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risa.12921

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Risk Analysis
DOI: 10.1111/risa.12921
Toward Probabilistic Prediction of Flash Flood
Human Impacts
Galateia Terti,1,∗ Isabelle Ruin ,1 Jonathan J. Gourley,2 Pierre Kirstetter,3
Zachary Flamig,3 Juliette Blanchet,1 Ami Arthur,4 and Sandrine Anquetin1
This article focuses on conceptual and methodological developments allowing the integration
of physical and social dynamics leading to model forecasts of circumstance-specific human
losses during a flash flood. To reach this objective, a random forest classifier is applied to
assess the likelihood of fatality occurrence for a given circumstance as a function of representative indicators. Here, vehicle-related circumstance is chosen as the literature indicates
that most fatalities from flash flooding fall in this category. A database of flash flood events,
with and without human losses from 2001 to 2011 in the United States, is supplemented with
other variables describing the storm event, the spatial distribution of the sensitive characteristics of the exposed population, and built environment at the county level. The catastrophic
flash floods of May 2015 in the states of Texas and Oklahoma are used as a case study to
map the dynamics of the estimated probabilistic human risk on a daily scale. The results indicate the importance of time- and space-dependent human vulnerability and risk assessment
for short-fuse flood events. The need for more systematic human impact data collection is
also highlighted to advance impact-based predictive models for flash flood casualties using
machine-learning approaches in the future.
KEY WORDS: Dynamic risk mapping; machine-learning predictions; probability of flash flood casualty
1. INTRODUCTION
globally.(1) Over the last two decades, from 1996 to
2015, the National Weather Service (NWS) reported
1,193 fatalities and 6,075 injuries due to flash flood
events across the United States.5 Population growth
in urbanized areas increases the exposure of individuals and their vulnerability during their everyday activities to fast-response events.(1,2)
Hydrometeorologists work on the challenging issue of modeling physical processes associated with
the occurrence and magnitude of flash floods. A suite
of hydrometeorological products operating at high
spatiotemporal resolutions has been developed to
support operational forecasters when issuing flash
flood warnings.(3) However, such advancements cannot yet address the occurrence of life-threatening
Technological advances in forecasting the potential for flash flooding have largely improved
watch-warning systems during recent decades. Yet,
flash flooding is recognized as one of the deadliest weather-related hazards in the United States and
1 Université
Grenoble Alpes, CNRS, IRD, Grenoble INP, IGE,
Grenoble, France.
2 NOAA/National Severe Storms Laboratory (NSSL), Norman,
OK, USA.
3 NOAA/National Severe Storms Laboratory, and Atmospheric
Radar Research Center, University of Oklahoma, Norman, OK,
USA.
4 Cooperative Institute for Mesoscale Meteorological Studies, University of Oklahoma, Norman, OK, USA.
∗ Address correspondence to Galateia Terti, Université Grenoble Alpes, CNRS, IRD, Grenoble INP, IGE, F-38000 Grenoble,
France; [email protected]
5 Estimates
based on the Storm Data reports available online at:
https://www.ncdc.noaa.gov/stormevents/ftp.jsp.
1
C 2017 Society for Risk Analysis
0272-4332/17/0100-0001$22.00/1 2
Terti et al.
situations emerging from the conjunction of the hazard, still difficult to predict, and social vulnerabilities
that evolve in space and time. Vulnerability science
aims at explaining the way individuals and societies
are affected by and manage all sorts of hazards. It
thus can complement hydrologic forecasts and contribute to a better realization of the complex and dynamic circumstances leading to human impacts from
flash flooding.(4)
Currently, social vulnerability modeling research
is dominated by the construction of indexes summarizing social dependencies and economic disadvantages of the population in geographic units varying from block groups to states.(5,6) While there is
a lot of research on analyzing flood impacts and
understanding the underlying causes of social vulnerability to flood hazards,(7–11) establishing specific
vulnerability metrics remains rare. Being strongly
influenced by pioneering studies,(5,6,12,13) social vulnerability quantification in cases of flooding relied
on either data-reduction techniques such as factor
analysis(14) or arithmetic methods such as standardization scores(15–17) to compose indicator-based aggregated social vulnerability measures and maps.
With these approaches, social vulnerability is treated
separately and is then merged with the hazard information (provided through maps or scores) only
as a final step to provide a static map of integrated
socioeconomical risk. Indicators are chosen based
on theoretical knowledge (deductive approach) or
data-driven analysis (inductive approach) whereas
links with impact-related observations are rarely
considered.(18,19) Zahran et al.(19) analyzed 832 flood
events in Texas from 1997 to 2001 to explore the intersection of population vulnerability characteristics
and aggregated flood casualties at the county level.6
Adopting a multiple regression analysis, their study
reveals that flood casualties are dependent on certain social vulnerability patterns. It was found that
flood deaths and injuries in Texas are positively correlated with socially vulnerable populations, whereas
they are reduced with the increase of structural and
nonstructural flood mitigation strategies in the exposed communities. Still, social vulnerability in that
analysis is described in a static way in terms of
racial minorities and economic status, inviting further research on the integration of more hazard and
circumstance-specific vulnerability predictors.
6A
county is a political and geographic subdivision of a state and
is used for the level of local government in the United States.
The median land area of U.S. counties is 1,610 km2 .
Rufat et al.(20) reviewed 67 flood hazard case
studies (1997–2013) to present the main factors considered when assessing social vulnerability to floods.
They found that the demographic and socioeconomic
characteristics, and health and coping capacity issues,
are the most frequently described ones in the quantification of social vulnerability. The frequency varies
depending on the flood type (e.g., riverine or flash
flood), disaster phase (e.g., response or recovery),
and place of application (e.g., developed or developing country).(20) In this perspective, studying social
vulnerability to a specific temporal and spatial context of the flood disaster is a key step to identifying
relevant and measurable indicators. It also helps to
explain the causative processes, avoiding generalizations and simplifications in vulnerability assessment
and mapping.
In this work, social vulnerability is studied in
terms of human life-threatening situations during
the “event’’ phase of flash floods, when most fatalities occur, together with the peak of the hydrological event.(10,21) Terti et al.(22) presented a conceptual model illustrating how interactions between
various social processes and their conjunction with
hydrological crisis create flash-flood-specific vulnerability paths associated with certain human
impacts.(22) Then, they analyzed 19 years of flash
flood fatality reports to investigate differences in
vulnerable situations as they emerge from the sociospatiotemporal conditions in various death circumstances (e.g., in vehicles, inside buildings).(23)
They found that circumstances associated with flash
flooding fatalities have specific characteristics related
to the time at which the event happens, the duration of the flood, and tend to be associated with specific age and gender groups, inviting a situational
approach when evaluating vulnerability and the subsequent risk of people to flash flooding.
The primary focus hereafter is the vehiclerelated circumstance where the majority of people
perish while inside their vehicle or are attempting to
escape from a vehicle being swept away in flash flood
waters.(10,23–25) An empirically guided, predictive approach is adopted to estimate the likelihood of one
or more vehicle-related fatality incidents to occur
in a specific flash flood event given the conjunction
of supplemented characteristics about the hydrometeorology of the event and the infrastructure and
demography of the exposed county. Random forest
(RF),(26) a well-known decision-tree-based ensemble machine-learning algorithm for classification and
regression, is adopted for this analysis. Tree-based
Toward Probabilistic Prediction of Flash Flood Human Impacts
models recursively split the data space into subspaces
according to the behavior of a target variable. The
succession of binary splits leads to a set of tree
branches subdividing the data space into disjoint partitions of the target variable. The splits are selected
to maximize the homogeneity or purity of the target
variable in the leaves.
Such modeling is a powerful tool with recent, increasing use in hydrological and meteorological research. Classification tree analysis has been used in
hydrograph analysis to identify the effect of various
hydrometeorological variables and certain thresholds on the type of catchment response,(27) as well as
in seasonal streamflow forecasting.(28) Clark(29) used
RF models to forecast the probability of flash flooding given a set of atmospheric and hydrologic conditions in the contiguous United States. Recently, regression tree models have been further applied in
assessing flood damage based on multiple variables describing the flooding hydrology and warnings, building characteristics and precaution measures, and the socioeconomic status of private
households.(30) Compared to other advanced statistical approaches such as logistic regression, the RF algorithm does not rely on any linear or other relationship between the input predictor variables and the
target variable, and it is not sensitive to outliers, being able to handle nonlinear and complex high-order
interactions.(26,30)
Building upon prior theoretical and empirical knowledge, this article addresses the following
questions:
(1) How can social and physical proxy variables at
the county level inform a circumstance-specific
human risk metric available at temporal and
spatial scales relevant to flash flood emergency
response?
(2) How to use historic fatal and nonfatal flash
flood reports as the basis to quantify the relationship between the magnitude of the flash
flood and proxies revealing the vehicle-related
vulnerability of people at the time of the
event?
(3) How can human risk predictions be estimated and mapped dynamically to reveal the
time-variant exposure to a given flash flood
forecast?
The article is structured in the following manner. First, the flash flood human impact data used
to create the target variable in the analysis and to
3
supplement extra variables treated as vehicle-related
risk predictors are presented. In that part, our conceptual and methodological framework for flash
flood and circumstance-specific human vulnerability
is explained. Section 3 describes the process to select certain independent predictor variables to insert
in the RF algorithm, and the performance of the final classifier on predicting flash flood events with
vehicle-related human losses is assessed. The next
section applies the built classifier for a series of flash
flood events that occurred in Texas and Oklahoma
during May 2015. This section presents a prototype
toward vehicle-related risk prediction by providing
dynamic maps at the county level. The final section
discusses the achievements and limitations of the current work and proposes key future steps for the improvement of machine-learning-based prediction of
human risk to flash flood threat.
2. DATA SETS AND METHODS
2.1. Flash Flood Event Database for
Binary Classification
Deadly and nondeadly flash flood reports
in the United States are obtained from the
NOAA/National Centers for Environmental Information (NCEI) Storm Events Database known as
Storm Data.7 Storm Data publishes two complementary files each year: (i) an event details file with information about the weather event and the respective event narratives and (ii) a fatality file with details
about the victims (e.g., age, gender, location) in each
event. In a previous study,(21) the location of 1,075 individual fatalities from 1997 to 2014 across the entire
United States was reclassified to compile a data set
with five main circumstances used as a base for the
development of data-mining approaches for assessing human losses due to flash flooding. It was found
that the majority of the reported fatalities could be
linked to: (i) vehicles; (ii) permanent buildings like
homes or businesses; (iii) mobile homes; (iv) campsites or recreational areas; and (v) outside/open air
and close to streams/rivers areas.
7 Although
not faultless,(81) Storm Data is the most extensive nationwide database in the United States, recording four types
of impacts (i.e., fatalities, injuries, and property and crop damages) for 48 weather-related events. Digital data are available at
http://www.ncdc.noaa.gov/stormevents/ftp.jsp.
4
Terti et al.
Fig. 1. (a) Percent of flash flood victims by circumstance. Percentages are estimated to the total 1,075 fatalities from 1996 to 2014 in the
whole United States and Puerto Rico (dark gray) and the 551 fatalities from 2001 to 2011 in the conterminous United States (light gray),
respectively. The values on the top of the bars indicate the raw number of fatalities in each circumstance. (b) Percent of fatal flash flood
events by circumstance. The values on the top of the bars indicate the raw number of fatal flash flooding events in each circumstance. Some
of the total 705 (1996–2014) and 385 (2001–2011) unique fatal events led to fatalities in more than one circumstance. Estimates are based
on the 1996–2014 reclassified Storm Data fatality files (reclassification proposed by Terti et al.(23) ).
Here, 10 years of the 19-year database8 are used
due to the restricted availability of radar-based precipitation reanalysis and hydrologic outputs9 used to
describe the hazard dynamics of past events. A total
of 551 fatalities resulting from 385 flash flood events
from 2001 to 2011 in the conterminous United States
are discriminated by circumstance and aggregated by
causative flash flood event to create a statistical sample for each of the circumstances. Although almost
half in size from the whole data set, the new sample presents a similar distribution of the fatality circumstances, dominated by vehicle-related incidents
(Fig. 1). The 385 fatal flash flood events are merged
with nonfatal events from the corresponding event
files, yielding a total of 38,106 reported flash flood
events in the study period. This allows classifying
each flash flood as event with fatality or as event without fatality when examining the number of fatalities
8 The
reclassified data set is available online at http://blog.nssl.
noaa.gov/flash/database/.
9 Only accessible for those years in the 48 states and the District
of Columbia.
in each flash flood event for each circumstance separately.
Storm Data events are recorded based on a specified point; however, for many flash floods early in
the study period the point was missing from the data
set and the flash flood event was considered to be
“countywide.” Although after October 2006 an effort has been made to report the locations of impacted regions using bounding polygons independent
of the county polygons, the accuracy of the stormbased polygons in unknown. Thus, the smallest reliable spatial reference for the flash flood events and
their impacts in the study period is the county. Hereafter, we called “exposed county” counties where at
least one flash flood event has been reported between
2001 and 2011. Over this period, 2,899 of the total
3,109 counties in the conterminous United States are
concerned, with a mean of about 13 events per county
and up to 224 events for the most exposed. The
latter are located in Arizona, southern California,
southeastern and central Texas, and southwestern
Missouri (Fig. 2). The 259 flash flood events that led
to one or more vehicle-related fatalities from 2001 to
Toward Probabilistic Prediction of Flash Flood Human Impacts
5
Fig. 2. County-level distribution of flash flood events represented as variation from the mean (i.e., 13 events per county) for the period of
2001–2011. Counties with fatal flash flood events that according to Storm Data led to one or more fatalities are highlighted with orange
dashed line, and counties in which the fatal flash flood events led to one or more vehicle-related death from 2001 to 2011 are filled with red
hatch.
2011 are distributed in 204 counties, mostly located
in the southern United States (red hatch in Fig. 2).
2.2. Indicators of Flash Flood and
Circumstance-Specific Human Risk
Today, there is no comprehensive catalog of
proxy data derived from the quantitative analysis
of human impact observations that can be used to
understand and predict the vulnerability of people
when facing flash flood events. The majority of flash
flood applications adopts generic vulnerability indicators that do not adequately describe the sensitivity
of people during the crisis phase; instead, they describe the social groups that are the most fragile from
an economic point of view (or as a result of other
types of marginalization processes).(16,31) These indicators are not sufficiently specific to deal with social and physical dynamics that emerge and/or interact during short-fuse and localized events like flash
floods. To fill this gap, we investigate nationwide
available data sets in the United States to quantify the main vulnerability factors influencing the
exposure, sensitivity, and coping capacity of people
during flash floods, as presented by Terti et al.(22) .
The spatial scale of the analysis and application
poses constraints on the representation of some of
the social vulnerability processes (especially cognitive ones).(22) Especially, no large-scale survey or
data set is available to directly provide up-to-date
information on the level of flash flood risk awareness or the capability of response from the exposed
population at the U.S. scale. Instead, questionnairebased local studies on flood risk knowledge, perception, and behaviors analyze the links to sociodemographic characteristics such as age and gender.(32–37)
Therefore, we propose to explore the suitability of
publicly available census data to be considered as
proxies for behavioral response in flash flood circumstances. In general, findings from flash flood
or flood human-impact studies(7–9,11,23,24,38–40) were
cross-checked with arguments from the literature
on social vulnerability to flooding and natural hazards in general.(5,14,41–45) The indicators quantifying
vulnerability and the prominent human risk during
flash floods are considered according to the following
criteria:
6
Terti et al.
r The
r
r
temporal phase of the event: Some indicators can be indicative of vulnerability in the
preparation or the recovery but not in the emergency phase of the hazard.(20,44) For example,
flood insurance cannot directly reduce vulnerability during flooding but may facilitate the recovery process after a flood disaster.(45,46) As
another example, gender is used as a proxy with
different meanings depending on the stage of
a disaster. Being female is often considered as
a factor of vulnerability because women generally have lower incomes, which may involve
more difficulties in the recovery phase.(5,47) But
during the “event” phase of flash flooding,
men have been observed to adopt riskier behaviors than women by entering floodwaters,
which make them more vulnerable during that
phase.(7,8,10,23,24,40,48)
The circumstance of the life-threatening incident: There are proxies that are specific to loss
of life circumstances. For example, characteristics of buildings such as their integrity and distance to a nearby stream relate to the indoor
loss of life circumstances. Other attributes such
as the road network density or travel time to
work are associated with daily mobility and environmental familiarity factors contributing to
vehicle-related incidents.(49)
The interaction between the social and flood dynamics: The rapidity and intensity of the runoff
plays an important role in shaping specific lifethreatening circumstances. The responses of
small and flashy catchments (few square kilometers) have sufficient power to trigger loss
of life among people who are not protected
by permanent structures. These include mobile
people (e.g., drivers, pedestrians, recreationists), campers, and residents in mobile homes.
It seems logical that data depicting the flow of
commuters at the time of peak runoff would
be very indicative of the potentiality of vehiclerelated accidents. Conversely, the location of
the nighttime population is more relevant to
evaluate vulnerability in cases of building collapses from extended flooding, when residents
may get surprised in their sleep.
In this study, the dynamics of the flash flood
event are represented by distributed hydrologicmodel-based discharge forecasts generated by the
Flooded Locations And Simulated Hydrographs
(FLASH) system.(3) The unit peak discharge (i.e.,
discharge normalized by the cell’s upstream drainage
area in m3 s−1 km−2 ) is computed by running the
Coupled Routing and Excess Storage (CREST) distributed hydrologic model(50) with kinematic wave
routing (at 0.01 × 0.01 degree resolution over the
conterminous United States). The hydrologic model
is forced with the National Severe Storm Laboratory’s (NSSL’s) Multi-Radar Multi-Sensor (MRMS)
five-minute precipitation rates and provides unit
peak discharge simulations on a daily scale from
2001 to 2011.(51) The maximum unit discharge in the
county where the event occurred is thus extracted
for each flash flood event reported from 2001 to
2011 in Storm Data. Multiple data sources were used
to complete the description of flash-flood-specific
human risk indicators (see Table A.I in Supporting
Information). About 400 proxies (not shown here)
from various sources were collected and processed (if
applicable) at the county level to depict the risk situation in each exposed county. Table I is an excerpt
of the total list of the gathered proxies proposed for
modeling the vehicle-related circumstance.
In the United States, private vehicle is the predominant transportation mode for work-related and
other travels.10 Integrating a proxy representing the
flow of commuters at the time of the event is crucial for this circumstance. Therefore, we combined
indicators concerning the “time of flash flood occurrence” and the “time arriving at work” to create a
new indicator referred to as “commuters.” Based on
the time of the simulated unit peak discharge for a
given flash flood event, each reported flash flood was
assigned to a 30-minute time step interval. Each flash
flood event was then supplemented with the number of workers who arrived at work by vehicle in
the exposed county during the certain time interval
that includes the occurrence of the flood peak. For
evening and nighttime hours, the temporal resolution
in the census data at the county level is reduced due
to confidentiality reasons, thus leading to increasing
bias during this period. To avoid further subjectivity
in the analysis, the number of commuters assigned
to the evening–nighttime events was kept constant,
assuming that this would be the highest possible
exposure of commuters for that event. Despite this
limitation, this new variable enables a more realistic representation of the people exposed in vehicles
10 U.S.
Department of Transportation, “Summary of Travel
Trends: 2009 National Household Travel Survey,” Technical Report No. FHWA- PL-11-022, 2011. Available at
http://nhts.ornl.gov/2009/pub/stt.pdf.
Indicator 7: Official emergency service
Number of local emergency operation centers
(count)
Area of moderate-to-low risk of flood hazard
(in km2 and % to the total county area)
Indicator 6: Flood severity
Mean flashiness (index)
Indicator 5: Flood hazard areas
Area of high risk of flood hazard (in km2 and
% to the total county area)
Indicator 4: Duration of the rainfall event
Maximum duration of precipitation (in hours)
Indicator 3: Magnitude of the rainfall event
Maximum accumulated precipitation (mm)
Indicator 2: Duration of the flash flood event
Duration of the flash flood event (in hours)
Indicator 1: Magnitude of the flash flood event
Daily unit peak discharge (m3 s−1 km−2 )
Supplemented Variables
Counted in each U.S. county using summary statistics on the
spatially joined 2010 TIGER counties and EOC shapefiles
from the HSIP.
Calculated mean flashiness index (i.e., values between 0 and
1) for each county. The original flashiness point data were
converted to a 1-km float raster and after to a 1-km
integer raster to calculate the mean in each U.S. county.
Calculated for each U.S. county after dissolving the flood
hazard areas geodatabase based on the 2010 TIGER
counties.
Estimated number of hours of the MRMS precipitation >1.0
mm on the day of the reported flash flood event.
Extracted from MRMS system providing precipitation rate
estimates across the CONUS at 1-km resolution with
updates every two minutes. Aggregated for the county in
which the event occurred at the reported day(s).
Estimated as the difference between the beginning and end
local time of the flash flood event (e.g., 4, 1.17 hours)
when provided by Storm Data.
Computed by running the CREST distributed hydrologic
model with kinematic wave routing (at 0.01 × 0.01 degree
resolution over the CONUS) from 2001 to 2011 with
MRMS five-minute precipitation rates. The unit peak
discharge for each day was stored, and has been
aggregated for the county in which the event occurred at
the reported day(s).
Processing (If Applicable)
(Continued)
The existence of local emergency services can likely
contribute to more timely and efficient response, leading
to successful rescues from vehicles.(9)
High flashiness index reveals the potentiality of
high-magnitude discharge in a short period of time
associated with severe flooding and limited anticipation
time for people.(55)
The existence of areas sensitive to flood risk indicates higher
likelihood of severe flash flooding and impacts on the road
network and its users.
The longer the precipitation lasts the more likely is the
occurrence of floodwaters on impervious surfaces such as
roads, creating dangerous conditions for motorists.
Maximum rainfall is associated with adverse weather and
road conditions, exacerbating traffic accidents and
vehicle-related risk.(54)
Short event durations are associated with fast and dynamic
flash floods surprising and trapping people during every
day commuting and traveling.
Higher magnitudes are associated with higher water levels
that inundate and expose a larger area to flooding.
Risk Hypothesis
Table I. Summary of Processing and Interpretation of Risk Indicators and Proxy Variables to Serve as Candidate Predictors for Flash Flood Events with
Vehicle-Related Human Losses
Toward Probabilistic Prediction of Flash Flood Human Impacts
7
Indicator 12: Gender
Males (count and % to the total county
population)
Median age of workers (in the workplace
county) (years)
Median age of workers commuting to work by
vehicle in the workplace county (years)
Median age of residents (years)
– 14 years or under (youth)
– 15–34 years (new drivers and young
adults)
– 35–59 years (middle-aged active adults)
– 60 years or over (retired and elderly)
Indicator 11: Age
People (count and % to the total county
residential population):
Indicator 10: River-road network intersections
Number of river-road crossings (i.e., probable
low-water crossings or bridges) (count)
Road density (km/km2 )
Indicator 9: Road network
Road length (km)
Indicator 8: Distribution of population
Daytime population density (people/km2 )
Supplemented Variables
Extracted from table DP05 of the county-level 2010 ACS
five-year estimates.
Extracted from table DP05 of the county-level 2010 ACS
five-year estimates.
Extracted from table B08503 of the county-level 2010 ACS
five-year estimates.
Estimated by grouping the carpooled and drove alone
classes from table B08503 of the county-level 2010 ACS
five-year estimates for workplace geography.
Estimated for each U.S. county by grouping age subgroups
provided in table DP05 of the county-level 2010 ACS
five-year estimates.
Calculated as the intersection points of the merged 2010
TIGER road and NHD river/stream network shapefiles
and aggregated for each U.S. county.
Calculated for each U.S. county by dividing the estimated
road length (km) by the county land area (km2 ).
Calculated for each road feature from the 2010 TIGER/Line
road shapefile and aggregated by 2010 U.S. county.
Calculated for each U.S. county by dividing the daytime
population (provided by the ORNL’s high-resolution [90
m cell] daytime population data) by the county land area
(2010 TIGER counties).
Processing (If Applicable)
Table I (Continued)
(Continued)
Males are supposed to be more likely to be involved in
emergency activities or to undertake risky behavior
associated with entering floodwaters in
vehicle(7,8,11,25,40,58,59) and, especially, driving through
already barricaded roads.(24,37)
Very young and old population is always susceptible due to
their physical constraints, and their dependency on others
to deal with or escape from floodwaters.(7,43,47,58) But in
majority, young and middle-aged active population is
more likely to be involved in vehicle-related
incidents.(23–25,49) Young drivers may be also less aware of
flash flood risk(33) and more confident to undertake risky
behaviors toward crossing flooded roadways.(32)
Crossings like bridges and low-water crossings are features
sensitive to flash flooding largely linked to vehicle-related
deaths in the United States.(25) Especially when
associated with low-visibility hours, drivers’ ability to
evaluate the conditions on high-risk locations of the road
network is subsequently reduced.(8,24)
The exposure of roads is inseparably linked with exposure of
vehicle users. Road network sensitivity to inundation
impedes rescue operations and limits the response
capacity of drivers and passengers during flooding.(57)
The daily mobility and routine that creates differences in
population density across space during the day defines the
distribution of exposure.(56)
Risk Hypothesis
8
Terti et al.
Indicator 16: Language
Number of people who speak other than
English languages at home, and speak
English less than ”very well” (for
population over five years) (count)
Number of commuters by private vehicle who
speak other than English languages at
home, and speak English less than ”very
well” (count) (for workers 16 years and
over in the workplace county)
Indicator 15: Ethnicity/citizenship
Number of foreign-born, not U.S. citizen
commuters by private vehicle (drove alone
or carpooled; in the workplace county)
People graduated from high school or
equivalent (count and % to the total
population 25 years and over)
Indicator 14: Educational attainment
People educated with less than 9th grade
(count and % to the total population 25
years and over)
Number of family households (i.e., families)
(count and % to the total number of
households)
Number of single-parent families (i.e., with
either male or female householder) (count
and % to the total number of households)
Indicator 13: Household family status
Average household size
Supplemented Variables
Estimated by grouping drove alone and carpooled classes
for workers who speak other than English languages at
home and speaking English less than ”very well” from
table B08513 of the county-level 2010 ACS five-year
estimates for workplace geography.
Extracted from table DP02 of the county-level 2010 ACS
five-year estimates.
Estimated by grouping the drove alone and carpooled
classes from table B08511 of the county-level 2010 ACS
five-year estimates for workplace geography.
Extracted from table DP02 of the county-level 2010 ACS
five-year estimates.
Extracted from table DP02 of the county-level 2010 ACS
five-year estimates.
Processing (If Applicable)
Table I (Continued)
(Continued)
Language difficulties can lead to limited or no reception of
warnings and emergency advice.(16,43)
Probable cultural or language constraints of foreign
commuters may hinder situational awareness related to
the forthcoming weather and driving conditions.(49,63)
Lower education may reduce the ability to understand
warnings.(5,43) People with less than a high school diploma
are the least likely (about 17.5% in 2004) to work in
occupations in which they are flexible to vary their work
schedules(62) and thus may feel the need to drive through
potentially flooded ways.
Family-care responsibilities and dependencies can lead to
unexpected mobility under extreme weather conditions.
Someone may try to cross flooded locations in the effort
to reach and help the rest of the household members
during flash floods.(43,60) Especially, single parents may
have more pressure for care giving that along with
parents’ tendency to ignore their self-protection to protect
their children can lead them to enter flashy waters.(61)
Risk Hypothesis
Toward Probabilistic Prediction of Flash Flood Human Impacts
9
Indicator 19: Commuters
Number of commuters who arrive at work by
vehicle in a time interval that covers the
time of the unit peak discharge associated
with a certain flash flood event in the
exposed county (for workers 16 years and
over who do not work at home in the
workplace county) (count)
Indicator 18: Travel time to work
Number of commuters who are travelling to
go to work from 5 to 90 or more minutes
estimated in 11 classes (e.g., five to nine
minutes, 10–14 minutes, . . . , 60–89 minutes,
more than 90 minutes) (count)
Aggregate number of vehicles used in
commuting by workers
Indicator 17: Vehicles
Aggregate number of vehicles available in the
total households (count)
Supplemented Variables
Estimated by assigning the number of workers arriving
during a given time interval at work to each flash flood
event for which the CREST simulated unit peak discharge
has been recorded in the same time interval. The drove
alone and carpooled classes of commuters were grouped
for each given time interval from table B08532 of the 2010
ACS five-year estimates for workplace geography.
Extracted from table B08303 of the county-level 2010 ACS
five-year estimates.
Extracted from table B08015 of the county-level 2010 ACS
five-year estimates.
Extracted from table B25046 of the county-level 2010 ACS
five-year estimates.
Processing (If Applicable)
Table I (Continued)
The conjunction of daily mobility related to professional
activity of people with the occurrence of unusual
hydrometeorological circumstances increases the
vehicle-related spatiotemporal exposure.(64,65)
Longer journeys suggest higher likelihood of exposure to
flooded roads. Also, commuters who are familiar with
long everyday travels on certain roads may be more likely
to underestimate the level of risk associated with
voluntarily entering floodwater.(38,49)
The number of vehicles used in daily commuting or being
available to be used to reach a destination or retrieve
family members (and/or property) during flooding can be
related to the likelihood of people to get trapped in a
car-related incident. Private four-wheel vehicles are
associated with driving through flooded ways, attributed
mainly to the drivers’ confidence in automobile safety or
personal driving capabilities and underestimation of
risk.(24,36–38)
Risk Hypothesis
10
Terti et al.
Toward Probabilistic Prediction of Flash Flood Human Impacts
during a flash flood event in the specific county. Commuting plays an important role in the overall vulnerability since work-related travels during a normal daily
routine are more likely to be continued under adverse weather conditions in contrast to leisure trips
that can be more easily rescheduled.(52,53)
2.3. Classification Method
Each flash flood event from 2001 to 2011 is classified in a dichotomous variable that takes the label
“EVENT” when one or more vehicle-related fatalities occurred, and “NO_EVENT” when no fatality was reported. RFs grow many binary classification trees that may be weak classifiers by
themselves. These are combined with the ultimate
goal of obtaining a learner with higher accuracy.(66)
Data consist of a given training set (X, Y) =
{(X1 , Y1 ), . . . , (XN , YN )} with N independent observations (e.g., flash flood events). The vector Xj is
p
composed of p input predictors (X1j , X2j , . . . , Xj ),
p
where Xj ∈ R and Yj is the target variable
that we are trying to classify or understand (i.e.,
“EVENT” or “NO EVENT”). Breiman(26) defines
RFs as “a classifier that consists of a collection
of tree-structured classifiers {h(X, k), k = 1, . . .},”
where k is the random vector generated for the
kth tree independent from the past random vectors
1 , . . . , k−1 but with the same distribution. Each
tree in the forest is grown with additional splitter
variables until all terminal nodes of the tree are
purely one class or the other. The main principle of
RFs is randomization, which is applied in two levels:
(i) each tree in the ensemble forest is built from a
new training sample drawn randomly with replacement (i.e., a bootstrap sample) from the N cases in
the original training set (X, Y), and (ii) the split in
each node during the construction of the tree is the
best split of a random subset mtry of all variables
(mtry < p).(67)
As a result of the inherent randomness, the individual trees are almost independent. Bootstrapping
makes the ensemble less immune to changes in data
and avoids overfitting. It also allows for an internal
validation during the model training. As the forest is
built on training data, each tree is tested on the samples not used in building that tree. Similar to a validation set, the predictions on the data points not included in the bootstrap sample (called “out-of-bag”
or OOB sample) are aggregated and the error rate is
thus estimated (OOB error).(68) The predictions of
the trees in the final forest are aggregated using a
11
data set that is independent from the training sample. Each tree provides, for instance, a classification
for each new flash flood event depending on where it
lands in the tree. At the end, the RF algorithm retains the classification having the most votes (over
all the trees in the forest). Probabilities of a vehiclerelated fatality are computed through the total number of votes. In our case, a probability threshold of
0.5 is used as a dichotomous event versus nonevent
definition.
3. RF MODEL FOR VEHICLE-RELATED
CASUALTIES
3.1. Selecting Candidate Predictors
Both conceptual and data-driven criteria need
to be considered when selecting proxy variables as
relevant predictors in modeling the occurrence of
vehicle-related flash flood fatalities. First, the proxies
proposed in Table I were screened for collinearity.
Correlation between some of the proxies is expected
either because the variables are compositional or
because they are different expressions of the same
underlying processes, though hidden dependencies
may also exist. For the variables for which percentages could be estimated, a two-sample Kolmogorov–
Smirnov (KS) statistic test was performed(69) to
compare the cumulative distributions of the proxies in the two data sets11 (i.e., fatal flash floods with
vehicle-related fatalities/nonfatal flash floods). It was
found that percentages did not present good behavior in terms of separated distributions between the
“EVENT” and “NO_EVENT” classes, so they were
excluded from the analysis.
Although the RF algorithm does not suffer from
multicollinearity issues, redundant variables complicate the evaluation of the effect of each variable to
the target variable.(26,70) To detect and remove dependent variables, the variance inflation factor (VIF)
is computed as: VIF = 1/(1 – R2 ), where R is the multiple correlation coefficient resulting from regressing
linearly a predictor variable against all other predictor variables.(70,71) VIF equal to 1 indicates no
collinearity, whereas increasing values (>1) entail increasing correlation between the variables. The procedure is described as follows: (i) compute VIF for
all the 41 variables from Table I, excluding the one
with the highest VIF; (ii) repeat the stepwise procedure until no variables with VIF greater than 2
11 The
null hypothesis was rejected for p-values < 0.05.
12
Terti et al.
Table II. Variance Inflation Factor (VIF) for the Proxy Variables
with VIF < Threshold = 2
Proxy Variable
VIF
1. Mean flashiness
2. Area of moderate-to-low risk of flood hazard
3. Median age of workers commuting by vehicle
4. Maximum duration of precipitation
5. Daily unit peak discharge
6. Average household size
7. Area of high risk of flood hazard
8. Number of local emergency operation centers
9. Daytime population density
10. Number of river-road crossings
11. Road density
12. Number of commuters who arrive at work
by vehicle at the time of the peak discharge
1.06
1.10
1.15
1.15
1.18
1.18
1.31
1.33
1.41
1.57
1.82
1.88
Note: The variables are sorted from the ones with the least to the
ones with the most variance explained by the other predictor variables in the regression.
remain.(72) At the end, 12 variables are found and
kept for further analysis (Table II). The Spearman’s
rank correlation coefficient is estimated to account
for monotonic (possibly nonlinear) relationships between the variables (see Fig. B.1 in Supporting
Information). Pairwise Spearman’s correlations indicated that daytime population density was highly
correlated with the commuters (rs = 0.85) who arrived at work close to the peak discharge time and
the road density (rs = 0.81), where the latter two
were also correlated to each other (rs > 0.6). These
three variables represent similar exposure aspects in
the vehicle-related vulnerability assessment. Therefore, we decided to keep only the number of commuters estimated at the hydrologic peak time as
input for the RF model. This variable is the most dynamic and flash-flood-specific one compared to the
other two correlated variables, and their distributions in the “EVENT” and “NO_EVENT” classes
present an adequate distinction (p < 0.05 in the KS
test). Similarly, the area of moderate/low risk of the
flood hazard variable is excluded as being highly correlated with the high-risk area variable. The final
set of candidate predictors consists of nine uncorrelated variables that are standardized as (numbermean)/standard deviation to avoid scale effects (see
Table B.I in Supporting Information).
3.2. Model Training
The set of 38,048 flash flood events was sampled randomly to provide a training set of 28,536
observations. The remaining 25% of the total events
comprises a separate test set, only used to assess
the prediction performance of the final model. The
main statistical problem is the extreme imbalance
between the frequency of flash flood events with
vehicle-related fatalities and nonfatal flash flood
events (i.e., 1% of class “EVENT” and 99% of class
“NO_EVENT”) from 2001 to 2011. A common approach to deal with severe class imbalance in which
the main interest is to forecast the rare class is
undersampling.(73) Therefore, the two classes in the
training set are randomly subset so that their class
frequencies match the least prevalent class. To increase variability of the subsamples, undersampling
is repeated for 20 bootstrap resamples (steps A to C
in Fig. 3). One thousand trees are built for each RF
in each bootstrap iteration according to the process
described in Subsection 2.3.(67) The algorithm compares five variables, selected randomly from the total number of nine predictor variables, to identify
the best splitter in each node. The optimal model
is selected to maximize the area under the receiveroperating characteristic (ROC) curve (AUC) across
the resamples(74) (see processes in step C in Fig. 3).
3.3. Model Performance and Variables Importance
The internal evaluation of the final RFs model
shows that the OOB error is about 39%. There is
no typical value to evaluate the OOB error rate
since it totally depends on the training data and the
model. Class probabilities are estimated for the independent test data set composed of 60 flash flood
events in “EVENT” class and 9,452 “NO EVENT”
cases (step D in Fig. 3). The model performance is
quantified based on the AUC, estimated as equal
to 0.7 for this classifier(75) (i.e., step E in Fig. 3).
An AUC value of 0.5 corresponds to random guessing (i.e., the diagonal line on the ROC curve) and a
value of less than 0.5 indicates discrimination worse
than random chance (Fig. 4). While the predicted
probability is a continuous value between 0 and 1,
it is often desirable to provide a binary prediction
of whether the event will or will not occur to better
understand the performance of the binary classifier.
The perfect model would be pointed in the left-upper
corner of the ROC area where both the sensitivity
(i.e., P(Ŷ = EVENT|Y = EVENT)) and the specificity (i.e., P(Ŷ = NO EVENT|Y = NO EVENT))
are equal to 1. The ROC curve illustrates the performance of the classifier system as its discrimination
threshold is varied. The end-users can then decide
Toward Probabilistic Prediction of Flash Flood Human Impacts
13
Fig. 3. Modeling steps (from A to F) for training (A–C), testing (D–E), and applying (F) the random forest classifier for vehicle-related
fatalities prediction.
what is the best tradeoff between the hit rate and
false alarms.
Fig. 4 shows that for a 0.05 probability cutoff, the
model classifies correctly the 73% “EVENT” and the
62% “NO EVENT” of the test data set. If hit rate
(i.e., sensitivity) and false alarm (i.e., 1-specificity)
have the same importance, for example, then the best
cut-off probability minimizes the Euclidean distance
between the ROC curve and the upper-left corner
of the graph, which in our case is close to the 50%
probability threshold (blue point in Fig. 4).(75) Forecasters and decisionmakers can further decide if they
prefer to maximize the hit rate at the cost of increasing false alarms when issuing warnings for flash flood
risk related to vehicles. In other words, they may select to warn and respond to vehicle-related threats
when the modeled probability of vehicle fatality exceeds 40%. According to Fig. 4, for this threshold, the
RF classifier assigns class “EVENT” when the predicted probability is >0.4 and by doing so, it classifies
correctly the 87% of “EVENT” of the test data set
(red point in Fig. 4). However, the probability of no
impact events to be classified as events with vehicle
fatality in the test data set is also increasing to 0.57.
The Mean Accuracy Decrease and Mean Decrease Gini through the training process is used to get
information about the contribution of each candidate
predictor to the model.(76) In every tree grown in the
forest, OOB samples are used to measure prediction
accuracy. After the values of each variable are permuted randomly, the OOB accuracy is reestimated
and subtracted from the OOB accuracy based on the
14
Terti et al.
Fig. 4. Receiver-operating characteristic (ROC) curve for the
final random forest model. Sensitivity in y-axis corresponds to
true positive rate (TPR), known also as hit rate. X-axis demonstrates specificity, known also as true negative rate (TNR).
False alarm cases in forecasting are estimated as 1-Specificity.
The area under the curve (AUC) is shaded in light gray. The
95% confidence intervals for the estimated AUC are computed with 2,000 stratified bootstrap replicates as defined by
DeLong et al.(77) The cut-off probabilities of 0.5 and 0.4, and
the corresponding specificity and sensitivity values, are illustrated on the plot. The best cut-off probability (closest top-left
point) for this model is estimated as 0.494.
Fig. 5. Rank of features by variable importance based on (a) the out-of-bag decrease of accuracy and (b) the out-of-bag decrease of Gini
index.
corresponding original variable. The differences for
each variable are averaged over all trees in the forest and normalized by the standard error to provide
permutation importance or “Mean Decrease Accuracy” of the variable. In Fig. 5(a), household size,
emergency operation centers, and median age appear
to be the least important for the estimated model.
The more the accuracy of the RF decreases due to
the exclusion (or permutation) of a single variable,
the more important that variable is ranked. Every
time a predictor variable is used to split a node, the
Gini index for the two descendent nodes are calculated and subtracted from that of the original node.
The Gini decreases are summed and normalized at
the end of the calculation to provide a measure of
node impurity for each variable.
Fig. 5(b) shows that peak unit discharge plays the
most important role in partitioning the flash flood
Toward Probabilistic Prediction of Flash Flood Human Impacts
Table III. Predictive Performance of Alternative Models on the
Test Data Set
Random Forest Model
AUC
Specificity
Sensitivity
Full model
Model without EOCs
Model without average
household size
Model without median age
Reduced model
0.703
0.700
0.691
0.620
0.620
0.614
0.733
0.733
0.717
0.703
0.694
0.620
0.621
0.700
0.633
Note: Full model is the selected optimal model including all of the
nine predictors (see Table B.I in Supporting Information). Additional models are built by removing one by one the least important
predictors (see Fig. 5). The reduced model includes seven predictors (i.e., EOCs, household size, and median age predictors are
excluded).
events in events with and without fatalities. Being
dynamic, this variable and the maximum precipitation both describe the magnitude of the natural
hazard. Especially, it is because these dynamic variables have been determined in much higher spatial
and temporal resolutions than the county-level demographics (i.e., commuters, median age, and household size), that they probably can inherently capture
some local conditions crucial for the occurrence of
life-threatening flash flood scenes. The fact that some
variables are ranked as less important than others
does not mean that they do not contribute in refining the “EVENT” class prediction in RF classifier.
To evaluate the effect of the least important variables
on the predictions, we estimated new models by removing, one-by-one, all of the last three variables of
Fig. 5. The model without the EOCs variable is almost indicative of the full model in terms of AUC
and sensitivity estimated based on the test data set
(Table III). When the “household size” and “median
age” variables are additionally excluded, the ability
of the model to predict the probability of flash flood
events with vehicle-related fatalities in the validation
set tends to decrease. Even if the variables do not
constitute very strong predictors, it appears that considering all possible interactions between them may
lead to a better model for vehicle incidents in flash
flood events. Since the number of variables is not
large enough to cause increase in the experimental
run time, the nine predictors are all kept in the final
model. In the following section, the model will be applied to a new set of flash flood events that occurred
in May 2015 in the conterminous United States, and
therefore they are independent of the training and
testing data sets used during the model building procedure (step F in Fig. 3).
15
4. BUILDING A PROTOTYPE: MAY 2015
FLASH FLOODS
4.1. Deadly Flash Floods in Texas and Oklahoma
in May 2015
May 2015 was the wettest May on record and
the all-time wettest month in 121 years of record
in the conterminous United States.12 Fortunately,
despite the severe flood damages across multiple
flooded states, fatalities were limited to three of
them: Louisiana, Oklahoma, and Texas.13 Oklahoma
and Texas each had their wettest month of any month
on record with precipitation totals more than twice
the long-term average. The Oklahoma Mesonet measured 367 mm of May total rainfall averaged over the
state compared to the prior monthly statewide record
of 273 mm observed in October 194114 (Fig. 6).
In Texas, the average accumulated rainfall in May
2015 (230 mm) also exceeded the previous record wet
month of June 2004 (170 mm).(78)
On May 14, 2015, prior to extensive flooding beginning around May 24, flash flood warnings were issued for counties in southeast Texas. At least 34 people lost their lives in flash floods from May 6 to 29,
including 30 victims in Texas and four in Oklahoma.
The majority of these fatalities occurred in the second half of the month. First-response authorities carried out hundreds of water rescues involving mainly
stranded motorists who attempted to drive through
high water. Especially, after May 16 Storm Data reported 27 victims in Texas and three in Oklahoma.
Fifty-seven percent of those deaths were related to
vehicles.15
The hydrologic conditions of the flash flood
events that occurred in May 2015 were described
on a daily basis by aggregating simulated unit peak
discharge in each county, accessible from May 16
and after. Values for all predictors were extracted
for each of the 3,109 counties in the conterminous
12 Climatological
rankings according to the NOAA, available
at: https://www.ncdc.noaa.gov/temp-and-precip/climatologicalrankings/index.php?periods%5B%5D=1&parameter=pcp&
state=110&div=0&month=5&year=2015&asof=on#ranks-form.
13 Considering the conterminous United States, Storm Data for
May 2015 reports one fatality in Louisiana, four fatalities in
Oklahoma, and 30 fatalities in Texas.
14 Rainfall records provided by Mesonet, available online at:
https://ticker.mesonet.org/select.php?mo=06&da=01&yr=2015.
15 Based on reclassification of fatalities circumstances reported
for May 2015 in the Storm Data fatality file, available at:
https://www.ncdc.noaa.gov/stormevents/ftp.jsp. Individual fatalities were reclassified as proposed by Terti et al.(23)
16
Terti et al.
Fig. 6. Monthly observed precipitation (mm) for May 2015 in the conterminous United States estimated by NOAA.
Source: Advanced Hydrologic Prediction Service, NWS. Available at: http://water.weather.gov/precip/.
United States.16 The model was then applied for each
day from May 16 and after to compute daily the
“EVENT” class probabilities, providing a probabilistic assessment of vehicle-related human risk in each
U.S. county.
4.2. Mapping Dynamic Human Risk Related
to Vehicles
In this case study, 16 daily risk maps were
constructed with a focus on the 254 counties in
Texas and 77 counties in Oklahoma. Fig. 7 presents
daily maps from May 23 to 26 when the majority
of vehicle-related fatalities occurred (12 of the 17
vehicle-related fatalities). The estimated probabilities are equally distributed in four categories (i.e.,
low: ࣘ0.25, moderate: >0.25–ࣘ0.5, high: >0.5–ࣘ0.75,
very high: >0.75). The RF classifier was trained on
reported flash flood events.17 To prevent overrepresentation of dynamic vulnerability in counties with
possibly high values of static (e.g., flashiness) or
semistatic predictors (e.g., commuters) but no ac-
tual flash flooding, the probabilities in counties with
low daily unit peak discharge (<2 m3 s−1 km−2 ) are
mapped in the low likelihood category.(79) The counties with vehicle-related victims are extracted depending on the fatality day reported in the Storm
Data fatality file and highlighted with red boundaries
on the produced daily risk maps. Local storm reports
(LSRs) are mapped with red dots to illustrate flash
flood emergency issues such as road flooding, closures, and rescues. LSRs are preliminary reports issued in near real time by local NWS forecast offices
and serve as the initial source for reports in Storm
Data.18
On May 23, the model predicts higher probabilities for vehicle-related incidents in two main areas
along the western Oklahoma–Texas boundary and
central Texas. In the first area, the eastbound and
westbound lanes of Interstate 40 were closed because
of flooding in counties predicted as high-moderate
likelihood (Fig. 7a). According to the media, nearly
every low-lying bridge in Elk City was flooded.19
Also, Oklahoma City’s and surrounding cities’ fire
16 Enumeration
based on the Census Bureau 2010.
flash flood event is reported by the NWS when it has posed a
potential threat to life or property, and had a report of moving
water with a depth greater than 0.15 m or more than 0.91 m of
standing water (http://www.nws.noaa.gov/directives/).
17 A
18 LSR
are available at: https://mesonet.agron.iastate.edu/lsr/.
news on https://weather.com/storms/severe/news/
southern-plains-flooding-texas-arkansas-oklahoma. Retrieved
on August 10, 2016.
19 Weather
Toward Probabilistic Prediction of Flash Flood Human Impacts
17
Fig. 7. County-level daily forecast of vehicle-related human risk for each day (from (a) to (d)) from May 23, 2015 to May 26, 2015 for Texas
and Oklahoma states: the likelihood of flash flood vehicle-related casualty for individuals is predicted by the random forest model for each
county and day (i.e., 1–100%), and is assigned to four categories from low to very high. Counties with daily unit peak discharge <2 (m3 s−1
km−2 ) are also assigned to the low likelihood category. Counties with fatal flash flood events that according to Storm Data led to one or
more vehicle-related fatalities are highlighted with red line.
18
departments responded to more than 100 vehicles
stuck in high water in the evening.20 In Texas, a 42year-old man died in his vehicle along the Blanco
River near downtown Blanco.21 In the same county,
another male victim (81-year-old) was swept away in
floodwaters while trying to escape his car.
The next day, on the 24th, high likelihood of
vehicle-related accidents is predicted from the eastern border of Oklahoma to the central-southern
counties of Texas. In fact, two vehicle-related fatalities occurred in the highlighted counties in Texas
(Fig. 7(b)). A 29-year-old man was washed away with
his vehicle and an 18-year-old girl was swept away
while driving back home in Hays and Medina Counties, respectively. In Oklahoma, no fatalities were
reported but several LSRs indicate numerous roads
flooding and submerged cars. On May 25, the spatial
pattern of the predicted risk remains similar and notable but with lower values. A cluster of higher probabilities occurred in central Texas where multiple
water rescues were reported. Two males (23- and
55-year-old, respectively) died when their vehicles
were swept away in Travis and Williamson Counties,
respectively (Fig. 7(c)).
On May 26, risk for motorists according to the
developed model is concentrated in southeastern
Texas with highest probabilities estimated around
Harris County in Houston, Texas (Fig. 7(d)). Actually, hundreds of vehicles were stranded in floodwaters after daylight in the Houston area. Four fatalities
occurred that were directly related to vehicles in Harris County. Three more fatalities resulted from the
capsizing of a Houston Fire Department rescue boat
while rescuing stranded motorists.22 In Fort Bend
County, a 73-year-old woman lost her life while driving to work and was found dead about 50 m from her
submerged car.23 It appears that the model performs
better for widespread precipitation and flash flooding
than for more localized events.
5. DISCUSSION AND CONCLUSIONS
This article explores an interdisciplinary approach to combine data related to meteorology,
20 Online
news, available at: http://newsok.com/article/5422080.
Retrieved on August 10, 2016.
21 According to Storm Data event narratives; available at: https://
www.ncdc.noaa.gov/stormevents/eventdetails.jsp?id=581620.
22 The three fatalities are classified as vehicle related based on the
reclassification of Terti et al.(23)
23 See http://edition.cnn.com/2015/05/27/us/severe-weather/.
Terti et al.
hydrology, and human geography in an effort to
produce probabilistic forecasts of human losses from
flash flooding at the county level and daily step. We
vision this approach as a forecasting system to anticipate potential human losses with a focus on the most
prevalent circumstance of fatalities: vehicle-related
incidents. The study builds on the social vulnerability
and risk analysis research with two main contributions: (i) human vulnerability aspects are integrated,
for the first time, with hydrological forecasts to
account for the evolution of human risk to flash flood
hazard in time and space, and (ii) historic losses are
involved in the modeling procedure to link vulnerability conceptualizations with human impact observations. RFs classifiers deal with nonlinear, complex interactions between the predictors and were selected
for this study. This method provides for the identification of the variables that best represent the interplay between the natural hazard and human vulnerability processes for commuters during flash flooding
events.
Validation of the developed model is not a
straightforward exercise. The same conjunction of
sociohydrological conditions identified as lethal in
past flash flood events may not result in fatalities
during a future event due to differing circumstances
at a very local level. The area under the ROC
curve used to evaluate the final classifier is estimated as 0.7, indicating a moderate predictive performance. Given that the coarse resolution of the predictors (i.e., county level) may be unable to explain
salient local natural and social processes, and that
fatal flash floods are extremely rare events to predict, this first result toward predicting vehicle-related
losses in a set of unseen flash flood events is encouraging. More precise impact data are needed to
calibrate and/or verify the model outputs. Integrating social media and crowdsourcing data sets in the
modeling process could provide a valuable contribution to the model performance. Based on the case
study, the model shows promising results in terms
of locating dangerous circumstances in space and
time. Higher probabilities are adequately predicted
for extended county-level flash flooding while the
model seems to overestimate vehicle-related vulnerability during very localized events. Critical thresholds for the prediction of vehicle-related incidents
need to be further investigated integrating local
sensitivities.
In summary, the results presented here are
subject to the following limitations and inherent
uncertainties:
Toward Probabilistic Prediction of Flash Flood Human Impacts
r Data
r
r
uncertainties: Flash flood events and
the recorded human losses in Storm Data
are subject to undercounting.(7,9) Because the
model uses a binary classification, underreporting is generally not a problem. Sociodemographic information from the U.S. Census
Bureau and other data sources used as inputs in this study may add further inaccuracies. Sometimes data provided by the American
Community Survey (ACS) are characterized
from large margins of error, adding further concerns about the quality and precision of model
inputs.(80) Finally, the hydrologic model simulations are subject to uncertainties due to inadequate model physics representations and forcing from weather radar-based rainfall estimates.
Scale limitations: The need for a large number
of observations to construct an adequate statistical sample for the machine-learning algorithm
necessitates the consideration of many years of
flash flood event observations within a large geographic area (i.e., the whole United States).
That means that regional differences and local
specificities that may convert an initially moderate risk flash flood event to a catastrophic
event are not considered in the current analysis.
Consideration of other types of human impacts
such as injuries or rescues could contribute to
a larger sample of impactful flash flood events
with vehicle-related incidents. This is a limitation in the current study since systematic classification of nonlethal circumstances is not available at the U.S. scale yet.
Resolution constraints: The fact that reports on
flash flood fatalities are not spatially explicit
complicates the supplementation with other extra data sets available at higher resolution than
the county. Local and sometimes dynamic information is aggregated, losing details that may
contribute to the occurrence of a lethal scene.
Given the locality and complexity of the flash
flood hazard, the practicality of the county-level
modeling is questionable. As mentioned above, at
the time of the analysis the most reliable spatial reference for the reported impacts in Storm Data was
the county. To avoid spatial vagueness and inconsistencies between Storm Data files, and to maximize the amount of available records, the county reference was used in the predictive modeling. When
the accuracy of the bounding polygons currently
adopted by the NWS to report impacted areas in
19
flash flood events allow for it, it would be interesting to bring all the data in finer resolution.
Furthermore, the reported bounding polygons could
be cross-checked with the extent of the hydrologic
forecast to delineate even more specific exposed areas. This would provide for the collection of more
spatially precise predictors to be used as input in the
machine-learning model training. For instance, data
that were already available on the order of a few
kilometers (e.g., population density, unit discharge)
could be then more valuable for describing the exposure related to a certain flash flood event. Census
data from the ACS could be then extracted and aggregated from smaller geographic units such as block
groups to allow for a better representation of the socioeconomic and demographic variability of the exposed people.
Casualties depend on many parameters such as
personal strengths and last-minute decisions. Discrimination between lethal and nonlethal events is
very difficult, especially for flash flood events with
less than five fatalities. Exploring other classification
criteria of the target variable might enable a more refined clustering of the severity of flash flooding. We
recommend that the flash flood disaster science community and practitioners conduct data collection with
more details and at finer resolutions to better capture local temporal and spatial complexities associated with human losses from flash flooding.
In this study, uncertainty in quantification of
human risk related to vehicles is accounted for by
treating the occurrence of flash flood fatalities in a
probabilistic way. Compared to previous studies, vulnerability is illustrated as an evolving likelihood of
vehicle-related incidents overcoming the one-sided
static generalization of social vulnerability from
county to county. Despite unavoidable biases and
scale issues, this work represents a first attempt to
provide a prediction system that supports emergency
preparedness and response to flash flood disasters.
Based on readily available data sets across the
United States, the adopted modeling approach can
support a nationwide prediction effort for forecasters
and emergency managers to target their warnings on
anticipated human impacts, forcing the model with
real-time hydrologic forecasts. Rather than presenting this model as an established relationship between
the selected predictors, we envision an adaptive approach that evolves with data updates and improves
with experience. Expert engagement is a necessity
to compensate the scarcity of large and suitable
data at the scale of the flash flood disasters. A
20
participatory approach, involving forecasters and
emergency managers, is a strong recommendation
not only to fit the model objectives and outputs to
their needs, but also to get feedback on potential
adjustments and improvements of the modeling itself
based on experts’ knowledge and experience in the
area of their responsibility.
ACKNOWLEDGMENTS
This work has been supported by a grant from
Labex [email protected] (Investissements d’avenir—
ANR10 LABX56, FRANCE). Partial funding for
this research was provided by the Disaster Relief Appropriations Act of 2013 (P.L. 113-2), which provided
support to the Cooperative Institute for Mesoscale
Meteorological Studies at the University of Oklahoma under Grant NA14OAR4830100. The authors
would like to acknowledge the comments provided
by the anonymous reviewer and the editors, which
ultimately led to an improved article.
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SUPPORTING INFORMATION
Additional supporting information may be found in
the online version of this article at the publisher’s
website:
Table A.I. Summary of Collected Data Types and
Sources, and Their Role in the Assessment of Human Vulnerability and Risk to Flash Flood
Table B.I. Description of the Nine Candidate Predictors
Fig. B.1. Spearman’s rank correlation for the 12 variables extracted after VIF analysis (see Table B.I in
Supporting Information for the description of the uncorrelated variables).
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