Sediment connectivity and fan formation processes in glacial and periglacial regions: the Ala Archa Valley in Tian Shan (Kyrgyzstan)

Periglacial areas are shaped by very active geomorphic processes resulting in a highly dynamic landscape. In this context, the hillslope-channel coupling plays a fundamental role in controlling catchment sediment dynamics. The sediment produced on hillslopes may reach the channel network and downstream propagation may have important effects on the environment, and create potential hazards. This is the case of the Ala Archa river basin (mean elevation 3160 m a.s.l.), located in the Tian Shan Mountains of Kyrgyzstan. Glaciers cover the upper part of the basin and, along the main valley, numerous fans are formed as a result of glacial lake outburst floods, debris flows, and colluvial processes. The Ala Archa river flows through the Kyrgyz capital city of Bishkek in the lower part, and, in the recent past, floods and debris flows triggered in the upper part of the catchment caused important damage to infrastructures and people. Therefore, the aim of this work is to improve the understanding of the hillslope-channel coupling in the Ala Archa basin, to support hazard and risk assessment, and to shed light into sediment connectivity in high mountain glacial and periglacial regions. A field survey was carried out in summer 2015 to analyse sediment sources and deposition sites (rock slopes, debris flow tracks, fans etc.), in addition, remote sensing and connectivity analyses have also been applied to map and characterize the main morphological characteristics of the basin. Connectivity results permitted to enlighten the role of the fans in coupling/decoupling the hillslpoes to main valley. Furthermore, a focus on periglacial areas allowed for a characterization of connectivity patterns and magnitude in such dynamic environments that will feature the most important changes in sediment dynamics and availability in the near future. This work has been supported by the Swiss National Science Foundation, through the joint SCOPES research project "Debris flow and outburst flood hazard in Tian Shan under impact of changing climate" (DEFENCC; grant no. IZ73Z0_152301).