Study of Intersection Coverage Area for Pedestrian Overpasses as Vertical Evacuation from Tsunami in Padang, West Sumatra

Padang City, as one of the highest vulnerable from earthquake and tsunami, has been transforming to become disaster smart city. However, the inadequacy of horizontal evacuation routes is caused by numerous tremors in 2007, 2009, 2010, and 2016 are indicating it is lack of occupancy for evacuee. Then, these condition is decreasing by traditional behavior evacuee are still using the vehicle and unwell planned evacuation as personally or in the community. The small number of vertical evacuation building and lack of awareness of community, and unmanaged the evacuation facilities make emergency response from earthquake and tsunami is uncontrolled in 0 – 20 minute for 30 minutes remaining time evacuate to inland in personally or community. Padang city has people density in the more than 1,317 people/km2 in the coastal area numerous potential for earthquake and tsunami risk. Pedestrian overpasses as primary facilities in many main roads in Padang City should be utilized for people to cross the road but it does not work properly but in fact, type of material, steel construction, was not durable with the climate in Padang that have coastal climate and a high number of behavior for crossing road in uncertain places. Regarding of the vulnerability in earthquake and tsunami risk, unmanaged construction and bad culture in crossing the road, pedestrian overpasses, especially in the intersection, will be redesigned to be a vertical evacuation. It will have a multifunction structure that is not simply for passing the pedestrian but also comprises remarkable facilities as a meeting point, commercial place and public facilities. Pedestrian overpasses for vertical evacuation from the tsunami will solve lack of area for construct vertical evacuation in the community. It can duplicate easily for any coastal cities that require vertical evacuation structures. Apparently, area availability will determine how vulnerable the site for vertical evacuation will suit for evacuee who living surrounding. Road intersection will be a good site for redesigning vertical evacuation Intersection of the road and have large space will be a good candidate for redesigning pedestrian overpasses as vertical evacuation structure. Road Intersection as vulnerable routes for horizontal evacuation is already happened in several occurrence of earthquake in Padang City. Based on google maps, every road will contribute a number of evacuee and mostly by using vehicle and fewer people who will evacuate by walking. The Study of coverage area intersection pedestrian overpasses as vertical evacuation from tsunami in Padang, West Sumatra will describe about how large the estimated capacity of pedestrian overpasses can be suit for vertical evacuation and how wide the area can be facilitated by this evacuation site. Remaining time of tsunami, walking space, readiness evacuation time, and time to reach upland. Those will be determined into how far the evacuee can reach the site. Based on the population density, it can observed the length of the radius can be serviced the evacuee to evacuation structure. People density will influence how large the coverage area for each site. According to this study, horizontal evacuation from tsunami in Padang city is still vulnerable for the people who living in coastal area. Bottleneck evacuation can be solved by build a vertical evacuation near by the bottleneck zone. Pedestrian overpasses for vertical evacuation is designing to accommodate the evacuee can save their life from the tsunami run up because incapability to reach inland. Keywords— Tsunami, vertical evacuation, pedestrian overpasses, intersection, and road. JIRS Volume XV Nomor 1, April 2018 e-ISSN: 2655-2124/ISSN: 1858-3695


I. INTRODUCTION
Earthquake gap along Mentawai Island contribute earthquake and tsunami potential risk directly to Padang city. This segment is one the gap Sunda mega-trench that is located next to Sumatra Island. According to seismology, Sunda mega-trench has probability exceedance of tsunami occurrence in every 30 to 200 years. Based on history tsunami occurrence in Padang city is exposed in table 1, Pedestrian Overpasses Bridge for Vertical Evacuation from Tsunami (POBET) becomes one of the substitute solutions for the lack of the availability if it compares to people density is concentrated in the coastal area [1]. The motivation POBET as the best alternative solution for vertical evacuation is, 1) To accommodate evacuation blockage triggered while community evacuate horizontally 2) To distribute evacuee to have alternative evacuation site with efficient and discrete evacuation or evacuation shelter. 3) To effortlessness the accessibility of the traffic ways that is passed by peoples, drivers or community nearby. POBET plan at intersection with the reasons below, 1) Intersection has proper area to build a pedestrian overpasses bridge and have a large space for vertical evacuation as a superstructure. 2) Intersection has a good access for pedestrian vehicle in many directions. 3) Intersection would become assembly point in some sub district and centre of evacuation blockage in emergency response.
Regarding to altitude of sub district in Padang City, it is publicized 34% from total area are inundated from tsunami, it shown on  table II   TABLE II  SUBS DISTRICT ALTITUDE OF PADANG CITY,  Based on estimated magnitude of earthquake and tsunami [17] West Sumatra claimed is inundated by 15 meter high and Padang city inundated in average by 9 meter high from tsunami with distance of prone area from 0.5 -1,5 kilometers from the shore line. The most vulnerable zone is in Padang Barat with lowest altitude from the level of sea and also the most protected vicinity to evacuate horizontally.

II. METHODOLOGY
This research is based on a road map of vertical evacuation study from tsunami study on figure 4 is shows about preliminary study for vertical evacuation is observed about the condition of prone area related to the vulnerability come up on the community and the environment that is measured the level of awareness community who's living in the prone area.
[1] Type of evacuation, horizontal and vertical evacuation, will be indicated from the behavior and evacuation culture of community to reach inland/high building, this factor will estimate travelable time due to remaining time for tsunami inundation. Location for evacuation would be nearest site for community to reach, good access, adequate to facilitated people to get inside. Then, the adequacy of evacuation (horizontal and vertical) should be compromising the population density, capacity of routes and capacity of shelters.
The existing evacuation feasibility study contributed the impact for searching for candidacy the site for horizontal and vertical

Subdistrics in Padang City
Tsunami Innundation Level from the shore line evacuation. It will be measured by the ratio from the evacuation (horizontal and vertical) infrastructure can be occupied by the community. Then, it also figure out by supporting facilities such as hospital, schools, government building, army facilities and any public building that should be withstand during the emergency response. These parameters will be determined into table IV about vulnerable index for evacuation.
POBET vulnerability parameters for tsunami is considering about location, buildings, social, infrastructure, shelters, and accessibility. Location vulnerability will indicate about buildings plan and coordinates for candidates of POBET by using Google earth application to estimate location and affected area would be accommodated by POBET [1]. Evacuations vulnerability, Routes or Buildings, will consider about distance from shoreline and riverside. Minimum distance from is approximate 1 km far from coastline. Parameters is used to identify the buildings candidate for POBET will be indexed to measure the vulnerability based on,

III. RESULT AND ANALYSIS
This Study is focus on how to determine and select potential site candidate for POBET.

A. Evacuation Routes Vulnerability Study
Regarding to evacuation map on figure 3 from BPBD Padang City, Padang City is divided into 6 (six) zone evacuation area [13]. It is shown on evacuation map that is produced by BNPB in 2010. The Evacuation zone is derived into, All this evacuation routes in these sector is eliminated for POBET site candidate by the evacuation routes have junction and 500 meters as minimum distance from the shoreline.
The candidate intersection evacuation routes for POBET are selected into 11 intersections from all evacuation routes that have been claimed by BPBD of Padang City. The candidates are,

B. Preliminary Study for POBET Site Candidate
According to type of tsunami in Padang, Samuel, 2012, is mentioned tsunami in Padang has 30 minute maximum for the remaining time. It has 200 km far epicenter and estimated have big tremor and devastated any buildings surrounded. It follows with big magnitude and tides.
Coverage area for evacuation based on tsunami mitigation guidelines for evacuation building specified on the travelable distance for evacuation, (L 1 ), from the relationship between the length of time which tsunami takes to reach ,(T), the walking space, (P 1 ), time take to start evacuation after the occurrence of an earthquake, (t 1 ), and time taken to reach uplands, upper floors, etc, (t 2 ). This correlation factors is designated into formula [7], Time taken to reach uplands, upper floors, etc ,(t 2 ), is estimated from the maximum run up of tsunami in certain place to speed for ascent/descent of stairs (minimum condition for elderly person), (P 2 ). This condition can derived into, Specification of walking space indicates the worst circumstances for a person to evacuate to inland or upper floors. It figure out into two condition, walking under normal conditions (P 1 ) and ascending/descending (P 2 ) based on table VII Regarding to the candidate intersection site for POBET, they need to measure the coverage area can accommodate people to evacuate to POBET as alternative vertical evacuation from tsunami. It derived from how large the estimation area intersection, elevation from the shore line, estimation of people density that will accommodate for POBET capacity with 1 people will accommodate for 1 meter square. These data is shown on table VIII,   (1) the travelable distance for evacuation is derived from the walking space for evacuation that condition is imagined crowed situation, coefficient of walking space, (P1) 1,29, the remaining time from tsunami (T) is taken for the minimum time in 20 minutes (1200 seconds), time take to start evacuation after the occurrence of an earthquake (t 1 ) is about 30 second and time taken to reach uplands, upper floors, etc, (t 2 ) is about 3,15 second. As the result, estimated coverage in every site POBET has 1505 meter or 1,5 km. Based on the field data, estimated POBET area and density of population will influence the accommodation coverage of POBET based on equation (3) in table

IV. CONCLUSIONS
This study shows horizontal evacuations routes is still need substitute infrastructure to accommodate evacuee to safe from the tsunami. Most of the routes are insufficient to deal out people to reach inland safely. Hereby, some wrapping up that commence POBET as a better solution for evacuation, 1) Elevation Padang city is about 34% is inundated by tsunami that have 9 meter high of run up 2) In the remaining time from tsunami in 20 -30 minute is not suitable for peoples in Padang reach inland safely by horizontal evacuation by walking. 3) Travelable time from shore line to the intersection in evacuation routes are still in excess of remaining time from tsunami. It is about 50% intersection is still reach more than 20 minutes. 4) Accumulation of travelable time (shoreline to intersectionintersection to inland) to inland is still over 30 minute. 5) Regarding to the behavior peoples in Padang City are still using vehicle than for evacuation will triggered evacuation bottleneck in every intersection. 6) As solution the evacuation bottleneck, POBET will accommodate people who get in trap during the evacuation to evacuate vertically. 7) Based on the remaining time tsunami, inundation high, the average coverage area of POBET have radius 1,5 km far from the POBET site. 8) The Accommodation Radius of POBET, it has 0,5 -2,5 km far based on the people density are living surrounding the POBET. 9) The large of radius of coverage area of POBET is influenced the people density, walking space, travelable time, inundation height and remaining time from the tsunami.

ACKNOWLEDGMENT
We would like to thank you for Dana DIPA Politeknik Negeri Padang for funding this research in Contract Number: 088/PL.9.1.4/LT/2017 and Google Map applications that have large contribution to gain any desk data for investigate every site candidate of POBET for free of charges. Then, Thanks to my former student Aufaa Rozaan and Afrina Roza who give big contribution for this research.