introduction to flood risk assessment & …introduction to flood risk assessment &...
TRANSCRIPT
Introduction to
Flood Risk Assessment &
ManagementBiswa Bhattacharya
Acknowledgement:
Flooding pictures of recent floods are taken from BBC World News website
Some pictures are taken from Prof Roland Price
Further reading:
FLOODsite (2009): Flood risk assessment and flood risk management: An introduction
and guidance based on experiences and findings of FLOODsite
APFM (2009): Integrated Flood Management, Concept paper, WMO Associated Programme
on Flood Management
FLOODsite (2009): Language of Risk
Flooding
Mississippi Flood 1993
Floods at other places
Teton Dam
China floods
Ganges Delta
Floods in central Europe 2013
Floods in central Europe 2013
Floods in central Europe 2013
Flood damages
Source: UN online publication, not dated
Aug 2006 flooding in Ethiopia, Awash.
Source: http://www.alertnet.org
13
Generation of floods & types
River floods normally are generated through prolonged and intense rainfall
– Monsoons, hurricanes, typhoons
Generation of flood: rainfall-runoff process
Propagation of flood water as a disturbance of the flow in open channels
Flooding types:
– Riverine/fluvial (floods large areas in lower catchments are
affected, usually slow rise, more on material damage)
– Pluvial floods (much influence on usually urban areas)
– Flash floods (localised, fast)
– Coastal floods (storm surges, huge area may be affected)
– …
Risk – Expenditure Cycle
No flood
Complacency
Reduced
expenditure
Increasing risk
Major flood
Damage,
distress, disruption,
disease, deaths
Review
Expenditure
Reduced risk
15
Management of flooding
The cause of the risk of flooding (rainfall) is often beyond control
The pathway of a flood can be managed to some extent
The impacted recipients (people and property) can be managed most
Flood management
RIBAMOD (EU) framework:
Pre-flood activities
Operational flood management
Post-flood activities
Managing floods
Post-flood measures
Flood event measures
Real time risk management
Pre-flood measures
Preventive risk management
Forecasting and warning, reservoir control, evacuation, rescue, etc.
Spatial planning, contingency plans, flood defence (mitigation)
measures, insurance, preparedness, etc.
Relief, clean-up, reconstruction,
regeneration, etc.
Pre-Flood Activities
Flood risk management for all causes of flooding
Disaster contingency planning (evacuation, etc.)
Construction of flood defence infrastructure
Forecasting and warning systems
Maintenance of flood defence infrastructure
Land-use planning and management within the whole catchments
Discouragement of inappropriate development within flood plains
Public communication and education of flood risk and actions to take in a flood emergency
Operational Flood Management
Detection of the likelihood of a flood forming (hydro-meteorology)
Forecasting of future river flow conditions from the hydro-meteorologicalobservations
Warning issued to the appropriate authorities and the public on the extent, severity and timing of the flood
Response to the emergency by the public and the authorities
Post-Flood Activities
Disaster relief, health and safety of people affected
Restructuring buildings, infrastructure and flood defence
Recover and regeneration of the environment and the economic
activities in the flooded area
Review of the flood management activities
Integrated flood management
Managing floods is very important; there are three ways
• Structural measures – embankments, diversions,
dams, barrages, etc.
• Non-structural measures – regulations, forecasting and
warning, flood insurance
• Do nothing – living with floods
A holistic approach to flood management is required,
which is called integrated flood management
WMO, through its Associated Programme on Flood
Management, among others, is pushing the IFM concept
Integrated flood management
IFM integrates land and water
resources development in a river
basin
IFM aims to maximize the net
benefit from floodplains
IFM aims to minimize loss to life
from flooding
IFM ensures a participatory
approach
Promotes decentralization of
decision-making Source APFM (WMO) website
Integrated flood management (2)
Global changes (population growth, urbanisation,
economic growth, climate changes) put pressure on
natural resources
Population growth and economic activities increase
the risk from flooding
Food security in developing countries: The degree of
vulnerability to such natural hazards is high in
developing countries where necessity tends to force
the poor to occupy the most vulnerable areas
Absolute safety from flooding is an illusion
Absolute protection from flooding is technically
infeasible and economically and environmentally
unviable
International initiatives
Associated Programme on Flood Management – an
initiative of WMO and GWP (www.apfm.info)
International Flood Initiative – an initiative of
UNESCO, WMO, UNU, International Strategy for
Disaster Reduction (http://www.ifi-home.info/)
EU project FLOODsite (www.floodsite.net, 2004-
2009), EU project RIBAMOD
Recent projects:
– KULTURisk
– WeSenseIt
– MyWater
– ……
Flooding risk originates from
Flooding risk changes due to
– Change in land use (urbanisation, deforestation, etc. which
are often driven by population growth or lack of policy on
spatial planning)
– Change in climate
– Risk mitigation measures
Flood risks are wholly a human or societal concern
rather than being an inherent characteristic of natural
system
No human, no risk
Use of floodplain
as a passage of
flood water
Use of floodplain
for human
activities
Conflict
Further reading: FLOODsite (2009). Language of risk
Presenting risk from flooding
Often the hazard-exposure-vulnerability
model is employed to represent risk
Hazard is defined as a threatening natural
event (with its probability/ magnitude of
occurrence)
Exposure is defined in terms of the
assets/humans that are present at the
location of the hazard
Vulnerability is defined in terms of the lack
of resistance to damaging/ destructive
forces
Risk is computed by combining hazard, exposure and
vulnerability
Risk increases with increase of any side of the risk triangle
Risk triangle (Kron, 2002)
Further reading: Kron, W. (2002). Food Risk = hazard x exposure x vulnerability. In B. Wu et
al. (Eds.) Proc. of 2nd Int. Conf. on Flood Defence, pp 82–97, Science Press New York Ltd.
Definition of risk
The risks from flooding can be computed as:
risk = hazard*exposure*vulnerability
Alternative definition of risk:
risk= probability * consequence
Risks may be due to a flood of high probability
(frequent flood, low magnitude) with not so severe
consequences
Risks may be due to a rare low probable flood with
very high consequences
Risk can be reduced by reducing any of the
constituent components
FRM basics
Primary objectives of flood risk management are:
To reduce flood risks at the current state of affair
– by adopting preventive and preparedness measures (structural
(dikes, etc.)
– by adopting non-structural (early warning systems, etc.))
To reduce further build up of risks in the future by adopting appropriate
measures (land use management, etc.)
To adapt to changes (CC, population growth, etc.)
FRM basics (2)
For efficient flood management practice we need:
knowledge of the present and probable future hazards and risks
This knowledge about the hazards and risks should include:
type of floods (fluvial/ coastal/ flash/pluvial/?)
the probability of a particular flood event
the flood magnitude
flood extent
water depth
flow velocity
consequences (on assets and people)
FRM basics (3) FRM aims at preventing/lowering damage and loss by
– preventing flooding
– lowering the exposure to flooding
Achieved by
– Lowering the flooding probability (thru’ a set of measures, e.g., an
embankment)
– Reducing vulnerability/ exposure (thru’ a set of measures, e.g.
flood insurance)
History:
– 1950s to 1970s: flood defence/ flood control: main efforts were on
engineering works
– Since 1980s: flood management: combination of engineering/
non-engineering but not considering entire societal perspective
With FRM: the main efforts are on analysing, mitigating and
managing risks to society
Risk assessment
Analyse the chain of causes and effects:
– rainfall or storms causing high water levels
– high water levels causing either a load on flood defences or the immediate flooding
of floodplains
– the load on the defences causing failure (breaching)
– Inundation of floodplain
– inundation drowning people or devastating property.
Risk assessment
Identify flooding types (fluvial/ coastal/ flash/pluvial/?)
Identify corresponding frequencies
Identify flooding extent, depth, velocities
Reliability of assets
Identify receptors (people and properties)
Assess damages
Estimate future changes
Assessment typically uses Source-Pathway-Receptor-
Consequence model
Source:Rainfall, snowmelt, storm surges and waves
Pathway:Bank failure, overland flow
Receptor:Property, people, possessions, environment
Consequence:Damages, distress, disease, death, degradation
Source-Pathway-Receptor-
Consequence model
Risk assessment: Frequency analysis
Flood frequency analysis
Requires past flood data
Provides flood magnitudes
with return period/
probability
Determining probability of
flooding in the protected
area is difficult
Risk assessment: Inundation modelling
Provides
information about
flood extent and
depth
corresponding to
a probability
May provide
information about
flood velocities
Identifying receptors: risk mapping
People: how many
people are affected?
What properties
(numbers, values) are
affected?
Often risk mapping is
used as a tool
Damage assessment
The assessment of flood damage in the connection with flood risk
management is a relatively new concept
In flood damage assessment we combine:
– Hydrological knowledge (~ flood frequency analysis >> hydrological
variables (e.g. discharge) with different frequencies are known)
– Inundation modelling (so that we know the extent and depth of flood
water corresponding to different flood frequencies)
Further reading: F. Messner, E. Penning-Rowsell, C. Green, V. Meyer, S. Tunstall, A. van der Veen (2007).
'Evaluating flood damages: guidance and recommendations on principles and methods' (FLOODsite report)
Climate change
European summer temperatures
observationsMedium-High emissions (modelled)
Rise in mean sea level
Source: Church, J.A. and White, N.J. (2006). A 20th century acceleration in global sea-level rise.
Geophysical Research Letter, 33
Relative change in runoff (%) of European rivers
during the twentieth century. Period 1971-1998
compared to 1900-1970. From Milly et al., Nature,
2005
Climate change (2)
Further reading: Milly et al., SCIENCE VOL 319 1 FEBRUARY 2008
• Climate is probably changing, consequently flood risk is also changing
• Risks from flooding in the future needs to be assessed
Modelling
Flood modelling is a key component of FRM and used in:
– Flood design: frequency analysis, regional flood frequency analysis, design flood
value, IDF charts
– Rainfall-runoff modelling
– Meteo forecasting
– Flood forecasting
– Inundation modelling
– Loss of life modelling
– Evacuation modelling
– Climate change studies
Flood forecasting and warning
Flood event management
Requires:
Data– Precipitation, ground
based– Water level– Discharge– Evaporation– Flood control structures’
operations– Asset conditions– Weather radar– Satellite based
information– GCM and LAM
Models GIS Responsible authorities Other gazettes
Communication
Forecasting is only useful if
there is adequate response
Communication is important
and the mode of
communication is location-
specific
Lydia Cumiskey (2013). Flood warning communication using mobile services in flash
flood communities of Bangladesh, MSc thesis, UNESCO-IHE
Source: Cumiskey (2013)
Source: Cumiskey (2013)
Uncertainty
“Uncertainty is a general concept that
reflects our lack of sureness about
something or someone, ranging from
just short of complete sureness to an
almost complete lack of conviction
about an outcome”
- National Research Council (US), 2000
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
0 10 20 30 40 50 60
Discharge m3/s
Sta
ge m
CES central estimate
Measured data
Credible upper / lower bands
Time
Pre
cip
ita
tion
Forecast
Now
Now
Fo
rec
as
tin
gM
od
el
Time
Pre
cip
ita
tion
Forecast
Now
Now
Fo
rec
as
tin
gM
od
el
~
Uncertainty in flood modelling
Uncertainty in stage-discharge
Uncertainty in flood frequency
“So far as laws of mathematics refer to reality, they are not certain; as far as they are certain, they do not refer to reality” Albert Einstein
• Uncertainty analysis is an essential
component of FRM
44
Studying uncertainty propagationy = M(x, p) + ε
s+ ε
θ+ ε
x+ ε
y
LZ
UZ
SM
RF
R
PERC
EA
Q=Q0+Q1Q1
Transform
function
SP
Q0
SF
CFLUX
IN
SF – Snow
RF – Rain
EA – Evapotranspiration
SP – Snow cover
IN – Infiltration
R – Recharge
SM – Soil moisture
CFLUX – Capillary transport
UZ – Storage in upper reservoir
PERC – Percolation
LZ – Storage in lower reservoir
Qo – Fast runoff component
Q1 – Slow runoff component
Q – Total runoff
LZ
UZ
SM
RFRF
RR
PERCPERC
EAEA
Q=Q0+Q1Q1Q1
Transform
function
SP
Q0Q0
SFSF
CFLUXCFLUX
ININ
SF – Snow
RF – Rain
EA – Evapotranspiration
SP – Snow cover
IN – Infiltration
R – Recharge
SM – Soil moisture
CFLUX – Capillary transport
UZ – Storage in upper reservoir
PERC – Percolation
LZ – Storage in lower reservoir
Qo – Fast runoff component
Q1 – Slow runoff component
Q – Total runoff
Research projects in hydroinformaticsSlide: Dimitri Solomatine
Reducing risk
Risks can be reduced either by
– reducing the flood probability
– reducing the consequences
The best solution is location-specific (also depends
on stakeholders)
Risk mitigation measures:
– interventions> structural and non-structural
– policy instruments (e.g. spatial planning)
Effectiveness of measures can be assessed as:
– Reduction of flood probability
– Reduction of flood extent (may include depth, velocity, etc.)
– Reduction in number of fatalities
– Number of people affected
– Expected annual damage
Risk mitigation measures
Managing residual risks
What happens if flooding occurs?
Flood event management / flood incident management: main goal is
prevent loss of life and damage if flooding occurs.
Consists of:
– flood forecasting and warning
– Operational flood control measures
– Evacuation of people and livestock
– Recovery
Thanks for your attention