road, and make it concrete in the parts most likely to erode. I don't think you could install enough culverts to pass that much water under the road. In that case you would plan to put the road on a substantial sized bridge instead of culverts.
IMO there are times when your best path is to call in a pro. I would get in touch with Bob L. or Mike Otto and ask for a referral to a good pond engineer. I agree with John F. That is a massive amount of water. Providing a robust solution will possibly require modeling, soil analysis, topography info, etc. not to mention some pretty intensive math.
After meeting with the Natural Resource guys, my biggest concern now is that the NRCS provided a watershed report for my location is showing a 665 acre watershed with a 712 cu ft/sec peak flow in a 1% (100yr) storm event. According to their numbers, even a 5 year storm will have 245 cfs peak discharge rate.
I'm thinking now to create a diversion dam and an auxillary water course to mostly bypass the pond in the case of anything more than a 5 year storm. Has anyone else needed to do something like that? I just don't think there is a way to handle that kind of flow THROUGH the pond.
NRCS must have trouble staying busy in your area - I couldn't get them to even shoot the elevations on a 4 acre pond that I had built in NE Texas some years back. They simply didn't want to deal with a pond that had 165 acres of watershed. Anyway, the pond worked out ok. It has 25'to 35' OV areas on both sides of the dam with a 12" siphon pipe that stays shut 90% of the time. We got a record 18" of rain this last October without problem.
I understand the importance of handling excess water from big rain events when building a pond, but I remember San Angelo as being a relatively hot and dry area of the state - maybe 18 to 20 inches of rain per year. I would think that the main concern might be just keeping enough water in the pond.
After I spoke with them about a pond location they were concerned the pond might go onto the neighbors land due to large water shed and big rains, so I changed my spot. If it would have worked out I could have had a small lake
Kinda of glad I did not go that route where I had to handle large rain events.