WA – Lefroy Brook Dam Remedial Works

In early 2016 works commenced on the dam that directs water to both the Pemberton Water Supply and the Pemberton Trout Hatchery; namely Lefroy Brook Dam. The dam has an interesting history having being constructed by the State Forestry commission as a source of water for the Pemberton Hydro Electric Scheme. In the intervening years the role of the dam and the ownership of the dam changed. Now it has a key role in the Pemberton Water Scheme.

In 2007 the dam experienced a “piping failure” whereby a concentrated leak formed under the concrete wall eroding material from beneath the dam and as a side effect forming a migration passage for Lamprey heading upstream. This erosion was promptly addressed through the construction of a reverse filter on the downstream area and the installation of a cuttoff on the upstream side. Some of the works are shown in the image below where, due to the nature of the works, some of Water Corporation Dams Section took to the tools – pictured with the shovel is Tristan De Lang, who is also the ANCOLD YP Representative for WA.

The initial repairs performed well and were stabilised in 2012 to ensure the continued protection of the dam.

In 2016 SRG commenced work on site to initiate a long term repair to the dam whereby the source of the failure would be identified and repaired and protected against further erosion. As part of the works, the outlet works through the dam were also upgraded – with life expired pipe being replaced.

The interesting aspect of this project was that the dam was unable to be taken off-line for any time beyond a couple of hours owing to concerns with supply to the trout hatchery downstream. The project team was able to put together a suitable and successful pumped diversion to keep the dam online and keep water flowing to the trout hatchery and the town for the duration of the works with no interruption in flow.

As the works progressed the source of the initial concentrated leak was identified. Towards the left abutment the dam had not been founded on rock, but rather had been founded on residual soil and a tree root had worked its way into the space under the dam, thus forming the source of the leak. This source is shown in the photo below.

The leak source was plugged, the pipes through the dam were replaced, a new valve pit was constructed and a new filter protection zone was installed by SRG over the course of the works.

The dam’s concentrated leak has now been repaired, new outlet works have been installed and the temporary diversion works removed.

Since completion the dam has seen one of the most significant overflows in recent years in early July.

Author: Michael Smith, Water Corporation