Lecture notes CT5314
Flood Defences
General
These lecture notes have been compiled from former lecture notes and are under constant review. In this sense these lecture notes are still DRAFT.
Some chapters still have to be filled in. In combination with the lectures and the slides as published on the "blackboard" these notes give sufficient information to study the principles of flood defences. In addition to these notes and the slides we will provide a series of example problems that may be dealt with during the oral examination.
Michel Tonneijck Joop Weijers February 2008
Table of contents
1. Introduction ... 9
1.1 History of water defences. ...10
1.2 The birth of the modern technology in water defences...12
2. Organisation in the Netherlands... 16
2.1 The system of water defence...16
2.2 Classification of dikes by category...17
2.3 Classification by type ...18
2.3.1 Dunes ...19
2.3.2 Soil structures ...19
2.3.3 Special water retaining hydraulic structures...19
2.3.4 Water retaining hydraulic structures...20
2.3.5 Combinations...21
2.3.6 Objects...21
2.4 Administrative framework...21
2.5 Legal basis...23
2.6 Policy implementation ...25
3. Design and other functions... 26
3.1 Values of Safety, Landscape, Nature and Cultural heritage...27
3.2 The vision on a dike improvement project ...28
4. Safety of the flood defence ... 30
Loads. ...31
Strength (Resistance of the water defences) ...31
Consequences...32
4.1.1 Catalogue of Landscape aspects ...32
4.1.2 Catalogue of Nature aspects ...32
4.1.3 Catalogue of Cultural aspects ...33
4.1.4 Valuation of other social functions...33
4.1.5 Valuation of the aspects ...34
4.1.6 Bottlenecks and solutions...34
4.1.7 Choice of options...35
5. Analysis of the structural design... 37
5.1 Analysis...37
Functions and values...37
Structural Design ...37
5.1.1 Longitudinal composition of a dike ...37
5.1.2 The composition of a dike in transverse profile ...38
Technical management limits ...39
The elements ...39
Elements and dike type ...39
Element groups ...39
5.1.3 Structural functions of the elements ...40
Crest ...40
Dike core...40
Slopes and berms...41
Outer slope ...41
Outer berm...41
Low outer berm/ transitional berm (plasberm/kreukelberm)...41
Inner slope ...41
Transitional Slope...42
Berm or dike ditch...42
5.1.4 Structural functions of the substratum...42
Dike base...42
Foreshore ...43
Hinterland ...43
5.1.5 Special elements ...44
Seal in water-bearing intermediate sand layer ...44
Screen in crest (continuous in the substratum) ...44
Drainage ...44
5.1.6 Connecting Structures ...44
Dike to dune connections ...45
Connection dike to high ground...45
Connection dike to hydraulic structure ...46
5.2 Limit states overview...47
5.2.1 Overflow...47
5.2.2 Wave overtopping...47
5.2.3 Sliding inner slope. ...47
5.2.4 Shearing. ...48
5.2.5 Sliding outer slope ...48
5.2.6 Micro instability ...48
5.2.7 Piping...48
5.2.8 Erosion outer slope...48
5.2.9 Erosion "first bank" ...48
5.2.10 Drifting ice...48
5.2.11 Settlement...48
5.2.12 Vessel collision. ...49
5.2.13 Failure of special structures and hydraulic structures ...50
5.2.14 Objects...50 Fault-tree ...50 5.3 Loading ...52 Permanent ...52 Variable...52 5.3.1 Permanent loading ...52
Own weight and soil pressure ...52
Extraction from the substratum...52
5.3.2 Hydraulic loading ...53
5.3.3 Normative high water level (NHW) ...54
Primary water retaining structures...54
Secondary Water-retaining structures...54
5.3.4 Water level development during normative loading ...54
River dikes ...55
Sea dikes ...55
Lake dikes...55
Dikes in the deltas ...55
Dikes in the estuaries ...55
5.3.5 Increase in normative high water levels. ...56
5.3.6 Local increases in water level...56
5.3.7 Wave overtopping height...56
Wind waves ...56
5.3.8 Vessel-induced hydraulic loading ...57
5.3.9 Wave boundary conditions under lower than normative water levels NHW ...57
5.3.10 Other types of loading. ...57
Wind...57
Influence of wind...57
Ice ...58
Traffic...58
Earthquakes...58
Collisions, floating objects ...58
Biological damage ...59
Chemical damage...59
Damage resulting from climate...59
Vandalism and shared use ...59
5.4 Probabilistic design methods. ...59
5.4.1 Dike ring areas. ...60
5.4.2 Distinction within a dike ring area ...62
5.4.3 The Delta committee’s safety principles...63
5.4.4 Developments in the safety approach ...64
5.4.5 Overload approach per dike section...65
Safety requirements ...65
5.4.6 Overload approach per dike ring area...65
5.4.7 Flood (inundation) probability approach ...65
5.4.8 Flood (inundation) risk approach ...66
Safety requirement ...66
5.4.9 From design water level to flood (inundation) risk approach...66
Current situation ...66
5.4.10 Future developments...67
5.4.11 Implementation ...67
6. Crest level of a dike. ... 69
6.1 Crest construction height ...69
6.2 Settlement...70
Crest ...70
Banks and berms...70
Residual settlement ...70
Settling...71
Decline in the soil level ...71
6.3 Horizontal deformation...71
6.4 Crest construction height in special structures ...71
7. Macro-stability of a water defence... 72
7.1 General consideration on macro-stability ...72
7.1.1 Overview...72
7.1.2 Loading scenarios ...73
7.1.3 Stability inner slope ...73
7.1.4 Stability outer slope ...73
7.1.5 Stability during construction...74
7.2 Slip plane calculations ...74
7.2.1 General ...74
7.2.2 Soil behaviour during shearing. ...74
7.2.3 Stability during uplift and heave...75
End phase...77
Design...77
7.3 Finite elements calculations...78
7.4 Resistance to shearing: stability factors ...79
Partial stability factors...80
Material factors ...81
Model factor ...81
Damage factor ...81
Safety and material factor in implementation ...81
Differentiation of the damage factor in shearing...81
7.5 Improvement measures ...82
8. Micro-stability... 83
8.1 Sand dike with a top layer of clay ...84
8.3 Calculation methods ...85
Sand core with clay revetment: testing for push up ...85
Sand core with clay covering: testing for shearing ...87
8.4 Improvement measures ...89
9. Stability during overtopping and overflow. ... 90
9.1 Infiltration and shearing...90
Clay core...91
Sandy core...91
9.2 Erosion of the inner slope ...91
9.2.1 Methods of calculation...92
Joustra and Edelman...92
Stability with infiltration ...92
9.2.2 Shearing ...93
9.3 Grass cover on the inner slope...94
9.4 Measures for improvement ...94
10. Piping. ... 95
10.1Introduction. ...95
10.2Process description...95
10.3Development of the classical rules. ...96
10.4Development of Sellmeijers rule. ...97
10.5Process description...99
Inward covering layer ...99
No inward covering layer ...100
10.6Calculation methods for uplift and heave...100
Influence of ditches and waterways ...101
10.7Calculation methods for piping ...104
Bligh...105
Lane...105
Sellmeijer ...105
10.8Improvement and management measures ...107
11. Stability of the foreshore ... 109
11.1Introduction ...109
11.2Shearing and settlement flow. ...109
11.3Calculation rules. ...110
11.3.1 Monitoring. ...111
11.3.2 Packing density ...111
11.3.3 Channel depth ...112
11.3.4 Erosion length...113
12. Limit state of dunes. ... 115
Seaside erosion...115
Erosion of the foreshore. ...115
Land sliding erosion...115
Dynamic equilibrium ...115
13. Stability outer slope... 117
13.1Introduction ...117
13.2Stability of clay and grass ...117
13.3Stability of placed revetments of concrete and natural stone ...117
13.3.1 Introduction. ...117
13.3.2 Functional analysis. ...117
Static equilibrium ...117
13.3.3 Geotechnical aspects ...117
The permeability of the top layer ...118
Clay layer...119
13.3.4 Loading by drifting ice and other floating objects ...120
14. Special water retaining structures... 121
14.1Introduction ...121
14.2Subdivision according to function in the water defence...121
14.3Selection ...124
15. Alternative solutions in the river bed ... 128
16. Non water retaining structures ... 129
16.1Trees. ...129
16.2Pipelines. ...129
16.3Buildings ...129
17. Alternative solutions in the river bed. ... 130
18. Water regulating structures. ... 131
19. Regional water defences. ... 132
20. Alternative solutions... 133
ANNEX 1
Soil-mechanical and geo-hydrological aspects ... 134
A1.1Soil-mechanical and geo-hydrological aspects ...134
A1.1.1 Introduction ...134
A1.2Geological description of the area ...134
A1.2.1 Introduction ...134
A1.2.2 Objective of the geological description of the area...135
A1.2.3 Geology Upper River area...135
A1.2.4 Geology of the Lower River area...137
A1.2.5 Coastal area geology...138
A1.3Soil behaviour and soil parameters ...139
A1.3.1 Introduction ...139
A1.3.2 Mass by unit volume...140
A1.3.3 Description of soil during shearing ...140
A1.3.4 Behaviour and strength in shearing...141
A1.3.5 Shear strength parameters...143
A1.3.6 Anisotropy...145
A1.3.7 Elasto-plastic behaviour of the Soil ...145
A1.3.8 Deformations and settlement...146
A1.3.9 Permeability ...150
A1.4Soil Analysis...151
A1.4.1 Introduction ...151
A1.4.2 Geophysical investigation...153
Explanation of Table A- 5 ...157
Application in the field...157
A1.4.3 Drilling...160
A1.4.4 Vane tests...160
A1.4.5 Field investigation to determine deformation parameters ...161
A1.4.6 Measuring pore pressure and hydraulic head ...161
A1.4.7 Electrical density measurement...162
A1.4.8 Pump and pit tests...163
A1.4.9 Infiltration tests ...163
Constant head test ...164
Falling head test ...164
A1.4.10 Compression test (odometer test)...164
A1.4.11 Measurement of shear stress parameters in the laboratory ...164
Direct shear test ...164
Critical shear plane...165
Recent developments...165
Anisotropy...165
A1.5Geo-hydrological aspects ...166
A1.5.1 Introduction ...166
A1.5.2 Groundwater flows in the Holland profile...166
A1.5.3 Modelling ...169
A1.5.4 Piezometric gauge observations and pressure flow measurements...170
A1.5.5 Pressure flow in water-bearing packages ...170
A1.5.6 Pressure flow in the weak layer package ...171
A1.6Pressure flow under design conditions ...171
A1.6.1 Introduction ...171
A1.6.2 Flood...172
A1.6.3 Extreme precipitation...172
A1.6.4 Outward stability ...173
1.
Introduction
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 250 to 10,000.
The Dutch flood protection framework is rather centralised. The Ministry of Transport, Public Works and Water Management sets the policy and legislation framework including safety standards. The Ministry is also responsible for managing the coastline 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.
Since time immemorial earth structures have been used as water defences in the Netherlands and, despite the fact that hydraulic structures and other special structures are being used to an increasing extent, they will undoubtedly continue to play an important role in the future. In this report the term ‘earth structures’ covers sea, lake and river dikes, dams, drainage canal embankments and polder dikes. Dunes are not considered to be earth structures.
The focus of this course on “Water defences” is not to teach new techniques, or to introduce a new way of analysing the stability of water defences. The focus is to show the students how to make practical use of all the techniques learned in other courses in Delft.
To determine the strength of dikes several aspects must be taken into account. A lot of these aspects can be analysed with geotechnical and geohydrological techniques. All the hydraulic aspects that are important for the load on these constructions are important as well.
In additions to these pure engineering aspects, the societal impact of the (re) construction of dikes is very big. This impact must be dealt with or no construction will be carried out.
The dikes form an important part of our Dutch history. This is true in engineering sense and in societal sense as well.
The focus of this course can be categorized as follows:
History of the protection against flooding in the Netherlands. o Why and how are these constructions built?
o How were they maintained and by whom.
o How disasters were the engine behind the development of knowledge. What is the system of the water defences?
o How do they perform? o What are water defences?
Influence of the water defences on daily life. o Safety.
o Landscape, Nature and Cultural in heritage. (LNC). o What purpose do they serve besides retaining water?
o How is the function of the water defence imbedded in legislation? The development of the safety against flooding in the Netherlands.
o The load on the dikes will be treated in a global manner. For a more detailed treated referred is to other courses
o The strength of the dikes will be treated in a global manner as well. For a more detailed treated referred is to other courses such as the several courses on soil mechanics.
o Some of the relevant soil mechanical and hydraulical techniques are treated in some detail in annexes to refresh the knowledge of the reader and to help the students who are not so experienced in some fields.
1.1
History of water defences.
More or less until the Middle Ages man adapted to nature. In coastal areas human settlements were in the higher dune areas or on man made hills, named "mounds" (in Dutch “terpen”). In the river areas people lived on the natural levees ("oeverwallen") of the rivers. As a result, the regular floods had little negative effects and even deposited fertile silt on the land, which approximately enabled the land to keep pace with the naturally rising sea level.
The rise in population meant that increasing numbers of lower lying areas were taken into use. Provisions were put into place in relation to agriculture to drain the land and peat was dug up in many places for fuel and salt. The consequence was a fall in the level of the ground surface, as a result of which flooding became a greater problem. In response the first dikes were constructed. Initially they only supplemented the natural heights; in the rivers area for instance, perpendicular to the bank walls to redirect river water flowing outside the banks along populated areas to lower-lying back lands. In the coastal area the influence of the sea increased steadily. In the southwest the great estuaries formed and in the north the Zuider Zee, which meant that more and more lowland had to be protected by dikes.
Figure 1-1 The fall in the peat surface (subsidence)
Further use of low ground led to more diking. The result was an increase of extreme water levels along the rivers and in the delta area: the loss of the flood level decreasing effect by inundation of flood retention areas. Improved drainage, first by ditches and canals, later by windmills, and
eventually by mechanical pumping systems, was the cause of a further fall in the level of the ground surface. The ultimate result, for example the peat areas of Holland, is that the land currently lies approximately three metres under sea level, as opposed to on average approximately three metres above sea level one thousand years ago. Only a small proportion of this change is due to a natural rise in sea level. It is mostly due to human influence (see Figure 1-1).
The situation is a little more favourable in low lying areas beyond Holland, where peat does not play any great role. The increasing problem of protection from the sea led to a reduction in the coastline by means of the Zuiderzee Works and the Delta Works in the twentieth century.
Nowadays almost all low-lying land in the Netherlands is densely populated. Lot of large-scale urban expansions are concentrated in low-lying polders. There is no more natural silting and the land has sunk a few metres compared with the water level. The consequences of a flood will therefore be much more serious than ever as the inundation depth and the economical value of the area’s did increase during the centuries.
The result of thousand years of efforts by our ancestors is a densely populated, highly developed, but low-lying area, where flooding could lead to the loss of human life, tens of billions of guilders’ damage and the breakdown of society. There is no way back and the only thing we can do is ensuring that we secure our residential areas prudently. That will be more difficult because the interventions
necessitated in the other functions of the flood defence and the adjacent countryside is increasingly found to be unacceptable.
In the past, when people first lived in these ‘low countries’, people lived with the water. There was no protection against floods coming from the sea, the lakes or the rivers. In case of a high water surge they ran if necessary and stayed on higher grounds if possible.
Somewhere in the Middle Ages, positive action was taken to influence the hazards of nature. The environment was changed to enable permanent occupation of the country along the sea and the rivers.
Figure 1-2 Man made hills (mounds) in coastal areas, mainly in Friesland Province
At first some small hills, named mounds where constructed and offered the population a relatively safe place to dwell. The photograph above shows one of those hills still existing in the north of the Netherlands. In that manner people and their livestock could survive a period of flood while their land around would get covered with a fresh and fertile layer of clay. When the population and the need for more space increased the development of dikes began. At first dikes protected only the settlements and would have a horseshoe shape. The water flowing down the river would be diverted to a place where it would cause less damage. In time, as the need for more space increased, the amount of protected area grew.
Figure 1-3 Settlements on natural river levees and early dike systems in Gelderland Province between the Rhine branches.
In the following centuries not only the protected area increases, but also the need for an increase of the level of protection was apparent. The value of the properties became greater and the number of human lives threatened by the water increased as well. In due time these little dams developed into the massive constructions we can find nowadays along our rivers, lakes and the sea.
In the past these dikes were designed, based on experience and ‘hands on’ skill of the builder. The experience and skill was gathered in practise. The cost was high. Many failures of the dikes were needed to learn the techniques. Practice was to construct the dikes high enough to withstand the highest water level known. This design water level and the construction techniques used had no scientific base. Experience and craftsmanship was the core of the business. No thorough analysis of the failures was performed so the possibility of missing the real cause of the failure is very high. When those dikes became bigger and the protected area did as well, the need for specialised organisations, that could oversee the whole problem, was apparent. Some 600 years ago this led tot the system of water boards still existing in the Netherlands. This organisation goes back further then the Dutch state.
1.2
The birth of the modern technology in water defences
In the last century things started to change however. The mathematical and statistical knowledge improved. Combined with the introduction in practice of fluid and soil mechanics the approach became more and more scientific. The hydraulic load on a water defence could be predicted more accurate and the strength of the water defences could be calculated. These techniques where not used at once. A disaster had to occur to make that possible.
Figure 1-4 Zeeland Flood 1953, flooded area
This disaster happened in 1953. The Dutch society got a severe wake up call. This event showed the vulnerability of the country and the lack of sound sciency to base de dike design on.
After this event the implementation of sophisticated techniques was accelerated.
Calculation models where developed to predict the water level and the wave action the constructions had to withstand. It was possible to optimise the height of a dike and to dimension the protection of the dike against wave action. The implementation of geo mechanics enabled the constructor to optimise the strength and to predict the resistance of a dike against the several ways it can collapse. (Failure mechanisms.)
Finally new exploration techniques where used to determine the characteristics of the layers in the subsoil. The “delta committee” (a committee that advised on the strategy needed to prevent disasters like the one that occurred in 1953), proposed a new way of looking at the safety of the Netherlands against flooding. This committee was aware of the fact that not all the knowledge needed to implement this new approach was available at that point, but the new way of thinking enabled the development of the scientific approach of the water defences.
One of the important changes was the development of the probabilistic techniques in hydraulic engineering. Using statistical techniques for the hydraulic load was the starting point. The realisation that the strength of a water defence is a parameter with an average and a standard deviation changed the way of designing dikes as well.
Figure 1-5 Closure of Grevelingen Estuary (Brouwer's Dam)
Construction techniques to close the estuaries like the one in Figure 1-5, still had to be developed. The Delta Committee proposed to start with the closure of the smaller estuaries and where
knowledge and experience would increase, it should be possible to increase the size of the dams to be built as well.
The science at hand is relatively young and not fully developed. This can be as exciting as frustrating. The exciting part is that ways are open for discovery and immediate implementation. The status of a Dutch “hydraulic engineer” in the world is still very good. The frustrating part may be that the
technique used in the construction of water defences will appear “old fashioned" in comparison with other fields of engineering. (i.e. mechanical engineering/physics etc.). An important reason for this is the fact that our subsoil is very unpredictable, the morphological processes are very uncertain and determination of the geotechnical strength is more inaccurate than the calculation of the strength of a steel construction.
After the realisation of the major part of the Deltaworks the public awareness of environmental issues rose. The design of the last closures needed to be changed to meet public demands for
environmental conservation. The closure of the Eastern Scheldt with a dam was altered into a semi-permeable construction instead of full closure.
In the mean time the program for river dike reinforcement had come to a virtual standstill altogether. Environmental issues like conservation of landscape, natural values and cultural heritage
(LNC-values) stopped the progress. The Brakel dike reinforcement project, where 90 houses had to give way for the new dike, appeared to be the last straw to break the camel's back.
In accordance with Dutch tradition a government commission was established to look into the matter and to formulate possible solutions to get the dike programme running again. This was the
“Commission on river dikes”, also called after its chairman, Becht Commission (1977).
Most important recommendation was to lower the safety requirement from 1/3000 per year to 1/1250 per year, allowing pure physical space for lower and particularly narrower dike reinforcements. This commission proposed a modern integrated approach that appeared to be ahead of time. The attitude of both dike constructors and environmentalists hardly changed. An adverse effect arose, when in 1986 research proved that in the case of design river floods, the morphological effects in the river bed would cause higher river water levels and require higher and wider dikes to ensure the flood
protection level of 1/1250. In fact the dikes needed to be raised and reinforced to a level
corresponding to the former 1/3000 per year. The situation escalated again to a complete standstill. Another government "Commission on re-assessment of river dike reinforcement" (Boertien
I
recommendations, which appeared to be more sustainable than in 1977. Boertien also proposes lower design water levels and used a trick to keep the food protection level at 1/1250 per year. They simply proposed a different frequency distribution for the extreme river discharges, resulting in
15,000 m³/s at Lobith instead of 16,500 m³/s, allowing to raise the dikes about 40 cm less than before. Then Mother Nature intervened. The Rhine and Meuse bought extreme floods considered as near disastrous. Adding these flood values into the (new) frequency distribution showed that the design discharge at 1/1250 per year rose back to 16,000 m³/s, almost compensating the relief brought by Boertien.
The 1993 and 1995 floods brought however back the sense of urgency for river dike reinforcement. The modernisation of the project approach with environmental impact assessment and regard for the LNC-values enabled environmentalists, dike engineers and administrators of central government, provinces and water boards to find a constructive way in completion of the remaining 600 km of dike reinforcement from an actual flood protection level of about 1/100 to the legally required level of 1/1250 or 1/4000 per year.
2.
Organisation in the Netherlands
2.1
The system of water defence
Three systems play a role in water defence, as set down in the Flood Defence Act (FDA): 1. The system of the protected dike ring areas;
2. The water management systems (outside the dike) bordered by the flood defences; 3. The system of the flood defences itself.
Figure 2-1 The system of the protected dike ring areas
In the protection against high water areas can be distinguished, each surrounded by an unbroken system of flood defences, possibly in combination with high grounds. High grounds are grounds that are high and broad to a very satisfactory degree to allow them to retain outside water without
management being necessary to maintain the situation. Encircled by flood defences and any high grounds, such an area is called a ‘dike ring area’.
The water management system (outside the dike) bordered by the flood defences
The water management system outside the dike sets the preconditions for the loads that work on the flood defences. The relevant water retaining aspects for these systems are handled in the following chapters.
The system of the primairy dikes itself
The system of flood defences around a dike ring area, including any dunes and locks, is called an encircling dike. (in Dutch ringdijk). The safety of people and goods in the dike ring area is dependent on the adequate performance of the whole encircling dike, perhaps in combination with the flood defences that are situated in front of the dike ring area. The encircling dikes of dike ring areas and any defences situated in front of it, are called primary flood defences.
Secondary or regional defences, such as the majority of compartment dikes, are beyond the
operation of the Flood Defence Act (FDA), and are therefore not handled here in detail. The effect of those water retaining structures will be treated elsewhere in this course however. The same applies to drainage/discharge canal embankments, which fulfil an important function in keeping large parts of the Netherlands dry.
2.2
Classification of dikes by category
In line with the Flood Defences Act, the primary flood defences can be subdivided on the basis of the following two characteristics:
1. A defence retains outside water or does not retain outside water. The concept ‘outside water’ is limited within the Act to the surface water where the water level is directly influenced by high tidal floods, high surface water of one of the major rivers, high water of
IJsselmeer/Markermeer or by a combination of these factors. Grevelingen lake for example, is therefore not outside water in the sense of the Act. Outside water accordingly indicates the most important threat. Inside water is all surface water except outside water.
2. A defence belongs to the system of flood defences that encircles a dike ring area or is located in front of a dike ring area.
The combination of these characteristics leads to the following four categories of primary flood defences:
1. The flood defence belongs to the system that directly encircles the dike ring area and retains outside water;
2. As category 1, but not intended for direct retention of outside water;
3. The flood defence is situated in front of a dike ring area and retains outside water; 4. As category 3, but not intended for direct retention of outside water.
In the Netherlands no land is located behind the flood defences of category 3 and 4, only water. Examples of category 3 are the IJsselmeer closure dam and the Tidal Flood Barrier in the
Oosterschelde, both of which retain high water levels on sea. In this context it makes no difference that the IJsselmeer closure dam always retains water and the Oosterschelde barrier only closes when the sea level is high. An example of category 4 is the northern part of Grevelingen dam, which has inside water on both sides. The function of flood defences of categories 3 and 4 is to prevent the occurrence of (too) high levels of water behind it, at least to greatly reduce the probability thereof. In doing so they limit the loads on the flood defences that separate the water behind them from the land in front of them.
The following distinction can be made within category 2: a) The flood defence retains outside water;
b) The flood defence only retains water if another defence has collapsed.
Examples of category 2a are the dikes of Goeree-Overflakkee and Schouwen-Duiveland on Grevelingen lake, an inside water separated from the sea by the Brouwers dam. In the case of
category 2b the flood defence separates the toe dike ring areas. An example is the Dief dike. This dike runs over land, on the provincial boundary between Zuid-Holland and Gelderland, from Everdingen on the Lek to Gorinchem on the Merwede. The Dief-dike separates two dike ring areas with a different protection level and has the status of primary flood defence. Finally, there is a fifth category, primary flood defences beyond the Netherlands. This situation is found where dike ring areas extend beyond the borders of the Netherlands. These are the dikes along the Rhine, Schelde and Eems. A collapse here can result in flooding in the Netherlands. It will be clear that international consultation is needed to realise the desired security level.
2.3
Classification by type
A large part of the Dutch coast is protected against tidal floods by natural dunes. The high grounds also form a natural protection against flooding. All other flood defences are manmade and are traditionally made of a combination of clay and sand, so-called soil bodies. The reason is obvious. The material is available in great quantities, is easy to process, flexible, easy to maintain and adept and very durable. In combination with grass, clay is reasonably stable to erosion. Structures as locks and cuts were designed in situations where the flood defence is crossed by (water) ways. These water retaining hydraulic structures were in the past most often made of wood and brickwork and later also concrete and steel.
The number of these structures is mostly limited in connection with the risk of not being able to close them (on time) and the problems of the watertight connection of these stiff structures to a soil body. There may be other reasons not to use a soil structure; usually following on from the other functions the flood defence has been given. For instance, the wish of vessels to moor on the flood defence demand a vertical wall, which leads to a retaining wall structure. In the reinforcement of a flood defence it can be decided to execute it completely or partially as a wall or screen structure, in order to gain space to save any buildings that may be considered to be of value for example.
This does not change the fact that initial thoughts are concentrated on the design of a soil structure when designing a new flood defence, and also when reinforcing existing flood defences, for the above reasons.
Figure 2-2 Main types of flood defence
On the basis of the above, there are four (main) types of structure for the protection of a dike ring area against high water. These are (see also Figure 2-2):
o Dunes;
o Soil structures (dikes, dams);
o Special water retaining structures (including cofferdam, retaining wall, sheet piling); o Water retaining hydraulic structures (including locks, cuts, tidal flood barriers, pumping
In addition, there may be objects in, on and alongside flood defences, such as pipelines, buildings and trees. These objects typically have no-water retaining functions, but could influence the water retaining capacity. For all flood defences, it must be said that the water retaining capacity must be assessed using the height and firmness of the whole structure, including all the objects in the vicinity of the dike.
2.3.1 Dunes
Dunes are natural landscape forms. They are formed by the wind from washed up sand in combined action with the vegetation that catches and holds the sand. The stabilisation can be accelerated or reinforced by marram grass (in dutch”Helm gras”). That vegetation is not meant and not able to stop erosion of the sand grains by waves at high water levels however. The effect of dunes as high water defence is solely based on the total mass of the sand. This mass must be so great that sufficient sand will stay put to retain the water level difference between sea and hinterland after sloughing by storm. After the storm the building up process by the wind can begin again. This dynamic character means that dunes demand special attention in terms of management and maintenance.
2.3.2 Soil structures
Dikes and dams are manmade soil bodies. In contrast to dunes, which can only withstand wave attack by eroding, dikes cannot be allowed to erode, due to their smaller dimensions. A dike derives that erosion stability from the materials used, clay with grass vegetation for example, or a revetment of stony materials or asphalt. A characteristic of these structures is the form of the soil body, which is trapezium shaped in sectional plane. The height and the form of the cross section supply the water retaining capacity of the structure. It must be ensured that there is sufficient resistance to shearing (firmness) and water tightness. The dike derives its firmness from the shear strength of the dike body and the subsoil.
2.3.3 Special water retaining hydraulic structures
Figure 2-3 The tidal flood defence in the Oosterschelde guarantees the safety of the area behind it,
while retaining the unique tidal eco-system. Photo 3.1
Special water retaining hydraulic structures have the same water retaining function as a soil structure, but the form and the materials can be very different. Examples are dike wall, cofferdam, and sheet pile. The special thing about these structures is that they make possible a greater freedom in form and functionality than a traditional dike design. Conversely, they do usually demand a great deal of attention in terms of management and maintenance.
These structures derive their strength from the materials used such as a steel, concrete and wood, which are able to withstand greater pressures than clay for instance. The general stability is due to friction (retaining wall with soil), by piles (retaining walls on piles, see Figure 2-3) or by wedging in the bottom (cofferdam).
Special attention in the design is demanded by the transition of the special water retaining hydraulic structure to the connecting soil structure.
2.3.4 Water retaining hydraulic structures
Water retaining hydraulic structures are made for another ('utilitarian') function that crosses the flood defence. Such a function may be:
Figure 2-4 Maeslant Storm Surge Barrier (Nieuwe Waterweg
o Shipping through a navigation lock (IJmuiden) or tidal flood barrier (Nieuwe Waterweg, Hollandse IJssel);
o Water through a pumping station (Katwijk), an outlet sluice (Haringvliet locks) or tidal flood barrier (Oosterschelde);
o Road or rail traffic through a cut (Lobith, Harlingen).
Figure 2-5 Gap in a flood defence for passing traffic, with log grooves. The
Owing to these utilitarian functions these hydraulic structures are usually provided with one or more moveable means of closure. In a closed state these means carry the forces that work upon them over to the inflexible part of the structure.
The Nieuwe Waterweg Storm Surge Barrier makes dike reinforcement in Rotterdam and Dordrecht superfluous (Figure 2-4).
2.3.5 Combinations
Figure 2-6 A house being a structural element, part of the water defence
The above clearly shows that the bounds between the various types of flood defence and objects they comprise is not very acurate. Special structure can reinforce, supplement or completely replace soil structures. Special structures may be fixed or moveable, whereas water retaining structures are actually always movable. Buildings can be saved by special structures or become part of a special structure. In some cases the building components with a specific water retaining function are easy to recognise. Figure 2-6 illustrates this.
2.3.6 Objects
‘Objects’ covers a large number of matters that are not introduced for the primary function of the flood defence, but are still part of it. These are buildings, roads, pipelines, trees et cetera.
Objects in the flood defence demand extra attention in the design and extra care in the management. Pipelines can form potential leaks in the soil body for example. Maintenance on those objects can weaken the primary function of a flood defence especially if that maintenance involves a lot of digging into the water retaining structure itself. This fact should be taken into account before one of these objects should be allowed into a dike. Buildings can lead to a weakened flood structure due to seepage capability, but may also be made in such a way that they form a special structure.
2.4
Administrative framework
The care of the flood defences in the Netherlands is spread over three administrative layers: the state, the provinces and the water boards. The municipalities are involved in spatial planning (as representative of other interests at flood defences like housing and traffic) and in the case of a threatened calamity.
A central role is reserved for the water boards. A water board is a functional administrative form, oriented to water management and flood defence management. The province has a regulatory task,
both for the municipality and the water boards and can therefore take binding decisions in the event of a difference of opinion (see box below).
DIVISION OF TASKS BETWEEN ADMINISTRATIVE ORGANS Water board/local
Water boards are responsible for the construction, management and maintenance of primary flood defences that surround a dike ring area. Water boards are controlled by an elected representation of landholders: the parties with interests in the protected area. The water boards have the power to issue the by-law needed measurements to secure the water retaining function. The management and maintenance is chiefly financed through taxation of landholders. The construction costs connected to the current round of dike reinforcement are too high for the majority of water boards however, and are therefore (largely) subsidised by the state. The construction subsidies for the river dike reinforcements were transferred to the provincial fund in 1993. The financing of integral flood defence management is a source of discussion, bearing in mind the limited terms of reference of water boards in the functional administration. It is expected however, that an integral vision on management will increasingly become common property and that financing will adapt to that. The aim is to form large and decisive water boards. This is connected to the ever increasing demands placed on the administrative and technical capacities of a water board, certainly in comparison to the situation just after the second world war. In less than fifty years the number of water boards has been reduced from around 2500 in 1950 to less than twenty now.
Province/regional
The provinces oversee the water boards. The Flood Defence Act distinguishes two specific tasks: (1) monitoring the technical quality of management, and (2) supervising proper harmony between municipal and water board policy. This last aspect is a guarantee within our polity for the adaptation of functional water board management in the general administration. The terms of reference of the water board is laid down by the province in the water board regulations. The plans for dike reinforcement and the flood defence manager’s five-year report prescribed by the FDA on the hydraulic state of the primary flood defences must also be submitted to the Provincial Executive. The regulating function also includes the national flood defences in the province. Furthermore, the province plays an important role in the organisation of the system of water boards, in the concentration of water boards mentioned for example. The setting of norms for drainage/discharge canal embankments and secondary flood defences are also provincial tasks.
State/National
The state has a number of responsibilities, including (1) legislation, (2) supreme control of the system of water boards, (3) the management of primary water defences that protect various dike ring areas (especially sea arm barriers) and the dune coast of the Wadden islands and (4) the management of the large waters and rivers. With respect to the sandy coast the state plays a specific role, of great importance to the flood defence managers along the coast. The state is responsible for maintaining the location of the coastline, one of the preconditions of the security of dunes and sea dikes. The river manager must ensure, among other things, that undesirable resistance is not created in the riverbed and that the water coming from upstream can be easily drained. The supreme control is expressed in the five-year report by the Provincial Executive to the minister of Transport, Public Works and Water Management on each dike ring area in its province, as prescribed in the tasks of the province.
Municipality/local
In the field of spatial planning the municipality draws up zoning plans in which flood defences must find a place. Whereas water boards are oriented to protection against flooding, bearing in mind their functional administration tasks, the municipalities are oriented to the other functions of water defence. In addition, the municipality has responsibilities in the case of a flood, including drawing up a contingency plan, maintaining public order and security and ensuring public health.
2.5
Legal basis
Article 21 of the Constitution calls the care of the habitability of the Netherlands a fundamental task of government. A number of legislative fields are especially important for flood defence management and improvement:
o Water management legislation (particularly the Flood Defences Act (1995), Water Control Act (1900), Delta Act (1958), Delta (Major Rivers) Act (1995), Rivers Act (1908), Water
Management Act (1989), Water Boards Act (1992), provincial regulations and water board by-laws;
o Planning legislation, particularly Spatial Planning Act (1962), Expropriation Act (1857); o Environmental legislation, including Environmental Management Act (1993), Soil Protection
Act (1986), Pollution of Surface Waters Act (1971). These acts are explained individually hereafter.
The main role of the Flood Defences Act (FDA, annex B) is to legally anchor protection from flooding by the outside water. The FDA is the legal foundation for construction, improvement and maintenance of flood defences and provides all dike ring areas with a safety norm (see annex I). The management of the flood defences is also regulated in the provincial regulations and the water board by-laws. The principal aim of the FDA is to guarantee security. It is a fact that social understandings of the risks of flooding decreases as the years elapse since the latest flood. Article 9 of the Act obliges the manager of the flood defence to report on the state of the defence in relation to the norm every five years. This is an attempt by the legislator to prevent the consequences of the process of a decreasing understanding of risks. The Delta Act regulates the damming of the sea arms in the South-West Netherlands and the improvement of the other flood defences along the whole Dutch coast (including the financing thereof). The Delta (Major Rivers) Act is an emergency act aimed at improving the weakest dikes along the great rivers in a short space of time after the high water of 1995.
The Rivers Act is oriented to ensuring that the water discharge function of the rivers continues to be guaranteed. This is also connected to the normative water levels for protection against flooding. A permit is therefore required for all works in (the winter bed of) the river.
The Water Management Act regulates in itself nothing with respect to flood defences, but the Fourth Policy Document on Water Management based on this act addresses in detail the relationship between water levels and the arrangement of the riverbed.
The Spatial Planning Act (SPA) is the basis of other zoning plans. A municipal building permit is needed for building of or on a flood defence, granted on the basis of the zoning plan. In the zoning plan the main purpose of the area of the flood defence is hydraulic. In turn, the water board draws up a by-law in which the permissible uses from a water retaining function are stated. The SPA set of instruments is also important at any building activities outside the dike.
The Expropriation Act regulates the legal procedures for ground acquisition, needed for improvement works, in the cases in which it is not possible to reach an amicable agreement.
The Environmental Management Act (EMA) regulates matters that are relevant to dike improvements. The EIA (environmental impact assessment report (MER in Dutch)) procedure, which is mandatory at improvement projects, is based on the EMA. The EMA is also applicable in the final execution of improvement works.
Materials used in flood defences must fulfil the Building Materials Decree based on the EMA. If this is not the case then a permit must be requested on the basis of the EMA.
The Soil Protection Act (SPA) covers both preventative protection and curative decontamination of the ground. The latter comes into play when a flood defence must be reinforced on soil that is very contaminated.
The Pollution of Surface Waters Act (PSW) is applicable when there may be pollution in the adjacent surface water, such as at bank facilities and use of clay screens in dikes.
2.6
Policy implementation
Numerous policy documents and regulations are important for the implementation of policy in the field of water defence. They will be addressed in the relevant chapters.
The point of departure in the implementation of policy is a multi-functional approach by which the water retaining function does not suffer, which means that the defence must fulfil the safety
requirements set. The social framework is sketched above within which dike improvements and flood defence management occur. Clearly, there is no ‘best solution’ for a specific case. There are always various possibilities that fulfil the many functions and aspects to a greater or lesser degree, and they are often (precisely) contradictory. The task of the manager and designer is to indicate that possibility and its consequences, including the financial ones. The challenge in the design is attempting to find a solution that fulfils its primary function and is well adapted socially and affordable.
The issue of sharing the financial burden is relevant not only to the design, but also to dike
improvement. Dike improvement literally intervenes in an existing situation. It is often a good moment to introduce other desired improvements in secondary functions, for instance with respect to traffic or recreational facilities. Not everything is on the account of dike improvement. An integral approach means that as many things as possible are taken into account, but not that the manager or the giver of a subsidy is left with the whole bill. The institutions or market parties responsible for the facilities usually pay for facilities that are not a consequence of, or necessitated by the improvement of the flood defence.
Another aspect of the sharing of the financial burden is the origin of the financial resources. Improvements usually come from a different financing source (subsidy from state or province) than maintenance (apportionment from residents). The prevention of improper choices at improvement works due to this difference in financing is the responsibility of all parties.
The multi-functional character of flood defences also plays a great role in management. The choice of broad dune flood defences, precisely to offer other functions more space, is one example. Another form of management of grass dikes, with less pasturing and manuring (fertilizing), creating a much more variegated and erosion stable vegetation also points to this. Good agreements on the other functions of the flood defence are needed to prevent conflicts. This is addressed in more detail later, ‘Management and Maintenance.
3.
Design and other functions.
This chapter provides an overview of how to handle the other functions of a flood defence than the retention of water. The Nature values have a central place here. Although they have played a role in dike improvement works for quite some time, these values were first given a clear place in the decision making process by the Boertien committee. (See 2.2)
The main function of flood defences is retaining water ofcourse. This chapter addresses the other functions and values. These other functions formally follow on from planning decisions taken by state, provinces and municipality. Values is used in the sense of the meaning allocated to landscape, nature and cultural heritage (LNC values). Some of these values are laid down in policy documents, but in this chapter it is also explained how to handle the Nature values for which no policy judgement has been made. The allocation of Nature values is often only expedient when activities are planned that may affect these values.
Sometimes the flood defence runs right through the town, like here in Voorstraat, Dordrecht. Figure 4-1.
Figure 3-1 Flood defence running through Dordrecht (Voorstraat)
This aspect is given a relatively great deal of attention. New insights have been rapidly developed since the Government Commission on Re-assessment of the River Dike Reinforcement Programme (Boertien
I
Commission 1993, Toetsing uitgangspunten rivierdijkversterkingen), and new policy andnew procedures formulated. Much experience has been gained for handling such planning functions as housing, traffic, agriculture, industry and recreation on and around the flood defence and various procedures exist. The significance of the dunes for nature and landscape was recognised a long time ago and fitted into policy and management. In terms of cultural heritage, the dunes are usually too young and too dynamic. As a result this chapter is chiefly oriented to Nature values of dikes in
drawing up a design and maintenance plan. The planning functions are described in spatial plans and plotted on plan maps accompanying municipal zoning plans, provincial regional plans and national policy plans (policy documents and plans for spatial planning, water management, nature and environment management, nature space, drinking and industrial water, traffic and transport). The functions housing, industry, traffic, agriculture and recreation are based on their contribution to employment and income. Their value can be expressed in market value. The functions landscape, nature and cultural heritage are based on a subjective allocation of value, for which no gauge exists in the form of a market price. The allocation of such subjective values must use the principle of the democratic selection process. The selection of value is based on a majority of votes or even better a consensus of opinion. Here, it is assumed that the person who votes has taken cognisance of the results of objective research into the aspects landscape, nature and cultural heritage. The competent authorities must ratify the allocation of values by an advisory group. This procedure is especially relevant in the design process and the Environmental Impact Assessment (EIA). In one of the
following chapters the steps in that process will be treated; this chapter addresses aspects of the content.
3.1
Values of Safety, Landscape, Nature and Cultural heritage.
The Values of landscape, nature and Cultural in heritage, also known as LNC values, are developed or preserved as a matter of national policy, in so far as that is reconcilable with the fulfilment of the desired safety. These policy judgements offer insufficient information as yet to work out a plan for a section of the flood defence to be improved along a river, the coast or a lake bank. On the scale of a few to a few dozen kilometres in length, choices must be made on the continued existence of a certain building, a specific landscape structure, a location with special flora or fauna, a campsite, et cetera.
Figure 3-2 Photo 4.2 A dike incorporated in the landscape.
Values may be allocated to characteristics that are considered important on a national, regional or local scale. Such valuations can be found in national and provincial policy documents and plans, such as the Environmental Policy Plan, the Gelderland River Dike Plan (GRIP) or the Brabant
LNC-guideline for dikes (LNC-richtlijn dijken).
A value on national or regional scale allocated to the dike, is that of ecological line of connection. This is found in the Environmental Policy Plan in the Ecological Main Structure. Because nature in the river forelands is increasingly being put under pressure, the insight has come into being that the dike slopes have an essential role in the protection of the typical floodplain flora and form an ecological connection zone between the nature areas in the river forelands. The function of ecological connection, thanks to the ribbon shape, has been allocated an authorised value in the policy.
Figure 3-3 ……
Also separate from the Ecological Main Structure, some objects or characteristics have also been valued. They are protected villages, the protected and endangered red list species, and the cultural-historic monuments. They have been allocated a value by political decision-making. On a regional and local scale there are typically many other LNC-objects with no authorised value. They must be catalogued separately for each improvement project. That is an overview of what is found where. That catalogue offers the possibility to select what is considered to be of value. Objective and subjective aspects play a role in the careful allocation of value. The objective aspect consists of care in cataloguing the Nature aspects. The catalogue also demands that the degree of rarity,
distinctiveness, and extent to which replacement is feasible, completeness, authenticity, cohesion and suchlike are checked. The knowledge of these characteristics is objective because it is gained with well-defined methods and is therefore independent of the personal preferences of the researcher. The subjective aspect is the evaluation. Objects of special value must be chosen from the large selection of objects in the catalogue. The allocation of value rests on a personal preference. It is also influenced by knowledge, but is primarily subjective. The allocation of values is needed because it enables the bottlenecks to be pointed out. A bottleneck is a situation in which the Nature value is threatened by an improvement project. Advisory groups are established for the allocation of Nature values. The members of those groups are representatives of the competent authorities and the interest groups. The advisory group usually has access to all disciplines needed for the project. If more information is needed experts are called in.
3.2
The vision on a dike improvement project
The adaptation of a design or management plan in a higher scale level than that of a dike section alone is very important for the continuity and cohesion of the project. To be in a position to name the spatial characteristics that must be taken into account, it is recommended that a vision be developed for a stretch of a few dozen kilometres. The values and functions will be allocated to the branch of the river.
They are rendered into the form of a vision at the level of the dike or coast route in question. The higher scale level provides direction in the reinforcement of a flood defence in a town or village centre. As a result the values and function in the improvement plan can be included in the policy decision for spatial planning and nature development.
To be in a position to develop a vision the catalogue must first be drawn up. The catalogue consists of an overview of the functions laid down by planning, of the allocated (that is authorised) Nature values landscape and cultural heritage. This description should not be made of the water defence only but of the whole area. That description of the area supplies knowledge of the nature aspects and
The vision is then formulated on the basis of the following steps: valuation, indication of bottlenecks and of potentials, priorities and mapping of solution orientations. The great importance of the vision on the flood defence improvement is that it explicitly shows what the aims are and where the hurdles are. The aim that always has priority is the fulfilment of the safety requirements set. It is also the aim of policy to preserve the Nature values to a maximum level and to develop them further.
The policy objectives set for the other spatial functions are also included in the vision. As a result it gives direction to the search for the optimal realisation of these objectives. The optimum for safety plus Nature values will often be different from that for safety plus housing, work, and traffic. The development of two or more options can then determine to what degree these interests can be reconciled and where the essential choices must be made.
The vision development is therefore a selection process that converges with every step on the basis of interests and bottlenecks. That occurs on the basis of existing knowledge and existing policy. The knowledge gaps are also exposed. If there is a lack of knowledge, that is needed to dissolve a bottleneck, an additional study will be needed. That knowledge is then used in the last phase of the vision development, that is the phase in which the alternative solutions are described. The vision development is connected with the start of the environmental impact report ( .. ).
4.
Safety of the flood defence
The protection level against the threat of water surges increased to a very high level. Especially in the last 60 years. Since the middle ages the protection was based on the level of the highest water level on record.
In the thirties of the last century scientists realised that a better way to deal with the threat of water surges was possible.
Mathematicians showed that a probability of exceedance could be calculated based on the existing records of high water as present in this country. Those records cover a period of several centuries so a reasonable estimation of the probability a set value would be exceeded could be given.
This technique was coupled to an economical analysis and it was possible to determine an economical optimal safety level for every Dutch region under threat of the high waters.
Although the above is theoretical true, it proved to be difficult to implement these techniques in practise. Some simplifications had to be adapted to facilitate the use of the technique.
In the first stage only the height of the crest was taken into consideration and the other limit states were omitted. (Overflow and overtopping were the only limit states under consideration and the strength was simplified with one parameter, “the crest height”)
In spite of this the technique showed that the advantages of this approach were too big to be ignored. The approach never made it to the decision makers because the recession of the 1930th was going on and the policy makers had other matters to attend to.
After this period the Second World War and the rebuilding of the country after it was consuming all the attention and nothing happened in this field.
In 1953 the contry got a wake up call however, with the disaster in the southwest. The public, press and the policy makers shifted their attention to the protection of the country against high water. The delta committee formulated their approach based on the pre-War developments. The lines of exceedance as shown below could be constructed quickly (see Figure 4-1).
Figure 4-1 Fig 4.1 probability of exceedance.
The protection level to choose presented the problem however. It was realised that the probability that a certain waterlevel combined with wave action was exceeded had another dimension than the acceptable economical risks.
The approach used was based on an analysis of the loads on the water defences, the strength (resistance) of those water defences and the consequences of a possible flooding.
Loads.
o The probability a certain waterlevel would be exceeded could be calculated with a reasonable accuracy.
o The probability of the wave actions could not be calculated with eaqual ease however. Reasonable estimates were given however.
o The correlation between thes data was obvious but couls not be calculated. It was assumed fully correlated as a probable safe assumption.
Strength (Resistance of the water defences)
o The strength of the water defences in terms of height could be measured quickly.
o The geotechnical stability against sliding/piping/seepage related failure mechanisms were based on hands on expierience in those days and could not be expressed in terms of probability.
o The effects of ice dams, ship collisions were not taken into account.
o The assumption was that a water defence should withstand the load of a normative high water in a safe manner. In terms of probability: The contribution of the uncertainties of the strength should be so little they can be neglected.
Consequences.
The consequences of a flooding could be estimated in terms of economical damage if the whole dike ring was flooded as a whole. (A maximum approach)
The effects of inundation speed were not considered.
The effects of compartments within the dike ring were not taken into account.
The loss of life was not taken into account because a lot of people considered that not ethical. (In a later stage the effects of the several solutions on the possible loss of lives were analysed as well. It proves to be difficult to implement this probabilistic approach in the practice of the 1960th and the decades after it.
An approach was chosen in wich the normative high water was chosen based on a probabilistic analysis. (Called level 1 approach)
The only choice to make was the level of protection to be achieved.
For the central part of the country a return period of inundation could be calculated as 1/125000 per year. The translated of this figure to normative high water eas not 1 to 1, because of al the limitations in the analysis as mentioned aboven. It was decided that the protection of the central part of the country against a flood that wil be exceeded with a probability of 1/10.000 would be sufficient if the construction inself could withstand this surge in a safe manner. This word “safe” is not specific onough however and this fact was the starting point of a lot of discussions of the years afterwards. In short:
The central part of the Netherlands should be protected against high water surges that will be exceeded with a probability not greater than 1/10.000.
The waves that will go with it can be calculated with a formula that was called “the Delft formula.” The water defences should be able to withstand these hydraulic loads in a complete safe manner. The safety of the remaining regions of the country was differentiated based on their economical value. The safety against a possible flooding by the rivers was described as well.
The way of thinking is developing in this country. The years to come will see big changes in the safety philosophy used in the field of water defences. In this moment the way the implementation of the safety in the daily practise is strongly influenced on the approach described above.
The Technical Advisory Committee on the Water Defences (now ENW) decided that if a probabilistic evaluation of another limit state than overflow or overtopping is possible the probability of failure should be less then 10 % of the return period of the considered area to be “safe enough” in terms of the considerations of the delta Committee.
This enables the researchers to go ahead with the development of the techniques to determine the complete risk picture with respect to the threat fron high water.
The possibilities to do this at present will be discussed later in chapter five of these notes.
4.1.1 Catalogue of Landscape aspects Still to be done!!!
4.1.2 Catalogue of Nature aspects
The first step in preserving and reinforcing Nature values is gaining knowledge. That consists of three components.
o The first is the collection of data from publications and archives on landscape, nature and cultural heritage.
o The second is the supplementation of missing data by field cataloguing.
o The third is the collection of judgements in policy documents in which value is allocated to LNC characteristics or elements in the study area.
