Spatio-Temporal Distribution of Flash Floods in Slovenia
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PAPERS Dela 46 ● 2016 ● 21–39 SPATIO-TEMPORAL DISTRIBUTION OF FLASH FLOODS IN SLOVENIA Tajan Trobec, PhD. Department of Geography, Faculty of Arts, University of Ljubljana Aškerčeva 2, SI-1000 Ljubljana e-mail: [email protected] Original scientific article COBISS 1.01 DOI: 10.4312/dela.46.1.5-39 Abstract This paper examines the spatial distribution of frequency of flash floods along with their seasonal distribution. The spatio-temporal analysis of past flash flooding covered 124 flash floods affecting areas of Slovenia between 1550 and 2015. Flash floods are most common in the mountainous and hilly area of northern Slovenia, which consists of alpine and a large part of subalpine landscapes. Autumnal flash floods occur across most of the country, while summer flash floods are seen mainly in the east. In most parts of the country autumnal flash floods predominate. Keywords: natural disasters, hydrogeography, prognostic geography, flash floods, river catchments, hydrographic area, Slovenia 1 INTRODUCTION Slovenia experiences riverine, karst and flash floods (Natek, 2005). Because of the various combinations of hydrogeographic factors underpinning runoff (such as land cover, soil, bedrock, and relief) in water catchment areas of watercourses, individual types of floods generally occur in certain areas only. Flash floods in Slovenia occur along most watercourses (Brilly, Mikoš, Šraj, 1999). They can present in the form of destructive raging of extreme volumes of water in large or small torrent channels and ravines, as well as debris and mud flows on alluvial fans and floodplains (Gams, 1991). On average, 1.3 flash floods occur annually in Slovenia (Trobec, 2015), although in individual basins they occur with varying frequency, as is evident from the regional over- view of flooding (Komac, Natek, Zorn, 2008). Due to specific seasonal weather patterns that lead to the formation of heavy and intense precipitation (Furlan, 1961; Pristov, 1982; Petkovšek, Trontelj, 1996; Vrhovec, 2002; Ogrin, 2008) and consequent flash floods, in Slovenia these occur mainly between June and November (during meteorological sum- mer and autumn; Trobec, 2015). Also in terms of the seasonal distribution of floods, there 21 Dela_46_FINAL.indd 21 13.3.2017 10:07:01 Tajan Trobec / Dela 46 ● 2016 ● 21–39 are significant differences between different river catchments in various areas of Slovenia as a result of different climatic characteristics (Ogrin, 1996; 2002). Regional differences in the frequency and seasonal distribution of flash floods with flash flooding occurring only in certain areas is also characteristic of other countries in the Alpine arc (Merz, Blöschl, 2003; Parajka et al., 2010). The concept of modern flood protection involves various levers for managing flood risks, including increasingly important preventive actions focusing on flood forecasting and raising awareness of the population about flood risks (Direktiva, 2007). Knowing the areas where flash flooding occurs can significantly contribute to raising the awareness of the population about flood risks in these areas. Namely, it is very likely that areas that were pre- viously affected by floods will again experience flooding in the future. Given the prognostic geographic assumption that flooding in specific areas is normal and indicative of the natural geographical characteristics of the local terrain (Radinja, 1983; Šifrer, 1983; Natek, 1992; 2002; 2003; 2007), it is possible to deduce from past flash floods the spatial patterns and dy- namics of future flash flooding. The latter already represents a step toward predicting floods. The purpose of this paper is to determine in which areas flash floods occur and attempt to reconstruct past flash flooding events on the basis of spatio-temporal analysis of 124 flash floods that affected Slovenia between 1550 and 2015. It presents the spatial distribu- tion of flash floods in Slovenia with reference to their frequency and seasonal distribution - both at the level of hydrographic areas of different orders (Hidrografska območja, 2016). 2 METHODS Data used to perform the spatio-temporal analysis of flash flood events in Slovenia were obtained from various records, descriptions and previous studies of flash floods. Thus, we included only those floods for which it was possible to discern, or at least indirectly infer that they would be classed as flash floods. As karst and lowland floods were not the object of our research, we excluded them from the analysis, despite the fact that the latter occasionally followed flash flooding, albeit with a lag, particularly follow- ing extended autumnal rainfalls. Since the analysis performed also incorporated a spatial component, the analysis only included those floods for which we were able to determine the approximate extent of flooding. Data on earlier floods were mainly gathered from secondary sources (Kolbezen, 1991; 1992; 1993; 1994; 1995; Jesenovec, 1995; Trontelj, 1997). Primary sources (public me- dia, records of provincial administrative bodies, archival records of daily newspapers, chronicles, archives of the Hydrometeorological Institute of the Republic of Slovenia - nowadays ARSO, etc.) which were referenced by the mentioned sources have not been listed. For floods, which occurred in recent decades, we also relied on primary sources, particularly on certain contributions to scientific and professional journals and antholo- gies, such as Geografski vestnik (Geographical Bulletin), Geografski zbornik (Journal of Geography), Ujma (Magazine on the protection against natural and other disasters), Mišič Water Day (forum and newsletter), and others, as well as on various reports and publica- tions from the ARSO website (Analize izrednih ..., 2016). 22 Dela_46_FINAL.indd 22 13.3.2017 10:07:01 PAPERS Spatio-Temporal Distribution of Flash Floods in Slovenia During less severe floods, which occurred in connection with short duration precipita- tion (usually in the warmer part of the year when precipitation is induced or enhanced by convection) there was a shorter time lag between precipitation and consequent flooding. Therefore, we were able to attribute individual floods to the exact day when they actually occurred. In contrast, when it comes to flooding that occurred as a result of slow travel- ling large frontal or cyclonic systems causing prolonged precipitation with different and varying local intensity (usually in the colder part of the year), several individual floods were considered as a single flood event, despite them occurring on different days and/or in different river catchments, and they were all attributed to the day when, in view of the current synoptic situation, flooding was most severe. In this way, we increased the accu- racy of the database, as individual scattered flooding was linked to a common cause, i.e. precipitation as part of the broader precipitation system. For each flood, we determined the approximate extent of flooding. In doing so, we did not limit our attention solely to the narrow floodplain areas along watercourses, but rather we actually examined entire river catchments (areas with small headwater streams, ravines, tributaries and alluvial fans), from where flood waters drain into river beds. In choosing the spatial unit to illustrate floods we relied on ARSO’s data layer “Hydro- graphic areas” (Hidrografska območja, 2016), which divides Slovenia into four hierar- chically arranged orders (Figure 1). The first, highest, order demarcates Slovenia into six main hydrographic areas that coincide with the river basins of major Slovenian rivers (Sava, Drava, Mura, Soča, and Kolpa). The Adriatic drainage basin (excluding the Soča Figure 1: Different orders of hydrographic areas in Slovenia. 23 Dela_46_FINAL.indd 23 13.3.2017 10:07:02 Tajan Trobec / Dela 46 ● 2016 ● 21–39 Figure 2: Determining the extent of flash flooding according to hydrographic areas for floods in September of 2007 (a) and 2010 (b). Source: Kobold, 2008; 2011; Polajnar, 2008; 2011; Večje nesreče …, 2008; Kuzmič et al., 2010; Sodnik, Mikoš, 2010; Poplave in zemeljski plazovi med …, 2011; Hidrografska območja, 2016. River) is also included at this level. In the second order, there are 31 hydrographic areas, whilst there are 155 and 582 third and fourth order areas, respectively. Hydrographic areas of the fourth order form the basic territorial units of ARSO’s data layer. Their aver- age size is about 35 km2 (in the Karst and on plains they are substantially larger, whereas in normal fluvial reliefs as well as in steeper terrain areas they are much smaller). Their borders conform to hydrographic watersheds of catchments and their parts. Depending on the extent of the area affected, each flash flood was depicted according to the correspond- ing hydrographic areas (Figure 2). In general, more recent floods (since around the end of the 1980s) are rather well and relatively accurately documented in the literature, which also meant that we did not have any major problems illustrating their reach. Determining the extent of some older, less well documented floods was more difficult. Due to a lack of information about the extent of flooding, occasionally we were forced to make a compromise in illustrating their spa- tial reach and depicted them at higher order hydrographic areas. Because of this there is a risk that we also included areas that were actually unaffected, or else overlooked some flood affected areas. We could sometimes fill in the gaps where sources were lacking by examining experiences garnered