Thirteen options were considered when we looked at the best way to get the pipe across the river. Open trenching was highlighted as one of the least favoured options by consultants due to the risks associated with this method.
In addition, when the council issued the public registration of interest (ROI) to the market, it specifically stated, "should there be a contractor who believes they have the capability and a proven track record to open cut trench across the river then this method will also be considered".
No applications of interest were received to install the pipe using this method.
Jac McIntyre refers to rule 27 in the Regional Water Plan which allows a "simple trench" to be laid across a water body without consent from Environment Southland. Unfortunately, getting a trench across Mataura River would not be simple. The advertisement mentions the conditions in 48A and B which would need to be met under this rule and unfortunately, an open trench across the Mataura would be unlikely to meet these conditions.
According to expert advice, open trenching of a large water body is complex and comes with significant risks and disruption to the water body to manage these risks.
Essentially the work would need to be carried out in sections, each section requiring a small dam (sheet pile) to redirect water around that section to enable a dry work area. This would involve driving large metal sheets into the riverbed.
Given the size of the river, volume of water and propensity to rise suddenly in rainfall events, the works would potentially need a widening of the river at the opposite bank or a complete bypass channel to be created on the floodplain. It is unlikely that this method would be palatable to iwi or the regional council.
A further complication is if the river were to flood during works, even with these staged areas in place, there would be a risk of compromising the work already done and having to start over, adding more cost to the project.
Horizontal directional drilling — the method we propose to use — is a widely accepted method for getting pipes across major water bodies and environmentally sensitive areas. It has the least disruption to the riverbed and is not disrupted by flood events.
The council has sought advice from consultants with significant expertise and experience in managing these types of sites.
Total costs and the ability to manage both project risks and environmental risks are considered when we select a method.
This council is committed to getting the best value and the best outcomes for our community.
— Gore District Council chief executive Debbie Lascelles.
Ms Lascelles was replying to an advertisement in The Ensign last week about the council’s plan rule 27 about cables, wires and pipes.
