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