Failure‐collapse process and hydrological triggers of a deep‐seated collapse of Nishime, Tsuruoka City, Yamagata Prefecture



A deep‐seated collapse occurred in an isolated hill of Nishime, Tsuruoka City, Yamagata Prefecture, at around 1a.m. on 31 December 2022. The height of the collapsed slope was approximately 25m, and the volume of sediment was approximately 20,000m3. 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 = 0.0142H1.99, 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 was8.6days for the slope height of25m in this case. This result, together with the winter rainfall situation, suggests24December 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 30mm/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.

Key words

winter precipitation, crack initiation, time lapse to collapse, effective rainfall model, collapse scale