ATTN: Geoff and Darren - Level sill spillways and erosion

Discussion in 'Designing, building, making and powering your life' started by gbell, Sep 21, 2008.

  1. alexizorbas

    alexizorbas Junior Member

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    I understand now. He does that because of the dam, obviously. The dam is dug much deeper than the swale, so an inch above bottom of the swale could still mean 20 ft or whatever of dam depth. This totally makes sense. I just had to talk it out. Thank you fkr help:)
     
  2. camwilson79

    camwilson79 Junior Member

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    If you can get your hands on 'Design and Construction of Small Earth Dams' (K Nelson, Inkata Press) there are some good tables for working out peak flows from the associated catchment, relevant to your location in Australia, along with the corresponding spillway width required. The figures they give are related to 500mm of water flowing over a spillway, so if your swale freeboard (difference between spillway and mound height) doesn't cater for that, you'll need to increase the spillway widths accordingly.
     
  3. gbell

    gbell Junior Member

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    Thanks Cam! I actually did get that book. I must've gotten lucky - old library copy - because I'm not seeing it now via booko.com.au.
    It's the most rigorous treatment of the subject I've seen. What we ended up doing is guessing and we just had a 1-in-20 year rain event here in Port Macquarie and things help up OK - water even went over the top of some swales, but the vegetation and the spread meant it didn't do much...
     
  4. Rhino

    Rhino New Member

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    For example:
    First: I'll build my dam with a compacted-clay freeboard-height of 200 meters (where the pegged shoreline is 199 meters)
    Second: I excavate my contour-swale at 199 meters and join it to the dam.
    Third: I excavate my level-sill-spillway somewhere along my swale (preferably on a ridge) at 199.1 meters.
    Fourth (optional): I make an emergency level-sill spillway at 199.2 meters.

    So, my dam will fill to 199 meters, then my swale will back-flood to 199.099 meters. Then the water will spill over the spillway at 199.1 meters.
    Do I understand the heights of the swale, dam-wall, & spillway correctly?
     
  5. songbird

    songbird Senior Member

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    if you are using the swale as part of the dam then
    you will also need to construct the swale as a dam.

    i think this makes it clear why you would not want your
    spillway to be too high up the swale. much more expense
    IMO for a very large swale if you don't have the right
    soils/materials on site.
     
  6. Rhino

    Rhino New Member

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    I believe with those measurements the swale-catchment will fill the dam and then back flood the swale to 99 mm before the water flows over the spillway. This allows the water to infiltrate the swale a few inches before discharging over the spillway. I am basing this off a video I watched of Geoff Lawton installing a pond on a property in Australia (). So, 99mm of water-height will soak into the swale-berm and then at 100mm the water will spill ;over the spillway. The non-compacted swale berm will be at least 600 mm high. So the water will only infiltrate 1/6 the height of the swale-berm until it spills over the spillway.
     
  7. songbird

    songbird Senior Member

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    to me that means that 500mm of swale is wasted effort for however
    long that is...
     
  8. Rhino

    Rhino New Member

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    Yea, I thought about that too but perhaps 100mm of soakage is adequate and you'd have less chance of swale-berm failure (Since it is not compacted). Once the planted roots penetrate 500mm, then they have access to that 100mm of rain (that is just sitting there on contour, slowly soaking into the subsoil & through the bottom 100 mm of the swale-berm). In the video posted above, Geoff made his dam freeboard 750mm above the shoreline. When the dam is at full capacity & the swale has back-filled 100mm, there will be 650mm of freeboard on the dam wall (because the water enters the berm from the "bottom" of the swale and the spillway is 100mm above that). If you soaked in 500mm of the swale-berm, than you'd only have 250mm of dam-wall freeboard.
    If anyone who has connected a swale to a dam could provide their input on this, it would be appreciated.
     
  9. Flatland

    Flatland Member

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    I totally agree. Peter Andrews work is amazing, turning particularly desert into productive land. What's also amazing is how the "experts" won't listen to him because they "know" better.

    My other comment is anyway where is likely to be erosion by water plant the dreaded kikuyu and there will be no erosion. I lived in the Adelaide Hills with a creek running through my property. My part of the creek was lined with kikuyu that i used as summer grazing for my animals and so it was controlled. Other property owners upstream and downstream did the "right" thing and removed all the non native vegetation and re vegetated the creek with natives. Every winter the creek would come down fast. That was the type of creek it was. My kikuyu would be flattened by the water and once the water level dropped it would spring back up and there would be not an inch of erosion. The native veg would be OK if it was only a small amount of water coming down and the owners got out and carefully righted their native plants and uncovered them from the general trash that had come down and replanted their lost plants, and filled in the areas of erosion. Every 10 years or so we would have a huge flood. Again my kikuyu would totally protect the banks and recover without me lifting a finger. The re veg sites would be totally scoured out, every plant would have been washed to the sea and the creek bed would have dropped to bed rock and widened by 5 or some metres. This happened at least 4 times that i was there but I was still the "baddie" for not planting natives "That protect the waterway". The last flood was a month ago and once again everywhere that was re veg has be scoured out and totally destroyed while my old kikuyu is still laughing at it all.
    My point is sometimes you should go with what works and forget the dogma preached by "experts"
    Sorry here endth the sermon
     

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