Perth’s landmark bridges are being given an £850,000 check-up.
Engineers are using a heavy duty machine to inspect the city’s Queen’s Bridge.
An under-bridge platform vehicle allows workers to climb down below the crossing and carry out a meticulous examination by hand.
The round-the-clock operation, which is causing some traffic disruption, is scheduled to continue for nearly two weeks.
Councillors have already budgeted £400,000 for the inspection. A further £235,000 check-up will be carried out at the Old Perth Bridge – also known as Smeaton’s Bridge – at a future date, while another £212,000 has been set aside for general repairs.
A local authority report revealed nearly £50,000 of unplanned maintenance work was carried out on both bridges last year.
The latest assessment has been timed to avoid minimum disruption for motorists.
A Perth and Kinross Council spokeswoman said the inspection and testing works to the underside of Queens Bridge were expected to last until Friday January 19.
She said: “During this time all lanes on the bridge will remain open during peak traffic hours. However, from 9.15am to 3.45pm and from 6pm to 7.30am each weekday, the nearside lane of the two eastbound lanes will be closed to allow the positioning of an underbridge access platform vehicle.
“There will be no diversion route required during the closure times. We apologise for any inconvenience caused.”
Last year, more than £120,000 was spent on assessing the crossings following a spate of floods.
During torrential downpours at the start of 2016, the Queen’s Bridge was forced to close three times as water levels rose to near record levels.
Although the 58-year-old structure was checked at the time and declared safe, council leaders said they wanted to push ahead with this year’s full inspection.
The A-listed Perth Bridge is the most historic of the two, built in 1771 and funded by the Thomas Hay, the 9th Earl of Kinnoull.
Details of each major flood that the bridge has survived are etched on one of its arches.
The crossing was widened to cope with an increase in traffic in 1869.
The Queen’s Bridge was opened by Queen Elizabeth in 1960.
Perth city centre’s flood defences — the most expensive of their kind — were opened in 2001.
The £25m project was installed after some of the worst flooding in Perthshire.
The defences along the River Tay were four years in the making are are made up of earth embankments and a stone-faced wall stretching more than 8km from the north to the south of Perth.
