Rebuilding a small dam

[Boondox]

For generations we've had a small stone and hemlock dam across a small stream above our pond. A 2 inch water line supplied fresh water flowing into the pond and kept things fresh. But last autumn we had a microburst that dropped 7.5 inches of rain on us in a little more than two hours. The resulting torrent ripped the old dam completely out of the ground and washed away the stones that formed the base.

I need to rebuild it. At first I was thinking of just digging into the banks by hand, placing a couple of 8x8 hemlock beams in place, then backfilling with stone. But if I drop a couple of trees I could actually use the tractor to build a ramp over some metal culverts that would serve as the overflow about a foot over my 2 inch waterline. That way I could capture a lot more water and not have to desilt so often!

Has anyone else done something similar or could point me to a primer on the topic?

 

[bczoom]

Boondox,

What is the normal water flow for your stream? How wide is it?

I had a similar setup for my pond (pipe into creek with dam setup so there was a pool). Mine wasn't near as elaborate. All I would do was make a stone dam from the content of the creek bottom using a combination of FEL and box blade.

If heavy rain was anticipated, I put a plastic cap over the pond intake to avoid silt/mud from coming in the pond.

This dam normally took about 2 hours to build but would last a full year. I took this approach since I kind of wanted it gone every year to wash out the silt basin created in the pool area I had.

EDIT: A couple other thoughts. If you get a decent (heavy) rain, would your proposed setup wash away carrying the culvert pipe downstream? If possible, you might want to wrap a chain or cable around one end of the culvert and tie off to a tree so you can find/get it the next day after a huge storm.

Does the DEP, DEC or whatever VT calls it care about you doing this?

 

[v8dave]

Don't dam the main channel. Make a low flow diversion channel. Dig/construct a channel at an angle from the main flow of the stream. Make a low dam for the pipe catchment basin. This is to divert low flow water to your pipe. The low dam will allow overflow to reenter the main channel.

Low flow water will follow the diversion and into your pipe for the pond. Heavy water flow, when it comes, will tend to follow the direct path, main channel, and make disruptions in the main channel. A very heavy flow may roll debris and boulders to block your diversion channel but is not likely to wash everything away. And it will be easier to clean out the diversion channel if that does happen.

What did you do on the cistern plumbing?

 

[Egon]

Pete:

From your comments the old dam has lasted for many years. A super heavy rainfall like you experienced does not happen every day.

Your ideas sound good. Installing the culverts would be a wise move for ordinary situations. Just make sure the culvert inlet is well sealed so no water can bypass or channel and make sure the outlet can not wash away materials on the dam.

Trying to build for the 7.5 in. deluge may be very expensive and probably a waste of time. It will depend on the size of the drainage basin involved and that may just be a whole lot of acre feet of water comming down all at one time just like a flash flood.

Whats with the hand digging when a backhoe sits available? /forums/images/graemlins/grin.gif

Stand back - look - think - wait a day or so and then follow your intuition.

Egon /forums/images/graemlins/grin.gif

 

[Boondox]

Dave, Brian and Egon -- Thanks for the excellent suggestions! I especially love the low flow idea, as silting has been a major problem. Though 7.5 inches was unusual, we do get a couple of heavy downpours (up to 1.5 inches in an hour) every year. The old dam handled that okay, though the silt was a pain to deal with.

The laws in VT are heavily grandfathered. I couldn't do a project like this where a dam never existed, but can repair or rebuild since the diversion goes to a pond which empties right back into the same stream.

I'm taking a couple of days off next week to deal with the springhouse. Will go plastic all the way. Thanks again! pb