The Latest Update from Post-Sunda Strait Tsunami Survey

Tsunami and Disaster Mitigation Research Center (TDMRC) of Syiah Kuala University sent a team to conduct a post-tsunami survey following the December 22, 2018 tsunami generated from Mt. Krakatoa complex in Sunda Strait. The tsunami survey was conducted from December 24, 2018 until January 1, 2019 (ten days). This team was led by Dr. Benazir and with team members Dr. Syamsidik (who is also a lecture at Civil Engineering Dept. of Syiah Kuala University) and Mumtaz Luthfi (see Figure 1). The team surveyed about 110 km coastline in Banten and about 14 km of Southern Lampung area that were affected by the 2018 Sunda Strait tsunami.

TDMRC Tsunami survey team led by Dr. Benazir. From left to the right: Mumtaz, Dr. Syamsidik, and Dr. Benazir.
Figure1 . TDMRC Tsunami survey team led by Dr. Benazir. From left to the right: Mumtaz, Dr. Syamsidik, and Dr. Benazir.

The team has successfully measured tsunami flow depths-directions, inundation limits, character of building damages, and tsunami boulders around Banten (from Anyer to Sumur) and around southern part of Lampung (Kalianda and Kiloan sub-districts). There were 113 points of flow depths/heights measured by the team in the affected area. The distribution of the flow depths can be seen in Figure 2. This figure shows that the higher flow depths were found around southeastern area from the Mt. Krakatoa, or around Panimbang. The highest flow depth was found at  around Cipenyu Beach-Tanjung Lesung of Padeglang District, which was 6.6 m from the ground or it was estimated about 9.6 from Mean Sea Level (flow height). Maximum tsunami run-up height was found at +14 m from Mean Sea Level. The location of the maximum run-up was also at an area between Cipenyu and Tanjung Lesung. There were some area where impacts of the tsunami were minor such as Citeureup of Pandeglang. The shape of the coastline created shelter effects on the area, causing minor impacts despite its surrounding areas were massively affected.

Map Flow Depth2
Figure 2. Distribution of measured tsunami flow depths from the 2018 Sunda Strait Tsunami Case, investigated by TDMRC Unsyiah started from December 24, 2018 until January 1, 2019.

The team also found that, at average, a flow depth as high as 2 m could destroy brick walls as these evidence shown in Anyer, Labuan and Carita of Banten, and in Kalianda of Southern Lampung.  This team could identify characteristics of building damages due to impacts of tsunami waves. Unlike what had been found in other post-tsunami surveys, this tsunami survey could specifically investigate impacts caused by the tsunami process only. Here, damages of the structures were shown due to foundation failures swept by the tsunami flow, impacts of debris, and the hydrodynamic forces (see Figures 3, 4, and 5).

Figure 3 A semi permanent house destroyed by the tsunami.
Figure 3. A semi permanent house destroyed by the tsunami.
Figure 4. A 3.8 m brick wall at SD Kunjir ( anElementary School) destroyed by tsunami.
Figure 4. A 3.8 m brick wall at SD Kunjir ( anElementary School) destroyed by tsunami.
Figure 5. A 3.8 m tsunami flow depth destructed a 4.0 m x 4.0 m brick wall in Kunjir of Kalianda.
Figure 5. A 3.6 m tsunami flow depth destructed a 4.0 m x 4.0 m brick wall in Kunjir of Kalianda.

The impacts on coastal communities in the affected area were massive. Boats and floating structures were swept away and damages. In one small bay in Kiloan sub-district of Lampung, the community lost 71 boats due to the tsunami and some floating nets were also destroyed. In coming months, tsunami recovery in the area could be a challenging process. Schools were still on vacation period when this survey was conducted. In the first week of January, the students will return to their school. However, large number of schools were severely damaged by the tsunami. The team found a number of elementary schools in Kalianda sub-districts will not be possible to be used by the student when the school’s vacation over on January 3rd, 2019. Figure 6 shows the impacts of the tsunami on Kunjir village in South Lampung. Tsunami inundation limits are shown in dashed-lines. Here, the tsunami wave attack the village at 60 degrees to the north (nautical direction). The tsunami could reach about 170 m from the coastline. A Sea dike structure protected the area from high waves has contributed to reducing the tsunami depths and velocity. However, some materials (rubble mound material) from the dikes were transported and destroyed some houses near the coast.

Figure 6. An aerial image of Kunjir in Kalianda that has been massively damaged by the Sunda Strait tsunami.
Figure 6. An aerial image of Kunjir in Kalianda that has been massively damaged by the Sunda Strait tsunami.

Figures 7 and 8 show a comparison of the area before and after the tsunami. This image was taken from an area at Kalianda city of South Lampung. Figure 8 clearly shows the different situation after the area was hit by the tsunami.

 

Figure 8. One area in Kalianda of South Lampung before the Sunda Strait Tsunami (image from Google Earth).
Figure 7. One area in Kalianda of South Lampung before the Sunda Strait Tsunami (image from Google Earth).
Figure 9. The area of Kalianda after severely damaged by the tsunami (image was taken from drone).
Figure 8. The area of Kalianda after severely damaged by the tsunami (image was taken from drone).

Analysis of these data will be continued to interpret some physical aspects of the impacts. We will attempt to infer the tsunami flow velocity from tsunami boulders identified around the area. Some boulders could be as large as 3.2 m in diameter could be found in the area as we found in Cipenyu Beach of Tanjung Lesung-Pandeglang (see Figure 9). Some of the boulders were identified as the revetment structure materials. This could produce a relatively more accurate estimation of the boulders’ origin.

Figure 7. A-3.2 m diameter tsunami boulder found at Cipenyu Beach of Pandeglang.
Figure 9. A-3.2 m diameter tsunami boulder found at Cipenyu Beach of Pandeglang.

TDMRC team will continue to process all the data and put them for public in coming weeks. Some remaining questions are still exist, such as detailed mechanism of the volcanic eruption induced tsunami. In some reports, the Mt. Anak Krakatau generated massive landslide at Northwestern to southeastern part of the Volcano. Interestingly, Kalianda which is located at the northern direction of the Krakatau Volcano site received severe impacts too from the tsunami. Tsunami wave diffraction and refraction could lead to this phenomenon. This complexity will require site investigation and numerical simulations to comprehensively provide the answer to the questions.

This survey is funded by a research grant from Partnership Enhanced Engagement in Research (PEER) Cycle 5 Grant No. 5-395, where Dr. Syamsidik is Principal Investigator (PI) of the research sponsored by United States Agency for International Development (USAID) and National Academies of Sciences, Engineering, and Medicines of US (NASEM). Results of this survey will also strengthen findings from other main research area of TDMRC PEER research. Further communication or correspondence regarding this survey can be retrieved from TDMRC of Syiah Kuala University or email at benazir[at]tdmrc.org.