PROGRAM
First COMRISK Pilot Study Workshop
Start-up meeting with focus on coastal flooding risk assessment
February 25th & 26th 2003
Ostend-Belgium
AIM :
Give a forum to discuss amongst COMRISK-partners and selected
others (scientists, other persons of our organisations who are
interested…). Give an overview of experience with coastal flooding
risk assessment. Introduction to the Flanders case.
PROGRAM : The workshop will comprise two 1/2 days of presentations and
discussion
 COMRISK - FLANDERS Session 1 : February 25th 13:00 - 14:30
- Introduction to COMRISK, Matthias Hamann 0:30
- Introduction of the Flanders case, Toon Verwaest(AWK)
0:30
- The sea defence structures in Flanders, prof. Juilen De
Rouck (Univ. Ghent) 0:30
 RISK ASSESSMENT EXPERIENCE Session 2 : February 25th 15:0017:30
- Failure behaviour of sea dikes and breach growth, dr.
Andreas Kortenhaus, Leichtweiss Institute 0:45
- discussion 0:15
- Approach to risk assessment in the Netherlands – Babette
Lassing & Mark Wehrung (DWW) 1:00
- discussion 0:30

RISK ASSESSMENT EXPERIENCE Session 3 : February 26th 8:30 10:00
- Approaches to risk assessment in the UK (including RASP),
Ian Meadowcroft (EA) and Paul Sayers (HR W) 1:00
- discussion 0:30

SPECIFIC TOPICS Session 4 : February 26th 10:30-12:15
- the Flemish methodology for assessing damage & casualties
Wouter Vanneuville, (Flanders Hydraulics) 0:45
- (general) discussion 1:00
LOCATION : The workshop will take place at AWK in Ostend (Vrijhavenstraat
3), which is on walking distance (10’) from the railway station.
The airport of Zaventem is connected by train to Brussels from
where a direct connection with Ostend exists. Travel time from
the airport of Zaventem to Ostend is estimated at 2h.
BACKGROUND : The workshop is organised as part of the COMRISK project
"Common strategies to reduce the risk of storm floods in coastal
lowlands". For more information see www.comrisk.org
ORGANISATION : The workshop is organised by the Belgian COMRISK partner,
which is the Coastal Waterways Division of the Waterways and
Maritime Affairs Administration of the Ministry of the Flemish
Community.
Comrisk Abstracts
Introduction of the Flanders case, Toon Verwaest(AWK)
The presentation will give some characteristics of the Flanders case and the questions arising for the risk
calculation.
Most important characteristics for a lowland are the defence structure (dunes, dikes, …), the topography, river
dikes & railways (possible secondary defence structures), …
In the process of risk assessment a lot of questions arise:
- what are the characteristics of the first defence structure – what are the failure mechanisms
- what is the probability that a section fails
- how to combine failure probabilities of different structures
- what happens when the structure fails : breach growth
- what kind of inundation model is necessary (1D with reservoirs and spills, 2D, 3D?)
- which scenarios should be modeled (eg calculating for different return periods the places where the
structure fails, and using these as sources, using the sections with the highest failure probability as
source (but maybe without knowing the corresponding water level , … (One can calculate the
probability of failure, searching the most likely water level/load corresponding to this failure and
than calculating the damage. Risk is the product of damage and probability. Or one can calculate
the probability of failure for different water levels, and for each case calculating the corresponding
damage. The probability of that damage is than the probability of the waterlevel multiplied by the
probability of failure given the waterlevel. Risk is than the integration of the product of probability
and damage over all water levels.
- how can inundation being linked with damage/casualties
- how can one calculate risk ?
Relevant project going on in Flanders and the Netherlands will be discussed together with a first evaluation of
the Flanders case.
The sea defense structures in Flanders, prof. De Rouck (Univ. Ghent)
Prof De Rouck will share with us his rich experience on the Flemish coastal dikes. He was also involved in
some measuring campaigns (e.g. on dike stability and wave run up)
Failure behaviour of sea dikes and breach growth, dr. Kortenhaus
(Leichtweiss Institute)
The number of natural disasters has increased significantly over the last decades so that questions on the
quantification of safety of coastal defences are getting increasingly important. Probabilistic methods can provide
answers to these questions. The main objective of this presentation is to provide probabilistic tools for sea dikes
on the basis of existing, newly developed or improved limit state equations including breaching of dikes.
Interactions between limit state equations are also considered. The proposed tools are exemplarily applied to a
set of typical North Sea dikes.
Using 25 failure modes an overall failure probability is achieved which is about one order of magnitude lower
than a failure probability calculated on the basis of simple fault trees with a significant lower number of failure
modes. The erosion processes at the seaward side of the dike represent the most important failure modes when
modern dike constructions and crest levels are considered.
The uncertainties related to the water level in front of the dike represent the most important influence on the
overall failure probability of the dike whereas most of the uncertainties of the limit state equations are of minor
relative importance.
Probabilistic methods help to quantify the remaining safety of coastal sea defences. On the other side the results
have shown that many assumptions regarding uncertainties and limit state equations are still necessary.
Requirements for further research are therefore concluded from the results. Overall, the study provides a basis
for embarking into risk analysis procedures for coastal sea defences.
Approach to risk assessment in the Netherlands – B.Lassing & M.Wehrung
(DWW)
In the Netherlands several case studies for risk assessment are going on or are completed.
In these studies the failure probability is calculated with PC-ring. Different failure mechanisms are incorporated
(dune erosion, stability of dikes, piping, delay in closing construction,…). With the FORM/SORM method
failure probabilities are obtained. For each ring ‘parts’ are defined. A part of the ring is a collection of sections,
which all have the same effect on inundation of the ring. For each part the total probability of failure is
calculated together with the most likely corresponding water level. Next scenarios are defined (eg failing of part
1, failing of part 3, combined failing of part 1 and 3,…) For each scenario the probability of occurrence is
known and an inundation model is applied). At the point of failure a breach growth model is used. When for
each scenario the inundation areas are known also the damage and casualties can be calculated. Integration of
this damage x probability of occurrence gives the risks.
During the presentation the methodology will explained in more detail together with a special focus on:
- how is the interdependency of the failure probability of sections being accounted for ? (e.g. if an
extreme water level occurs for a given section, probably the water level is also very high for the
other sections ?
- how are failure probabilities combined (combination of failure mechanisms/combination of events/
how are parts/scenarios defined ?
- how are uncertainties being accounted for (uncertainties on structural parameters/uncertainties on
extreme value distributions of water level ?
At the end a case study will be presented.
Approaches to risk assessment in the UK (including RASP), Ian
Meadowcroft (EA)
See attachment
The Flemish methodology for assessing damage & casualties Wouter
Vanneuville, (Flanders Hydraulics)
When inundation maps are available the expected damage and number of casualties can be calculated. The main
input for these calculations are GIS-data of land use and relations between water depth and damage for each type
of land use.
Different types of land use are distinguished:
- buildings
- agriculture
- population
- industry
With these data the value /m2 can be calculated (which requires a lot of assumptions)
In the next step the loss of value in relation to the water level is investigated.
Combining these data gives the complete damage and number of casualties.
All these steps will be presented, including the assumptions that had to be made
First COMRISK Pilot Study Workshop
February 25th and 26th 2003, Ostend, Belgium
Approaches to Risk Assessment in the UK (Including RASP)
Ian Meadowcroft, Environment Agency
Paul Sayers, HR Wallingford
Frameworks for Risk Assessment and Management
The language is changing, from Flood and coastal defence, to flood and coastal risk management, and
this reflects a real shift in the way policy is applied. In particular, risk management risk focuses
attention on outcomes - such as economic damage and harm to people, and risk reduction aims to
reduce the likelihood and consequences of harmful outcomes.
In the UK, the Government is promoting best practice in handling risk among government
departments and Agencies, and the following key principles are likely to be promoted:
Proposed Principles of managing risks to the public (Summarised from
Strategy Unit Consultation Document http://www.strategy.gov.uk/2002/risk/risk/home.html )
Government will be open and transparent about its understanding of the
nature of risks to the public and about the process it is following
Government will seek wide involvement of those affected by risks in the
decision process
Government will act proportionately in dealing with risks to the public, and
will take a precautionary approach where necessary
Government will seek to base decisions on all relevant evidence
Government will seek to allocate responsibility for managing risks to those
best placed to control them
A well-developed approach for environmental risk assessment and management has been established1.
This includes the standard process of hazard identification and assessing magnitude and probability of
consequences. It also stresses the importance of:
 social aspects of risk (including risk perceptions, trust and credibility, risk communication and
stakeholder participation)
 risk screening and prioritisation (eg scoring and ranking and prioritisation of effort)
 dealing with Uncertainty (weighing up a range of possible outcomes, communicating uncertainty)
 monitoring and review (post project appraisal)
Approaches to flood risk have been developed within this general framework2,3 In flood and coastal
defence we have adopted the ‘Source-Pathway-Receptor’ conceptual model to ensure that risk
management takes account of the whole flooding system eg
Source - eg extreme river flows, coastal water levels, wave heights, boundary conditions
Pathway - eg defence reliability and performance, flood inundation
1
Guidelines for Environmental Risk Assessment and Management. DETR / Environment Agency.
The Stationery Office, July 2000
2
Risk, Performance and Uncertainty in Flood and Coastal Defence - A Defining Review. I Meadowcroft et al,
in 37th Defra Flood and Coastal Management Conference 2002.
3
Risk Performance and Uncertainty in Flood and Coastal Defence - A Review. Report FD2302/TR1, in the
‘risk’ section of www.environment-agency.gov.uk/floodresearch
Receptor - eg infrastructure, buildings, people
Consequences - eg the degree of harm or damage.
Flood risk management activities
Flood management activities by the Agency and other operating authorities include:
 Managing the magnitude / frequency of the source (eg climate change mitigation, catchment
management for source control)
 Managing reliability and performance of barriers to potential pathways (eg improving and
maintaining defences)
 Managing the consequences to receptors (development control, flood forecasting and warning,
raising awareness)
All of these activities can reduce risk. Some have the greatest impact on the probability of flooding,
others affect mainly the consequences or impacts. Quantifying risk, and understanding the degree of
risk reduction associated with each measure enables the best combination of risk management
measures to be developed in each case. This requires an integrated approach which takes account of
the whole flood system – for example the planning of a flood warning service may depend on the
presence and performance of flood defence structures.
Challenges
Along with the promise of improvement, there are challenges. These include:

Defining a consistent language and concepts. Even the simple terms such as 'risk', 'performance'
and 'uncertainty' are used in a variety of ways.. A defining Review has recently been carried out
which includes a glossary of terms, outlines the main tools and techniques available for carrying
out risk assessment, including risk-based decision-making under uncertainty.

In our experience it is particular difficult to communicate the difference between ‘collective’
(also known as societal or group risk) and ‘individual’ risk. Collective risk is given in terms
such as ‘annual average damage’ whereas individual risk is in terms of probability of a certain
degree of harm. In a particular area there may be a high collective risk but low individual risk and
this needs very careful communication.

Risk calculation is often constrained by data availability and limitations on the level of analysis.
Large scale ‘national’ assessments must make use of simplified approaches. Individual site
specific assessments can take advantage of more intensive study and data collection. These
different levels or ‘tiers’ should be carefully nested to ensure consistency.

There is a lack of information on how structures perform under a range of loads. Reliability
analysis provides the framework but further development is needed to establish, for example the
effect of condition and deterioration on performance (and hence on risk)

Different decision-makers require specific information. For example, an asset manager needs
to know the impact of different performance management activities on the risk - in other words,
how should money be spent to gain the most ‘risk reduction’. Activities include inspection,
maintenance, repair or replacement. Crucial information therefore relates to the levels of risk
under the range of asset management activities.

Risk assessment and management requires data and modelling. Large scale planning such as
Catchment Flood Management Plans and Shoreline Management Plans should be incorporate best
practice in their approach to risk assessment (see for example www.mdsf.co.uk for information on
the Management and Decision Support Framework which supports the production of CFMPs)
Figure. Assessment of “damage / probability” function
Flood loading (source – shown as a probability distribution) acts on pathway (e.g. flood embankment) to
generate a flood probability. This in turn generates a “damage / probability” curve.
RASP R&D Project and applications
In practice, risk assessment requires the development and application of improved tools to support
decision-makers. A major issue, which is the subject of ongoing research, is assessing the
performance of defence systems and the associated risk of failure. Traditional approaches assume a
simple ‘one to one’ mapping between a defence and a flood area. New approaches within the RASP
(Risk Assessment for Strategic Planning, www.rasp-project.net) project are being developed which are
better able to how risk varies across a flood plain, depending on performance of individual defences
and groups of defences. This research has included new approaches to characterise the ‘performance’
of a defence at loads above and below crest level, with the use of ‘fragility curves’. RASP uses a
‘tiered’ approach - at the ‘high’ level4, approximate methods are used to process national data and this
method has been used for a national assessment of flood risk (annual average damage). At the detailed
level, we are evaluating the application of PC-Ring to sites in England and Wales.
The government is taking a long term view of flood risk in the Foresight project
(www.foresight.gov.uk). This is addressing the questions:
 How might changes in land use, demographics, and climate affect river and coastal flooding over
the next 30-100 years?
 How should the Government and business respond to those challenges?
A risk-based approach is being adopted, including the use of RASP to estimate future risk under a
range of future scenarios.
Lincshore
As well as discussing these issues, the paper will also present a summary of the ‘Lincshore’ risk
assessment study (SP8) including data requirements, the proposed methodology, and a discussion of
issues to be resolved.
4
Risk Assessment of Flood and Coastal Defence Systems for Strategic Planning (RASP) - A High Level
Methodology. P Sayers et al, in 37th Defra Flood and Coastal Management Conference 2002.