Morphometry of alluvial fans in an alpine valley basin in southeast Tibet: Spatial heterogeneity, controls, and implications

Abstract

The morphometry of alluvial fans is a fundamental yet significant aspect of understanding fan dynamics and controls, offering potential benefits for fan type discrimination and mitigation of geo-hazards such as flash floods and debris flows. However, alluvial fans in alpine regions have received considerably less attention than their counterparts in non-alpine areas, despite their importance. Using Google Earth Pro, remote sensing images, targeted field surveys, and statistical analysis, we examine the morphometry, spatial heterogeneity along with underlying causes, and primary types (fluvial, debris-flow, and colluvial) of alluvial fans in the Parlung Tsangpo Basin (PTB), a large alpine tributary basin of the Yarlung Tsangpo River (upper Brahmaputra River) characterized by deep valleys in southeastern Tibet, China, based on a dataset of 129 fan samples. Our results reveal that: (1) Significant spatial heterogeneity exists in the morphometry of alluvial fans in the PTB, particularly regarding fan area. (2) Fans exhibiting distinct morphometries correspond to different primary processes (debris flow, fluvial, or colluvial dominated), with varying links to their source basins. (3) Discriminant analysis based on three variables (fan slope, source basin area, and Melton’s R) can effectively distinguish between fan types and their primary processes. The development of alluvial fans in the PTB is collectively determined by various allogenic and autogenic factors, with primary processes and tributary-junction effects (distal confinement) acting as the main controls. The primary processes, influenced by regional relief (slope), climate (precipitation and temperature), and sediment supply conditions (volume and caliber), play an essential role in fan development, particularly regarding fan slope. In light of global climate change, new fans have been developing in the basin, while some previously stable fans have entered a new active phase. Consequently, debris flows have become a dominant primary process for alluvial fan development in PTB, driving the evolution of fan morphology.