Construction of Wairoa Dam

Wairoa Dam is a water supply dam in the Otau Valley in the Hunua Ranges southeast of Auckland. It was built by the Auckland Regional Authority and is now owned by its successor Watercare Services Ltd.

History  Design  Construction

Photos:

Tunneling  Dam Construction  Valve Tower  Valve Tower Bridge  Delivery Pipeline  Finishing  Spillway  Spillway Model  Lake Clearance  Moumoukai

It was built in the early 1970's. The series of photographs on this site were taken by the site staff at the time of construction. This website was created by Garry Law who was one of the designers of the dam and worked on site during the construction as part of the resident engineer's staff while living at Moumoukai. He took some of the photos. The site camera was a Canon Canonet rangefinder. The colour images here were taken by the author as slides, on a Petri 7s rangefinder camera.

Thanks to Graeme Doherty and Huia Mitchell for contributions.

There are four other dams in the ranges.

The site is open to the public in daylight hours. there are toilet facilities for visitors.  Unfortunately the bronze plaque that marked the opening by Authority Chairman Tom Pearce on May 23, 1975 and recorded some information on the dam construction has been removed by vandals.

The brochure produced for the opening (thumbnails).

The Dam on Google Maps:

View Wairoa Dam in a larger map

A Little History:

Wairoa Dam was originally intended to be the fourth dam in the ranges. The tunnel system to carry water to Ardmore Treatment Plant and thus to the city had gone past the site connecting through to the second built dam, Upper Mangatawhiri. A flow gauging and interim intake weir was built above the proposed Wairoa Dam site and an interim run of stream supply commenced from that in 1960. The Mangatangi dam site had a similar weir and interim supply. Mangatangi dam was supposed to be next in the sequence and work commenced on that, but contractual difficulties with the construction of the diversion tunnel had it running behind schedule. In a period where demand growth averaged over 4% per annum this presented a risk to Auckland's water supply. Consequently on the suggestion of long term resident engineer Eric Scanlen, Wairoa Dam was advanced in the sequence and was built in parallel to Mangatangi, entering service two years earlier than the completion of Mangatangi. The other sites mentioned here are shown on the Google map below.  

Design:

The design was lead by the ARA water department design engineer, Don  Wilson. Other contributors were Eric Scanlen (hydrology, general diversion and spillway arrangement), John Crabtree (spillway preliminary, site investigation), Garry Law (spillway and delivery system), Graeme Barnard (geotechnical) and Remi Gobet (structures). All calculations were by hand assisted only by an early electronic calculator. Computer aided design in civil engineering was then in its infancy. 

Construction:

Work commenced on the diversion tunnel utilising a labour-only form of contract that had been much used for tunneling work by the Regional Authority and its preceding organisations. The Authority provided all plant and materials for the tunnel. A concrete batching plant was established at the site. John Crabtree was the staff engineer in charge of this work, reporting to Eric Scanlen. The main work was let by competitive tender to Downer and Co through a conventional construction contract. The delivery pipeline from the tunnel mouth to the supply tunnels was laid by an ARA pipe gang. There was an on-site earthworks control laboratory.

While building the dam the ARA staff also did a site investigation for Lower Mangatawhiri Dam (never built) and undertook the Ardmore supply tunnel duplication.

 

The pictures following are all thumbnails.

	Stripping of the dam site has commenced. Pipe for the supply line stacked in the middle distance over the stream bridge. The stream diversion was not yet in place.
	The stream diverted and the dam part finished - the valve tower at full height but with no bridge.
	The dam a little higher and the bridge to the valve tower under construction - the aggregate bins for the tunnel concrete lining have gone. The tree in the center has had too much root disturbance and is near death.
	The dam nearing full height in the second earthworks season
	West abutment from the east - during site investigation

 Tunneling:

	Tunnel form at the downstream portal. The cement silo is over the portal, the concrete mixer below that which discharged into the concrete carriage vehicle in the tunnel. The mixer was fed from a trolley mounted weigh hopper discharging onto a belt. The three aggregate and sand hoppers are above. The central building is the compressor shed. The air extract fan pipe can be seen left of the portal, but has been disconnected.
	Tunnelers - John Crabtree second from left.
	Damaged photo of the tunnel junction form
	Shifting the tunnel form
	Taking pipe into the tunnel - taken from the tunnel portal - the form is parked to the right
	Placing pipe in the tunnel floor.

 Dam Construction:

	Site stripping commencing - photo from upstream. The miners adit into the manganiferous rock can be seen at the right.
	Later in site stripping.
	Downstream shoulder stripping
	Core trench, covered drains from springs to the pump well
	Spring pump well
	Right abutment shaping
	Grout pipes to close pump wells
	Filling
	Down stream shoulder filling
	Piezometer tubes
	Dam site Panorama
	General view looking downstream - early in the fill placement
	Compacting the filter drain - constructed by excavating a trench through fill down to the last lift.
	Filter drain construction
	Filter drain construction with core placement upstream
	Filter drain construction - spillway shaft to the left
	The west abutment - soft sensitive alluvial material required stripping her to get to competent rock and weathered rock

 Valve Tower:

	Tunnel form being positioned at the upstream portal. The inverted siphon of the interim supply pipe can be seen and the under-floor permanent supply pipe
	Link section from the upstream portal to the valve tower. Inverted siphon on temporary supply pipe goes under.
	Reinforcing tieing for the valve tower base
	Shuttering for the valve tower base.
	Rock anchor tendons.
	Anchoring, pre-diversion. The weir supply pipe diverted into the tunnel
	Slipforming the valve tower underway
	Valve tower top
	Installing an intake gate valve
	Concreting in an intake in the valve tower
	Fixing the lake level markers to the valve tower
	The bottom outlet pipe in place.
	Pouring the tunnel plug in the valve tower base.
	About to close the bottom outlet gate valve - from the left Bruce Smithson, Don Wilson, John Crabtree, ?Keith Hay, Fergie Schiska, ?(ARA photographer), ?.
	The bottom outlet closed, Eric Scanlen facing the camera, George Wyman on the right on the platform.
	Installing the discharge valve on the bottom outlet.

Valve Tower Bridge:

	Foundations for the bridge pier
	Pier bases
	Pier construction from the tower.
	Pier construction
	Pier construction
	Concrete pour for the pier
	Launch beam
	Delivery of a Tee beam for the valve tower bridge
	Ditto
	First beam ready to launch
	First beam about to be lowered
	Ditto
	Third beam
	Last beam in place

Delivery pipeline:

	Delivery pipe for the downstream portal to the control structure under way
	Supply pipe being laid
	Control valve chamber inner form. Link structure between the two tunnels in the background. Damaged photo.
	Control valve being installed, pipe to right is the overflow for an emergency drawdown. The valve has special porting allowing fine control over part of the travel and opening a large port area over the latter part for emergency drawdown..
	Completed control structure

 Finishing:

	Piezometer house under construction
	Log recovery
	Hydroseeding
	Rock facing on the lower part of the upstream embankment
	Junction chamber - outlet to left
	Inlet from dam berms to right, one collector of two from the downstream shoulder drainage blanket below - perforated pipe.
	Proof grouting under the core - from the east abutment.
	Quarry
	Crusher / screening plant
	Garry Law in the piezometer house.
	John Crabtree, ?, Graheme White. Phil Salmon, ?, ?   Graheme was a local contractor operating bulldozers forming the waterline track during lake clearing..
	John Crabtree and George Wyman
	Messy!
	Guests after valve closure. Bruce Smithson, Eric Scanlen, Fergie Schiska, Don Wilson, maybe Graheme Tait
	Downer's fleet

 Spillway:

	Spillway elbow shutter - in two halves second in back right.
	Spillway outside former
	Stilling basin excavation
	Stilling basin excavation
	Stilling basin construction

 Spillway model:

The 1:40 scale hydraulic model of the spillway stilling basin running at full capacity.  The model was built by the Authority staff and run at the Auckland University Engineering School hydraulics lab. It was used to confirm the bellmouth approach and fins were sufficient to suppress vortexing in the down shaft, to check the choke size needed at the bottom of the shaft, the pressures in the shaft, the diversion tunnel transition, and the stilling basin form. A large number of varieties of the stilling basin were tried, mainly to get the teeth positioning and sizes right to control the position of the hydraulic jump. The shaft pressure checks were important as the transition from the bellmouth to the vertical shaft was kept short to limit the amount of double curvature shuttering that needed to be built. With this a length of the shaft would run below atmospheric pressure. If pushed too far the pressure would fall to full vacuum and the flow might separate. The model confirmed pressures stayed above full vacuum. The choke at the shaft base limits the flow so the exit tunnel remains as free surface flow. Otherwise the whole tunnel might fill expelling air and run as a full pipe, increasing the flow beyond the stilling basin capacity.

 The Lake:

There were small areas of original bush in the lake area but most of it had been cleared for farming between the wars. After the Council had bought it for catchment land it had reverted to scrub - some of it high kanuka by the 1970s. The vegetation had to be removed from the lake area for water quality reasons. The task was performed by staff. The initial task was cutting a waterline road then felling the vegetation within the lake area and burning it. The burning was done in stages. One fire got out of control and burned a small area of bush above the waterline.

	The lake partly cleared. There were two ARC Land Rovers allocated to the site. They were short wheelbase Series III models bought for the project. During clearance slit trenches an ammunition cases were found, from the time in WW2 the valley was used by US troops for training.
	Burning the felled trees, cut below the waterline track - starting the fire at the top, which allowed better control. The blue Ford Cortina was the resident Engineer's car
	Helicopter on fire control
	The diversion weir lake as a source of water. The waterline track marks the edge of the clearing.
	Screamer fire pump
	After a burn cutting up remaining logs to pile and burn. A dirty job.
	Pump duty - note the hoses feeding up the hill beyond the stream. Peter Andrew, one of the ARC caretakers.

Moumoukai:

	The Mangatawhiri Valley, over the hill from Wairoa with Moumoukai in the middle distance and Upper Mangatawhiri Dam beyond it.
	The Moumoukai houses in the Mangatawhiri Valley where several of the staff working on the dam lived and where there was a depot and garage to support the Hunua catchments' construction.
	The house at Moumoukai where I lived.