My Sites  > Wairoa Dam Construction

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


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


A modern view of Wairoa Dam -  Simon Lieschke

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.


Wairoa Dam
Completed:                    1975
Lake area:                      98 hectares
Capacity:                        11.6 Gigalitres
Catchment area:              1303 hectares
Dam height:                     47m
Dam type:                       Earth fill
Fill volume:                     685,000 m3
Service spillway:             Bellmouth
Spillway capacity            122 m3/s
Mean yield:                     32Ml/d
Contractor:                     Downers
Contractor's Engineer:    George Wyman
Cost:                              $3.9M

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.

scan0014.jpg (947962 bytes) scan0014x.jpg (2618026 bytes) scan0017.jpg (2352529 bytes) scan0018.jpg (3562325 bytes) scan0019.jpg (3480056 bytes)
scan0020.jpg (3366340 bytes) scan0021.jpg (3621162 bytes) scan0022.jpg (3594980 bytes) scan0023.jpg (683815 bytes) scan0024.jpg (1183321 bytes)

The brochure produced for the opening (thumbnails).


Comments and contributions are welcome!

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.  


plan.jpg (581066 bytes)
Plan and section - thumbnail - the information on the control valve chamber is incorrect.

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. 


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.

ARA Team:
Resident Engineer:           Eric Scanlen
Earthworks Engineer:      John Crabtree
Structures Engineer:        Garry Law
Surveyor:                        Phil Salmon
Laboratory Technician:    Peter Thomas
Assistant Engineer           Neil Boys
Survey Assistant              Chuck Bird


Construction Sequence  
Tunnels / spillway / intake Dam

The diversion tunnel was driven including the spillway up to the elbow but not including the shaft. 


Tunnel lining was completed, not including the elbow


The trench for the delivery pipe was excavated in the tunnel floor and the pipe laid in it and concreted in - integrally with the tunnel floor.

Site stripping commenced

 The valve tower base was built (tendon anchored into the rock)

The temporary supply was diverted through the supply pipe as far as the downstream portal

The stream was diverted through the diversion tunnel  

- Site stripping completed
Spillway shaft excavated from above Dam earthworks
Liner for upper spillway shaft set into fill
Valve tower slipformed
Valve tower bridge completed
Spillway elbow lining, shaft lining and bellmouth crest completed.
Bottom outlet pipe put in place, weir off take diverted through it, tunnel plug shuttered up and poured  
Valve tower pipework completed, supply pipeline completed   
Bottom outlet gate valve closed - lake filling commenced  
Bottom outlet discharge valve installed  
Tunnel junction training wall and spillway stilling basin constructed  


The pictures following are all thumbnails.

scan0005.jpg (311405 bytes) 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.
scan0004.jpg (558266 bytes) The stream diverted and the dam part finished - the valve tower at full height but with no bridge.
scan0006.jpg (567282 bytes) 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.
scan0001.gif (223277 bytes) The dam nearing full height in the second earthworks season
scan0012.gif (387189 bytes) West abutment from the east - during site investigation


scan0002.gif (319457 bytes) 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.
scan0006.gif (271922 bytes) Tunnelers - John Crabtree second from left.
scan0009.gif (352845 bytes) Damaged photo of the tunnel junction form
scan0008.gif (331778 bytes) Shifting the tunnel form
scan0010.gif (295011 bytes) Taking pipe into the tunnel - taken from the tunnel portal - the form is parked to the right
scan0011.gif (256617 bytes) Placing pipe in the tunnel floor.

 Dam Construction:

scan0013.gif (305823 bytes) Site stripping commencing - photo from upstream. The miners adit into the manganiferous rock can be seen at the right.
scan0014.gif (255476 bytes) Later in site stripping.
scan0003.gif (375240 bytes) Downstream shoulder stripping
scan0005.gif (351330 bytes) Core trench, covered drains from springs to the pump well
scan0016.gif (286347 bytes) Spring pump well
scan0015.gif (307293 bytes) Right abutment shaping
scan0017.gif (388754 bytes) Grout pipes to close pump wells
scan0019.gif (345806 bytes) Filling
scan0020.gif (360283 bytes) Down stream shoulder filling
scan0021.gif (366565 bytes)  
scan0023.gif (355026 bytes) Piezometer tubes
scan0085.gif (449052 bytes) Dam site Panorama
scan0024.gif (266283 bytes) General view looking downstream - early in the fill placement
scan0025.gif (376572 bytes) Compacting the filter drain - constructed by excavating a trench through fill down to the last lift.
scan0027.gif (374261 bytes) Filter drain construction
scan0028.gif (295924 bytes) Filter drain construction with core placement upstream
scan0029.gif (309483 bytes) Filter drain construction - spillway shaft to the left
scan0030.gif (363912 bytes) The west abutment - soft sensitive alluvial material required stripping her to get to competent rock and weathered rock

 Valve Tower:

scan0007.gif (324122 bytes) 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
scan0031.gif (385654 bytes) Link section from the upstream portal to the valve tower. Inverted siphon on temporary supply pipe goes under.
scan0032.gif (344142 bytes) Reinforcing tieing for the valve tower base
scan0033.gif (398796 bytes) Shuttering for the valve tower base.
scan0036.gif (339379 bytes) Rock anchor tendons.
scan0034.gif (380098 bytes) Anchoring, pre-diversion. The weir supply pipe diverted into the tunnel
scan0037.gif (322311 bytes) Slipforming the valve tower underway
scan0035.gif (374391 bytes) Valve tower top
scan0039.gif (272426 bytes)  Installing an intake gate valve
scan0038.gif (261212 bytes)  Concreting in an intake in the valve tower
scan0040.gif (290047 bytes) Fixing the lake level markers to the valve tower
scan0041.gif (339421 bytes) The bottom outlet pipe in place. 
scan0042.gif (286235 bytes) Pouring the tunnel plug in the valve tower base.
scan0043.gif (220571 bytes) About to close the bottom outlet gate valve - from the left Bruce Smithson, Don Wilson, John Crabtree, ?Keith Hay, Fergie Schiska, ?(ARA photographer), ?.
scan0044.gif (211720 bytes) The bottom outlet closed, Eric Scanlen facing the camera, George Wyman on the right on the platform.  
scan0045.gif (274233 bytes) Installing the discharge valve on the bottom outlet.

Valve Tower Bridge:

scan0048.gif (344024 bytes) Foundations for the bridge pier
scan0049.gif (431927 bytes) Pier bases
scan0050.gif (240261 bytes) Pier construction from the tower.
scan0051.gif (343612 bytes) Pier construction
scan0052.gif (260860 bytes) Pier construction
scan0053.gif (281466 bytes) Concrete pour for the pier
scan0054.gif (217851 bytes) Launch beam
scan0047.gif (303381 bytes) Delivery of a Tee beam for the valve tower bridge
scan0055.gif (284933 bytes) Ditto
scan0056.gif (276872 bytes) First beam ready to launch
scan0057.gif (245845 bytes) First beam about to be lowered
scan0058.gif (206586 bytes) Ditto
scan0059.gif (405716 bytes) Third beam
scan0060.gif (361155 bytes) Last beam in place

Delivery pipeline:

scan0074.gif (355175 bytes) Delivery pipe for the downstream portal to the control structure under way
scan0080.gif (330949 bytes) Supply pipe being laid
scan0061.gif (353869 bytes) Control valve chamber inner form. Link structure between the two tunnels in the background. Damaged photo.
scan0062.gif (257653 bytes) 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..
scan0063.gif (365024 bytes) Completed control structure


scan0064.gif (235587 bytes) Piezometer house under construction
scan0067.gif (330775 bytes) Log recovery
scan0068.gif (352276 bytes) Hydroseeding
scan0046.gif (240240 bytes) Rock facing on the lower part of the upstream embankment
scan0069.gif (397465 bytes) Junction chamber - outlet to left
scan0070.gif (384809 bytes) Inlet from dam berms to right, one collector of two from the downstream shoulder drainage blanket below - perforated pipe. 
scan0071.gif (357529 bytes) Proof grouting under the core - from the east abutment.
scan0072.gif (348628 bytes) Quarry
scan0073.gif (343388 bytes) Crusher / screening plant
scan0076.gif (234729 bytes) Garry Law in the piezometer house.
scan0077.gif (297989 bytes) John Crabtree, ?, Graheme White. Phil Salmon, ?, ?


Graheme was a local contractor operating bulldozers forming the waterline track during lake clearing..

scan0971.jpg (6559103 bytes)  
scan0078.gif (270676 bytes) John Crabtree and George Wyman
scan0075.gif (335450 bytes) Messy!
scan0079.gif (341734 bytes) Guests after valve closure. Bruce Smithson, Eric Scanlen, Fergie Schiska, Don Wilson, maybe Graheme Tait

scan0081.gif (341942 bytes)

Downer's fleet


scan0066.gif (368588 bytes) Spillway elbow shutter - in two halves second in back right.
scan0065.gif (218797 bytes) Spillway outside former
scan0082.gif (305447 bytes) Stilling basin excavation
scan0083.gif (338791 bytes) Stilling basin excavation
scan0084.gif (374448 bytes) Stilling basin construction

 Spillway model:

scan0001.jpg (80697 bytes) 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.

Photo0005.jpg (88184 bytes) 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.

Photo0002.jpg (83379 bytes) 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
Photo0004.jpg (78556 bytes) Helicopter on fire control
Photo0009.jpg (125852 bytes) The diversion weir lake as a source of water. The waterline track marks the edge of the clearing.
Photo0003.jpg (90087 bytes) Screamer fire pump
Photo0007.jpg (97203 bytes) After a burn cutting up remaining logs to pile and burn. A dirty job.

Photo0011.jpg (145934 bytes)

Pump duty - note the hoses feeding up the hill beyond the stream. Peter Andrew, one of the ARC caretakers. 


Photo0008.jpg (65899 bytes) The Mangatawhiri Valley, over the hill from Wairoa with Moumoukai in the middle distance and Upper Mangatawhiri Dam beyond it.
Photo0006.jpg (81274 bytes) 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.
Photo0010.jpg (111938 bytes) The house at Moumoukai where I lived.