While most people are familiar with walking the plank, it's probably only Mid Cantabrians who know about walking the siphon.

About 700 people visited the Rangitata Diversion Race (RDR) open day on Sunday to walk the longest siphon on the race at Surrey Hills.

RDR Management Limited chief executive Tony McCormick said the Surrey Hills siphon was one of nine siphons on the RDR and the longest at 2.7km. It was last opened to the public about 10 years ago.

Donations from the successful Mothers' Day event raised $3000 for the Hinds and Districts Lions, with the club assisting with parking and transporting walkers back to their cars.

Sausages, ice cream, and coffee were provided, with contractors and consultants sponsoring the refreshments.

"It was such a pleasure to see the enjoyment all our visitors got from the event, and we appreciate their generosity with donations.

“We say the RDR is the lifeblood of Mid Canterbury and the turnout on Sunday shows that our community recognises and appreciates the importance of the scheme to their region.”

We even had a group of mountain bikers ride the tunnel and back at the end of the day, McCormick said.

It took the bikers about seven minutes while walkers were in the tunnel for about 40 minutes.

McCormick said all the water in the RDR goes through the siphon and takes 16 minutes to get through the tunnel.

​​​​​​ "Water coming out of the tunnel would fill an Olympic swimming pool in 90 seconds," he said.

Surrey Hills siphon facts

Construction of the Surrey Hills section of the RDR was in progress when the construction gang went on Christmas break. When they returned to work, the crew found a slip on the hillside had taken part of the canal with it.

It was decided that Surrey Hill was too unstable and an inverted siphon was designed to route the canal along the base of the hill.

A pipe factory was constructed nearby to make more than 700 sections of pipe that were fitted together to form the Surrey Hills siphon.

"The lowest point of the tunnel is approximately 40 metres below the inlet, but it then rises up to the outlet which is only five metres lower than the inlet. Given the resources of the day, it is an amazing engineering accomplishment," McCormick said.

By Sharon Davis