This study involves using 3D GIS for landslide prediction on a slope between Beykent and Büyükçekmece towns, northwest Istanbul, Turkey. The main reason for studying Büyükçekmece town is that it is a rapidly growing suburban area in Istanbul with a serious problem of landslides made worse by of dense and inorderly settlement. I ask the questions: ‘’What is the landslide potential in this area’’; and ‘’How can we mitigate the hazard to humans?’’ I collected data on the slope, soil, geology, settlement and vegetation using maps and aerial photographs. One outcome of this project will provide officials, from local municipalities, government and private institutes, useful information about where to settle new sites, so that they are far from landslide hazard areas.

Landslides are common on the eastern side of Büyükçekmece Lake, along the coast near the Marmara Sea, because of the geological and geomorphological features. Landslides are caused by a number of factors based on the interaction of geology, hydrology, climate, and anthropogenic activities ( Demirci, 2001 ).

INTRODUCTION

DESCRIPTION OF STUDY AREA

Büyükçekmece, Turkey is located in the western part of the district of Istanbul (Figure-2). It covers 213 square km. It is surrounded to the south by the Marmara Sea, to the east by Avcýlar, to the north by Çatalca, and to the west by Silivri. Most of the land is plains.In the region, the average elevation is 75-85 meters(Arýkan,2002).In addition, study area on the aerial photo covers 1.124.789.94 sq.(Figure-3).

Figure-2: Location map of Büyükçekmece, Istanbul,Turkey

Figure-3: Aerial photo of study area

Büyükçekmece is located on the E-5 Highway, one of the most important and busiest roads in Turkey. The convenience of E-5 and its many buses to other regions of Istanbul are attractive factors of Büyükçekmece for the population. It is also a summer holiday place for the people of Istanbul. Although it has potential risk from landslide, day by day people continue to build summer houses there.

GEOLOGY OF STUDY AREA

Gürpýnar Formation:

‘’ This formation is composed of 150-400 m thick clays and is the most problematic land of Istanbul in terms of landslide. It developes landslides and flows in places with steep topography. This formation extends from Büyükçekmece Lake to Silivri and has a high water content. Gürpýnar means abundant springs. Geomechanic values which belongs to this accumulation show weak ground charecteristics. Although the Gürpýnar Formation is the dominant outcropping in Büyükçekmece , it is not good land for building development. Before building, the land should be analyzed in terms of ground, landslide and earthquake hazards. This formation is relatively young age, less than 30 million years old, and it occurs mostly on the west-southwest side of Ýstanbul ‘’(Istanbul Plan Raporu,1995) .

Figure-4-5: Geology maps of study area at colorful and black-white form

Figure-6:General geology map of European side of Istanbul(involvesBüyükçekmece)

LANDSLIDES

Landslides are a downslope displacement of masses of regolith and rock. However, it is more properly called mass movement, because these displacements do not always occur on land but on the seafloor as well,and many move by creeping or falling, rather than sliding ( K.Coch ,1995 ).

“ Landslides are among the most important processes of a catastrophic type. In general, the landslide activity is related to a number of factors like lithology, structure, land-use and morphology. There should seen the relationship of important geo-environmental factors to landslide activity and to developed an approach for landslide hazard zoning by using multi-data sets “ (R. P. and B.C. , 1990).

“ Landslides can be existed by a variety of different factors. There should be high-intensity or long-duration rainfall, snowmelt, seismic shaking, land-use changes, or human actions. Movements change in size and character from small rocks falling off a cliff to deep-settled landslides, for these reasons, landslides should be identify both societal and individual risk meaning “ ( Guzzetti, 2000).

Mass movement can be a serious hazard where expanding populations build homes on steep slopes. The hazards are especially severe when hillside development is done in a seismic zone, where ground shaking is a constant threat (K.Coch,1995).

“ The seismic threat to Istanbul comes from possible movement along the section of the North Anatolian Fault (NAF), under the Sea of Marmara and to the west of Izmit .The North Anatolian Fault is not only one long single break in the rock but also made up of lots of sections. When an earthquake occurs, the stress which led to it is passed along to the next section, a process called "stress pumping". Examination after the August earthquake showed: There had been no displacement on the NAF section under the Sea of Marmara. The fear is that this section is now bearing the additional stress passed on by the other earthquakes to the west.Istanbul is expected to withstand any earthquake far better than the areas hit recently. Some buildings would be damaged but much of Istanbul is built on better, firmer ground. But not all of Istanbul is safe as some parts of the city suffered in the past by ground collapses during the recent earthquakes even though they were relatively far away like Avcýlar. There are areas of poor ground which have been built upon. Seismic waves had became greater when they travel through softer ground made of sands and clays. Whole areas can shake like slow-motion. This produces a much greater damaging effect than quakes when they hit houses built on hard rock “ (ref-7-8).

Table-1: Istanbul soil characteristics map Table-2: Earthquake reasons

Mass movements include mechanisms both of flowage and sliding. Flowage is in velocity of the flow and it is greatest at the surface ( H. Bryant,1997 ). ‘’ To being the most fluid and having the highest velocity show the biggest hazard. The geomorphological analyses of the main slopes interested by landslides.The difference of level between the tpo and the foot of the slope and the dip of the slope determine the landslide capacity ‘’ ( G. and A.,1999 ). ‘’ Sliding is in the velocity at the base and it is similar at the top. In either case, the downslope movement of materials is a result of shearing stresses. It caused by gravity and the weight of the material and soil water. These forces increase with increasing angle and height of slope. Slopes can be described in plane view to see of increasing angle and height of slope clearly in Figure-1 ‘’ (H. Bryant, 1997 ).

Resistance is provided to stresses by the cohesive properties of soil particles and their internal friction . This depends on water content in pore spaces of soil . Water plages a vital role in mass movements . (H. Bryant ,1997)

Landslides are rock, earth, or debris flows on slopes due to gravity (Demirci,2001). ‘’ They usually occur fairly rapidly. Mostly aftar heavy rain and excess ground water reduce soil strength. A weak layer or bed of clay provides zone to landslide. Sometimes landslides may be started by the removal of support from the front of a slope or by sea erosion or artificial excavation ‘’ ( H. Bryant,1997 ).

“ In other hand, sometimes landslide can be useful or serve to nature and earth’s surface geology. They supply to redistribute soil and sediments around the ground. It can be abrupt collapses or gradual slides such as mudflows, debris flows, earth failures or slope failures. They can be existed not only by rains, floods, earthquakes, and other natural causes but also by human-made causes such as excessive development, cutting and filling. Landslides can be geophysical or human-made because of affecting factors. They can occur in developed areas, undeveloped areas, or altered areas for roads, houses, utilities and buildings “ ( Demirci, 2001 ).

‘’ Why do some mass movements travel downslope very fast, whereas others migrate so slowly that people notice the displacement only over a long period? Why are many areas free of major mass movements whereas others are continually at risk? How can we tell if mass movements are a potential risk when buying a home or seeking a building site? We will answer these questions by comparing factors that trigger mass movement with factors that prevent such movement “ ( K. Coch, 1995).

Mass movements involve two classes of forces ( Table-3 ).

Table-3: Factors affecting mass movement ( ref-6)

“ First of them is driving forces which promote them such as gravity, water. Secondly is resisting forces which deter movement such as water. Driving forces can be increased by natural processes and human activity. For example, water erosion, human excavation, deforestation, overgrazing, soil degradation or desertification processes can remove a portion of the lower slope, thus steepening the slope. Alternatively, people may build structures on the upper slope, increasing the gravity force (its weight). Both can cause a landslide “ (K.Coch,1995).

“ Types of mass movement are divided to three major types that are falls, slides, and flows according to the type of the movement, rock or sediment. Falls move through the air and land at the base of a slope, slides move in contact with the underlying surface, flows are plastic or liquid movements in water and more rarely in air which the mass breaks up and flows during movement “ ( K.Coch, 1995).

All types of mass movement are based on the material, moisture content, speed of movement ( K.Coch,1995).