Point of access; Double Hill Road – 500m on right beyond gate below Forest HQ. 45 minutes return. Moderate.
The Seacliff Mental Hospital was competed in 1884 and at the time was the largest public building in New Zealand. Water for the complex was originally supplied from a spring behind Warrington. However, as the hospital developed, this supply became inadequate. An alternative supply was sought and it was decided that a dam and pipeline at Double Hill would be suitable.
As a consequence, this dam and an associated pipeline were built around 1912 for the purposes of supplying water to the Seacliff Hospital. The dam is some 6m high and approximately 1.5m thick at the base. It is estimated that 80 cu m of concrete were necessary to build the structure. In addition, the construction required 17,000m of 150mm pipe to link the dam with the storage tanks at Seacliff. The flow was gravity fed, the dam being at a greater elevation than the hospital.
All equipment to build the dam and pipeline was packed into the upper reaches of the Whaitiripaka Stream on horseback or horse-drawn sled. The pipeline was laid along the stream-bed to Evansdale Glen and thence over Porteous Hill to Omimi and Seacliff.
Problems were associated with the scheme however, primarily due to silting up of the dam and a subsequent loss of capacity. The tragic fire at the Seacliff Hospital in 1942, which resulted in the loss of 37 lives prompted greater concern over the reliability of the Double Hill water supply. Corrosion of the pipeline and unstable ground over Porteous Hill compounded the problems.
An auxiliary water supply was installed at Evansdale in 1944 to augment the supply during the summer period. However the condition of the pipeline continued to deteriorate. Major repairs to the pipeline were considered too expensive and in 1952 it was replaced by a new line from Cherry Farm. Thus ended the useful life of the Double Hill dam and water supply.
Since 1981 the N.Z. Forest Service staff have improved access to the damaged sections of Whaitiripaka Stream where remnants of the water supply scheme remain. The walk to the dam is quite short and well worth a visit in light of its historical background.