Potential flood impacts of a second Grafton Bridge

What National Party politicians don’t usually mention when bragging about the plan for a second crossing of the Clarence River at Grafton.

Excerpts from Additional Crossing of the Clarence River at Grafton: Flood Impact, Levee Upgrade, and Structural Considerations (June 2014):

4.1. Potential Adverse Flood Impacts

Flood levels within Grafton and South Grafton are largely dictated by the volume of floodwater overtopping the respective levee systems. Upstream of the existing Grafton Bridge, the Grafton and South Grafton levees extend for approximately 10km before tying into naturally high ground. Due to the long length of these levees, slight changes in flood level within the main Clarence River have the potential to considerably alter the volume of water overtopping the levee, possibly resulting in significant variations in flood level behind the levee systems. This has the potential to adversely affect the populations of Grafton and South Grafton, increasing their flood risk exposure. All upgrade options for an additional crossing of the Clarence will increase flood levels. Public concern of this was evident through submissions received during community consultation…….

4.7. Flood Risk Considerations

A problem often experienced by communities protected by levees is complacency regards the likelihood and associated consequences of flooding. When a levee is constructed, the frequency of flooding is reduced, resulting in residents forming the option that the levee has eliminated the flood threat completely. This complacency increases the flood risk within leveed communities. Maddocks et al. (2007) reported that the March 2011 flood was predicted to overtop the levees in Grafton and consequently an attempt was made to evacuate some 12,000 residents. The evacuation was unsuccessful, with only approximately 10% of the people actually evacuating the town. Pfister (2002) stated “the residents of Grafton, having experienced few direct effects of flooding since the construction of the levees, are likely to have developed a low consciousness of the flood threat, and are therefore less ready to act.” Consequently, it appears that although a high levee will provide a high level of protection, it is likely to also induce a high level of community complacency, and when the levee does finally overtop evacuation procedures can be severely hampered and consequences of flooding may be amplified. Community flood education is therefore critical in leveed towns. Flood education aims to increase flood awareness. In times of flood, a flood aware community will be more likely to respond appropriately during an emergency situation. As such, community flood education is an effective means of reducing flood hazard……

4.9. Land Use Controls

…..There is an area that sits outside of the levee system on Carrs Island. Properties on Carrs Island were impacted by Council’s recent upgrading of the South Grafton levee system. As a result, Council has paid to lift several dwelling to place the floor level above the 100yr ARI flood level, as well as building some flood mounds for stock. As such, consideration also needs to be given to the fact that with raising the levee wall, properties sitting just outside of this system have greater impacts during floods. If upgrade of the Grafton and South Grafton levees is proposed as part of the bridge duplication project, further consideration of mitigation measures for the residents of Carrs Island will be necessary……

5. SUMMARY

Roads and Maritime Services (RMS) is currently investigating options for an additional crossing of the Clarence River at Grafton. Without mitigation an additional crossing of the Clarence River will increase upstream flood levels, with adverse impacts to the communities of Grafton and South. RMS intends to maintain the current level of immunity and mitigate any adverse impact from piers and structures within the Clarence River by raising current levees. Although the primary objective of the additional crossing aims to address short-term and long-term transport needs, secondary flood risk management benefits will be an outcome of the project. The additional crossing will significantly improve the flood evacuation situation in Grafton. Furthermore, potential opportunities exist to further reduce flood risk within the respective townships as part of the levee raising exercise which is proposed to mitigate the flood impacts associated with the bridge design. These issues are currently being considered as part of the design process.

I’m sure there are more than a few long time Grafton residents who are hoping that the Baird Government gets the new bridge design right if they do go ahead with construction.

Just as many residents upstream from Grafton are probably hoping that upgraded levees protecting the city don't cause a bottleneck which spreads flooding across their land and, downstream residents hoping this upgrade doesn't funnel more fast moving flood waters towards their towns, villages and farms.

While Clarence Valley Council in its turn is probably hoping that the "good faith" defence in Local Government Act 1993 will cover it should any of the poorly maintained sections of its levee network give way under the pressure of any change in flood behaviour.