International Journal of GEOMATE, July, 2020, Vol.19, Issue 71, pp. 235 - 241 ISSN: 2186-2982 (P), 2186-2990 (O), Japan, DOI: https://doi.org/10.21660/2020.71.75161 Geotechnique, Construction Materials and Environment INVESTIGATION OF TSUNAMI IMPACTED AREA FROM ANAK KRAKATOA VOLCANIC ERUPTION Agus Dwi Wicaksono and *Fadly Usman Department of Urban and Regional Planning, Faculty of Engineering, Brawijaya University, Indonesia *Corresponding Author, Received: 05 Aug. 2019, Revised: 24 Jan. 2020, Accepted: 13 March 2020 ABSTRACT: The eruption of Anak Krakatoa Volcano is assumed to be the cause to generate tsunami wave; it can also be assumed that the wave source is only about 50.3 km from the coast. The interesting point is that the propagation of the tsunami wave was not caused by an earthquake but rather by volcanic material from Anak Krakatoa Volcano which slid into the sea. An investigation was conducted using spatial analysis of coastal areas around Tanjung Lesung, Banten with GIS analysis. While the propagation of tsunami waves was calculated from Anak Krakatoa Volcano that was projected towards Tanjung Lesung, Banten. These analyses were done to obtain assumptions of the wave velocity and propagation time from the center of the wave to the coastal area. The primary purpose of this study is to investigate the effectiveness of the vegetation belts as tsunami mitigation. This study used a 2D numerical simulation wave based on a VOF method. The numerical simulation calculated the interaction between the bore wave propagation and the vegetation belts. Numerical analysis was done to estimate the altitude and velocity of tsunami waves toward the coastal area of Tanjung Lesung. By comparing the results of the spatial and numerical analysis, the delineation of the tsunami-affected area on 31 December 2018 can be determined. Keywords: Anak Krakatoa, Tsunami, Vegetation belts, Numerical simulation, GIS Analysis 1. INTRODUCTION In this study, a spatial analysis was carried out using ArcGIS to delineate the impacted topographic The tsunami that hit South Lampung and Banten areas in the coastal area of Tanjung Lesung, Banten. Province on December 31, 2018, is an unusual Field observations were done with the help of aerial phenomenon in the case of a tsunami disaster. The photographs using unnamed aerial vehicle (UAV) tsunami happened without any earthquake, taking or drones. The numerical analysis is also used in this the coastal residents by surprise. study, which is useful to determine the height and In general, tsunamis that hit coastal areas begin velocity of the tsunami waves range entering the with an earthquake with a fairly shallow epicenter land. point. The official state institution that provides early warning of potential tsunamis in Indonesia is 2. LITERATURE REVIEW the Meteorology, Climatology and Geophysics Agency (BMKG), which, at the time of the tsunami Numerical simulations on waves are often used in Banten and South Lampung, did not even record in several studies, and in this research, it was earthquakes with specific strengths before the conducted to verify and compare the results of the tsunami. tsunami with several scenarios such as wave height, According to several past research reports, earthquake center, topography, etc. [2]. tsunamis caused by volcanoes are quite rare. The A satisfactory correlation was reached between eruption of Krakatoa Volcano in 1883 was the only the results of the explosion and the deposits caused series of eruptions that had enough data for detailed by the eruption, as was geologically determined to studies of tsunamis. Historical records estimated the be close to the eruption source. Thus, to determine tsunami to originate from a volcanic eruption, at a the tsunamigenic potential of a certain volcano, a distance along the coastline to a tide gauge specified index can be estimated to be in specific calculating 15 minutes of travel time to the coast, or planes to calculate the tsunami amplitude, and equivalent to the range of a tsunami wave in the therefore can also serve as a basis for tsunami risk observed interval time against the distance of contingency plan [1]. Krakatoa Volcano towards the shore [1]. Volcanic tsunamis are rare compared to seismic More in-depth studies need to be conducted to tsunamis. Volcanic tsunamis are characterized by determine the relationship between the Anak short to medium wavelengths, and their effects are Krakatoa eruption and tsunami wave propagation. often limited. By considering the diversity of source The mechanism of tsunamis triggered by a volcanic mechanisms, the prevention of volcanic tsunamis eruption is slightly different from the tsunami through monitoring is very challenging and must be triggered by an earthquake. combined with the preparedness policies of 235 International Journal of GEOMATE, July, 2020, Vol.19, Issue 71, pp. 235 - 241 potentially affected communities. In the case of hazard, so it can be assessed using spatial multi- hazard evaluation and warning, the worst case criteria [10]. Physical vulnerability parameters such scenario is from tsunamis caused by activity or as topographic data, distribution of settlements, etc. instability of submarine volcanoes, most of which are then analyzed and evaluated where all the are not monitored [3]. parameters in tsunami vulnerability and the tsunami The profile of the simulated waves from the risk assessment are analyzed through the overlaying tsunami caused by volcano avalanche (Fogo, Cape technique for each attribute in the geospatial Verde) can reach 50-100m. The comparison of two analysis [10, 11]. tested avalanche scenarios (massive or retrogressive) based on the numerical simulation results shows that the failure mechanism of landslide material can be simulated to determine the impact based on waves and rise of seawater [4]. The eruption of the Krakatoa Volcano in 1883 showed that the velocity of a pyroclastic flow was 15 minutes faster than a tsunami wave. While the pyroclastic flow that hit the coastal area, which is as far as 60 km from the center of the explosion, have killed around 1,000 people [1, 5]. As the tsunami waves have occurred, all the available data for the assumptions of wave propagation velocity and wave height in coastal areas use numerical simulations and GIS-based spatial analysis. Fig.1 Tsunami affected areas in Banyuasih Village, Previous research has shown that the use of Pandeglang, Banten, Indonesia numerical simulations can help researchers to calculate wave velocities as well as wave heights at The ability of the tsunami wave height to predetermined measurement points [6-9]. While the propagate to a higher location is also limited. Thus, use of satellite imagery or aerial photo processing topographic data in spatial analysis becomes very to identify tsunami risk have been conducted by important, and the range of the waves will also be previous researchers with a variety of variables [10, limited by how much volume of seawater enters the 11]. land. Thus, analysis needs to be done The reality faced by developing countries, like simultaneously, namely (1) analysis based on Indonesia, is the absence of a good early warning tsunami events, (2) numerical analysis to determine system which is closely related to information on the water level height (inundation depth), and (3) tsunami risks which should be widely accessible to spatial analysis to determine the estimated the community. Like Japan with the Japan inundation based on the results of the previous Meteorological Agency (JMA) is being the official analysis. state institution that provides reliable information One variable can be found in the tsunami- and is easily accessible to all communities [12]. affected area in Tanjung Lesung, Banten which is a This study shows the trigger for the tsunami terrestrial area. There is a great deal of vegetation wave was the activity of Anak Krakatoa Volcano. If distribution that can hinder tsunami waves to go a straight line projected from Anak Krakatoa further inland. Field observations show that shrub Volcano to Tanjung Lesung, Banten, the distance vegetation dominates the tsunami-affected area, but would be around 50 km. Another preliminary data other vegetations can be significant barriers to for numerical simulation is the velocity of the withstand tsunami waves, thus decreasing the tsunami wave, knowing the depth of the Sunda hydraulic force and the destructive force will also Strait being about 100m can determine the assumed decrease dramatically. tsunami wave velocity before it hit the coastal area The distribution of vegetation at the research in Tanjung Lesung, Banten, Indonesia. This study location as shown in Figure 1 above is drawn the will use CADMAS Surf/2D software with spacing between vegetations and resitential area. measurement parameters such as wave velocity and Two regions contrast, namely in areas with dense inundation height. The results of the numerical vegetation cover and coastal areas with low-density simulation can also show how big the horizontal vegetation cover. Based on the aerial photos in force is when compared between wave height and Figure 1, it can be assumed that the tsunami waves wave velocity. were dampened. Thus the destructive force was also Tsunami risk can be defined as a mathematical significantly decreased. The aerial photo above will calculation by entering the relevant variables and be analyzed based on topography and land cover. the vulnerability of the tsunami or the tsunami Thus the initial assumption that tsunami