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A snow shed, snow bridge or avalanche gallery is a type of rigid snow-supporting structure for avalanche control (avalanche defense) or to maintain passage in areas where snow removal becomes almost impossible. They can be made of steel, prestressed concrete frames, or timber.〔(Photographs of avalanche defences ), FAO corporate document repository〕〔("Avalanche and torrent control in the Spanish Pyrenees" )〕 These structures can be fully enclosed, like an artificial tunnel, or consist of lattice-like elements. They are typically of robust construction considering the environments they must survive in. Snow protection is particularly important when routes cross avalanche "chutes", which are natural ravines or other formations that direct or concentrate avalanches. Snow sheds or avalanche galleries are a common sight on railroads in mountain areas, such as the Donner Pass in the United States or many of the Swiss mountain railways, where tracks are covered with miles of shedding. Although unused today, the Central Pacific railroad had a complete rail yard under roof in the pass. They are also found on especially hazardous stretches of roadway as well. The Trans-Canada Highway between Revelstoke and Golden in British Columbia has a snowshed covering both carriageways to cope with the heavy snow. Snoqualmie Pass previously had a snow shed for the westbound carriageway of I-90 near Keechelus Lake, though it was replaced with a bridge in 2014.〔("New bridges get green light to replace snowshed east of Snoqualmie Pass" )〕 The I-90 snow shed was the last remaining snow shed along an Interstate highway when it was removed.〔("The I-90 snowshed retires after 64 years!" )〕 == Snow bridge == Snow bridges may superficially look similar to snow fences, but they act differently. Snow fences are built vertically and accumulate snow on their downwind side, while snow bridges are slanted or horizontal and hold snow on their top side. Snow bridges are fastened to the slope on the upslope side by tension anchors and on the downslope by compression anchors.〔("Steel snow bridges" )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Snow shed」の詳細全文を読む スポンサード リンク
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