Download Chapter 4 - Natural Hazards: An Overview

Chapter 4 - Natural Hazards:
An Overview
Effects of hazards on humans
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scope: $50 billion/year
avg 150,000 dead/year
social loss - employment, anguish,
productivity
humans located in the way of natural
processes
Problems
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hazardous zones
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complex response & threshold crossings
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dramatic effect of “small”hazard
effects
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between tectonic plates
between land and water - population centers
biological
geological
meteorological
good vs bad - depends on POV
few if any places are free from all hazards
magnitude and frequency
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magnitude: size of event
frequency: recurrence interval
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% chance per year
hi magnitude, low frequency usually
most dangerous
Service functions: natural results of processes
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nutrients to flood plains
sediment to beaches
flushing of pollutants (often end up elsewhere)
landslide dams for lakes
sinkholes form ponds
volcanoes
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create new land
add nutrient rich ash
support life
earthquakes
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affect ground water
lift mountains
catastrophe potential
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Latin and Greek - overturn or overthrow
extraordinary or violent change
any great or sudden calamity, disaster, or
misfortune
any event that disturbs or overthrows the
order of things
importance is debated by geologists
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esp. with respect to long term development of
earth’s surface and uniformitarianism
table p 106
Evaluation of hazards
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purpose - to minimize loss
methods
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identify susceptible areas - probability of
occurrence
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based on
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past events - history of area
studies of process
 understanding of geology
 linkages
physical location
scale
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global
regional
local
Evaluation of
Hazards
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prediction
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forecast
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general location
magnitude range
chance of
occurrence
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not specific
ratio = 1:100 or 100
yr flood
percent - 50% over
next 15 yrs
warning – this will happen
specific
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time
place
magnitude
based on
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precursors
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non or pseudo science beware
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ie heavy rain = flood
often wrong
certain to be correct
occasionally
dangers
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boy who cried wolf
affects people and businesses
Scientists, media, & Hazards
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media - human impact
scientists
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conservative
reluctant to make statements without disclaimers
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based on
it is likely
lack 100% agreement
communication problems
Risk assessment
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probability x consequences
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qualitative - determine factors
quantitative
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Acceptable risk
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based on
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assign # values to risk
# values may be hard to determine
personal control
public perception
problems
opportunities
Impact of and recovery from
disasters
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impact
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direct
indirect
recovery - figure p 115
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emergency work
restoration
reconstruction I: recovery to pre-disaster
reconstruction II: may plan to decrease effects of
repeat disaster
Adjusting to hazards
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reactive - after the fact
proactive - before the fact
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avoidance
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land use planning
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identification and probability
predictions and forecasts
risk assessments
hazard studies and zoning
insurance
evacuation plans
disaster preparedness
bear the loss - ride it out
artificial control
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deflect/redirect the hazard
stabilize problem areas
Climate change, land use
change, and hazards
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effects floods, erosion, landslides,
drought, fires
alters locations and probabilities
normal, long-term change
Population increase and
natural hazards
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increases demands on land and resources
pushes people into marginal areas
Chapter 5: Earthquakes &
Related Phenomena
EQ features
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epicenter
hypocenter (focus)
seismic waves
fault
rupture
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below ground
surface
Magnitude
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amount of shaking
normalized to set distance
Richter magnitude
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largest amplitude S-wave
logarithmic scale
energy is 30X for each level
Moment magnitude
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seismic moment based on
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average amount of slip on fault
area actually ruptured
strength of rx that failed
more quantitative and accurate
Intensity
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based on personal observations of
severity of shaking
quantifies damage – mag. doesn’t
Shows variation for different areas
affected by EQ
modified Mercalli scale
Faults
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cause
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plate boundary - may be far from actual boundary
intraplate - weak zones
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types
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Dip slip
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former plate boundaries
Addition or removal of material
normal
reverse & thrust
Strike slip - right lateral, left lateral
oblique slip
buried/blind faults - no surface trace
zone - related faults may be of several types
Seismic activity
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Identification of activity
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date movements of soils and other features
study stress field and measure stain
trench across fault
other portions (segments of fault)
top seds are not offset
tectonic creep - constant movement (small or
no EQs)
classification (table p 137
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active fault zone - Holocene (10K yr)
potentially active - Quaternary (2M yr)
inactive - no activity for 2M yr
Seismic Waves
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Body waves - hi freq 05 -20hz
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P-wave
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S-wave
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fastest
thru solid only
Surface waves - lo freq <1hz
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Love - shear (side to side)
Rayleigh - oscillation - fig p 139
Seismology
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Measuring seismic waves
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seismograph
seismic station
seismogram
Location by triangulation
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S&P wave arrivals
Distance radios for 3 stations
Shaking
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frequency
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building vs EQ wave
harmonics - natural freq of vibration
low building - hi freq
 tall buildings - low freq
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materials - natural freqs vary
distance
hi freq wave decay most quickly
 tall bldgs are damaged at greater distances
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Shaking
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amplification
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ground acceleration
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material - most intense in unconsolidated
material!!!
directivity - most intense in direction of fault
rupture
acceleration of ground as EQ waves pass
horizontal & vertical
distance
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depth of focus
horizontal distance
EQ causes
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EQ cycle - Elastic rebound theory
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stress builds up
exceeds strength
rocks snap back
vibrations = EQ
recurrence depends on rock strength
Human induced EQs
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addition of water
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reservoirs (increases pressure and lubricates fault
fluid injection
explosions & nuclear tests
Primary Effects of EQs
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ground motion
Fault rupture - very localized
Shaking
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collapse buildings
knock things down
bend things
Secondary effects of EQs
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liquefaction
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landslides
fires - broken power and gas lines - result loss
of life
water bodies
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water saturated material
material acts as a liquid
tsunamis - long wavelength, fast
seiches
changes in land elevation
disease
Estimation of seismic hazard
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Max. magnitude/intensity
soil/bedrock conditions
estimated fault location
Probability
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recurrence interval
expected magnitudes
all based on
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fault assessment
historical record
earth materials
stress field measurements
EQ prediction
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Precursors - don’t always occur
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micro earthquake swarms
preseismic deformation of ground surface
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radon gas release may increase
seismic gaps (locked fault
magnetic fluctuations
electrical resistivity
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rates of uplift or subsidence
varies with earth materials, groundwater, and others
changes before EQ
animal behavior
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not reliable
could relate to other precursors
EQ prediction
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Prediction models
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stress buildup and avg recurrence
med to long term probabilities
not certainties
EQ hazard reduction
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mapping
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active fault zones
earth materials sensitive to shaking
research to predict and control EQs
develop and improve adjustment
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building design
land-use planning & hazard assessment
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siting assessment for new facilities
hazard assessment for existing facilities
Insurance and relief
warning systems
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small seismic sensors
15sec - 1min warning
EQ Hazard perception
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denial
acceptance
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why?
education
 experience
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response
move away
 prepare
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Chapter 6: Volcanic Activity
Volcanoes
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Magma rises to surface
lava extruded - eruption
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Eruption type
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Gas content (hi gas = explosive
Si content (hi Si content = explosive
hi viscosity = explosive
pyro clastic
landform
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vent
cone
caldera
rift
volcano types and eruption
manner - table p 176
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Shield - quiet
Cinder - explosive
Composite - quiet/explosive
Volcanic domes - explosive
Flood basalts - quiet
Origins
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mid-ocean ridge
hot spots
subduction zones
Volcano Effects
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Caldera - forming eruptions
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vary in size eg Crater Lake 7K yrs ago,
Yellowstone, 600K yrs ago
massive release of material
collapse of overlying material
dormant result may linger for a long time
Long Valley, CA
hot springs & geysers
Volcano Effects
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Lava flows
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Aa, slow blocky
Pahoehoe, fast ropey
Volcano Effects
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Pyroclastic activity
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tephra blown from vent into air
ash fall
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wide spread
buries, contaminates H2O, collapses structures,
respiratory problems, kills vegetation
ash flow
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supported by gas
huee ardente
lateral blast (one type Mt St Helens
cloud collapse
Volcano Effects
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gases
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types
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water vapor
CO2
CO, SO2, H2SO4
emission
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during eruption
during dormancy
1986 Lake Wios, Cameroon
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heavier than air
dissolved in H2O
released quickly due to agitation
Volcano Effects
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debris flows and mudflows (lahars)
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ash and water esp. from snow and/or ice
landslide hazard
may be large and fast
may dam rivers or more far downstream
during eruption and after eruption
Fires
Identification of volcanic hazard
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activity
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active
dormant
inactive
hazardous areas
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identify effects of previous eruptions
examine current conditions
prediction of eruptions
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Geophysical monitoring
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hydrologic
topographic changes
tilting
gas emissions
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seismic monitoring
magnetic
thermal
geochemistry
quantity
geologic history
Adjustment to and perception
of hazard
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mapping - land use planning
evacuation
warning system: table p 201
diversion of lava flows
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bombing - of lava in a channel - blocks
channel
water - chilling creates lava wall
walls
Chapter 7: Rivers & Flooding
Basics of rivers
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flowing surface water within a channel
source of water – precipitation via:
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overland flow
groundwater
Basics of rivers
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basin (watershed)
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area drained by stream
characteristics
size
 drainage density
 relief
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Basics of rivers
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channel
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shape - width and depth
gradient
velocity
discharge - volume/time
pattern
braided - bars
 sinuous/meandering - fig p 217
 pools and riffles
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Basics of rivers
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sediment load
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suspended load
bed load
dissolved load
erosion and deposition
Basics of rivers
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dynamic equilibrium
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describes relationship between all of the
above
disturbing one disturbs all
stream will alter until a new balance is
reached
land use change - fig p 215
 dam - fig p 216
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Flooding
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overbank flow
causes
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precipitation rate (or snowmelt rate)
exceeds infiltration capacity, affected by
soil/rock type
 preceding rainfall
 freezing
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dam failure
floodplain
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plain adjacent to river, subject to
flooding
geomorphic definition
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engineering/legal definition
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formed by migration of river
overbank deposition includes natural
levees
area covered by flooding
stores water –esp. wetlands
types of floods
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upstream
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short intense rainfall
small area
dissipate downstream
downstream ie. 1993 Mississippi flood
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long duration, wide spread storms
cumulative effect of med-lg flows on many
streams
long duration of downstream events is done, in
part, to flood plain storage (travel time)
dam failure instant release of stored water
What hazards do floods pose?
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primary effects
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human injury and death
water damage
sediment damage
erosion - note bank erosion
secondary effects
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hunger
disease
displacement
fires
What effects the amount of
damage caused by a flood?
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land use
flood magnitude
rate of rise
duration - seepage behind levees
season
sediment load
effectiveness of warming
identification of flood prone
areas
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topography
soils
wetlands
vegetation zones
historical development
historical floods
Magnitude and Frequency of
Floods
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flow events - hydrograph
gaging station
stage & discharge
recurrence interval
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express as ___- year flood or % chance/year
R = (N+1)/M
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N = number of years of record
M = rank of flow in array: pick highest flow from
each year and rank or rank all flows exceeding a
given stage Plot on log-normal paper
recurrence interval of largest flood is always
years of record + 1
Importance of the flood record
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quality of the record
more record = better analysis
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flood deposits
vegetation
climate change
flood populations
floodplain development
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why develop the floodplain?
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good farming - soils - water
near transportation
flat
flood control
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levees, dams, channelization
restricts floodwaters, increases stage
encourages more development
Urbanization & Flooding
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alters rainfall to runoff relationship
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increases drainage density
decreases permeability and infiltration
capacity
results
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increases frequency
increases flood stages
flashier floods
Channelization - fig p 229
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adverse effects
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habitat - consider biology with dynamic equilibrium
flow
erosion - incision and/or widening - alters dynamic
equilibrium
increases downstream flooding usually
benefits
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improves navigation
reduce flooding
some try to mimic natural systems
river restoration
redirection of erosion and deposition
Flood prevention
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fight nature - often results in increase of flood
magnitude
methods
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levees
dams
channelization
retention ponds
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mimic lost infiltration
store water - fig p 228
adjustment to flood hazard
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work w/ nature
flood proofing
regulationss based on calculated magnitude
and frequency
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flood hazard maps
zoning areas
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floodway - provides passage of 20 or 100 yr flood without
elevation increase and allows for few if any structures
floodway fringe - limited development, subject to 100 yr
flood back water
relocation of people
special flooding problems
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building in the path of over-land flow
bank erosion
perception of flooding
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accurate knowledge does not inhibit all
development
maps not always effective
communication
upstream development is scapegoat
personal knowledge varies
Chapter 8: Slope Processes,
Landslides, and Subsidence
Mass wasting
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Down slope movement of material
(dynamic - mat’l moving down)
Elements - fig p 243
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crest - convex
free face
debris slope
wash slope - concave
Classification of slope failures
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basis
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material - rock vs soil
water content - wet vs dry
rate - slow vs fast
shape - rotational vs translational
types
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flows - incoherent
slides - coherent
falls
creep
subsidence
snow avalanche
factors effecting slope stability
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Forces on slope
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driving vs resisting
weight vs shear strength (W + Mass X Gravity
load vs support
Material Type
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soil & weak rock - rotational and frequent
strong rock - translational & infrequent
orientatation of layers - (esp w/ planes of
weakness
factors effecting slope stability
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Slope and topography
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slope angle - steeper = more slides
low = slips = slow processes
steep = falls, avalanches - fast processes more energy
Climate
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moist flows, wet, weathering, material
removal
factors effecting slope stability
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Vegetation
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positive & negative
provides cover
roots – binding & breaking
removes water
adds weight
concerns
vegetation type
 hydrophobic soils: infiltration retarded due to
fire causing waxy organics to accumulate
 loss of vegetation
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factors effecting slope stability
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Water (Very important)
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weathering reduces shear strength
quantity - dry, moist, saturated
increase = >pore pressure - decrease shear
strength, > weight
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rainfall & snowmelt
liquefaction of clay
loss of shear strength due to disturbance
seepage of water onto slopes
removal of slope by erosion, and humans
Time
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seasonal changes
reduction in strength
What causes slope failure?
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long-term changes (core cause)
trigger – immediate cause
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EQ’s
vibration
rapid moisture increase
What causes slope failure?
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external
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increase shear stress
loading
steepening
shock
internal
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reduce shear strength
increase water pressure
decrease in cohesion
slopes and humans
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humans building in the way
enhanced by humans - humans induce longterm changes and triggers
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timber harvesting
urbanization/development - fig p 256
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septic fields
loading
toe removal
humans create unstable situations
Hazard recognition
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slope stability maps
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geology
slope angle % of slope
landslide inventory
landslide risk and land-use
location of property
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base of slope
top of slope
mouth of valley - debris fan
What features are evidence of
an unstable slope?
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buildings - cracked, stuck doors
crooked fences and retaining walls
broken underground pipes
uneven pavement
uneven ground
cracks in ground
trees - tilted - buttressed
rockfalls
slump features
Preventiing slope failure
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Careful planning of human activities AVOID
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sensitive slopes
loading
cutting
wetting
drainage and dewatering - gutters & french
drains
grading and benching
retaining walls
bolting, netting, spray crete
Response to unstable slopes
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Warning systems
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surveillance
tilt meters
geophones
Landslide correction
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stopping active slide
removal of water - drainage
What causes land subsidence?
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withdrawal of fluids - oil or water - p
263-264
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compaction due to lower fluid pressure
cannot reverse
uniform materials
mining - coal, fluid, salt, other
Karst - limestone and dolomite,
dissolving rock, loss of rock/H2O
Land subsidence effects
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large areas - zones above mines & wells
sinkholes
identification of subsidenceprone areas
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look for historical evidence
look for danger signs
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mines
soluble rock
Chapter 9: Coastal Processes
characteristics of the coast
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Transitional zone - continent and coast
population concentration
coast types
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erosional vs depositional
ocean vs Great Lakes
wave generation
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wind
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velocity
duration
fetch
earth movement
gravity
wave types
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open ocean
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oscillation
movement is to a depth of ½ wave length
advance until they hit coasts
shallow water - fig p 275
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translation
waves touch bottom
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turn toward coast
focus on headlands
break
rip currents - fig p 279
wave erosion
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


water pressure
abrasion with sediment
entrainment
forms - fig p 281


cliff
platform
wave transportation




longshore drift
sediment moves along the coast because waves
approach at a slight angle
constant movement
littoral cell

source





river
coastal erosion
moves along beach
moves off shore
beach budget - seasonal/annual
beach form - fig p 278
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

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cliff or dune
berms (old beach faces) if any
beach face
swash zone
surf zone
breaker zone (longshore bar
note zone of littoral transport
Coastal Erosion

causes

storms





storm surge
waves
human interference
sea level rise: worldwide 2-3mm/yr, 1"/10yr,
1ft/100yr
effects


sea cliff erosion
beach erosion


seasonal
long term
storm surge

local rise in sea level





wind and high pressure push water onto coast
added to tide
waves on top
moves waves farther on shore
solutions


build well above sea level
build barriers
tropical cyclones

powerful storms



tropical storms - winds up to 60 mph
typhoons and hurricanes - winds greater than 60
mph/100 kph
damage

initial damage (coastal




high winds
heavy rainfall - flooding
storm surge - shoreline flooding
secondary effects (inland


heavy rains - flooding
slope failure
Responses to coastal hazards


bear the loss
engineering

types







Groin
seawall, revetment
break water
jetties
Beach nourishment
Dune building
problems


enhanced erosion
disruption of littoral drift
adapt behavior


e-zones - p 297
principles



coastal erosion is a natural process
shoreline construction causes change
structural stabilization
high cost
 limited benefit
 eventually destroys beaches
 encourages poor development trends
