A Case Study of Debris Flow Mitigation of No. 3 Valley at Shitou

摘要

The geological structures of Taiwan are complex, weak and young. The rock slopes of many river valleys were damaged by earthquakes, which caused a great deal of cracks and unstable rock blocks. The torrential rain brought by typhoons usually induced serious debris flow. The collapse of slopes, erosion of rivers and expansion of watercour ses which were caused by debris flow would induce casualties of mankind and properties. It turns out a most important mission to deal with debris flow by engineering facilities properly. A case study of No. 3 valley at Shitou, which includes the debris flow investigation, analysis, plan and detailed design that serve as an example to other similar debris flow treatment projects. The length of No. 3 valley is 2419m and the watershed has an area of 85. 1 ha, including the main stream and north branch. The total rainfall of Typhoon Toraji in 24hrs was 615mm which destroyed 11 sabo dams and damaged down stream facilities. Based on observation from geological survey and borings, the valleys in these areas were filled with broken shale and colluviums. The topography at the source of the valley is steep, with a slope angle of more than 35 degrees. It is easy to trigger debris flow owing to the contribution of heavy torrential rain and steep slopes. Aiming to the mitigation of debris flow disaster, ecological engineering methods such as vegetation work, flow division and drainage work are used at the source of debris flow valley. At the main stream of No. 3 valley, it is planned to set up 2 sabo dams (5m high), 2 comb-type dams, a large-sized grit chamber and 800m compound section stream way with 10m width. At the branch, it is designed to set up 5 sabo dams (3m high), a medium sized grit chamber and 287m stone masonry. A planned automatic pre-warning system which includes 2 rain gauges, 2 wire sensors, a laser sensor, a ground phone sensor, a CCD camera and control center, is used to provide emergency reaction to ensure the safety of local residence.