SIGNIFICANCE OF HISTORICAL RECORDS FOR AVALANCHE HAZARD ZONING IN NORWAY Krister Kristensen*, Carl Harbitz* Norwegian Geotechnical Institute, Oslo Alf Harbitz** Norwegian Institute of Fisheries and Aquaculture Ltd, Trorns0 ABSTRACT: In avalanche hazard zoning, it is common practice to investigate the previous avalanche history for the area considered. Historical observations of avalanches serve as an aid in the classification of the terrain, and may also serve as verification of estimates of avalanche runout. Conditions influencing the avalanche occurrences may change significantly over time and it is important to take these changes into account when using historical avalanche observations in hazard ioning today. A number of the most extensive avalanches recorded in Norway, are found during the eigthteenth and nineteenth century. The catastrophes may be linked to weather as well as to socio-econornic conditions, in particular deforestation of mountain slopes. The implications of using or disregarding historical avalanche observation are shown in an example of statistical estimation of avalanche runout. KEYWORDS: Snow avalanche, run-out distance, -probability, extreme value statistics, Western Norway 1. INTRODUCTION Given enough time, nature will again provide the conditions that caused the avalanche in the past. According to the Norwegian Building Assuming that the avalanches were mainly Regulations, the "safe" areas for habitations in the related to the weather conditions, the avalanche snow-avalanche prone regions of Norway are de­ probability can be related to the probability of fined as areas where the nominal annual prob­ such weather conditions. ability of a house bein:p hit by an avalanche One problem with this approach is that it should be less than 10-. This means that not may often not be possible to reconstruct the snow only the obvious paths with recurring avalanches conditions and the weather sequences with have to be considered, but also areas where enough detail. Another problem, which will be avalanches are known to have occurred very discussed in this paper, is that there may be rarely, or not at all. other, non-weather related, factors influencing the Land use planning in avalanche prone ar­ avalanche activity. This makes it important to eas is usually an interdisciplinary task. It includes have a fairty broad historical perspective when avalanche dynamics calculation, geomorphologic using historical avalanche observations. It may interpretation, meteorology, climatology and his­ also be that some of these factors are not present torical research. The historical records of ava­ today, and this may have implications on the lanches often serve as an indication of what may estimates of avalanche probabilities. be potential starting zones and as a verification of estimates of avalanche runout. 2. USE OF HISTORICAL DATA The existence of a historical observation is often seen as proof of an existing hazard today. Historical data of avalanche occurrences in Western Norway vary in quality and availability. In many cases the interpretation of them demand Corresponding author addresses: use of historical research methods, particularty the *Krister Kristensen and Carl Harbitz: procedures of source validation. The sources Norwegian Geotechnical Institute, P.O. Box include farm and family histories, newspaper 3930 Ullevaal Stadion, N-0806 OSLO, reports, official reports of damage appraisals, per­ Norway sonal letters, etc. Problems with the sources E-mail: [email protected]@ngLno include lack of coherent terminology: e.g. they **Alf Harbitz: Norwegian Institute of Fisheries may not distinguish between snow and slush ava­ and Aquaculture Ltd., 9291 TROMS0, lanches. It may also be that the messenger has Norway underlying motives, such as pleas for tax reduction E-mail: [email protected] or financial support that could lead to overstate­ ments of the magnitude of the avalanche. 475 Unless photos or maps exist, reconstruct­ damage. Both settlements are known to "-e ing the spatial distribution of an avalanche in the been permanently settled before 1600 AD. F past at a given site, will often include rather sub­ thermore, Gj0rven and Tenden are still 0CCu~ jective interpretation. If it is possible to identify today, more or less at the same sites as in 1868 ruins or obtain old maps of building sites, this can and none have suffered any avalanche dartlage provide data on certain points, e.g. which buildings since then. were destroyed and which were not. But often the It may be of interest to look at the COrtcI­ determination of the avalanche's area of influence tions that led to this unusual avalanche activity in is left to interpretation and avalanche dynamics 1868. Was it only the unusual combination ~ calculations and this is clearly a source of error. A weather sequences or were other factors also im­ claim about what a snow avalanche debris looked portant? like 100-200 years ago, is unlikely to be verified nor disproved even by detailed sedimentology or 4. CONDITIONS THAT LED TO AVALANCHES archeological methods. IN 1868 Westem Norway is fortunate to have a good record of avalanche events since the "land 4. 1 Weather conditions rent", a measure of taxability, was assessed for individual farms on the grounds of, among other The disasters of 1868 have been linked to things, their vulnerability to avalanche hazard a unique combination of a cold early winter Wilb (Grove and Battagel, 1983). Claims of tax reduc­ prevailing storms from the Norwegian Sea foIl_ tion could be made after avalanche damages to a ing in February. The weather data available, farm, and all such claims were assessed by official although limited, support this (Kristensen, 1999). tax commissions and thus recorded for the after­ It is likely that a widespread instability in tile time. A voluminous and fairly accurate record of mountain snow cover developed during the eatt avalanche damage from around 1650 to 1815 of winter because of a shallow snow cover and krf exists. After 1815 the tax assessment system temperatures. In this period a snow cover was changed, but similar data can be extracted from present also in the lowland. From around JTid. other sources. The records available show sev­ January, the low-pressure activity over W~ eral major avalanche winters during the latter part Norway increased markedly with Northwest winds of the nineteenth century and large amounts of snow. The extraordil'Vly snowfall is mentioned in many eyewitne$ll 3. THE AVALANCHES IN 1868 accounts from the time. Since local weather data from Stryn lie The most disastrous avalanche winter in not available, it is difficult to estimate the retlJII terms of loss of lives occurred in the winter of period for similar weather situations. Preliminay 1867/68 when a total of 161 people were killed. analyses of data from nearby stations indicate ttd This winter, avalanches started running down into there is hardly any reason to conclude that ti! the inhabited areas of the north region of Westem conditions were sufficiently unique to not hat th Norway (figure 1) on February the 6 , with several occurred several times during the preceding 2lI disasters following the next twenty days until the years with known settlement or during the tid th 26 . The communities that suffered the greatest after 1868 (Kristensen, 1999). In an analysi~' losses were Stryn with 35 and Oppdal with 32 major avalanche winters by Fitzharris aad fatalities. All of the victims were caught inside Bakkeh0i (1986) several winters with similar S'II' houses that were destroyed by the avalanches optic characteristics as the 1867/68 winter­ (Kristensen, 1998). reported. The avalanches of 1868 in Stryn have been examined closer (Kristensen, 1999). 4.2 Other factors Avalanches struck clusters of farms that both had, and did not have a previous history of avalanche The socio-economic conditions in NolVlIIf damage. One of the settlements affected (13 in the 1860-ies were very different from toda'tl victims) had been struck by an avalanche in 1718, The 1860-ies were generally hard times in 150 years before. However neither of the other West-Norwegian countryside. The size d two settlements, Gj0rven, where four farms were popUlation had reached a level where the destroyed and 11 people lost their lives, nor production could hardly keep up. This led to Tenden, where the destruction of two farms also intensification of the agricultural practices claimed 11 victims, had any history of avalanche involved large-scale use of the mountain 476 f forest cutting and for grazing livestock on tion is if and how this influenced the avalanche ormmer pastures above the timberline. A large activity. su rtion of a farm's production was actu~lIy m~ved m the valley floor up into the mountains. Since r 5000 t~~ Number of cotters. production of the sum":,er dairy farms near the SF, Western Norway. timberline increased, so did the demand for fire­ 4000 (Timberlid 1981) WOOd for heating and making brown whey cheese. There are several contemporary accounts of the 3000 lack of forest, which sometimes led to abandon­ 2000 ment of the summer dairy farms. An increasing number of cotters (Iandren­ 1000 ters usually farming the outskirt~ of a r~ular farm) sometimes led to settlements In marginal areas, 0 no doubt also leading to increased exposure to 1800 1850 1900 1950 2000 avalanche danger. 250 The forester A. T. Gll2lersen (1868) com­ Fatalities in buildings affected by avalanehes (10 year sums). ments that although the winter conditions were 200 WeslBrn Norway (SF,MR,HO,RO). special, they were not all that unusual. Simil~r snow and weather conditions had been experi­ 150 enced before without the same dire conse­ 100 quences. A new and unique factor must therefore have been added, he argues. 50 Gll2lersen leaves no doubt about what he thinks this is: "I have come to the conclusion that 0 a large number of the avalanches last winter is to 1800 1850 1900 1950 2000 blame on the thoughtless, fast spreading destruc­ tion of the birch forest in the upper mountain Figure 1.