The Fernhill rising main bursts - What went wrong and what was done about it?

In July and August 2009 a rising main in the Fernhill water system in Queenstown burst on three separate occasions. The bursts resulted in significant damage due to washout of tracks and scouring around a reservoir, with significant costs being incurred for repairs of both private and council-owned infrastructure.

An investigation to determine the likely causes of these three bursts indicated two issues were of concern: the condition of the rising main itself, including the material it was made of and its bedding; and the absence of automated shut-off of the pumps and upstream reservoir in the case of a burst. The results of a pressure analysis of the rising main indicated operational pressures nearly equal to the pressure rating of the pipe. In addition a surge analysis indicated the potential for negative surge pressures in the area of the rising main where the majority of the bursts occurred. This could result in collapse of the main.

The Queenstown Lakes District Council has put in place a number of upgrades to the system, including the installation of automatic burst control valves on both rising mains and automatic shut-off of the pump station in the case of a burst. These upgrades were tested when a fourth burst of the rising main occurred shortly after commissioning.