Infrastructure Resilience in the Face of Extreme Weather Events: 'Domino Effect'

INTRODUCTION Many communities around the world are facing unexpected and compounding impacts from extreme weather events, driven by climate change and changing weather patterns. Infrastructure — particularly water, wastewater and stormwater assets — designed on the basis of historical events and existing climate records is being found to be highly prone to catastrophic failure, often with a 'domino' effect on other assets and the health and welfare of the communities served. This paper will cover the catastrophic impacts of a major tropical mega-storm — Cyclone Gabrielle — which impacted the east coast of the north island of New Zealand in March 2023 (Refer Figure 1 for location map and Figure 2 for weather map). This brought some of the highest rainfall intensities, river flows and winds that the country has ever experienced in over 250 years of European settlement and recorded data. The paper will highlight the compounding impacts of the cyclonic event on the region's water systems, and how existing land use policies and practices (forestry, horticulture, agriculture and urban development) contributed negatively to the devastating outcomes of this storm event. HISTORICAL BACKGROUND Cyclone Gabrielle is not the first major tropical storm to hit this region. In 1988, a similar weather system generated Cyclone Bola (see Figure 3) which caused widespread land slips and river overflows in the Gisborne region. Farmland and forestry lands were devastated as natural vegetation was swept away by the slips and in erosion by floodwaters. As part of the recovery, a government-funded regional forestry replanting project was instigated — using pine trees — to stabilise the slopes and create a long-term economic benefit once the pine trees were harvested. It should be noted that pre-European settlement and introduction of primary industries to the region — primarily sheep and beef raising — most of these hills were covered in native forests which were felled and replanted with grass for grazing. COMPOUNDING CONSEQUENCES OF 2023 EVENT — THE 'DOMINO EFFECT' Unfortunately, some 35 years later, this Bola recovery project was one of the root causes for the widespread damage caused by Cyclone Gabrielle. Plantation tree husbandry practices left much of the branches (referred to as slash) on the ground, to be swept off the steep slopes into the flooded rivers, sweeping downstream to destroy or block bridges and destroy utility services carried by the bridges. The 30km long main water supply to Gisborne City (40,000 population) was destroyed in a number of locations, cutting off this supply for six weeks. The shallow-rooted pines were easily uprooted and swept into the flooded rivers, adding to the volume of slash, and ultimately ending up on the regions beaches as mountains of woody debris. The accompanying silt and soil loads from the now naked and destabilised hill country blanketed low lying productive land (vineyards, kiwifruit and citrus plantations, and vegetable farms), ruining the majority of the current seasonal crops either being or about to be harvested. The knock-on economic and social impacts on the whole region have been huge as it relies very heavily on the agricultural and horticultural industries. The silt blanketing the land did not finish there, as the rivers carried the huge volumes of silt into the nearshore ocean waters, smothering the easily accessible seashore and seabed from which many forms of marine life have been harvested as a traditional indigenous food source. An unanticipated outcome of this impact was the increased cost of living for many sections of the Gisborne community who relied on this readily available food source. While the city's wastewater system was overwhelmed by the heavy rain, with many system overflows to the three rivers running through the city, there was little long-term damage to the network or the wastewater treatment plant — even though the latter was in the middle of a major upgrade. The telecommunication network was not so lucky, with cell towers knocked out or without power, with Starlink satellite phones being the only viable alternative once enough were procured and distributed to essential lifeline services. Major damage occurred to the roading network, through the loss of many bridges and massive wash-outs, which severely limited local and regional transport of emergency services and maintenance crews and equipment LESSONS LEARNED With the Cyclone Gabrielle response well in the past, regional public and private sector leadership is now working in the recovery phase with support from central NZ government. Everyone involved realises that similar or even worse cyclones will hit again, driven by changes in the wider Pacific weather systems, driven by global climate change. Revegetating and restabilising the steep hill country is essential, but it is unlikely to see replanting of the pine plantations, focussing instead on a greater diversity of tree and shrub species. The resilience of water supply assets is being addressed, including more appropriate carriage of water supply pipes (and other utilities) over rivers subject to flooding. It is impossible for communities and individuals to insure against the scale of losses and damages incurred by the Gisborne region; the best investment is to envisage the very worst that can happen and to plan accordingly, fully recognising the 'knock-on' or 'domino impacts' that these mega-events can produce.