Kingcombe Stonbury were contracted to carry out comprehensive repairs to an existing outfall structure at a lake in the South West. The original scope of works involved desilting around the outfall pipework. After initial investigation work, voids within the dam structure were revealed at over 1m deep.

Working closely with our client, their structural engineers, and a reservoir panel engineer to provide the best solution for the repair of the dam. We were asked to reinstate the dam structure where leakage and piping had eroded the centre, without causing any damage to the downstream habitats, as the reservoir was located at the head of the River Otter.

A controlled drain down of the reservoir was completed to gain access to the working area and once the water had been lowered by 2m, our fisheries team attended site to net and electrofish the reservoir. A temporary cofferdam was installed around the immediate working area and our fisheries team were on site to rescue any fish behind the cofferdam during to the dewatering process. All fish were dip netted by hand and were put into an oxygenated transportation tank, before returning to the top lake.

Silt was pumped into dewatering bags around 100mtrs away from the outfall structure, due to a build up around the outfall pipe, this allowed us access with a mini excavator to break out the existing pipe work and prepare for the installation of a new pipework design.

Further investigation work revealed some of the voids were over 1000mm deep, and so it was agreed with the client that repairs would include grouting and sprayed concrete application. Steel mesh reinforcement set 50mm off the prepared surface of the wall was fixed into place and short lengths of pipe were installed into the wall at selected positions. Concrete was sprayed onto the mesh to the required thickness. After the curing period, grout was pumped through the pipes into the voids behind the walls. Once complete the short sections of pipe were broken out and the concrete repaired. The large voids on the top of the dam structure were also filled, using a pumpable grout.

The temperature and dissolved oxygen readings were monitored twice a day, including weekends for the duration of the project. Our site team had an emergency process in place including pre-installed aeration equipment with our fisheries team on standby, incase the readings became critical for the remaining fish. Working methods and techniques were reviewed by the site ecologist to ensure minimal impact to the environment during works and for reinstatement after the reservoir had been completed.

On completion of the project, barley straw sausages and vegetated floating islands were installed. The barley straw sausages were used to help combat algae growth within the reservoir and the floating islands to create habitat and fish refuge, aswell as helping combat algae growth.

Following installation, the dam was handed back to the client for inspection and sign off.