2023 Volume 76 Issue 4 Pages 25-32
A deep-seated collapse occurred in an isolated hill of Nishime, Tsuruoka City, Yamagata Prefecture, at around 1 a.m. on 31 December 2022. The height of the collapsed slope was approximately 25 m, and the volume of sediment was approximately 20,000 m3. More than a dozen houses were destroyed, and two people lost their lives as a result of the collapse. The question is why the deep collapse occurred in early winter when there was no heavy rain. One possible trigger is that total precipitation throughout December 2022 was 1.8 to 1.9 times higher than usual. Two models were used to answer this question. First, since the number of elapsed days until collapse depends on the scale of the collapse, the following equation Te = 0.0142 H1.99, or roughly Te ∝H2, was obtained from the known data on the height of the failed slope H and the number of elapsed days Te. Based on this equation, the author concluded that the number of days to collapse was 8.6 days for the slope height of 25 m in this case. This result, together with the winter rainfall situation, suggests 24 December as the date of crack initiation. Second, the effective rainfall model predicted the limit of water storage in the slope to be 303 mm on the day of the crack initiation, and the coefficient (a) of this model was indicated as a = 0.96. The hydrological model can be applied to heavy summer precipitation as well as winter precipitation by setting the maximum water inflow into the landmass as R0.That is, R0 was 30 mm/d before the 2019 earthquake and increased to 60 mm/d after the earthquake. The water storage reached a value close to the limit Dc on 24 December. After the subsequent process, it can be inferred that a deep-seated collapse occurred on 30-31 December.
