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Ridge Dam Construction at Zaytuna

Posted by Gordon Williams about 2 years ago

The most recent PRI earthworks course saw the construction of a new ridge dam and swales at Zaytuna Farm.

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 project for the 25 ton excavator was to construct a ridge point dam connected to the end of an existing swale that would increase the 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 required 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 other end of the dam wall. 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 allows not only for increased catchment but when the dam fills 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 finished dam half full with the cover crop of cow pea doing well.

The dam with the swale entering to the right of the tree.

The spillway that will only overflow when the swale is full and there is still water coming in.

The dam has been full continually since nine weeks after completion

Comments (5)

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jordan lowery : what a beautiful spot for a dam/pond. also done very well.

Posted about 2 years ago

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Darren J. Doherty : No lockpipe? For such a great position this would have been a fantastic addition...Of all the dam types Ridge, Hillside or Contour Dams the installation of a lockpipe serves such an important function lest the dam is so much less utile. Nice job though...Also how many days did it take and what was the cost? Cheers, Darren

Posted about 2 years ago

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Gordon Williams : 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

Posted about 2 years ago

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Darren J. Doherty : G'day, <br /><br /> Thanks for this Gordon. Fair enough then...Nice job for that amount of cash....All the best, Darren

Posted about 2 years ago

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