Skeiðarársandur. 아이슬란드(Iceland)

Skeiðarársandur. 아이슬란드(Iceland)

The New Gígjukvísl Bridge

Information

In November 1996 the road crossing the Skeiðarársandur sandflats was washed away in a huge glacial flood following an eruption north of Grímsvötn in the Vatnajökull glacier. The foodwater rose rapidly, reaching its highest level in 15 hours and subsiding over the following two days. A maximum flow rate of about 20,000 ㎥/s had been expected; in fact it reached nearly 50,000 ㎥/s. Huge blocks of ice were swept along by the flood, causing great damage to the road: the 376-m Gígjukvísl Bridge was completely washed away. The Skeiðará Bridge was also badly damaged: the easternmost unit, 176 m long, and the central pillar of the westernmost unit, were swept away. The road was washed away to the west of the bridge, and the guide bank was mostly destroyed, and the 12-m land bridge to the west disappeared in the flood. The bridge across the river Súla was not damaged. It is estimated that the icebergs that reached the bridge at Gígjukvísl weighed up to 1,000-2,000 tons (measuring 10 x 10 x 20 m), and those that reached the Skeiðará Bridge weighed up to 100-200 tons.

The Gígjukvísl Bridge before the flood

The collapse of the Gígjukvísl Bridge, 5. 11. 1996

The course of Gígjukvísl on 8. 11. 1996 after the flood

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Skeiðarársandur

Damage

What is to be expected

The Icelandic road system

What happened on Skeiðarársandur showed how sensitive the Icelandic road system is in many places to natural hazards. Over long stretches, the No. 1 "Ring" Road is the only means of land transport. If the road is destroyed or blocked at any one point, it can result in everyone having to travel "the othe way", destruction or blockage at two points can result in the complete interruption of road transport for a whole area. The volcanic centre beneath the Vatnajökull glacier, which fed Grimsvötn with the 1996 floodwater, is in a very critical location in terms of where the floodwater will emerge. An upsurge of volcanism only a few kilometres in a different direction could result in glacier floods in completely different locations from Skeiðarársandur: the rivers Skaftá and Hverfisfljót could also be affected. A shift in volcanic activity towards the west could affect the rivers Tungnaá and Köldukvísl - these are rather further away - or Jökulsá á Fjöllum, which is rather closer if the volcanic activity shifts to the north. Ther river Skjálfandafljót also rises in this area, and could also be swollen by floodwater, though the other rivers are more likely to be affected. An eruption under ther glacier in this region could therefore have very serious effects for the road system in more than one location. Another glacier-capped volcano, Katla, is known to have caused spectacular floods, far larger than one on Skeiðarársandur.

The Mýrdalssandur sandflats and the neighbouring districts would then be at risk. Floods there have tended to last only a short time, but they would probably cause great damage. It is therefore clear that Iceland has not seen the last natural hazard in this region.

Bridge foundations

The bridge over the river Súla, Gígjukvísl and Skeiðará were all of the same type, steel girders with timber deck on concrete pillars. Each pillar stands on twelve 11-metres-long cast piles, with a section measuring 300 x 300 mm,d riven into the sand. The bridge consist of 176-m units, each of which consists of four 44-m spans. They are designed in such a way that even if individual units collapse, the remaining units will continue to stand unless they have also been damaged.

The height of the lower edge of the girders of the Súla, Gígjukvísl and Skeiðará bridges above the water was about 6 m, the intention being that this would ensure clearance of the icebergs washed down in floods.

The flood in the river Gígjukvísl swept the bridge away and washed away

a large section of the road

The view towards the mountain Lómagnúpur at the height of the flood. The Gígjukvísl bridge is gone and the guide banks are covered by the floodwater. Note the icebergs scattered across the road in the foreground.

The view towards the mountain Lómagnúpur after the flood. In many places there was not much left of the main road across the sands after the flood receded. Of the 32.8 km long road from Núpsstaður to Skaftafell 6.4 km were destroyed, 5.3 km were seriously damaged, little damage occured to 0.7 km and 20.4 km remained undamaged.

Many large blocks of ice were carried across the sandflats by the flood, some even out to sea.

Those that were carried as far as the site of the bridge across the Gígjukvísl are estimated to have weighed as much as 1,000 - 2,000 tons, while the ones that reached the bridge over the river Skeiðará were up to 100 - 200 tons. This caused immense destruction; when the floodwaters receded it was clear that road repairs would be very expensive. Steel girders from the Skeiðará bridge were left half buried in sand far downstream from the bridge after the flood.

The damage caused was very evident when the flood was over and it was clear that there was costly rebuilding ahead. The picture shows a steel beam from the bridge over Skeiðará half buried in mud far down the river.

Reconstruction

Work on rebuilding the section of teh Skeiðará Bridge that was damaged in the 1996 flood ended in July 1997. As before, it is designed to withstand a maximum flow rate of 9,000 ㎥/sec. It is expected that the embankment system would be broken by a greater flow rate. The new bridge over the Gígjukvísl was completed in summer 1998. It is designed to withstand conditons under a major flood in Gígjukvísl (with a flow of 30,000 ㎥/sec.). with 25 ㎥/sec. flowing under the bridge for each metre of the bridge's length. All the structures on Gígjukvísl (the bridge, the road and the guide banks) are designed to withstand flow rates of 3,000㎥/sec. without sustaining damage. The new bridge is 336 m long.

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