Debris-flow hazards on tributary junction fans,Chitral, Hindu Kush Range,northern Pakistan |
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| Institution: | 1. Department of Geosciences, University of Tübingen, Rümelinstraße 19-23, 72070 Tübingen, Germany;2. CEMSE Division and PSE Division, King Abdullah University of Science and Technology, Thuwal, Jeddah, Saudi Arabia;3. Dipartimento di Scienze della Terra e del Mare, Università degli Studi di Palermo, Via Archirafi 22, 90123 Palermo, Italy;1. Department of Civil Engineering, University of Seoul, 9- Cheonnong-dong, Tongdaemun-gu, Seoul 130743, Republic of Korea;3. University of Portsmouth, Portsmouth, United Kingdom;1. Hydro geochemistry Laboratory, Department of Environmental Sciences, Faculty of Biological Sciences, Quaid-i-Azam University, Islamabad, PO 45320, Pakistan;2. National Centre of Excellence in Geology, University of Peshawar, 25130, Pakistan;3. Department of Botany, Hazara University, 21300, Pakistan;1. Center for Quaternary Research, Department of Geography, Royal Holloway University of London, Egham, Surrey TW20 0EX, United Kingdom;2. Key Laboratory of Western China''s Environmental Systems (Ministry of Education), College of Earth and Environmental Sciences, Lanzhou University, Lanzhou, 73000, China;3. Department of Earth Sciences, Villavägan 16, Uppsala University, 75236 Uppsala, Sweden |
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| Abstract: | The Chitral district of northern Pakistan lies in the eastern Hindu Kush Range. The population in this high-relief mountainous terrain is restricted to tributary-junction fans in the Chitral valley. Proximity to steep valley slopes renders these fans prone to hydrogeomorphic hazards, including landslides, floods and debris flows.This paper focuses on debris-flow hazards on tributary-junction fans in Chitral. Using field observations, satellite-image analyses and a preliminary morphometry, the tributary-junction fans in the Chitral valley are classified into (1) discrete and (2) composite. The discrete fans are modern-day active landforms and include debris cones associated with ephemeral gullies, debris fans associated with ephemeral channels and alluvial fans formed by perennial streams. The composite fans are a collage of sediment deposits of widely different ages and formed by diverse alluvial-fan forming processes. These include fans formed predominantly during MIS-2/Holocene interglacial stages superimposed by modern-day alluvial and debris fans. Composite fans are turned into relict fans when entrenched by modern-day perennial streams. These deeply incised channels discharge their sediment load directly into the trunk river without significant spread on fan surface. In comparison, when associated with ephemeral streams, active debris fans develop directly at composite-fan surfaces. Major settlements in Chitral are located on composite fans, as they provide large tracts of leveled land with easy accesses to water from the tributary streams. These fan surfaces are relatively more stable, especially when they are entrenched by perennial streams (e.g., Chitral, Ayun, and Reshun). When associated with ephemeral streams (e.g., Snowghar) or a combination of ephemeral and perennial streams (e.g., Drosh), these fans are subject to frequent debris-flow hazards.Fans associated with ephemeral streams are prone to high-frequency (～10 years return period) debris-flow hazards. By comparison, fans associated with perennial streams are impacted by debris-flow hazards during exceptionally large events with return periods of ～30 years. This study has utility for quick debris-flow hazard assessment in high-relief mountainous regions, especially in arid- to semi-arid south-central Asia where hazard zonation maps are generally lacking. |
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