HKO Research Forum 2019

Landslides Hazards Associated with Super Typhoon Mangkhut and

Severe Historical Rainstorms

Edward Chu

Geotechnical Engineer, Standards and Testing Division, GEO

3 May 2019

1

Content

• Landslide hazards associated with Super Typhoon

Mangkhut and other rainstorms in 2018

• Counterexamples of historical tropical cyclones causing

heavy rainstorms and severe landslide hazards

• Previous extreme rainfall scenario-based study

• Further study of characteristics of tropical cyclones leading

to severe rainfalls

2

Landslip Warnings Issued in 2018

Rainstorm Warnings IssuedHighest Max Rolling

24hr Rainfall

Number of

Landslides

Reported

323 mm

(N41, Tai Po)

381.5 mm

(N05, Fanling)

343.5 mm

(N37, Tsuen Wan)29

07 June 2018 13

16 September 2018

6529 August 2018

3

Annual Rainfall Distribution and Locations of Reported Landslides in 2018

4

• Total number of landslides: 253

• Total number of major landslides: 11

Reported Landslides (1997–2018)

559

228

402322

214

138

201

69

481

193

83

863

101

206

77

163

241 237

161

226

147

253

6026

6445

16 9 11 1

82

5 2

160

2 15 1 8 9 14 10 21 8 11

3,343

2,565

2,129

2,752

3,092

2,490

1,942

1,739

3,215

2,628

1,707

3,066

2,182

2,372

1,477

1,925

2,847

2,638

1,875

3,027

2,572

2,163

0

100

200

300

400

500

600

700

800

900

0

500

1000

1500

2000

2500

3000

3500

4000

4500

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018

Nu

mb

er o

f Re

po

rted

La

nd

slid

es

An

nu

al

Ra

infa

ll (

mm

)

Mean Annual Rainfall Between 1981 and 2010 ( mm )

Annual Rainfall ( mm )

Total No. of Reported Landslides

= 169

169

11 Total No. of Major Landslides (≧ 50 m3)= 11

Average no.

300

LEGEND :

5

Distribution of Landslides in 2018

4 incidents

≥ 50 m3 to ≤ 500 m3 ≥ 50 m3 to ≤ 500 m3

7 incidents

193 incidents

< 50 m3

49 incidents

< 50 m3

Total No. of Landslides in 2018 = 253

Landslides on Man-made slope

(Total number = 197)

Landslides on Hillside

(Total number = 56)

6

• Chung Hom Kok, Stanley Bay

• Landslide on natural hillside

• Failure volume: 80 m3

• Consequence:

• Emergency action not necessary because the landslide

was located at a relatively remote area away from the

footpath and existing buildings

• Pai Tau Village, Shatin

• Landslide on a 2.5 m high cut slope

(70° inclined)

• Failure volume: 1.5 m3

• Consequence:

• Permanent evacuation of a squatter

Landslides Associated with Super Typhoon Mangkhut

7

22282229(upper)

2229(lower)

2229(upper)

2229(lower)

2228

• 3 landslide clusters at Fan Kam Road

• Sourced from steep terrain typically inclined at 30° to 40°

• Source volume of 250 - 500 m3

• Open hillslope failure and channelized debris flow

• Landslides along Fan Kam Road

• Road section inundated by landslide debris

• Vehicles and container pushed downstream

• Storage structure adjoining a village house reported to have been

entirely washed away

• Abandoned structure partially demolished with the remnant wall

observed at about 20m downstream

Landslides in August 2018

8

Typhoon Sam in August 1999

• No. of landslides: 303

• No. of major landslides: 52

• Major landslide incidents:

• Sham Tseng San Tsuen

• Failure volume up to 600m3

• 1 fatality and 13 injuries

• Shek Kip Mei

• Failure volume of 2,500m3

• No fatality and injury

9

Landslides Associated with Historical Tropical Cyclones

Typhoon Helen in August 1995

• No. of landslides: 109

• No. of major landslides: 16

• Major landslide incidents:

• Fei Tsui Road

• Failure volume of 14,000m3

• 1 fatality and 1 injury

• Shum Wan Road

• Failure volume of 26,000m3

• 2 fatalities and 5 injuries

Previous Extreme Rainfall Scenario-based Analysis

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Scenario 1 - June 2008 Lantau rainstorm striking Hong Kong Island

Scenario 2 - 2009 Typhoon Morakot striking Hong Kong Island

• About 2,000 landslides• 200 to 300 impacting on buildings or

roads• Consequence:

Emergency system stretched to limit• Solution:

Streamline emergency system

• About 50,000 landslides• 4,000 to 9,000 impacting on buildings or

roads• Consequence:

Existing system completely overwhelmed• Solution:

Non-engineering approach

Further Study on Characteristics of Tropical Cyclones and Rainfalls

• Serious landslide incidents occurred due to Typhoon Sam in 1999 and

Typhoon Helen in 1995

• However, no serious landslide incidents occurred due to Super Typhoon

Mangkhut in 2018

• What characteristics of tropical cyclones would delivere large amounts of

rainfall and cause substantial landslide impacts?

• Typhoon magnitude

• Typhoon size

• Wind speed

• Track speed

• Eyewall size at landfall

• Track direction

• Track location

11

Thank You

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