A regional analysis of tide-surge interactions during extreme water levels in complex coastal systems of Aotearoa New Zealand

Abstract

Tide-surge interaction (TSI) is a critical factor in assessing flooding in shallow coastal systems, particularly in estuaries and harbours. Non-linear interactions between tides and surges can occur due to the water depth and bed friction. Global investigations have been conducted to examine TSI, but its occurrence and impact on water levels in Aotearoa New Zealand (NZ) have not been extensively studied. Water level observations from 36 tide gauges across the diverse coast of NZ were analysed to determine the occurrence and location of TSI. Statistical analysis and numerical modelling were conducted on data from both inside and outside estuaries, focusing on one estuary (Manukau Harbour) to determine the impact of TSI and estuarine morphology on the co-occurrence rate of extreme events. TSI was found to occur at most sites in NZ and primarily affects the timing of the largest surges relative to high tide. There were no regional patterns associated with the tide, non-tidal residual, or skew-surge regimes. The strongest TSI occurred in inner estuarine locations and was correlated with the intertidal area. The magnitude of the TSI varied depending on the method used, ranging from -16 cm to +27 cm. Co-occurrence rates of extreme water levels outside and inside the same estuary varied from 20% to 84%, with TSI modulating the rate by affecting tidal amplification. The results highlight the importance of investing in a more extensive tide gauge network to provide longer observations in highly populated estuarine coastlines. The incorporation of TSI in flooding hazard projections would benefit from more accurate and detailed observations, particularly in estuaries with high morphological complexity. TSI occurs in most sites along the coast of NZ and has a significant impact on water levels in inner estuarine locations. TSI modulates the co-occurrence rate of extreme water levels in estuaries of NZ by affecting tidal amplification. Therefore, further investment in the tide gauge network is needed to provide more accurate observations to incorporate TSI in flooding hazard projections.

Tide-Surge Interaction near Singapore and Malaysia using a Semi-empirical Model

“…This means that while the residuals exceeding the 99th percentile can occur close to HW, the peak of each cluster of exceedances is unlikely to be found in the time window close to HW. Across the locations, this time window generally begins 2-Otago in New Zealand (Costa et al, 2023), Shijiusuo, Lianyungang and Xiamen along the coast of China (Feng et al, 2019), Hiron Point at the Bay of Bengal India (Antony and Unnikrishnan, 2013), and Aberdeen, North Shields, Immingham, Cromer and Sheerness at the North Sea (Horsburgh and Wilson, 2007). At Kuantan, Cendering and Geting, the primary mode is found after HW, at +6, +6 and +4 hours respectively.…”

“…In existing studies, a uniform distribution is chosen as the null distribution (Horsburgh and Wilson, 2007;Haigh et al, 2010;Antony and Unnikrishnan, 2013;Feng et al, 2019;Costa et al, 2023). However, the uniform distribution is not the most suitable null distribution to represent the null hypothesis.…”

“…At Kuantan, Cendering and Geting, the primary mode is found after HW, at +6, +6 and +4 hours respectively. Outside Southeast Asia, such signals have appeared less commonly but have been found at Onehunga in New Zealand (Costa et al, 2023) and at Kaohsiung and Zhapo in China (Feng et al, 2019).…”