Coastal flooding exposure under future sea-level rise for New Zealand Technical Report
- Author
- Paulik, R., Stephens, S.A., Wadhwa, S., Bell, R.G., Popovich, B. &; Robinson, B.
- Year
- 2019
- Journal / Source
- Deep South Science Challenge
- Publisher / Organisation
- NIWA
- Month
- March
- Pages
- 76
- Keywords
- flood, sea-level rise
- Summary
- This study presents New Zealand’s exposure to 1% annual exceedance probability (AEP) coastal flood inundation under present-day and future higher sea levels. The investigation comprised: ▪ Estimating 1% AEP extreme sea level elevation (ESL1) at present-day mean sea-level (MSL) around the New Zealand coastline, resulting from combination of tide, storm-surge, mean sea-level anomaly and wave setup. ▪ Mapping coastal flood inundation on low-lying coastal land by projecting ESL1 onto both high-resolution airborne LIDAR and lower resolution satellite derived MERIT digital elevation models (DEM). On land with LIDAR DEM coverage, inundation is mapped for +0.1 m sea-level rise (SLR) increments up to +3 m above present-day MSL. On land without LIDAR DEM coverage, inundation is mapped for +3 m SLR above present-day MSL. A composite LIDAR and satellite DEM with complete national coverage was used to map land exposed to ESL1 +3 m SLR around the entire New Zealand coastline. ▪ Mapping “elements at risk” to ESL1 including: population, buildings (count and 2016 NZD replacement value), transport infrastructure (roads, railways, airports), electricity infrastructure (transmission lines, structures, sites), three-waters infrastructure (nodes, pipelines), and land cover (built, production, natural or undeveloped). These elements at risk provide a representative sample of built assets and land cover types exposed within New Zealand’s coastal floodplains for each coastal flood inundation scenario. ▪ Developing national and regional level exposure profiles for elements at risk to ESL1. Exposure for the present-day MSL and with SLR of +0.3 m, +0.6 m, +0.9 m and +1.2 m for areas with LIDAR DEM coverage as presented in Tables 0-1 to Table 1-5. These sea-levels were related to four projected future SLR scenarios for New Zealand beyond 2120 based on three greenhouse gas concentration pathways (RCP2.6, RCP4.5 and RCP8.5). ▪ Developing combined national and regional level exposure profiles of elements at risk to ESL1 with SLR of +3 m SLR for the composite LIDAR and satellite DEM as presented in Table 1-6. The exposure mapping results showed that: ▪ At a national level, elements at risk are subject to a linear ESL1 exposure increase in response to rising sea levels. At region and territory levels, ESL1 exposure of elements exhibits non-linear behaviors, with acceleration or deceleration in response to increasing SLR increments and the characteristics of elements at risk in these areas. ▪ Population and built assets in some regions and territories with major coastal urban areas experience an initial rapid increase of ESL1 exposure in response to SLR. In Hawkes Bay (i.e. Napier City), Wellington (i.e. Lower Hutt City) and Canterbury (i.e. Christchurch City), exposure of these elements accelerates rapidly to +0.9 m SLR above present-day MSL, before decelerating under higher SLR thereafter. In other populous Coastal Flooding Exposure Under Future Sea Level Rise for New Zealand 7 regions (e.g. Auckland, Waikato, Bay of Plenty), population and built asset ESL1 exposure increases at an approximately constant rate in response to SLR. It is also noted that: ▪ Coastal flooding from ESL1 is considered an unusually large and rare event at present-day MSL, having only a 1% annual exceedance probability. The effect of future rising sea-levels is to increase the frequency of ESL1 being reached or exceeded. This means that elements at risk to present-day ESL1 will be more frequently exposed to coastal flood inundation in future. ▪ The information presented in this study provides researchers and practitioners with locations to focus more detailed investigation on the potential impacts and management implications of coastal flooding under future sea-level rise. ▪ It is recommended that coastal flood inundation maps and element at risk exposure information produced in this study should be updated as new LiDAR data are made available.