Assessing the effects of consecutive sediment-control dams using a numerical hydraulic experiment to model river-bed variation

In the Kotaki River Valley, Japan, the Fossa Magna has produced a large-scale collapse and landslide zone. Debris flows occur annually in the Kotaki River, which has headstreams in the Hakuba range (altitude: 2267 m) located in Niigata Prefecture. Sediment-related disasters occur over extensive areas from headwater to downstream cities and in a variety of forms. This will produce large quantities of upstream sand in the future, which may cause flooding through the deposition of sediment near downstream junctures. In such a river, it is necessary to control sediment runoff from the upper stream and prevent extreme changes in the height of the downstream riverbed. To protect people and properties from sediment-related disasters, consecutive sediment-control dams are erected. These dams are built into the upstream areas of mountain streams to accumulate sediment, suppressing its production and flow. On the Kotaki River, in areas of the river where the MTL is exposed, the potentially large quantity of sediment discharge during floods was reduced by installing three sets of slit barriers to prevent landslides while allowing the passage of fish. This dam system was modelled after extant slit barriers of this nature. Previous investigations of temporal changes in sediment-regulating function and sediment outflow in river sections with continuous slit barriers have incorporated many unknown factors. We performed a numerical simulation of a section of the Kotaki River containing slit barriers to prevent landslides and examined the characteristics affecting sediment-regulating functions.