Abstract
Abstract. Coastal flooding is a major global hazard, yet few studies have examined the spatial and temporal characteristics of extreme sea level and associated coastal flooding. Here we analyse sea-level records around the coast of New Zealand (NZ) to quantify extreme storm-tide and skew-surge frequency and magnitude. We identify the relative magnitude of sea-level components contributing to 85 extreme sea level and 135 extreme skew-surge events recorded in NZ since 1900. We then examine the spatial and temporal clustering of these extreme storm-tide and skew-surge events and identify typical storm tracks and weather types associated with the spatial clusters of extreme events. We find that most extreme storm tides were driven by moderate skew surges combined with high perigean spring tides. The spring–neap tidal cycle, coupled with a moderate surge climatology, prevents successive extreme storm-tide events from happening within 4–10 d of each other, and generally there are at least 10 d between extreme storm-tide events. This is similar to findings from the UK (Haigh et al., 2016), despite NZ having smaller tides. Extreme events more commonly impacted the east coast of the North Island of NZ during blocking weather types, and the South Island and west coast of the North Island during trough weather types. The seasonal distribution of both extreme storm-tide and skew-surge events closely follows the seasonal pattern of mean sea-level anomaly (MSLA) – MSLA was positive in 92 % of all extreme storm-tide events and in 88 % of all extreme skew-surge events. The strong influence of low-amplitude (−0.06 to 0.28 m) MSLA on the timing of extreme events shows that mean sea-level rise (SLR) of similarly small height will drive rapid increases in the frequency of presently rare extreme sea levels. These findings have important implications for flood management, emergency response and the insurance sector, because impacts and losses may be correlated in space and time.
Highlights
Coastal flooding is a major global hazard with historical events killing hundreds of thousands of people and causing billions of dollars in damage to property and infrastructure (e.g. Lagmay et al, 2015; Needham et al, 2015; Haigh et al, 2016)
mean sea-level anomaly (MSLA) is derived from the low-frequency non-tidal residual sea level (Merrifield et al, 2013), which is dominated by seasonal heating and cooling of the sea (Bell and Goring, 1998; Boon, 2013) and inter-annual climate cycles such as El Niño–Southern Oscillation (Goring and Bell, 1999) and is often not considered in extreme storm-tide analyses because in most places it is small compared to tide and skew surge – we show that it has an important influence on the timing of extreme storm-tide and skew-surge events in New Zealand (NZ)
At 0.52 m the skew surge was relatively large for NZ and coincided with a predicted “red alert” tide, and a background MSLA of nearly 0.05 m, noting that ongoing sea-level rise (SLR) increasingly adds to the impact or frequency of flooding (Stephens et al, 2018)
Summary
Coastal flooding is a major global hazard with historical events killing hundreds of thousands of people and causing billions of dollars in damage to property and infrastructure (e.g. Lagmay et al, 2015; Needham et al, 2015; Haigh et al, 2016). It has been estimated that up to 310 million people are already exposed to a 1 in 100-year flood from the sea (Jongman et al, 2012; Hinkel et al, 2014; Muis et al, 2016). In New Zealand this includes 72 000 people – 1.5 % of the population (Paulik et al, 2019, 2020) This will get worse, since without adaptation, it has been estimated that 0.2 %–4.6 % of global population will be flooded annually under 25–123 cm (RCP2.6–RCP8.5) of global mean sealevel rise (SLR; Hinkel et al, 2014). Improved understanding of extreme sea level and coastal flooding events is important. D. Haigh et al, 2010, 2014) and global
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