Matt Todd
With silt seeping into the Waiau River as a result of damage to Eastland Group’s Waihi hydro electricity scheme near Wairoa, I wanted to share the background to this problem with The Gisborne Herald’s readers and also reassure you that we are doing all we can to find a solution.
Eastland Group has owned the Waihi hydro scheme since it was purchased in 1999 as part of our Wairoa Power (electricity network) acquisition. Located at Lake Ruapapa on the Waihi River 26km north west of Wairoa, the dam was completed in 1986. Its reservoir is 5.2km in length, with an area of 42ha holding almost two million cubic metres of water.
Annual electricity output is around 11GWh — enough to power about 1400 houses. The dam has one intake gate and three radial arm flood/sluice gates built into the face of the dam. Their function is to open automatically in times of high lake water level, to drop the lake level and relieve pressure on the dam itself.
During the extreme rains and flooding of September 2015, all three gates opened as they should to cope with the additional inflow, but even then the water was flowing over the top of the dam. Along with the water, large amounts of silt, logs and other debris were carried downstream and these rendered the gates inoperable. Over the subsequent weeks more silt and debris compounded the problem and clogged the gates up completely. During this time we started to work on ways of repairing the gates and getting them fully operational.
Diving and engineering specialists in hydro dam maintenance carried out a full underwater survey of the gates and confirmed the damage to them. They advised us that the scale of damage meant the gates must be removed and rebuilt in a suitable workshop.
Hawke’s Bay Regional Council (HRBC) was notified of the issues regarding the gates and the proposed action to remedy the situation. During December, our team worked seven days a week on the gates and managed to get one of the three sluice gates open just before Christmas. For safety reasons the gate was initially left in the open position, but in compliance with the abatement notice for HBRC we attempted to close it. However, the damaged gates are still not sealing properly and water that would normally be captured in the reservior is flowing through, taking silt with it and discolouring the Waiau River system.
Eastland Group then engaged additional internationally-recognised engineers to help us plan the best way to return the dam to a workable state and how to dispose of the sediment built up behind the dam in a manner that will minimise the environmental impacts.
No work will be undertaken without the agreement of HBRC. Some of the remedial work may require variations to existing resource consents or new resource consents, and we will work with HBRC to obtain them if these are necessary.
We hope to get started within the next few days. Unfortunately this will not be a quick fix — it is going to take some time to complete the works and more silt is likely to be released in the meantime.
Once again, Eastland Group apologises for any inconvenience. We are working as hard as possible to remedy the situation at the Waihi Dam.