Flood risk assessment and management: main concepts and tools

1. Flood risk assessment and management: main
concepts and tools
15 December 2017
Course in “Risk-based design”, Master of Science in Building Architecture
Daniela Molinari, DICA, Politecnico di Milano (daniela.Molinari@polimi.it)

2. 2Outline
•Flood risk in numbers
•What is flood risk?
•Flood risk assessment
•Why evaluating flood risk?
•Risk mitigation
•Spatial planning and building codes

3. Flood risk…an evergreen topic 3
19 Novembre 2013

4. Source: EMDAT
The “weight” of flood risk:
a world view

5. Source: EMDAT
The “weight” of flood risk:
a world view

6. Source: EMDAT
The “weight” of flood risk:
a world view

7. Source: EMDAT
Overview of Natural Disasters in Europe over the last 30 years (1980-2008):
occurrence, economic damage and affected people
The “weight” of flood risk:
the European view

8. Source: EMDAT
Overview of Natural Disasters in Italy over the last 30 years (1980-2010):
occurrence, economic damage and affected people
The “weight” of flood risk:
the Italian view

9. Source: ISPRA
70 % of Italian Municipalities are prone to
hydrogeological risk
Cinque Terre, 2011
Piemonte, 1994
Sarno, 1998
The “weight” of hydrogeological risk:
the Italian view
Sardegna, 2013

10. The “weight” of flood risk:
the Italian view
TOT: 16.878.058 inhabitants
(28.4% of Italian population)

11. Floods: what does it mean? 11
A flood is: “A general and temporary condition of partial or complete inundation of
normally dry land areas”
1) Flooding occurs most commonly from heavy rainfall, when soil is saturated. In such
conditions, soil can not absorb all the rain which is converted into surface runoff. When it
reaches water bodies (rivers, lakes, channels, pipes, etc.), runoff is converted into water
discharge/volume. This process is named rainfall runoff-process.
- Natural floods (natural watercourses/reservoirs do not have the capacity to convey excess
water)
- Urban floods (because of insufficient drainage systems)
The water balance
2) Flooding can result from other phenomenon,
particularly in coastal areas where inundation can
be caused by a storm surge associated with a
tropical cyclone, a tsunami or a high tide
- Coastal floods
3) Dam failures can result in flooding of the
downstream area

12. Riverine and Flash floods 12
Riverine floods: Water rises over time due to
prolonged rain in region or in response to snow
melt from above average winter storms. They
develop slowly, sometimes over a period of days
- Big catchments / floodplain
- Slow velocity / low debris load
Flash floods: Fast response to severe storms or
dam failure (too much water in very short
duration and little space). They develop quickly,
usually less than 6 hours.
- Small catchments / mountain
- High velocity / high debris load

13. 13What is risk?
H= Hazard: intrinsic characteristics of the natural phenomenon
E= Exposure: items potentially at risk
V= Vulnerability: items propensity to be damaged (i.e. fragility of systems)
FLOOD RISK ASSESSMENT REQUIRES THE ESTIMATION OF ALL RISK’s
COMPONENTS
RISK meaning the expected number of lives lost, persons injured, damage to property
and disruption of economic activity due to a particular natural phenomenon (e.g. floods)
)(),,( pDdpVEHfR ∫ ⋅==

14. 14
There is no a common agreement among terms like damages, losses, impacts
“Injury, harm; esp. physical injury to a thing”
“The sum of money claimed or adjudged to be paid in compensation for loss
or injury sustained”
The interest lies in all the harmful effects of a flood on a community (i.e.exposed items):
 impacts on humans
 impacts on humans’ health and belongings
 impacts on public infrastructures and costs to face the emergency
 impacts cultural heritage and ecological systems
 impacts on industrial production and the economy
Quantifying risk: damage models
glossary

15. 15Quantifying risk: damage models
Kinds of damages
DIRECT  losses resulting from direct contact with the hazard (e.g. flood
damage to building)
INDIRECT  losses are those resulting from the event but not from its direct
impact (e.g. business losses due to activity disruption)
TANGIBLE  losses concern things with a monetary value (e.g. buildings,
livestock, etc.)
INTANGIBLE  losses regard things that cannot be bought and sold (such as
lives, heritage and environmental items, memorabilia, etc.)

16. Quantifying risk: damage models
Current state of the art
16
(i) direct damages are usually present in any damage assessment
(ii) indirect losses are often roughly estimated
(iii) intangibles are frequently ignored or simply mentioned, without any attempt of
evaluation.
TYPE MODELING APPROACH
DAMAGE
direct indirect intangible
EXPLICIT
AVERAGING APPROACH:
mean unit values (e.g. average loss per flooded
dwelling, average loss per km of inundated road,
loss of value added, etc.)
x
FUNCTIONS APPROACH:
relative or absolute hazard-loss (typically depth-
damage) functions
x
SURVEYS:
field surveys of event impacts x x
INDIRECT
PERCENTAGES:
fixed or variable (e.g. as a function of warning time,
depth of flooding) ratios of potential/direct damages
x x
ADHOC
FROM OTHER DISCIPLINES OR
EXPERIMENTAL:
Surrogate values, Opportunity Cost, Human Capital
Approach, Hedonic price, Contingent valuation,
Replacement costs, etc.
x x

17. Quantifying risk: damage models
Current state of the art (direct damage)
17
(i) Damage to residential sector is the most investigated, along with agriculture
(ii) Few (simple) models for damage to people, industrial & commercial sector, roads
(iii) Local (ex-post) studies for the other sector
costi PC

18. averaging methods: an average loss per flooded unit is supplied
e.g. RAM - Australia
stage-damage curves (otherwise called “depth-damage” curves or stage-damage
“functions”): model of the relationship between the expected loss in the unit and the
varying depth of the flood water
Quantifying risk: damage models
direct damage
18
0,000
0,200
0,400
0,600
0,800
1,000
1,200
0 2 4 6 8 10 12 14
damage(-)
depth (m)
STANDARD METHOD
damage to buildings (content + structure)
low rise
single and farm
intermediate
high rise

19. 1919
Molinari D.
Depth-damage curves are the
standard tool to estimate direct
damage to buildings
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4 5 6
damage(%)
water depth (m)
DAMAGE TO BUILDING STRUCTURE
(Source: USACE)
one storey - basement
more storeys - basement
one storey - no basement
more storeys - no basement
R = f ( H , E , V )
Quantifying risk: damage models
Depth-damage curves
DAMAGE TO BUILDING (structure + contents)
(Source USACE)

20. 2020
Molinari D.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4 5 6
damage(%)
water depth (m)
DAMAGE TO BUILDING STRUCTURE
(Source: USACE)
one storey - basement
more storeys - basement
one storey - no basement
more storeys - no basement
R = f ( H , E , V )
Depth-damage curves are the
standard tool to estimate direct
damage to buildings
Quantifying risk: damage models
Depth-damage curves
DAMAGE TO BUILDING (structure + contents)
(Source USACE)

21. 2121
Molinari D.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4 5 6
damage(%)
water depth (m)
DAMAGE TO BUILDING STRUCTURE
(Source: USACE)
one storey - basement
more storeys - basement
one storey - no basement
more storeys - no basement
R = f ( H , E , V )
Depth-damage curves are the
standard tool to estimate direct
damage to buildings
Quantifying risk: damage models
Depth-damage curves
DAMAGE TO BUILDING (structure + contents)
(Source USACE)

22. 2222
Molinari D.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4 5 6
damage(%)
water depth (m)
DAMAGE TO BUILDING STRUCTURE
(Source: USACE)
one storey - basement
more storeys - basement
one storey - no basement
more storeys - no basement
R = f ( H , E , V )
Depth-damage curves are the
standard tool to estimate direct
damage to buildings
Quantifying risk: damage models
Current state of the art
DAMAGE TO BUILDING (structure + contents)
(Source USACE)

23. 0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 1 2 3 4 5 6
damage(%)
water depth (m)
DAMAGE TO BUILDING STRUCTURE
(Source: USACE)
one storey - basement
more storeys - basement
one storey - no basement
more storeys - no basement
2323
Molinari D.
R = f ( H , E , V )
Depth-damage curves are the
standard tool to estimate direct
damage to buildings
Quantifying risk: damage models
Depth-damage curves

24. An example: hazard assessment
Valmalenco
Hydraulic modelling  flooded area + hazard variables of inerest
(water depths, velocities, etc.)

25. An example: exposure assessment
• People
• Residential buildings
• Economic activities
• Infrastructures
• Environmental & Cultural
heritage
• Strategic buildings

26. Vulnerability factors for buildings
Building structure
(e.g.. wood,
concrete, masonry),
Number of
floors/presence of
basement
Yera of
costruction/level of
maintainance
Use
An example: vulnerability assessment

27. N.B. Risk is due to the combination of different damage scenarios
Damage model  hazard, exposure and
vulneability
An example: damage assessment

28.  percentages of direct damages
 surrogate values (e.g. the cost of renting an equivalent home)
 ad-hoc methods grounded on economics (e.g. loss of “value-added”, opportunity
cost, etc.) as well as other scientific disciplines (e.g. the origin-destination matrix for
the evaluation of road disruption costs)
 detailed field surveys
28
IMPLICIT  infer indirect damages from the knowledge of direct ones
Quantifying risk: damage models
indirect damage

29. 29
MAIN DIFFICULTIES:
ethical objections 
availability of data 
How can we prize a life or an historical monument?
How can we value a worsening in the landscape?
the few existing data usually refer only to the
number of injured (or dead) people with the
problem of gaining information for the modelling
of other types of intangibles
FEW EXPERIMENTAL METHODS FOR CERTAIN CATEGORIES OF LOSS
Quantifying risk: damage models
intangible damage

30. 3030
Molinari et al.
Jongman et al. (2013), Comparative flood
damage model assessment: towards a
European approach,Nat. Hazards Earth Syst.
Sci., 12, 3733–3752, 2012
Uncertainty in damage estimation

31. Dealing with damage variability 31
Damage depends on both hazard and vulnerability factors
assessment procedures have historically focused on a small
number of explanatory variables
(i.e. the depth of flooding and few vulnerability features)
flood damage assessments are currently associated with large
uncertainties just because these few variables are not able to
describe the variability of damage data

32. Dealing with damage variability 32
André et al. (2013): Contribution of insurance data to cost assessment of coastal flood
damage to residential buildings: insights gained from Johanna (2008) and Xynthia (2010)
storm events, Nat. Hazards Earth Syst. Sci., 13, 2003-2012

33. Dealing with damage variability 33
Scorzini A. (2014), Analisi e Gestione del Rischio Idraulico: valutazioni economiche a
supporto della pianificazione di bacino, Tesi di Dottorato – Univesrità degli Studi dell’Aquila

34. 34Why estimating risk?
1. To define long term risk mitigation strategies  on
the base of cost-benefit analyses
2. To define emergency management strategies  on
the base of priority for intervention
3. To support (private/public) fund
allocation/compensation
4. To define priority for intervention in the emergency
phase
5. To learn from past events (i.e. understand risk
drivers)
EX-ANTE
EX-POST

35. Short term (e.g. EWS,
emergency plans)
Long term (e.g.
spatial planning)
Hazard (e.g.
banks, dams)
Exposure (e.g.
spatial planning)
Vulnerability (e.g.
building codes,
insurance)
Structural (e.g. banks,
buildings features)
Non structural (e.g. spatial
planning, communication
A good risk reduction strategy should foresee a mix of all the above
Risk reduction strategies
Temporal scale
Components
Typology

36. The European Directive 2007/60/CE:
the “Floods Directive”
36
Purpose (Art 1):
to establish a framework for the assessment and management of flood risk, aiming at
the reduction of the adverse consequences for
 human health
 the environment
 cultural heritage and
 economic activity
associated with floods in the Community

37. The European Directive 2007/60/CE:
3 steps process
37
22 Dec 2011
Preliminary flood risk
assessment
22 Dec 2013
Flood hazard maps
and flood risk maps
22 Dec 2015
Flood risk
management plans
Revision by
22 Dec 2018, 2019 and
2021(respectively) and
every six years
thereafter

38. The European Directive 2007/60/CE:
3 steps process
38
22 Dec 2011
Preliminary flood risk
assessment
22 Dec 2013
Flood hazard maps
and flood risk maps
22 Dec 2015
Flood risk
management plans
FLOOD RISK MANAGEMENT
PLANS:
SHALL ADDRESS ALL ASPECTS
OF FLOOD RISK MANAGEMENT
MUST BE BASED ON FLOOD
HAZARD MAPS AND FLOOD RISK
MAPS

39. The European Directive 2007/60/CE:
3 steps process
39
22 Dec 2011
Preliminary flood risk
assessment
22 Dec 2013
Flood hazard maps
and flood risk maps
22 Dec 2015
Flood risk
management plans
FLOOD RISK MAPS:
SHALL SHOW ALL THE
POTENTIAL ADVERSE
CONSEQUENCES ASSOCIATED
WITH FLOOD SCENARIOS

40. The European Directive 2007/60/CE:
competent authorities
40
22 Dec 2011
Preliminary flood risk
assessment
22 Dec 2013
Flood hazard maps and
flood risk maps
22 Dec 2015
Flood risk
management plans
Competent authorities:
River basin district authorities

41. The European Directive 2007/60/CE:
competent authorities
41
22 Dec 2011
Preliminary flood risk
assessment
22 Dec 2013
Flood hazard maps and
flood risk maps
22 Dec 2015
Flood risk
management plans
Competent authorities:
River basin district authorities
Water Framework Directive
“River basin district" means the area of
land and sea, made up of one or more
neighboring river basins together with their
associated groundwaters and coastal
waters, which is identified as the main unit
for management of river basins.

42. Short term (e.g. EWS,
emergency plans)
Long term (e.g.
spatial planning)
Hazard (e.g.
banks, dams)
Exposure (e.g.
spatial planning)
Vulnerability (e.g.
building codes,
insurance)
Structural (e.g. banks,
buildings features)
Non structural (e.g. spatial
planning, communication
The role of spatial planning
Temporal scale
Components
Typology

43. Italian regulation on spatial planning:
historical pathway
43
Law 183/1989 % “Sarno” Law (1998)
 River Basin e PAI
DLgs. 152/2006
 River Basin Districts
Floods Directive (2007)
Water Framework Directive (2000)
DLgs 49/2010

44. Italian regulation on spatial planning:
historical pathway
44
Law 183/1989 & “Sarno” Law (1998)
 River Basin e PAI
Floods Directive (2007)
Valtellina flood, 1987
Sarno flood, 1998
Elbe flood, 2002

45. Italian regulation on spatial planning: :
the law 183/1989
45
Purpose (Art.1): to ensure
• hydrogeological risk prevention
• water quality restoration
• use and management of water resources
It is a legislative framework of unique importance,
although it mixes water restoration and risk prevention objectives

46. Italian regulation on spatial planning: :
the law 183/1989
46
Competent authorities (Art. 12)
It creates the river basin authorities (national, interregional and regional) as
competent authorities
Its strength consists in focusing on the river basin scale
instead of administrative boundaries.
sezione di chiusura
bacino idrograficoRiver basin

47. The European Directive 2007/60/CE:
Italian river basin districts
47

48. Italian regulation on spatial planning: :
the law 183/1989
48
Tools (Art. 17)
It Introduces the river basin plan, a “territorial” plan with rules and restrictions to be
implemented at the local level
sezione di chiusura
The law allows to proceed by acting on different basin priorities one at a time,
by means of the so called “piani stralcio” (thematic plans).

49. The River Po basin:
Piano stralcio delle fasce fluviali – River zones plan (1998)
49
Goals:
• Classifying flood plain areas on the basis of the flood risk (river zones)
• Setting development rules for each zone

50. The River Po basin:
Piano stralcio delle fasce fluviali – River zones plan (1998)
50
Zone A:
Portion of the river bed usually housing of
the reference flood water discharge
Zone B
External to the A zone, it corresponds to
the areas usually flooded in case of the
reference flood; it generally coincide with
area inside secondary levees system
Zone C:
External to the B zone, it corresponds to
the areas usually flooded on occasion of
catastrophic floods, more severe than the
reference one
River zones

51. The River Po basin:
Piano stralcio delle fasce fluviali – River zones plan (1998)
51
Zone A:
Portion of the river bed usually housing of
the reference flood water discharge
Zone B
External to the A zone, it corresponds to
the areas usually flooded in case of the
reference flood; it generally coincide with
area inside secondary levees system
Zone C:
External to the B zone, it corresponds to
the areas usually flooded on occasion of
catastrophic floods, more severe than the
reference one
River zones
Actually, river zones are identified on the basis of the only flood hazard.

52. The River Po basin:
Piano stralcio delle fasce fluviali – River zones plan (1998)
52
Zone A:
• New building and farming not permitted,
only: renaturalization
• Relocation incentives
Zone B
• New building not permitted, only:
temporal activities (storage),
renaturalization
• relocation incentives;
Zone C:
• Compulsory contingency plans;
• structural measures incentives
(demanded to local planning decisions).
It actually consists in defining areas where
there is an “acceptable” risk
Development rules

53. 53The River Po basin:
PAI– hydrogeological assessment plan (2001)
Goal:
improving river zones plan
Reduce hydro geological risk
News:
It considers both landslides and floods
Zones are defined according to risk and not only to hazard
It considers both structural and non structural prevention measures

54. The River Po basin:
PAI– hydrogeological assessment plan (2001)
54
Risk Atlas
R1
R2
R3
R4
Damage to objects
Damage to people

55. Short term (e.g. EWS,
emergency plans)
Long term (e.g.
spatial planning)
Hazard (e.g.
banks, dams)
Exposure (e.g.
spatial planning)
Vulnerability (e.g.
building codes,
insurance)
Structural (e.g. banks,
buildings features)
Non structural (e.g. spatial
planning, communication
The role of building codes
Temporal scale
Components
Typology

56. The role of building codes: waterproof buildings 56