The Impact of Tides on Tsunami Inundation Hazard

Transcription

1 The Impact of Tides on Tsunami Inundation Hazard Lane, E. M. 1, 2, P. A. Gillibrand 1 1 NIWA, Christchurch, New Zealand 2 e.lane@niwa.co.nz

2 Tides and Tsunamis Motivation Do tides matter? South Pacific Tsunami Chilean Tsunami Probabilistic Tsunami Hazard Assessment - Monte Carlo simulations - Tides? - Examples Conclusions Acknowledgements

3 Motivation

4 Motivation Infrequent Devastating NZ has a long history of tsunamis Historic (1855, 1868, 1960) Maori oral traditions and archaeology Geologic record Need to be able to plan for them Photo by James Goff

5 Do Tides Matter? Southern coast of Western Samoa

6 Do Tides Matter? Southern coast of Western Samoa

7 Do Tides Matter? Southern coast of Western Samoa Sometimes no.

8 Amanave, American Samoa Do Tides Matter?

9 Do Tides Matter? Pagopago, American Samoa

10 Do Tides Matter? Photo by Geoffroy Lamarche Tsunami arrived at close to low tide Could have been up to 0.8 m higher had it arrived at high tide Photo by Mme Hasser Lamarche et al. Impact of the 29 September 2009 South Pacific tsunami on Wallis and Photo by Bernard Pelletier Futuna, Marine Geology: 271 (2010)

11 Do Tides Matter? Closer to home

12 Do Tides Matter? Closer to home Maximum wave height 0.99 m What would have happened if the earthquake and tsunami had occurred at a different time? Maximum sea level 1.83 m

13 Do Tides Matter? Closer to home 1.40 m 2.09 m 1.52 m 2.21 m

14 Do Tides Matter? Closer to home Probability of higher sea level Possibly 40 cm higher than occurred Predicted highest tide m HAT m Actual maximum sea level 1.83

15 Do Tides Matter? Closer to home Probability of higher sea level Possibly 50 cm higher than occurred Predicted highest tide m HAT -1.3 m Actual maximum sea level 1.46 m

16 Probabilistic Tsunami Hazard Assessment Where will likely be inundated by a tsunami at least once every 2500 years? 100 years? 500 years? How deep? How fast?

17 Probabilistic Tsunami Hazard Assessment WHAT ABOUT TIDES? Step 1: Randomly select 100,000 years worth of tsunamis using source characteristics Step 2: Model the inundation caused by these tsunami 100,000/2,500 = 40 Step 3: Count the number of times an area is inundated. Any area inundated 40 times or more will probably be inundated at least once every 2500 years

18 Probabilistic Tsunami Hazard Assessment with tides(?) Standard practise is to assume that the maximum tsunami wave arrives at mean high water spring (MHWS) Most of the time (>95%) the tidal level is below MHWS

19 Probabilistic Tsunami Hazard Assessment with tides Monte Carlo Simulation with tides: Select tsunami scenarios and also randomly select a start time. Model the tsunami and the tide together Currently computationally too expensive and too many simulations would need to be done to get a representative sample.

20 Probabilistic Tsunami Hazard Assessment with tides Use time series and inundation information from simulations together with tidal information for each location to calculate the effect of the tides.

21 Probabilistic Tsunami Hazard Assessment with tides Step 1: Randomly select 100,000 years worth of tsunamis using source characteristics Step 2: Model the inundation caused by these tsunami at HAT (i.e. maximum inundation), also save time series at areas of interest. Step 3: Calculate PDF of possible heights given tide and tsunami time series for each area and each simulation Step 4: Count the number of times an area is inundated. Adjust the inundation and weight it according to the PDF.

22 Probabilistic Tsunami Hazard Assessment with tides

23 Probabilistic Tsunami Hazard Assessment with tides

24 Probabilistic Tsunami Hazard Assessment with tides

25 Probabilistic Tsunami Hazard Assessment with tides

26 Probabilistic Tsunami Hazard Assessment results

27 Probabilistic Tsunami Hazard Assessment results

28 Conclusions Tides can make a significant difference to the impact of a tsunami This should be accounted for in Probabilistic Tsunami Hazard Assessment Using time series and tidal information as well as inundation information from simulations we can account for the tides

29 Acknowledgements With thanks to: William Powers Xiaoming Wang Greg Holland Jade Arnold Julian Sykes