Morton Buttress Dam
The Morton Dam, in the hills of Wainuiomata, was completed in late 1911 and decommissioned from Wellington's water supply system in October 1988.
By the mid-1880s the water supply provided to Wellington city from the Lower Karori Dam was added to by a small dam in the Wainui headwaters. The augmented supply nevertheless was soon insufficient to meet demand. In 1900 the City Engineer R.S. Rounthwaite recommended further works, which included constructing a new concrete dam in the Wainuiomata headwaters, at a site known as Solomon’s Knob. This was endorsed by a 1902 special Commission, but rejected by ratepayers. It was not until 1904 that ratepayers were prepared to accept the endorsement by City Engineer, William Hobbard Morton, of Rounthwaite’s proposals, and tenders were finally received in mid-1908.
The valleys east of the Wellington faultline have been tilted westwards and built up with sediments as a result. At Solomon’s Knob a buried river channel existed, which no doubt influenced Morton to use a concrete buttress dam, therein spreading the water load on the foundation. A concrete cut-off wall had to be excavated down into the buried channel to control leakage under the dam.
The dam is 164 m long and at the spillway crest is 12.5 m above the floor of the reservoir. The buttresses are spaced along the dam at about 3.6 m intervals, giving the dam a cellular appearance from downstream since the “cells” are open on that side except at the spillway.
Of the tenders received to build the dam, O’Donnell's was accepted. However, this tenderer withdrew in October 1908 on the grounds of errors made in pricing. With some reluctance the Council agreed and awarded the contract to Messrs Martin, Hurrel and Snedden. A cableway with two moveable tail towers was among the equipment used in building the dam. The contractors were still working on the dam in July 1911, but by November the Mayor reported in Council that the dam had been completed.
In 1988, after nearly 80 years’ service, it was considered that the dam did not comply with modern safety standards. Decommissioning of the dam was achieved by removing a section of the dam wall next to the spillway. Most of the structure remains intact as a memorial to its past. New water storage was created at Te Marua beyond Upper Hutt.
Images of the Morton Dam, and other Wellington water supply features, by Simon Oldham, are viewable on the Panoramio website.
Wainuiomata Water Collection Area, Wainuiomata, Wellington.
Access is restricted – permission may be obtained from Greater Wellington, the Regional Council.
Peter Cooke, Our Water History - On Tap: Water supply in the Wellington region, 1867-2006, Wellington, 2007.
Additional image gallery details
The water supply of Wellington City: A general view of the recently-completed dam and reservoir at Solomon's Knob, Wainui-o-mata, 9 November 1911. Sir George Grey Special Collections, Auckland Libraries, AWNS-19111109-14-4.