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Slovakia, a country with an area of 49 thousand km in the heart of Europe, is also called a land of landslides. Floods and landslides are the two most occurring natural hazards, the fact being reflected in fifty years of systematic geologic research and mapping of slope deformations. According to the Slovak State Geological Institute, more than 5% of the country is already affected by rock, debris and earth movements. There have been more than 21, 000 slope distortions recorded.
The reasons why the earth starts to move are both below and above the surface. Highly important are geologic factors. The greatest risk is pronounced in the so-called Carpathian Flysch Belt, an arched tectonic zone stretching through the territory of Czech Republic, Slovakia, Poland, Ukraine and Romania. It is made up of alternating marine deposits of claystones, shales and sandstones. When eroded, the layered geological strata are a perfect predisposition for landslides and rockfall displacements. The other deciding agent is precipitation, being in the form of rain or melting snow. Weather anomalies and heavy rainfall were the triggers that initiated the three most extreme landslide events of the last few decades in the country.
Catastrophic Debris Flows in Vratna
The Vratna valley is one the most visited tourist and ski destinations in Slovakia located in the Mala Fatra mountain range. Within two July days in 2014, Vratna received over 70 mm of rain, a quantity that exceeds long-term monthly precipitation amounts in the local area. The meteorological event had its climax on the second afternoon when a torrential downpour of 50 mm fell within a single hour. The whole watershed was already saturated from the previous rainfall and could not absorb the additional precipitation. Soaked soil layers lost their compactness, overcame the friction and resistance of bedrock and started to rush down the mountain slopes. The main scarps were formed on upper parts of steep grassy slopes, their heights ranging from 25 to 140 cm. The layering of bedrock even exaggerated the movement of mud and debris, since the bedrock deposits are inclined towards the valley. Overland water flow, together with a huge mass of mud and rocks, were concentrated in stream channels, taking down the vegetation and everything else standing in the way, adding other sediments, and leading to further accumulation of volume and density of catastrophic debris flows. The biggest flow reached velocity of 40 km per hour. Having great momentum, it rolled deep into the valley of Vratna, severely damaging the infrastructure and burying parts of the gondola lift base station. Experts estimated that the deposits in the base area had 6 m of depth and 100 000 m3 of volume. Dozens of cars were destroyed, as was a road leading from Terchova village to the Vratna valley. The natural disaster, of massive proportions considering the national circumstances, caused $12 million worth of damage.
The Benefits of Lidar
A few days after the calamity, a team of geologists set out to explore the affected area. Helicopter flights permitted the visual inspection and localization of scars formed by debris flows on mountain uplands, as well as the investigation of muddy deposits lying in the valley floor. Field surveys were conducted to evaluate the situation with regards to present slope instabilities and to provide prognosis for future scenarios of development. Due to physical, time, and manpower constraints, field reconnaissance was focused on accessible parts of head slopes and on critical areas around the infrastructure in the valley bottom. The smaller channels eroded by water-laden mass of soil, rock and vegetation in forested areas went therefore mostly undetected. Hazards geologists and landslides experts sought a tool that would help them to identify slope deformations in zones covered by trees and to detect potential landslide spots that could pose further risk. A crisis management group decided to make use of novel lidar technology to localize unexplored debris flows that were inaccessible to the crew of geologists for field inspection.
A consortium of governmental institutions, universities and commercial companies were approached to acquire aerial and lidar data of the area struck by the sliding and moving mass. The flight time was chosen when the leaves of deciduous trees had already fallen and the ground was not yet covered by snow to get the maximum reflection of a laser beam off the ground surface and to derive an optimal digital terrain model. Aerial photographs were collected with the pixel resolution of 0.15 m, although the cloudy conditions obscured a part of the overall mosaic. Airborne lidar data were acquired with the density of 47 points per square meter, which was sufficient to obtain a high quality bare earth surface model for identifying geologic features. Imagery and lidar data were processed in a regular workflow and digital terrain model was generated revealing the variability of terrain in remarkable detail. In order to ease and automate the delineation of features related to the summer land movements, geomorphometric analysis was run to quantitatively analyze the shape of structures on the ground.
Locating the Hidden Debris Flows
Several traditional land surface classification methods were applied to identify elements representing ridges and valleys, slopes, peaks and pits. The outcomes were not satisfactory, because the computation results were greatly affected by coherent noise, blur and spatial displacement of feature boundaries.
As a solution, the technology for digital data analysis–Proxima–was used. It employs numerical mathematics to fully describe the shape of digital surface. Database creation and geometric analysis of all objects are also a part of the processing steps. The computation is fully automated and is run on several levels of detail, therefore providing multi-scale information about the surface geometry.
The final workflow solution that makes use of lidar data and that automatically identifies structural features on a digital surface was much valued by geology experts, since they could clearly visualize and identify channels eroded by debris flows. The methodology proved to be especially beneficial in forested areas where the mass movements were hidden by trees. Quantitative analysis of lidar data proved invaluable in determining and quantifying the extent of terrain failure event. The airborne-acquired data and the results of analysis will support the planning activities for further field surveys scheduled for the summer of 2015.
Martina Szabova is an experienced geomatics professional, currently advancing the use of lidar in commercial applications and delivering new analytical insights with geospatial technology at YMS (www.yms.sk, martina.szabova@yms.sk).
Stanislav Hroncek focuses on applications of surfaces analysis in various industries including geoscience. He currently leads a team of developers, mathematicians and geoscientists as a Research Manager at Proxima R&D (www.geoproxima.com, stano.hroncek@proximard.sk).
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