Hydrological Analysis of High Waters and Flash Floods Occurred in September 2007 in Slovenia
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HYDROLOGICAL ANALYSIS OF HIGH WATERS AND FLASH FLOODS OCCURRED IN SEPTEMBER 2007 IN SLOVENIA Mira Kobold, Mojca Sušnik, Mojca Robič, Florjana Ulaga, Bogdan Lalić Environmental Agency of the Republic of Slovenia Vojkova 1b, Ljubljana, Slovenia [email protected] Abstract Heavy and abundant precipitation which captured the western, north-western and northern parts of Slovenia on 18 September 2007, caused quick rise of river discharges especially in the region of Baška grapa, Davča, the Cerkljansko and Škofja Loka hills. In that area the streams Selška Sora, Davča and Kroparica caused real destruction. The torrential streams and rivers flooded also in the region of Karavanke and foothills of the Kamnik-Savinja Alps, Kranj and Domžale fields, the Tuhinj valley and extensive Celje region. The Savinja was high in the middle and lower stream. The discharges of the tributaries Hudinja and Ložnica exceeded 100- year return period. The Dravinja flooded in the middle and lower stream. The return period of flood was between 50 and 100 years. On the most affected area the peak discharges exceeded the periodical maximum discharges measured at the water stations. The Sava was high mostly because of the high tributaries of the Sora, Bohinjska Bistrica, Lipnica, Tržiška Bistrica, Kamniška Bistrica and Savinja. The return period of the flood wave was around 20 years. Besides flooding many landslides were triggered, what is usual for Slovenia at such hydrological situations. The result of this catastrophe was enormous economic damage and loss of six people's lives. Keywords: flash flood, precipitation, HEC-1 model, return period, hydrograph, runoff coefficient, September 2007. 1 INTRODUCTION Almost every year local heavy rain showers and flash floods appear in different parts of Slovenia. Flash floods in September 2007 caused, besides enormous material damage, loss of six human lives. Meteorological forecast predicted precipitation for Tuesday, September 18, but not in the quantity and intensity as it happened. It is impossible to predict such events in advance, correctly and with reliability, in an alpine region with distinctive topography and climate variability. The paper presents description of meteorological conditions which led to catastrophic hydrological situation and raging power of the nature. It follows the description of hydrological situation, showing the hydrographs of high waters recorded on the measurement sites of national hydrological monitoring network. The situation was the worst in the catchment of Selška Sora in Železniki where the measuring equipment stopped to work and flood wave was not recorded. For that reason the hydrograph was simulated by the HEC-1 model. During the event hydrological forecasting service issued hydrological warnings and data, and predictions regarding to meteorological forecasts. Field measurement groups were trying to measure the flood discharge by ADCP measurement equipment on some locations. It was turned out that the performing of measurements were nearly impossible in such meteorological circumstances combined with high velocity of water and debris flow. 2 REVIEW OF METEOROLOGICAL CONDITIONS There was a region of low air pressure over the Northern Europe on 18 September 2007. Cold front moved over western and middle Europe towards the Alps. At the same time a high valley of cold air moved over western Europe towards east. South- western wind was getting stronger and stronger over Slovenia. Main reasons for strong precipitation were the diversity of terrain, constant inflow of moist air from southeast, very unstable atmosphere and wind shearing in higher atmospheric layer. In the morning of September 18, an extensive convective system was formed in the north-western part of Slovenia and that system remained there for a few hours. The first precipitation zone moved over western Slovenia towards east on 18th September 2007 between 5 and 7 a.m. in the morning. A short brake has followed and after 8 a.m. some thunderstorms appeared in the hilly area of western Slovenia. Very heavy rain showers arisen after 9 a.m. (Figure 1). A strong thunderstorm line was established from Posočje over Idrija-Cerkljansko hills and Škofja Loka hills to northern part of Ljubljana basin. The thunderstorm was located there nearly two hours. Next distinctive stationary thunderstorm line was built on 18th September at 13:30 in a direction of Tolmin – Radovljica. Precipitation weakened at 5 p.m. in Bohinj, but has not stopped yet. New thunderstorm cells were formed over northern Slovenia over and over during the afternoon and precipitation strengthened in northeast part of Slovenia (Figure 1). Wind had started to blow in lower atmosphere from northeast to northwest. Thunderstorms were formed when the cold front was passing, and moved towards south. Precipitation stopped in western part of Slovenia around 9 p.m., in northwest at midnight, and in southeast during 2 and 3 a.m. next morning. Figure 1. Radar images of precipitation intensity over Slovenia at 9:30 and 14:30 local time. Unexpectedly heavy rainfall fell in a few hours on the most affected area, mostly within six hours. On some precipitation measurement stations more than 300 mm of rain was recorded. The highest precipitation was recorded in the region from Bohinj and hills of Cerkljansko to Celje valley (Figure 2). The spatial distribution of precipitation was very high. High differences in the amount of precipitation were at small distances. The return period of the highest precipitation was more than 100 year. The amount of precipitation decreased to the east. Above 100 mm of precipitation fell in north part of Ljubljana valley and in some parts of Štajerska region, where the highest precipitation fell in the surroundings of Celje. Figure 2. Daily amount of precipitation from 18 September at 8.00 to 19 September 2007 at 8.00 (source: ARSO, 2007b). 3 DESCRIPTION OF HYDROLOGICAL EVENT There was not a lot of precipitation in Slovenia from the beginning of September 2007 (Figure 3) and discharges of the rivers were low early in the morning of 18 September. Only some rivers in eastern and western part of the country had mean yearly discharges. A night before, rivers in south-western part of the country raised, but in the morning they were all decreasing. Regarding to the meteorological situation on 18 September (Figure 2), the northwest part of Slovenia got the highest amount of precipitation and the hydrological situation was the worst in the catchments of the Selška Sora, Cerknica, Bistrica, Lipnica, Tržiška Bistrica, Pšata, Kamniška Bistrica, Dravinja and some tributaries of the Savinja (Figure 4). 250 Kobarid Rateče 200 Davča Železniki ] m Poljane nad Škofjo Loko 150 m [ Kamniška Bistrica n o i t Ljubljana a t i p i 100 Celje c e r P 50 0 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. Septem ber 2007 Figure 3. Daily amount of precipitation from 1st to 20th September 2007. H D u ra Trž.Bistrica d vi i n K L Savinja o n ja a j ž a Li m n pn ic ica . P B Dreta a Bistrica š i a s t t r a i ča Selška Sora c Ba a ica ora rkn S Ce ska ljan Po Figure 4. Catchments affected the most by the floods on 18 September 2007. The Sora catchment Under the influence of very intensive precipitation on the area of northwest Slovenia, water streams from the catchments of Davča, Cerkljansko hills and Bohinj ridge started to rise. Bača and Cerknica rised very quickly in only a few hours between 10 to 12 o’clock and they started flooding. At the same time the Selška Sora and its tributary Davča started to rise. On precipitation measurement station in Davča 80 mm of rain was registered in only 50 minutes. The Davča torrent in Davča and the Selška Sora in Železniki rose extremely quickly and caused destruction and enormous material damage, the most in Davča and Železniki (Figure 5), where three people lost their lives. The water measurement station at Železniki was not damaged, but the equipment stopped to work during the flood and the flood wave was not recorded entirely. The highest water level 551 cm was determined after the flood according to catchment (Figure7). data into the Watershed model Modelling was System (WMS, precipitation 1997; Kobold from and Sušnik, the 2001). The meteorologicalinput drainage area of the stations catchment to Železniki in is 104 The km the flood in the Selška Sora catchment was an extreme hydrological event. The observation period1991-2006is148m exceeded 100-year return period of maximal floods.discharge The highest was discharge from the estimated to the 300 highest m water flood level trace and it happened from around 13:30. In the that event the Selška periodSora exceeded of observation 1991-2006. The peak 213.8 km flood (photo: M. Burger). flood (photo:M. traces of 2007and flood on18September afterthe inŽelezniki Figure 5.Destruction 2 . The simulation of flood at Železniki was done by HEC-1 model using the Figure 7. The HEC-1 model for the Sora catchment. modelforthe Sora Figure 7.TheHEC-1 3 3 ŽELEZNIKI /s by extrapolation of rating curve, and it /s occurred in1995(Figure6). the SelškaSoraat Železniki. maximumFigure 6.Annual discharges of Q max [m3/s] 100 150 200 250 300 50 0 VEŠTER 1991 w .s.Železniki ZMINEC 1992 1993 SORA 2 1994 and downstream to Vešter 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 The simulated hydrograph for Železniki is shown in Figure 8. The peak of flood wave is estimated to 278 m3/s at 13:45, what means 2670 l/s/km2 of maximal specific runoff. The cumulative areal precipitation for the catchment to Selška Sora amounts to 219 mm, while the effective precipitation which caused direct runoff was only 57 mm.