土砂管理研究グループ

雪氷チーム

雪崩・地すべり研究センター

Snow and Ice Research Team

Snow Avalanche and Landslide Research Center

短時間の多量降雪による雪崩危険度評価に関する研究
Hazard Assessment for Avalanches Caused by Intense Heavy Snowfall
平成18年豪雪以降、北海道や東北、北陸、中国地方と各地で豪雪が続いています。特に、平成26年2月
は雪の少ない関東甲信などの広い地域で、短時間の多量降雪により記録的な大雪となりました。短時間の多量降雪は、雪崩の多発につながり、豪雪地帯はもとより豪雪地帯以外の地域でも突発的に雪崩が発生する可能性が高くなります。
土木研究所では、短時間多量降雪による雪崩発生の危険度評価技術に関する研究に取り組みました。

短時間多量降雪による雪崩事例
An avalanche caused by intense snowfall

Since the Heavy Snowfall of 2006, there have been heavy snowfall events in many areas of Japan, including Hokkaido,Tohoku, Hokuriku, and Chugoku. In February 2014, a record intense heavy snowfall occurred in large parts of the Kanto
and Koshin areas, which ordinarily have little snow. Intense heavy snowfalls lead to many avalanches. In such cases,
unpredictable avalanches become highly likely in areas within and beyond the areas of heavy snowfall.
The Civil Engineering Research Institute for Cold Region has been conducting research on technologies to assess the riskof avalanche occurrence from intense heavy snowfall.

雪崩発生の気象・積雪条件

雪崩発生の地形・植生条件

平成26（2014）年2月の関東甲信地方の大雪事例や北海道野塚峠の雪崩事例等について、短時間多量降雪を12時間の最大降雪深を用いて表し、樹林帯における雪崩発生条件や同時多発するときの条件を明らかにしました。

雪崩発生箇所の現地調査と文献レビューの結果、立木間隔と斜面勾配の関係など、樹林内における雪崩発生の植生と地形の条件を明らかにしました。短時間多量降雪時は、樹木が密でも雪崩が発生する特徴があります。

For the heavy snowfall events in the Kanto and Koshin areas in February 2014 and theavalanches at Notsuka Pass in Hokkaido, extreme high-intensity snowfall was expressedusing the maximum snowfall in 12 hours, and the occurrence conditions were clarified foravalanches in the forest zone and for multiple simultaneous avalanches.

As a result of the field survey and a literature review of the avalanche sites, we clarified thevegetation and topographic conditions for avalanche occurrence in forests. The findings included
the relationship between tree spacing and slope gradient in the forest. Avalanches characteristicallytend to occur even in dense forest when an area has extreme snowfall in a short period.

Weather and snowfall conditions for avalanche occurrence▲樹林帯での雪崩発生箇所の例（平成26年2月、R48関山峠(宮城県)）
Example of an avalanche site in a forest zone (February 2014,
Sekiyama Pass on Route 48 in Miyagi Prefecture)

雪崩の発生頻度

6050403020

同時多発する雪崩発生条件

Occurrence conditions formultiple simultaneous avalanches

SI=1.52.0

Avalanche occurs-8-4-20

Air temperature (12-hour mean)

3

Fujikawaguchiko Town

富士河口湖町2非発生

Non-occurrence

既存研究による表層雪崩の発生条件(12時間平均)気温(℃)▲12時間降雪深と気温を用いた雪崩発生
条件（SI: 斜面積雪の安定度）
Avalanche occurrence conditions expressedusing 12-hour snowfall and air temperature(SI: stability of the snowpack on the slope)030▲雪崩発生箇所の現地調査例（秩父市）
A site of the onsite survey on avalanche occurrence（Chichibu City)

Occurrence conditions forsurface avalanches in previous studies

354045

Slope gradient50

斜面勾配(°)

雪崩の到達距離と衝撃圧

Frequency of avalanches

奥多摩町

秩父市

1-6

Okutama Town

Chichibu City

3.0

100-10

雪崩発生

4

Avalanche occurrence
conditions ina forest zone

Tree spacing

Snowfall (12-hour max.)(12時間最大)降雪深(cm)70

5

樹林帯の雪崩発生条件

立木間隔(m)80

Topographic and vegetation conditions for avalanche occurrences▲雪崩発生箇所の立木間隔と斜面勾配の関係
Relationship between tree spacing
and slope gradient at avalanche sites

Runout distance and impact force of avalanches

気象庁アメダスと北海道開発局道路テレメータデータを用いて、雪崩発生条件を満たす降雪事例の発生頻度を解析し、樹林帯において雪崩が発生すると考えられる頻度の地域分布（雪崩発生危険度図）を提示しました。

樹林の影響を考慮した雪崩運動モデルを考案し、樹林の有無、斜面勾配、発生層厚等の様々な条件で樹林帯を流下する雪崩の到達距離と衝撃圧を算出する手法を提案しました。

Using AMeDAS data from the Japan Meteorological Agency
and road telemeter data from the Hokkaido Regional
Development Bureau, we analyzed the frequency of snowfall
events that satisfied the avalanche occurrence conditions andpresented the regional distribution of the frequency at whichavalanches are considered to occur in forest zones(avalanche occurrence risk map).

We devised an avalanche motion model that takes into account the effects of forestsand proposed a method for calculating the runout distance and impact force foravalanches that flow through forest zones under various conditions, such as thepresence or absence of trees, different slope gradients, and different thicknesses of theavalanche layer.
Without trees

雪崩の危険度評価

SnowfallHorizontal distanceTree density

Horizontal distance

Avalanche impact force▲雪崩発生条件を満たす降雪事例の発生頻度分布12-hour max. snowfall Occurrence frequency distribution for asnowfall event that satisfies theconditions for avalanche occurrenceLongitudinal
profile of the site

Thickness of the avalanche layer
Avalanche impact force
Slope height

Total snowfall of
the snowfall event(Once in 8.3 years)

With trees

Thickness of the avalanche layer
Slope height

Occurrence frequency(times/year)(times/year)

Low

Example in Minakami,
Gunma Prefecture▲

雪崩発生条件を満たす降雪事例の発生頻度解析の例
Example of occurrence
frequency analysis for asnowfall event that satisfies
the conditions for avalanche
occurrenceHigh Occurrence frequency

Longitudinal
profile of the site

▲雪崩の到達距離と衝撃圧の算出例
Calculation examples for avalanche runout distance and impact force

Risk assessment for avalanches

この研究により、短時間多量降雪時において、従来発生しにくいと考えられてきた樹林帯での雪崩発生評価が可能になりました。
また、頻度解析から求めた降雪深を雪崩運動モデルの発生層厚に適用することで、対策施設の設計条件となる頻度（例：３０年確率値など）で発生すると考えられる雪崩の到達距離と衝撃圧を算出（雪崩危険度評価）することが可能になりました。

This study enables avalanche occurrence assessment for forest zones, which had been regarded as areaswith rare avalanche occurrence, when the area has extreme snowfall in a short period. This study alsoenables avalanche risk assessment, i.e., calculation of the runout distance and impact force of an avalanchewith a probable occurrence frequency that is specified for the design conditions of avalanche control
structures (e.g., an avalanche with a 30-year probability). In this calculation, the snowfall obtained from thefrequency analysis was applied to the thickness of the avalanche layer in the avalanche motion model.
2022.7

安全・快適

北の道Safe, pleasant driving on Northern Roads

国立研究開発法人土木研究所 寒地土木研究所

National Research and Development Agency Public Works Research Institute Civil Engineering Research Institute for Cold Region