the consequences of snow and sleet in slovenia · vremščica, the foothills and slopes of...
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»With the contribution of the Civil Protection Financial Instrument of the European Union«
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Newsletter nº 8 – September 2016
The consequences of snow and sleet in Slovenia
Introduction
This newsletter presents a brief overview of incidence and consequences of snow and sleet in Slovenia in February 2014. For better understanding, the formation of sleet is presented first followed by gathered historical data and description of the vulnerability to high wind in combination with snow and sleet that occur in critical time in 2014. The assessment of risk of areas to high winds in combination with snow and sleet is based on acquired existing data and reports from local, regional and national agencies and responsible institutions. The vulnerability towards snow, sleet and winds is assessed in five major aspects: population, infrastructure, transport, buildings and forests.
The formation of sleet
Sleet is a smooth, transparent and homogeneous ice coating that forms either on ground or on other
objects. It occurs when it rains at temperatures below freezing or when the precipitation in liquid
form falls on subcooled ground. At such a temperature it usually snows, but in some weather
conditions the snow or the mixture of snow and rain become liquid. Sleet is often associated with the
approach of a warm front, when subfreezing air (temperatures at or below freezing) is trapped in the
lowest levels of the atmosphere while warm air advects in aloft. As it accumulates a lot of cold air in
the lowlands during a clear and calm winter nights, the warm air can hardly move it by the absence
of stronger winds. The exchange of cold air with warmer is even more difficult in mountainous areas,
where the cold air lake stay at farthest.
Next, a cold air flow in lower layers and warmer and moist air flow in higher layers of the atmosphere
may also cause the phenomenon of sleet. This phenomenon is characterized by formation of
secondary cyclone over the southern side of the Alps, which occur when atmospherics passing over
Alps to the east. This is typical when from above the Mediterranean area over Slovenia at higher
altitudes flowing moist subtropical air, and cold eastern air to the ground layer. Although in such
weather conditions the temperature in the valleys remains several degrees below freezing, the zero
isotherms in warmer layers maybe on altitude of the highest Alpine peaks (see Fig. 1).
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Figure 1: The formation of sleet (e-schooltoday.com)
Historical data
Slovenia is one of the countries that belong to sleety areas in Europe. In Slovenia, the sleet appears in
the cold half of the year. The medium strong sleet occurs every few years while strong sleet, which
causes high economic damage, occurs approximately every ten years. The greatest damage is caused
to trees as well as to electrical and telephone communication lines. The sleet is characteristic above
all for the southwestern part of Slovenia, along dinar barrier, either on the mainland or coastal side.
The most commonly areas in Slovenia affected by sleet are Brkini, the area around Senožeče with
Vremščica, the foothills and slopes of Snežnik, Javorniki, Nanos, Trnovo forest and Čičarija (ARSO,
2002). In Figure 2 the map of areas threatened by sleet during time period 1961-2006 is shown. The
short explanation of zones is presented next to the map.
Zone 1: The area where sleet does not occur or occurs rarely in thin layers (it does not cause considerable damage) Zone 2: The area where sleet usually occurs, but very rarely causes minor damage (once every 10 years) Zone 3: The area where sleet appears frequently and on 3 years average causes damage Zone 4: The area where sleet causes damage and occurs on 1 to 2 years average relatively often with even greater damage
Figure 2: Map of areas threatened by sleet in time period 1961-2006 (Slovenian Environmental
Agency, 2006).
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For the purposes of the Wind Risk project, we asked Slovenian Environmental Agency for the
historical data of sleet. They are gathered in Table 1. The notable events with high damages are in
bold type.
Table 1: Historical data of sleet damage
Time period
(month, year)
Area Damage to forest
[m3/area]
Note
1896* Zgornja Pivka, Knežak
December 1899 Zgornja Pivka, Vremska dolina
1900** Pivka
1933 Brkini, Košana Majority damage in altitudes between 500 and 700 m
January 1952 Vremščica, Brkini
March 1952 Gorenji Kras
December 1953 Idrijsko hribovje 153.000 Majority damage in altitudes between 500 and 800 m
1958 Litijsko hribovje 1.150
1958 Haloze, Boč, Tisovec 7.000 Majority damage in altitudes between 500 and 600 m
1960 Rudnica, Sotelsko, Haloze 7.930 Majority damage in altitudes between 400 and 500 m
November 1963
Area of Logatec 7.000
1966 Area of Vrhnika and Škofljica Sleet and snow damage
November 1968
Idrijsko hribovje, Trnovski gozd, Krekovše
75.080 Majority damage in altitudes between 600 and 800 m
January 1972 Kras in the edge of Divača, Trstelj 40.000 Majority damage in altitudes between 400 and 550 m
1973 106.000
March 1975 Idrijsko hribovje 20.000
November 1975
Idrijsko hribovje, Trnovski gozd, Hrušica, Snežnik, area of Kranj
378.860 Majority damage in altitudes between 800 and 1200 m
February 1976 Area of Razdrto
November 1980
Brkini, Čičarija, area of Idrija and other areas in Slovenia
673.644 Majority damage in altitudes between 500 and 800 m, inside the country in altitudes between 400 and 700 m
November 1984
Idrijsko and Cerkljansko hribovje, the area of forestry organization of Ljubljana
More than 100.000 Majority damage in altitudes between 700 and 1000 m
November 1985
Idrijsko and Cerkljansko hribovje, the area of forestry organization of Kranj and other areas in Slovenija
More than 500.000 Majority damage in altitudes between 700 and 1000 m
At the beginning of January 1996
The central Slovenia, Štajerska, Notranjska
680.700, 87.440 ha of damaged area, that is 8.1% of all forests in Slovenia
Majority damage in altitudes between 400 and 900 m; sleet and snow damage
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At the end of January 1996
Kras, Goriška Brda, Kambreško, Banjščice, Trnovski gozd, Brkini
Majority damage in altitudes between 400 and 900 m
December 1996, January 1997
whole country 867.400, 81.8100 ha of damaged area, that is 7.5% of all forest area In Slovenia
sleet and snow damage
February 2009 Bloke, Brkini
January 2010 Brkini, in places of Postojna and Brežice
Affected area 5150 ha from that 3720 ha of froests, damaged 850 m3 or 3 % in Brkini
Majority damage in altitudes between 600 and 750 m
February 2014 whole country except Vipava valley, Brkini, Kras, coastal area and Prekmurje
9.386.776, 601.900 ha or 50 % of all forest area In Slovenia
Majority damage in altitudes between 300 and 1100 m
December 2014 Trnovski gozd, Banjščice Sleet damage only on electric installation
Source: Slovenian Environment Agency, 2016; Summarized after Administration for Civil Protection and Disaster Relief, 2015
In February 2014 the worst sleety event in living memory occurred in a great part of the country. A
brief description of this critical event is presented additionally.
Weather situation in the end of January 2014
In the end of January 2014, the weather situation in the area above Europe was characterized by high
contrast between deep cyclonic area above the eastern Atlantic and partly over the Mediterranean,
and expressive anticyclone with the center above Russia. The difference in air pressure between both
formations was occasionally more than 100 hPa. The Slovenia was in large part in the area of contact
between anticyclone and cyclone, where the frontal zone was renewed constantly. In frontal zone,
the cold air of polar origin in a thin
boundary layer of the atmosphere and a
strong flow of warm and moist air at
altitudes above North Africa and the
Mediterranean were composed. In
February during 30th and 5th intense
precipitation occurred in large areas of
Slovenia, widely combined with high
southwestern wind speed and wind gusts
of intensity category 8 Beaufort, which is
nearly 70 km/h. This kind of synoptic
situation constitutes a recipe for abundant
rainfall, snowfall and sleet formation on
the southern edge of the Alps.
Figure 3: Surface pressure chart over Europe in February 1st (National Meteorological Services, 2014)
In Slovenia, the worst situation was in Postojna city, which is situated in south-west part of the
country. The wind speed properly was not so high, but in combination with sleet caused a great
damage to infrastructure, forests, transport, buildings and to population as well.
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Population
The sleet damage caused a lot of problems to the population. About 153.400 of the households did
not have electricity for at least one day and about 23.270 of the households had to live without
electricity for more than two days. The Civil Protection of the Republic of Slovenia in cooperation
with the electricity distributors had made a plan of priority electricity supply. Consequently, the
electrical generators, which were connected to the operators of public telecommunications services,
were assured by Civil Protection (ACPDR, 2014).
The great part of the population was not able to get to their workplaces due to collapse of many
lattice towers and trees. Kindergartens and schools in the area of Pivka and Postojna and some other
parts of the country were closed for one day, while students remained stranded overnight at a local
school at Vojsko before access was unblocked in two days.
Emergence wards in hospital, as a secondary factor, had also seen a huge surge in emergency cases.
Hundreds of patient with injuries received emergency care at the two main hospitals, namely, in
Ljubljana and Maribor. There were also some injuries of electricians, firefighters and civil rescue
teams who helped to unblock roads and provided emergency relief. Due to consumption increase of
electrical generators and its irregular use, there were also some poisoning by carbon monoxide.
Transport
In most critical days, the electric traction was disabled in some electrified lines due to power failure.
The worst situation was between Pivka and Borovnica where trains got stuck due to broken-down
trees, demolition of masts and beams of overhead lines (ACPDR, 2014). Where it was possible, the
electric trains were exchanged by diesel trains. Because of the worsening of situation, further falling
of trees, demolition of devices and signaling, the trains with diesel traction had to finally stop
(Slovenian Railways, 2015)
The railway stations of Postojna, Pivka and Prestranek
and the other stations were fettered in sleet for several
days. These railway stations were totally damaged. All
cantilevers of the running network and a few columns
were broken. The danger in the station area was
accounted by broken and damaged vegetation
(Slovenian Railways, 2015). The complete passenger
traffic between Borovnica and Divača was moved to the
bus lines for more than a year. The railway track at that
segment was repaired recently in June 2015.
Figure 4: Sleet damage at railway station in Pivka (Paolo Visintini)
Infrastructure
The disaster caused by sleet only in the area, which is administrated by Elektro Ljubljana, damaged
3000 wooden and 92 metal masts, 23 of them were on 110 kV voltage network and other in medium
voltages network. It was damaged more than 4000 cantilevers and 6000 insulators. It was also
affected 1800 km of medium voltages and 1200 km of low voltages.
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Figure 5: Amount of sleet on the conductor (left) and on the insulators (right); Rebolj 2015
The assurance of electricity was the biggest problem, since the overhead power lines collapsed under
the weight of wet snowfalls and sleet. The difficulties occur due to the lack of electrical generators.
Therefore, the Civil Protection of the Republic of Slovenia in cooperation with the electricity
distributors had made a plan of priority electricity supply. On the initiative of the Telekom Slovenija,
who is the most accessible mobile service provider in Slovenia, the 36 telecommunications facilities
were included at 3rd of February. In all telecommunications facilities operated by the Administration
for Civil Protection and Disaster Relief and the Department of Informatics and Communications of
Ministry of Defence, the supply of electrical generators, which were connected to the operators of
public telecommunications services, was assured by Civil Protection. The helicopter transport of
aggregates as well as fuel for aggregates was also assured to the aggravated places. The customers of
Telekom Slovenia got reduced invoices for telecom services due to loss of cable network
infrastructure caused by sleet. The value of reduced invoices was depended on time and type of
services (Božičić, 2014).
Buildings
In news media we find some reports of building damage during sleety event, however, as a
consequences of fallen tree or fallen lattice steel power, which hit the roof of the building. However,
luckily no major damages were recorded during sleety event.
Forests
The greatest damage to forests arose as early as in January 31st when at least 20.000 ha of forests
were hit by snow, sleet and wind. The most severe regions were Notranjska, the southwest edge of
Ljubljana Basin, Cerkljanska and Idrijska region (ACPDR, 2014). According to the Slovenian Forest
Service the sleet damaged near 601.900 ha or 51 % of forest sites in various types of forests cross the
country (ACPDR, 2015; Marinšek, 2015). Such natural disaster in Slovenian forests has not been
observed.
It was damaged about 9.386,776 m3 of wood mass and at least 660 ha of forest was completely
damaged (ACPDR, 2014). Most damaged wood mass was in the regional unit GGO Ljubljana (about
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2.4 million m3), GGO Postojna (about 2.1 million m3), GGO Tolmin (1.8 million m3) and GGO Kranj
(about 900,000 m3). Among damaged trees, the third of all were coniferous tress (3.137,122 m3) and
the rest (6.178,403 m3) were deciduous trees (Veselič et al., 2015; ACPDR, 2014).
Figure 5: Sleet damage to fruit trees (Slovenian Press Agency)
Summary
The newsletter comprises a brief description of incidence of sleet in Slovenia and their consequences. In February 2014 the worst sleety event in living memory occurred in a great part of the Slovenia, but the worst situation was in Postojna, which is situated in south-west of the country. For better understanding, the formation of sleet is presented first followed by gathered historical data and description of the vulnerability to high wind in combination with snow and sleet that occur in critical time in 2014. Due to this, the newsletter is focused on sleet damage in Postojna area. There it was damaged a lot infrastructure facilities, forests, and buildings; due to road closures, the mobility was strongly districted. Therefore, the vulnerability towards snow, sleet and winds in five major aspects: population, infrastructure, transport, buildings and forests, was briefly presented.
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Literature: Administration for Civil Protection and Disaster Relief - ACPDR (2014): Poročilo o posledicah poplav, visokega snega in žleda v Republiki Sloveniji med 30. januarjem in 9. februarjem 2014. Božičić, D. (2015): Kritična telekomunikacijska infrastruktura v primeru naravnih nesreč. Diplomsko delo. Online
pdf: file:///C:/Users/Urska/Downloads/Bo%C5%BEi%C4%8Di%C4%87_DanijelKriti%C4%8Dna_telekomunikacijsk
a_infrastruktura_v_primeru_naravnih_nesre%C4%8D.pdf http://www.sos112.si/slo/tdocs/ocena_zled.pdf (20.11.2015)
Marinšek, A. (2015): Kakšna je povezava med gozdnim rastiščem in poškodovanostjo drevja zaradi žleda? Mednarodna konferenca: Obnova gozdov po žledu, Postojna, 19. - 20. March 2015. Rebolj, J. (2015). Vpliv bistveno večje žledne obtežbe vodnikov od projektirane na statično stabilnost DV stebra, [Ice load on conductors, significantly greater than planned, and its effects on static stability of transmission line towers], 12. Konferenca slovenskih elektroenergetiokov. CIGRE ŠK B2-10. Portorož. Slovenian Railways (2015). Poročilo o izrednih dogodkih. [Report of extraordinarily events in 2014], June 2015. Veselič, T., Grecs, Z., Kolšek, M., Oražem, D., Matijašić, D., Beguš, J. (2015). Žled v slovenskih gozdovih in njihova sanacija, [Icebreak in Slovenian forests and salvation of consequences], UJMA 29. Website Slovenian Environment Agency (2002): Poročilo o stanju okolja 2002. http://www.arso.gov.si/varstvo%20okolja/poro%C4%8Dila/poro%C4%8Dila%20o%20stanju%20okolja%20v%20Sloveniji/povzetek.pdf (created 2002, accessed 30.12.2015) Website e-schooltoday: http://www.e-schooltoday.com (accessed 30.12.2015)