»With the contribution of the Civil Protection Financial Instrument of the European Union«

1

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).

»With the contribution of the Civil Protection Financial Instrument of the European Union«

2

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).

»With the contribution of the Civil Protection Financial Instrument of the European Union«

3

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

»With the contribution of the Civil Protection Financial Instrument of the European Union«

4

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.

»With the contribution of the Civil Protection Financial Instrument of the European Union«

5

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.

»With the contribution of the Civil Protection Financial Instrument of the European Union«

6

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

»With the contribution of the Civil Protection Financial Instrument of the European Union«

7

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.

»With the contribution of the Civil Protection Financial Instrument of the European Union«

8

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)