HYDROLOGICAL ANALYSIS OF HIGH WATERS AND FLASH FLOODS OCCURRED IN SEPTEMBER 2007 IN

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 , Davča and Kroparica caused real destruction. The torrential streams and rivers flooded also in the region of Karavanke and foothills of the - Alps, Kranj and Domžale fields, the and extensive region. The Savinja was high in the middle and lower stream. The discharges of the tributaries and Ložnica exceeded 100- year return period. The 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 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 . 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 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 flood trace and it happened around 13:30. In that event the Selška Sora exceeded the highest water level from the period of observation 1991-2006. The peak discharge was estimated to 300 m3/s by extrapolation of rating curve, and it exceeded 100-year return period of maximal floods. The highest discharge from the observation period 1991-2006 is 148 m3/s occurred in 1995 (Figure 6).

300 w .s. Železniki 250

200 ] s / 3 m [

x 150 a m

Q 100

50

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 9 9 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 9 9 9 9 9 9 9 9 9 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2

Figure 5. Destruction in Železniki after the Figure 6. Annual maximum discharges of flood on 18 September 2007 and traces of the Selška Sora at Železniki. flood (photo: M. Burger).

The flood in the Selška Sora catchment was an extreme hydrological event. The drainage area of the catchment to Železniki is 104 km2 and downstream to Vešter 213.8 km2. The simulation of flood at Železniki was done by HEC-1 model using the Watershed Modelling System (WMS, 1997; Kobold and Sušnik, 2001). The input data into the model was precipitation from the meteorological stations in the catchment (Figure 7).

ŽELEZNIKI

VEŠTER SORA

ZMINEC

Figure 7. The HEC-1 model for the Sora catchment. 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.

300 0

10 250 20 ]

s 200 / ]

3 30 m m m [ [

n e

150 40 o g i r t a a t i h c p i

s 50 i c

100 e D r 60 P 50 70

0 80 18.9.2007 0:00 18.9.2007 12:00 19.9.2007 0:00 19.9.2007 12:00 20.9.2007 0:00

P P effective Q Železniki (HEC-1 model) Q max Q mean Figure 8. Simulated hydrograph of the Selška Sora at Železniki using HEC-1 model, periodical mean and maximal discharges, and areal precipitation to Železniki.

In next few hours flood wave of the Selška Sora moved downstream towards Škofja Loka. At the water measurement station Vešter, situated 15 km downstream from Železniki, the maximum discharge of 335 m3/s was registered at 16:15 (Figure 9), what is 25-year return period of maximal floods considering the period of observations from 1988 onward. The amount of precipitation decreased downstream on the Selška Sora catchment. In the catchment from Železniki to Vešter the average areal precipitation amounts to 187 mm. That is 32 mm less precipitation than in the upper part to Železniki. In late afternoon the water was decreasing (Figure 10).

450 0

400 10

350 20 ] ] 300 s m / 30 3 m [

m [ n

250 o e i t

g 40 a r t i a

200 p h i c c s e i 50 r D

150 P 60 100

50 70

0 80 18.9.2007 19.9.2007 20.9.2007 21.9.2007

P Davča Q Vešter Q max Q mean Figure 9. Hydrograph of the Selška Sora at Vešter with periodical mean and maximal discharges and hourly intensity of rainfall in Davča. Further analysis of flood waves and calculation of runoff coefficients showed that the coefficients were very small in the Sora catchment (Figure 11). The coefficients of direct runoff were about 0.27 and the coefficients of total runoff about 0.31. Regarding to small antecedent soil moisture and consequently low flows before flood event, the infiltration into the soil was very high and that was also a reason for triggering of numerous landslides. Precipitation caught and held by vegetation presented the remaining losses.

70 w .s. Vešter 60 w .s. Železniki 50 ]

m w .s. Suha catchment form m [

Železniki to f 40 f

o Vešter n u r

t 30 c

e w .s. Zminec r i

D 20

10

0 100 150 200 250 Precipitation [mm] Figure 10. The Selška Sora at Vešter on 18 Figure 11. The dependence of direct runoff September 2007 at 17:30 (photo: M. Kobold). from precipitation in the Sora catchment.

The Poljanska Sora was not so high. The peak discharge at Zminec was 122 m3/s what is less than 2-year return period of flood. The maximal discharge of Sora at Suha was 440 m3/s, what is 10-year return period of flood.

The catchments of the Lipnica and Tržiška Bistrica

Discharge with a return period over 100 years was recorded on the Lipnica at Ovsiše (Figure 12) and Tržiška Bistrica at Preska as well (Figure 13). The maximum discharge in Tržiška Bistrica was 155 m3/s at 16:30 (Figure 14).

Figure 12. The Lipnica at Ovsiše on 18 Figure 13. High water of the Tržiška Bistrica at September 2007 at 17:13 (photo: M. Burger). Preska on 18 September 2007 at 16:39 (photo: M. Burger). 160 0

140 5

120 10 ] ] s m / 100 15 3 m [

m [ n

o e i t

g 80 20 a r t i a p h i c c s e i 60 25 r D P

40 30

20 35

0 40 18.9.2007 19.9.2007 20.9.2007 21.9.2007 P Zg. Jezersko Q Preska Q max Q mean Figure 14. Hydrograph of the Tržiška Bistrica at Preska with periodical mean and maximal discharges and hourly intensity of rainfall on Zgornje Jezersko.

The Bohinjska Bistrica catchment

The Bistrica at Bohinjska Bistrica gathers water from southern side of the Bohinj ridge. On 18 September the extreme amount of precipitation fell in that area which caused huge rise of the Bistrica and flooding of the surrounding buildings. The peak of flood wave reached 108 m3/s at 17:30 and exceeded the periodical maximum discharge at this location. The return period of flood was more than 100 years (Figure 15). Other rivers and streams in Bohinj and its surrounding rose significantly. A number of land slides closed the roads and the railway traffic was impossible too.

120 0

10 100 20

30 ] ] 80 s m / 3 m

40 [

m [ n

o e i t

g 60 50 a r t i a p h i c 60 c s e i r D

40 P 70

80 20 90

0 100 18.9.2007 19.9.2007 20.9.2007 21.9.2007

P Bohinjska Češnjica Q Bohinjska Bistrica Q max Q mean Figure 15. Hydrograph of the Bistrica at Bohinjska Bistrica with periodical mean and maximal discharges and hourly intensity of rainfall in Bohinjska Češnjica. The catchment of the Pšata and Kamniška Bistrica

In the afternoon on 18 September the precipitation moved towards northeast of Slovenia. The streams increased in the foothill of Kamniške Alps, on the field of Domžale and in the Tuhinj valley. The at had a maximal discharge of 68 m3/s late in the evening with a return period of 100 years. The Pšata flooded in Komenda and its neighborhood. At Topole the runoff reached 52 m3/s, what is 100- year return period (Figure 16). The Kamniška Bistrica at Kamnik had 10-year return period with maximum discharge of 140 m3/s, but downstream the discharge increased to 50 to 100-year return period due to the high Pšata inflow. The Rača with 5 to 10-year return period additionally contributed to the increase of discharge of the Kamniška Bistrica on the confluence with the Sava river.

60 0

50 5 ] ] 40 s m / 3 m

10 [

m [ n

o e i t

g 30 a r t i a p h i c 15 c s e i r D

20 P

20 10

0 25 18.9.2007 19.9.2007 20.9.2007 21.9.2007

P Krvavec Q Topole Q max Q mean Figure 16. Hydrograph of the Pšata at Topole with periodical mean and maximal discharges and hourly intensity of rainfall on Krvavec.

The Savinja catchment

In the late afternoon when rivers in western part of Slovenia started to decrease, the majority of precipitation moved to central and eastern part of Slovenia, to the Savinja and Dravinja catchments. Several smaller streams and torrents flooded in the Savinja catchment, as well as the Savinja river in lower stream. The discharges of the tributaries Hudinja and Ložnica exceeded 100-year return period. The maximal discharge of the Hudinja at Škofja vas was 173 m3/s and Ložnica at Levec 120 m3/s (Figure 17). The Dreta, and had peak discharges with lower return period and they did not exceed the periodical maxima. 140 0

2 120 4

100 6 ] ] s m / 3 m

8 [

m 80 [ n

o e i t

g 10 a r t i a p h 60 i c 12 c s e i r D P 40 14 16 20 18

0 20 18.9.2007 19.9.2007 20.9.2007 21.9.2007

P Levec Q Levec Q max Q mean Figure 17. Hydrograph of the Ložnica at Levec with periodical mean and maximal discharges and hourly intensity of rainfall in Levec.

The Savinja at Solčava had a maximal discharge of 29 m3/s on 18 September at 9 pm. This is only mean high discharge in a period. The flood wave was increasing downstream due to the high tributaries. During the night the Savinja in Celje reached the top of the embankment, but it did not flood the city. The maximal discharge at Laško was 1254 m3/s reached on 19 September at 3 am. The return period of flood is 50 to 100 years. The maximal periodical discharge at Laško is 1406 m3/s occurred in November 1990.

The Dravinja catchment

The Dravinja in its upper part at Loče exceeded the periodical maximum discharge on 18 September at around 10 pm. The peak discharge was 78 m3/s and it is the highest recorded discharge in 25-years long observing period at that site. The return period is 20 to 50 years (Figure 18). In Makole which lies 18 km downstream, a flood vawe reached a pick of 118 m3/s early in the morning of 19 September, what is 50 to 100-year return period. The same return period of flood was at Videm with peak discharge of 272 m3/s and was reached on 19 September at 14:30.

90 0

80 5 70 ] ] 60 10 s m / 3 m [

m [ n

50 o e i t

g 15 a r t i a

40 p h i c c s e i r D

30 20 P

20 25 10

0 30 18.9.2007 19.9.2007 20.9.2007 21.9.2007 P Sl. Konjice Q Loče Q max Q mean Figure 18. Hydrograph of the Dravinja at Loče with periodical mean and maximal discharges and hourly intensity of rainfall in . The Sava river

Due to the high discharges of the Sora, Bohinjska Bistrica, Lipnica, Tržiška Bistrica, Kamniška Bistrica, Savinja and other rivers, Sava was raising too. The maximal discharge at Šentjakob was 1157 m3/s, what is around 10-year return period (Figure 19). The return periods of the flood wave in the middle and lower Sava were about 20 years. The peak discharge in Hrastnik was 1669 m3/s.

1600

1400

1200 ] s /

3 1000 m [

e

g 800 r a h

c 600 s i D 400

200

0 18.9.2007 19.9.2007 20.9.2007 21.9.2007

Q Šentjakob Q mean Q max Figure 19. Hydrograph of the Sava at Figure 20. Discharge measurement of the Šentjakob with periodical mean and maximal Sava in Hrastnik on 19 September 2007 at discharges. noon (photo: M. Burger).

4 CONCLUSION

Heavy rainfall with high intensity caused huge floods on 18 September 2007 in north- western part of Slovenia. Beside the mentioned rivers the others streams (Bača, Cerknica) and torrents in northwest Slovenia were flooding too. The time of concentration was very short and discharges of the rivers rose very quickly on the most affected area and caused destruction of infrastructure, homes and business. The situation was the worst in the catchment of Selška Sora in Železniki where floods caused enormous economic damage and loss of human lives. Observed discharges of some rivers like Selška Sora, Bistrica at Bohinjska Bistrica, Tržiška Bistrica, Pšata, Dravinja, Ložnica and some others exceeded periodical maximum discharges. That was an event with more than 100-year return period on that area. The transport of sediment load was also very high during that event and the measured values of sediment load at some locations exceeded periodical values.

Due to a large amount of precipitation and strong intensity, several landslides were triggered. One of them destroyed the hospital Franja, a historical monument from the Second World War. The estimated damage caused on the streams is around 91.500.000 EUR and the whole damage was estimated on almost 200.000.000 EUR.

The floods have not been predicted and people have not been warned against floods to protect their property. Meteorological forecasting service predicted precipitation for the September 18 on the base of meteorological models, but not in the amount and intensity of precipitation which than happened. It is difficult to predict flash floods for the alpine and mountainous region due to the complex topography and high variability of precipitation. On the other side, the resolution of meteorological models is still not high enough for mountainous terrain.

References

ARSO, 2007a: Poročilo o vremenski in hidrološki situaciji 18. septembra 2007. http://www.arso.gov.si/vode/publikacije%20in%20poro%c4%8dila/visoke_vode-0070918.pdf ARSO, 2007b: Izjemen padavinski dogodek 18. septembra 2007. http://www.arso.gov.si/vreme/poro%c4%8dila%20in%20projekti/padavine_18sep07.pdf ARSO, 2008: Visoke vode in poplave 18. septembra 2007. http://www.arso.gov.si/vode/publikacije%20in%20poro%c4%8dila/Visoke%20vode%20in%20poplave %2018.%20septembra%202007.pdf Kobold, M., Sušnik, M., 2001: Hidrološki model Save do Zidanega mosta v programskem okolju WMS. Hidrometeorološki zavod RS, Ljubljana. Sušnik, M., Robič, M., Pogačnik, N., Ulaga, F., Kobold, M., Lalić, B., Vodenik, B., Štajdohar, M., 2007: Visoke vode in poplave v septembru 2007. 18. Mišičev vodarski dan 2007. Zbornik referatov, VGP, Maribor, 10. december 2007. WMS Manual, 1997: Watershed Modeling System. Reference manual, Version 5.0. Provo, Utah, Brigham Young University, Engineering Computer Graphics Laboratory.