User:Ngatimozart/Tsunami and seiching occurring simultaneously in a rock walled inlet
In this article the effects of a tsunami occurring simultaneous with seiching in a rock walled inlet is discussed and it is possible that this is an answer to the question of why Lyttelton Harbour's response to tsunami is five times greater than anywhere else in New Zealand. A search of the scientific literature and Wikipedia database has resulted in no previous attempts to explore seiching and tsunami in a rock walled inlet.
New Zealand (Aotearoa) has a short human history of tsunami compared to other nations like Japan. Even though the 2010 Chilean tsunami was small compared to the 1868 or 1960 tsunami we are slowly building a database of these events. This article looks at the effect that the 28th February 2010 (NZDT) tsunami had on Lyttelton Harbour which is located in western flank of the Banks Peninsula which is adjacent to the city of Christchurch (Otautahi) in the South Island (Te Waipounamu). Lyttelton Harbour may be unique in the way that it responds to tsunami due to a variety of reasons. In this article some of these will be touched upon.
Even though this article is about one specific location, it does have international ramifications. For example the fjords of Norway, Sweden and Alaska, or collapsed calderas like Santorini. So whilst the discussion is about Lyttelton Harbour and the processes that occur within it in relation to tsunami and seiching, the basic processes could easily refer to any other rock walled inlet in the world. Each inlet is unique in its own way and in certain specifics, could react differently. This will be because of things like, for example, morphological differences, size and water depth (Fine et al. 2009).
The 28th Feb 2010 (NZDT) tsunami has resulted in a wealth of data to work with in order to attempt to answer the following questions. So by discussing the possible reasons for Lyttelton Harbours high response rate to tsunami, the seiching and tsunami interactions will be addressed.
- 1. Is the wave refractive effect of the Chatham Rise a causality factor?
- 2. Or does it also act as a wave guide? Or a combination of both.
- 3. Is the resonance of Pegasus Bay a causality factor?
- 4. Is the geomorphology of the harbour responsible?
- 5. Is resonance within the harbour an agent?
- 6. Do the seiches have an effect?
- 7. Is the harbour in itself actually unique?
Banks Peninsula and Pegasus Bay are the western extent of the Chatham Rise (Figure 1) which is a large geomorphological feature that is a dominating factor of any tsunami analysis with regard to Pegasus Bay, Banks Peninsula and Christchurch. The Chatham Rise can be defined as that area within the 1000m isobath stretching eastwards from Banks Peninsula over a distance of 1300km (Fenner et al. 1992). Wood et al define the Chatham Rise as "... submarine rise that extends due east of the South Island for over 1100km. The region described ... includes the submerged portion of the rise shallower than 1500m and its land area between 1750 W and the Southern Alps (Wood et al. 1986)p4.
Figure 1. Chatham Rise & New Zealand
Lyttelton Harbour (Figure 2) has been defined as a rock walled inlet (Curtis 1985). Heath used Cottons (1949) definition that Lyttelton Harbour was 16km long and 2km wide, had a depth of 16m at the harbour mouth and the head of the harbour comprised of shallow water and mudflats (Heath 1976). It is the remnants of an extinct collapsed caldera volcano on the northeastern side of Banks Peninsula, close to Christchurch. It has an ENE - WSW axis with the ENE end of the harbour being the mouth opening to Pegasus Bay. Eruptions occurred during a period from approximately 11million years ago to 9.2million years ago. The seaward end is constructed of basaltic lava, with the head predominately ryholithic and andesitic rock types. The present harbour is the third known breach of the caldera (Weaver et al. 1985).
Figure 2. Pegasus Bay and Banks Peninsula.
There exists a previous history of tsunami events in Lyttelton Harbour with Canterbury having reported 19 tsunami between 1840 and 1982. The most destructive or memorable events are the 1868, 1877, and 1960 events all originating in Chile (de Lange & Healy 1986). To this can now be added the 2010 event which can be classified as a memorable event. In each case the wave trough arrived first which is in agreement with research by Mazova and Ramirez (1999) that concluded that most of the large and highly destructive recorded in Chile were waves where the trough arrived first (Mazova & Ramiirez 1999). It has been suggested that Lyttelton Harbours high response rate to far field tsunami is because of resonance in Pegasus Bay, refraction due to the Chatham Rise, and resonance within the harbour itself (de Lange & Fraser 1999) p9. Heath suggests that the Chatham Rise could act as a wave guide (Heath 1976) p81-82, (Heath 1985) p104).
At 0634 UTC on 27th Feb 2010 a magnitude 8.8 earthquake at a depth of 55km occurred near the coast of central Chile with a subsequent tsunami sweeping the Pacific. At 1045 UTC the Pacific Tsunami Warning Center issued a tsunami warning. The estimated time of arrive at Lyttelton, NZ was 2040Z 27 Feb 2010 (0940 NZDT 28 Feb 2010). (Pacific Tsunami Warning Center 1045Z 27 Feb 2010). A further bulletin was issued at 1843Z 27 Feb 2010 giving tide / wave gauge measurements that denoted wave heights and periods. (Pacific Tsunami Warning Center 1843Z 27 FEB 2010) Note here that the Z included in the PTWC messages time is Zulu time which is UTC. NZDT is New Zealand Daylight Time (UTC + 13 hours) and one hour ahead of New Zealand Standard Time (UTC + 12).
The Pegasus Bay seiche was already excited when the tsunami arrived which amplified the seiche and this started at the forecast time. When it reached Lyttelton Harbour the fundamental mode of the existing seiche in the harbour was already overly excited because of a possible prior weather event. This is shown in Figs., 3 & 5 and Fig., 4 shows the excitation continuing into the next day and decreasing over time. There was double amplification of this when the tsunami arrived, which peaked 6 hours after the tsunami arrival. The normal seiche periods within the harbour are:
- • Longitudinal harbour seiche 1st mode period 99 mins
- • Longitudinal harbour seiche 2nd mode period 40 mins
- • Transverse seiche 10 mins
- • From Pegasus Bay 205 mins
(Mulgor Consulting Ltd 2010)
In the 2010 tsunami the largest contributor to changes in water depth was the Pegasus Bay seiche. This is shown in Table 1. The maximum wave height in the harbour throughout the event was 1.822m. This is the crest to trough height. If all of the three types of waves had arrived simultaneously the outcome would have been far different with extreme seas which would have been destructive. As a result the minimum sea level drop was -0.550m below Chart datum and the maximum sea level rise was 3.226m above Chart datum (Mulgor Consulting Ltd 2010).
Table 1 Maximum wave heights and occurrence. (Mulgor Consulting Ltd, 2010, p7)
| Wave Type | Wave Height (m) | Time of Occurrence NZST |
|---|---|---|
| Pegasus Bay Seiche | 1.573 | 12:43 |
| Harbour Seiche | 0.628 | 14:02 |
| Long Waves (Tsunami) | 0.491 | 15.47 |
Figure 3. Lyttelton Tide for 28/02/2010 (Available Online: http://www.lpc.co.nz/RP.jasc?Page=N0P11)
Figure 4. Lyttelton Tides for 1/3/2010 (Available Online: http://www.lpc.co.nz/RP.jasc?Page=N0P11)
Figure 5. Seiches and Long Waves (Mulgor Consulting Ltd, 2010, p4)
Figure 6. Pegasus Bay & Lyttelton Harbour Seiches 28/2/2010 NZST (Mulgor Consulting Ltd, 2010 p5)
In this tsunami event Lyttelton Harbour and Christchurch were quite lucky in that the tsunami arrive during a low spring tide and the actual tsunami wave crest to trough height was 1.822m which places it well within the normal tidal range of 2.3m. As shown in Fig., 6 the seiches at Sumner head and at Lyttelton Harbour, as well as the tsunami wave were out of phase and this remained so throughout the event. If the two seiches and the tsunami had phase coupled the story would have been completely different with amplified wave heights that would have meant possible inundation to low lying areas. The 1960 tsunami was the most destructive but the area hasn't sustained a fatality because of a tsunami.
Not mentioned yet is a fourth possible phase coupling and that is with an atmospheric event. Phase coupling between tsunami and the atmospheric events have been known to have occurred. The most famous would have been the Krakatau volcanic event of 1883. (de Lange & Prasetya 1999). It has also been suggested that the c186 -200AD Taupo eruption may have phase coupled with the ensuring tsunami (Lowe & de Lange 2000).
Of the five tsunami recorded as destructive or memorable, four (1868, 1877, 1960 & 2010) were all far field tsunami that originated in Chile. The 1923 event was a locally generated tsunami, the result of the Rangiora earthquake. Rangiora is approximately 30km north of Christchurch. This tsunami was not large and is only memorable by the fact that three swimmers had to be rescued from the water in Pegasus Bay (de Lange & Healy 1986).
At the start of this article seven questions were asked. The wave refraction effect of the Chatham rise cannot be discounted as a possible cause. There is yet to be more work needed specifically upon this question. Also what cannot be discounted is that in conjunction with the refraction it could also act as a wave guide. The resonances of Pegasus Bay and Lyttelton Harbour are in fact seiches and they are affected by incoming tsunami. The geomorphology of the harbour itself does play what could be a major effect, in that not only is wave refraction occurring within it, but wave energy is being reflected off the rock walls. If the seiches and incoming tsunami phase couple then a destructive result is very probable. Every rock walled inlet is unique in its own way but what has been discussed here could very well apply to any rock walled inlet. This is an area of study that needs to be further investigated because of possible hazard potential to human populations.
A video showing the location can be viewed on You Tube: http://www.youtube.com/watch?v=QsyWdMP8rt8 Also on You tube a video taken in Christchurch of the surge arriving http://www.youtube.com/watch?v=1zG_ARyrEKM&feature=player_embedded
Acknowledgements.
I am very thankful to the following who gave me a lot of help and advice . Dr Derek Goring, Mulgor Consulting Ltd. http://www.mulgor.co.nz/ and Mr Neil McLennan, Engineering Manager, Lyttelton Port Company http://www.lpc.co.nz/RP.jasc?Page=N0P11
Sources
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