A hidden relic of the past, this 19.2-metre-high concrete well and its 43-metre brick-lined tunnel remain concealed by dense forest near Reservoir Road.
Colemans Tunnel was built by William Frank Oakes around 1882 as part of his contract to build a covered concrete culvert (raceway) for £15,000. But the tunnel section was lined with bricks by John Coleman, after whom the tunnel was named. The well and tunnel was abandoned by 1899 due to the old raceway’s inefficiency, as it leaked badly. The raceway and tunnel were later bypassed with pipes running from the Lower Reservoir Dam to the existing pipeline at the tunnel’s terminus.
The concrete well, standing at 19.2 metres (63 feet) high, remains intact today. But its brick-lined tunnel, extending 43 metres, is hidden from public view. In the early 1980s, the tunnel entrance was sealed with bricks and later buried under rocks. Over time, thick scrub and vegetation have completely concealed the site, making it difficult to locate. The tunnel entrance was situated near Reservoir Road with the tunnel going under the road.
The well served as a water conduit, and the attached horizontal tunnel directed the water into a pipe. Beginning at the dam, water flowed along concrete raceway, driven by the weight of the water in the dam and the gravitational force acting on the pipeline. Upon reaching the well, it simply poured down the shaft maintaining a descent. Consequently, when the well reached capacity and potentially overflowed, it served dual purposes.
Primarily, the shaft offered volume and gravity-driven water pressure to boost the water flow in the pipeline to Wellington. This pressure enabled the water flow to ascend over the Wainuiomata Hill to Petone.
Secondly, it played a crucial role in barring the introduction of air into the initial section of a sealed pipeline. The presence of air, being compressible, poses significant challenges by generating considerable back pressure and triggering substantial hammering issues in plumbing. If not properly addressed, these problems can potentially lead to severe fractures.
Further, along the pipeline, various air traps and relieving valves were strategically placed. These installations aimed to address the inevitability of some air ingress, necessitating the bleeding out of air pockets at high points throughout the pipeline route.
Credits
Special thanks to Jeremy Foster for some information in this article.
Special thanks to Tony Weir for his help with explaining the function of the shaft.
