Canton to reassess situation after rockfall at Villenquartier

After Tuesday's rockfall, the rock bands above Oberwil must be re-analysed by geologists. It may even be necessary to draw up a new maintenance concept for the protective forest.

Anyone who sees the pictures of last Tuesday evening's rockfall in Oberwil (we reported last week) can understand why the local forest needs to be well looked after - without the trees, the boulders and rocks would probably have reached the houses. But even despite the forest: the largest boulder, measuring almost five cubic metres and weighing twelve tons, rolled right up to the Weidlistrasse, just above the houses in the Räbmatt villa district.

Martin Ziegler, head of the Office of Forests and Game (Amt für Wald und Wild) confirms: "We were a bit surprised by the size of the boulder”. A geologist will now have to clarify whether the so-called ‘blockfall’ with boulders of this dimension was a one-off event, or whether it could happen again.

Do we need larger, more powerful trees?
If the canton's geological analyses show that rockfalls with large boulders are still likely in Oberwil, the forest maintenance concept will have to be rethought. "In order to stop large boulders, large and strong trees are needed. This results, however, in is less space for trees - and small stones can slip between the trunks more easily," says Martin Ziegler, explaining the situation. Until now, this protective forest was designed for smaller stone dimensions, and more trees with smaller diameters were planted. This increases the probability that a stone will hit a trunk when rolling downhill, and thereby be slowed down.

This boulder thundered onto the Weidlistrasse
View from above of the new gap in the forest         Photos: PD

Before the "tree question" is clarified, the focus is currently on the current dangerous situation. The initial geological assessment has shown that there is another rock package of a similar size next to the site from which the rocks fell, but also that it is not in immediate danger of falling. According to Martin Ziegler, this rock package will be removed for safety reasons during the road closure. This measure will probably be carried out next week, weather permitting.

30 years for reforestation
Due to the rockfall, there is now a gap in the forest, and trees should therefore be planted here again as soon as possible. If necessary, this reforestation will be supported by planting, but it will take time before the same level of stability is recreated in the protection forest at this point. Marti Ziegler adds:

"It could easily take 30 years, or even more, before the new trees fulfil the protective function again."

In the meantime, the affected area is to be secured with technical measures. The Zug Corporation has currently erected a metal palisade on the Weidlistrasse road to protect the buildings below during the rock-clearing work. In addition, a permanent protective wall of stones and earth is being erected above the road to stop stones from rolling down. Depending on the risk assessment, rockfall nets could also be used.

The area above Oberwil is criss-crossed by rock bands from which stones can come loose again and again. A great deal of effort is thereby made to maintain the protective forest in the transit area. As the forest owner, the Zug Corporation carries out regular felling operations to maintain this important forest. Unstable trees are removed and stable trees are favoured. In the past, felled trees have been removed from the stand by rope crane or helicopter in order to guarantee optimal protection - with success, as was shown on Tuesday.