The occurrence of mudslides in China is mainly caused by long-continued
rainfall and heavy rainfall, especially the heavy and concentrated rainfall.
Therefore, the time law of debris flow is in keeping with the regularity of
concentrated rainfall, which is obvious seasonal. It usually happens
in rainy summer and autumn.
The time is different between concentrated rainfalls. Most of the rainfall
in southwest China, such as Sichuan and Yunnan, is concentrated
from June to September. So the debris flow in the southwest region always occurs
in the three months. In the northwest, rainfall is concentrated
in June, July and August.
The rain is concentrated and heavy, so the debris flow of westnorth region
mostly occurs in July, August.
According to incomplete statistics, the debris flows of the two months
account for more than 90% of all debris flow in the region.
The occurrence of debris flow is affected by rainstorm,
flood and earthquake, and storm, flood, earthquake always appears
periodically. Therefore, the occurrence of debris flow and development
are also periodicially, and the activity cycle is generally consistent
with the cycle of rainstorm, flood and earthquake.
When the cycles of heavy rains and floods overlay each other,
the climax of debris flow will occur. For example, the strong earthquake
period in Dongchuan region of Yunnan province in 1966 made the
more rapid development of the debris flow.
Only on dongchuan railway, there were more than 250 mudslides
in 11 years from 1970 to 1981. Also, in 1981, the mudslide in Dongchuan Dade,
the mudslide in Chengkun railway, Baocheng railway and Baotian railway
all occurred in the case of periodical and heavy
The occurrence of debris flow is usually in the peak of a rainfall
or after the continuous rain.