Renato U. Solidum

Philippine Institute of Volcanology and Seismology –

Department of Science and Technology

Lahar Management at Pinatubo

2nd VUELCO Workshop

November 7-8, 2013

Clues of real lahar problem prior to eruption

• Pinatubo eruptions characterized by large volume

pyroclastic flow deposits (source of lahar)

• Communities downstream built on widespread

lahar deposits

Hazard map released prior to eruption

Mapping-based hazards map for authorities and the

public: released May 1991 -> for Evacuation/disaster

response activities

Information and Warning Flow

Clear immediate danger: threat of post-eruption lahars

Lahar source materials: Loose debris on the slopes • Pyroclastic flow – 5.5 km3 volume; affected slopes within 16

km of summit covering 400 km2 area

• Ash fall

• Old erodible volcanic materials pre-1991

Marella River at SW slope before eruption

23 May 1991

Marella River at SW slope after eruption

June 1991

1991 Pinatubo Pyroclastic Flow Deposits

Marella River at SW slope 3 years after eruption: significant erosion of pyroclastic flow fan

05 Oct 94 June 1991

Ash fall covered slopes

Gumain River Watershed area

• How big is the lahar problem?

• How long will the problem last?

• What areas will be at greatest

risk?

Major questions about lahar risk

Projected lahar volumes as a basis for planning intervention

• Rivers draining ash fall covered slopes will have short term lahar

problems

• Rivers draining pyroclastic fields will have long term lahar

problems

Year Estimated Sediment Yield (in Mcm)

1991 500-1,000

1992 300-500

1993 180-350

1994 110-210

1995 65-110

1996 39-81

1997 23-48

1998 14-29

1999 8-17

2000 5-11

Total 1,240-2,450

• 2.5 billion m3 of ash eroded from slopes in 10 years

Pierson and others, 1992

Post eruption hazard map for lahars

Lahar impacts: destruction of roads and bridges

Before

After

Lahar impacts: burial of river channels

River buried after one lahar event

Burial, excessive

sedimentation by LAHARS

October 1995

27 July 1994

TYPHOON MAMENG LAHARS -

BACOLOR, PAMPANGA

Lahar impacts: burial of communities

Orani

• Lowermost

reaches of

Pasig-Potrero &

Porac-Gumain

clogged by new

sediments

• Back flooding

• Worsen problem

for flood prone

areas

Lahar impacts: deposits worsen flooding

Pinatubo landscape and lahars

NASA, 1995

By 1995 lahars have:

• Remobilized more than 2 billion m3 of 1991 deposits

• Have buried 400 km2 of lowland areas

• Displaced > 50,000 people permanently

Mitigation decisions (1) 1.Engineering intervention to dredge channels,

build dikes

• big debate: protect all communities or

sacrifice some not affected yet by lahars

• dikes initially beside river channels but

distances widened later; right of way problem

for some locations

• done by phases, cannot do everything right

away - nature (lahar events) faster than man

(construction, dredging of channels) due to

magnitude of problem

Mitigation decisions (2) 2. Resettlement – transfer of families,

communities to safe areas • Housing types

• Livelihood

• Cultural sensitivity – indigenous people

3. Lahar monitoring and warning • lahar manned watch points upstream of rivers

through Disaster Coordinating Councils

(police, villagers) • augmented by monitoring through rain gauge

and acoustic flow sensors, with data given to

Regional Disaster Coordinating Council

Mitigation decisions (3)

• Establishment of a

Mount Pinatubo

Commission to

coordinate recovery

of the Pinatubo

affected area,

including

engineering

intervention,

relocation,

livelihood

Dike protecting cities at SE

slope of Pinatubo; also used as

secondary road

Awareness and Preparedness Campaign for Lahars

• Flyers, posters, leaflets,

primer, booklet on

nature, effects,

preparedness for lahars

• Video presentation of

volcanic hazards

(including lahars) by M

Krafft (IAVCEI)

• Briefings, meetings

• Seminars - Workshops

• Media: print, radio, TV

Lahar Mapping and Assessment: Need for continuous monitoring of lahar and its effects

• Document rapid changes in topography in the

area affected by lahars to determine areas to be

immediately affected by next lahars

• Document lahar deposit volumes with time to

assess potential volume of future lahars per river

which is important for appropriate planning and

design of engineering interventions

• To document and study active lahar flows by

actual observations on site to understand lahars

better

Lahar deposit mapping and hazard assessment

Lahar Deposit Map Lahar and Flood

Hazards Map

Pasig-Potrero River System, SE of Pinatubo Volcano

Updated lahar hazards map: a decade after

Formation of lakes due to lahar dam

MAPANUEPE LAKE

(formerly a river)

MARELLA

LAHAR

DEPOSITS

Drain canal

Isolated community due to lahar dam

MAPANUEPE LAKE

DIZON MINES

COMMUNITY

Jun 1991

Pinatubo Crater Lake breakout: A slowly developing threat

04 Nov 96

MARAUNOT

NOTCH

Lake Breakout a threat to NW communities after 10 years

Threat of lahars, flashfloods

Planned controlled breaching, drain lake (2001)

Maraunot Drain Canal

Idea: prevent catastrophic lake breakout Goal: construct canal to initiate erosion down Maraunot notch bedrock Results:

• no breaching (design gradient not followed by engineers)

• but prevented accumulation of additional 15 M m3 volume of water

Pinatubo Crater Lake Breakout (2002)

Lake level drop of 23 meters

Lahar deposits downsream

• Heavy rainfall initiated

breaching • 23 meters: actual lake drop

based on high water mark • 65 M m3: volume of water

released from the

• 160 M m3: approximate

volume of breakout lahars

conveyed along the Bucao

River

• Lahar deposit did not

overtop dike protecting

villages. Lesser volume of

water released (and

consequent lahars) due

to drain canal!

Continuing threat 20+ years after eruption

• Siltation, threat of floods

overtopping or destroying

dikes; river beds have

higher elevation than

surrounding areas

• Threat of lahars during

extreme rainfall events

• Increased siltation caused

long term flood problems

for rivers affected by lahars

Considerations in lahar risk management

Identify/

Assess

Hazards

and Risks

Monitor

Volcanoes

and

Forecast

Activity

Communicate

Hazards and

Risks /

Warning of

Activity

Prepare

and

Mitigate/

Respond

to Warning

And

Or

Versus

Scientific and

Technological

Dimension

Human

Dimension

Uncertainties in

forecast Risk Perception

Thank you!