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An update on the bridges and the development of a district-wide bridge strategy was presented to councillors at a meeting of the Central Otago District Council in Alexandra yesterday.
Council roading manager James McCallum presented a report on the bridges: Bridge 121 at Scott Lane, spanning the Kyeburn River, and two bridges that cross the Taieri River — Bridge 145 in Maniototo Rd (Halls Ford) and Bridge 160 in Linnburn Runs Rd.
All three bridges were closed due to damage caused by flooding as a result of heavy rain from January 1-4.
In his report, Mr McCallum said the flooding accelerated existing deterioration on the old timber structures, which were already nearing the end of their useful lives.
The council did not have in-house staff to undertake structural inspections and due to the damage in Linnburn Runs and Maniototo Rds it outsourced that work to specialist engineering consultant Beca.
The structural inspections had taken longer to complete than anticipated due to vacancies in the roading team and demand on a small resource pool of structural engineers in the South Island.
That was further complicated by flood events in Canterbury and Marlborough, he said.
There had been further delays due to high flow in the Taieri River and the second Covid-19 lockdown.
Initial inspections found at least one of the primary load-bearing timber beams on the 91-year-old Maniototo Rd (Halls Ford) bridge showed significant cracking along its length.
It was near the end of its useful life and had a traffic weight restriction before the flooding. A principal inspection would determine if it could be repaired or, as the name ‘‘Halls Ford’’ suggested, a ford could be constructed in its place, Mr McCallum said in his report.
At the Linnburn Runs Rd bridge, the at least 70-year-old structure had unsupported beams in two places but a principal inspection would determine if it could be reopened to light vehicles.
A ford was not possible at the site due to the depth of the river.
The 91-year-old bridge at Scott Lane had four of 13 sections of its span swept away meaning there was no point in undertaking a principal inspection, he said.
Before the flood, the bridge was restricted to heavy vehicles and its repair was not viable.
Beca’s investigations centred on more cost-effective options and users were using a ford crossing in the meantime.
A concrete wash-over box culvert was an option.
Beca was expected to complete structural inspections in the coming weeks.
The bridge closures highlighted the need for district-wide bridge strategy, Mr McCallum said.
The council had 179 maintained bridges and the its roading team had been working to put together a strategy since March that would take time to complete.
It was anticipated district-wide inspection would be completed by early to mid 2022 and a draft bridge strategy would be completed by late next year.
That would be followed by consultation in early 2023, development of long-term plan funding requests and a final approved bridge strategy by mid 2023.