Landform Control on Debris Flow Hazards Hindukush Himalayas Chitral District, N. Pakistan M. Asif Khan & M. Haneef National Centre of Excellence in Geology University of Peshawar Pakistan Objectives: • Identify Debris Flow Hazards on Alluvial Fan Landforms Approaches: • Satellite Images and Field Observations • Morphometric Analysis of Drainage vs Depositional Basins Outcomes: • Develop Understanding of Debris Flow Processes for General Awareness & Mitigation through Engineering Solutions 2 Chitral District, Hindukush Range • Physiography • Habitats • Natural Hazards Quaternary Landforms Mass Movement Landforms • Types • Controlling Tributary Streams • Morphometery Landform Control on Debris Flow Hazards Kohistan N Tibet Himalayas Chitral R. Pamir Knot Hindu Kush 4 Pamirs CHITRAL 5 Physiography •Eastern Hindukush 5500-7500 m high (Tirich Mir Peak 7706 m) Climate •Hindu Raj 5000-7000 m high •34% are above 4500 m asl, with 10% under permanent snow cover •Minimum altitude 1070 m at Arandu. •Relief ranges from 3200 to 6000 m at the eastern face of the Tirich Mir. Climate •high-altitude continental, classified as arid to semi-arid 6 CHITRAL 7 Hindu Raj Chitral Valley Tirich Mir (5706 m) SE NW 8 Upper Chitral Valley, N. Pakistan N 9 10 11 DEBRIS FLOW HAZARD Venzuella Debris Flow- 1999 Deaths: 50,000 Persons affected: 331,164 Homeless: 250,000 Disappeared persons: 7,200 Housing units affected: 63,935 Housing units destroyed: 23,234 12 13 Debris Flow Hazards Settings in Chitral Habitation restricted to River-Bank Terraces Terraced Landforms Flood Plain Recent Alluvial/Debris Fans Remnants of Glacial Moraines Remnant Inter-glacial and Post- Glacial Alluvial fans Chitral R. Recent Debris Flow BUNI 14 N Bedrock Lithologies: PF Purit Fm (S.St; Congl,Shale) DF Drosh Fm (Green Schist) MZ Mélange Zone (Ultramafic blocks, AF6 volcanic rocks, slate) Alluvial Fan Terraces AFT-1-4 Remnant Fans AFT-5,6 Active Fans LFT Lake sediment Terraces Multi-Stage Landform Terraces, Drosh, Chitral, Pakistan 15 Classification of Landforms, Chitral, N. Pakistan (Modified after Kamp et al, 20040) Formation Description Process Sediment Age Shishi Fan • Perennial-stream Stream-flow Silt, sand, gravels Contemporary Formation related dominated debris flow and rare boulders. • Distal toe fans • Lowest terraces Daryano • Ephemeral stream Mass movement Variable (silt, Contemporary Fan • Superimposed on supported debris flow sand, gravels and Formation older landforms/ boulders). Higher contemporary flood clay content in plains case of derivation • Lowest terraces from older landforms. Urghuch • Lower Terraces Mass-movement Slope debris, MIS-2 and Fan • Variable thickness redeposited younger Formation up to ?200 m. diamictons Ayun Fan • Middle terraces. Fluvial terraces. MIS-2 Formation • 70-200 m thick Broz Fan • Upper terraces Glaciofluvial and mass Diamictons with 55-31Ky (MIS- Formation • ~350 m thick. movement sharp, angular 3-2) • Paraglacial, formed boulders in fine between the trunck matrix. glacier and valley Poorly sorted slopes. conglomerates, 16 garvels and sands ALLUVIAL FAN LANDFORMS & ASSOCIATED STREAM TYPES Talus Slopes/Cones Rills/Gullies Debris Fans Ephemeral Tributary Stream Debris Fan Chitral Alluvial Fan Perennial Tributary Stream 17 Alluvial Fan Alluvial Fan Landforms, Chitral Valley, N. Pakistan 18 Morphometric Relations DRAINAGE BASIN • Catchment Area • Feeder Channel Length • Feeder Channel Gradient Apex Proximal DEPOSITIONAL BASIN • Fan Surface Area Medial • Fan Gradient Distal • Distributary/ Incised Channel Gradient 19 Morphometry Direct proportional relation between TC the catchment area, feeder channel length and fan-surface area. This implies that as catchment area increases, the talus/debris cones RAF DF grade into debris fans, and debris DF RAF fans into alluvial fans. TC The fan-surface slope, which has almost 1:1 ratio with the gradient of feeding gullies in the case of the talus cones, decreases proportionally TC as the cones evolve into fans and RAF DF DF feeding gullies, into ephemeral and TC perennial channels. RAF Inverse proportional relationship between the catchment area and the surface slope on associated fans. 20 Case Studies Steep Rocky Slopes Debris Slopes/ Talus Cones .Associated with Talus Cones Ephemeral Steep Gullies .High Debris Flow Hazards Chitral River Valley .Low Vulnerability 21 Debris Fans Active Abandoned Distributary Distributaries Debris Fan Chitral R. 22 23 Debris Flow Hazards on Debris Fans: Contributing Factors Typically associated with Ephemeral Tributary Streams: High sediment/water Ratio Poor Fan-Surface Drainage: mostly radiating distributaries Levee-Bounded distributaries mostly perched high on fan surface. Common channel avulsions, threatening settlements/crops on fan flanks High Hazard, High Vulnerability Return period 3-5 years 24 Remnant Alluvial Fans Two types: Remnant Alluvial Fans drained by Ephemeral Streams. Remnant Alluvial Fans drained by Perennial Streams. 25 Remnant Alluvial-Fan Landforms Associated with Ephemeral Tributary Channels Snowghar – Site of 2007 Debris Flow Disaster 26 a Mori Lusht Remnant Fan, Debris Flow Middle Chitral Main Valley Channel Incised through-going ephemeral channel Major Debris-flow Chitral R. event in 1986, causing channel avulsion. Several dozens of b houses destroyed with some causalities 27 Remnant Alluvial-Fan Landforms Associated with Perennial Tributary Channels Large catchment with enormous debris material Efficient transport of sediment through water-saturated streams Equally efficient discharge through fan surface by through- going entrenched channel Direct flushing of debris into trunk river rather than on fan surface. Perennial, incised through- going Channel Buni 28 Reshun Gol afternoon of August 14, 1975 Bridge Aftermath Bridge view after Return7pm, Period: during the30 Years debris flow peak After Wasson, 1978 13 Active Alluvial Fans Associated with Perennial Streams Typically forming at stream mouths at junction with trunk rivers Subject to common toe trimming by river action High debris flow hazard but low vulnerability Shishi Gol Remnant Fan Terraces Active Tributary Alluvial Fan Chitral River 30 CONCLUSIONS About 2 million people in Chitral live on river-bank terraces exposed to debris flow hazards Debris-flow hazards are most common on relatively immature landforms associated with ephemeral streams i.e., talus cones and debris fans Alluvial fans associated with high-energy Perennial Streams, less prone to debris flow hazards due to efficient discharge of debris load direct into the trunk river rather than on the fan surface. Debris flow hazards have a return period of 3-5 years on alluvial fans related with Ephemeral Tributary streams, and >30 years is characteristic of perennial-stream related fans. Ephemeral Fans need elaborate Engineering Solutions to trap debris in the catchments or entrain the flows through specified channels on fan surface. Perennial Fans simply need protection from encroachments. 31.