Tsunami Modelling

In tsunami modelling, the application of the "standard procedure" of numerical modelling is very difficult to observe, as on the one hand some phenomena, e.g. tsunami source and inundation, are still not fully understood, and on the other hand detailed validation data are not available, especially when conditions change during the flood event. Basically, one has to account for three phases:

Wave Generation

There are numerous works which deal with source modeling so far. Among them, Okada's (1985) double-couple method plays an important role, because it has been the first easy to use sequence of closed equations. It accounts for deformations, strain and inclination in a half-space for point sources and bounded rectangular sources. Usually, it is applied to the water surface as an initial condition. However, after an earthquake, guessed seismic parameters can be used which are available from Internet within short time.

wave generate

Figure 1: Wave generation. (Source: Okada, Y. (1985) "Surface deformation due to shear and tensile faults in a half space", Bulletin of Seismological Society of America, 75 (4), 1135-1154.)

In case of using several sub-faults for earthquake modeling one way is to use the landslide option in MIKE 21 (see Kofoed-Hansen et al., 2001). Combining Okada's double-couple method with the landslide module comprises the possibility to account for horizontal displacements, to control the period of an earthquake itself and to include sub-faults.

sub fault model

Figure 2: Subfault model of the 17 July 2006 Java Earthquake tsunami (Sources: Fujii, Y. and Satake, K. (2006) "Source of the July 2006 West Java tsunami estimated from tide gauge records" Geophys. Res. Let., Vol. 33, L24317; Leschka, S., Hesselink, S., Kongko, W. and Larsen, O. (2008) "A tsunami generation tool for dynamic sea bottom deformation and its application to the 17 July 2006 Java Earthquake tsunami", E-Proc. of the International Conference on Tsunami Warning (ICTW), DMS16DE, 12-14 November 2008, Bali/Indonesia)

Subfault model of the 17 July 2006 Java Earthquake tsunami (sources: Fujii, Y. and Satake, K. (2006) "Source of the July 2006 West Java tsunami estimated from tide gauge records" Geophys. Res. Let., Vol. 33, L24317; Leschka, S., Hesselink, S., Kongko, W. and Larsen, O. (2008) ,A tsunami generation tool for dynamic sea bottom deformation and its application to the 17 July 2006 Java Earthquake tsunami", E-Proc. of the International Conference on Tsunami Warning (ICTW), DMS16DE, 12-14 November 2008, Bali/Indonesia)

Wave propagation and transformation

The most popular models for tsunami wave propagation are the so-called non-linear shallow water equation models like MIKE 21. Among others, they account for wave-bottom interaction and vortices which can be smaller than the distances between calculation points. In order to avoid numerical dispersion and to account for characteristics of the sea bottom adequately, one needs to select this distance very carefully.

Wave runup and inundation

Near the shoreline and on land, a variety of processes needs to be considered when calculating the water motion. In high resolution applications like hazard mapping, not only the water depth but also flow velocities have to be determined. From both parameters, fluxes as a measure of hazard in hazard assessment have to be defined. This can only be achieved using a very high resolution, sufficient to describe heterogeneous characteristics of a city. In recent numerical models not all types of energy losses can be considered. Instead, heterogeneous onshore roughness maps can be developed in order to overcome the gap between recently applied knowledge in onshore flooding processes and the requirements in hazard mapping. Onshore, the model should make use of aerial photos, digital terrain data, road maps, land use/land cover data, building masks (see Leschka et al., 2009b, Figure 3).

cilacap tsunami

Figure 3: Example for roughness map generation and implementation into tsunami model

References
Federal Emergency Management Agency (1979). The floodway: A guide for community permit officials. US Federal Insurance Administration, Community Assistance Series, N. 4, 1979.
Gayer, G., Leschka, S., Nöhren, I., Larsen, O. and Guenther, H. (2010) "Tsunami inundation modelling based on detailed roughness maps of densely populated areas", Natural Hazards and Earth System Sciences, 10, 1679-1687.
Kaiser, G., Scheele, L., Kortenhaus, A., Lovolt, F., Römer, H. and Leschka, S. "The influence of land cover roughness on high resolution tsunami inundation modeling", Natural Hazards and Earth System Sciences, in review.
Kofoed-Hansen, H., Cifres Giménez, E. and Kronborg, P. (2001) "Modelling of landslide-generated waves in MIKE 21", 4th DHI Software Conference, Elsinore, Denmark, 6-8 June 2001.
Kongko, W., Leschka, S., Larsen, O., Gayer, G., Noehren, I. and Guenther, H. (2008). "A sensitivity test of tsunami modeling using various data: Case study in Cilacap Indonesia", E-Proc. of the International Conference on Tsunami Warning (ICTW), DMS07ID, 12-14 November 2008, Bali/Indonesia.
Leschka, S., Hesselink, S., Kongko, W. and Larsen, O. (2008) "A tsunami generation tool for dynamic sea bottom deformation and its application to the 17 July 2006 Java Earthquake tsunami", E-Proc. of the International Conference on Tsunami Warning (ICTW), DMS16DE, 12-14 November 2008, Bali/Indonesia.
Leschka, S., Kongko, W. and Larsen, O. (2009a). On the influence of nearshore bathymetry data quality on tsunami runup modeling, part I: Bathymetry, In Proc. of the 5th International Conference on Asian and Pacific Coasts (APAC 2009), Vol. 1, pp 151-156, Eds. Soon Keat Tan, Zenhua Huang, 13-18 October 2009, Singapore.
Leschka, S., Kongko, W. and Larsen, O. (2009b). On the influence of nearshore bathymetry data quality on tsunami runup modeling, part II: Modelling, In Proc. of the 5th International Conference on Asian and Pacific Coasts (APAC 2009), Vol. 1, pp 157-163, Eds. Soon Keat Tan, Zenhua Huang, 13-18 October 2009, Singapore.
Leschka, S., Petersen, C. and Larsen, O. "On the requirements for data and methods in tsunami inundation modeling-Roughness map and uncertainties", In Proc. of the 3rd South China Sea Tsunami Workshop, Penang/Malaysia, 3-5 November 2009, USM Press, in print.
Okada, Y. (1985) "Surface deformation due to shear and tensile faults in a half space", Bulletin of Seismological Society of America, 75 (4), 1135-1154.
Slingerland, R.L. and Voight, B. (1979) "Evaluating hazard of landslide-induced water waves" In "Developments in Geotechnical Engineering" 2nd edition, Ed. B. Voight, Elsevier

Example of Results

17 July 2006 Java Earthquake tsunami

On 17 July 2006 an earthquake of magnitude M7.8 off the southern coast of western Java, Indonesia, generated a tsunami that affected over 300 km of coastline and killed more than 600 people, with locally focused runup heights exceeding 20 m. This slow earthquake was hardly felt on Java, and wind waves breaking masked any preceding withdrawal of the water from the shoreline, making this tsunami difficult to detect before impact (see Fritz et al., 2007). Tsunami generation has been done using the sub-fault model of Fujii and Satake (2006), derived from inverse modeling using tide gauges. The model data has been used as an input for generation of a time-variable bathymetry (Leschka et al, 2008).

References
Fritz, H. M., et al. (2007) "Extreme runup from the 17 July 2006 Java tsunami" Geophys. Res. Lett., 34, L12602, doi:10.1029/2007GL029404.
FujiiY. And Satake, K. (2006) "Source of the July 2006 West Java tsunami estimated from tide gauge records" Geophys. Res. Let., Vol. 33, L24317.
Leschka, S., Hesselink, S., Kongko, W. and Larsen, O. (2008) "A tsunami generation tool for dynamic sea bottom deformation and its application to the 17 July 2006 Java Earthquake tsunami", E-Proc. of the International Conference on Tsunami Warning (ICTW), DMS16DE, 12-14 November 2008, Bali/Indonesia.

Indonesian Tsunami Early Warning System

The Sumatra earthquake of 26 December 2004 was one of the largest ever detected ruptures in the Earth's crust. Only a few minutes after the earthquake, the first tsunami waves hit the coastline of Northern Sumatra, subsequently also affecting Thailand, Sri Lanka, India and the east African coast (Figure 4).

Shortly after the tsunami disaster, DHI participated in the implementation of a tsunami early warning system in the Indian Ocean. In a joint cooperation between Germany and Indonesia, such an early warning system has been established in Indonesia (GITEWS ). The implementation was mostly completed in 2009. The activities are fully integrated into the overall UN plans and strategies for the establishment of global and regional early warning systems (see Rudloff et al., 2009).

Because such short early-warning times do not allow numerical computations, it relies on a data base of thousands of pre-computed Indian Ocean Tsunami scenarios.

The task of DHI-WASY and its partners in the GITEWS project was to calculate the detailed tsunami run-up and inundation - for all of those scenarios with wave heights higher than 1m at the coast - in three priority areas, namely Padang (Sumatra), Cilacap (Java), and Kuta (Bali), to provide the basis for the preparation of high resolution hazard and risk maps (Gayer et al., 2011).

indonesia map

Figure 4: Tsunami following the Sumatra earthquake of December 26, 2004.

References
Rudloff, A., Lauterjung, J., Muench, U., and Tinti, S. (2009) Preface "The GITEWS Project (German-Indonesian Tsunami Early Warning System)", Nat. Hazards Earth Syst. Sci., 9, 1381-1382, doi:10.5194/nhess-9-1381-2009, 2009.
Gayer, G., Leschka, S., Noehren, I., Larsen, O. and Guenther, H. (2010) "Tsunami inundation modelling based on detailed roughness maps of densely populated areas", Natural Hazards and Earth System Sciences, 10, 1679-1687.