Calculation of the moving landslide masses volume from air images
The landslide Slano blato is of great dimensions, longer than 1 km and wider than 300 m. The movements of 10 m per day mostly happen in heavy rainy seasons and afterwards calm down, while the landslide progresses for a few 100 meters at the time. Because of the size and the inaccessibility of the landslide, common surveying was not possible. So the observation of the sliding masses movements was only possible by successive photography from a plane. For this purpose we carried out two special photo shoots with a special plane equipped for remote sensing. The existent snap shots from regular cyclic remote sensing prior to landsliding were also applied. On the basis of the snaps, TIN meshwork was created for each photo shoot separately. Geodetic maps in 1:2000 scale with contour lines 1 m apart were also produced for this purpose. It is important to know the volume of the moving masses so we can determine which measures are significant for stopping the landsliding (mudflow) that threatens the village of Lokavec. As we had available data of the area size before landsliding, we could easily calculate the mass volume sliding by cross-sectioning the area of the two conditions at different times of aerial photo shoots. The problem in the calculation was the landsliding masses that joined the mudflow from the ground. These were the masses from the previous older slidings, known to had happened at least twice - 100 and 200 years ago. The volume of the old landslide was estimated with the help of geological evaluation which we used to access the depth of the slope base. Geological evaluation was partially based on field drilling and partially on presumption of the depth of weathering cover. The landslide depth data were interpreted with the help of two longitudinal sections and transverse sections each 25 m apart. We put the surface lines from each remote sensing on each cross-section and calculated the volumes of the landslide between two cross-sections. By means of this procedure we assessed that the volume of all the sliding masses was 684.000 m3 (April, 2001). With regard to this and other parallel results we determined that we should stop the sliding before it gets to the village by draining and pushing the masses aside. Part of the masses was impossible to withhold on the slope (between 300.000 m3 and 400.000 m3), so it was removed by means of vehicles to the deposit. It was confirmed that the calculation of the volumes with the help of remote sensing is a very suitable method for large landslides, but will only give the right results by detailed geological interpretation of landsliding.