The existing bridge has been slowly deteriorating for more than 60 years, despite regular maintenance work. It is now in a state where it needs to be replaced.
The new structures will be constructed around the existing arch bridge, reducing the length of time the road will be closed to traffic from the original estimate of 12 months to just 10 weeks.
The River Bridge will be constructed in stages, with the new arches cast above the old arches to support a new deck of transverse steel beams and a reinforced concrete which will then be built. New footpaths will be cantilevered to the outside of the reinforced concrete arch.
The Abbey Bridge replacement scheme was one of the schemes given funding approval last year as part of the Spending Review process. Work can now start on construction with the new bridge scheduled to open in December 2013.