Similarities and differences in tsunami and storm surge ... - IIIRR
Similarities and differences in tsunami and storm surge ... - IIIRR
Similarities and differences in tsunami and storm surge ... - IIIRR
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<strong>Similarities</strong> <strong>and</strong> <strong>differences</strong> <strong>in</strong> <strong>tsunami</strong> <strong>and</strong> <strong>storm</strong> <strong>surge</strong>mitigationN. Nirupama 1 <strong>and</strong> T.S. Murty 21Disaster & Emergency Management, York University(Toronto, Canada)Email:nirupama@yorku.ca2Department of Civil Eng<strong>in</strong>eer<strong>in</strong>g, University of Ottawa(Ottawa, Canada)Email:smurty@hotmail.comTsunamis <strong>and</strong> <strong>storm</strong> <strong>surge</strong>s belong to the class of long gravity waves like ocean tides <strong>and</strong> they can cause substantialcoastal <strong>in</strong>undation, lead<strong>in</strong>g to loss of life <strong>and</strong> extensive damage. Superficially, the coastal effects from <strong>tsunami</strong>s <strong>and</strong><strong>storm</strong> <strong>surge</strong>s are quite similar, but dynamically they are very different. Tsunamis propagate through the deep oceans<strong>and</strong> strike the coastl<strong>in</strong>es, whereas <strong>storm</strong> <strong>surge</strong>s are only coastal phenomena <strong>and</strong> do not exist over the deeper part ofthe oceans. Tsunamis can affect 100s to 1000s of kilometers along the length of a coastl<strong>in</strong>e, whereas <strong>storm</strong> <strong>surge</strong>s atmost can affect few tens of kilometers along a coast. The effects of <strong>storm</strong> <strong>surge</strong>s are to some degree hemispheredependent,<strong>in</strong> the sense that <strong>in</strong> the northern hemisphere, the peak <strong>surge</strong> occurs to the right side of the hurricane track<strong>and</strong> <strong>in</strong> the southern hemisphere on the left side. For <strong>tsunami</strong>s, there is no such hemispheric dist<strong>in</strong>ction, except thefact that most of the <strong>tsunami</strong> energy propagates <strong>in</strong> a direction perpendicular to the fault l<strong>in</strong>e caus<strong>in</strong>g the earthquake.The hurricanes that generate <strong>storm</strong> <strong>surge</strong>s can be seen several days earlier, even though the prediction of the exactlocation of the l<strong>and</strong>fall is quite difficult <strong>and</strong> is prone to errors. No <strong>tsunami</strong> prediction can be given until the underocean earthquake actually happens, <strong>and</strong> <strong>in</strong> this sense the warn<strong>in</strong>g time for <strong>tsunami</strong>s is substantially smaller than for<strong>storm</strong> <strong>surge</strong>s. In the case of <strong>tsunami</strong>s, <strong>in</strong>itial ocean withdrawal is often observed while no such phenomenon hasbeen reported <strong>in</strong> the literature for <strong>storm</strong> <strong>surge</strong>s. Tsunamis <strong>and</strong> <strong>storm</strong> <strong>surge</strong>s are different <strong>in</strong> the duration of the oceanwater stay<strong>in</strong>g on the l<strong>and</strong> dur<strong>in</strong>g <strong>in</strong>undation. In Japan, <strong>in</strong> some <strong>in</strong>stances, only few m<strong>in</strong>utes of elapsed time isobserved between the occurrence of the earthquake <strong>and</strong> the <strong>tsunami</strong> impact on the coastl<strong>in</strong>e. In such cases, no<strong>tsunami</strong> warn<strong>in</strong>g system would be very effective, <strong>and</strong> the earthquake itself has to be used as early warn<strong>in</strong>g. Theenergy conta<strong>in</strong>ed <strong>in</strong> <strong>tsunami</strong> <strong>and</strong> <strong>storm</strong> <strong>surge</strong> wave systems differs substantially.Key Words: <strong>tsunami</strong>, <strong>storm</strong> <strong>surge</strong>, mitigation, Japan1. INTRODUCTIONThere is a class of ocean waves referred to as long gravity waves (Figure 1) which <strong>in</strong>clude tides, <strong>storm</strong><strong>surge</strong>s <strong>and</strong> <strong>tsunami</strong>s. Only long gravity waves can climb l<strong>and</strong>, cause coastal <strong>in</strong>undation lead<strong>in</strong>g to severeloss of life <strong>and</strong> substantial damage to coastal <strong>in</strong>frastructure. Short period w<strong>in</strong>d waves, although could bedangerous over the ocean, cannot climb l<strong>and</strong> as they break at the beach. Out of these, <strong>tsunami</strong>s have theshortest periods rang<strong>in</strong>g from few m<strong>in</strong>utes to a couple of hours. In terms of the frequency <strong>and</strong>geographical extent of coastal <strong>in</strong>undation, tides are the highest; however, s<strong>in</strong>ce they occur at regular<strong>in</strong>tervals, the humans have factored them <strong>in</strong>to their lives <strong>and</strong> will not live or build any structures <strong>in</strong> the<strong>in</strong>tertidal zone (Bernard 2005). Hence, <strong>in</strong> this study, we will compare <strong>and</strong> contrast <strong>storm</strong> <strong>surge</strong>s <strong>and</strong><strong>tsunami</strong>s with regards to coastal <strong>in</strong>undation lead<strong>in</strong>g to loss of life <strong>and</strong> damage to properties.-186-
Fig.1 Ocean wave spectrum for long gravity waves (Platzman 1971).2. SIMILARITIES BETWEEN STORM SURGES AND TSUNAMISStorm <strong>surge</strong>s <strong>and</strong> <strong>tsunami</strong>s are long gravity waves, which get amplified considerably <strong>in</strong> shallow water<strong>and</strong> on wide cont<strong>in</strong>ental shelves. Tsunamis have large wavelengths but rather small amplitudes (of theorder of 50 cm) while propagat<strong>in</strong>g over the deep ocean. However, as they enter the shallow water thewave gets compressed <strong>and</strong> the amplitude <strong>in</strong>creases as shown <strong>in</strong> Figure 2. Storm <strong>surge</strong>s do not exist <strong>in</strong> thedeep ocean. Similar to <strong>tsunami</strong>s, they also amplify (Nirupama et al 2006) considerably <strong>in</strong> shallow water.deep oceanFig.2 Amplification of a typical <strong>tsunami</strong> approach<strong>in</strong>g shallow water at the shore.3. DIFFERENCES BETWEEN STORM SURGES AND TSUNAMIS-187-
Storm <strong>surge</strong>s occur much more frequently than <strong>tsunami</strong>s, which are relatively rare. However, if <strong>and</strong>when the <strong>tsunami</strong>s occur, the loss of life <strong>and</strong> the damage could be much greater than <strong>in</strong> a <strong>storm</strong> <strong>surge</strong>.Tsunamis are much more energetic than <strong>storm</strong> <strong>surge</strong>s, <strong>and</strong> are three dimensional <strong>in</strong> nature which can be<strong>in</strong>ferred from the considerable bottom scour<strong>in</strong>g they do dur<strong>in</strong>g their propagation (Murty 1984; Murty et al1987; Luick 1994).The reach of a <strong>tsunami</strong> is <strong>in</strong> thous<strong>and</strong>s of kilometers as can be seen <strong>in</strong> Figure 3, which shows thecoastl<strong>in</strong>es affected by the Indian Ocean <strong>tsunami</strong> of December 26, 2004.Fig.3 The Indian Ocean <strong>tsunami</strong> of December 26, 2004 affected thous<strong>and</strong>s of kilometers of coastl<strong>in</strong>ealong many countries. (Wikipedia).Storm <strong>surge</strong>s have hemispheric dependence. As can be seen <strong>in</strong> Fig 4, <strong>in</strong> the Northern Hemisphere (NH),the w<strong>in</strong>ds are towards the coastl<strong>in</strong>e on the right side of the <strong>storm</strong> track <strong>and</strong> away from the coastl<strong>in</strong>etowards the ocean on the left side because the hurricane rotates <strong>in</strong> a counter clockwise direction. These onshore w<strong>in</strong>ds push the water from the ocean <strong>and</strong> pile it up at the coastl<strong>in</strong>e, thus creat<strong>in</strong>g the <strong>storm</strong> <strong>surge</strong> onthe right side of the track. In the Southern Hemisphere (SH), the rotation is clockwise, <strong>and</strong> as such thepil<strong>in</strong>g up of the water occurs on the left side of the track. While <strong>tsunami</strong>s are not hemisphere dependent,their energy propagation predom<strong>in</strong>antly occurs <strong>in</strong> a direction perpendicular to the fault l<strong>in</strong>e (Figure 5).-188-
Fig.4 Schematic of hurricane track <strong>and</strong> w<strong>in</strong>dfield.Fig.5 A significant part of the energy propagatedperpendicular to the fault dur<strong>in</strong>g the December26, 2004 Indian Ocean <strong>tsunami</strong> (Lomnitz <strong>and</strong>Nilsen-Hofseth, 2005).For <strong>tsunami</strong> prediction, travel time charts are prepared <strong>in</strong> advance because the speed of travel of <strong>tsunami</strong>due to gravity, <strong>and</strong> D = water depth. On the other h<strong>and</strong> for the <strong>storm</strong> <strong>surge</strong>s, one does not talk about traveltime charts because the <strong>surge</strong> ma<strong>in</strong>ly develops at the coastl<strong>in</strong>e itself. While it takes about a day for a<strong>tsunami</strong> to cross the Pacific Ocean, it only takes a few hours for an Indian Ocean <strong>tsunami</strong> to arrive at mostpopulated areas (Bhaskaran et al 2005). This means, reflected waves from distant coastl<strong>in</strong>es do notcontribute significantly <strong>in</strong> the Pacific Ocean, while reflections must be factored <strong>in</strong> the Indian Ocean(Figures 6-9).-189-
Fig.6 Distribution of the <strong>tsunami</strong> amplitude <strong>in</strong>the Indian Ocean at 2 h 50 m<strong>in</strong> from the <strong>tsunami</strong>onset. The wave reflected from the India <strong>and</strong> SriLanka propagates back to the source region(Kowalik et al 2006; Murty et al 2006)Fig.7 Distribution of the <strong>tsunami</strong> amplitude <strong>in</strong>the Indian Ocean at hour 4 from the <strong>tsunami</strong>onset. Along with the reflection shown <strong>in</strong> Figure21, the reflection from the Maldives also sendsenergy eastward (Kowalik et al., 2006; Murty etal 2006)Fig.8 Reflected waves from the LakshadweepIsl<strong>and</strong> ridge arriv<strong>in</strong>g <strong>in</strong> Kerala, India dur<strong>in</strong>g theIndian Ocean <strong>tsunami</strong> of December 26, 2004(Kurian et al 2008).Fig.9 Reflected waves from the Somalia coastdur<strong>in</strong>g the Indian Ocean <strong>tsunami</strong> of December26, 2004 (Kurian et al 2008).-190-
A typical <strong>tsunami</strong> could consist of 3-10 waves as can be seen from Figure 6, whereas a <strong>storm</strong> <strong>surge</strong>generally has only one s<strong>in</strong>gle peak. Another difference is the presence of forerunners <strong>in</strong> a <strong>tsunami</strong>whereas these are not present <strong>in</strong> a <strong>storm</strong> <strong>surge</strong>. Dur<strong>in</strong>g a <strong>tsunami</strong>, at some locations, the ocean can recedeup to a kilometer or more, which is referred to as <strong>in</strong>itial withdrawal; no such phenomenon is apparent <strong>in</strong> a<strong>storm</strong> <strong>surge</strong>. Although, negative <strong>storm</strong> <strong>surge</strong> can occur caus<strong>in</strong>g boats suddenly get grounded or cutt<strong>in</strong>g ofwater supply to coastal <strong>in</strong>stallations, no reference is made of negative waves <strong>in</strong> <strong>tsunami</strong>s. A typicalnegative <strong>storm</strong> <strong>surge</strong> is shown <strong>in</strong> Figure 10 <strong>in</strong> eastern Canada.Fig.10 A negative <strong>storm</strong> <strong>surge</strong> <strong>in</strong> December 1972 <strong>in</strong> eastern Canada (Gonnert et al 2001).Generally speak<strong>in</strong>g, several days <strong>in</strong> advance are available for <strong>storm</strong> <strong>surge</strong> prediction once the tropicalcyclone is spotted over the ocean several days before it makes a l<strong>and</strong>fall, although the exact location <strong>and</strong>tim<strong>in</strong>g of the l<strong>and</strong>fall is somewhat uncerta<strong>in</strong>. On the other h<strong>and</strong>, no <strong>tsunami</strong> prediction can start unless<strong>and</strong> until the earthquake actually occurs. For <strong>tsunami</strong> prediction, there is great <strong>in</strong>ternational collaborationas can be witnessed from the fact that some 26 countries around the Pacific Rim are <strong>in</strong>volved <strong>in</strong> the effort(Murty et al 2006). However, for <strong>storm</strong> <strong>surge</strong>s, which are mostly local, no such large scale <strong>in</strong>ternationalcollaboration is required. Another important difference between <strong>surge</strong>s <strong>and</strong> <strong>tsunami</strong>s is the predictabilityof their sources (Titov et al 2005). The weather systems that generate <strong>surge</strong>s can be seen several daysahead before the occurrence of the <strong>surge</strong> whereas there is no earthquake (which generates the <strong>tsunami</strong>)prediction at this time. In Japan, <strong>in</strong> certa<strong>in</strong> <strong>in</strong>stances the <strong>tsunami</strong> arrives at the coast only a few m<strong>in</strong>utesafter the earthquake occurs. In such cases, the earthquake itself is used as the warn<strong>in</strong>g for the <strong>tsunami</strong>.While both <strong>storm</strong> <strong>surge</strong>s <strong>and</strong> <strong>tsunami</strong>s <strong>in</strong>teract with the long period tides, there is significant <strong>in</strong>teractionwith short period w<strong>in</strong>d waves only for the <strong>storm</strong> <strong>surge</strong>s. The coastal <strong>in</strong>undation from <strong>storm</strong> <strong>surge</strong>s appearsto last several days as can be seen from Figure 11, whereas, usually <strong>tsunami</strong> coastal <strong>in</strong>undation is of theorder of a few hours.-191-
Fig.11 Calculated water level (tide plus <strong>surge</strong>) at (a) Sagar Isl<strong>and</strong> <strong>and</strong> (b) Pussur River entrance <strong>in</strong> the Bayof Bengal for a hypothetical <strong>storm</strong> modeled after the November 1970 <strong>storm</strong> (Gonnert et al 2001).Additionally, the follow<strong>in</strong>g physical processes are important for <strong>tsunami</strong> for either amplify<strong>in</strong>g the wavesor ma<strong>in</strong>ta<strong>in</strong><strong>in</strong>g wave activities for a longer period; while they are not important for <strong>surge</strong>s. Quarter wave resonance Helmholtz resonance Energy trapp<strong>in</strong>g through OFC <strong>and</strong> OSCFigures 12 <strong>and</strong> 13 illustrate the phenomena of quarter wave resonance amplification <strong>and</strong> Helmholtzresonance (Miles 1975) respectively. The highest possible amplitude (anti-node) occurs at one quarterwave length (Fig 12), mean<strong>in</strong>g that as the <strong>tsunami</strong> approaches the coastal bay from the ocean, when 1/4 thof its wavelength matches the l<strong>in</strong>ear dimension of the coastal bay or gulf, there will be significant <strong>in</strong>crease<strong>in</strong> the <strong>tsunami</strong> amplitude, which is referred to as quarter wave resonance amplification. Figure 13illustrates that <strong>tsunami</strong> energy can enter a harbor through a narrow entrance channel <strong>and</strong> can get subjectedto multiple reflections <strong>in</strong>side the harbor, but the energy cannot easily get out. This is referred to asHelmholtz resonance <strong>and</strong> could cause serious disruption to ship movements <strong>in</strong>side a harbor, as happened<strong>in</strong> Kerala, India dur<strong>in</strong>g the 2004 <strong>tsunami</strong>.-192-
Fig.12 Quarter-wave resonance amplification <strong>in</strong> coastal bays, gulfs <strong>and</strong> <strong>in</strong>lets.Fig.13 Helmholtz resonance <strong>in</strong> harbors.4. DISCUSSION AND CONCLUDING REMARKSBased on the argument <strong>and</strong> evidence presented <strong>in</strong> the paper, it can be <strong>in</strong>ferred that <strong>tsunami</strong>s <strong>and</strong> <strong>storm</strong><strong>surge</strong>s have both similarities <strong>and</strong> <strong>differences</strong>. The similarities occur <strong>in</strong> the area of shallow water behavior,coastal <strong>in</strong>undation characteristics. Both are long gravity waves with a potential of high <strong>in</strong>tensitydestruction. In terms of <strong>differences</strong>, advance warn<strong>in</strong>g time availability, <strong>in</strong>ternational collaboration,hemispheric dependence, <strong>in</strong>l<strong>and</strong> reach, physical processes, prediction of their sources, number of wavesconta<strong>in</strong>ed, energy content, <strong>and</strong> energy trapp<strong>in</strong>g. Tsunamis are three dimensional <strong>in</strong> nature caus<strong>in</strong>gsignificant bottom scour<strong>in</strong>g; whereas, <strong>storm</strong> <strong>surge</strong>s are ma<strong>in</strong>ly surface phenomenon. In the mitigation <strong>and</strong>socio-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. <strong>and</strong> 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 <strong>and</strong> Siefert, W (2001). Global Storm Surges: Theory, Observations <strong>and</strong> Modell<strong>in</strong>g, DieKûnste, Germany, 637pages.4) Kowalik, Z., Knight, K, Logan, T <strong>and</strong> Whitmore, P (2006). Numerical Model<strong>in</strong>g 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 <strong>and</strong> Murty, TS (2008). Analysis of the Tsunami of 26 December 2004 on theKerala Coast of India -Part I: Amplitudes, Mar<strong>in</strong>e Geodesy, 29 (4), 265-270, 20066) Luick, J.L., Murty, T.S. <strong>and</strong> Hubbert, G. (1994). Isl<strong>and</strong> Sea Level Response to Cyclones: A Numerical Case Study, Advances<strong>in</strong> Mar<strong>in</strong>e Science <strong>and</strong> Technology, 241 - 249, Ed. O. Bellwood, H. Choat, N. Saxena.7) Miles, J.W. <strong>and</strong> Lee, Y.K. (1975). Helmholtz Resonance of Harbors. J. Fluid Mech., 67, 445-464.-193-
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