High-Water Alerts from Coinciding High Astronomical Tide and High Mean Sea Level Anomaly in the Pacific Islands Region

Abstract

A technique to produce high-water alerts from coinciding high astronomical tide and high mean sea level anomaly is demonstrated for the Pacific Islands region. Low-lying coastal margins are vulnerable to episodic inundation that often coincides with times of higher-than-normal high tides. Prior knowledge of the dates of the highest tides can assist with efforts to minimize the impacts of increased exposure to inundation. It is shown that the climate-driven mean sea level anomaly is an important component of total sea level elevation in the Pacific Islands region, which should be accounted for in medium-term (1-7 months) sea level forecasts. An empirical technique is applied to develop a mean sea level-adjusted high-water alert calendar that accounts for both sea level components and provides a practical tool to assist with coastal inundation hazard planning and management.

Spatial and temporal analysis of extreme storm-tide and skew-surge events around the coastline of New Zealand

“…We found that extreme storm-tide elevations are strongly correlated with MHWS elevation. This affirms current practice in NZ of forecasting "red-alert" tide dates when high-tide peaks are predicted to be unusually high (Bell, 2010;Stephens et al, 2014) (see NZ Storm-Tide Red-Alert Days; National Institute of Water and Atmospheric Research; https://niwa.co.nz/ our-science/coasts/tools-and-resources/tide-resources, last access: 10 March 2020). The red-alert tide concept works in New Zealand because the semi-diurnal tides dominate sea-level variability and storm surges are limited to mostly < 0.5 m, which is approximately 25 % of the average tidal range (Stephens et al, 2014).…”

“…This affirms current practice in NZ of forecasting "red-alert" tide dates when high-tide peaks are predicted to be unusually high (Bell, 2010;Stephens et al, 2014) (see NZ Storm-Tide Red-Alert Days; National Institute of Water and Atmospheric Research; https://niwa.co.nz/ our-science/coasts/tools-and-resources/tide-resources, last access: 10 March 2020). The red-alert tide concept works in New Zealand because the semi-diurnal tides dominate sea-level variability and storm surges are limited to mostly < 0.5 m, which is approximately 25 % of the average tidal range (Stephens et al, 2014). Coastal or hazard managers are advised to keep a close watch on the weather for lower barometric pressure and adverse winds during the red-alert tide days, as even a minor storm or swell event could lead to inundation of low-lying areas, especially if accompanied by waves (Bell, 2010).…”

“…NZ does not experience tropical cyclones (hurricanes) and has a relatively deep and narrow continental shelf and so does not experience very large storm surges on a global scale -the semi-diurnal tides dominate sea-level variability and storm surges are limited to mostly < 0.5 m, which is approximately 25 % of the average tidal range (Stephens et al, 2014). Rueda et al (2017) classify NZ's coastal flood hazard climate as being "lightly unbounded" (extreme storm-tide shape parameter weakly positive), "macro-level" (total water level ≥ 1 m), "highly variable" (high interannual variability) and "tidedominant" (tidal range dominates the total water level height compared to storm surge and wave set-up).…”

“…However, numerical storm-surge models for climate change typically do not include MSLA, which is an important component of extreme storm tide (for tidally dominant coasts) and hence flooding exposure (e.g. Stephens et al, 2014;Sweet and Park, 2014), and which we have shown to be an important influence on the timing of extreme events. Efforts to forecast MSLA are being made (Widlansky et al, 2017), but work is required to include MSLA into models for the development of probabilistic assessment of extreme water levels to assess coastal hazard risks, taking into account SLR.…”

A monthly tidal envelope classification approach for semi-diurnal regimes with variability in S&lt;sub&gt;2&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt; tidal amplitude ratios

“…Daily tidal water level variations are a key control on shore ecology; access to marine environments via boat and shipping infrastructure such as ports, jetties and wharves; drainage links between the ocean and coastal hydrosystems such as lagoons and estuaries; and the duration and frequency of opportunities to access the intertidal zone for recreation and food harvesting purposes. Fortnightly and monthly tidal envelope variations, such as those associated with spring-neap and perigean-apogean cycles, have similar moderating roles on human usage of intertidal and shoreline environments, and additionally these medium term variations in tide levels are important factors in coastal inundation risks (Menéndez & Woodworth, 2010;Stephens 2015;Stephens et al, 2014;Wood, 1978Wood, , 1986). High perigean-spring tides, for example, interact with extreme weather events (including low pressures, strong winds and extreme rainfall) to produce significant coastal inundation in low-lying coastal settlements such as on deltas (Hart et al, 2015).…”