Lessons from Gabrielle: Building greater forest resilienceFriday 16 Aug 2024
The company lost around 3% of its forest estate as well as roads and surrounding infrastructure in the cyclone that hit the North Island’s east coast in February 2023. The damage to the forestry side of Pan Pac’s business cost around $40 million. One of the key learnings from the disaster was the awesome power of water, says Damon Wise, Forests Manager – Operations. “Pre-cyclone, our infrastructure was designed to cope with normal winter storms, not catastrophic events,” says Wise. “No one really appreciated the damage big volumes of water could do. Water dissolves roads, floats concrete blocks and woody debris and acts in direct competition with the blocked culverts and multispan bridges. While the power of Gabrielle meant in some cases damage was unavoidable, we are now designing our infrastructure to cope with much more severe weather events than normal. “We need to allow more space for water to flow through. At some point, culverts will block, so you have to think about where the water goes and what damage it could do downstream. Water cascading over the road will quickly eat it away. You only need to have one or two culverts in a row block up to lose a section of road. As well as investing in better infrastructure, you also need to be regularly checking it. Preventative maintenance is the key to minimising damage associated with bad weather.” Pan Pac’s Forests Operations team has added more culverts in certain soil types and high rainfall areas. “We know through experience what each catchment can now produce in terms of water and debris. There’s no point investing heavily in roading infrastructure just to lose it all because the culvert pipe was too small.” Culvert installations lost to the cyclone that were traditionally designed for a 5% AEP (Annual Exceedance Probability) storm event are being replaced with larger pipes that will cope with a 2% AEP storm event. This equates to a 20% – 25% increase in pipe size. A 5% AEP storm event has a 5% chance of occurring in any given year and is likely to occur once every 20 years. The lower the percentage, the more intense the rain and associated flood event. Therefore, a 2% AEP event is equivalent to a 1 in 50-year storm event. Between March 2023 and June 2024, Wise says Pan Pac has built or replaced 247 culverts and restored access to over 200 roads. In some cases, single span bridges have replaced large culverts to allow more area for water to flow. “You might say we’re over engineering, but we understand the ramifications of not being prepared.” In other cases where the cost of building a bridge would be prohibitive, the team has installed box culverts with concrete armoured decks that will withstand overtopping in high flows. “Catastrophic weather events are quickly becoming the new normal and often involve large volumes of water,” says Wise. “We’ve learnt water will always find a way and when contained has little regard for infrastructure as it travels. Providing a route with as few restrictions as possible minimises damage and mitigates disruption.” Source and image credit: Pan Pac Forest Products | ||
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