International Symposium on Mitigative Measures against Snow ...

<strong>International</strong> <strong>Symposium</strong> on Mitigative Measures against Snow Avalanches
Egilsstaðir, Iceland, March 11–14, 2008
3.2 Deflecting dam below Búðargil in Bíldudalur
The mountain Bíldudalsfjall on the Westfjords peninsula, rises up to 460 m a.s.l. above the
town Bíldudalur. The gully Búðargil is deep and steep and cuts Bíldudalsfjall above the north
part of the settlement. A large part of the residential area sits on the alluvial fan underneath
the gully. A few snow avalanches down the gully have been recorded since the town’s
settlement along with more frequent events of waterfloods, mudslides and slushflows.
Proposed defence structures for the area were designed in 2005 and consist of a 300 m long
deflecting dam, to deflect snow avalanches to the north away from the more densly populated
part of town directly below the gully and into the ocean through a less populated part of town
(houses in this area which will be abandoned during winter) as shown in Figure 3 (Sigurðsson
and others, 2005). The dam is highest 20 m above ground level close to the mouth of the gully
and there its deflecting angle is 22°. The height of the dam decreases gradually towards the
settlement. The north bank of the avalanche channel was enhanced on a 20 m stretch at the
downstream end of the channel in order to prevent avalanches from spreading out to the north.
The dam is scheduled to be built in 2008.
Figure 3 A contour map of the Bíldudalur deflecting dam superimposed on an area photo
and a photograph looking up into Búðargil.
The height of the dam was determined based on the shallow-layer theory. The confined
geometry of the gully leading to the deflecting dam creates a thick, fast flowing avalanche
stream out of the gully. The design avalanche had a return period of 1000 years with a 5 m
thick dense core flowing at 38 m s -1 as it flows out of the gully (Fr = 6). The shallow-layer
theory predicts that a stationary oblique shock with a thickness of 18 m will form at an angle
of 10° to the dam in the interaction. It was additionally assumed that the snowcover on the
ground was 3 m deep, reducing the dams´ effective height. A dam with a maximum height of
21 m was therefore predicted by the theory, compared with an 18 m high dam using the
traditional design methods.
3.3 Splitter in Fljótsdalur
The switchgear house for the Kárahnjúkar hydroelectric plant sits under the mountain Teigsbjarg
in southern Fljótsdalur. The mountain stretches up to 500 m a.s.l. The switchgear
house is positioned in a 10° slope only 60 m from the steep hill. Under normal conditions
there is not much snow accumulation in the mountainside, but occasionally large amounts of
82 The design of avalanche protection dams based on new design criteria:
Three different case studiew