PRI Ridge Point Dam Earthworks

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!