Waihi dam sluice gate damage and repair

Waihi dam | Sluice gate damage and repair

The Eastland Group-owned Waihi dam, which sustained damage during a severe weather event in September last year, has now been repaired and will shortly resume full function as a hydro-electricity station generating enough power for 1600 homes.

The damage to the dam, which is 26km north-west of Wairoa, resulted in silt being released into the Waiau River system. Understandably, there are some people in the area who are concerned about the impact this event has had on the river.

However, there has been some media coverage in recent weeks and months in regional and national outlets which does not accurately reflect the work that has been undertaken to repair the sluice gates and get the dam operational. A team of engineers, divers and other expert personnel, has been on site since the HBRC issued a consent allowing repair work, and that team has worked in arduous and dangerous conditions to ensure repairs took place as urgently as possible. Temporary gabion baskets were also installed downstream from the dam to catch sediment prior to entering the Waiau River, with silt collected in the area transported to council approved clean-fill sites on nearby farmland.

Eastland Group has consistently maintained open lines of communication with all key stakeholders including local and regional Councils and members of the farming and local community, with a view to supporting affected parties where possible with up-to-date information.

Despite the ongoing efforts we have made to keep all relevant parties informed, many of the comments that have been made to media are incorrect. Since last year, we have been in communication with the Mayor of Wairoa, other members of Council and residents who have affected by the incident.

Matt Todd, CEO of Eastland Group was yesterday offered the opportunity to be interviewed by Radio New Zealand or to make a statement. The latter was supplied and the reporter read the statement back to Mr Todd to confirm her understanding. At no time did Mr Todd decline to engage with Radio NZ.

The Eastland Group team would like to reiterate that we sincerely regret the sediment releases that have occurred over summer. Now that repairs are complete, all three of the dam’s sluice gates are closed and the reservoir is now full.
Eastland Group will shortly be recommencing generation of electricity.

To date, Eastland Group has not received any substantive claims for damages – if we receive such claims they will be dealt with responsibly.

It is difficult to be more exact at this point as there have been a number of wide ranging statements in the media, but as yet no information has been supplied to Eastland Group to support the claims being made.

It is our understanding that over the summer period the water treatment system in Wairoa has still been fully operable although there may have been additional treatment required.

It is hard to determine whether there has been any ecological damage to the Waiau river system although mullet were observed in the Waiau River in the vicinity of the Waihi powerhouse earlier this week. It is important to note that these are normally high sediment river systems and, with the winter rains coming, any residual sediment is likely be moved through quite quickly.

Hawke’s Bay Regional Council is undertaking an investigation into this event and they have our full cooperation. They are the appropriate regulatory body to undertake this work. Eastland Group has also initiated a project with HBRC to consider whether there need to be changes to resource consent conditions and operating procedures to help ensure this sort of situation is avoided in the future.

ENDS

ADDITIONAL INFORMATION

Outlined below are details of the event which caused damage to the dam gates, the process of repair and the work that has been undertaken to resume operations. This information was supplied to the HBRC.

September 2015 weather event

The storm that hit the Waihi Dam catchment area in late September 2015 was a significant and severe weather event. Eastland Group recorded 196mm of rain across 20 and 21 September, and inflow levels upstream of the dam rose from 0.189m to at least 1.919m during the storm. While we do not have access to records of rainfall in the catchment area upstream of the dam, one upstream farmer has anecdotally advised us that he recorded rainfall of 400mm in 24 hours.

Considerable debris travelled down the Waihi stream, overwhelming the log boom for the dam, and a large amount of sediment and debris was pushed up against the dam. The damage to the surrounding area was extensive, which is highlighted by the fact that we were not able to access the dam immediately following the storm because of trees blocking the access road. Other infrastructure in the area was also affected, such as roads and bridges, as seen in local media at the time. The three sluice gates to the Waihi Dam suffered varying degrees of damage as a result of this event.

Damage to the gates

The damage to the Waihi Dam sluice gates was unprecedented and entirely unexpected. Prior to the September 2015 event, and since acquiring the Waihi Dam in 1999, Eastland Group had not experienced any problems with the operation of the sluice gates.

In all previous storm events and times of high inflows our experience has been that the gates have operated as intended and as provided for in the relevant consents. That is, the gates have opened to (a) release water during periods of flood and (b) release any sediment and or debris that was being carried by flood waters or that had built up behind the gates. From 2010 our data indicates that the sluice gates have operated up to 10 times per year, though some years only infrequently.

The nature and extent of the damage to the gates was not immediately apparent in the days and weeks following the September storm. However, given the importance of the sluice gates to the safe operation of the dam, we closely monitored their operation following the storm event. When the extent of the damage became clear, and the risks that posed to the safe operation of the dam, we promptly sought expert assistance, engaged experts to assess what was required to fix the gates, and got the recommended repairs underway.

We engaged with the HBRC in November 2015 to inform them of the damage that the gates had sustained and the repair work that was required. As is described in more detail below, the HBRC gave approval to carry out the repairs and indicated it did not require anything further from us.

On 24 December 2015, part way through the repair work, the HBRC issued an abatement notice that required us to close (and not re-open) the gates, stopping repairs being made to the gates.

The notice also required that we engage an independent engineer to provide a report by 29 January 2016 outlining recommended steps to be taken to remove and dispose of the sediment built up behind the dam. As advised to HBRC at the time, it was not however physically possible to close the gates at that point, given the damage sustained and the stage that the repairs had reached. Having received this advice HBRC then confirmed Gate 3 could remain open. A new abatement notice, (essentially the same as the first), was issued on 7 January 2016. Eastland sought to comply with that notice, but Gate 3 could not be closed.

The HBRC refused to allow any further repair work to be done on the gates until 15 February 2016, when, following receipt of the engineer’s report on 26 January 2016 and a subsequent resource consent application shortly thereafter, it issued a resource consent permitting some in situ repairs to be made. Those repairs (which are effectively the same works as those we had originally commenced) are now complete.

We have at all times acted responsibly in response to what is unprecedented and entirely unforeseeable damage to the Waihi Dam. We have taken all steps possible to fix the damage caused by the September event, as quickly as possible, and to address effects flowing from that.

Purpose of the sluice gates

In order to understand the difficulties that Eastland Group has faced in identifying the damage to the sluice gates and the importance of the repair work, it is necessary to understand the purpose of the gates, the important role that they play in dam safety, and the way that they are designed.

The three sluice gates are essential for the safe operation of the Waihi Dam. Without properly operating sluice gates, it is not possible to release sediment laden water and debris from the bottom of the dam in times of high flow, which can compromise the structural integrity of the dam structure. That ‘sluicing’ function is their purpose, and they are consented to operate in that way under the relevant resource consents held for the dam (being ‘grandfathered’ resource consents originally granted in 1979). Experts have instructed us that, in order to operate the dam safely, it is necessary to have at least two of the three gates operational at all times.

The Waihi Dam has some some key differences to other New Zealand hydro dams:

• The sluice gates on the Waihi Dam are, somewhat unusually, located at the very bottom of the dam face. It is impossible to conduct a detailed inspection of the gates when the dam is operational because the gates are entirely under water; and

• The sluice gates are also located at the lowest point of the dam. Any sediment that builds up behind the dam will build up behind the sluice gates. Consequently, if there was an issue with sediment build up behind the dam prior to the September 2015 event, we would have expected to have had difficulty operating the gates in previous periods of high inflow. That has not been the case.

In the days following the September event, we adopted the measures that have at all previous times been successful in clearing and sealing the gates following floods:

• In respect of Gate 3, we opened and closed Gate 2 (the adjacent gate) in an attempt to dislodge sediment/debris around Gate 3;

• In respect of Gate 2, we opened and closed the gate in an attempt to remove what was believed to be debris caught in Gate 2. We used grappling hooks to try to dislodge any debris that was caught upstream of the gate.

Following a dive inspection on 3-4 November, we had a clearer understanding of the possible damage to all three gates. Having established that the safety requirement to have at least two operational gates was compromised, our engineers worked with Greenfield Diving Services to agree what remedial works should be carried out on the gates. A plan was developed and agreed, during a number of meetings and discussions between Eastland Group and Greenfield Diving Services, to remove the gates, fully repair them and return them to work. This plan was agreed on 9 November and steps were taken to get the necessary experts onsite to commence the work as soon as possible.

Eastland Group informed the HBRC on 19 November 2015 of the issues that had been identified during the inspection of the gates on 3-4 November and the repair work that it planned to carry out.

In addition to liaising with Greenfield Diving Services, we also attempted as early as 11 November 2015 to engage a relevant dam engineering expert to assist with repairing the gates. The number of relevant experts in New Zealand is very limited however and, in light of the approaching Christmas holiday period, the first time a dam engineering expert was able to attend the dam and provide independent advice was 16 January 2016, when Grant Murray Associates was formally engaged.

We have, at all times since the September 2015 event, been focused on ensuring the safe operation of the dam. We have acted as quickly as possible to investigate all issues identified with the sluice gates, and to work with appropriate experts to determine a solution to return them to operation. It cannot be overstated that it is essential that the sluice gates are in proper operation for the safety of the dam.

Eastland Group has a regular maintenance program for its operation of the dam. This program includes:

• Comprehensive Safety Reviews of the dam carried out by independent experts every ten years (last CSR was in September 2014), which include consideration of the impact of sediment on the dam. Eastland Group has implemented the recommendations set out in these reviews;

• Intermediate Safety Reviews carried out by independent experts approximately two to three times in the years between Comprehensive Safety Reviews (last ISR was in April 2013). Again, Eastland has implemented the recommendations set out in these reviews;

• Regular inspections of the gates by specialist divers, usually scheduled to coincide with the Intermediate Safety Reviews;

• Periodically opening the gates during times of high flow (as permitted by the resource consents for the dam), which allows any sediment or debris that builds up behind the gates to be cleared;

• Conducting regular (five yearly) deformation surveys (last survey completed in March 2014);

• Implementing an extensive water monitoring program at the dam, designed to pick up structural issues with the dam. This involves monthly monitoring, recording and analysis of water level data from standpipe collection points installed in the dam abutment, penstock slope, tunnel cut section and the Waihi Stream bank;

• Carrying out a profile survey of the dam in 2013-2014.

Eastland Group has also actively managed sediment within the reservoir by implementing a generation and production regime that primarily utilises the top 1/3 of the water in the reservoir. This regime helps ensure that any sediment deposits are undisturbed by the use of water for generation.

Eastland Group has sought to engage with the HBRC in relation to the conditions of the resource consents held for the Waihi Dam.

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