E3S Web of Conferences 415, 05008 (2023)
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The Emerging Widespread Debris Flow Disasters in Tropical
Terrain of Peninsular Malaysia: Understanding the Risk andPolicy Intervention
Abd Rasid Jaapar1*, Mohamad Faruq Syahmi Md Aripin2, Ibrahim Komoo3, Che Aziz Ali4, Zamri Ramli5, Abd Rahim
Harun5, James Bachat2, Zakaria Mohamad2, Rohayu Che Omar6, Khamarrul Azahari Razak7 and Choun Sian Lim8
1

Institute of Geology Malaysia, 43600 UKM, Bandar Baru Bangi, Malaysia

2Geomapping Technology Sdn Bhd, 43000 Kajang, Malaysia
3Academy of Professors Malaysia, 62000 Putrajaya, Malaysia
4Geopark & Geotourism Creative Solutions Sdn Bhd, 43000 Kajang, Malaysia
5Department of Mineral and Geoscience Malaysia, 62000 Putrajaya, Malaysia
6Universiti Tenaga Nasional, Institute of Energy Infrastructure (IEI), 43000 Kajang, Malaysia
7Universiti Teknologi Malaysia, Disaster Preparedness and Prevention Centre (DPPC), Malaysia-Japan International Institute ofTechnology, 54000 Kuala Lumpur, Malaysia
8Universiti Kebangsaan Malaysia, Institute for Environment and Development (LESTARI), 43600 Bandar Baru Bangi, Malaysia

Abstract. Several widespread debris flow disasters stroke Peninsular Malaysia in between 2021 and 2022,
scattered across the country. It was the first time for this hazard being nationally recognised as a type of
geological disaster in the country. Debris flow disaster is not a new phenomenon. It had been recorded since
1995, however, in the past they were isolated incidents and did not attract much attention. The recent
widespread debris flow disasters started in Gunung Jerai, Kedah on 18 August 2021; followed by similar
incidents but in 3 different states concurrently in (1) Bentong, Pahang, (2) Hulu Langat, Selangor, and (3)Jelebu, Negeri Sembilan on 18 December 2021; Kenyir, Terengganu on 27 February 2022; and finally at
Gunung Inas, Kedah on 4 July 2022. These disasters sparked numerous technical and social issues where
heated debates on whether they were purely due to natural processes or caused anthropogenic activities.
Previously, debris flow phenomena in Malaysia were so rare, that the term ‘debris flow’ was not familiar tothe lay public. The term of “debris flow” was often disregard or used interchangeable with ‘water surge’
phenomena by the media and public, especially for sudden sediment water surge from hilly upstream after
intense rainfall at hillside river catchment areas, normally occurs during the monsoon season. The ordinarywater surge has significantly lesser level of destruction and momentum as compared to a debris flow withhigher debris-type sediments. This paper discusses the emerging issues of debris flow in Malaysia and some
initiatives being carried out in in facing the geohazard. The Department of Mineral and Geoscience Malaysia
and a team of local geohazard experts have taken the lead to carry out studies to understand the sudden
emergence of widespread and cascading debris flow events in Malaysia. As a result, proposal on policy
intervention was put forward to the relevant government ministries; post-disaster programme has been
continuously carried out to educate the communities on the geological disaster risks through communitybased disaster risk management (CBDRM).

1 Introduction
It is a general misconception by the lay public thatchannelized fast flowing landslide or debris flow eventsoccur at the hillside river catchment is being considered
as ordinary riverine water surges from upstream
headwaters after a heavy rain. During the debris flow
disaster in Gunung Jerai, the event was termed as ‘riverwater surge phenomenon’ by the communities and mass
media. The water surge phenomenon commonly occurs
at the foot of mountainous region or hillside rivercatchment after extreme rainfall events in the upstream,
causing the rapid increase of river water level creating
torrent flows rapidly to the downstream. However, thisprocess does not carry debris materials and the water

rarely overflows over the riverbanks. Due to the
misconception and lack of knowledge, the communities
at risk are getting false information and taking incorrect
countermeasures.
In the recent years, this country had observed
multiple events of debris flow occurred at the same timewith varying degree of destruction where communities
were badly affected, socially and economically. The
Department of Mineral and Geoscience Malaysia, thegovernment agencies responsible geological hazards,
has structured multiple initiatives to communicate theunderstanding of hazards and risks of debris flow, as
well as strengthening the communities through policy
intervention
andlocalisedcommunity-based
programme in the disaster prone areas.

* Corresponding author: arjaapar@gmail.com© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

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1.1 Debris Flow Events in Malaysia
The earliest record of debris flow in Malaysia pointed to
the Genting Sempah Debris Flow in 1995, which killed
20 people and 3 were still missing. A year later, ‘Pos
Dipang Mud Flood’ and ‘Keningau Greg Tropical
Storm’ occurred three months later which killed 44
people and 303 people, respectively.
Between the years 2000 to 2015, approximately 20
debris flow disasters occurred throughout the country.
However, most of these events were not recognised asdebris flows but commonly being treated or labelled as
mud floods, floods and some with landslide connotation
which were perceived as slumping type landslides at thecrown areas instead of the overall debris flows. Table 1
shows the list of some debris flow events in Malaysia
after re-examining past disaster events.
In 2021, the first major debris flow disaster occurredin Gunung Jerai, Kedah caused major destructions anddamages to most of its district of Yan and 5 were killed.
Four months later, the debris flows occurredconcurrently at least 3 States in Peninsular Malaysia, in
Bentong, Pahang; Hulu Langat, Selangor; and Jelebu,
Negeri Sembilan; consequently 23 lives perished in total
and major disturbance with economic loss of near
RM1.0B (USD210M). Four months later, another debris
flow disaster occurred at Kenyir, Terengganu without
any fatality but had disrupted the operation of thehydroelectrical dam. Lastly, on 4 July 2022, the Gunung
Inas, Kedah debris flow disaster with run-out distanceapproximately a few kilometres. It caused 3 fatalities
and the cascading impact of debris and mud floodinundated up to 20 km at the downstream.

Date

Location

Fatalities

1

30
June
1995
29 Aug
1996

KM 38.6 Kuala Lumpur–
Karak Highway, Genting
Sempah, Selangor
Pos Dipang, Kampar, Perak

20

3

26 Dec1996

Keningau, Sabah

300

4

4 Jan
2000

Cameron Highland, Pahang

6

5

22
Sept
2001
28 Dec2001

Kampung Chinchin,Gombak, Selangor

1

Pulai River, Gunung Pulai,
Johor

5

7

28 Jan
2002

Ruan Changkul, Simunjan,
Sarawak

16

8

8 Nov
2002

Taman Hillview, Hulu
Kelang, Selangor

8

9

7 Aug
2011

Sungai Ruil, Cameron
Highlands, Pahang

7

2

6

23 Oct2013

Bertam Valley, Cameron
Highland, Pahang

1

11

18 Aug2021

Gunung Jerai, Yan, Kedah

5

12

18 Dec
2021

23

13

27 Feb2022

Bentong, Pahang, Hulu
Langat, Selangor and Negeri
Sembilan
Hulu Terengganu,
Terengganu

14

4 July2022

Gunung Inas (Sungai
Kupang, Baling), Kedah

3

Total fatalities-442

1.1.1 Gunung Jerai Debris Flow
Debris flow disaster in Gunung Jerai was the turning
point of debris flow study in Malaysia. The level ofdestruction and disruption had never been seen before inthis area or even the country. The concern washeightened by whether the hazard might affect the status
and integrity of current Gunung Jerai as both the
National Geopark and archaeological heritage site.
The debris flow disaster was reported triggered by
an extreme rainfall (281 mm in 6 hours) [2]. This had
caused multiple landslides to occur on the steep slopesof this isolated mountain (1,217m). Residents describedthe ‘water surge’ carries rock boulders as big as onestorey house along with uprooted trees, sand and mud.
Signs of temporary dam bursting can also be observedat the upstream where these dams accumulated loads
and burst. The energy carried by the debris flow also
scoured and widened the river channel up to 5-7 times
in width than its original size. As the new debris scoured
and eroding the riverbanks, it also exposed the evidenceof old debris-flow colluvial deposit horizons. The
exposed old colluvium suggested there werepossibilities of larger debris flow events in the past
before this incident. This evidence was an eyes openerof researchers that this area might has undergone morethan one debris flow event and it was never recorded.

Table 1. List of major debris flow events, its distribution andfatality count in Malaysia (modified from [1]).
No

10

44

1.1.2 Bentong Debris Flow/Hulu Langat Debris
Flow/Jelebu Debris Flow
These events occurred three months after Gunung Jerai
Debris Flow in the same year. There were consecutive
heavy rain events for three days due to regional tropicaldepression meteorological phenomenon. As a result,hundreds of landslides occurred on the Main Range of
Peninsular Malaysia (Titiwangsa Range) and fell into
the main tributaries and rivers flowing into threedifferent states of the country. The water level in the
major rivers in the areas, among them the Kerau Riverrose above critical level [3]. Aerial observation foundthat hundreds of landslide scars spreading across themountainous range including the mountain primary
forest areas. The extreme rain and water surge had swept
through the landslide debris ‘avalanche’ to downstream
carrying uprooted trees and sediments. The debris were2

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1.2 Lack of Research and Knowledgethen deposited in the narrow rivers at the downstream
and clogged the water flow causing mud flood as well
as destroying houses in the low-lying settlement areas.

Four recent debris flow incidents taught us that researchand understanding on debris flow in Malaysia is still at
infancy stage. The failure to identify the correct
terminologies and processes could lead to improper andincomplete database of event reporting, ultimately
keeping track of past disasters. On the other hand, there
were constraints to conduct analysis of the current
events when the infrastructures including sensors fordata measuring such as rain gauges were not available atthe disaster areas and many highland areas in general.
In addition to that, proper knowledge onunderstanding the mechanisms of the process is very
important in order to structure the best approach for any
mitigation works. This can only be obtained if all the
involved parties such as government technical agencies,
research institutions, stakeholders, communities, andprivate consultation firms are ready to work incooperation for the sake of public concerns.

1.1.3 Kenyir Debris Flow
Kenyir debris flow occurred on 27 February 2022. Thereis no casualty reported for this event. However, this area
is located at one of the most important hydroelectric dam
and power plant in Peninsular Malaysia. The debris flow
not only deposited in the dam’s reservoir but also
destroyed multiple transmission towers, parts of power
plant, accessibility roads and jeopardised the dam
integrity.
The source of the debris flow was mainly due torockslides of granitic rocks (weathering grade III-IV)[4]. The sliding blocks were huge and massive in size
measuring up to 30 m in length. These rockfalls of large
boulders can also be observed at the site and simulation
was carried out to understand the force and kinetic
energy of the rock fall. The Limit Equilibrium Method(LEM) analysis found the blocks were sliding at anextreme condition where the water infiltrate the joint
opening at 100% filled.

2 Understanding the Hazards and Risks
2.1 Cascading Geological Events and Major
Causal Factors

1.1.4 Gunung Inas Debris Flow

It is important to understand the debris flow processes,
as well as the impacts of that processes in order tounderstand the risks. Recent debris flow events showedthat the occurrence were due to the cascading geologicalevents which triggered by extreme and prolonged
rainfalls. Cascading Geological Processes is a termcoined to educate the communities on the compounding
and transitioning of geological processes from landslide
to debris flow, subsequently debris flood and mud flood(Fig. 1). In the context of geomorphology, themountainous or hillside catchment, they are the terrain
conditions that susceptible to the debris flow events. The
communities need to understand the geologicalprocesses and environment in order to be resilient andsafe.
This large-scale disaster starts as landslips orlandslides that triggered by rainfalls. When the rain
prolonged and the volume of water increases, the debris
will start to move rapidly downstream and destroying
anything in its path, or known as debris flow. Reaching
to the foot of the mountain, the slope gradient becomesgentler and the velocity decreasing as the debris mixes
with water thus depositing the heavy debris to thesurrounding area and causing flood (this phenomenon iscalled debris flood). The water continues to flow
downstream to the main river carrying mostly sand andsilt materials which at the same time flooding the area inthe vicinity of the river (mud flood).

The most recent incident that occurred was on 4 July
2022 causing 3 fatalities, destroying 17 residential
houses, affecting 3,546 residents, and costing
approximately RM25.9 million of economic losses. This
event was very controversial due to the appearance andbelieves of logging and plantation activities at thehillside since 2010 could be related.
The disaster was triggered by heavy rains, causing
the landslides to occur at the hillside in the upstream
followed by debris flows that carry all the debris
materials along the river channels and then deposited at
the nearby villages. Weather satellite images showed therate of rainfall at the mountainous area during the eventswas as high as 150 mm/hr. There was a bridge at thedownstream acted as temporary dam where the burstingof this temporary dam, resulted in wider spread of debris
flood and mud flood at the downstream of the main river
channel and plain areas [5]. The logging and plantation
activities in the past were among the public suspicion as
the culprit to this tragedy even though their activities hadceased for years, however the activities might had
contributed a fraction of the debris materials fromaccelerated erosion and siltation.
Investigation report by government agencies
estimated the total volume of debris deposits about 7.3
million m3, where 0.3 million m3 was from landslide, 2.6
million m3 from the debris flow processes, 3.3 million
m3 from debris flood, and 1.1 million m3 from mudflood. Comparing to that volume of soil loss via erosion
and siltation by the logging activities (since 2010) only
contribute about 1.5 million m3. Hence, the geologicalhazard process alone could cause the major destruction
without the existence of the human activities.

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3.1 Short-Term Actions
These sets of actions were proposed to understand themechanisms of the geological process and developmentof mitigation measures on the currently affected areas.
Some of the suggestion includes:• Detailed geological hazard mapping.
• Installation of early warning system.
• Adaptation and mitigation measures for disasterrisk reduction.
• Capacity building and outreach via public forum.
3.2 Long-Term Actions
The long-term actions were proposed to develop thegeological hazard database, through governance toinstitutionalize expert groups and infra-/info-structuresof integrated system for prevention and mitigation
measures. The suggestions are as listed below:• Establishment of National Research Centre for
Geological Hazards.
• Mapping programme on potential debris flow inhigh risk areas based on river sub-basins.
• Networks of devices for detection and Early
Warning System (EWS) on mountainous areas.
• Integrated geological disaster mitigation measures.

Fig. 1. Cascading geological processes starting from (a) alandslide or many landslides; (b) debris flow; (c) debris
flood; and (d) mud flood.

2.2 Public vs Scientific Understanding
Public often relates the debris flow disaster withanthropogenic activities such as unsustainable logging,mining and quarrying, and uncontrolled hillsidedevelopment. This is probably due to the public usually
only had experienced the flooding part of the wholeprocess, i.e. debris flood and mud flood. Hence, theperception was on land erosion and logging activities.
The recent debris flow phenomena were extraordinaryand have never happened before in their lifetime.
Public perception is sometime based on hearsay,
experiential learning and observation, sometime could
be skewed away from the actual scientificunderstanding. On top of the extreme rainfall as thetriggering factor, natural elements such as geological,
geomorphological, and hydrological processes are themain causing factors leads to debris flow disasters,
where a debris flow can be originated from a naturallandslide in the highlands. In many cases, anthropogenic
activities might only contribute a fraction, either
positively or negatively, to the impact of the disaster.
Public awareness on debris flow hazard is a naturalprocesses that can repeat within certain period of time isvery crucial.

3.3 Community-Based
Disaster
RiskManagement for Debris Flow DisasterOne of the first action planned by the government was
the development Community-Based Disaster Risk
Management (CBDRM) modules for all affected areaand debris flow prone areas. The CBDRM programme
was designed and developed to address the aspects ofdisaster education, awareness, preparedness andresilience. Effective high impact CBDRM programme
can only realise when it was co-designed, co-developed,
and co-implemented by empowering local knowledge,
understanding the local disaster risks (coupled withtechnical/scientific information), and its cascading
impacts to the social, economic, and environment.
The CBDRM program emphasised on the strategies
in communicating risk information or ‘knowledgetransfer’, by sharing the relevant technical information
about cascading geological processes (landslides, debris
flow, debris flood, and mud flood) as part ofunderstanding the risks. The evacuation route should be
identified, evacuation centre should be made known to
the local communities. It is important to empower the
local stakeholders on their roles and responsibilities aswell as special attention to vulnerable communities inbuilding disaster resilient in local stakeholders andcommunities. The risk nature of debris flow is of highimpact (if it occurs) and low-frequency in occurrence,
hence regular and continuous drills and awarenessprogrammes should be practiced.

3 Policy Intervention
Following the Gunung Inas debris flow tragedy,
proactive measures have been taken by the government
technical agencies and authorities to structure a policy
to recognize debris flow as a major geological hazard in
the country that could occur more frequently in view ofcurrent trends of climate extreme and variability. Shortterm and long-term measures are being planned to
improve the technical knowledge, disaster risk
management and public awareness.

4

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4 Future Planning and Way Forward
Malaysia is still in the infancy stage of understanding
hazards especially geohazards, managing disasters, anddeveloping the mitigation measures. However, withclose cooperation between government technicalagencies,
research
institutions,stakeholders,
communities, and private consultation firms, the rate ofunderstanding can be improved significantly to be on
par with other countries that have experienced thisdisaster on yearly events.

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