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Slowing the flow. Why doesn't the water just erode around the edges of dam?

 
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If your goal is to slow the flow, so you build dams such as one rock dams, but the water really comes down hard during rain season, what is preventing the water from just eroding around the edges of the dam (potentially causing more issues)? Thanks!
 
pollinator
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That's challenging. I'm not sure about your situation. Pics would be helpful.

Usually, riprap and breakwater structures are used to capture and dissipate the energy of the flow, which would otherwise cause erosion.
 
zurcian braun
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Hi Douglas, thanks for the response. It's mostly a general question so I can better understand how these kinds of dams implemented. And it sounds like the idea is to dissipate the energy enough that you don't run into this issue of erosion/undercutting? The creek that I'm interested in working with is quite steep (maybe around 45 degrees). No pics at the moment.
 
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with small rock dams, id say the edge rocks should be higher than all the center rocks. so water doesnt go around edges and stays flowing over rock, until plants are established
i think 45 might be kind of steep. may have to work the angle to be less. also, cover with good size rocks to stop erosion. sides higher than center to hopefully keep water flow over stable surface
 
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I had a recent wash out. This was an undercut that I knocked down then rocked. Failures will happen (especially when all one has is round rocks) but that's why you keep it at one rock status and play the numbers game.  If it cuts around the side the embed is cut for the next time one wanders up the wash.  Happy accidents?

Proud to say that all the other washes were washed out by the time the water reached the bottom of mine.
20220813_085330.jpg
rock catch dam
rock catch dam
 
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Smaller is better.

If I build a huge structure, and it washes out around the edges, then it often takes the whole structure with it.

If i build dozens of structures that are only one rock high, then they usually stay put, and fill with sediment during every runoff event. Then I build another one rock high structure on top of that, which fills with sediment during the next runoff event. If one washes away, i don't weep over it, cause it was easy to build.

 
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https://www.youtube.com/watch?v=RDCFbfcRcUE

Hi. In this video, Andrew Millison explains how to slow, spread and sink water in dams.
I'd swear he had another video with more specifics, like where to dig so your dam doesn't have to face all the strength of the stream.
Brief, you dig a large area in one side so when the flood comes, it fills that area, making the stream much shallower. The bigger the flow, the larger the dam.
 
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