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( Scott Burns ) 대한지질공학회 2019 대한지질공학회 학술발표회논문집 Vol.2019 No.2
As the climate warms, the world is experiencing more forest fires. Following forest fires, debris flows occur very commonly when the first major rains hit an area if the vegetation is normally dry. In rain forests, there is a delay in the timing of debris flows. In the late summer of 1991 there was an extensive forest fire in the Columbia Gorge, USA, on the Oregon side of the river that was started naturally by lightning. We learned from this fire that this steep terrain underwent three basic erosion/landslide processes in the next ten years as a result of the fire. After the fire was out in the autumn, the first rains brought abundant surface erosion of burnt soil and vegetation. A lot of this ended in the streams. Second, extensive enhanced rock fall occurred in the burned area. One classic area was next to Multnomah Falls where a Brugg cable fence had to be installed to protect the trail leading to Benson Bridge from rock fall onto hikers. Third, we learned that in a period of 5-10 years after the forest fire, areas of intensive burning of the forest would produce very large debris flows. It takes 5-10 years for the roots of the trees burned to disintegrate. Seven large debris flows in 1996 at Dodson and one large one near there in 2001 are examples of this delayed debris flow generation when a “Pineapple Express” would come into the area. This is a rain forest getting over two meters of precipitation per year. This differs from dry climate forest fires where debris flows are generated with the first major storm after the fire. After the 2017 Gorge fire which also occurred on the Oregon side, but started by two teenagers, we noted the same things. First, there was extensive surface erosion for a week after the first rainfall. Also, all of the roads and trails were closed until checked for rock fall hazards. We now expect debris flows in the next 5-10 years to come down the following drainages that had extreme burning in the headwaters: Tanner Creek, Eagle Creek, Oneota Creek, and Horsetail Creek.