Methodology Results and Discussion

33 Where: Q d = Discharge of the river C d = Discharge coefficient . – . g = Acceleration due to gravity = .8 ms B = the width of the spillway m = m B = the width of the upper spillway m = B + m.h w M = spillway slope h w = water level m then using equation : 8 . . ,8 . . . After the equation being executed, there is only one variable undefined which can be determine by using trial and error test. The result shows that B is . m. Free board design: The function of the free board is to avoid water to be over‐spilled into side of the river. Free board is determined based on the flow‐rate of the river. From the calculation, and based on the design rainfall, the free board height is obtained about . m. Thickness of main dam spillway: The thickness of the spillway must reflect from the stability and the probability of impact caused by the debris. The thickness is determined to be m. Depth of dam sub‐structure: This equation below is used to determine the depth of the dam structure p = sd ¼ h w + h m Where: h w = water level above spillway h m = effective height of the main dam Therefore from the equation it could have two different value of p which is: . p = h w + h m ; where p = . m . p = h w + h m ; where p = . 8 m From the results can be obtained that p value is in range between . 8 ‐ . m, where in this design the average value of p is . m. Slope of the main dam design: The slope of the main dam is designed for avoiding an intense impact from upstream’s debris because it might cause bigger damage into the dam and ignite an abrasion problem in downstream of the dam. Other than that, the slope will give a big influence to the stability of the main dam. Downstream slope of the dam is based on the material and the critical velocity that passing through the spillway and free falling on the apron floor. The downstream slope is determined with ratio of : . . 34 Apron design: We obtain that the length of the apron is 8. m therefore we could even the number and use m as the length of the design. Figure . Uteki Dam and Apron Sketch From overall calculation that has been conducted, the design of Sabo dams of Mulki River and Uteki River are typical. Below are proposed designs for Sabo dam in each river. Figure . Proposed Sabo dam for Uteki River Figure . Proposed Sabo dam for Mulki River hm = 6,34m hm = 2,5m hm = 2,8m H j =2,92 m H 2 = 2,5m 3 4.3. River normalization Normalization of both rivers could be done by removing sediment deposited along the stream, place doloscoarse materials at the toe of the river bank and the fix eroded areas above to protect surrounding infrastructures road, bridge and barracks . t is also necessary to remove large boulders in the upstream part which might triggered to be swept away when the peak discharge occurs. Normalization can be done using existing natural rocks arranged according to the regime figure a and in more flat areas, it can be proposed using the vertical gabions along the river side or embankment as shown in figure b. a b Figure . a Normalization using natural rocks, b Normalization using vertical gabions

5. Conclusion

This study has shown approaches to decrease flood velocity which implicates in reducing the peak flood discharge, reduce the momentum of debris transport, and design normalization of rivers to give assurance of safe water passage. Reducing river bed slope to get stable regime will have impact to the protection the residentialurban area from excess debris and boulders. nstallation of series of check dams to trap debris material and use it to build mild bed slope, along with providing sufficient channel capacity to convey at least year flood return period, with additional consideration of using dolos and gabion to stable the river banks. t is also suggested to normalize the river at selected sections and critical banks. Acknowledgements A sincere gratitude to Mr. Soeryantono for the guidance, also to Mr. Achdiana and all supporting team who contributed to this study. References Knight, D. W., and Samuels, P. G. Eds River Basin Modelling for Flood Risk Mitigation. Proceedings of an EU Advanced Study Course on River Basin Management, Birmingham, Oct . Nyssen, J. et.al. . The Effectiveness of Loose Rock Check Dams for Gully Control in Tigray, Nothern Ethiopia. Soil Use and Management : – . 3 U.S. Fish Wildlife Service FWS . . Debris Basins. Retrieved January USAD, , Making Cities Work: USAD’s Urban Strategy. http:www.makingcitieswork.orgfilesdocsMCWMCWurbanstrategy .pdf Visser, W.A., and ermes, J.J. . Geological Results of the Exploration for Oil in Netherlands New Guinea; Koninkl. Netherlands Geol. Mijnbouw Genoot., Verh., Geol. Ser., v. , spec no. p. https:en.wikipedia.orgwikiDebris_flow http:www.fao.orgdocrep ad 8 eAD 8 e .htm Joint Scientific Symposium IJJSS 2016 Chiba, 20‐24 November 2016 37 Topic : Disaster Management Vulnerability of Volcanic Ternate Island: Towards Ecosystem Based Disaster Risk Management Dyah Rahmawati izbaron a , Estuning Tyas Wulan Mei a , Arry Retnowati a , Desy Wahyuning Tyas a , Muh. Aris Marfai a a Faculty of Geografy, UGM, Yogyakarta, Indonesia 55281 DREAM Project of Community Resilience andEconomic Development Programme, UGM Abstract Ternate is a volcanic island amongst North Moluccas sland, which belongs to North Maluku Province. t is a volcanic ecosystem island centered on Gamalama Volcano. The island and its inhabitants are prone to volcanic eruption; hence vulnerability studies are essential for this area. This study applied vulnerability assessment using ndonesian Disaster Management Board standard regulation known as Regulation of Peraturan Kepala BNPB No. about risk assessment. There are four vulnerabilities assessed namely economic, social, physicalinfrastructure, and environment vulnerability. Weighting method is implemented on each vulnerability score, where social vulnerability considered as the highest weighting factor. Environment vulnerability is the lowest weighting factor in consideration that environment suffered less compare to other element at risk. Based on the calculation, the highest vulnerability is in the urban area while the rural area less vulnerable. As volcanic ecosystem based island, Ternate inhabitants depend on their ecosystem services to support their livelihood. Therefore, this study argued and proposed to apply ecosystem based disaster risk management upon Ternate as volcanic island to minimize its vulnerability. Keywords Ternate; volcano; vulnerability; ecosystem; Ternate; Indonesia

1. Ternate island and its volcano

Ternate island is located administratively in North Maluku Province, ndonesia. ts island is formed by Gamalama volcano activities and lied above a low angle subduction zone of Sangir and almahera arc amilton, USGS Prof. Paper., see Figure . The activity of plate tectonic surrounding North Maluku islands, including Ternate, triggers earthquake occurrences and volcano eruption when Gamalama is in the active level endrayana, et al. . Ternate inhabitants activities mostly rely on the natural resources nurtured by volcanic materials. The island was well known as the spices island during Dutch government colonialism in the 8 s ‐ s. The spices trail still can be found, particularly in the northeast to northwest of the island. Agriculture activities rely basically on the volcanic ecosystem of Gamalama. After the eruption and lahars Corresponding author. Tel.: + + 8 8 , fax: + ‐ ‐ E ‐mail address: dyah.hizbaronugm.ac.id 38 floods, sand and gravel mining occur in several river outlets. Due to its historical background as center of spices trade, Ternate sland contributes to the development of North Mollucas island in term of distribution of goods and services to Sofifi, the capital city. Figure 1. Ternate Island see red square and its surrounding tectonic plate Gamalama volcano is one of five volcanoes in North Maluku Province, which is still in active phase. The last eruption was in and shut down local airport for several days due to its volcanic ashes. Transportation accessibility in Ternate rely not only on air transportation, it also make use sea transportation as secondary option Ternate Spatial Plan, ‐ . There are at least three main ports in Ternate which support the inhabitants and visitors mobile accessibility when the air transportation close due to Gamalama eruption. Ternate sland is basically the volcano itself. Gamalama eruption in 8 s once forced inhabitants in the north part area to evacuate to closest island namely Tidore and iri sland. Since then, there was no adequate record of eruption that imposed people to leave their village and the main island. This study aims to analyze vulnerability of the island towards volcanic eruption. The method proposed highlights ndonesian National Disaster Management Board on vulnerability assessment Perka BNPB no. 22012, which applied analytic hierarchy process AP . n more details, this study targeted ecosystem based disaster risk reduction application to minimize vulnerability in the area.