Flooding risk in coastal areas

Road and Hydraulic Engineering Division

Flooding risk in coastal

areas

Risks, safety levels and probabilistic techniques in

five countries along the North Sea coast

December 2000

Richard Jorissen Judith Litjens - van Loon Anabel Méndez Lorenzo

Road and Hydraulic Engineering Division

Table of Contents

1 Introduction

4

2 Scanning the horizon

6

2.1 Inventory of flood defence policies 6

2.2 Focusing on some policy aspects 10

2.3 Developments and research activities 12

2.3.1 Developments 12

2.3.2 Research activities 13

3 Probing the depth

16

3.1 Pettemer Zeewering (The Netherlands) 16

3.1.1 Safety standards and crest levels 17

3.1.2 Design procedures and safety margins 18

3.2 Fremskudt Dige (Denmark) 20

3.2.1 Safety standards and crest levels 21

3.2.2 Design procedures and safety margins 22

3.3 Summary and analysis 23

4 Conclusions

25

5 Recommendations

27

Appendices

1 Members of North Sea Coastal Managers Group

2 Belgium

3 Denmark

4 Germany

5 United Kingdom

1 Introduction

1.1 Background

The countries along the North Sea coast enjoy both the advantages and disadvantages of this shared neighbour. All countries face the threat of coastal floods to some extent, although the potential consequences of a flooding disaster varies significantly. Each country has developed a system of flood protection measures according to the nature of the threat, potential damages, and its historical, social, political and cultural background. These measures may range from coastal zone planning to evacuation in emergency situations. In all cases, however, construction and maintenance of flood defence structures is the core of these measures.

At first glance, the safety offered by flood defence structures seems to vary quite a lot in the different countries. Safety levels are generally expressed as return periods of extreme water levels that the flood protection structure must be able to withstand. In the United Kingdom, no safety levels are prescribed. Indicative safety levels range from less than 200 years to 1,000 years. In The Netherlands, on the other hand, the legally prescribed safety standards range from 2,000 to 10,000 years.

The return period of an extreme water level however, is only one indication of the actual safety provided by the flood defence structures. In practice, the applied data, design procedures, criteria and safety margins determine actual safety. There are also differences in the way structural aspects like crest level and stability are dealt with.

In addition to all this, significant historical, social, cultural and political differences contribute to the variety of flood protection policies, especially with regard to the authorities involved and their responsibilities.

A wide range of safety levels and differences in structural aspects makes it difficult to appreciate the situation in Belgium, the United Kingdom, Germany, Denmark and the Netherlands. Communication between both policy-makers and engineers of these countries would improve significantly if the observed differences can be explained.

1.2 Scope

The North Sea Coastal Management Group (NSCMG) has agreed to conduct a joint study on the different approaches to safeguarding against coastal flooding. The primary goal of research is to improve communication between the various countries on this subject. The study is limited to coastal defence structures in the five countries participating in the NSCMG.

NSCMG (appendix 1) brings together professional managers and advisers of government and public authorities from the countries along the Southern North Sea coast (Belgium, the United Kingdom, Germany, Denmark and the

Netherlands). The group aims, via international co-operation, to improve the efficiency and effectiveness of flood and coastal defences and make them sustainable in the long term. These aims will be achieved by exchanging experiences, sharing knowledge, addressing common problems, co-operating on research and improving public awareness. It provides a forum for the discussion of issues of common concern in the implementation of coastal management policy and the provision of defences to alleviate flooding and erosion.

The study was conducted in the Netherlands by Richard Jorissen, Judith Litjens-van Loon and Anabel Méndez Lorenzo (Ministry of Transport, Public Works and Water Management). Contributions and comments were given by the following people from other participating countries: Ian Meadowcroft (Environment Agency, United Kingdom), Christian Laustrup (Danish Coastal Authority, Denmark), Peter De Wolf, Toon Verwaest (both from the Ministry of the Flemish Community, Belgium), Volker Barthel and Jacobus Hofstede (both from Germany).

The authors wish to thank the people mentioned above for their contribution to the report. Their efforts have taken away much of the misunderstanding on the authors’ behalves.

1.3 Contents

In chapter 2, the countries’ coastal flooding policies are analysed on the basis of information collected in the appendices. Chapter 3 presents the results of two case studies. These case studies compare the safety levels the countries would probably adjudge to two sea dikes, and the resulting differences in crest level. The differences in crest level due to design procedures and safety margins in the various countries are also examined. The chapters 4 and 5 contain conclusions and recommendations.

The appendices contain an extensive description by country of the

organisational framework, risk assessment methods, safety levels, technical models, probabilistic techniques and future developments concerning flooding risk in coastal areas. In fact, the appendices are the foundation on which the final report is based.

Although it has been already mentioned in the scope of the report, it must be stressed that the study has been focused on coastal flooding only. Fluvial or other type of flooding haven’t been addressed

2 Scanning the horizon

The countries analysed in this study all cope with flooding risks in coastal areas. The scale of the risk differs greatly however as is shown in figure 1. In the Netherlands, for example, protection against flooding is of national importance, because the entire country would be seriously disrupted in the event of a major flood. This is not always the case in the other countries. But also other aspects like history or culture may have contributed to the diversity in flood protection policies. The various policies are described in this chapter. Section 2.1 briefly describes the situation in the countries, and section 2.2 reflects on some aspects of policy. Section 2.3 deals with more recent developments and research activities in the field of flood protection.

2.1

Inventory of flood defence policies

The Netherlands

Two-thirds of the Netherlands (25,000 km2 ) is at risk of flooding. This

flood-prone area comprises very large, densely populated polders accommodating most of the Dutch population and economy. The very existence of the Netherlands is dependent on reliable flood protection structures; protection against flooding is thus an important national issue and political task

embedded in the constitution. Failure of flood defence structures would have devastating human and economic consequences not only in the stricken area. The entire country would be seriously disrupted. Due to the obvious need for flood protection structures and awareness of the extent of the problem, a statutory safety level (laid down in legislation) is taken into account when designing and managing flood protection structures. This centralised and prescriptive type of policy is unique in Europe. The statutory safety levels (expressed as return periods) range from 2,000 to 10,000 years and appear quite high compared to safety levels in other countries.

The Dutch flood protection framework is rather centralised. The Ministry of Transport, Public Works and Water Manages sets the policy and legislation framework including safety standards. The Ministry also is responsible for managing the coast line and a limited number of flood protection structures. Engineering guidelines are prepared by a technical advisory committee and issued by the Ministry. Local water boards play a key role in actually accomplishing the prescribed safety by constructing and managing the vast majority of flood protection structures.

United Kingdom

In the United Kingdom, 2,200 km2 of land is at risk of flooding by the sea.

Five percent of the population live in this area. Although the United Kingdom coastline is approximately 10 times longer than the Dutch coast, the area at risk of flooding is 10 times smaller.

The legislative and organisational structure is decentralised and permissive, meaning that the relevant authorities are given powers to carry out flood defence and coastal protection activities but are not required to do so.

Appropriate standards for new defences are assessed on the basis of an economic analysis that compares the present value cost of different standards of protection to the present value of damage that would be avoided. Indicative safety levels vary from 200 to 1,000 years. For London, as an exception to the rule described above, a statutory safety standard of a 1,000 years is applicable

The Ministry of Agriculture, Fisheries and Food (MAFF) has overall policy in England. In Wales, the National Assembly has the policy responsibility. These national authorities play two roles. The first is to establish a policy framework within which the other organisations can plan and execute their own operational strategies. The second is the provision of government funds for cost-effective flood defence and coastal protection works and flood warning systems. Implementation is largely carried out by the operating authorities. A variety of authorities have been assigned permissive powers for the implementation of flood and coastal defence policy and the construction of defence works.

Denmark

The total length of Denmark’s coastline is comparable to that of the United Kingdom. However, the area at risk of flooding is much smaller. Along the Kattegat and the Baltic Sea coasts the flooding risk is very limited. In the Wadden Sea area from Esbjerg to the German border the situation is different. Although most towns are situated on higher ground, a few

exceptions (Thyborøron, Højer, Tønder and Ribe) cause significant flooding risks. These towns are protected by major dikes, with a 1,000 (Thyborøron) and 200-year safety level. The safety levels are comparable to those in the United Kingdom.

The Danish Coastal Authority, an organisation under the Ministry of Transport is the key player in the field of coastal defence, although local authorities typically contribute 30-50% of the cost for coastal and flood protection. Germany

Coastal (flood) defence in Germany is in the responsibility of the coastal states. For the North Sea these are Niedersachsen, Bremen, Hamburg, and Schleswig-Holstein. However, as coastal defence has national consequences, the federal government co-finances capital measures with 70% of total costs (maintenance is financed to 100% by the states). The technical and financial concepts for coastal defence are described in masterplans for each coastal state. In the state of Niedersachsen local water board are responsible for implementing state measures against flooding.

In all, 11,240 km2 (i.e., 17.5% of the total area of the four states) is at risk of flooding. As a result of land reclamation during the last centuries, most of the 1,580 km long coastline may be divided into flood units, so-called "köge". The safety levels for the sea walls that protect each of these flood units are determined without consideration of population density and economic values in each unit. The safety levels are expressed as a combination of design water level, wave run up, an additional safety value and (indirectly) slope criteria. The design water level has to meet the following three criteria:

• return period at least 100 years;

• not lower than the highest recorded storm surge including sea level rise

since than;

• not lower than the sum of mean high water, spring tide set up and highest

surge height recorded (single value method.

In practice the return period will exceed a 100 years, but to what extent is more or less depending on the highest record water or surge level. The actual safety levels therefore vary slightly from state to state.

Belgium

The Belgian or Flemish coastline is only 65 km long. Approximately 3% of Belgium’s surface area is at risk of flooding. The legislative and

organisational structure with regard to coastal defence is centralised (at the level of the Flemish region) and permissive. In the recent past safety levels for beach nourishments were calculated using two successive storms with return periods of 100 years. This is approximately equivalent to at least a 1,000-year safety level. At the moment a minimum required safety level of a 1,000 years is prescribed according to the Dutch methodology. However, a comprehensive study has been started with regard to safety levels and coastal protection in general along the Flemish coast. One of the aims of this study is to determine statutory levels of protection.

Table 1 compares some specific aspects of flood protection policies.

Table 1 : Overview of flood protection policies

Netherlands United Kingdom Denmark Belgium (Flanders)

Germany

flood-prone areas The coastline is 350 km long. Two-thirds of the country (25,000 km2) is at risk of coastal flooding. The flood-prone area comprises densely populated polders. The capital value at risk is estimated at 2,000 billion euros (1992).

The coastline is 4500 km long. 2,200 km2 ( with 5% of the population), is at risk of coastal flooding: some large urban and agricultural areas, but also very many small areas. The capital value at risk is estimated at 250 billion euros (2000)

The coastline is 7,300 km long. A few towns and some agricultural areas are at risk of coastal flooding.

The coastline is 65 km long and about 3% of total area of Belgium is at risk of coastal flooding.

11,240 km2 (17,5% of the area) of land at risk of coastal flooding in the coastal states Niedersachsen, Bremen, Hamburg, Schleswig-Holstein. types of sea defence • dunes (70%) • embankments • storm surge barriers

• sea walls • embankments • dunes, beaches, some

shingle

• gates and storm surge barriers • embankments • beaches, some sandy • embankments • dunes and beaches • embankments • dunes • combination embankments and dunes organisation / responsibilities

centralised policy framework, decision making and engineering, decentralised operational management

centralised policy framework, decentralised engineering and decision making

centralised centralised (at the level of Flanders)

centralised (at the level of coastal states)

legislation prescriptive legislation (Flood Protection Act,1996)

permissive legislation permissive legislation (Act of Reinforcement of Ribe dike, 1976; Fremskudt Dige, 1977) permissive legislation (Regionalisation Act, 1988) permissive legislation (State Water Act)

decision criteria legal safety standards economic efficiency, indicative standards

size of the population at risk

absolute standard absolute standard

safety levels Statutory standards by dike ring area. Standards are expressed as return periods of extreme water levels. Safety standards in the coastal area range from 2,000 to 10,000 years.

No target risk or flood defence standard; general aim of reducing risks to people and the environment, and requirement to achieve value for money spent. Indicative standards range from less than 200 to 1,000 years.

Safety levels are proposed by the DCA and approved by the Ministry. Safety levels are based on a cost/benefit analysis. Safety standards range from less than 50 to 1,000 years.

A minimum safety level of at least a 1,000 years is prescribed according to the Dutch metho-dology.

Safety levels expressed as a combination of design water level, design wave run-up and slope criteria. In practice this standard will exceed a 100 years.

2.2

Focusing on some policy aspects

The overview as shown in table 1 shows some different aspects of flood defence policies in the five countries. The following section analyses some of these aspects.

Organisation / responsibilities

All countries have a centralised responsibility for flood protection policy. In Germany and Belgium ‘centralised’ is considered at the level of the German states and the Flemish Region.

In the United Kingdom the centralised responsibility is limited to a policy framework within which the other organisations plan and execute their operational strategies and the provision of government funds for cost-effective measures.

In the other countries the centralised responsibility extends also to planning and decision making. In Belgium and Denmark the role of local authorities is practically absent. In Germany and the Netherlands local and regional authorities play an important role in achieving the goals of flood protection policies by construction and managing flood protection structures. The water boards in these two countries are examples of such local authorities. The centralised responsibilities are reflected by the financing arrangements, which are nearly all centralised. In the United Kingdom however, this is limited to government funds for cost-effective measures. Two specific items need to be addressed :

• in Germany 70% of capital investment (construction costs) is being paid

for by the federal government;

• in the Netherlands 70% of maintenance costs is being paid for by water

boards via local taxes.

An advantage of a centralised organisational framework is that a certain uniformity in flood protection policy is guaranteed and priorities for the country as a whole can be easily decided. There is a certain rigidity to this system, though, making it is more difficult to act flexibly in local situations. With a decentralised organisational framework, the reverse is true.

The inventory shows that countries facing relatively large potential flooding damages, such as the Netherlands and Germany, have a more centralised approach. The United Kingdom with relatively small flooding damages has a far more decentralised approach. However, the scale of potential flooding is not the decisive factor. Historical, social, cultural and political aspects play their role as well.

Legislation

The survey shows that all countries except the Netherlands have a

permissive legislation with regard to safety standards. For the Netherlands two major arguments are applicable :

• The widespread awareness that another disaster like the 1953 flood is to

be avoided at all costs. Based on a national risk assessment safety standards were derived.

• The water boards are responsible for construction and maintenance of the vast majority of flood protection structures. The prescriptive

legislation sets clear and uniform goals for this task and provides regional and local authorities with sufficient powers to accomplish these goals. Safety standards which are legally prescribed provide a very solid base, but may result in a significant administrative and organisational burden. For example, in the Dutch situation the safety standard is expressed in technical terms and calculated using technical data, models and assumptions. Ongoing research leads to new insight regarding these aspects. Once accepted, research results are introduced more or less automatically in practice. This leads to a rather technical approach of safety levels and related structural aspects. However, an advantage of prescriptive legislation is that the importance of flood protection becomes very eminent and can’t be

negotiated. It is well known that if there has not been a flood in many years, people tend to forget about the risk. Only when a new flood occurs does everyone become aware again that flood protection is an important issue.

When flood defence legislation is permissive, there is no legal right to protection against flooding. This is in some ways a disadvantage, particularly where flood defence competes with other (local) expenditure needs for resources. These issues may not be consistent with national or strategic aims. Obviously, this is the most likely to occur if decision making takes place locally.

However, permissive legislation also means that flood defence decisions can be made on the basis of economic efficiency, rather than absolute standards which might conceivably lead to inappropriate, inefficient or environmentally damaging schemes. This ensures value for money spent compared with other government expenditures.

Decision criteria

Flood defence decisions can be made on the basis of absolute standards or on the basis of economic efficiency. The required safety level can be laid down in law(the Netherlands) or can be otherwise agreed (Denmark, Germany, Belgium). No further decision making is required.

Safety levels can also be assessed on the basis of an economic analysis that compares the present value cost of different standards of protection against the present value of damage that would be avoided (United Kingdom). Decisions based on economic efficiency have the advantage of being flexible and adaptive. Absolute standards can conceivably lead to out-of-date or inappropriate safety levels.

Safety levels and design procedure

All of the countries studied express safety levels as a return period of extreme water levels (and associated hydraulic loads) that flood protection structure must be able to withstand. In most countries (United Kingdom, Belgium, the Netherlands, Denmark) so-called joint return periods for water levels and waves are used.

The concept of return periods makes the degree of offered safety appear easy to compare; yet each country’s elaboration is very different. The various

policies and their practical applications are difficult to compare using a single parameter.

One disadvantage of expressing safety levels in terms of a return period is that the actual risk of flooding is unknown and cannot be compared to other risk-bearing activities. The probability that a water level will be exceeded in a particular year is not equivalent to the probability that a dike ring area will flood; the probability of a dike breach followed by flooding is the most tangible measure of safety.

Another disadvantage of expressing safety standards as return periods of extreme water levels is that safety against flooding can only be improved by increasing the height of flood protection structures. If safety standards are expressed in terms of risk (probability of failure times the consequences of failure), then safety against flooding can also be improved by taking other measures. Measures which reduce the probability of a dike breach or which limit damage caused by flooding can make just as great a contribution to protection as raising the height of the dike itself. This makes policy and its implementation more flexible.

2.3

Developments and research activities

2.3.1 Developments

The survey focused primarily on the standard of practice as applied by the different countries. Obviously the state of the art will produce some new developments. These developments can be divided into two main categories:

• technical developments aimed at improving the deterministic and

probabilistic models used for design;

• conceptual developments aimed at applying a quantitative risk analysis in

policy preparation. Technical developments

In all countries studied design conditions for flood protection structures are based on a prescribed return period or some historical reference. Initially, the flood level was considered to be the most important parameter, and the application of flood level frequency exceedance curves has become widespread. Other hydraulic loads, such as wind and waves, are still mostly treated in a deterministic way. Sometimes expected values are used; in other cases ‘best’, ‘educated’ or ‘conservative’ guesses. All countries are to some extent trying to improve the models they use for deriving hydraulic boundary conditions. Joint probability distributions of flood levels, and wind and wave parameters are being derived and in some cases are already applied in practice.

Application of these developments, however, may lead to practical

difficulties. In recent years it has been shown that the technical developments mentioned above may lead to higher hydraulic loads, which in turn may lead to massive reconstruction works. On the other hand, probabilistic techniques would be very welcome if certain traditional, conservative design rules were replaced with a modern, cheaper variety. A major issue in discussing the application of probabilistic models is the way in which we deal with uncertainty.

Uncertainties can be classified in three categories:

• implicit uncertainty, because the variables studied have a stochastic nature;

• model uncertainty, because our description of natural phenomena is always insufficient;

• statistical uncertainty, because the number of observations of extreme events is too low.

Some recent studies on uncertainties have shown that all of the uncertainties mentioned above can be incorporated into our design procedures. However, if these uncertainties are just treated as additional stochastic variables and the safety level remains unchanged, this will lead to enormous increases in required crest levels. These increases may vary from 1.0 to 2.0 meters. Appendix 6 contains some examples.

In all countries, strength parameters are still treated deterministically using design criteria for wave run-up or geotechnical stability of an embankment. Design criteria are largely based on experience, laboratory tests or field tests. Acceptable damage to flood protection structures (except dunes) is generally limited to superficial damage and will not lead to flooding of the hinterland. Research into structural failure and probabilistic design criteria is used to determine actual flooding probabilities, especially in the Netherlands. However, these developments need to be incorporated into a different framework for evaluating flood protection.

Conceptual developments

The second major issue developing in flood protection is quantitative flooding risk analysis and the application of this analysis within the framework of risk management. Risk is defined as the product of flooding probabilities, and the associated flood damage. Risk management aims to:

• assess and compare the effectiveness of flood protection measures; • optimise strategies and measures;

• prioritise measures;

• compare flooding risks with other societal risks;

• monitor relevant developments within flood-prone areas.

In the United Kingdom, quantitative flooding risk analysis is applied in practice, while economical optimisation of flood protection measures is sought. In the Netherlands, a risk-based flood protection policy is in preparation. In Germany, the federal states of Schleswig-Holstein and Niedersachsen are preparing quantitative models to describe flooding risks in the coastal zone. These models can be used to support risk management. In Belgium also a comprehensive study has been started to determine

economically optimised safety levels which may vary along the coast by taking into account the different potential flooding damages.

2.3.2 Research activities

Research is needed to realise the changeover from the current safety philosophy to a safety philosophy based on a flooding risk approach. Research programmes in the United Kingdom, Germany, Belgium and The Netherlands aim to make this changeover possible. In order to develop an accurate safety philosophy based on the risk of flooding, it is essential that the probability of flooding and its consequences can be calculated sufficiently accurately. It is also important to establish what is felt to be an acceptable

level of risk.

Probability of a dike breach

The probability of a dike breach is not adequately defined under the current safety standards, even though the probability of a dike breach followed by flooding is the most tangible measure of danger. After all, flooding results in economic damage and, depending on the situation, can claim victims. Measuring dike height alone provides insufficient information about protection from flooding. Two technical arguments for this can be cited: the geotechnical stability of dikes and the correlation between the failure of different dike sections. For example, if dikes lose their resistance to sliding during periods of high water, a dike breach could occur without the water flowing over the crest of the dikes. This contributes to the probability of a dike breach or flooding. The required resistance to these mechanisms (largely geotechnical failure) cannot be expressed in terms of a hydraulic load standard or crest height. In the current situation, additional requirements are established for the probability of a dike breach occurring at water levels below the prescribed water level.

The larger the polder, the more dike sections are needed to protect the area. If these sections were fully correlated with the hydraulic load on them, the safety of the area could be expressed as the safety of a single dike section. In practice, this is not the case. Both the strength of and the load on the dike sections around the area are not fully correlated. Other types of construction, such as discharge sluices, are to a large extent responsible for this. The probability of a dike breach in any dike section, followed by flooding, is thus always greater than the probability of a breach in any given dike section.

Consequences of flooding

Flooding usually results in extensive material damages. The extent of the damages depends on the nature of the threat (sea water or fresh water, short or long period of flooding, expected or unexpected) and the characteristics of the flooded area (depth, built-up areas, industry, exact location of the dike breach). In particular, deep floods or fast-flowing water can have serious consequences in terms of victims, damage, and disruption to normal life and infrastructure. In calculating the consequences of flooding, research

concentrates on developing an instrument by which damage and number of victims for each dike ring area can be calculated in a uniform and practical manner. Information and warning systems help both the government and individual citizens to take the right measures at the right moment. Applying these types of instruments decreases the consequences of flooding. In the Netherlands for example, a Flood Information System is currently being developed. This can be used before and during floods for predicting the way in which a flood will occur; monitoring water levels, waves, the dike

conditions and the availability of the road network; determining the effects of any measures taken; and announcements and communication.

Safety standards

Another research item are the present standards. In the Netherlands these are laid down in the Flood Protection Act which dates from 1960. These standards relate only indirectly to flood damages. Whether the standards are still sufficient is a matter of social and political debate. To support this debate tools need to be developed to answer such questions as:

• what is an acceptable risk?

• can such an acceptable risk be expressed in quantitative measure(s)?

• is it possible and meaningful to compare flooding risks with other societal

risks?

• what is the role of economic optimisation?

Communication

Communication will be a vital item to address. The general public is scarcely aware of flooding as an actual risk. It is therefore very difficult to raise the flood protection issue in a public debate without taking people by surprise. Furthermore, the technical scope of the flood protection issue is very wide and at the same time very complicated. The result is the limited participation of the general public so far. It is a challenge for the competent authorities and researchers to develop a well-balanced communication strategy. This

strategy should aim to clarify the main problems we are facing now and in the future. Once the public has become aware of potential flooding problems, communication may be focused on the full range of possible solutions and their effects.

For example, in England and Wales the Environment Agency has embarked on a major publicity campaign to rise awareness of flood risk. In future, the campaign will give information to different areas depending on, for example, the likelihood of flooding.

3 Probing the depth

Return periods of hydraulic boundary conditions are just one indication of the actual level of safety provided by flood defence structures. In practice, the applied data, criteria, design procedures and safety margins determine actual safety. Therefore it is necessary to work out some practical examples. This section presents case studies of two sea dikes: the Pettemer Zeewering in the Netherlands and the Fremskudt Dige in Denmark. The case studies is aimed at two goals :

• comparing the safety levels most likely adjudged by each country to both

sea dikes;

• comparing required crest levels according to these standards. A

distinction is made between crest levels due to design procedures and due to safety margins.

Sections 3.1.1 and 3.2.1 provide estimates of safety standards for the Pettemer Zeewering and the Fremskudt Dige, expressed as return periods of extreme water levels. The required crest level is calculated using Dutch design formulae and safety margins (see appendix 5).

Sections 3.1.2 and 3.2.2 compare crest levels of both dikes using design procedures and safety margins of each country (see appendices 1 to 5) and a uniform safety standard.

The information from the case studies is collected and analysed in section 3.3.

3.1

Pettemer Zeewering (The Netherlands)

The Pettemer Zeewering is located on the Dutch coast as shown in Figure 3.

3.1.1 Safety standards and crest levels

The stretch of Dutch coast along which the Pettemer Zeewering is situated has a safety standard based on a 10,000-year return period. The prescribed hydraulic boundary conditions for this location, using a 10,000-year safety standard, are:

• flood level : MSL + 4.70 metres;

• wave height : 4.70 metres.

If the United Kingdom, Denmark or Belgium were to adjudge a safety level to the Pettemer Zeewering, it is plausible that they would employ a 1,000-year safety level. For Germany a 100-year safety level is applied. An explanation of this supposition follows directly below.

In the Thames Estuary through London there are statutory defence standards based on an historic assessment of nominal 1 in 1000-year water levels plus an allowance for sea-level rise and wave action. Apart from this, there are no statutory defence standards in the UK. Appropriate standards for new

defences are assessed on the basis of an economic analysis that compares the present value cost of different standards of protection with the present value of damage that would be avoided.

It is estimated that the value of damage that would be avoided in the area of the Netherlands protected by the Pettemer Zeewering is comparable to the value of damage expected in London. It is reasonable to assume that a

Figure 3 Location of the Pettemer Zeewering

1,000-year safety standard would be used when designing the Pettemer Zeewering according to United Kingdom practice.

In Denmark three different safety levels are used, varying from 1,000 years (near the town of Thyborøron) to 50 years (for other important dikes). Considering that the Pettemer Zeewering protects a vital part of the Netherlands, it is reasonable to assume that a 1,000-year safety standard would be adjudged to the Pettemer Zeewering when using the Danish practice.

In Belgium, coastal protection works are generally designed fto withstand storm conditions with an estimated return period of a 1,000 years, according to the Dutch methodology.

When designing in Germany (Schleswig-Holstein), it is common practice to use a design water level with a return period of 100 years.

When using a 1,000-year safety level, the hydraulic boundary conditions are:

• flood level : MSL + 4.10 metres;

• wave height : 4 metres.

For a 100-year safety level the flood level is reduced to MSL + 3.40 meters. This more or less corresponds to the highest recorded water level at this location : MSL + 3.25 meters.

Table 2 : Required crest level for a 10,000, 1,000, and 100-year safety level overtopping discharge Þ 1 l/m/s 10 l/m/s Netherlands (10,000 years) 14.9 m 12.2 m United Kingdom (1,000 years) 13.4 m 10.7 m Denmark Belgium Germany (100 years) 11.9 m 9.2 m

3.1.2 Design procedures and safety margins

The applied data, criteria, design procedures and safety margins also determine actual safety in practice. This section compares these aspects to determine the real differences in actual safety from flooding (expressed as crest level) provided by the flood protection structures in the various countries, when using the same return period.

It was not possible to calculate the hydraulic boundary conditions for certain safety standards according to methods used in other countries. As a result the hydraulic boundary conditions as applied in the Netherlands for the Pettemer Zeewering are used in the case study. The prescribed hydraulic boundary conditions for this location are:

• flood level : MSL + 4.70 metres;

• wave height : 4.70 metres;

• peak period: 12 seconds.

These boundary conditions are derived only partially in a probabilistic way. The water level has a return period of 10,000 years. The wave height is the expected wave height at the toe of the dike associated with this water level.

Design criteria depend on the individual circumstances of each defence structure. Where overtopping is considered (as in the two case studies) the design criteria are based on failure caused by damage to the rear face from overtopping. Critical overtopping discharges normally used in the

Netherlands are applied in this case study. The overtopping discharges used are 0.1 l/m/s, 1 l/m/s and 10 l/m/s.

The Pettemer Zeewering is a sea dike with a berm slope (see figure 4). Above the berm, the outward facing slope is 1:3; below the berm, the slope is 1:4. The berm is approximately at storm surge level.

The required crest level, according to the methods used in the different countries, is given in the table below for three different overtopping criteria. Belgium and Germany are not considered, as there was not sufficient information to make the calculations.

Table 3 : Required crest level for various overtopping discharges

required crest level without taking safety margins into account overtopping discharge Þ 0.1 l/m/s 1.0 l/m/s 10 l/m/s Netherlands 17.2 m 14.6 m 11.9 m United Kingdom 16.3 m 13.6 m 10.8 m Denmark 12.8 m (2% overtopping) 9.2 m (30% overtopping)

Additional safety margins lead to higher required crest levels. Safety margins for sea level change, land subsidence, seiches and uncertainties in design are normally used. The safety margin for land subsidence is always based on local geotechnical calculations. Commonly used values for the other safety margins are given in the table below. It must be emphasised that these represent approximate values.

Table 4 : Safety margins

safety margins Þ sea-level change seiches design

per century uncertainties Netherlands 0.2 m 0.1 - 0.8 m -

United Kingdom 0.4 - 0.6 m - 0.3 m

Germany - - 0.5 m

Denmark 0 - 0.15 m - -

The table below shows the required crest levels obtained when additional safety margins (sometimes averaged) for sea level change, seiches and uncertainties in the design are included.

Table 5 : Required crest level including safety margin overtopping discharge Þ 0.1 l/m/s 1 l/m/s 10 l/m/s Netherlands 17.5 m 14.9 m 12.2 m United Kingdom 16.9 m 14.2 m 11.4 m Denmark - 12.9 m 9.3 m

The differences in crest levels due to design procedures and safety margins generally range from 2 meters (overtopping discharge 1 l/m/s) to 3 meters (overtopping discharge 10 l/m/s).

3.2

Fremskudt Dige (Denmark)

The Fremskudt Dige is located on the Danish coast as shown in figure 6.

Figure 6 Location of the Fremskudt Dige

3.2.1 Safety standards and crest levels

The Fremskudt Dige has a 200-year safety level and was designed in 1977. The prescribed hydraulic boundary conditions for this location were

approximately:

• flood level : MSL + 5.35 metres;

• wave height : 1.80 metres;

• wave period : 7 seconds.

If the Fremskudt Dige were designed by other countries, the safety level would be different. Germany would use a 100-year safety level. It is plausible that the United Kingdom would use the same safety level as Denmark (200 years). Belgium would probably adjudge a 1,000-year safety level to the Fremskudt Dige. The Netherlands would most likely employ a 2,000-year safety level. An explanation of these suppositions follows directly below. When designing in Germany, it is common practice to use a design water level with a return period of 100 years, taking into account the highest recorded water level as well. The 100-years water level is still somewhat higher than the highest known water level in front of the Fremskudt Dige, which is 4.92 m + MSL (1976).

In the United Kingdom, appropriate standards for new defences are assessed on the basis of an economic analysis that compares the present value cost of different standards of protection against the present value of damage that would be avoided. However, indicative standards are provided based on different categories of land use. When urban areas (like Højer and Tønder) are at risk of flooding, the indicative safety levels in the United Kingdom vary from a 100 to a 300-year return period of hydraulic boundary conditions. It is

thus plausible that a 200-year safety level would be used for the Fremskudt Dige when following United Kingdom practice.

In Belgium, coastal protection works are generally designed for withstanding storm conditions with estimated return periods of a 1,000 years, according to the Dutch methodology.

In the Netherlands, flood protection structures bordering the Wadden Sea coast have a safety level ranging from 2,000 to 4,000 years. Based on the area at risk, it is estimated that a 2,000-year safety level would be used when designing the Fremskudt Dige.

Using a 2,000-year safety standard, the hydraulic boundary conditions would be approximately:

• flood level : MSL + 6.15 metres;

• wave height : 2.70 metres.

Table 6 : Required crest level for a 2,000 and 200-year safety level

overtopping discharge Þ 2% 30% Denmark, United Kingdom (200 years) 7.1 m 5.7 m

Netherlands (2,000 years) 8.6 m 7.2 m

3.2.2 Design procedures and safety margins

This section compares the design procedures and safety margins of the various countries to determine the differences in calculated crest level for the Fremskudt Dige when using different calculation methods, but the same return period.

For the Fremskudt Dige a 200-year safety level is employed. The hydraulic boundary conditions for this location are:

• flood level : MSL + 5.35 meters;

• wave height : 1.80 meters;

• peak period: 7 seconds.

It was not possible to calculate the hydraulic boundary conditions for a 200-year safety level according to methods used in other countries. Therefore the hydraulic boundary conditions as applied in Denmark for the Fremskudt Dige are used in the case study.

Design criteria depend on the individual circumstances of each defence structure. The critical overtopping volumes normally used in Denmark vary from a 2% to 10% overtopping allowance. In specific cases with mild slopes and/or clay covers however 30% to 90% overtopping can be allowed. In this case only 2% and 30% criteria are applied.

The table below shows the required crest level for the two different

overtopping criteria, according to the methods used by the different countries. Belgium and Germany are not considered, as there was not sufficient

information to make the crest level calculations.

Required crest level without taking safety margins into account overtopping discharge Þ 2% overtopping (1.0 l/m/s) 30% overtopping (10 l/m/s) Netherlands 7.1 m 6.5 m United Kingdom 7.6 m (1) 6.9 m (1) Denmark 7.1 m 5.7 m (1)

The Fremskudt Dige has a smooth, level (1:10) slope. The United Kingdom formula for the wave overtopping margin is not meant for such mild slopes. The coefficients in this formula are

extrapolated.

Additional safety margins lead to higher required crest levels. Safety margins for sea-level change, land subsidence, seiches and uncertainties in design are normally used. The safety margin for land subsidence is always based on local geotechnical calculations. Section 3.1.2 gives commonly used values for the other safety margins.

The table below shows the required crest levels obtained when additional safety margins (sometimes averaged) for sea level change, seiches and uncertainties in the design are included.

Table 8 : Required crest level including safety margin Overtopping discharge Þ 1 l/m/s 10 l/m/s Netherlands 7.4 m 6.8 m United Kingdom 8.2 m 7.5 m Denmark 7.2 m 5.8 m

Differences in crest level up to 1.7 meters can be expected due to variety in design procedures and safety margins.

3.3

Summary and analysis

As in the case studies a distinction is made between the effect of safety standards and the effect of design procedures, including safety margins. Effect of safety standards

The results of the case studies are partly summarised in table 9. The effect of different safety standards is largely determined by the hydraulic boundary conditions at the different return periods. Therefore it not surprising that for the Pettemer Zeewering and the Fremskudt Dige the effect is the same and equal to 1.5 meters. The contribution of water level and wave height is approximately equal. For the Pettemer Zeewering the Dutch design

procedure was applied as was the Danish procedure for the Fremskudt Dige.

Table 9 : Effect of safety standards

Crest level Pettemer (NL) Fremskudt (DK) 10,000 14.9 m -

1,000 13.4 m - 200 - 7.1

Effect of design procedure and safety margins

To separate the effect of different safety standards from the effect of design procedures and safety margins, both cases studies were repeated with fixed safety standards. The results of this part of the case studies are summarised in table 10. Between brackets the crest levels without safety margin are displayed.

For the rather strict overtopping criterion (1 l/m/s) the due to design procedure and safety margins vary from 1.2 to 2 meters. Without safety margin these differences are slightly reduced.

For the Pettemer Zeewering the Dutch safety standard (10,000 years) was applied as was the Danish standard (2,000 years) for the Fremskudt Dige.

Table 10 : Effect of design procedures and safety margins Crest level (1 l/m/s) Pettemer (10,000) Fremskudt (2,000) United Kingdom 14.2 m (13.6 m) 8.2 m (7.6 m) Denmark 12.9 m (12.8 m) 7.2 m (7.1 m) The Netherlands 14.9 m (14.6 m) 7.4 m (7.1 m)

If a more lenient overtopping criterion (10 l/m/s) is applied for the Pettemer Zeewering the difference due to design procedures and safety margins increases from 2 to 3 meters. For the Fremskudt Dige the difference increases from 1 to 1.7 meters.

The results as summarised in table 10 also show that the effect of the safety margin is rather arbitrary. Given the different background of these safety margins this result might have been expected.

Combined effects

Although the combined effect of safety standards, design procedures and safety margins hasn’t been determined properly, some expected results can easily be deducted.

For the Pettemer Zeewering the combined effect would lead to even bigger differences than observed in the case studies. This is due to the fact that both the Dutch safety standard and the Dutch design procedure lead to the highest crest level. Therefore, the difference compared to the other countries will increase.

For the Fremskdt Dige the combined effect will lead to a smaller difference between the United Kingdom and the Netherlands. The difference between the Netherlands and Denmark however will increase.

4 Conclusions

The study consist of two main parts : a global inventory and a further technical analysis. The following conclusions can be drawn from the global inventory:

• The five countries analysed in the study all cope with flooding risks in

coastal areas. The risk, however, is totally different in the various countries. For example, the area subject to flooding ranges from a few percent of the total area up to nearly 70%.

• Due to this enormous difference, the relative and absolute importance of

flood protection varies significantly. In the Netherlands, for example, protection against flooding is of national importance, because the whole country would be seriously disrupted in the event of a major flood. In the other countries, the consequences of flooding are relatively limited. Flooding in these cases would have a regional, rather than national, impact.

• The organisational framework for flood and coastal defence in the

different countries varies due to differences in the state constitutions and governmental relations. In the United Kingdom, national policy is defined in terms of the general aim of reducing risks to people and the natural environment, and the requirement to achieve value for money spent. This promotes a flexible organisational approach. In the Netherlands, national statutory safety levels are laid down in legislation, leading to a centralised and more rigid organisational approach.

• Different types of flood protection structures guarantee protection against

flooding. However, there is great similarity in flood protection structures used by the different countries.

• As an exception to this rule the Dutch Ministry is responsible for keeping

the coastline at the 1990 position and in most cases local water boards maintain the dune as a natural flood protection structure to protect the hinterland. In the other countries such a distinction between coastline and dunes isn’t made. The latter situation allows a more flexible approach.

• Safety levels for flood protection structures vary considerably in the

different countries. The lowest safety level varies from 50 (United

Kingdom) to 2,000 years (the Netherlands). The highest safety level varies from 1,000 (United Kingdom, Denmark and Belgium) to 10,000 years (the Netherlands). In Germany the safety level is at least a 100 years. This variety in safety levels is due largely to differences in the scale of flood risk in the different countries.

• The various methods of deriving hydraulic boundary conditions for the

design of flood protection structures have not been investigated. However, the survey shows that both deterministic and probabilistic boundary condition models differ greatly. These models are being developed on a large scale. The different models for boundary conditions may affect the results of the study significantly.

• In most countries, quantitative risk analysis is being applied for present or future policies. In all cases, risk is defined as the probability of flooding multiplied by the consequences.

As a part of the study two case studies were carried out to investigate the practical differences of the various flood protection approaches. From these case studies the following conclusions could be drawn :

• If only differences in safety levels are considered (for the same flood

protection structure using the same design procedure), differences in crest level of up to 1.5 meters can be expected.

• If only differences in design procedure and additional safety margins are

considered (keeping the safety level constant), differences in crest level up to 2 (1 l/m/s) and 3 meters (10 l/m/s) can be expected.

5 Recommendations

The following are recommendations for further study:

• The study focused on the structural aspects of coastal dikes. It would be

valuable to extend the study to the structural aspects of dunes and other flood protection structures, including storm surge barriers.

• It was not possible to calculate hydraulic boundary conditions according to

methods used in all countries. It would be useful to study this aspect, since these boundary conditions are extremely important for the final results.

• During this study it was not always be easy to gather the information

required. Improved, and especially harmonised, communication and the availability of basic information are necessary factors for any further study or inventory in this field.

• The goal of the project is not to achieve uniformity of safety levels. The

results show, however, that differences in safety standards can be less significant than differences in calculation procedures. Harmonisation of these technical procedures would help both technicians and policymakers to clarify discussions on safety levels in Europe.

• The study focused on coastal areas. Given the results, it would be