Reaction:

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Reaction Could Forests Stop Floods? The Case
of Tignoan, Agos and Dingalan Watersheds
Philippine Forestry Development Forum June 2-3,
2005 Asian Development Bank
Renato U. Solidum, Jr., PhD Philippine Institute
of Volcanology and Seismology Department of
Science and Technology
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Pattern of Civilization

• In general, most of human settlements in the
  Philippines grew outward from and along major
  rivers. As a consequence, barangays, town and
  even cities occupy
• River deltas
• Alluvial fans
• River valley floors

Many settlements in river deltas, alluvial fans
and valley floors are bounded by steep mountain
slopes.
The net result is the high vulnerability of our
human settlements to floods/flashfloods and
landslides.
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PHYSIOGRAPHY OF REINA/DINGALAN AREAS
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WHAT HAPPENED IN THE REINA AND DINGALAN AREAS?
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Debris Flow, Subsob River, Dingalan
Courtesy of DENR
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Landslides in Real, Infanta and Nakar
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Flashflood in Infanta and Nakar
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How do Flashfloods Form?
Flashfloods commonly occur 1) when excessive
runoff accumulates in the stream over a short
period of time 2) by breaching of landslide dams.
Landslide deposits can form dams across
constricted segments of a river valley and
impound runoff and river water upstream. The
failure of relatively large landslide dams causes
the release of their impounded water to bring
about flashfloods. 3) by lake breakout, reservoir
dam failure and logjam breaching.
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Logjam caused by landslide Luzon, 1990
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Flashflood formation mechanism
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Flashflood formation mechanism
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Flashflood formation mechanism
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Flashflood formation mechanism
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Flashflood formation mechanism
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Can FAULTS be faulted for the landslides?
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Active Faults Near the REINA, Dingalan Areas
Metro Manila
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Answer NoDo not fault the FAULT for the
landslides.
No recent significant movement of the faults. No
significant strong earthquake affecting
the areas. Many landslides not along the fault.
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Do forests prevent landslides?
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Landslide in Real, Quezon
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Landslides in Infanta, Quezon
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Grasslands along the Infanta Road
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Trees carried by the flashflood in Infanta, Quezon
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A chaotic pile of tree trunks, branches and roots
in Infanta
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Root system with bedrock chunks
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The Answer
No, forests do not prevent landslides. They
prevent surface erosion, but not mass wasting due
to saturation of the soil with water. In
soil-saturation landslides, trees may even
contribute to the process by adding additional
weight to the sliding mass.
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WHAT IS THE CAUSE?
Excessive rainfall resulted into the
over-saturation of thick soil in steep slopes
Very high rainfall Thick soil Steep slope
Landslides
Landslides -gt -gt Debris Flow/Flashflood
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(No Transcript)
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Landslide Flashflood Mitigation Strategies

• Hazard mapping assessment
• Land-use management
• Early warning
• Public education
• Disaster preparedness systems