Engineering Geology and Geohazards Lecture 8: Pyroclastic Flows and Lahars Flashcards
Define a Pyroclastic flow
Pyroclastic Flow (Nuée Ardente) - A laterally directed(coming from the side) , ground hugging fluidised suspension of ash, glass shards, pumice and volcanic
fragments.
Can move at up to 200km/hr and
up to 800°C.
May travel up to 50km from
vent.
Pyroclastic flows are released in several ‘pulses’ from volcano
Describe characteristics of a pyroclastic flow
Concentrated (dense) gas - solid
dispersion
Flow durations rarely more than a few minutes
Velocities may be up to 160m/s
Emplacement Ts: >100 and up to
900 degrees C
Small flows are topographically controlled, can travel distances of 5 - 10km
Larger flows travel in all directions between 50 - 100 km
Pyroclastic surges can detach from main flow.
Velocity and temperature of flow decrease with distance
In what kind of eruptions do pyroclastic flows occur?
Moderate to Large explosive eruptions. E.g. Vesuvius 79 ad.
VEI 3 to 8
Define a pyroclastic surge
Pyroclastic Surges - turbulent low density clouds of debris and gas that move at high speeds
Describe the characteristics of a pyroclastic surge
Pyroclastic surges are significantly less dense and less hot and than pyroclastic flows
With or without abundant
matrix fines
Faster moving than Pyroclastic flows, so generally come before them
Can overcome larger
obstacles than pyroclastic flows
Can move over water
Turbulent
Describe impacts of pyroclastic flows and surges
Impacts to life:
* Asphyxiation and suffocation
* Incineration.
* Crushing from impacts.
Impacts to property & facilities:
* Ignition
* Destruction by impact/shock
* Burial
Describe the damage potential of flows and surges
Above ignition temperature of many materials
Force of impact extremely
destructive
High velocity ensures no
possibility to out run
Can overcome 1000m high
topography
Surge can travel across water
Generate co-pyroclastic flow ash
fall
Deposits may source lahars
Describe mitigation strategies for Flows and Surges
Barriers (F. Dobran model). These barriers will be overwhelmed therefore just serve to increase evacuation time.
Land engineering (trenches and changes in slopes)
Pyroclastic Density Current protective structures, Under and overground.
Evacuation and warnings (civil officials based on forecasts from monitoring teams)
Relocation of people
EDUCATION
Define a Lahar
Indonesian word for volcanic mudflow
Main causer of death along with Pyroclastic density currents
Rainfall induced (e.g. mass movement of water saturated pyroclastic debris deposits
on steep surfaces)
Eruption induced (e.g. through crater lake)
Occurs syn-eruptive (mid eruption) and post-eruptive
Describe the characteristics of lahars
Full range from viscous to
diluted
Has a consistency of cement to
flood(more liquidy)
Velocities of 10s km/h
Travel for 10- 100 km ( more water = travels further )
Deposits typically metres
to 10s m thick
May be hot or cold. They are also acidic
If a lahar is very energetic it may pull in more debris/material as it flows, making it larger and more viscous.
Describe lahar damage potential
Drowning
Chemical burns
Erosion
Deposition (Burial)
Impact (can contain house-size
blocks)
May clog rivers, overspill banks
and block channels
Describe mitigation for lahars
Engineering:
– Sabo dams & walls
– Refuges
– Draining of crater lakes
Early warning:
– Detection
– Monitoring
– Warning
Planning and Education:
– Hazard maps
Explain lahar detection
The automated detection system relies on a series of acoustic- flow
monitor (AFM) stations installed downstream from a volcano.
Each station consists of a seismometer sensing ground
vibrations from an approaching & passing lahar & microprocessor
that analyzes the signal.
A radio at the station sends & receives information from a base
station, usually a volcano observatory.
