Experiences from cost-effective

instrumentation for early warning systems

installed in some hot-spot areas in South and

South East Asian Countries

Rajinder Bhasin, Jose Cepeda, Suzanne LacasseNorwegian Geotechnical Institute – NGIHong Kong, 5 December 2018

Experiences from EWS in some hot-spot areas

Landslides are often triggered in areas with past landslidehistory

Monitoring landslides using community-based approaches have proven to be highly effective in developing countries

Slow moving slides on hill slopes need constant monitoring as many developing countries cannot afford to undertakemitigation measures or resettle people to safer areas

Presentation shows experiences from NGI projects in Vietnam, India, Sri Lanka and Bangladesh from 2014 to present

Early Warning Systems

Mitigate the risks by reducing the consequences

A successful early warning system should be able to identify and measure small but significant indicators (precursor*) that precede a landslide

The components of a cost-effective early warning system are:─ the sensors and measuring devices,

─ a real-time data acquisition unit with communication link

─ software to process and analyse the measurements.

Principal activities in an EWS

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• Monitoring / Data acquisition

• Analysis and forecasting

• Warning

• Response

The key to a successful EWS

The key to a successful EWS is to be able to identify and measure the precursors to a landslide.

Typical examples of precursors are:• Intense rainfall• Earthquakes and ground vibrations• High rate of movement of the slope• Rapid increases in pore water pressure• Erosion at the toe of a slope, etc.

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Instruments

Data acquisition

NO

YES

NO

YES

Trouble shoot,

maintenance

Analysis and

Forecasting

Warning

Monitoring

Response

Issue an alert of warning

Is data

within specified

bounds?

Will there be

a landslide?

Emergency plans

Flow diagram of a EWS

Two major problems with EWS

The most difficult problem in designing a EWS is to be able to specify proper threshold values for the events that trigger landslides.

Avoiding false alarms. The consequences of false alarms are so serious that every possible action must be taken to avoid them. Some steps to take:

• Redundancy in instrumentation

• Data quality control measures

• Make maximum use of human intelligence and “engineering judgment” in decision making.

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Importance of rainfall-induced landslides

Significant in total number of landslides:

~ 80%

Numerous transform in debris flows:

~ 90%

Debris flows:

─Long runout distances: several kilometres

─High velocities: >> 18 km/h

www.usgs.gov

Thresholds for rainfall-induced landslides

Threshold:• Boundary curve separating landslide-triggering and non-

landslide triggering events.

• It is defined by predictors for initiation of a landslide

Predictors:• Precipitation conditions (pore pressure conditions)

Types of thresholds:• Physically-based

• Empirical

Basic frameworkTwo usual variables for defining threshold:• Mean rainfall intensity• Duration of a rainfall event

This may be good enough for shallow-landslides and debris flows, but not for deep-seated landslides (due to significant influence of antecedent rainfall)

Duration (h)

Intensity (mm/h)

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Cepeda and Devoli (2008)

Cost-effective EWS developed at NGIRaingauge connected to GSM logging and alarm unit. Powered by solar panel so can be installed anywhere there is mobile telephone coverage. Data downloaded from internet can be processed to find the rainfall events associated with landslides to determine alarm thresholds

Alarm sent directly to up to 20 mobile phones as soonas thresholds for rainfall eventsexceeded. Allowsfor immediate response

EWS

Threshold eventstriggered alarm messages

Example from testing of system at NGI.

Landslide early warning system in Tram Tau

and Ha Giang Province Vietnam

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Initial Set-up for EWS for Vietnam

Further research being performed to fine tune rainfallthresholds for alarms.

Initially:

• 3 hours – 150 mm

• 24 hours – 300 mm

• 3 days – 600 mm

• VIGMR’s study shows that the relationships between the daily rainfalls and the 7 days-antecedent rainfalls are very important to trigger series of disastrous events in Bac Kan province

Landslide early warning system in the lower

Himalayas and in Nilgiris, India

KunjethiLandslide

Linga landslide in Nilgiris, South India

More than 50 peopledied in 2009

Development & monitoring of cracks in the houses on Linga landslide

SAR Interferometric observation

Bhutan Hydroelectric Project

The Å knes rock slope in Norway

•900 m high rock slope•Unstable area: 800,000 m2

•Unstable Volume: 30-40 mill. m3

•Observed disp. 3-10cm/year

Artist’s depiction of tsunami at Geiranger

30-70 (?) mill. m3

8-16 mill. m3

Early warning centre operating 24hrs a day at Å knes, Norway

Alarm thresholds criteriabased on:

• Total displacements

• Velocity in defined time periods

• Acceleration

• Threshold values need to be defined and updated

Sirens in all the villages located in the tsunami hazard zone

Phone messages

Evacuation procedures and routes

The police responsible for the evacuation

Sri Lanka, Arayanayaka landslide that killed 130 people in May 2016

Sri Lanka, hot-spot area

Sri Lanka

Near Riverstone

Sri Lanka, World Bank Project 2017-2018A preliminary threshold has been calibrated with a landslide database and rainfall data using Machine Learning techniques for the Badulla district (Cepeda 2018):

Where:

A20d is the antecedent and cumulative 20-day rainfall in mm.

Rain24h is the triggering 24-hour rainfall, in mm.

This threshold can be used as a component of an early-warning system as follows:

1 Calculate 20-day precipitation before today using observations from a station.2 Estimate threshold value using equation Rain24h threshold

3 Estimate forecast for the next 24 h, Rain24h and compare with R24h threshold

4 If R24h << R24h threshold, no warning is issued.

5 If R24h < R24h threshold , issue of a warning level depending on how far the forecasted value is from the threshold value.

6 If R24h >= R24h threshold , a warning level must be issued, but the acute level of the response depends on how far the forecast is from the threshold value.

7 The number and position of the warning levels, both below and above the threshold can be set based on a back-analysis of historical landslides in the region.

Rohingya Refugee Camps, Bangladesh (UNDP)

Sandstone

Weakly cemented sandstone

Similar to porous sandstone type that can soak up moisture and dissolves the cemeting agents like clay, calcite, silica etc.

Loses much of its strength (up to 70%) from rain

Rainfall Threshold Values for Chittagong City

3 Hour – 100mm 24 Hour – 200 mm

3 Day – 350 mm

Sandstone

Concluding remarks

The type of instrumentation and mode of operation of the EWS have allowed a straightforward transfer of the systems to thelocal partners.

The installed EWS have operated satisfactorily issuing warningsto stakeholders and allowing for timely actions.

The experiences of EWS projects for landslides in India, Sri Lanka, Vietnam and Bangladesh have been positive both for stakeholders, local partners and NGI.

The cooperation between the local partners and NGI has gonebeyond the topic of landslide EWS and has included also otherurgent areas in geosciences.

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Thank you for your attention