Damming the flood

Ross Reid, contractors on the Pukekohe Hill dam project, had to learn to take the rough with the smooth.   BY ANDY MORRIS

Dam_5.jpgHeavy rain in the winter of 2008 came as no surprise for Ross Reid Contractors, but it certainly presented an extra challenge when completing the $1.25 million Pukekohe Hill dam project for the Franklin District Council.

The fact that Pukekohe had such a problem with stormwater suited Ross Reid just fine as it landed both contracts in a $3.25 million council project. It had just signed off on a two kilometre long pipeline construction to take stormwater away from several roads in the town when it was awarded the second project in late January last year.

That project was to build a dam at Pukekohe Hill that was capable of dealing with the worst storm in a century, as well as help the council improve the quality of the water flowing into nearby streams.

But as it proved, awarding a scheduled 20-week project so late in the summer season meant the contractors would be battling with the elements.

Dam_4.jpgConsultant Lorenzo Canal of Urban Solutions and the Ross Reid team were charged with the task of diverting stormwater that had been flowing down several streets neighbouring the Pukekohe racetrack. And having signed off on the project in mid-December he declared himself satisfied with having solved a number of challenges.

“Overall, the net result was good,” says Canal. “It’s come in within the original budget, and we’re out the other side of a number of problems with a very good result.

“The project was scheduled to take 20 weeks and it’s actually taken longer than that, but that’s through overcoming the challenges.”

First of the setbacks for the contractor was dealing with a pre-existing and un-maintained farm pond on the site. It had been estimated that some 2000 cubic metres of sludge would need removing, and this material would then be dried and used in the dam.

What Canal and Ross Reid project manager Ross Ferraby discovered was that the estimate was wrong by about a factor of four – they ended up removing around 8000 cubic metres from the pond.

Dam_2.jpg“It was quite challenging to dry a pond site with the amount of space we had, so we worked together with the council and came up with a solution of capping one of their nearby landfills with that material,” says Canal.

“We used sealed trucks to remove it off site, which was good because it freed up more space on what was a pretty constrained site to use for other construction activities.”

A temporary cofferdam was built to exclude water from the area.

The next stage called for importing material with which to construct the dam, and a borrowpit was used at a nearby quarry. But then their initial luck with the weather ran out.

From late April it started to get very wet so the contractors had to come up with an alternative source to finish off the filling, turning instead to Stevenson’s Quarry in nearby Drury.

Prutvi Kumar, Ross Reid business development manager, says, “We were able to obtain dry clay from there throughout the winter, so we continued working throughout the winter – it saved another season of the contract period.”

Dam_3.jpgMore than half the 15,000 cubic metres of clay imported was delivered during the winter, and the Auckland Regional Council proved receptive to the contractor’s need to extend the earthworks. The Ross Reid team were able to address their structural compaction issues by working when the weather gave them a window of opportunity.

“Another of the challenges was the issue of access,” Kumar adds. “We had to build temporary access through the narrow areas to get the six-wheelers and artics through.

“The initial source of materials identified in the tender also proved not to be feasible because of traffic management issues.”

The dam was not only needed to contain stormwater, which it does in what is currently a 20,000 cubic metre pool, but also address water quality issues.

Dam_1.jpgThe water coming off the hills and roads is compromised by agricultural residue, rubble and metal as well as the likes of diesel fuel, brake fluid and rubber from the nearby roads.

“Some of the land close to the dam is being rezoned for rural residential use, so some of the work was basically future-proofing that part of town,” says Ferraby. “All that residue needs treating and part of the idea of the pond that has been built is to get those heavier solids to settle out of the water before it flows out into the streams.”

Future-proofing is very much part of the design, as the dam is designed to cope with whatever rain the heavens can throw at it.  

A 200mm diameter pipe caters for everyday rain events, then there is a further 22,000 cubic litres of capacity built in, which caters to a one in five year flood event.

To insure against an even worse storm, Ross Reid constructed a spillway which starts at the level where water might reach once in a decade.

“The spillway’s made of Reno mattressing and gabion baskets to prevent scouring during flood events,” says Ferraby. “In a severe weather event the water goes down that spillway and into a stilling basin before flowing though a new culvert thrust under Kitchener Road. If this culvert cannot cope with the volume of water, then it will flow down a new rock lined channel built down Kitchener Road.”

Dam_6.jpgAround 60 percent of the filling work was completed by the end of May, and in the winter saw Ross Reid completed the rest of the works that included building access tracks and putting a thrusting pipe under Kitchener Road, which borders the site.

The client is well satisfied. Franklin District Council’s stormwater asset manager, Godfrey White, says Manukau Road (situated below this attenuation dam) in Pukekohe used to flood twice a year and the area has suffered severe flooding in the past with a major flooding event in 1999.

“Council has been working towards improving the overall stormwater protection since 2001 and the current work is part of the solution. There is still some linking work to occur, however Pukekohe has a much better outlook when it comes to stormwater protection.  It has been a real success story.” 

Contractor Vol.33  No.1  February 2009
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