Recent two runoff-triggered debris flow events in Tibet Plateau, China

In Tibet Plateau, China, debris flows have been commonly documented in the marine monsoon climate region, yet few cases were recorded in continental climate regions, such as the mountain belt of Kallash and Back-Himalayas. In July 2021, prolonged low-intensity rain in Angla Mountain, another short-duration intensive rain in Purang County, initiated runoff-triggered debris flows. In this work, we are mainly concerned with the differences in the triggering rainfall condition and local landscape dissection with respect to the freeze-thaw state. The results reveal that the low triggering rainfall of debris flows at Angla Mountain may attribute to the long freeze-thaw duration. The high triggering rainfall of debris flows in Purang County may attribute to the high saturated hydraulic conductivity and short freeze-thaw duration. Nevertheless, freeze-thaw duration, together with the low saturated hydraulic conductivity, may result in heavy landscape dissection of Angla Mountain. The Melton's ruggedness number of catchments in the two areas is greater than some suggested criteria, indicating they are susceptible to debris flows. Debris flow deposits have low proportions of grain size fractions < 0.1 mm and higher fractions of 1-10 mm in size. Field measurement of a slope debris flow found that the mean diameter decreases as the slope gradient decreases and channel geometry widens. Such changes in gradient and channel geometry may lead to a volumetric concentration decrease, and the deposits become finer. This work preliminarily contributes to know about the effect of a freeze-thaw state on the triggering rainfall of debris flows and local landscape dissection in the continental climate region, Tibet Plateau, China.