Debris-flow fan development and geomorphic effects in alpine canyons under a changing climate

Debris flows in alpine environments are prone to occur in the context of global climate change (i.e., elevated air temperature and higher frequency of strong precipitation events). (Alluvial) Fans often develop at the outlet of tributaries after high-intensity debris flows. Most debris-flow fans in alpine canyon areas extend directly to the main river channel and become the forefront of the interaction between the tributary gully and the main river channel. Clarifying the development processes/dynamics, evolutionary mechanisms and driving factors of alluvial fans would shed light on understanding the geomorphological effects and genesis of river valleys in alpine canyon areas. Here, we report the development of debris-flow fan at the outlet of the Tianmo Gully, a formerly hazard-free but currently hazard-active tributary of the Parlung Tsangpo Basin, Southeast Tibet, where debris flows have occurred frequently in the last two decades. Combining remote-sensing images, DEM data, UAV aerial photography, RTK topographic survey and other fieldwork, the development processes and morphological characteristics of the Tianmo fan under the influence of four large debris flows were analysed. Both primary events (described as episodic debris-flow events characterized by high-magnitude mass movement) and secondary events (corresponding to perennial stream flow processes with much lower sediment concentrations) affected the development of the Tianmo fan. Episodic debris-flow events drastically shape the macroscopic morphology of the fan, with rapid deposition and expansion of the fan body, whereas perennial stream flow processes slowly modulate the fan during the intermittent period between debris flows, mainly with gradual retrogressive incision and lateral migration of flow channel on the fan body. Influenced by the strong sediment-transport process of debris flows and the alluvial fan development, the planform of the Parlung Tsangpo River evolved from a relatively narrow and single-thread pattern to an alternating-wide-and-narrow pattern, with a corresponding staircase-like longitudinal profile. We investigated alluvial fan development and geomorphic effects of Tianmo Gully in Southeast Tibet, an alpine canyon area where debris flows have been active in recent decades under a changing climate. Debris flow is the key driver for the fan development, whereas runoff reworking is non-negligible, and strong interactions exist between the fan and the main river. Fan development exerts big impacts on the morphology evolution of the main river, forming an alternating-wide-and-narrow planform pattern with a staircase-like longitudinal profile.image