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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
Physiography•Eastern Hindukush 5500-7500 m
high (Tirich Mir Peak 7706 m)
•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
Climate
6
DEBRIS FLOW HAZARD
Deaths: 50,000
Persons affected:
331,164
Homeless: 250,000
Disappeared persons:
7,200
Housing units affected:
63,935
Housing units
destroyed: 23,234
Venzuella Debris Flow- 1999
12
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
N
BUNI
Recent
Debris Flow
Chitral R.
14
Bedrock Lithologies:
PF Purit Fm (S.St; Congl,Shale)
DF Drosh Fm (Green Schist)
MZ Mélange Zone (Ultramafic blocks,
volcanic rocks, slate)
Alluvial Fan Terraces
AFT-1-4 Remnant Fans
AFT-5,6 Active Fans
LFT Lake sediment Terraces
AF6
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
Formation
• Perennial-stream
related
• Distal toe fans
• Lowest terraces
Stream-flow
dominated debris flow
Silt, sand, gravels
and rare boulders.
Contemporary
Daryano
Fan
Formation
• Ephemeral stream
• Superimposed on
older landforms/
contemporary flood
plains
• Lowest terraces
Mass movement
supported debris flow
Variable (silt,
sand, gravels and
boulders). Higher
clay content in
case of derivation
from older
landforms.
Contemporary
Urghuch
Fan
Formation
• Lower Terraces
• Variable thickness
up to ?200 m.
Mass-movement Slope debris,
redeposited
diamictons
MIS-2 and
younger
Ayun Fan
Formation
• Middle terraces.
• 70-200 m thick
Fluvial terraces. MIS-2
Broz Fan
Formation
• Upper terraces
• ~350 m thick.
• Paraglacial, formed
between the trunck
glacier and valley
slopes.
Glaciofluvial and mass
movement
Diamictons with
sharp, angular
boulders in fine
matrix.
Poorly sorted
conglomerates,
garvels and sands
55-31Ky (MIS-
3-2)
16
Alluvial Fan
Perennial Tributary
Stream
Debris Fans
Ephemeral Tributary
Stream
Talus Slopes/Cones
Rills/Gullies
ALLUVIAL FAN LANDFORMS &
ASSOCIATED STREAM TYPES
Debris Fan
Alluvial Fan
Chitral
17
Morphometric Relations
Proximal
Medial
Distal
Apex
DRAINAGE BASIN
• Catchment Area
• Feeder Channel Length
• Feeder Channel Gradient
DEPOSITIONAL BASIN
• Fan Surface Area
• Fan Gradient
• Distributary/ Incised
Channel Gradient
19
TCDF
RAF
RAF
RAF
RAFDF
DF
DF
TC
TC
TC
Direct proportional relation between
the catchment area, feeder channel
length and fan-surface area. This
implies that as catchment area
increases, the talus/debris cones
grade into debris fans, and debris
fans into alluvial fans.
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
as the cones evolve into fans and
feeding gullies, into ephemeral and
perennial channels.
Inverse proportional relationship
between the catchment area and the
surface slope on associated fans.
Morphometry
20
21
Talus
Cones
Steep Rocky
Slopes
Chitral River Valley
Case Studies
Debris Slopes/
Talus Cones
Associated with
Ephemeral Steep
Gullies
High Debris Flow
Hazards
Low Vulnerability
24
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
25
Remnant Alluvial Fans
Two types:
Remnant Alluvial Fans drained by Ephemeral Streams.
Remnant Alluvial Fans drained by Perennial Streams.
26
Remnant Alluvial-Fan Landforms
Associated with Ephemeral Tributary
Channels
Snowghar – Site of 2007 Debris Flow Disaster
27
a
b
Main
Channel
Debris Flow
Chitral R.
Mori Lusht
Remnant Fan,
Middle Chitral
Valley
Incised through-going
ephemeral channel
Major Debris-flow
event in 1986,
causing channel
avulsion.
Several dozens of
houses destroyed with
some causalities
28Buni
Perennial,
incised
through-
going
Channel
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.
Reshun Golafternoon of August 14, 1975
view after 7pm,
during the
debris flow
peak
Aftermath
Bridge
Bridge
13After Wasson, 1978
Return Period:
30 Years
30
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
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
CONCLUSIONS