Conducting Damage Survey

Andy Bailey
National Weather Service Forecast Office
Pleasant Hill / Kansas City
Agenda
• Why we do surveys
• Tools we use to conduct surveys
• What the damage can tell us
about the storm
• Anatomy of a Survey
• Wind damage processes
• EF Scale and EF Kit
• How to give us a headstart
Why do we bother to do surveys?
• Evaluate our performance
• Learn from events-improve our
understanding of the atmosphere
• Maintain a climatological
database
• Satisfy media/public hunger for a
label
(or someone to blame…May 1&2)
The Tools We Use
• Damage Survey “go” Kit
The Tools We Use
• Computer
–EF Kit software
–GPS mapping
(if no iPhone)
–Pretty clunky
The Tools We Use
• Camera/Video Cam
–Document Damage
visually
(again if no iPhone)
–Capture eye-witness
accounts
–Gadget
management fatigue
The Tools We Use
• Binoculars
–Helpful to see
what is stuck in
fence line ¼ mile
away
–Saves time
• Maps
The Tools We Use
–Detailed road
networks
–Draw crude
damage path
directly on
–Work with no
power!
Clipboard –Paper –Pens
• Take notes
– Explain pictures
– Justification of
ratings
– Capture eye
witness
accounts, times,
etc.
– Can’t
accidentally
delete
The Tools We Use
• iPhone
–Communications
•Voice/ twitter/
facebook/ sms/
–GPS and compass
–Camera and Video
Cam
–Internet access and
mapping
–Miracle tool
The Tools We Use
• iPhone
–MotionX GPS app
• Allows photos to be
geotagged with notes
added, then shared via
email, twitter, and fb
• $2.99
The Tools We Use
• Example MotionX GPS output
What the Survey Can Tell About Storm
• How strong were winds
Vs.
What the Survey Can Tell About Storm
• Tornadic vs. Straight-line Winds
Tornado vs. Downburst
Differentiating between the two….
• MYTH: Twisted trees or signs, or trees laying in
different directions must mean tornado damage.
• FACT: No tree is perfectly symmetrical and will
fall differently depending on wind direction,
weight of canopy, and integrity.
• FACT: Signs may also twist because of unequal
footings or posts. Could also blow around after
initial break and appear twisted
• Tornado winds are essentially straight line to any
one particular object. Tornado would have to
have similar diameter to bring winds from several
directions.
Tornado vs. Downburst
Differentiating between the two….
• MYTH: Witness reported a “roaring” sound with
passage of storm…must be a tornado.
• FACT: Strong wind produces the roaring sound,
and can occur with either tornadoes or straightline wind storms.
• MYTH: “My house was damaged. It must have
been a tornado”.
• FACT: Straight line winds can produce EF0 to EF2
tornado damage.
What the Survey Can Tell About Storm
• What would damage path look
like for a tornado?
What the Survey Can Tell About Storm
• What would damage path look
like for a tornado?
Damage path is “convergent”,
relatively narrow and strongest
on right side of the damage path
Tornado vs. Downburst
• Tornado damage is convergent.
• Most tornado damage on right flank.
Tornado vs. Downburst
• Best way to
differentiate
is an aerial
survey
• Tornado
damage is
convergent.
What the Survey Can Tell About Storm
• What would damage path look
like for straight-line winds?
What the Survey Can Tell About Storm
• What would damage path look
like for straight-line winds?
Damage path is “divergent”,
wider, and strongest on along
the damage path
Tornado vs. Downburst
• Downbursts exhibit straight
or divergent damage
patterns, and are often fairly
widespread.
What the Survey Can Tell About Storm
• Was the warning received and
understood?
• Timing of events
• Readiness of community
Anatomy of a Damage Survey
Anatomy of a Damage Survey
• NWS Gets word of damage
– Usually we understand the potential
• Use radar and reports to identify
target locations, at least as a
start
• Contact local EMD
• Drive to site
Anatomy of a Damage Survey
• Upon arrival, we attempt to get a local
EMD or law enforcement escort
• Try to get a feel for the damage path
and nature (aerial support helps)
• Begin methodically, but quickly moving
from site to site, documenting damage
• We note: Location, photos, degree of
damage, direction of debris
Anatomy of a Damage Survey
• Note quality/consistency of DIs
• Begin to send data back to office
• Gather eye-witness accounts
• Affix a rating
• Coordinate with the NWS office
• Coordinate with local officials
• Conduct media interviews
Damage Surveys on the Web
Google Earth Output
Wind damage processes
• Buildings are designed
so gravity holds them in
place
• Forces from extreme
wind act against the
designed gravity-based
load path
• This misunderstanding
bred the “open window”
myth.
Wind damage processes
• Recall the “Bernoulli”
effect
• Air has to flow faster
above the object,
with lower pressure
causing lift.
• This same process
works on walls and
roofs of houses.
Wind damage processes
• Weak point failures such as
windows and garages compound
the problem. This internal
pressure aides the outside lift.
FEMA 342
Wind damage processes
• Garage doors are common
weak points, and are often
the first failure point.
• This can result in
downstream projectile
damage to structures
which otherwise may not
have occurred
(Important F-scale
considerations)
F-scale: History and Definition
• Developed in 1971 by University of Chicago professor
Dr. T. Theodore Fujita
• Wind vs Damage Table, mainly correlated to wood
frame houses (wind at standard anemometer height of
10 meters)
• Rating actually done by assessing damage. Wind
speeds inferred by damage.
• Drawbacks
– Difficult to estimate in open areas or with lack of
standard structures
– Wind speeds appear to be too strong in higher end
of scale
– Too few # of damage indicators
EF Scale
• An improvement over the old Fujita scale
• 28 separate damage indicators
• Based on damage assignment on more than
one structure (if available)
• Needed for consistent tornado ratings
EF Scale
• Difficulties
– Identical tornadoes moving at different speeds
produce different damage
– Tornado size cannot be used to rate damage
– Tornado hits nothing in open area
– Gaps in damage path
• Likely caused by weakening of near surface
wind due to friction from structures and trees
producing “tree top” level damage.
• Video does not support skipping or quick
withdraw back into clouds.
EF Scale
• Identification of appropriate damage indicators is
first step
• Structural integrity or integrity of tree should be
considered however.
• Shoddy construction or weak connections between
foundation, walls, and roof can reduce the EF-scale
rating by 1 or more
• Orientation of weak points such as garage doors
relative to wind can reduce F-scale rating
(failure caused indirectly by weak point)
Residences
Commercial/retail
structures
Schools
Professional
buildings
Metal
buildings/canopies
Towers/poles
Vegetation
DOD
Damage Description
1
Threshold of visible damage
2
Loss of roof covering material (<20%), gutters and/or awning; loss of
vinyl or metal siding
3
Broken glass in doors and windows
4
Uplift of roof deck and loss of significant roof covering material (>20%);
collapse of chimney; garage doors collapse inward or outward; failure of
porch or carport
5
Entire house shifts off foundation
6
Large sections of roof structure removed; most walls remain standing
7
exterior walls collapsed
8
Most walls collapsed except small interior rooms.
9
All walls collapsed
10
Destruction of engineered and/or well constructed residence; slab
swept clean
DOD for a Framed House, DI2
EXP
LB
UB
y An easy to use computer program
y Allows for easy application and use of EF Scale
y PC based, even from thumb drive
y Provide street addresses and lat/lon
coordinates of worst of damage
y Describe nature of damage
y Estimate damage path width
y Provide photos
(not just close ups)
Conclusion
• A good quality damage survey
requires:
– a knowledge of meteorology / radar
interpretation
– basic construction knowledge
– Ability to work quickly, yet thoroughly
– Organization
– Sensitivity