SOME levee banks on the north and south sides of the Clarence River may be raised 20cm when the second Grafton Bridge is built.
The Environmental Impact Statement released by Roads and Maritime Services earlier this month shows the construction of the new bridge will cause upstream river heights in a one-in- 20-year flood to increase by 10cm.
The study also looked at the effect of the increased flood heights on 58 rural properties outside the levees.
It said these increases are typically less than 0.1m in a 100-year ARI event.
The report recommended further investigations to quantify this risk and identify additional mitigation measures (if possible) to be completed during detailed design.
This would include a habitable flood level survey and comparison to existing and predicted flood levels, followed by consideration of additional mitigation options for these properties.
The proposed mitigation measures also include a detention basin and pumping arrangement to provide a flood-free approach road to Grafton during a 20-year ARI event.
Climate change will make the communities of Grafton and South Grafton vulnerable to impacts but the proposed scheme does not significantly alter this increase in future risk.
Instead it recommends future stages of this project match future "climate adaptation" strategies adopted by Clarence Valley Council.
The report notes existing flood evacuation arrangements within Grafton will generally benefit from the proposed project, which will improve access to higher ground in South Grafton, subject to constraints of traffic congestion within the business district of Grafton.
Raising the levees
Mitigation Option 2: Raising 3.7km of levee on the north bank and 7km of levee on the south bank of the Clarence River by 0.2m.
Mitigation Option 4: Raising 3.7km of levee on the north bank, 4.7km of levee on the south bank and raising a 550m length of the Gwydir Hwy to an elevation of 7m AHD (current elevations are between 5-6.5m AHD).