Flood risk Understanding concepts

Transcription

1 Flood risk Understanding concepts Ministry of Transport, Public Works and Water Management

2 Foreword Whether we are managers or politicians, entrepreneurs or teachers, flood risk concerns us all. In our country a lot of land is below sea level, and it is not always straightforward to stay dry. In the past, after a flood dikes were repaired and their levels were raised to just above the last known water mark. Structural measures were generally only taken after a major disaster. Since then, there has been a decisive change in approach. Population increases, high economic growth and climate change taught us that things must be done differently. Preventing floods is the top priority. We are bringing flood risk up to date under the auspices of the Flood risk for the 21st Century policy. The Committee for the Sustainable Development of Coastlines, that is the new Delta Committee, provides advice on long term policies and on implementing enduring protection of the coasts. Many groups are involved in discussing sustainable flood risk, whether it concerns safety norms or how to keep the consequences of a possible flood under control should an unfortunate event happen. We talk with experts in hydraulic engineering as well as with lay people not versed in the technical jargon. F L O O D R I S K

3 Confusion caused by the terminology arises very quickly. To prevent this from happening and to understand more clearly certain concepts used in hydraulic engineering, we have put together this booklet. I strongly and sincerely recommend using this booklet. Then a common understanding between all of us can be created. DIRECTOR GENERAL WATER AFFAIRS ir. A.G. Nijhof MBA U N D E R S T A N D I N G C O N C E P T S

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5 Introduction The level of the sea is rising, rains are stronger and more icewater is flowing down from the Alps. You can hear about it everywhere. The risk of leaks and floods is steadily increasing. It is time to cast a realistic and practical eye on the future. The consequences of more floods and leaks will also increase. And there are new technical insights on the strength of dikes that need to be considered. All of this makes it clear that we have to look at flood risk in a new way. Are our current policies equipped to deal with large scale floods? Can they respond adequately to their consequences? Is the current legal framework still adequate? Many stakeholders are concerned with flood risk, such as municipalities, provinces, dike surveyors or the state. Others include experts in hydraulic engineering, project developers, research institutes, gardeners or farmers. These groups often have to deal with water and flood risk. For this reason, it is important that we understand each other well in particular when we are discussing the development of the Flood risk for the 21st Century policy. For a successful dialogue, it is important everyone speaks the same language and uses the same terminology, which is why this booklet has been produced. This booklet starts by giving a broad overview of the most important developments in the fields of water and flood risk. This is followed by an introduction to the main terminology where, if you need it, references to an explanatory glossary can be found at the back of the booklet. This enables you during your discussions to search for words and terms you don t know quickly and easily. This way you know for sure that you are talking about the same things! U N D E R S T A N D I N G C O N C E P T S

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7 Table of contents HOOFDSTUK Water safety through 1 the years The Zuider Sea flood 11 The flooding of the Rhine 11 The infamous flood of The Delta Plan 14 The Brecht and Boertien River Committees 15 Coastal policy 15 (Near) Floods in 1993 and The Delta Plan for the Major Rivers 16 Water management in the 21st century 17 The Maaswerken project 18 The Space for the Rivers project 19 Weak links along the coast 20 Project to assess Flood Risks and Safety in the Netherlands 20 Hurricane Katrina 20 The Water Act 21 The European Flood Directive 22 The Flood Surge Protection Programme 23 Flood risk in the 21st Century (WV21) 24 Climate change 25 U N D E R S T A N D I N G C O N C E P T S

8 The concept of flood risk What is the risk management cycle? 27 What are the different zones? 28 What is a flood? 30 What is an exceeding frequency? 30 What is the probability of flooding? 31 What is the relationship between the exceeding frequency and the probability of flooding? 32 What are the possible consequences of flooding? 32 What is the risk of flooding? 34 What is a dike ring? 35 What safety standards are there in the Netherlands? 35 What laws protect the people of the Netherlands from high water levels? 37 Probability of flooding What causes a water defence to fail? 39 - Mechanisms that cause a dike to fail 39 - Mechanisms that cause a dune to fail 40 - Mechanisms that cause a hydraulic structure to fail 41 What effect do emergency measures have on the probability of flooding? 42 What is a system effect? 43 The system effect near national borders 43 Uncertainties in probability of flooding 43 F L O O D R I S K

9 The consequences of flooding How does a flood occur? 45 How do we determine the consequences of flooding? 45 What are the factors that determine the severity of the consequences of flooding? 47 How do we estimate the extent of the material damage and the number of victims? 48 Do we implement evacuation plans? 48 The risk of flooding How do we determine the risk of flooding? 51 Does the Netherlands revise the safety standards in light of the risks? 52 How do we determine the economically optimal safety level? 52 How do we minimise the risk of flooding? 53 Flood risk awareness Flood risk awareness and water-aware behaviour 55 Glossary Glosssary 57 U N D E R S T A N D I N G C O N C E P T S

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11 Flood risk through the years The Zuider Sea flood The year 1916 is stormy from the start. The storm rages for days in the north of the Netherlands. When the storm surge meets the high water discharge coming down the rivers, the dikes around the Zuider Sea give way. On 13 and 14 January the dikes fail in dozens of places. The wind reaches speeds of up to one hundred kilometres an hour and water flows across the land. The extensive damage to the inner slopes of the dikes and the dike revetments makes the dikes even less capable of withstanding the pressure of the water. Besides the considerable material damage, there are also sixteen casualties. The Zuider Sea flood has lasting consequences for the Netherlands. Due to the number of people affected and the extent of the damage, the government immediately decides to close off the Zuider Sea from the North Sea. A plan drawn up by the engineer Cornelis Lely forms the basis for construction of the Afsluitdijk or Closure Dike, which is completed in The flooding of the Rhine On 3 January 1926 the Rhine reaches its maximum discharge capacity. The river discharges more than 12,000 cubic metres of water per second. This extreme river discharge is generated by a combination of high meltwater discharge and heavy rainfall. Also the Meuse has to cope with a high river discharge of approximately 3,000 cubic metres of water per second. Large areas of land along the Rhine, the Meuse and the Oude IJssel are under water. On the morning of 31 December 1925 one of the main dikes along the Meuse, near Overasselt and Nederasselt, U N D E R S T A N D I N G C O N C E P T S 11

12 fails. Water and ice damage and destroy at least 3,000 homes and the costs run into tens of millions of guilders. The floods along the Rhine lasts for fifteen days. The floods along the Meuse lasts for eleven days. After this disaster several bends of the Meuse are cut off and the height of various dikes is increased. 12 F L O O D R I S K

13 The infamous flood of 1953 Spring tide and a northwestern storm drives the waterlevel of the North Sea to record heights during the night between 31 January and 1 February There are floods and casualties in England, Belgium, the Netherlands and Germany. In the Netherlands 400,000 hectares of land are flooded and at least 40,000 buildings are damaged. More than 1,800 people die and 70,000 people have to be evacuated. There is massive damage to livestock, buildings and infrastructure. A plan to prevent another similar disaster is clearly urgently needed. The Delta Committee, which is appointed later the same year, proposes a series of protection measures that come to be known as the Deltaworks. U N D E R S T A N D I N G C O N C E P T S 13

14 The Delta Plan The Delta Committee submits its first recommendations to the government a few months after the infamous flood of The committee recommends that the height of the dikes around the island of Schouwen, the island most prone to flooding, be raised to five metres above the Amsterdam Ordnance Datum. It also recommends that a flood surge barrier be constructed across the mouth of the Hollandse IJssel, near Krimpen aan de IJssel. The government immediately acts on both proposals. A year after the flood, the committee presents its most important proposal: the Delta Plan. The plan proposes the construction of large closure dams across the mouths of the four main coastal inlets in the southwest of the Netherlands. These barriers will prevent flooding from sea in that area. But the delta region is not the only part of the Netherlands that is at risk other areas along the coast are 14 F L O O D R I S K

15 also prone to. So the committee also advises the government to raise the height of the dikes and dunes along the rest of the coast and along the coastal inlets that are not closed off from the sea. In 1960 the committee presents its recommendations in a final report. Almost 40 years later, in 1997, with the completion of the Maeslantkering flood surge barrier in the New Waterweg waterway, the Deltaworks are complete. The Brecht and Boertien River Committees In the seventies the Brecht and Boertien River Committees define standards for water defences along the rivers. The standards are based on the standards established by the Delta Committee. For the river region the standards are based on an exceeding frequency of one in 1,250 per year, given that freshwater does less damage as sea water. In establishing the standard they also take into account the importance of the values of the landscape, nature, cultural history of the area and the predictability of high water levels along the rivers. For the transitional zones between the rivers and the coast (including the IJsselmeer region), the committees base the standard on an exceeding frequency of one in 2,000 per year. In 1996 the government incorporates these standards in the Flood Defences Act, which applies to the coast, estuaries, rivers and the transitional zones (tidal rivers and the IJsselmeer region). Coastal policy The coast is a dynamic environment. During calm periods there is gradual accretion, but during storms the coast is eroded. In 1990 there are two heavy storms that sweep away large sections of the dunes along the Dutch coast. To prevent further erosion, in a white paper entitled Dynamic coastline management the Cabinet announces its intention to maintain the coastline as it is in 1990, which is regarded as the basic coastline. This is mainly achieved by means of beach nourishment. Later the Cabinet expands these measures with the suppletion of sand loss in deeper water. U N D E R S T A N D I N G C O N C E P T S 15

16 (Near) floods in 1993 and 1995 Due to heavy rainfall in 1993 and 1995 the Rhine and the Meuse have to contend with very high water levels. Floods along the Meuse in Limburg causes extensive damage and in some places people need to be evacuated. In many parts of Brabant and Gelderland the dikes are barely able to cope with the volume of water. In January 1995 more than 250,000 residents have to be evacuated because of the risk that the dikes will fail. This time there are huge economic losses as many businesses are forced to suspend their activities. As soon as things return to normal, the Cabinet draws up the Delta Plan for the Major Rivers. The Delta Plan for the Major Rivers The flood surges in 1993 and 1995 lead to the formulation of the Delta Plan for the Major Rivers. The dikes and embankments are to be improved without delay to ensure that they are of the required structural strength in the short term (these improvements have actually been overdue for some time). The Delta Major Rivers Act enters into effect on 21 April The act is unique in the way that it provides for concentrated decision-making, public consultation and the safeguarding of legal rights. It 16 F L O O D R I S K

17 enables the government to take over privately owned land with immediate effect and suspends all other statutory regulations that would normally apply, including the obligation to produce an environmental impact assesment. This makes it possible for the most urgent dike reinforcements and embankment construction to be tackled without delay. Five years later eighty percent of the works have been completed. The last dike section is due to be completed in Water management in the 21st century At the end of the nineties the committee Water Management 21st centuary appointed to address the issue of water management in the 21st century concludes that the water management system currently being implemented in the Netherlands will not be able to cope with the anticipated developments. In a policy document entitled A Different Approach to Water Management (2000) the Cabinet outlines a radically different approach. Water is to be given more space rather than less. The object of this new policy is to reduce the probability of disasters caused by flooding, to reduce inundation caused by heavy rainfall, and to store water for anticipated periods of drought. With the introduction of a U N D E R S T A N D I N G C O N C E P T S 17

18 strategy referred to as retain, store and drain, the policy makers break with the traditional approach which is to pump and drain as fast as possible. This new approach will help to ensure that water problems are not simply passed on to lower-lying areas of the Netherlands. The Maaswerken project, the national Space for the Rivers project and the organisation of regional water storage facilities are examples of this approach being put into practice. The Maaswerken project In 2006 the Directorate-General of Public Works and Water Management and the provincial authorities in Limburg start working on a massive infrastructural project known as the Maaswerken project. The project is designed to improve flood risk management in the catchment area of the Meuse in Limburg, North Brabant and Gelderland. The project is divided into two 18 F L O O D R I S K

19 parts the Sand Meuse and the Border Meuse. Excavators will widen and/or deepen long stretches of the river bed. The plan also provides for the improvement of navigability. Besides reducing the risk of flooding and meeting the demand for gravel, the Maaswerken project also involves the creation of hundreds of hectares of new natural ecosystems and the construction of two flood channels in North Limburg. Near Roermond there will be a retention area with two large water basins, and more than forty kilometres of embankments will protect those who live near the banks of the Meuse from rising river water. The Sand Meuse and Border Meuse parts of the project are due to be completed in 2015 and 2017 respectively. The Space for the Rivers project The area behind river dikes is subject to increasingly intensive use as more homes, business and industrial premises and farms are developed in these areas, which means that the consequences of flooding are likely to be all the greater. At the same time these areas also become more vulnerable as climate change increases river discharge. While new dike reinforcements reduce the probability of flooding, if there is a flood, the consequences will be that much more extreme. In an attempt to ensure that the Netherlands remains sufficiently safe, habitable and attractive, the government has come up with a remarkable alternative in the form of the Space for the Rivers project. In 2006 the Cabinet adopts a Key Physical Planning Decision to create space for the rivers. The object of the Space for the Rivers project is threefold: In 2015 the branches of the Rhine must safely be able to accommodate a normative river discharge of 16,000 cubic metres of water per second. The measures that need to be implemented to bring this about must also improve the spatial quality of the river region. The extra space that the rivers are likely to need during the U N D E R S T A N D I N G C O N C E P T S 19

20 course of this century (as climate change progresses) must remain available. Work on the project starts in 2007 and includes measures such as the creation of floodplains, the lowering of breakwaters (deepening of the foreland) and the relocation of dikes. The project is due to be completed in Weak links along the coast Waves hit the coast with considerable force. In 2003 new insights into wave lengths lead to a reassessment of all of the water defences along the North Sea Coast. The assessment reveals that there are ten weak links along the Dutch coast. These links will fail to meet the flood risk standards before the year Some of these links are reinforced in 2003 and But there needs to be a structural solution for all ten weak links before Eight of these weak links have priority status, which means that the measures implemented to reinforce the coastal defences, must also enhance the nature, landscape, economic function and recreational values of these areas. Project to assess Flood Risks and Safety in the Netherlands What is the probability of flooding? And what are the likely consequences? These are the two main questions that a project set up to assess flood risks and safety in the Netherlands (FLORIS) sets out to answer. The project launched in 2001 seeks to identify the failure mechanisms that contribute to the occurrence of a flood. The project also uses a new method to calculate the consequences of dike failure. The FLORIS project is due to be completed by the end of 2009 and the study it produces will be used to update policy. Hurricane Katrina On 25 August 2005 the United States experiences one of the worst natural disasters in its history: Hurricane Katrina. All 500,000 of the people who live in New Orleans have to be evacuated. The storm surge causes between 30 and 40 dike sections to fail, 20 F L O O D R I S K

21 which leaves approximately 75% of New Orleans under water. There are more than 1,300 fatalities and the cost of the damage exceeds 100 billion dollars. The disaster reminds the Netherlands of what it means to live on a delta. And it also serves as a warning that we would be well advised to think more carefully about our own safety. The United States has since shown a great deal of interest in the Netherlands knowledge of flood surge protection. In the days following the disaster the Netherlands sends materials and manpower to assist with the reconstruction. The Water Act The Water Act replaces eight existing laws on water management. Including the Flood Defences Act and the Public Works (Maintenance of Engineering Structures) Act, both of which are important for flood risk policy. From now on the management of surface water and ground water and the improvement of U N D E R S T A N D I N G C O N C E P T S 21

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23 the interrelationship between water policy and spatial planning will be governed by the Water Act. The act ensures that central government, water boards, and municipal and provincial authorities are better equipped to prevent flooding, water shortages and water pollution. The act also provides for the assigning of functions for different uses of water, such as shipping, drinking-water supply, agriculture, industry and recreational activity. Depending on the function, the government imposes certain requirements that apply to the quality and handling of the water. The European Flood Directive Water does not stop at national borders. Hence to some extent the risk of flooding along the rivers in the Netherlands also depends on developments in Germany, France and Belgium. In 2004 the Netherlands proposes the development of an EU flood prevention programme with a view to promoting collaboration with neighbouring countries on flood surge protection. In 2007 the European Parliament approves a Flood Directive that establishes the framework for this collaboration. The member states are required to respect the principles of solidarity (not passing on the problem), the necessity of dealing with the entire catchment area, and a range of measures based on the risk management cycle. Member states must also endeavour to reduce risks, and in doing so they must consider various factors, such as water, spatial planning, nature and the economy. All stakeholders must be actively involved in the planning. The member states must analyse which areas are at serious risk by 2011 and must produce flood risk maps by 2013 and flood risk management plans by the end of The member states are also free to implement other initiatives in addition to the measures required by the Flood Directive. The Flood Surge Protection Programme Approximately 65% of the Netherlands Gross Domestic Product is produced below sea level. So effective protection of this low- U N D E R S T A N D I N G C O N C E P T S 23

24 lying land is absolutely vital. This is an ongoing task, partly because of climate change, among other things. A new regulation introduced in 1996 means that the authorities responsible for managing the water defences (the water boards and the Directorate-General of Public Works and Water Management) are now legally required to assess the quality of the primary flood defences once every five years. The Transport, Public Works and Water Management Inspectorate reviews the assessment. If the defences fail to meet the requirements, the authorities must take action. The government provides subsidy funding for the implementation of measures that are considered to be necessary via the Flood Surge Protection Programme. So far, this has led to the improvement of more than one hundred projects, including the weak links along the coast. The programme is updated from one year to the next. Flood risk in the 21st Century (WV21) Climate change, economic growth, the increase in the population and new insights into the probability and probable consequences of dike failure are forcing the Dutch government to re-examine its flood prevention policy. The policy is now being updated 24 F L O O D R I S K

25 within the context of the Flood risk in the 21st Century project (WV21). Initial explorations in 2006 and 2007 identify three main issues. How do we update the prevention policy so we reduce the probability of flooding? How do we minimise the consequences of flooding? And how do we increase flood risk awareness and water-aware behaviour? Climate change The climate is changing. Temperatures in Europe are set to rise by six degrees Celsius this century. The future will bring with it a hotter climate, more drought, more precipitation, more and heavier wind and a rise in sea level. The higher temperatures will result in an increase in meltwater and the existing water will expand. According to the Royal Netherlands Meteorological Institute (KNMI), sea level will rise by 35 to 85 centimetres during the course of this century. And more violent storms will drive higher waves further over the coastal defences. At the same time the increase in precipitation in the winter will mean that the rivers have to contend with higher peak discharges. And more intense short storms in the summer will cause more frequent floods. The Netherlands is preparing itself for this scenario by keeping a close eye on developments. And by designing flood defences that will be able to contend with the rise in sea level. U N D E R S T A N D I N G C O N C E P T S 25

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27 The concept of flood risk 1.1 What is the risk management cycle? The risk management cycle is a systematic approach to dealing with flood risks. It consists of five links. Proaction The prevention of high-risk situations. In the earliest phase of the planning, measures are implemented to prevent or avert dangers. The Major Rivers policy is a good example of this, in that it sets out rules that apply to building on floodplains in order to minimise fatalities and damage in the event of a flood. Prevention Prevention of disasters. Firstly by ensuring that existing risks do not escalate into a full-blown disaster, by managing the water defences and reinforcing or raising the height of the dikes for example. And secondly by endeavouring to minimise the consequences of a disaster, by constructing intermediate dikes, for example. Preparation Being fully prepared to deal with a disaster. By drawing up plans, maintaining systems, ensuring that the right materials and equipment are to hand and that districts and buildings are accessible, and by training and drilling emergency services staff and informing the public. U N D E R S T A N D I N G C O N C E P T S 27

28 Response Containing and dealing with accidents and disasters. In the case of a flood, the response involves saving victims, evacuating people from the area affected by the disaster, and draining areas that have been flooded. Follow-up All of the actions necessary to ensure that things return to normal as fast as possible. These include things such as physical repairs, accounting for what happened, evaluation of policy and the provision of psychosocial care. What are the different zones? The causes of a flood differ from one zone to another. There are three different zones: coast, rivers and lakes. Coast The Dutch coast consists of three different zones which have different characteristics. The delta region in the southwest of the Netherlands, where several major rivers meet the sea (estuary), the coast of the provinces of South and North 28 F L O O D R I S K

29 Holland, where there are extensive dunes, and the dynamic intertidal area of the Dutch Wadden Sea. The main danger of flooding comes from the sea and depends on the tide, and the direction and speed of the wind. A flood surge along the coast can only be predicted a day and a half in advance. This makes it very difficult to prepare for a coastal flood. Rivers Rivers have to contend with peak surges, consisting of meltwater and rainwater, mainly in the winter and the spring. The hinterland is protected by dikes, embankments and high-lying land. River surges can be predicted relatively rapidly by measuring water levels and predicting precipitation and discharge levels in the catchment area upstream of the river. Lakes Lakes receive water from rivers and regional drainage systems. River surges can cause the water level of a lake to rise too high. And in the case of very large lakes, such as U N D E R S T A N D I N G C O N C E P T S 29

30 Lake IJsselmeer and Lake Markermeer, the wind also plays a significant role by increasing the height of the waves. What is a flood? A flood occurs when an unmanageable quantity water sweeps across an area of land. The water may come from a river, a lake, or the sea, if there is a hole in a water defence for example. Or so much water may flow over a dike that sand bags and other emergency measures are unable to contain it. So a lock that is leaking or submerged, without causing an unmanageable situation, is not a flood. Water that sweeps across the land during or after heavy rain is referred to as inundation rather than flooding. And in the case of immersion, which is not synonymous with inundation, an area of land is deliberately covered with water. What is an exceeding frequency? The probability that a water defence will be unable to withstand water levels and waves, because the water level and the waves reach heights that exceed the circumstances that the water defence is designed to cope with. These circumstances are referred to as normative circumstances and are identified for each dike section. The use of the term exceeding frequency has to do with the way safety standards are established in the Netherlands. The design of a dike is based on a defined exceeding frequency. For example, an exceeding frequency of one in 2,000 per year means that the water defence must be able to withstand all combinations of water levels and waves that have an occurrence probability of one in 2,000 per year. In other words, the exceeding frequency is not a measure of the strength of water defences. In theory, a higher, and therefore less probable, water level would cause the water to flow over the top of the dike and sweep across the area behind dike. But this is only in theory, because in reality the design of the dike also includes a safety margin or freeboard, which 30 F L O O D R I S K

31 ensures that no waves will break over the dike. This is also referred to as residual strength. dike section exceeding frequency dike ring probability of flooding What is the probability of flooding? The probability that a dike ring is unintentionally flooded with an unmanageable quantity of water because a water defence fails in one or more places. The hydraulic load (normative circumstances) and the height and strength of a water defence are factored in when calculating the probability. If probability of flooding is said to be one in 1,000 per year, the probability of a flood happening is 0.01% in any one year. However this does not mean that a flood will only occur once every 1,000 years. Circumstances change, and as they change so does the probability of flooding. The calculation of the probability of flooding allows for the fact that: Several failure mechanisms are possible. In principle, the probability of flooding applies to a dike ring as a whole, irrespective of the point from which the area is flooded. The probability of flooding is the probability that the water defence fails not the probability that a critical load occurs. U N D E R S T A N D I N G C O N C E P T S 31

32 What is the relationship between the exceeding frequency and the probability of flooding? The exceeding frequency is an expression of the hydraulic load that a water defence must be able to withstand and applies to the external dike. The probability of flooding says something about the strength of the water defence and applies to the internal dike. When calculating the probability of flooding, various failure mechanisms and uncertainties are taken into account. In other words, besides allowing for extremely high water levels when calculating the exceeding frequency, we also allow for the fact that a dike may be instable or that a hydraulic structure may fail to close in time. The exceeding frequency is therefore one of the factors in the probability of flooding. The combined probabilities of all of the possible failure mechanisms determine the probability of flooding. If the water defence is in order and safely able to hold back the normative high water levels, the probability of flooding will be less than the exceeding frequency. The residual strength of the dike plays a primary role in this. This principle is also enshrined in the Flood Defences Act. Dikes must strong enough to be able to withstand a certain high water level and a certain amount of wave overtopping. What are the possible consequences of flooding? The possible consequences of flooding include fatalities, injuries, material damage and psychological trauma. People, pets and livestock can be wounded or killed or may experience psychological trauma. Homes and buildings suffer water damage and, in the days following a flood, there are economic losses, such as production losses and loss of income, as business are forced to suspend their activities. A flood also has a knock-on effect beyond the area that is directly affected. Haulage companies incur additional costs as they have to drive around the area that has been flooded. Companies that deliver to customers in the area that has been flooded sustain losses, while others may benefit from 32 F L O O D R I S K

33 the situation, if they take over other businesses in the area that has been flooded. Possible negative consequences of a flood Losses incurred due to damage in the area that has been flooded. Damage in the area that has been flooded due to suspension of business activities. Loss of income experienced by retailers, hotels and other such businesses, production losses experienced by businesses Social disruption Non-material damage Fatalities, physical injury, damage to ecosystems, damage to cultural heritage, loss of irreplaceable items such as photographs. Social disruption Stress and grief experienced by people other than the residents in the area that has been flooded. Stress of being evacuated Material damage Damage to property (such as homes, factories, fields, roads, cars), cost of repairing water defences Loss of income experienced by retailers, hotels and other such businesses, production losses experienced by businesses Production losses experienced by businesses outside of the area that has been flooded due to a shortage of materials, the lack of a sales market, or loss of infrastructure Need for emergency services, evacuation U N D E R S T A N D I N G C O N C E P T S 33

34 large probability large consequences large dike probability ring small probability consequences of flooding small probability large consequences small probability small consequences What is the risk of flooding? Besides the probability of flooding the possible consequences of flooding are also important. If a river floods in an empty field, for example, the consequences will not be as great as they would be if a river floods in a residential area. The risk of flooding is a measure of the probability of flooding and the consequences. We calculate the risk of flooding by multiplying the consequences of a flood by the probability of flooding. In other words, the risk of flooding increases as the probability of flooding increases, possibly because of a higher river discharge or a higher sea level, or as the consequences 34 F L O O D R I S K

35 of flooding increase because of the presence of many residents and businesses. Conversely, the risk of flooding decreases if we reduce the probability of flooding, by raising the height of the dikes for example. Or by reducing the consequences by designing more effective evacuation systems or constructing buildings on mounds or stilts. The risk of flooding is expressed as an average amount of loss per year. What is a dike ring A dike ring is a connected ring of water defences (dikes, dunes and/or hydraulic structures) that protect an area from flooding. Some dike rings are surrounded by other water defences and high ground, such as the area between the Veluwe and the IJssel. The area within a dike ring is known as the dike ring area. A dike ring is divided up into sections, hydraulic structures, dike sections and dune sections that are more or less the same height. What safety standards are there in the Netherlands? The water defences in the Netherlands have to meet certain safety standards. These safety standards are based on a series of recommendations made by the Delta Committee in light of the optimal probability of flooding. Having conducted a cost-benefit analysis, the committee calculated the optimal probability of flooding as one in 125,000 per year. However, since we do not know enough about the effect of the strength of the retaining structure and the dike section to be able to assess the probability of flooding in practice, rather than basing safety standards on the probability of flooding, we base them on the exceeding frequency. The committee calculates the maximum water level that the water defence must be able to withstand. The Flood Defences Act specifies the exceeding frequency for each dike ring in the Netherlands. These safety standards were first introduced after the infamous flood of 1953 and have applied ever since. U N D E R S T A N D I N G C O N C E P T S 35

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37 The norms of the probability of exeeding per region, per year Exceeding frequency standards Region 1/250 Dike rings along the Meuse and south of Nijmegen 1/1.250 Rivers 1/2.000 Transitional zones between the rivers and the coast and the Dutch West Frisian Islands 1/4.000 The delta region, the north of the Netherlands, the island of Texel and the IJsselmeer region 1/ The coast of the provinces of South and North Holland What laws protect the people of the Netherlands from high water levels? The measures that protect the people of the Netherlands from high water levels are stipulated in the Flood Defences Act. The act stipulates that the authorities responsible for managing the water defences must assess the quality of the primary water defences once every five years. This is done per dike ring area and on the basis of normative circumstances. The water boards send the results to the provincial authorities, who submit a report to the State Secretary for Transport, Public Works and Water Management. Since 2001 the production of the report of the assessment has been coordinated by the Transport, Public Works and Water Management Inspectorate. The State Secretary then informs the Lower House of the Dutch parliament of the content of the report. The Dutch Ministry of Transport, Public Works and Water Management reviews and, if necessary, revises the limiting conditions and the assessment criteria every five years prior to the carrying out of the assessment. This constant reviewing and revising is necessary because the sea level is rising, there is more precipitation, more ice is melting and the storm climate at sea is changing. The Flood Defences Act also stipulates that the Directorate-General of Public Works and Water Management is required to notify the responsible authorities of impending high water levels and storms at sea. U N D E R S T A N D I N G C O N C E P T S 37

38 38 FLOOD RISK

39 Probability of flooding What causes a water defence to fail? The factors that cause a water defence to fail differ from one water defence to another. Dikes, dunes and hydraulic structures all have their own failure mechanisms. If a water defence fails, there is a breach in the structure and water floods into the area. Mechanisms that cause a dike to fail: Overflowing or wave overtopping A large volume of water flows over the dike, or waves break over the dike. The resulting erosion of the inner slope of the dike eventually causes the dike to fail. Bursting open and piping erosion The pressure of the water first causes a covering layer of clay to burst open, then washes away sand leaving the dike prone to piping erosion and subsequent collapse. Damage of the dike revetment and erosion of the outer slope Waves damage the dike revetment leaving the dike prone to erosion and subsequent failure. Sliding of the inner slope of the dike Persistently high water levels raise the level of the water table within the dike. This causes the ground to become unstable and the inner slope of the dike eventually slides, causing the dike to fail. U N D E R S T A N D I N G C O N C E P T S 39

40 overflowing and wave overtopping erosion outer slope instability inner slope piping Mechanisms that cause a dune to fail: Dune erosion The onslaught of the waves during a storm washes away a large part of the dune. Seawater may also wash through a dune if there is not enough sand in the profile of the dune. 40 F L O O D R I S K

41 dune base Amsterdam Ordnance Datum foreshore beach primairy flood defence dune area Mechanisms that cause a hydraulic structure to fail: Overflowing or wave overtopping High water levels or waves cause water to flow over a hydraulic structure and this causes the hydraulic structure to fail. Non-closure The hydraulic structure fails because it does not close. The cause may be human (the lock operator is absent or makes a mistake) or technical failure (the lock gate jams). overflowing and wave overtopping instability non-closure failure U N D E R S T A N D I N G C O N C E P T S 41

42 collision risk piping Instability Parts of the hydraulic structure give way and eventually the entire hydraulic structure fails. What effect do emergency measures have on the probability of flooding? Flood bags, sand bags and/or membranes all serve as an emergency measure in the event of an impending flood. Dike monitoring is essential to identify weak areas without delay. Emergency measures are especially effective when wind is driving waves against the dike. Studies show that the dike rings on the right bank of the IJssel and along the Waal are most likely to benefit from emergency measures. 42 F L O O D R I S K

43 Emergency measures are not possible along the coast or in the IJsselmeer region, because storm surges and high water levels can only be predicted a short while in advance. And violent weather conditions make it impossible to climb onto a dike during a storm. What is a system effect? A flood in one dike ring can affect the water in another dike ring. This is known as the system effect. A system effect can manifest in different ways. Dike failure reduces the flood surge downstream, which reduces the water level and the probability of flooding. Conversely there might conceivably be a cascade effect, which will do the opposite. In this case the water will flow through the dike ring to another dike ring or another (branch of the) river, causing water levels to rise thereby increasing the probability of flooding. The system effect near national borders Like a river, the system effect shows little respect for national borders. If, for example, a dike fails along the Lower Rhine, the flood water may enter the crossborder Rhine and IJssel, Ooij and Millingen, IJssel or Waal dike rings via the Oude IJssel valley (past Doetinchem). This will increase the probability of flooding. Uncertainties in probability of flooding The calculation of the probability of flooding is simply an estimate of the probability of flooding. This estimate takes into account the effect of various uncertainties. These uncertainties are due to gaps in our knowledge. If, for example, we are uncertain (lack knowledge) about the substratum, we work with conservative values. The probability of flooding then increases. We choose to do this because underestimating the probability of flooding could result in a false sense of security. By gaining more knowledge about the most important variables, we are able to reduce the probability of flooding. U N D E R S T A N D I N G C O N C E P T S 43

44 44 F L O O D R I S K

45 The consequences of flooding How does a flood occur? It is difficult to predict precisely what will happen. The way in which a flood occurs will depend on the speed with which the water sweeps into the area, the depth of the water and the size of the area that is flooded. The nature of a flood will differ considerably from one region to another. The pattern of a flood also depends on the volume of water and size of the breach in the water defences. The river discharge and the duration of the flood surge determine the influx from a river. If water defences are breached by the sea, the effect of the tides determines how much water sweeps into the area. High tide peaks and subsides more rapidly than the flood surge on a river and lasts only a few hours, rather than several days. How quickly the size of a breach increases depends on the materials used to construct the dike (sand or clay), the nature of the substratum and the difference in the water table landward and seaward of the dike. Secondary dikes and other obstructions also determine how much of an area is flooded. In some polders it is virtually impossible for a whole dike ring to be flooded. How do we determine the consequences of flooding? We determine the consequences of flooding by estimating the depth of the water and the size of the area that will be flooded. In other cases we model the pattern of a flood. This shows what will happen if a dike fails and how deep the water will be. In some dike rings the point at which the water defences U N D E R S T A N D I N G C O N C E P T S 45

46 fail determines how much of the dike ring area is flooded. This applies to the large dike rings in particular. We also estimate the consequences of a certain depth of water or flood scenario. In the Netherlands we do this with the aid of the Standard Method for Calculating Flood Damage and Victims. 46 F L O O D R I S K

47 What are the factors that determine the severity of the consequences of flooding? The pattern of a flood, the speed with which the water sweeps into the area and the duration of the flood determine the severity of the consequences. The speed with which the water rises and the maximum depth of the water are also determining factors. These variables depend on the point at which a dike fails, the water level outside the dike ring and the volume of water involved. The socioeconomic characteristics of the area are also significant. Does the area maintain important economic relations with other regions? Do a large (or small) number of people live in the area and how do they use the land? Once U N D E R S T A N D I N G C O N C E P T S 47

48 it becomes apparent that a flood is inevitable the advance warning, the condition of the approach roads and the presence of higher-lying land are important factors. The time at which a flood occurs is also important. Will it happen at the night, during the day, on a public holiday? What season is it? Is there both heavy rainfall and high winds? Is the water polluted? Are there any emergency measures that can be implemented? In other words, it is possible to conceive all kinds of scenarios. How do we estimate the extent of the material damage and the number of victims? We estimate the extent of the material damage and the number of victims with the aid of the Standard Method for Calculating Flood Damage and Victims. Once the pattern of a flood becomes apparent, we calculate the flood damage with the flood damage module. The entry data consists of a map of the area with the anticipated depth of the water, and other data such as the maximum speed at which the water is expected to sweep into the area, the speed at which the water is expected to rise and any storm warnings. The result is a map of the number of victims and the expected flood damage. Yet even if the pattern of a flood has become apparent, some things still remain uncertain. This is especially true of the number of victims. How many people need to be evacuated as a preventive measure? And how many people will flee from the area or head for high-rise buildings? To allow for these different possibilities, the number of victims in the scenarios includes a margin. Do we implement evacuation plans? Municipal authorities are responsible for drawing up evacuation plans. Prompt evacuation reduces the number of victims, and can even result in zero victims. Evacuation means that people and animals have to be moved out of the area that is (likely to be) flooded, but it also means that everyone has to be able to find safe refuge within a dike ring area. The Evacuation Calculator is a tool that can be used to explore the possibilities. 48 F L O O D R I S K

49 This calculator shows the dike rings that are difficult to evacuate. It also explores different traffic management systems. That way we know how much traffic the approach roads can cope with. And we also know where extra manpower and resources will be needed. U N D E R S T A N D I N G C O N C E P T S 49

50 50 F L O O D R I S K

51 The risk of flooding How do we determine the risk of flooding? We determine the risk of flooding by multiplying the probability of flooding by the possible consequences of a flood. Hence the risk of flooding is a measure of the probability of flooding and the possible consequences. Each probability of flooding relates to a different outcome. This also applies to the damage risk. For example, extremely high river discharge is not likely to occur very often, but when it does occur, it is likely to have greater consequences than low river discharge. The sum of all of the scenarios determines the total risk of flooding. What is the fatality risk? The fatality risk is the risk of an individual dying. This risk calculates the probability of an individual dying in a certain place as a result of (in this case) a flood. For all of the people of the Netherlands the fatality risk averages at one in ten million per year. Naturally this will depend on where a person lives and the steps they are able to take to save themselves. An athletic twenty-year-old will be more likely to be able to save themselves than an elderly person who is not steady on their feet. The fatality risk generally refers to a group risk. When calculating the group risk we calculate the probability of a flood that will cause some of the people present to lose their lives. In calculating this risk we take into account the wider social impact of incidents that involve large numbers of victims. U N D E R S T A N D I N G C O N C E P T S 51

52 What is the damage risk? The damage risk is the average material damage caused by flooding in any one year. Or the value of the material damage caused by flooding multiplied by the probability of flooding. Does the Netherlands revise the safety standards in light of the risks? The safety standards per dike ring vary from one in 250 to one in 10,000 per year. These standards are based on the cost-benefit analysis conducted by the Delta Committee for Central Holland. We calculate the exceeding frequency on the basis of these standards. However, we need to consider whether the current standards are still appropriate given the value that exists behind the water defences. The determination of a socially acceptable risk is ultimately a political consideration. How do we determine the economically optimal safety level? We determine the economically optimal safety level on the basis of a cost-benefit. It costs money to implement measures to reduce the risk of flooding. In calculating the risk of flooding we weigh up these costs against the benefit of preventing a flood. In the case of the damage risk this calculation is straightforward: the cost of the measures can be directly compared with the cost of the material damage that can be prevented. This cost is expressed in euros. In the case of fatality risks the comparison is more difficult. For how do we work out how much a person s life is worth? Studies have shown that the Netherlands is prepared to invest 2.2 million euros to prevent one fatal traffic accident. This is an indication of how much economic value society places on a person s life. So for the moment we are working with the sum of 2.2 million euros per human life in our cost-benefit analyses. 52 F L O O D R I S K