SPATIAL MODELLING OF TSUNAMI INUNDATION ZONE IN THE SOUTHERN COASTAL AREA OF WEST JAVA INDONESIA

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1 Journal of Engineering Science and Technology Special Issue on AASEC 2016, October (2017) School of Engineering, Taylor s University SPATIAL MODELLING OF TSUNAMI INUNDATION ZONE IN THE SOUTHERN COASTAL AREA OF WEST JAVA INDONESIA NANDI*, DERI SYAEFUL ROHMAN Department of Geography Education, Universitas Pendidikan Indonesia, Jalan Dr. Setiabudi No. 229 Bandung Indonesia *Corresponding Author: nandi@upi.edu Abstract The southern coastal area of West Java is the region that has the potential of tsunami. It has geological conditions that face the active margin boundaries and historical data of tsunami events which occurred in southern part of West Java. The purpose of this research was to measure the spatial modelling of the tsunami hazard zone as a prediction and better mitigation in the near future. Exploratory method used in this research was through observation and measurement of research variables, where both physical and social parameters were taken directly from the field representing the population. The sample area was Cipatujah, an area located in West Java province, Indonesia. Analytical Hierarchy Process (AHP) was used in this research in order to make value and weight of each tsunami s indicator and the further analysis using GIS approaching in order to make spatial modelling of tsunami inundation zone with the elevation indicators. The results show that the Cipatujah has two types of tsunami hazard; medium and high-level zone. Keywords: Tsunami hazard; Disaster mitigation; Coastal area; West Java Indonesia. 1. Introduction Indonesia is the largest archipelagic state in the world with 16,056 islands and ± 81,000 km of coastline length [1]. Moreover, demographically, Indonesia is one of the regions with high population and about 60% of its population lives on coastal region. Based on physical condition and historical data, Indonesia is the 34

2 Spatial Modelling of Tsunami Inundation Zone in the Southern Coastal Area region which has potentials of the tsunami disaster events. In total, 110 fatal tsunamis occurred in Indonesia during , which caused casualties and have killed about 244,000 people in total [2] Geological setting According to Jones et.al [3], The Southern part of Java Island was located on active margin boundaries of the Sunda convergence margin. The Sunda convergent margin extends for 5,600 km from the Bay of Bengal and the Andaman Sea. This tectonically active margin is a result of the India and Australia plates converging with and subducting beneath the Sunda plate at a rate of approximately 50 to 70 mm/year. However, this subduction zone exhibits a gap seismicity from 250 to 400 km, interpreted as the transition between extensional and compressional slab stresses. Historical example of large interplate events include: the 1903 M8.1 event, 1921 M7.5 event, 1977 M8.3 event, and August 2007 M7.5 event. Indonesia was divided into 6 (six) zones in terms of the composition of its tectonic plate-convergence, the depth of seismicity variation, and the characteristics of tsunami generation [2]. The southern coastal area of west Java was located in Zone-B where tsunamis were generated by two types of earthquake, i.e. subduction of Indian Ocean Plate beneath the Eurasian Plate and back-arc-thrusting lies east-west in the north of the Bali-Lombok-Sumbawa islands [2]. Nonetheless, Southern Coastal Area has complexity of physical features. Geological and the records of tsunami event in southern coastal area of West Java indicate that the area has a high-level area of exposure to tsunami. Therefore, it needs to make a spatial modelling of tsunamis as a projection for the future in order to reduce the effects both the material damages and the death toll Social setting Social condition of Cipatujah sub-district demographically is an administrative region from Tasikmalaya Regency with population of people and total area that reaches ,6 Ha [4]. Population density of Cipatujah sub-district is 266 population/km 2. Most of the pople cultivate the land for a living and also some of them as fisherman with low economic condition [4] The 2006 Pangandaran tsunami sequence The last tsunami which occurred in southern Java Island was July 17 th 2006 with earthquake s magnitude 7,7 SR and earthquake s intensity VI MMI. In that tsunami, more than 550 people died [5]. According to the record of Tasikmalaya Regency Government, total victims that died and were missing because of tsunami in Cipatujah Sub-district and Cikalong Sub-district were 56 people and 218 houses were damaged [6].

3 36 Nandi and D.S. Rohman 1.4. Tsunami Disaster management in Indonesia According to Regulation of the National Disaster of Indonesian National Board for Disaster Management (BNPB) No. 2, 2012 on General Guidelines for Disaster Risk Assessments [7] the complexity of organizing disaster management requires an arrangement and planning. Countermeasures conducted so far have not been based on systematic measures and planned, so as to avoid overlapping and sometimes important steps are untreated [8-15]. Earthquake and tsunami in July 17th 2006 have the total amount of the victims in 4 areas (Ciamis, Tasikmalaya, Cilacap, dan Kebumen) which have reached 341 people died, 220 people were missing and 40 percent of the buildings in these area were damaged. The most affected areas in Tasikmalaya, were Cipatujah and Cikalong Sub-districts. At least 191 houses in this areas were damaged [6]. Based on those conditions, this study has an objective to measure the spatial modelling of the tsunami hazard zone as a prediction and better mitigation for the near future. The assessment of the tsunami threat level is critical in providing essential preliminary information for tsunami disaster management planning [16]. In disaster management, the threat level is one of the important parameters for determining the risk level of a disaster or the degree of probability of the occurrence of damaging consequences or expected loss to the community or population, or a system. The threat itself is often described as an event or phenomenon caused by natural or non-natural factors that potentially cause loss of life. Utilization of Geographic Information System (GIS) one of which is analyzing spatial data with the aim of making a modeling a distribution map of the area that has the consequence of damage caused by a disaster [17], in this case is the tsunami threat level map in Kecamatan Cipatujah. With the goal of implementing tsunami disaster mitigation appropriately, accurate information based on the characteristics and impacts that can be indispensable is needed in this study. Therefore, an analysis of the tsunami threat level should be developed based on tsunami threat level indicators that is aimed at gaining a clear picture of the nature of the study area, spatially or temporally, which refers to the historical data of a study [18]. In connection with the explanation, it is necessary to formulate a strategy for determining the tsunami hazard level as one of the main factors of planning to reduce the effects of both material and non-material damages or losses. Therefore, disaster mitigation planning requires specific information to be implemented. 2. Experimental Methodology 2.1. Location and materials The location in this research is administrative area of the Cipatujah. The method used in this research was exploratory. Exploratory method is a research method conducted by observation and measurement of research variables both physical and social are taken directly from the field representing the population [8]. Data and information as materials of this study are collected from topographical maps

4 Spatial Modelling of Tsunami Inundation Zone in the Southern Coastal Area issued by Geospatial Information Agency (BIG) and also uses Shuttle Radar Topographic Map (SRTM) imagery Parameters of tsunami hazard Sets of parameters that describe the tsunami hazard are classified into six parameters: Elevation, Tsunami waves known as an extreme wave, which has a greater wave height than the first wave that swept the coastal area. The elevation is classified into the parameters of tsunami hazard because elevation is one of the principal datasets required for the model to generate inundation of the tsunami [9]. We classified the elevation parameter into three classes of hazard. Slope, the coastline with flat morphology may cause higher for tsunami s run-up than the steep coastline [5]. We classified the slope parameter into three classes. Earthquake Intensity, Source of the tsunami which is associated with subduction zone are located in the southern part of the study area. Therefore, a wave of the tsunami has a relationship with the magnitude and the depth of the earthquakes. To determine the earthquake intensity, it can use the measurement of seismic intensity scale of Mercalli or the Modified Mercalli Intensity. We classified the earthquake intensity parameter into three classes. River Proximity, the river proximity parameter became an important part to determining the hazard level of the tsunami because the propagation of tsunami waves penetrated into the mainland through the water bodies, in this case the rivers. Therefore, we classified the parameter into three classes. Coastal Roughness, Geologically, the roughness of the coastal area has a very important role when the tsunami waves hit the land. The roughest surface of the coast will affect the weakening of the power of a tsunami. Population Density, Regulation of the National Disaster of Indonesian National Board for Disaster Management (BNPB) No. 2, 2012 on General Guidelines for Disaster Risk Assessments [7] concluded that the population density can be put into the parameters as an affected object of the tsunami. Table 1. Pair-wise comparison of tsunami hazard indicators. Elevation Slope Roughness Earthquake intensity River proximity Population density Elevation 1 5 1/ Slope 1/ Roughness Earthquake intensity River proximity Population density /5 1/5 1/5 1/ /5 1/5 1/5 1/5 1 1 All the parameters are analysed by using Geographic Information systems (GIS). Figure 1 shows that the steps of analysis start from gathering the data that

5 38 Nandi and D.S. Rohman support in this study and ends with spatial analysis using GIS approach analytical hierarchy process as a process that allows us to make structure of a system and its environment [10, 11]. By using the pair-wise comparison, we can make a weight of these parameters. The assignment of all parameters in this study through AHP is shown in table 1. Fig. 1. Research flowchart analytical hierarchy process and GIS analyses Measuring the distance range of tsunami inundation The distance of the buffering itself can be determined by the calculation method that combines several factors, including historical event of the tsunami in order to determine the height of the tsunami in the study area. Mathematical equation to generate the distance range of multi-buffering is [10]: logx max = log log(y 0 10 ) (1) Where X max shows the maximum range of the waves of the tsunami that can penetrate in, an Y 0 shows height of a wave in the coast (based on historical data). The hazard classification can be developed by the characteristics of interaction of hazard-forming and environment [12, 13]. In this study, the classification of the multi-buffering tsunami hazard zone can be shown at table 2.

6 Spatial Modelling of Tsunami Inundation Zone in the Southern Coastal Area Table 2. River proximity hazard zone. No. Distance from the Coastline Hazard Level meters High meters Medium meters Low 3. Results and Discussion Figure 2 represents the results of study using Geographic Information System (GIS) approach created two spatial models for tsunami. Figure 2(a) shows six meters height tsunami and Figure 2(b) is 12,5 meters height tsunamis, which are marked by red colour in the maps. To create these height parameters, an elevation map by using contour lines derived from Shuttle Radar Topographic Map (SRTM) imagery is used. The elevation data is the basic parameter of the tsunami hazard modelling, especially for the inundation factor [14]. In the Spatial information of the tsunami hazard with height of six and 12,5 meters are showed that all the area of Cipatujah Sub-District has potentially inundated by the tsunami. Based on the results of exploratory method, field ground check and calculation over Analytical Hierarchy Process, overall, the area of the study identified as a medium-high threat by tsunami. The areas with medium-level of hazard are Ciheras, Ciandum, Cipatujah, and Sindangkerta villages. And the area with high-level of the tsunami hazard is Cikawunggading Village. Fig. 2. Six-meter height (a) and 12,5-meter height (b) tsunami s inundation map. Fig. 3. Tsunamis hazard zone based on coastal proximity.

7 40 Nandi and D.S. Rohman Figure 3 shows the tsunami penetration to the main land. It was predicted that the tsunami will spread to the area of Cipatujah Sub-District from southern part to the norther part. Coastal proximity factor is related with the tsunami s inundation to penetrate on land. Calculation of the distance between the coastlines to the land on this study is by using multi-buffering analysis through GIS approach that generates the differences of zoning level of the tsunami hazard. The data that used to calculate the height of waves at the coast in this research study are based on the historical data of the tsunami events on July 17th 2006 in Southern part of Java [9]. The information shows that the maximum height of the tsunami on July 17th 2006 was 7,3 meters. Therefore, by using the eq. (1): logx max = log log(7,3 10 ) logx max = X max = 10 2,9638 X max = 920 meters With these calculations, it is concluded that the maximum distance of the tsunami can penetrated in to the land is as far as 920 meters from the coastlines. The red colour shows the inundation which reaches the area as high impact of tsunami. While the yellow and green colour shows the inundation area of tsunami classified in medium and low hazard level. Based on the field observation, the characteristics of the land use is main factor that influence of the range of inundation [15], where at the area study most people use it for cultivation. There is no barrier between coastal area to the settlement area. This means that the prone area is very vulnerable to tsunami hazard. Regarding this condition, the people who life in this area should increase their awareness and mitigate to prevent tsunami. 4. Conclusion AHP calculation and GIS approach can be useful to make assessment of the tsunami hazard level. According to the calculations of Analytical Hierarchy Process (AHP), Cipatujah Sub-district area has two levels on tsunami hazard level. The area of medium-level of hazard including are Ciheras, Ciandum, Cipatujah, and Sindangkerta villages. And the area with high-level of the tsunami hazard is Cikawunggading Village. GIS approaches in this study can make a description about tsunami prediction model in the near future in order to mitigate the threat of the tsunami or to increase awareness of the local people. For the government, instead, the tsunami hazard assessment can be followed by better mitigation programme in Southern Coastal of Java in the near future. References 1. Badan Informasi Geospasial. (2017). Hasil survey geografi dan toponimi. Retrieved January 20, 2017, from

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