SCIENTIFIC SYMPOSIUM GEOGRAPHY AND SUSTAINABLE DEVELOPMENT OHRID, REPUBLIC OF MACEDONIA; 22-25.10.2009 APPLICATION OF REMOTE SENSING AND GIS IN DETECTION OF POTENTIAL LANDSLIDE AREAS Ivica MILEVSKI, Blagoja MARKOSKI, Svemir GORIN, Milorad JOVANOVSKI* University “Ss. Cyril and Methodius” - Skopje, Republic of Macedonia FNSM- Dept. of Geography, *Faculty of Civil Engineering ivicamilevski@gmail.com ABSTRACT In this work a GIS procedure of potential landslide areas assessment, based on digital elevation model (DEM), satellite imagery and other digital data analyses is presented. The research area is Gevgelija-Valandovo basin (1077.0 km2), which is located in the southern part of the Republic of Macedonia. This basin is very heterogeneous in regard to topography and vertical relief (44 m to 2112 m), geology (from erodible clastic sediments to very solid limestones), climate (especially with altitude), vegetation, and human impact as well. Thus, as a consequence of suitable natural-geographic factors (geology and soil structure, topography, climate, vegetation) and significant human impact, some sites in this area have severe erosion with numerous landslide occurrences. For that reason, in GIS procedure several landslide-related factors are weighted and analyzed, and with cluster classification, areas with different potential to landslides are identified. Key words: GIS, remote sensing, landslide potential INTRODUCTION Because of large area with erodible rocks, steep slopes, semi-arid climate and weak vegetation, processes of mass movements and especially landslides are very often in Macedonia. They typically appear on steep and south inclined hillslopes in the lower mountain sides, generally below 1000 m of altitude, were human activity is significant. On these altitudes (on the rims of depressions-basins), geology is very favorable factor, because lacustrine sands and sandstones Pliocene in age (which fulfill most depressions), usually are superimposed over inclined lacustrine clays as a non-permeable layer. Climate is additional factor, especially in regard to precipitations, with mean annual value of only 500-700 mm. That inhibit vegetation growth, and frequent storms and heavy rains in summer half of the year contribute to excess runoff and high surface water absorption. Another cause is weak vegetation cover heavily changed and degraded under human impact, which permit over wetting of soils and surface layers of clastic or weathered rocks. However, the leading trigger of landslides is human activities in the landscape with road and channel construction in susceptible terrain, or by building of heavy objects on sloped terrains. Followed consequences of landslides can be significant not only in regard to economic costs, but sometimes with casualties; like in landslide of Gradot hill (Manakovic, 1960). For that reasons, there is a real need for estimation and mapping of potential landslide areas in Macedonia, which is very complex task, because of many natural and anthropogenic factors related with landslide processes. But with newer GIS technologies, digital elevation models and remote sensing tools, offer better possibilities for such research. Exactly these tools are used in landslide susceptibility estimation in Gevgelija-Valandovo basin, as a case study area. In this research, several parameters as a topographic, geologic and vegetation indices were chosen, although worldwide there are many different approaches for selection of best fitting factors. According to the some authors, depending on the characteristics of the study area, at least three factors which are topography, lithology and land use have to be included in GIS analysis. Nefesliogly 455 НАУЧЕН СИМПОЗИУМ ГЕОГРАФИЈАТА И ОДРЖЛИВИОТ РАЗВОЈ ОХРИД, РЕПУБЛИКА МАКЕДОНИЈА; 22-25.10.2009 et al. (2008) take into account elevation, slope, plan and profile curvature, geology and stream power index. However, Terzaghi (1950) group landslide factors in two categories, namely internal and external causes. The former include those mechanisms within the mass that bring about a reduction of its shear strength to a point below the external forces imposed on the mass by its environment, thus inducing failure. External mechanisms are those outside the mass involved that are responsible for overcoming its internal shear strength, thereby causing to fail. RESEARCH AREA OVERVIEW Gevgelija-Valandovo Basin is located in the southern part of the Republic of Macedonia, on the border with Greece. On the southwest and west side it is bordered with Kožuf Mountain (2166 m), from northeast side with Konečka (1159 m), Gradeška Mountain (1031 m) and Plavuš (996 m), while from east side with Belasitsa Mountain (2031 m) and several hills up to 720 m high. On the north, with deep Demirkapija Gorge on Vardar River, this basin is separated from large Tikveš Basin, while on south it is opened toward Greece. Gevgelija-Valandovo Basin is generally elongated in NW-SE direction i.e. along the Vardar River which flow through the middle of the basin. However, because of complicated tectonic in this region, Valandovo sub-basin is extended in east-west direction, and Gevgelija subbasin have undefined elongation. METHODOLOGY Landslide risk assessment in study area of Gevgelija-Valandovo Basin is performed trough the detailed analyses of several digital datasets: DEM (Digital elevation model) for topography acquired from 3”SRTM DEM; raster grids for vegetation cover acquired from Landsat ETM+ satellite imagery and from Corine Land Cover 2000-CLC2000; digitalized geologic (lithology) map etc. In this way, influence of most relevant topographic indices (hypsometry, slopes, curvatures, aspects), vegetation index (vegetation cover) and lithology hardness is estimated. Then, with clustering module incorporated in SAGA GIS software, and superimposing of several layers, sites (clusters) with different potential of landslides were identified, especially showing high risk areas. These computations resulted in digital map of landslide potential, which compared with real indicators and measures show satisfactory fitting. Certainly, many other factors influenced risk of landslides, but because of avoiding complexity, they are not considered here. However, previous procedure may be helpful to relatively fast and accurately predict landslide risk in the landscape. TOPOGRAPHIC INDICES OF GEVGELIJA-VALANDOVO BASIN Topographic indices of Gevgelija-Valandovo Basin are computed from 3”SRTM model (version 3 from CGIAR-CSI, Jarvis et al, 2006), though SAGA GIS v.2, MicroDEM v.10 and Surfer v.8 software. Used digital model, according his spatial resolution (3” or about 70x90 m in this latitude) and quality is just acceptable for analyzed area (Markoski & Milevski, 2005; Milevski, 2005a). On other side, SAGA GIS and Surfer v.8 software, together gained incredible results. Before analytic procedures, original DEM model for practical purposes is preprocessed and re-interpolated to square pixel resolution of 30m. From numerous topography indices which can be extracted from DEM, only several landsliderelated were selected: hypsometry, slopes, curvatures, aspects, relief, topographic wetness 456 SCIENTIFIC SYMPOSIUM GEOGRAPHY AND SUSTAINABLE DEVELOPMENT OHRID, REPUBLIC OF MACEDONIA; 22-25.10.2009 index and stream power index. On the end, a cluster classification of principal topography features is made. Hypsometry of Gevgelija-Valandovo basin has generally indirect influence on landslide processes, thought climatic and biogenic zonality (vegetation cover), as well as intensity of human impact. The basin area lay between 45 m (riverbed of Vardar River), and 2112 m (peak on Kožuf Mountain), which is altitude difference of 2067 m, and mean altitude is 400.4 m. Largest part of area lay from 45-300 m (48.0%), and from 300-500 m (23.6%), when higher elevations are lesser: 500-1000 m (22.2%), 1000-1500 m (4.9%), and 1500-2112 m (1.3%). On lower altitudes (45-1000 m) which cover great area, the mean annual precipitations is 600-700 mm, there are high temperature amplitudes (Lazarevski, 1993), sparse (human changed) vegetation, and significant human activity (roads, land-use, constructions). So it is obvious that here we can expect elevated landslide potential, which is shown on further analysis. However, except for elevation analysis, DEM model may serve for processing of mean temperature, precipitation and solar radiation grid with means of vertical gradient algorithms. Outputs can represent indexes of climate potential in some equations for landslide modeling. One of the most important topographical factors conditioning landslides is the slope gradient. In the regional landslide susceptibility or hazard assessments, several researchers (i.e. Maharaj, 1993; Jager and Wieczorek, 1994; Atkinson and Massari, 1998; Baum et al., 1998; Guzzetti et al., 1999) took into consideration statistical techniques for the assessment of slope gradient in terms of landslide activity. In the present study, the slope gradient is considered as a conditioning factor during the analyses. Slopes like first hypsometry derivatives, has strong effect on landslide processes, especially by slope angle, slope length, and slope curvature, so these parameters must be considered in every research of landslide potential. The influence of slope angle on landslide occurrence is the easiest factor to understand in the way that generally, steeper slopes have a greater vulnerability to landsliding. This does not prevent failures from occurring on gentler slopes. Other factors may make a gentle slope especially sensitive to failure, and thus in this situation could be determined to have a relatively high hazard potential. However, slopes on Gevgelija-Valandovo basin was obtained from preprocessed 30m DEM, and corrected for simple empiric coefficient in form of α=α*(1+(α/150). Further analysis show that slope values varies from 0° (flats in central part of the basin) to >60° (steepest slopes on highest areas of Kožuf Mountain), with mean slope of 13°. Almost 45.6% of area cover slopes lower than 10°, while largest areas has slopes of 0-5° (26.3%), and 5-10° (19.2%). Those are gentle slopes and flats usually related with deposition. Moderate slopes of 10-20° and 20-30° cover 31.3% and 17.1% respectively, usually related with deeper gully erosion, denudation, landslides and rock falls. Higher slopes (more than 30°) cover only 6%, and generally represent areas of severe erosion, landslides and rockfalls. Similar like elevations, slopes are applied in landslide models, commonly as a main parameter, which is the case in our model too. The term curvature is generally defined as the curvature of a line formed by intersection of a random plane with the terrain surface (Wilson and Gallant, 2000). The influence of plan curvature on the landslide processes is through the convergence or divergence of water during downhill flow. In addition, this parameter constitutes one of the main factors controlling the geometry of the terrain surface where landslides occur (Evans, 1998). Profile curvature is in steepest direction, while planar curvature is along contour direction, and correlate with degree of azimuth change. According to Ohlmacher (2007), statistical analysis of plan-curvature and landslide datasets in Appalachian Plateau indicate that hillsides with planar plan curvature have the highest probability for landslides in regions 457 НАУЧЕН СИМПОЗИУМ ГЕОГРАФИЈАТА И ОДРЖЛИВИОТ РАЗВОЈ ОХРИД, РЕПУБЛИКА МАКЕДОНИЈА; 22-25.10.2009 dominated by earth flows and earth slides in clayey soils. The probability of landslides decreases as the hillsides become more concave or convex. Hollows have a slightly higher probability for landslides than noses. According to Komac (2006) terrain curvature indicates that landslides tend to occur in concave areas where the concentration of pore water is higher. Planar and profile curvature for Gevgelija-Valandovo catchment are calculated in SAGA software, where convex terrains has positive values, concave terrains negative values, and straight terrains (flat hillslopes) near zero values (with units m/m). Table 1. Value for planar and profile curvature in Gevgelija-Valandovo basin Planar curvature Profile curvature Description-landslide vulnerab. Value Area in % Value Area in % < -0.002 8.4 < -0.002 6.9 Valley bottoms-high -0.002 to -0.0005 16.5 -0.002 to -0.0005 21.1 Bottom concave hillslopes -very high -0.0005 to 0.0005 45.5 -0.0005 to 0.0005 49.4 Flat hillslopes-moderate 0.0005 to 0.002 19.5 0.0005 to 0.002 16.2 Bellow-ridges convex-low > 0.002 10.2 > 0.002 6.4 Ridges-very low According to our research, most suitable curvature for landslide processes are moderately concave (-0.002 to -0.0005) which cover 21.1% of the Gevgelija-Valandovo basin area. Inclination or terrain aspect is another valuable parameter related to landslide potential, where south aspects are dryer, hotter, barer, and more eroded in contrast to north aspects. According to the research of 205 landslides examples in Three Gorges region in China, it is found that the slope contributes most whose aspect is towards south, southeast and southwest aspect contribute moderately, and other five aspects contribute little (Caiyan, Jianping and Meng, 2006). Similarly, Tanaka (2005) find that many small landslides occurred on the southfacing slopes, while landslides on the north-facing slopes were less frequent, but larger. Terrain aspects of Gevgelija-Valandovo watershed are outputted in Surfer software from original 3”SRTM DEM, and values are in azimuth angle. Analysis show that in general, south sided terrains has greater fraction (51.5%) than north sided (48.5%). According to 4 main inclinations, east (29.2%) and south (27.5%) aspects prevails, then west (23.2%), and last north aspects (20.1%). Terrain (vertical) relief is parameter frequently extracted from DEM’s indicating maximal altitude differences in some area, which is typically square with 1km sides. This parameter is closely related with intensity of neotectonic movements and river (valley) incision, where greater values show higher elevation differences, thus higher erosion and landslide potential (Marković, 1983). Terrain relief in the Gevgelija-Valandovo catchment is computed from digital elevation model thought MicroDEM software, in m/km2 square areas. Values range from 3 m (flats and alluvial plains) to 807 m (source area of Konjska River on Kožuf Mountain). Largest areas cover terrains with moderate relief between 100 and 300 m (58.5%), especially from 100-200 m (32.5%). Low relief representing flats or near flats (0-50 m) occupies 11.4%, while high relief (300-807 m) cover 18.0%. Topographic wetness index (TWI) is parameter which show tendency of runoff dispersion in the catchment and represent ratio between upstream area and slope. Regions of the landscape that drain large upstream areas or that are very flat give rise to high values of the index; thus areas with the highest values are most likely to become saturated during a rain or snowmelt event and thus are most likely to be areas that contribute surface runoff to the stream (Moore et al, 1991). This parameter is computed from DEM model in SAGA hydrology module. Areas with high values for TWI tend to be with higher landslide potential. Values for wetness index in Gevgelija-Valandovo catchment range from 2.9 to 17.9, with 458 SCIENTIFIC SYMPOSIUM GEOGRAPHY AND SUSTAINABLE DEVELOPMENT OHRID, REPUBLIC OF MACEDONIA; 22-25.10.2009 average of 7.7. Well saturated areas (above value of 10) cover 12.6% of entire area, while moderate saturated (values from 6 to 10) cover 71.9% and poorly saturated (values below 6) cover 15.5%. Together with slope angle, curvature, and SPI factor, topographic wetness index can indicate areas with higher potential of landslides. Fig. 1, Maps of hypsometry, slope angle, aspects, terrain relief, TWI (topographic wetness index) and SPI (stream power index) of Gevgelija-Valandovo basin 459 НАУЧЕН СИМПОЗИУМ ГЕОГРАФИЈАТА И ОДРЖЛИВИОТ РАЗВОЈ ОХРИД, РЕПУБЛИКА МАКЕДОНИЈА; 22-25.10.2009 Stream power index (SPI) is also very indicative about landslide potential, and represent upstream catchment area multiply with slope. This index is related to erosion processes, constituting an indicator of the capabilities of a flow to generate net erosion (Moore et al., 1991; Olaya, 2004). Although, the minimum and the maximum ranges of the SPI values on the grid cells with landslides and without landslides are observed as close, the mean SPI value for the grid cells with landslides was calculated more than that for the grid cells without landslides (Nefeslioglu et al., 2008). SPI of Gevgelija-Valandovo catchment is obtained from SAGA hydrology module, like derivative of slope and catchments area. Values have large range, and mean value is 169.5. However, values up to 100, cover 71% (767 sq. km) of entire area, from which 26.6% are areas with SPI values below 20. SPI, rise highly with elevation, especially above 1000 m. Generally, high SPI indicate greater potential of landslide occurrences (Gokceoglu et al., 2006). ANALYSIS OF GEOLOGY (LITHOLOGY) COMPOSITION Lithology has great influence on landslide processes through the type, structure and texture of the rocks. Compact rocks are less suitable to landsliding in contrast to soft, permeable and porous rocks, which can store greater amount of water. In general, more permeable sedimentary rocks (sands, sandstones, clays) are with higher risk of sliding because of greater pore space and water capacity. Similar is with cracked or weathered schist’s, which can collect great amount of water and lose stability on steep slopes. In this sense, GevgelijaValandovo basin is composed by variable rocks: from gneiss and other Precambrian and Paleozoic schist’s, through Mesosoic limestones to the Cenozoic sand, sandstones, Quaternary deposits etc. For the purpose of the work, according to the geologic maps which cover study area (sheets: Gevgelija, Kavadarci and Kožuf), digital geologic map is made. This map is then rasterized in such way that for each lithological unit, proper index value is given in range from 0.1 (limestones, compact vulcanite’s) to 1 (sands, deposits). Thus, higher values designate more erodible and unstable rocks. Rasterized digital geologic map is used as one of the layers in cluster classification of the terrain in relation to the landslide potential. Here must be stress-out that constitution of the upper layer of rocks is most significant for landslide processes than lithology as such, so frequently this factor can be estimated only by field analyses or from detailed engineering-geologic map. Other approach is use of Landsat ETM+ satellite imagery and proprietary spectral bands. Thus, the use of RGB 457 FCC, pan sharpened with panchromatic Band 8 and contrast stretched with a 99.9% transform in all bands and increased contrast show bare soil and rocks in the resulting image from light to dark blue depending on light incidence and moisture content, meaning that landslides and areas of erosion were clearly highlighted. Here, little difference in color can be noted between landslides and other areas of bare soil and rocks (Petley et al., 2002). SATELLITE IMAGERY ANALYSIS Aside from topography analysis, for potential landslide areas prediction, satellite imagery has essential importance. There are many approaches of using satellite imagery in potential landslide area assessment: from vegetation (land cover) identification as an indicator of more or less suitable areas for landslide occurrence; to direct analyses of high resolution satellite imagery which allow precise identification of active and potential landslides itself. In our study, first approach is used through Landsat ETM+ imagery (acquired in 2000), precisely the square area containing Gevgelija-Valandovo basin, within exact limits as previously applied 3”SRTM DEM. From overall 8 ETM+ spectral bands, only 3 (red) and 4 (near 460 SCIENTIFIC SYMPOSIUM GEOGRAPHY AND SUSTAINABLE DEVELOPMENT OHRID, REPUBLIC OF MACEDONIA; 22-25.10.2009 infrared) band was used, which give NDVI (Normalized Difference Vegetation Index; where Vi=((TM4-TM3)/(TM4+TM3)). Then values of vegetation index are transformed in such way that lowest values (near zero) represent dense vegetation cover (forests), and highest values (up to 1) represent bare soils and areas with sparse vegetation cover and high overland flow. Table 2, Vegetation index and corresponding land cover (CLC2000) in Gevgelija-Valandovo catchment Corine Land Cover 2000 Landsat ETM+ 2 Code Description Veg. ind. Area km Area % Description Area km2 Area % 35-40 0.7 0.1 wetland; waters 0.0-0.2 25.0 2.3 water surf. 23-25 356.7 33.1 forests (bl; c; m) 0.2-0.3 98.4 9.1 dense forest 0.3-0.4 176.4 16.4 medium forest 0.4-0.5 170.0 15.8 sparse forest 26-29 259.4 24.1 scrubs 0.5-0.6 167.3 15.5 grasslands 28 124.9 11.6 scrub-sclerophy 0.6-0.7 173.2 16.1 sparse grass. 19-21 188.2 17.5 croplands 0.7-0.8 159.5 14.8 croplands 30-34 3.7 0.3 bare rocks 0.8-0.9 94.2 8.7 bare soils 1-11 11.3 1.0 artificial 0.9-1.0 13.2 1.2 anthrop. constr. other 132.1 12.3 agricultural 0.0-1.0 1077.0 100.0 - Table show that areas with vegetation index greater than 0.6 cover significant 40.8% (440.1 sq. km), representing sites with sparse vegetation or cultivated areas (lower altitudes of Gevgelija-Valandovo basin), whereas areas with dense, medium or sparse forest cover about 41.3% (higher altitude on mountain areas on: Kožuf, Konečka, Gradeška Mountain and Plauš). These data’s tell about weak overall vegetation. If other elements are appropriate (slopes, aspects, curvature, soils, lithology), which is case in the edge of depression-mountain areas (altitude of 100-500 m), sites with high landslide potential appear. LANDSLIDE RISK ASSESSMENT Analysis of topography indices extracted from DEM, lithology from digital geologic map, and vegetation or land cover from Landsat ETM+ satellite imagery, give us some ideas about spatial distribution of areas with higher landslide risk. But taken individually, these analyses often produce mistaken conclusion. For accurately results it is necessary to put several indices in combination, with some kind of overlapping. For example, there are some attempts to assessment potential landslide areas with models where only parameters are slopes and vegetation index. In our study, cluster classification of mentioned indices is performed, with aim to classify terrain features is small number of as much is possible homogenous landslide-risk related classes. Procedure is made with SAGA discretisation module, where Hill-Climbing algorithm automatically classifies closest homogenous terrain units from several grid layers. Among previously elaborated parameters were selected slopes, lithology index, transformed NDVI and profile curvature. The result is shown on fig. 2. From cluster classification is obvious that classes with ID 8 and 1 as some areas in class 7 are with highest landslide potential. Thus, according to this model, areas with high landslide potential occupy 397.0 km2 or 36.9% from the entire area. On the other side, areas with low landslide potential occupy 434.9 km2 or 40.4%, while areas with moderate potential 245.1 km2 or 22.7%. 461 НАУЧЕН СИМПОЗИУМ ГЕОГРАФИЈАТА И ОДРЖЛИВИОТ РАЗВОЈ ОХРИД, РЕПУБЛИКА МАКЕДОНИЈА; 22-25.10.2009 Table 3, Landslide related cluster classes of lithology, slopes, profile curvature and vegetation index in Gevgelija-Valandovo basin C.ID Area km2 Lith.Ind. Slope Prof. Cu. Veg.Ind. Risk d. Description high 0 156.7 0.247 7.7 -0.00015 0.678 hilly areas-sparse veg. 1 48.7 0.350 15.8 -0.00477 0.436 low-mod. valley bottoms very low 2 214.8 0.946 2.8 -0.00008 0.774 flat plain in the central part low 3 51.3 0.350 15.9 0.00438 0.471 ridges and ridges areas low 4 168.8 0.222 12.9 -0.00003 0.362 hilly and mount. Areas-solid r. 5 115.0 0.245 29.1 -0.00001 0.341 moderate forested steep hillslopes-solid r. 6 81.4 0.726 21.6 -0.00008 0.377 moderate forested steep hillslopes-soft r. high steep concave hillslope-sparse v. 7 91.1 0.257 22.0 -0.00020 0.621 8 149.2 0.810 7.5 -0.00012 0.522 very high conc. hillslopes-soft r., sparse v. Previous mean that according to the model, about 1/3 of the Gevgelija-Valandovo basin area is under higher treat of landslide occurrences. That are hilly areas with moderate to steep slopes and concave shape (where surface water percolate faster), sparse vegetation (usually weak grasslands), and cracked, weathered or even very soft rocks. Fig. 2, Lithology index, vegetation index, landslide potential map and landslide risk area map of Gevgelija-Valandovo basin 462 SCIENTIFIC SYMPOSIUM GEOGRAPHY AND SUSTAINABLE DEVELOPMENT OHRID, REPUBLIC OF MACEDONIA; 22-25.10.2009 Field research confirms that in those areas most of the landslides occur or that there are many potential landslides. Certainly, these results must be validated with very detailed field research, but generally this approach is acceptable on large-scale level. If other parameters are included like TWI (topographic wetness index) or SPI (stream power index), the final results may be even more accurate and precise, but overall model will be more complex for use. Also, in further studies it is better to weightening all of the included factors, according to their significance, which will also bring-up better final results. CONCLUSION In this work is presented one possible approach to assessment potential landslide area on the area of Gevgelija-Valandovo basin. This approach must be further validated and checked with field research and landslide inventarisation. From many factors that influence landslide processes, for this purposes only several are selected, which according to our opinion has great importance for the area. Climate factors, especially amount, intensity and seasonality of precipitation is not taken into account, because here it is relatively homogenous (around 600-900 mm/y). Topography, especially slopes and curvature has high significance for landslide processes, and for this reason they both are included in the model. Some authors propose that aspects, TWI and SPI are preferable to be included in assessment of landslide risk, but because of model simplicity, they are not incorporated now. Rock composition (lithology) is represented by lithology index, based on rock propreties (hardness-softness, permeability etc.), and from the existing geologic maps (100 000K). Vegetation index is extracted from Landsat ETM+ through NDVI relation. Other possibility is use of Corine Land Cover (2000; 2006), but for our purpose, it is not too suitable for the model used. Normally, the accuracy of the results depend also of DEM quality and resolution used to extract topographic indices which is 3”SRTM DEM. An alternative is 1”ASTER GDEM (30 m), but because of some quality issues it is not applied now. Also, better results will be obtained with high resolution satellite imagery (like IKONOS), but these are yet very expensive for our research purpose. However, the final results show that about 1/3 of the Gevgelija-Valandovo basin is under higher landslide potential. Main factor of such landslide potential are natural environment, but human activities may increase the natural tendency for a landslide to occur. Landslides which result from development activities are usually the result of increasing moisture in the soil or changing the form of a slope. Development activities such as cutting and filling along roads and the removing of forest vegetation are capable of greatly altering slope form and ground water conditions (Swanson and Dyrness, 1975). These altered conditions may significantly increase the degree of landslide hazard present (Varnes, 1985, and Sidle, Pearce, and O'Loughlin, 1985). For example, converting a forested area to grassland or one where crops are cultivated can increase the moisture in the soil enough to cause landslide problems (DeGraff, 1979). Or building a road which cuts off the toe of a steep slope can increase landslide susceptibility. It is possible to reduce the potential impact of natural landslide activity and limit development-initiated landslide occurrence by early consideration of these effects (Kockelman, 1985). All of those are largely present in Gevgelija-Valandovo basin. 463 НАУЧЕН СИМПОЗИУМ ГЕОГРАФИЈАТА И ОДРЖЛИВИОТ РАЗВОЈ ОХРИД, РЕПУБЛИКА МАКЕДОНИЈА; 22-25.10.2009 ACKNOWLEDGMENT This research was undertaken as a part of bilateral Macedonian-Bulgarian project entitled “Estimation of contemporary (erosional-denudation) geomorphological processes on Ograzden, Malesevo and Vlaina Mountain, and their relation with land use change”, financed by Ministry of Education and Science of the Republic of Macedonia (Agr. No. 03-1587/1 from 26.06.2009), and Ministry of Education of Bulgaria. The project is headed by Ass. Prof. Dr. Ivica Milevski from Macedonian side and Prof. Dr. Hab. Georgi Baltakov from Bulgarian side. REFERENCES Andonovski T. (1982): Erosion sites in SR Macedonia. XI Congress of geographers in SFRY, Budva (on Macedonian) Andonovski T., Kolcakovski D. (1989): Geomorphological characteristics of Kozjacija nad Sredorek. 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