Similarities and differences in tsunami and storm surge ... - IIIRR

Similarities and differences in tsunami and storm surgemitigationN. Nirupama 1 and T.S. Murty 21Disaster & Emergency Management, York University(Toronto, Canada)Email:nirupama@yorku.ca2Department of Civil Engineering, University of Ottawa(Ottawa, Canada)Email:smurty@hotmail.comTsunamis and storm surges belong to the class of long gravity waves like ocean tides and they can cause substantialcoastal inundation, leading to loss of life and extensive damage. Superficially, the coastal effects from tsunamis andstorm surges are quite similar, but dynamically they are very different. Tsunamis propagate through the deep oceansand strike the coastlines, whereas storm surges are only coastal phenomena and do not exist over the deeper part ofthe oceans. Tsunamis can affect 100s to 1000s of kilometers along the length of a coastline, whereas storm surges atmost can affect few tens of kilometers along a coast. The effects of storm surges are to some degree hemispheredependent,in the sense that in the northern hemisphere, the peak surge occurs to the right side of the hurricane trackand in the southern hemisphere on the left side. For tsunamis, there is no such hemispheric distinction, except thefact that most of the tsunami energy propagates in a direction perpendicular to the fault line causing the earthquake.The hurricanes that generate storm surges can be seen several days earlier, even though the prediction of the exactlocation of the landfall is quite difficult and is prone to errors. No tsunami prediction can be given until the underocean earthquake actually happens, and in this sense the warning time for tsunamis is substantially smaller than forstorm surges. In the case of tsunamis, initial ocean withdrawal is often observed while no such phenomenon hasbeen reported in the literature for storm surges. Tsunamis and storm surges are different in the duration of the oceanwater staying on the land during inundation. In Japan, in some instances, only few minutes of elapsed time isobserved between the occurrence of the earthquake and the tsunami impact on the coastline. In such cases, notsunami warning system would be very effective, and the earthquake itself has to be used as early warning. Theenergy contained in tsunami and storm surge wave systems differs substantially.Key Words: tsunami, storm surge, mitigation, Japan1. INTRODUCTIONThere is a class of ocean waves referred to as long gravity waves (Figure 1) which include tides, stormsurges and tsunamis. Only long gravity waves can climb land, cause coastal inundation leading to severeloss of life and substantial damage to coastal infrastructure. Short period wind waves, although could bedangerous over the ocean, cannot climb land as they break at the beach. Out of these, tsunamis have theshortest periods ranging from few minutes to a couple of hours. In terms of the frequency andgeographical extent of coastal inundation, tides are the highest; however, since they occur at regularintervals, the humans have factored them into their lives and will not live or build any structures in theintertidal zone (Bernard 2005). Hence, in this study, we will compare and contrast storm surges andtsunamis with regards to coastal inundation leading to loss of life and damage to properties.-186-

Fig.1 Ocean wave spectrum for long gravity waves (Platzman 1971).2. SIMILARITIES BETWEEN STORM SURGES AND TSUNAMISStorm surges and tsunamis are long gravity waves, which get amplified considerably in shallow waterand on wide continental shelves. Tsunamis have large wavelengths but rather small amplitudes (of theorder of 50 cm) while propagating over the deep ocean. However, as they enter the shallow water thewave gets compressed and the amplitude increases as shown in Figure 2. Storm surges do not exist in thedeep ocean. Similar to tsunamis, they also amplify (Nirupama et al 2006) considerably in shallow water.deep oceanFig.2 Amplification of a typical tsunami approaching shallow water at the shore.3. DIFFERENCES BETWEEN STORM SURGES AND TSUNAMIS-187-

Storm surges occur much more frequently than tsunamis, which are relatively rare. However, if andwhen the tsunamis occur, the loss of life and the damage could be much greater than in a storm surge.Tsunamis are much more energetic than storm surges, and are three dimensional in nature which can beinferred from the considerable bottom scouring they do during their propagation (Murty 1984; Murty et al1987; Luick 1994).The reach of a tsunami is in thousands of kilometers as can be seen in Figure 3, which shows thecoastlines affected by the Indian Ocean tsunami of December 26, 2004.Fig.3 The Indian Ocean tsunami of December 26, 2004 affected thousands of kilometers of coastlinealong many countries. (Wikipedia).Storm surges have hemispheric dependence. As can be seen in Fig 4, in the Northern Hemisphere (NH),the winds are towards the coastline on the right side of the storm track and away from the coastlinetowards the ocean on the left side because the hurricane rotates in a counter clockwise direction. These onshore winds push the water from the ocean and pile it up at the coastline, thus creating the storm surge onthe right side of the track. In the Southern Hemisphere (SH), the rotation is clockwise, and as such thepiling up of the water occurs on the left side of the track. While tsunamis are not hemisphere dependent,their energy propagation predominantly occurs in a direction perpendicular to the fault line (Figure 5).-188-

Fig.4 Schematic of hurricane track and windfield.Fig.5 A significant part of the energy propagatedperpendicular to the fault during the December26, 2004 Indian Ocean tsunami (Lomnitz andNilsen-Hofseth, 2005).For tsunami prediction, travel time charts are prepared in advance because the speed of travel of tsunamidue to gravity, and D = water depth. On the other hand for the storm surges, one does not talk about traveltime charts because the surge mainly develops at the coastline itself. While it takes about a day for atsunami to cross the Pacific Ocean, it only takes a few hours for an Indian Ocean tsunami to arrive at mostpopulated areas (Bhaskaran et al 2005). This means, reflected waves from distant coastlines do notcontribute significantly in the Pacific Ocean, while reflections must be factored in the Indian Ocean(Figures 6-9).-189-

Fig.6 Distribution of the tsunami amplitude inthe Indian Ocean at 2 h 50 min from the tsunamionset. The wave reflected from the India and SriLanka propagates back to the source region(Kowalik et al 2006; Murty et al 2006)Fig.7 Distribution of the tsunami amplitude inthe Indian Ocean at hour 4 from the tsunamionset. Along with the reflection shown in Figure21, the reflection from the Maldives also sendsenergy eastward (Kowalik et al., 2006; Murty etal 2006)Fig.8 Reflected waves from the LakshadweepIsland ridge arriving in Kerala, India during theIndian Ocean tsunami of December 26, 2004(Kurian et al 2008).Fig.9 Reflected waves from the Somalia coastduring the Indian Ocean tsunami of December26, 2004 (Kurian et al 2008).-190-

A typical tsunami could consist of 3-10 waves as can be seen from Figure 6, whereas a storm surgegenerally has only one single peak. Another difference is the presence of forerunners in a tsunamiwhereas these are not present in a storm surge. During a tsunami, at some locations, the ocean can recedeup to a kilometer or more, which is referred to as initial withdrawal; no such phenomenon is apparent in astorm surge. Although, negative storm surge can occur causing boats suddenly get grounded or cutting ofwater supply to coastal installations, no reference is made of negative waves in tsunamis. A typicalnegative storm surge is shown in Figure 10 in eastern Canada.Fig.10 A negative storm surge in December 1972 in eastern Canada (Gonnert et al 2001).Generally speaking, several days in advance are available for storm surge prediction once the tropicalcyclone is spotted over the ocean several days before it makes a landfall, although the exact location andtiming of the landfall is somewhat uncertain. On the other hand, no tsunami prediction can start unlessand until the earthquake actually occurs. For tsunami prediction, there is great international collaborationas can be witnessed from the fact that some 26 countries around the Pacific Rim are involved in the effort(Murty et al 2006). However, for storm surges, which are mostly local, no such large scale internationalcollaboration is required. Another important difference between surges and tsunamis is the predictabilityof their sources (Titov et al 2005). The weather systems that generate surges can be seen several daysahead before the occurrence of the surge whereas there is no earthquake (which generates the tsunami)prediction at this time. In Japan, in certain instances the tsunami arrives at the coast only a few minutesafter the earthquake occurs. In such cases, the earthquake itself is used as the warning for the tsunami.While both storm surges and tsunamis interact with the long period tides, there is significant interactionwith short period wind waves only for the storm surges. The coastal inundation from storm surges appearsto last several days as can be seen from Figure 11, whereas, usually tsunami coastal inundation is of theorder of a few hours.-191-

Fig.11 Calculated water level (tide plus surge) at (a) Sagar Island and (b) Pussur River entrance in the Bayof Bengal for a hypothetical storm modeled after the November 1970 storm (Gonnert et al 2001).Additionally, the following physical processes are important for tsunami for either amplifying the wavesor maintaining wave activities for a longer period; while they are not important for surges. Quarter wave resonance Helmholtz resonance Energy trapping through OFC and OSCFigures 12 and 13 illustrate the phenomena of quarter wave resonance amplification and Helmholtzresonance (Miles 1975) respectively. The highest possible amplitude (anti-node) occurs at one quarterwave length (Fig 12), meaning that as the tsunami approaches the coastal bay from the ocean, when 1/4 thof its wavelength matches the linear dimension of the coastal bay or gulf, there will be significant increasein the tsunami amplitude, which is referred to as quarter wave resonance amplification. Figure 13illustrates that tsunami energy can enter a harbor through a narrow entrance channel and can get subjectedto multiple reflections inside the harbor, but the energy cannot easily get out. This is referred to asHelmholtz resonance and could cause serious disruption to ship movements inside a harbor, as happenedin Kerala, India during the 2004 tsunami.-192-

Fig.12 Quarter-wave resonance amplification in coastal bays, gulfs and inlets.Fig.13 Helmholtz resonance in harbors.4. DISCUSSION AND CONCLUDING REMARKSBased on the argument and evidence presented in the paper, it can be inferred that tsunamis and stormsurges have both similarities and differences. The similarities occur in the area of shallow water behavior,coastal inundation characteristics. Both are long gravity waves with a potential of high intensitydestruction. In terms of differences, advance warning time availability, international collaboration,hemispheric dependence, inland reach, physical processes, prediction of their sources, number of wavescontained, energy content, and energy trapping. Tsunamis are three dimensional in nature causingsignificant bottom scouring; whereas, storm surges are mainly surface phenomenon. In the mitigation andsocio-economic aspects, both are quite similar.REFERENCES1) Bernard, E.N. (2005). National Tsunami Hazard Mitigation Program: A Successful State-Federal Partnership, Natural Hazard,35, 5-24.2) Bhaskaran, P.K., Dube, SK, Murty, TS, Gangapadhyay, A., Chaudhuri,A. and Rao, AD (2005). Tsunami Travel Time Atlasfor the Indian Ocean, Indian Institute of Technology Kharagpur, India, p. 279.3) Gonnert, G., Dube, SK, Murty, TS and Siefert, W (2001). Global Storm Surges: Theory, Observations and Modelling, DieKûnste, Germany, 637pages.4) Kowalik, Z., Knight, K, Logan, T and Whitmore, P (2006). Numerical Modeling of the Global Tsunami: Indonesian Tsunamiof 26 December 2004, Science of Tsunami Hazards, 23(1), 40-56.5) Kurian, N.P., Baba, M., Rajith, K, Nirupama, N and Murty, TS (2008). Analysis of the Tsunami of 26 December 2004 on theKerala Coast of India -Part I: Amplitudes, Marine Geodesy, 29 (4), 265-270, 20066) Luick, J.L., Murty, T.S. and Hubbert, G. (1994). Island Sea Level Response to Cyclones: A Numerical Case Study, Advancesin Marine Science and Technology, 241 - 249, Ed. O. Bellwood, H. Choat, N. Saxena.7) Miles, J.W. and Lee, Y.K. (1975). Helmholtz Resonance of Harbors. J. Fluid Mech., 67, 445-464.-193-

8) Murty, T.S. (1984). Storm Surges-Meteorological Ocean Tides, Bulletin 212, Canadian Journal of Fisheries and AquaticSciences, Ottawa, 920 pages.9) Murty, T.S., Saxena, NK, Sloss, PW and Lockridge, PA (1987). Accuracy of Tsunami Travel Time Charts, Marine Geodesy,11, 89-102.10) Murty, TS, Kurian, NP and Baba, M. (2006). Trans-Oceanic Reflection of Tsunamis: The Kerala Example, Dusaster &Development, Vol 1, No 1, 65-76.11) Nirupama, N., Murty, TS, Nistor, I. and Rao, AD. (2006). A Review of Classical Concepts on Phase and AmplitudeDispersion: Application to Tsunamis. In: Indian Ocean Tsunami, Eds: Murty, Aswathanarayana, Nirupama, A.A. BalkemaPublication, The Netherlands, p63-72.12) Platzman, GW (1971). Ocean Tides and Related Waves, 239-291, In: W.H. Reid (ed.), Mathematical Problems in theGeophysical Sciences, AMS, Providence, Rhode Island.13) Titov, VV, Gonzalez, FI, Bernard, EN, Eble, MC, Mofjeld, HO, Newman, JC and Venturato, AJ (2005). Real-Time TsunamiForecasting: Challenges and Solutions, Natural Hazard, 35, 41-58.14) Wikipedia at www.wikipedia.org/en (accessed February 15, 2012).-194-