LANDSLIDE RISK MAPPING ALONG RANTEPAO – PALOPO ROAD SECTION, SOUTH SULAWESI PROVINCE

Rantepao – Palopo Road Section is 61 km long, which provides a connection between Palopo City and North Toraja District. Geographically, the road is winding, located in a hilly area with steep canyon. This condition makes the road vulnerable to soil movement or landslide. On 8 November 2009, a landslide, which caused a huge material and immaterial loss, took place. The scale of the disaster was so large that it was classified as a national disaster. In the landslide disaster management, the mitigation and preparedness effort will be more focused if complemented with spatial data in the form of landslide hazard map in Rantepao – Palopo road section. This research uses Public Works Minister Regulation No.22/PRT/M/2007 concerning landslide hazard area spatial planning as the reference which is modified and assisted with the application of Geographical Information System (GIS). Based on the topographical condition, the research location can be classified into three, namely typology A (> 1000 m msl), typology B (500 – 1000 m msl), and typology C (<500 m msl). Each typology consists of natural physical aspects with slope indicator, soil type, geology, rainfall, distance from the river/slope water system, distance from seismic fault and vegetation. Human activity aspects comprise planting pattern indicator, slope cutting, pond, population density, and mitigation effort. Based on the regulation, several indicators are difficult to apply in the road section study. Therefore, in this research, some modifications are made to several indicators. In the slope cutting indicator, to obtain slope cutting map, overlay process on topography map, slope variation map, and road section map was performed. The distance from the river/slope water system was obtained by calculating the distance from the river to the road, the closer the river to the road, the bigger the vulnerability. Meanwhile, the distance from the fault was evaluated based on the existence of seismic faults in the research location. Landslide hazard map was obtained by applying overlay process to natural physical aspects map and human activity map. To obtain a hazard map for Rantepao – Palopo road section, a modification by applying overlay to road section map and landslide hazard map was performed. Risk map on Ranteo-Palopo is divided into three types, i.e. low risk, medium risk, and high risk.


INTRODUCTION
Palopo City is a city in South Sulawesi Province which lies between latitudes 2° 53′15″ S -3° 04′08″ S and longitudes 120° 03′10″ E-120° 14′34″ E. It has an area of 247.52 km².Palopo City is divided into 9 sub-districts which are connected to North Toraja District (Rantepao) via the 61 km long Rantepao -Palopo road section.The road section is winding, located in a hilly area with steep canyon, all of which causes the road to be vulnerable to soil movement or landslide.
Based on Directorate General of Bina Marga (2009), the landslide taking place on 8 November 2009 in Rantepao -Palopo road section had 40 landslide spots with damage scale from medium to high and 80 landslide spots with low damage scale.To anticipate a more severe damage, a countermeasure / mitigation effort to anticipate disaster risk is vastly necessary.One of the efforts is by conducting disaster risk management through an evaluation of landslide potential/risk, which result is then used to formulate landslide hazard map in Rantepao -Palopo road section.
The research is aimed at conducting mapping on landslide hazard along the Rantepao -Palopo road section, particularly in West Bara sub-district as the research location to minimize the risk from the landslide.The purposes of this research are to give description on the condition of the controlling factor and triggering factor of landslide disaster in Rantepao -Palopo road section, to conduct the landslide hazard mapping in Rantepao -Palopo road section, to give recommendation as disaster mitigation efforts in Rantepao -Palopo road section.

Public
Works Minister Regulation No.22/PRT/M/2007 defines landslide as soil/rock mass movement process that slides from the original position so that it is separated from solid mass due to gravity while Karnawati (2005) defined landslide as soil, rock mass movement or slope composing material namely combination of soil and rock which moves downward due to gravity force.The trigger of landslide or soil movement is the loss of soil stability in a slope or certain location due to several factors, through natural process, unnatural process, or the combination of both processes.Karnawati (2005) explained that landslide occurs due to the existence of movement controlling factors, among others geomorphology, soil, geology, geohydrology, and land use, and movement triggering process such as water infiltration to the slope, seismic activity, human activity or land use change.Based on Law No. 24 Year 2007 on Disaster Management, mitigation is a series of efforts to reduce disaster risk through physical construction and improvement of community capacity in anticipating disaster risk.Disaster mitigation is a cycle of disaster management divided into 4 steps, namely, efforts taken long before disaster occurs, preparedness when disaster is about to occur, post disaster emergency response, and rehabilitation effort.

Public
Geographic Information System (GIS) is an information system which is designed to work with data referenced by spatial or geographic coordinates.In other words, GIS is data base system with special capabilities for handling spatial referenced data working in conjunction with a set of operations (Barus & Wiradisastra, 2000).

Research Location
The research scope of work is Rantepao -Polopo road section which is included in the administrative area of Palopo City, an autonomous city in South Sulawesi Province with an area of 247.52 km².However, spatial analysis and study uses regional data focusing on West Wara sub-district area.

Data Collection
Data collection needed for this research is primary and secondary data.The primary data is data from direct observation in the landslide area.The secondary data is rainfall data, topographic map, geology map, land use map, and road network map.

Field Survey
Field survey is an activity where the primary data is collected directly in the research location through direct observation in the research location or interview with local community.The data obtained from this activity is inventory data at every landslide spot, comprising landslide spot identification, landslide type characteristic, landslide causing factors, and identification of efforts that have been conducted as landslide mitigation efforts in the road section in the research location.The susceptibility, vulnerability, and risk analysis used GIS (using ArcGis 9.3) with analysis phases as follows (Danoedoro, 2004;2008

Susceptibility Based on Natural Physical Aspects
The susceptibility indicators based on natural physical aspects constitute of 7 indicators (Ministry of Agriculture, 1980) which weights are 30% for slope variation, 15% for soil type, 20% for slope composing material, 10% for rainfall, 7% for the distance from the river /slope water system, 5% for the distance from seismic fault and 13% for vegetation.The scoring on the weight of each indicator based on natural physical aspects is conducted by multiplying indicator weight with scoring weight that can be seen in Figure 1.

Vulnerability based on Human Activity Aspects
The vulnerability indicators based on human activity aspects comprise 5 indicators (Ministry of Agriculture, 1980), i.e. 16% for planting pattern, 22% for slope cutting, 16% for pond, 30% for population density, and 16% for mitigation effort.The scoring on the weight of each indicator based on human activity aspects is conducted by multiplying the indicator weight with scoring weight as seen in Figure 2.

Landslide Hazard Map and Hazard Map of Rantepao -Palopo Road Section
The landslide hazard map at Rantepao -Palopo road section is a combination of landslide susceptibility map based on natural physical aspects and landslide vulnerability map based on human activity aspects which was obtained through spatial analysis by multiplying indicator weight with scoring weight as seen in Figure 3. Rantepao -Palopo Risk Map is a combination of landslide hazard map and Rantepao -Palopo road section map, through spatial analysis using ArcGis 9.3 program as seen in Figure 4.     a) The result of this research is non-technical; therefore, if a technical management will be conducted, a further research is necessary.b) An analysis on the slope stability in the road design at the research location is needed.c) Formulation of regional regulations and policies in landslide prone area spatial planning and management is needed and should be followed by real action by the community and related government institutions.
Data analyzing process through digitization of every landslide vulnerability indicator with reference to Public Works Minister Regulation No.22/PRT/M/2007 (Ministry of Public Works, 2007) on landslide prone area spatial planning, classifies disaster prone area into two aspects that are susceptibility level based on natural physical aspects and vulnerability level based on human activity aspects based on the typology of the landslide prone area.

Figure 1 .
Figure 1.Landslide susceptibility map based on natural physical aspects (Ministry of Public Works, 2007).

Figure 2 .
Figure 2. Landslide vulnerability map based on human activity aspects (Ministry of Public Works, 2007).

Figure 3 .
Figure 3. Landslide hazard map in West Wara Sub district, Palopo City.