Course Unit 3 Flood Risk
Flood Risk Assessment and Uncertainty H.P. Nachtnebel
Dept. of Water-Atmosphere-Environment Univ. of Natural Resources
and Life Sciences
hans_peter.nachtnebel@boku.ac.at
Structure of presentation
Objectives and introduction
Methodological concept
Risk assessment (options and uncertainties)
Risk management and mitigation strategies
Conclusions
Objectives
Critical review of the evaluation concepts of flood risk and of mitigation options
considering trends and
various sources of uncertainties
Introduction
River floods are the most frequent and costly natural
hazard, affecting the majority of the world’s countries on a regular basis (Jongman et al., 2012; UNISDR, 2011).
At the global scale (Kundzewicz, 2010) it is estimated that, on average, floods affect more than 115 million people each year
The respective economic damages are about $19 billion.
Between 1998 and 2009, Europe suffered over 213 major damaging floods (Kryzanowski et al., 2014).
Flood 2002 caused 43 victims and 15-16 billion € damages
Flood damages
The total (CEA, 2007) estimated flood damages in
Europe are about 100 billion € of economic losses only over the period 1986–2006.
The economic damage from flood events have increased during the past few decades in most regions of the world (de Moel et al., 2009, Barredo, 2009; Bouwer et al.,
2010; Kreft, 2011; UNISDR, 2011).
This is surprising because many countries, especially in Europe, have annually invested over the last decades substantial amounts in physical flood protection
measures
The Risk Management Cycle
Consultation and Implemenatation
Consultation and Planning
Risk Analysis Recovery and
Post Disaster Works Flood Event
Management Flood
Prepardness
Definitions: Reliability and Failure
Resistance Load Q
X
Q is a random variable with pdf f(Q) Reliability:
Failure rate: V(X*)=1-Z(X*)
0 * ( )*)
(X X f Q dQ
Z X*
Risk analysis
Identification of hazards
Identification of consequences (damages or fatalities)
Risk assessment
Quantifying the risk of a specific event or alternative
Comparison and ranking of alternatives
Some definitions
Hazard
Disaster
Flood Risk: Course Unit 4 H.P. Nachtnebel
Revised definition
The damage D(Q) can be analysed in more detail:
exposure of populations and property (who and what)
and the vulnerability of those exposed e.g., sensitivity to the hazard (how)
Flood risk assessment
What is a flood ?
Define the flood probability
Define the flood impacts (exposure, vulnerability)
Estimate the risk
Identify risk reduction measures
Risk elements
A hazardous event
A probability distribution function (pdf)
The consequences (damages, victims,..)
f (Q)
Q
Potential Damages D (Q)
Q X*
old
Definition of the risk
Floods (load) Q and pdf (Q)
Loss function (potential damages) D (Q)
Risk R is an expectation value
Damage function dependent on Q flood probability
0
) ( )
(
() f Q D Q dQ
R
Hazard analysis
Estimation of the frequency and magnitude of a flood events
Annual flood series (annual flood maxima)
Partial duration series (all events above a threshold level)
Partial duration series
Impact assessment
Analysis of impacts (ex ante or ex post)
analysis of previous floods (ex post analysis)
Simulation of possible floods by using a rainfall-runoff model (ex ante analysis)
Analysis of possible failure cases in the area
2 D hydraulic model (to identify exposed objects)
Damage analysis and assessment (to identify vulnerability of exposed objecs)
Impact assessment (ex ante)
Establishing a DTM
Generation of an incoming flood (hydrology)
Hydraulic model to calculate propagation of flood in the project area
Calculation of inundated area, water depth and flow velocity
Overlay with cadastre map
Identification of exposed objects
Classification of objects
Damage potentials
Buildings in a GIS
Representation of the scenarios
Assignment damage functions to classes Individual estimation of damages via interviews and local analysis
Damage estimation Building
Equipment
Creation of value losses (duration, €) Environmental hazards
Resultant effects Not monetary damages Damage estimate about combination with flood
depth of the scenarios Unity damages per
object (Method point
values)
representation of damages
Additional survey Industry, larger companies All buildings:
Reference values
Attributes of the object qualities, classification, point layer
Damages per area unit (Method area
values)
Attributes of the flood depths of the scenarios, post-processing
From Laserscan data to a Digital Terrain
Model (TDM) by mesh generation
Comparing a DTM with areal photos
Geländenetz (SMS) Orthophoto
Consideration of cross sections
is very helpful in generation the DTM
Application of a hydraulic model
Initial conditions: water depth and flow velocity at t=0 an every location
Boundary conditions: Inflow hydrograph
Model parameters: roughness coefficients for each element
Results from the hydraulic model
Water depth and flow velocity at each location (grid element)
Delineation of inundated areas and boundaries of inundation
Which scenarios (discharges) ? EU Flood risk directive
a frequent flood HQ30 a HQ100
an extreme event HQ300
Spatial distribution of water depth for a given
time slice (0,1-2m)
Exposed objects for HQ 30/100/300
Damage estimation
Classify objects (one family houses, multi familiy houses, frams, garages, companies, enterprises,
infrastructure…)
Estimate the value of the object and its vulnerability
Companies: need individual analysis
Damages
How to evaluate the objects
Typology of flood damages
(Messner et al. 2006, Penning-Rowsell et al. 2003, Smith and Ward 1998)
Measurement
Tangible Intangible
Form of damage
Direct
Physical damage to assets:
Buildings Contents Infrastructure
Loss of life Health effects
Loss of ecological goods
Indirect
Loss of industrial production Traffic disruption
Emergency costs
Inconvenience of post-flood recovery
Increased vulnerability of survivors
Property damages
Building, heating systems, electric and electronic infrastructure.
Vehicles
Goods, products, stock levels Operating equipments, EDP ...
Loss due to service interruption: losses in sales volume and profit Location disadvantages
Environmental consequences
Classification of damages of enterprises
Vulnerability of objects and uncertainty
On site inspections
Different set of loss functions are available (absolute or relative values)
Damage estimates are subjected to a large uncertainty
Example HOWAS database (Merz et al., 2004)
An example
Description of the area
Example
Flood area before implementation of
flood control structures Raab: Qmax = 200 m3/s Rabnitz: Qmax = 40 m3/s probability: ~1/100 p.a.
ZT Turk 1995 & 1997
Development
Land survey 1787
GIS Styria, http://www.gis.steiermark.at/07 -2005
Dykes
Flood reservoir
Reservoir outflow
Inflow to reservoir
Dykes
Flood protection project 97-99
Analysis of the Flood Series
Flood series Feldbach
0 50 100 150 200 250
1951 1956 1961 1966 1971 1976 1981 1986 1991 1996 2001 Annual maxima of the drain Q [ m 3 /s]
HQ
Trend straight
3 /s]
Analysis of the Flood Series
Flood series Feldbach
Flood series Takern
0 50 100 150 200 250
1951 1956 1961 1966 1971 1976 1981 1986 1991 1996 2001 Annual maxima of the drain Q [ m 3 /s]
HQ
Trend straight
0 20 40 60 80 100 120 140 160
1968 1974 1979 1984 1989 1994 1999 2004 Annual maxima of the drain Q [ m 3 /s]
HQ
Trend straight
Scenario 2
Flood areas, Depths
Raab: Qmax = 200 m3/s Rabnitz: Qmax = 40 m3/s probability: ~1/100 p.a.
Scenario 3
Existing flood protection Depth of inundation
log jam at the bridge
Scenario 4
inundation area and depth
Raab: Qmax = 245 m3/s Rabnitz: Qmax = 56 m3/s flood probability:~ 1/300
Scenario 5
Inundation area and depth
Raab: Qmax = 310 m3/s Rabnitz: Qmax = 82 m3/s flood probability: ~ 1/1000
Scenario 6
Inundation area and depth
Raab: Qmax = 400 m3/s Rabnitz: Qmax = 97 m3/s flood probability: ~ 1/5000
List of exposed objects
# of endangered objects
total
agricultural buildings residential buildings small scale enterprises garages
industrial firms
Damage potential
Method to BUWAL (1999) & BWG (2002)
Converted & discounted Austria p., 2004
Damages in €/building & damages in €/m2
Low Intensity h< 0,5 m Medium Intensity h> 0,5 m
Specific Damages Classification scheme
Single familiy houses Appartment buildings Small/med enterprises Industrial firms
Damage potential in industrial sector
Damage types
damages of property losses in production
Competition disadvantages subsequent damages
...
Analysis at the sites
Information (presentation and informative material) Contacting
common inspection at the company's premises Damage estimation.
Estimation of
damage potential
Questionnaire
1st what can happen?
2nd description in monetary units
Damage potential in the industrial sector
Results from interviews 10 companies responded
among them the 4 largest ones:
Management and insurance companies are interested
one company: internal mitigation measures
some of them have an insurance: property and losses in production
sensible topic (image losses when the companies vulnerability would be identified)
difficult to get reliable response from the comapnies
Risk management
Risk management compares different alternatives, quantifies them and ranks them
Assist in selecting a preferred alternative
Options for risk mitigation
Possible decisions refer to
Reducing damages Actions Ai to control D(Q):
• Revise building codes
• Harmonisation of risk maps with local/ regional development
• Early warning systems
• Raising awarness about risk exposure
• Avoid secondary damages
• Resettling people outside the flood plain (buying land..)
• Reduction of the uncertainty in D(Q,t)
Options for risk mitigation
Possible decisions refer to
Changing pdf Actions Ai to control f(Q):
• Increase natural retention capacity
• Consider surface and groundwater systems
• Reduction of the uncertainty in f(Q)
• Consideration of human interventions
• Consideration of sediment transport and discharge
Options for risk mitigation
Possible decisions refer to
Changing protection level Actions Ai to control X*:
•Increase the reliability of the resistance
•Temporary protection systems
•Dikes require spillways like dams to protect the dike from collapse and to
ensure a controlled flooding and drainage of the floodplain
Options for risk mitigation
Possible decisions refer to
Risk transfer Actions Ai to control R(X*):
• Insurance system vs catastrophic funds
• Clear seperation of responsibilities among individual and public authorities
• Risk zonation and individual responsibilities
Consideration of a dynamic environment
Nothing remains as it is
Flood frequency may change (Climate change, human impacts on the water cycle,..)
Land use changes and thus changes in teh damage potential and thge number of exposed people
Trends
Flood damages are increasing
Trends
Flood damages are increasing
Why ?
Are hazardous events more frequent ?
Trends
Flood damages are increasing
Why ?
Are hazardous events more frequent ?
Trend analyses do not reveal any evidence of an intensification of the hydrological cycle (GRDC
report No. 33, 2004)
There is a likelihood of increasing extremes of precipitation in some regions and thus in flood peaks (IPCC 2007)
What happens after building a levee ?
Land use will change
What happens after building a levee ?
Land use will change
More houses will be built
What happens after building a levee ?
Land use will change
More houses will be built
The value of properties increases
What happens after building a levee ?
Land use will change
More houses will be built
The value of properties increases
The damage potential increases
What happens after building a levee ?
Land use will change
More houses will be built
The value of properties increases
The damage potential increases f (Q)
Q
Potential D (Q)
Q X*
old new
Consequences
The expected damages may be larger after implementation of flood protection measures
Land management and development strategies are required
Safety of levees ?
Protective structures may fail already before the critical load is reached
Definition of the remaining risk
Design level for a dyke X* (resistance)
Remaining risk R (X*) because of exceedance of X*
Loss function flood probability
X* is the design value
*
) ( )
(
*) (
X
dQ Q
D Q
f X
R
Conclusions
Strategies are needed which are reasonable in the short and the mid term
Conclusions
Strategies are needed which are reasonable in the short and the mid term
+ communication of hazards
+ removing of highly vulnerable objects (hospitals, Kindergarden, chemical firms
Conclusions
Strategies are needed which are reasonable in the short and the mid term
+ communication of hazards
+ removing of highly vulnerable objects (hospitals, Kindergarden, chemical firms + Improving the reliability of systems
+ integration of spillways into dikes
+ restriction on land use in riverine areas
Conclusions
Strategies are needed which are reasonable in the short and the mid term
+ communication of hazards
+ removing of highly vulnerable objects (hospitals, Kindergarden, chemical firms + Improving the reliability of systems
+ integration of spillways into dikes
+ restriction on land use in riverine areas + improved forecasting systems
+ …..
