Labyrinth Weir

Client: Central Plains Water Limited

Collaborators: Hamish Tait Projects Ltd, Riley Consultants, and Smith Crane and Construction

Taylors upgraded the existing standard weir and channel to a labyrinth weir to allow for more capacity during higher river flows.  This upgrade allows farmer shareholders to irrigate when the river is in flood after rain on the Main Divide.

During the procurement phase, Taylors suggested to Central Plains Water that Taylors meet with the designer and mutually agree on a concrete subcontractor. Working together at the design phase and choosing a subcontractor together meant that everything progressed much more smoothly, with no interface issues and better collaboration between the contractors on the project.

During the materials procurement phase, Taylors and Central Plains Water chose Smith Crane and Construction as the concrete prefabrication subcontractor. Making this choice early on in the project meant that Smith Crane and Construction could get on and construct the concrete panels for the weir, allowing more time and flexibility for the completion of the overall project.

From the client’s point of view, potential problems and delays were mitigated by the contractors working together in this way from an early stage.

For this particular project, there was a very strict timeframe for closing the canals to undertake the work. This could have posed a significant risk to timely completion, but working together during the planning and design phase helped alleviate these risks.

Taylors successfully adhered to the tight pre-construction and construction programme during the winter shutdown period, offset by Smith Crane and Construction precasting the concrete weir panels in Christchurch. Assembling these panels on site compressed the programme’s timeframe to minimise exposure to flood events.

Accurately positioning the new weir at the exact location and height of the old weir was also a challenge. This was achieved using a combination of modern survey total station and a traditional dumpy level.

The concrete panel sections were poured into purpose-built concrete moulds at Smith Crane and Construction’s yard. It took a lot of work to get the bull nose shape on top just right as well as the angle of each panel.

Interesting facts and figures

A total of 113m3 pre-cast panels were poured and trucked to site from Christchurch, with 266m3 of 50MPa concrete poured on site, and trucked in from Firth Concrete’s plant in Darfield.

There was 23,000m3 of material excavated to widen the channel to allow for the increase in flow across the new weir. This was achieved while the earthworks crew completed their usual winter maintenance work on site, excavating approximately 20,000m3 of silt out of the sediment pond/rowing course. Another 2500 tonnes of rock was placed around the new weir in July. All work was completed well ahead of the programme’s project deadline.