Surcharging of the 3.6m diameter northern culvert by flood levels in the River Derwent has pressurised its lower end, lifting the brick arch by around 40mm, causing horizontal and vertical stress fractures and raising the risk of collapse.
The weakened section will be replaced with 150m of reinforced concrete box culvert.
To avoid obstructing the north culvert during construction the new box will be built parallel to the damaged section and connected after completion.
The £600,000 contract is out to tender and due to start in April.
Animal life, in particular the white-claw crayfish, has been caught and relocated upstream during the work.
It is hoped the crayfish will return to recolonise the dredged ponds.