GIS BASED NATURAL DISASTER MAPPING: A CASE STUDY O. Avsar a, *, Z. Duran a , D. Z. Seker a , M. Hisir a , M. Shrestha b a ITU, Civil Engineering Faculty, 80626 Maslak Istanbul, Turkey – (avsara, hisirm)@yahoo.com, (duranza, seker)@itu.edu.tr b River Basin Research Center, Gifu University 1-1, Yanagido, Gifu-city 501-1193, Japan - madhu@ina.janis.or.jp KEY WORDS: Landslide, Geology, Geo-referenced, Digital Elevation Model, Climate, Water, Block ABSTRACT: Sebinkarahisar Township which has been selected as study area is located between Black Sea and Central Anatolian Regions. Not only geological characters but also social life in the township had been affected by these two regions. History of the settlements goes back to early Hittite period. Even it was a province in the first years of Turkish Republic, today it is a township belongs to Giresun province. Landslide is one of the most effective natural disasters in the region. Due to wrong decisions of local authorities caused lots of damage, during the last two decades. Firmly some part of the city, which has the highest landslide risk, is prohibited for new settlement, and some studies (still at beginning level) had been carried out. In this study, morphology, geology, land cover, slope inclination, elevation of the region is investigated. 1:25000 standard topographic and 1:25000 standard geological maps are scanned and referenced geographically by using Topol software. Contours, settlements, roads, meadows, and forests are digitised from the geo-referenced topographic map and different geological formations are digitised from the geo-referenced geological map. These layers are saved as blocks. By extrapolating method, the third dimensions of these blocks are reedited from the contours using QuickSurf module of AutoCad software. These blocks are added to ArcView software and digital elevation model is produced. Also the reasons of the occurred landslides are investigated and they are added to ArcView software. Finally the region classified by the amount of their potential landslide risks and risk map is produced. The aim of this study is; producing a landslide risk map and giving some precautions before the planning of the settlement. * Corresponding author. 1. INTRODUCTION Landslide is defined as a slide of a large mass of dirt and rock down a mountain or cliff. Landslide is the exterior and downward movement of the natural rocks, every sort of soil and artificial filling. In another words, the movements of the masses on the slope sides by the grounds. Usage of industrial, agricultural and settlement areas causes some geological events. Landslides are examples of these geological events. Naturally occurred landslides may cause lost of lives and wealth and also communication and transportation interruptions. Therefore; stages of landslide constitutions and causes should be researched. In this study, the occurred landslides in Sebinkarahisar Township were examined. There are many active and potential landslide zones in this region. Morphology, geology, land cover, slope inclination, elevation of the region is considered. The aim of this study is; producing a landslide risk map and giving some precautions before the planning of the settlement. 2. CAUSES OF LANDSLIDES For the landslides, water flow, excavation, dissociation of the rocks and changing of plant cover are the main reasons. The along effects of these causes facilitate the landslide occur. According to topographical conditions and excavations, sliding movements turns to fall downs. Because of not doing stability analysis for excavations for settlement and transportation constructions, there are many landslide occurs in potential landslide zones. Also, changing the plant cover for earning new agricultural fields or growing up more beneficial crops improves the landslide risk. In addition to them natural (earthquakes), artificial (explosions for road and mine constructions) vibrations, rain dose, fountains and churn waters are the other effects. (Tarhan, 1991) According to researches; landslides are caused by %42 water effects, 26 % excavations, 26 % dissociation, 4 % changing of plant cover and 2 % other effects. When these effects are considered together, the numbers on table 1 are appeared. Causes Percentage Dissociation 3 Water 9 Excavation 11 Water + Excavation 20 Water + Dissociation 22 Excavation + Dissociation 11 Water + Excavation + Dissociation 10 Water + Dissociation + Plant Cover Changing 9 Others 5 Table 1. Causes of Landslides 3. FORMS OF LANDSLIDES Snaps As a result of snapping of soil and rocks, mass falls, leap, hop or roll freely. There is a little or non effect between the units during the snapping. Movement may change from fast to very fast. There may be little movements before the main movements. The main reasons for snaps are climate, frost, and dissociation of the blocks by hydrostatic effects, dynamite explosions and carvings of the waves.