Roger, J.H.M.; Power, W.L. 2024 Combined effects of tsunami, storm surge and tide in Wellington Harbour. Lower Hutt, N.Z.: GNS Science. GNS Science report 2024/30. 43 p.; doi: 10.21420/M8TY-ZD05
Abstract
Tsunami and storm surges have been widely studied independently within the last few decades. While the numerical modelling of storm surge often considers simultaneously dynamic variations of tide, it is rarely the case for tsunami simulations, for which only statictide levels (e.g. mean sea level or MSL, mean high water spring or MHWS) are normally considered. New Zealand is subject to both potentially hazardous tsunami and storm surges, and to large variations of tide, depending on the considered region. However, little is known about the interaction of tsunami with dynamic tides, and the interaction between tsunami, dynamic tides and storm surges, in the relatively plausible case a storm surge occurs at the same time of a tsunami. In this study, Wellington Harbour was selected as a pilot site to perform preliminary investigation of these interactions because (1) records of tsunami, tides and storm surges were available in and around the harbour and (2) it hosts the capital city of New Zealand and is one of the largest commercial ports, where large ships may be affected by such phenomena. Simulation of two tsunami scenarios, the 2016 Mw 7.8 Kaikoura tsunami and hypothetic extreme-case Mw 8.9 Hikurangi megathrust scenario, have been performed at four different sea-level conditions: (1) static MSL, (2) static MHWS, (3) dynamic tide and( 4) dynamic tide plus storm surge, the latter being based on the historical 2 February 1936 storm surge. Notably, the comparison of simulation results at static MHWS and dynamic tide show that simulating tsunami at a constant MHWS level, the common practice to simulate tsunami waves for tsunami hazard assessment, is a good, conservative, approximation as it provides slightly larger wave amplitudes and flow depth on land. However, the comparison of tsunami currents between the two approaches show that the dynamic tides may increase the current speed, especially in the case of a relatively small tsunami like the Kaikoura one. According to the few tested cases studied here, the simulation with dynamic tide plus storm surge does not reveal any particular additional effects when compared with the one with the dynamic tide only. (auths)
