Jökull


Jökull - 01.12.2006, Síða 9

Jökull - 01.12.2006, Síða 9
Avalanches in coastal towns in Iceland 2001-2002). There are three main evacuation levels (Magnússon, 1996): Evacuation level 1: An area where avalanches have occurred following moderate snow accumula- tion. The extent of the evacuation area may be smaller than indicated by the avalanche history, where ex- treme conditions are included. Frequent evacuation of homes can be expected. Evacuation level 2: An area which is predominantly determined by known avalanches and topographic conditions that are similar to known avalanche paths. Avalanche hazard is associated with heavy accumula- tion of snow. The area will be evacuated during im- pending weather conditions which are known to im- pose a serious threat of avalanches. Evacuation level 3: An area which is considered threatened by catastrophic avalanches that need not be included in the known avalanche history, but are considered possible; meteorological conditions with extreme snow accumulation and extreme winds. Ar- eas threatened during extremely rare meteorological conditions are included. Evacuation level 2 1/2: In Neskaupstaður, level 3 reaches down to the ocean in an extensive part of the town. Furthermore, the starting zones of the vari- ous avalanche paths have similar aspects resulting in avalanche hazard a large area at the same time. To be able to evacuate smaller parts of the town, an in- termediate level between levels 2 and 3 was defined. Although permanent avalanche protection has been constructed in some towns, level 3 evacuation plans are still in effect, as the risk can never be elimi- nated completely. Where the hazard is mainly due to wet avalanches or slush flows, evacuation is usually not predefined, but partly based on a subjective evaluation of the situ- ation on each occation. WEATHER LEADING TO AVALANCHES IN THE TOWNS AT RISK A general description of weather related to avalanches for each town is given in the evacuation plans (Veðurstofa Íslands, 1997). The weather description is based on the studies mentioned previously in this pa- per, as well as unpublished studies relating local con- ditions and particular weather characteristics. Details on avalanche paths and weather characteristics prior to avalanches in individual towns are summarized in Tables 3–17. The evacuation levels are included to show the scale of the danger in the zone below the relevant source area. A short overview is given for each town. Ólafsvík The two known avalanche events in Ólafsvík (Figure A1) occurred when relatively large amounts of snow had accumulated during winds from S and SE. Patreksfjörður The most severe avalanche hazard in Patreksfjörður (Figure A2), especially at Vatneyri, seems to be re- lated to intense snowfall and/or snowdrift in easterly wind directions, i.e. from 60◦ to 130◦. This is asso- ciated with frontal zones moving slowly northwards over the area. Intensive rain and/or melting can cause serious slush flow danger. Bíldudalur During most winters there is not much snow accumu- lation on the slopes above Bíldudalur (Figure A3). Al- though rare, avalanche hazard may develop along the mountainside above the town, during intense snowfall and persistent westerly winds. Wet avalanches, slush flows, landslides and water floods are more likely than dry avalanches. Intensive rain or sudden thawing can create a severe avalanche danger in Bíldudalur. Tálknafjörður The town is mainly threatened by slush and water flows, caused by intense rain or thawing. Evacua- tions due to risk from dry avalanches were not consid- ered necessary and there are only a few recent mapped avalanches. Predefined evacuation plans are designed with respect to flooding along two gullies. Northern part of Vestfirðir: Suðureyri, Flateyri, Súðavík, Ísafjörður, Hnífsdalur and Bolungarvík The most severe avalanche hazard in Northern- Vestfirðir (Figures A4-A8) is related to snowfall with strong winds from northerly directions, mainly N and JÖKULL No. 56 7
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