Posted by & filed under Dams, Earth Banks, Irrigation, Land, Material, Natural Swimming, Swales, Water Conservation, Water Harvesting.

by Gordon Williams

On the 31st of January the Permaculture Earthworks course at Zaytuna Farm began with good weather and a group of enthusiastic students ready to see the process of laying the groundwork for functional rainwater harvesting features in landscapes. During the week a variety of works were conducted across the property, including a new dam and swale, swale pipe crossings, building site levelling and, to make everyone’s life a little bit easier, the excavator divided some clumping bamboo.

The first task for the 25-ton excavator was to construct a ridge point dam connected to the end of an existing swale, so as to increase catchment. If the dam were to be built independent of the swale it would not naturally fill. The primary purpose of this dam is to increase the volume of water stored on the property at a height where it can be gravity fed to areas below for use.

The first step was to remove the topsoil from the dam site and set it aside so that it could be laid out over the dam wall once it was finished. This gives the best conditions for a cover crop to be sown to protect the dam wall from erosion and get ahead of any weeds that are in the soil.

Once the site was cleared work began on digging the keyway which serves as a means to lock the dam wall into the subsoil. The keyway consists of a trench dug into the undisturbed subsoil along the line of the wall and is filled, ideally, with the best clay from the dam excavation. This material is then compacted into the trench to make it impervious. In this case the track of the excavator was used.

Once the keyway has been compacted the process of building the wall begins. As the material is excavated it is spread out in layers and then compacted down onto the one below in a repeating process until the wall reaches the desired height.

Because the dam was constructed at the end of an existing swale, the high water mark of the swale (300mm of water in the swale) determines the height of the spillway, which is located at the opposite end of the dam wall from the swale itself. The wall is then raised further than this to create a freeboard to stop water flowing over the wall.

In large rain events water can be coming into a dam faster than it can exit and as a result of this the water can back up at the spillway and rise to a few feet in depth. As this ridge dam has a small catchment the wall was built up to obtain a freeboard of 700mm. For dams with larger catchments a freeboard of upwards of a meter would be ideal.

By having the swale attached to the dam, it allows not only for increased catchment but when the dam fills up, the swale will back-flood and rehydrate the land below.


Once the wall is completed the stockpiled topsoil is laid out to cover the wall
and the cover crop sown

 


The dam half filled with water and cover crop on top of wall

 


The swale that’s connected to the dam (on the right hand side of the tree)

 


The spillway

 


…a little later, one full ridge point dam!

16 Responses to “PRI Ridge Point Dam Earthworks”

  1. Jason Gerhardt

    Gordon, Geoff,
    The subtropics make this look so easy! In the Front Range of Colorado, USA I am working on a large water harvesting project and am lusting after the conditions of the subtropics after reading this. Just drop cover crop seed on the ground, it’ll grow, no need to mulch and overhead irrigate. Where I am, not the case. We are in a year long drought having only received a tiny fraction of our average annual precip (17 in). I’d like to see more examples on the PRI site of people in challenging climates doing the essential work of permaculture. Guess I better get to writing an article myself… As always nice work, and thanks for taking the time to share! Geoff, maybe you could make a new film on permaculture in challenging climates? Just a thought. I love all your other videos too!

    Reply
  2. Anthony Teitzel

    Nice article Gordo! Would have loved to have seen it full but missed it by a couple of days, dam!

    Reply
  3. Rob

    Amazing! I wish I was there to see it flow! Geoff, where are you picking up the overflow? Is there a culvert by the new shed, or did you run a new swale along the bottom of the sheep paddock?

    Reply
  4. Gordon Williams

    Hi Rob,
    The overflow is indeed picked up by a new swale along the bottom of the former sheep paddock. The swale spills just above the small dam perched above the house dam.

    Reply
  5. aslanded

    Ive done a couple of projects with the 20odd tonner for a day or two and as a result I now have a couple of spring fed dams, a house site and a well formed drive I can use all winter. The earthworks are such an exciting part of the project and the way a good excavator drivers artistry with those beasts brings your vision to life, then enhances it, is nothing short of miraculous. Your dam is beautiful and the amount of life this machinery brings to our worlds is something we should all take advantage of while we can afford such awesome technology.

    Reply
  6. David Bartlett

    Would love to see a rough map of what earthworks have been added since we were there Geoff! Not to mention a pic of the trees on the bottom swale next to the extended main crop. Wishing you all well from South Africa

    Reply
  7. maria

    nice work chuckles! can’t believe its so full already. we built that dam (well we cheered glenn on as he did) and we are all proud. one more down and another 29 to go to make zaytuna the most drought resistant property in the whole of oz!!! : )

    Reply
  8. Darren J. Doherty

    G’day,

    So to repeat my queries of before?

    1. What was the cost of this structure, how much time did it take etc.?
    2. Was there a reason for this structure not having a lockpipe installed so that such a useful position can be taken full advantage of?

    Otherwise as mentioned before very nice job in a lovely position.

    All the best,

    Darren

    Reply
  9. Darren J. Doherty

    G’day again,

    Too many sites to keep up with obviously…thanks for answering my question on the PC Global site with the same article….To repeat the answer there from Gordon:

    ‘….I wondered the same thing about the lockpipe but Geoff didn’t feel the need as there are three more valley dams at the same level all with pipes. Would have been a good opportunity to demonstrate the installation of a pipe to students. The excavation cost roughly $1800 and took about a day and a half to build with the machine operator stopping for a few chats with students about the process along the way. It has about a 600,000L capacity and makes a pretty neat natural swimming pool with a nice view. Gordon…’

    Thanks,

    Darren

    Reply
  10. Gordon Williams

    Sorry, I missed the questions over here, thanks for putting that up.

    Gordon.

    Reply
  11. Daniel Parra Hensel

    Jason Gerhardt* If you are looking to more dryland approaches look up some of the work by Brad Lancaster he wrote the book “Rainwater Harvesting for Drylands and Beyond” also check out some of the articles by Quail Springs Permaculture, they have a high desert arid landscape with about 4-6 inches of annual rain. They have a dam there and many little ponds that are more dryland appropriate (many small dams highly vegetated, rather than one big one…). Good luck with your projects!

    Reply
  12. Daniel Parra Hensel

    Gordo thanks for the article! It is great to see the process and different stages of the dam building. Especially now being able to walk up to the dam and take a look, it was hard to imagine what the landscape was like before the dam was installed and the pictures really help!

    Reply
  13. Mattias Ackermann

    Thanks for the article,

    I have a question regarding the slopes of the dam wall as well as inside of the dam. They both look very steep. Is they width to height ratio more than 2?

    Reply
  14. Gordon Williams

    well spotted Mattias, the walls are fairly steep. Because this is a ridge dam the wall is long so it wraps around and contains the water, therefore it required a lot of material to build. As you can see the machine got every last bit if soil that it could reach out of the hole and that is how much material was available to build the wall. It would have be nice if there was more to make the outside gradient less steep.
    We didn’t work out the exact gradient at the end of the build so I couldn’t tell you.
    Clumping bamboo was planted on the wall so that the roots hold the soil together.
    I haven’t been there for a couple of years now but am sure it is still holding up well.
    Gordon.

    Reply

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