flood risk assessment for emergency preparedness and response. paolo reggiani & nathalie...
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Flood Risk Assessment for Emergency Preparedness and
Response. Paolo Reggiani & Nathalie Asselman
WL | Delft Hydraulics
Symposium on Multihazard Early Warning Systems for Integrated Disaster Risk Management
WMO- Geneva 23-24 May 2006
Definition of Flood Risk:
Risk = damage x probabilityRisk = damage x probability
Damage
Damage is function of:• type and number of buildings• land use• infrastructure• flooding characteristics….
Important flooding characteristics:• time of inundation (winter / summer)• rate of rise• duration of inundation• water depths• flow velocities
Flooding characteristics
Damage andcasualties
model developed at
Delft Hydraulics for
the Dutch situation
General
• Development of a standard method to compute costs and casualties caused by flooding.
• Implemented in a Hydrological Information System (HIS).
• Initiated by Road and Hydraulic Engineering Division, Directorate-General of Public Works and Water Managementm, The Netherlands.
• The method is applicable for situations world-wide.
Aim and applicability
• Modelling system to estimate/predict damage caused by low-frequency floods.
• Floods and inundations originating from “larger” water bodies.
YES: dike breaks.
NO: local inundation from sewage systems.• No difference between salt and fresh water.
What is modelled?
• Casualties
• Damage
• loss of capital goods (houses, infrastructure, etc.)
• reduced productivity
• loss of income (businesses, shops, restaurants, etc.)
Procedure
Model•damage functions
Data•land use type•nr of inhabitants•etc.
Flooding scenario
damagedamage function
water depth
land use
Total damage estimation:
with: i = damage-factor for category i (e.g. houses, roads, etc.)
ni = number of units in category i
Di = max. damage per unit in category i
1
n
i i ii
D n D
Damage factor i
Varies from 0 to 1
Function of:
• depth of inundation
• (critical) flow velocity
• storm(waves)
• rate of rise (important for number of casualties)
Problems:
• Number of casualties depends on water depth and flow velocities after dike break, but also on possible warning and evacuation beforehand
• Damage relations for other categories mainly based on theory as little ‘experimental’ data are available form previous flood experience
• Use of experimental data still poses problems
Utrecht
Amsterdam
Den Haag
Rotterdam
NorthSea
Rhine
Meu
se
December 1993 - January 1995
3700
3800
3900
4000
4100
4200
4300
4400
4500
4600
0 200 400 600 800 1000 1200 1400 1600 1800
tijd (uren)
wat
er le
vel (
m +
msl
)
h-dec93 h-jan95
Damage claims
Cathegory (aggregated)
1993 (million guilders)
1995 (million guilders)
Private properties
96 48
Agricultural companies
39 42
Other companies
71 62
Government 61 39
Total ca. 270 ca. 190
Damage data private properties: houses
0
5000
10000
15000
20000
25000
30000
35000
40000
45000
0 0.5 1 1.5 2 2.5 3
water depth (m)
da
ma
ge
(fl
)
HD-95 HD-93 FD-theory
Damage data private properties: furniture
0
10000
20000
30000
40000
50000
60000
0 0.5 1 1.5 2 2.5 3
water depth (m)
da
ma
ge
(fl
)
FD-95 FD_93 FD-theory
Why are the costs for the second flood lower ?
• People take measures to reduce the damage:• tiled floor instead of parquet floor• put furniture on first floor when probability for
flooding increases
--> difficult to include in damage model !!
Improvement of the damage models: current research
• adjust damage functions• economic – loss of income (TU Twente)• houses and roads (TNO)
• environmental risks (Alterra, TNO, GeoDelft, CSO, WL)
• casualties / victims (WL, DWW)
Case study area ‘Zuid-Holland’
Flooding characteristics: water depth
Flooding characteristics: flow velocities
Isochrones of inundation (in hours after dike break)
Is it possible to escape ?• based on evacuation model for different area
0
10
20
30
40
50
60
70
80
90
100
0 5 10 15 20 25 30 35 40 45 50
time (hours after dike break)
perc
enta
ge o
f pe
ople
tha
t ha
ve e
scap
ed
empirical relationships established for different areas
Percentage of inhabitants that are able to escape (based on data on road network)
Estimated number of casualties (based on 1953 flood data, Zeeland province)
The Problem – time to escape
First estimate
City (suburb) Rotterdam-Hilligersberg
available time (hours) 60
route A20 Gouda
distance (km) 27
driving speed (km/h) 20
driving time needed (hours) 1,5
time elapsed before departure (hours)
4
total time (hours) 5,5
risk ? very low
Second estimate
City (suburb) Rotterdam-Hilligersberg
bottleneck road to highway, highway
available time (hours) 1
distance to bottleneck (km) 1,5
total time needed (hours) 4,1
risk ? very large !
Main lesson learned?
• A traffic / transport model is needed to obtain the most realistic estimate of potential number of casualties
• Good instructions beforehand may reduce the risk of wrong decisions with respect to choice of route