E3S Web of Conferences 415, 06009 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506009

Mitigation Strategy for Low-frequency Large-magnitude Debris
Flows in Hong Kong
Florence Ko1, Chris Chan1*, Hoi-yan Ho1, and Kevin So1
1Geotechnical Engineering Office, Civil Engineering and Development Department, Government of HKSAR, China

Abstract. Potential hazards arising from low-frequency large-magnitude debris flows have become
apparent under the adverse impact of climate change and extreme weather. Hong Kong has systematicallymanaged landslide risks arising from man-made slopes and natural hillsides since 1977, owing to its uniquegeographic location, steep terrain, deep weathering profile and post-war rapid urban development. In recentyears, evaluation of potential hazards arising from low-frequency large-magnitude debris flows and their
possible dire consequences has developed into a key part of the landslide mitigation effort in Hong Kong inlight of the occurrence of more frequent and more severe extreme rainfall events. Valuable experience isgained in identifying potential low-frequency large-magnitude debris flow hazards, assessing their potential
impacts to the public located at the densely populated foothills and developing a holistic strategy to mitigate
against potentially catastrophic consequences. This paper sets out the mitigation strategy for mitigating lowfrequency large-magnitude debris flows in Hong Kong and gives a case study to demonstrate the approach.

1 Introduction
The landslide problem in Hong Kong is chronic, owing
to its unique geographic location, steep terrain, deep
weathering profile and post-war rapid urbandevelopment. Since 1940s, landslides have caused over470 fatalities in Hong Kong, majority of which havehappened before the implementation of thecomprehensive Slope Safety System by theGeotechnical Engineering Office (GEO) established in1977 [1]. Recently, extreme rainfall events, which havebecome more frequent and severe in Hong Kong, havetriggered sizeable landslides in terms of source volumeand mobility that have resulted in unprecedented
disruption and damage.
Hong Kong is located at the south-eastern tip ofChina and frequent with tropical cyclones and lowpressure troughs between May and September, bringing
heavy rain and thunderstorms. It has an average annual
rainfall of 2,400 mm, which is highly seasonal,
short-duration, high-intensity (24-hour intensity
exceeding 300 mm and 1-hour intensity exceeding
70 mm are not uncommon) and unevenly distributed
across Hong Kong. It has a land area of about 1,100 km2,
with some 75% steeper than 15 and over 30% steeper
than 30. The terrain is mantled by weak saprolitic or
residual soils, or colluvial deposits derived from pastlandslides and erosion processes. Upon the impact ofshort-duration high-intensity rainfall, the steep hillsides
are highly susceptible to rain-induced landslides as they
are subject to continual degradation [2].
Hong Kong has a population of 7.3 million as in
mid-2022. Intense urban development has taken placeon flat ground and in foothill areas, and is progressively
encroaching on the steep hillsides where landslides fromman-made slopes and natural terrain could pose asignificant hazard. With the highly populated
developments along the foothill areas, even a smallfailure on natural terrain can cause devastating life andeconomic consequences.
In June 2008, a severe rainstorm hit Hong Kong andtriggered over 3,000 landslides on the natural hillsides.
Many of the landslides were sizeable debris flows withmuch greater runout distance than those that had
previously occurred in Hong Kong and resulted indevastating disruption and damage (Fig. 1). The Hong
Kong Observatory has indicated that extreme rainfall
events have become more frequent and more severe. The
hourly rainfall record at the Hong Kong ObservatoryHeadquarters was broken several times in the last few
decades, whereas in the past, one record was only
broken in several decades (Fig. 2). In light of thelooming impacts of extreme weather, the GEO has
recognised the need to adopt a proactive approach toidentify, study and mitigate against those sizeable debris
flows, though relatively rare, with a view to reducing thelandslide risk to the public to a level as low asreasonably practicable (ALARP). These sizeable but
relatively rare events are termed low-frequency
large-magnitude (LFLM) events in Hong Kong.

* Corresponding author: chriscwchan@cedd.gov.hk© 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/).

E3S Web of Conferences 415, 06009 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506009(b) Flow capability: The presence of a major
drainage line in hillside catchments has been
identified as an adverse site setting that areprone to the development of LFLM debris flowswith watery debris of high mobility in Hong
Kong [2]. These drainage lines are associatedwith large catchments and long debris flow
paths where a large amount of storm water andentrainable materials may be available formixing with the landslide debris under extreme
rainfall scenarios.
(c) Landslide volume: Based on the characteristicsof the landslides occurred in the June 2008
rainstorm, best-estimated, upper-credible andlower-credible models were proposed toestablish the relationship between themagnitude and the notional return period (NRP)
for catchments with adverse site settings
pertaining to debris flows [3]. The results
suggested that the probable orders of magnitude
for LFLM debris flow events could be up to tensof thousands cubic metres.
(d) Landslide mobility: Based on the results of backanalyses of selected LFLM debris flows
occurred in the June 2008 rainstorm using 2dDMM [4-5], the back calculated apparent
friction angle (ϕa) and turbulence coefficient (ξ)
of Voellmy model could be as low as 8° and
500 m/s2 respectively [6].

Fig. 1. Landslide debris caused the closure of North Lantau
Highway, the critical transport corridor to the Hong Kong
International Airport, in June 2008.

Fig. 2. Hourly rainfall record at the Hong Kong Observatory
Headquarters.

2 LFLM debris flows
2.1 Characteristics
Numerous sizeable debris flows of notable volumes(e.g. an active debris volume in excess of 10,000 m 3)
with long runout distances (>1 km on plan) occurred in
the June 2008 severe rainstorm (Fig. 3). The normal
range of volumes of natural terrain landslides in Hong
Kong is in the order of a few tens to a few hundreds
cubic metres. The large-magnitude events havepreviously not occurred in Hong Kong as far as thehistorical records have shown. The triggering June 2008
severe rainstorm was equally a rare event, with its
maximum rolling 4-hour rainfall at the western Lantau
Island of Hong Kong having a return period of about
1,000 years [2]. In addition, this rainstorm is ranked thefourth in terms of 4-hour maximum rainfall in thehistorical records kept by the Hong Kong Observatory(Table 1). The detailed mapping and back analyses ofthese LFLM debris flows have diagnosed theircharacteristics as:(a) Hillside topography: The catchments are hilly
with elevation differences more than 300 m [2].
The upper terrain is usually steeper than 30° andunderlain by in-situ weathered volcanic or
granitic rocks, while the foothill is generally
gentle sloping (< 15°).

Fig. 3. An example of a LFLM debris flow occurred during
the June 2008 rainstorm.
Table 1. Top ranked 4-hour rainfall events at the Hong Kong
Observatory Headquarters [7].
Rank1234Date and Time(yy-mm-dd hh)
1889-05-30 021926-07-19 04
1966-06-12 06
2008-06-07 07

4-hour Maximum
Rainfall (mm)
302.3
292.4
273.4
246.3

2.2 Need for mitigation
The GEO has recognised the need to adopt a proactive
approach to identify, study and mitigate against thosesizeable debris flows, though relatively rare, for thefollowing considerations:(a) As extreme weather is poised to increase, andfrom the lessons learnt in the mapping of theJune 2008 landslides, LFLM debris flows may

2

E3S Web of Conferences 415, 06009 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506009

not be as rare as one might have perceived. At
the same time, rapid urban development has
resulted in denser population encroaching into
steep hillsides, many of these with adverse sitesetting favourable to the occurrence of LFLM
debris flows (Fig. 4). If a similar event as theJune 2008 rainstorm hit these vulnerable areas
again, dire consequences (e.g. building
collapse or multiple fatalities) are probable. Itis plausible to think and plan ahead how best
the Government should and could protect
public lives from landslide hazards arisingfrom LFLM debris flows, knowing that actions
take time to materialise.
(b) With continuous technical development in theunderstanding of debris flow initiation andmotion in the past one to two decades, themethodology developed to identify anddetermine landslide source areas and volumes
as well as entrainment potential has proved
effective and practicable. Advanced numerical
tools are also available to analyse and estimate
debris mobility and debris-barrier interaction
with higher degree of estimation and level ofconfidence [8]. The current state of knowledgeand technical capability have enabled thedevelopment of some feasible solutions tomitigate LFLM events.
(c) In a very developed society as Hong Kong, thepublic has high expectation on the Government
to protect them well and has low degree oftolerance to life loss, social disruption andproperty damage due to natural disasters.
Despite there is a need to take a balance
between what should be done and what could
be done, and the need to strengthen public
adaptation and resilience against adverse
impacts of extreme weather, the Government
always has a duty of care to protect public lives
and property as far as practicable.

Fig. 4. A view of natural terrain on Hong Kong Island
overlooking the adjoining dense developments.

(b) LFLM event - The mitigation measuresprovided for the Design Event should endure
for the LFLM event while debris volume
beyond the Design Event is allowed tooverflow downstream. This is to acknowledge
the fact that it is impractical, costly andenvironmentally undesirable to construct asuper-structure to fully contain a rare event,which is not credible.
(c) Overflow debris - Other prescriptive andsacrificial mitigation measures in the form ofsegregation, retention, deflection and/or energy
dissipation of the overflow debris are adopted
to further reduce the landslide risk to the public
at the downstream as far as practicable.
Depending on the different topography of different
terrain, there is considerable room for engineering
innovation for improving the design, construction andmaintenance of natural terrain landslide risk mitigation
measures, bearing in mind the importance ofcommunicating risk to the public, minimising disruption
to the environment and facilitating future maintenance.

4 Case study

3 Mitigation strategy

One of the vulnerable hillside catchments with adversesite setting is the Fei Ngo Shan study area. The study
area comprises a channelised catchment with a major
drainage line of approximately 700 m long, overlooking
a residential development (Fig. 5). Methodology
suggested in Section 2.1(c) was adopted for theestimation of the magnitude and frequency of potential
landslides up to a NRP of 10,000 years in the study area.
Debris mobility modelling using 2d-DMM [4-5] wascarried out to determine (1) whether debris would reachthe residential development concerned, and ifaffirmative, (2) the corresponding debris impact on theresidential development. Vollemy parameters given in
Section 2.1(d) were adopted in the modelling.
Mitigation strategy including a rigid debris-resisting
barrier for the Design Event and a series of prescriptive
impediments (e.g. baffles) to reduce the mobility of thedebris and damage beyond the Design Event wasadopted.
In parallel, a quantitative risk assessment (QRA) wascarried out to assess the risk level posed to theThe approach for Natural Terrain Hazard Study (NTHS)
and design of mitigation measures for hazards up to a
notional return period of 100 years (namely DesignEvent) are outlined in [9]. With a view to systematicallydealing with the risk of LFLM events in Hong Kong, athree-tier approach is adopted:(a) Design event - This event, which is expected tooccur during the design life of the commonlyadopted mitigation measures, such as rigid orflexible debris-resisting barriers, is assessed in
an NTHS and the consequences mitigated on
the basis of the current state of best knowledgeof typical debris flows and design requirements[9]. A single debris-resisting barrier together
with some deflection structures or energy
dissipation structures at the lower portion of
the terrain is usually effective.

3

E3S Web of Conferences 415, 06009 (2023)
DFHM8

https://doi.org/10.1051/e3sconf/202341506009

residential development concerned under extreme
rainfall scenarios as compared to the risk tolerability
criteria [10] and to verify the reasonableness of themitigation strategy as proposed. The QRA generally
followed the framework by [11] and the key modules ofwork suggested in [12] for the Ling Pei site.

Fig. 7. Schematic model of the mitigation strategy for the Fei
Ngo Shan study area.
This paper is published with the permission of the Head of theGEO and the Director of Civil Engineering and Developmentof the Government of the HKSAR.

Fig. 5. General view of the Fei Ngo Shan study area.

The results of this site-specific QRA revealed thatboth the calculated Personal Individual Risk (PIR) andSocietal Risk (SR) arising from the Design Event with aNRP of 100 years are unacceptable (Fig. 6). However,the calculated PIR and SR arising from the LFLM eventare acceptable and within the ALARP region
respectively. A rigid debris-resisting barrier for theDesign Event and prescriptive measures should be
proposed to deal with both the calculated and residual
risks arising from the Design Event and LFLM event
respectively (Fig. 7). The QRA results confirmed thatthe mitigation strategy as proposed is appropriate andreasonable.

References
1.

H.N. Wong, Forty years of slope engineering in
Hong Kong, in Proceedings of the HKIE GD
Annual Seminar 2017, HK, pp i-x (2017)
2. H.N. Wong, Rising to the challenges of natural
terrain landslides, in Proceedings of the HKIE GD
Annual Seminar 2009, HK, pp 15-53 (2009)
3. H.Y. Ho, F.L.C. Lo, N.M.C. Yim, Discussion Note
No. DN 3/2020, GEO, HK, 39 p (2020)
4. J.S.H. Kwan, H.W. Sun, Can. Geotech. J. 43, 531539 (2006)
5. R.P.H. Law, F.W.Y. Ko, GEO Report No. 332,
GEO, HK, 98 p (2018)
6. GEO, Technical Guidance Note (TGN) No. 29,GEO, HK, 6 p (2011)
7. B. Lin, GEO Report No. 331, GEO, HK, 121 p(2017)
8. K.K.S. Ho, R.C.H. Koo, J.S.H. Kwan, Debris
Flow Mitigation – Research and Practice in Hong
Kong, in Proceedings of the 7th ICDFHM,Golden, Colorado, USA, 8 p (2019)
9. H.Y. Ho, K.J. Roberts, GEO Report No. 138 (2 ndEdition), GEO, HK, 173 p (2016)
10. ERM, GEO Report No. 75, GEO, HK, 183 p (1998)
11. R. Fell, K.K.S. Ho, S. Lacasse, E. Leori, A
framework for landslide risk assessment andmanagement, in Proceedings of the ICLRM,Vancouver, pp 3-25 (2005)
12. H.N. Wong, Landslide risk assessment forindividual facilities, in Proceedings of the ICLRM,
Vancouver, pp 237-296 (2005)

Fig. 6. Calculated F-N curve for the Fei Ngo Shan study area.

4