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

Tibet

Pamir KnotHindu Kush

Kohistan

Himalayas

Chitral R.

N

4

PamirsCHITRAL

5

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

CHITRAL

7

Tirich Mir (5706 m)Chitral ValleyHindu Raj

NW

SE

8

Upper Chitral Valley, N. Pakistan

N

9

10

11

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

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

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

Alluvial Fan Landforms, Chitral Valley, N. Pakistan

18

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

22

Debris Fans

Chitral R.

Debris Fan

AbandonedDistributaries

Active Distributary

23

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