Material source characteristics and formation mechanism of debris flows in soft rock-semi-lithologic areas： A case study of Dajing River debris flow in Nanjian County， Yunnan Province

Abstract:

The area of Nanjian County， Yunnan Province， is a typical area of soft rock and semi-lithologic， and debris flow disasters are developed behind the county town. Based on field investigations and combined with multi-source and multi-period remote sensing interpretation and topographic mapping， the material source characteristics and activity characteristics of debris flows in the soft rock-semi-lithologic area were summarized， and the debris flow formation mechanism were analyzed. The results show that there are sources of debris in the Dajing River Basin： slope surface erosion source， collapse and landslide source， and channel accumulation source. Among them， the erosion source on the slope surface is in a wide range and has become the most important source type in this river basin. The mudstone in the northern gully develops over a wider range， while the semi-rock in the southern gully is more developed. The different resistance to weathering and erosion result in more developed material sources in the northern gully. Due to the lithological combination and strength differences of soft rocks and semi-rocks in the north and south gullies， there is a significant spatial differentiation of material sources. The middle reaches of the main gully have become the most effective material source recharge area， characterized by many points and wide distribution. In the historical record， seven debris flow disasters have occurred in this gully， and short-duration concentrated rainfall is the main factor triggering debris flow activities. The Dajing River debris flow is a typical rainstorm gully-type hydraulic debris flow disaster， and the possibility of debris flow occurrence still exists under extreme rainfall conditions. The gully mouth is the core area of the county town， so the hazard and risk of debris flow remain prominent.