Crisis struck this year's spring release on the Mangahao River. Heavy rains caused the dam to spill on the day before the release scheduled for Saturday 30 October. The generating company operating the Mangahao site are allowed to call off a release due to adverse hydraulic conditions (drought or flood). Cancelled releases mean that an extra flow date is provided the following year.
The power company feared that a spill was likely during the release. Deciding that this would present an unacceptable risk to kayakers they gave notice on Friday that the Saturday's release would not proceed as scheduled. Their point of contact is the Palmerston North Canoe Club. The word went out on the grapevine from there and hit Wellington that evening.
The put-in for the Mangahao is a long way from anywhere and the shuttle is a 90km drive. In today's climate of rodeo boating a Mangahao trip is no mean feat of organisation. It means leaving town early, driving miles on back roads with sketchy directions, waiting hours in a paddock and getting back late. Any paddlers that actually turned up at the river have just cause for insane rage and consideration of extreme measures like converting to gas and solar to get back at the power companies.
The Mangahao power scheme is one of the oldest operating in the North Island. Water is taken from the Mangahao River and discharged through the powerhouse into the Mangaore Stream on the other side of the hill. During generation the sum of the natural Mangaore flow and the tail-race flow is not to exceed 70 cumecs. During a flood when the dam inflow exceeds the allowable generating capacity the reservoir level rises until water fills a counterweight which opens a totally automatic and otherwise uncontrolled trip gate. This large wooden structure takes up a significant proportion of the dam face and literally releases a wall of water down the natural river bed, an artificial flash flood potentially up to 300 cumecs. Our recreational flows of 30 cumecs come via an-other structure, the bypass tunnel.
I've never seen the gate trip but I have heard stories of a couple of runs made when this was happening. Prior to the current resource consent being negotiated the only way to paddle the Mangahao was while the dam was spilling. General consensus is that this is quite dangerous. Small groups of half a dozen or so who are fully aware of the conditions have so far been able to get away with running spill flows without tragedy. However a spill while the regular Mangahao crowd of a hundred plus paddlers are on the river would be an invitation to an Interlaaken-style disaster.
It gripes me that I flew down to Wellington with my boat and ended up paddling the Hutt with my friends who came across the Strait for something special. However, had we all turned up and had there been water, even with the certain knowledge of a spill, how many would have forgone the trip? It is difficult to imagine every paddler in the regular crowd making an informed and accurate judgement of their abilities and the conditions in the event of encountering spill flows. The dam did trip on Saturday at about 6.00pm, this would have presented little actual risk to paddlers.
The cancelled Mangahao release highlighted problems in the way that the power company informed kayakers of the flow. Discussions are underway between the NZRCA, the Palmerston North Canoe Club and the power company on how releases dates are decided and publicised. Watch the website rivers.org.nz for details. See you in autumn and twice next spring.
12 September 2000: An extra release has been scheduled for the Autumn of 2001.