Central Otago's ageing bridge network is being placed under pressure as the number and weight of trucks increase.
"The community faces a significant issue with 65 bridges forecast to reach the end of their economic lives within the next 30 years,'' the Central Otago District Council says in its transportation activity management plan.
"Replacement of these bridges to retain the original levels of service is unlikely to be economically viable or affordable to the community.''
Major bridge renewal work has been deferred by the council until 2018 so structural reports can be obtained and consultation carried out with the communities affected.
About $200,000 has been budgeted for the structural investigation of "critical'' bridges.
This work was vital for informed decisions to be made on the future of the bridges, the plan said.
The community would be consulted on a long-term bridge strategy, which would include replacement of essential bridges, installation of culverts and the "retiring'' of some bridges.
The council maintains 1886km of roads, of which 73% (or 1374km) are unsealed.
The population in the district is growing at just under 1% a year, resulting in the growth of subdivisions and new roading assets being vested in the council, the management plan said.
Increased traffic movements and heavy vehicle use were having a high impact on gravel roads and bridges.
"Increased dairy farming ac-tivity is occurring in the Man-iototo, Manuherikia and Roxburgh areas,'' the plan said.
"The conversion of significant areas of high tussock land into pasture is resulting in demand for heavy vehicle movements year-round on roads that had traditionally only provided a very low, dry-weather, back-country level of service access.''
All freight was transported in and out of the area by road, so an increase in the use and weight of heavy vehicles was important for the district's economic prosperity, the plan said.
The council recently reviewed the weight limits on nine bridges.
In some cases, stricter load restrictions were needed, roading manager Julie Muir said in a recent report to the council.
The Clutha River bridge at Millers Flat, Clutha river bridge at Roxburgh and Green Bridge (the bridge over the Taieri River in Patearoa Rd) were among those inspected with a view to strengthening.
Repair work was already planned on the Roxburgh bridge to fix a defect which affected its seismic resilience but that work would not necessarily improve the bridge's structural capacity, she said.
Any work or restrictions on the two Clutha river bridges had to be considered together as limitations on one would affect the other.
The Green Bridge was used by dairy tankers.
Heavy vehicle use of this section of sealed road had drawn complaints from other road users who believed the road was too narrow for this type of traffic.
There was an alternative route available but the length of the detour was at least 10km, Ms Muir said.
No decision had been made yet about heavy vehicle restrictions on the bridge.
Bridge strengthening work on Central Otago and the Queenstown Lakes districts' highways had also been on the agenda for the New Zealand Transport Agency.
It spent $1.6million last year strengthening four bridges - the State Highway 8 Alexandra bridge and three on SH6, between Cromwell and Queenstown.
Bridging issues
• 175 bridges in Central Otago roading network
• 65 expected to reach the end of their economic life within the next 30 years
• A further 30 require significant renewal work
• Number and weight of trucks increasing as land productivity improves
• Bridge repairs/replacements costly
