Countermeasures against a large landslide dam with a high head and a long frontal slope : Case of countermeasures for Kuridaira landslide dam

Wataru SAKURAI, Ryo SAKAI, Takahisa MIZUYAMA, Akihiko IKEDA, Nobuo SUGIURA, Yoshinori ARAKI,
Takayuki HAMADA, Yoshiaki KASHIWABARA and Kosuke YOSHINO

A large landslide dam at Kuridaira, in Totsukawa Village, Nara Prefecture, which resulted from a deep]seated landslide following torrential rain brought on by Typhoon No. 12, had a 100]meter head with a 600]meter long downstream slope, creating a 1 in 5 slope that made it highly vulnerable to erosion by water overflowing the landslide dam reservoir. An issue was what degree of inclination was suitable for the slope to prevent severe erosion. Therefore, we modeled a one]dimensional riverbed fluctuation calculation, which indicated that a 1 in 12 inclination would be suitable downstream of the landslide dam to avoid severe erosion. In order to realize such an inclination, we intended to reduce the overflow elevation of the landslide dam, install a sabo dam at the foot of the landslide dam, and reclaim the space above the sabo dam to install a series of groundsels on the slope. The effectiveness of this plan was verified by means of hydraulic experiments. This clarified that the reclaimed portion would be severely eroded, so a channel with protective sides and bottom on the reclaimed portion was added to the planned countermeasures.