Cosseys Creek Dam
Cosseys Creek Dam, completed in 1955, was the first of a series of rolled earth water supply dams in Auckland's Hunua Ranges. Rolled earth dam design in New Zealand was significantly advanced by these dams constructed between 1950 and 1977.
Cosseys Creek Dam is situated in a young gorge valley about 2.5 km upstream of the Hunua Falls. At this point the stream was originally incised in its gorge about 62 m upstream of the site. At the time of construction, the valley was in its original mature state with very favourable storage conditions.
The dam was built using rolled fill consisting entirely of residual clays and rubbles from greywacke-argillite rock. All materials were existing and within a haul distance of five km. The clays and clay-rubble for the core and rock rubbles at the faces were easily selected at the borrow area, and the core construction readily controlled by the use of standard moisture content and compaction density checks. Use of the newly introduced Euclid T24 earthmovers, and tractor-drawn sheep’s-foot rollers made for speedy completion – 411,000 cubic metres of material in 190 placing days over three seasons.
Diversion of the stream took place during construction through a 4.3 m tunnel, 235 m long, which was subsequently sealed with the valve tower and its five intakes. A bellmouth type spillway feeds to a spillway tunnel discharging to a lined channel below the downstream toe of the dam. The spillway capacity is 15,000 cusecs (441 cumecs).
Filling of the reservoir commenced on 13 May 1955, and was completed by 12 July 1956 following heavier than normal rainfall.
Crest length: 168 m
Height above foundation: 41.2 m
Catchment area: 2,200 ha
Safe yield: 40,800 cu.m./day
Reservoir area when full: 119 ha
Storage volume: 14.460,000 cu.m.
Hunua Ranges Regional Park, Auckland.
The dam and reservoir can be seen from the Cosseys-Massey Loop walk. Access information is available on the Auckland Council website.
Additional image gallery details
Cossey's Dam on the headwater of the Wairoa River, Hunua Ranges, Auckland Region [12 February 1965]. Whites Aviation Ltd: Photographs. Ref: WA-63447-F. Alexander Turnbull Library, Wellington, New Zealand.
Permission of the Alexander Turnbull Library, Wellington, New Zealand, must be obtained before any re-use of this image.