Climate Change Requires Smart Resilience
The recent tropical cyclones Hale and Gabrielle have demonstrated the increasing impact of extreme weather events that are becoming more frequent as the effects of global climate change become more apparent. The consequences for built infrastructure from climate change is huge and will be a key focus for local and central government agencies as well as all organisations that manage built infrastructure. We all need to consider how we can develop assets to be more resilient to the adverse effects of climate change.
At Frame Group, we are often involved in design of assets in parks, reserves and forestry land that has a higher than average risk of being subjected to natural hazards such as landslide, flooding, coastal attack and high winds. This land is often consigned to recreation reserve or forestry use because it is more risk prone than land normally used for building development. Whilst initiatives to address the human induced causes of climate change are now critical, any reductions in greenhouse gas emissions in the near future will not reduce the natural hazard risks to this land. We are faced with developing management strategies for facilities and assets on this land that are sustainable and affordable in a changing global environment.
It is now very clear that as well as global warming and sea level rise, climate change is also resulting in more frequent and more intense rainfall events. The rainfall intensity of a 100-year return period storm has increased. Whereas the rainfall intensity, measured in millimetres per hour, of a 100 year intensity storm previously had a 1% probability of being exceeded in any one year, this probability figure is increasing as a result of climate change and is probably closer to 2% and may even become a 4% chance of being exceeded in any one year. Similarly, stream flood flow rates and floodwater levels have increased. In the past we have comfortably designed structures to accommodate 100 year or 200 year return period events in the belief that the probably of these being exceeded during the life of the asset is at an acceptably low level, however the goalposts have shifted. Structures designed with a 1% probability of damage or loss in any year, may now have a 2% change of damage or loss. As an example, this means that for a bridge with an intended life of 50 years and designed to resist a 100 year flood event, the probability of it being damaged or lost at some stage during its design life has shifted from 39.5% to 63.6%. This represents a 60% increase in assets that may be lost before they reach the end of their economic life.
The standard engineering response to this shift in probabilities is to increase the return period of the event that we design for, or to add additional conservative design factors in the design process. Both these responses generally mean higher initial capital investment (higher and longer bridges, more retaining walls, and more robust foundations). These measures alone will not necessarily fully eliminate the possibility of asset loss, in fact it means that the monetary value of premature loss is even greater. Landslides and coastal erosion are extremely difficult to predict on a probability basis. Councils and Government will simply not have enough funds to continually replace damaged parts of the infrastructure network with assets that are more conservatively designed. A different approach will be needed in many cases.
Providing climate change resilience for infrastructure in parks, reserves and forests can take different forms. The conservative engineered solution based on new probability data is one approach, however we need to look at alternatives that are affordable and do not compromise the other important factors such as the environmental, cultural and aesthetic values of these sites. We do not want all the stream banks in our parks to lined with heavy retaining walls, nor can we afford to construct numerous large bridges high above streams to be clear of the worst possible flooding, or away from possible landslide or ground settlement.
Retreat from rapidly eroding coastal margins is now a heralded preference over the armouring of our coastline and cliffs with artificial seawalls. This approach applies to stream margins as well. As stream channels broaden and adapt naturally to accommodate more frequent larger flood flows, they will claim the stream edge. Shifting tracks and structures away from erosion prone stream edges is preferable to resorting to extensive retaining walls or deep foundations.
Much new investment in infrastructure, no matter how resilient it is designed, will have some risk of damage or loss in extreme events, especially on parks and reserve sites that have a heightened vulnerability to natural hazards. There is an incentive to carefully manage the capital investment on such sites to limit the potential for loss. We need to consider if there are alternatives to building a bridge or a track in vulnerable sites. Are there alternative routes or is there a better solution to providing the desired amenity or the required functionality. Most recreation tracks and cycleways are classified as non-critical infrastructure, and brief period of closure during severe storm events can be tolerated. An example in forestry is the low-level stream crossing. Frame Group has designed a significant number of low-level battery culvert stream crossings for wood harvesting truck access over streams at a much lower cost than bridging, and providing a structure that can sustain major floodwater overtopping with little or no damage. With careful design, a good standard of fish passage and stream ecology can be maintained for such structures. The sacrifice is that the crossing may not be accessible for a few hours on several occasions each year when the stream is in flood. Forest operations can accommodate this and are planned around this limitation. A similar approach may be adopted for recreation access. Construction of bridges and boardwalks that are occasionally inundated, but not destroyed in severe events may be preferable to raising all structures high above potential flood levels.
Where assets are prone to damage from landslide, tree fall, floodwater impact, or wave attack, there is merit in designing structures that utilise components that are readily available from locally sourced materials rather than bespoke elements, and that incorporate construction details that facilitate easy replacement of damaged components. This approach enables repairs to be undertaken quickly and cost effectively allowing early re-opening of storm damaged structures and facilities for ongoing use. Long periods of closure cause disruption, and reduce amenity. The impact on community wellbeing arising from people being shut out of local recreation areas can be significant.
The use of natural asset protection measures should be encouraged. Planting with land stabilising species and surface flow drainage management are preferred options for the improvement of slope stability, ahead of the construction of high cost invasive retaining walls. If infrastructure can be designed to be easily repaired after land movement, this reduces the potential economic loss. With considered design, foundations that settle or are displaced during severe events may be satisfactorily and economically remediated to a state where the remaining economic life of the asset is not lost.
The onset of climate change is a signal for us to design smarter, which may not necessarily be bigger, stronger and more expensive. At Frame Group we pride ourselves on finding pragmatic solutions that offer affordable climate change resilience as well as protection of wider environmental and community values.