The influence of compartmentalisation on flooding in Central Holland

MSc Thesis

E.P. de Bruine April 2006

Author: E.P. de Bruine

Graduation Committee:

Prof. Dr. Ir. H.H.G. Savenije (DUT) Dr. Ir. M.J. Baptist (DUT) Ir. S.N. Jonkman (DUT) Dr. A.W. Hesselink (RIZA WRE)

Copyright:: E.P. de Bruine.

4 Compartmentalisation in Central Holland

Preface

......

This report is the result of graduation research for the Master study Watermanagement at the faculty of Civil Engineering and Geosciences of Delft University of Technology. The graduation research consisted of a internship at Rijkswaterstaat RIZA-WRE. The author is solely responsible for the content of the report.

I would like to thank Gerben Spaargaren of the water board Delfland, Rob Taffijn of the water board Schieland en de Krimpenerwaard, Frans van Kruiningen of the water board Rijnland and René Piek of the Province of South-Holland for providing me with the information I needed. I also would like to thank Paul Visser of DUT for giving me more insight in breach growth processes in dikes; Marcel van der Doef and Plony Cappendijk-de Bok of DWW for helping me with the calculations of damage and casualties; my colleagues at RIZA-WRE for creating the perfect atmosphere to work in and especially Sacha de Goederen for creating ‘boundary conditions’-files the NDB-model, Johan van Zetten for helping me with the schematisation in Sobek and Aad Fioole for teaching me all about GIS-applications; my girlfriend Angela Paulus for supporting me and reading all my drafts.

Special thanks go to Alex Roos, Stephanie Holterman and Marcel van der Doef of DWW for supplying me with the model schematisation of the study area. This study would not have been possible without it. Special thanks also go to my graduation committee: Professor Savenije (DUT) for his sharp view on the matter; Martin Baptist (DUT) for his enthusiasm and indispensable knowledge of hydrodynamic modelling; Bas Jonkman (DUT,DWW) for his technical input and for constantly providing me with relevant reports and Annika Hesselink (RIZA-WRE) for reading all my drafts very carefully, helping me to improve the texts and for teaching me to create a network of interested and helpful people.

Erik de Bruine, April 2006.

5 Compartmentalisation in Central Holland

6 Compartmentalisation in Central Holland

Summary

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The Dutch landscape is characterised by polders of low elevation next to the North Sea and the rivers Rhine and Meuse. This landscape makes it necessary for the Dutch to protect themselves against flooding by building dikes around the polders. These dikes are called primary dikes, the area that is protected a dike-ring area. The safety level for dike-ring area 14: Central Holland is 1/10.000 per year, meaning that a flood event statistically can occur every 10.000 years. The policy against flooding is based on taking measures to guaranty this low chance of occurrence of a flood event. However, after the flood event in New Orleans, USA, a discussion has arisen about the Dutch policy on water safety. This discussion is held in the project Waterveiligheid 21ste eeuw (WV21). Other options to reduce the flood risk are taken into consideration, not only measures to decrease the chance of flooding, but also measures that decrease the consequences of a flood event. Compartmentalisation is one of these options, but only little information is available about the ability of compartments to reduce the consequences of a flooding. In 2008, the (new) policy on water safety will be determined.

This report focuses on the hydraulic effect of compartmentalisation to reduce the consequences of a flood event. Compartmentalisation stands for the division of the dike-ring area into smaller units (compartments) by the construction of dikes. Only a few of these compartments are supposed to flood in case of a large flooding. The floodwater is retained within these compartments, preventing flooding in the rest of the area. Investigations are also made into the influence of canals in the area and the possibility to redirect floodwater to a target location rather than retaining it.

A hydrodynamic model schematisation in Sobek-rural for the study area Central Holland is developed in the project Veiligheid Nederland in Kaart (VNK). This schematisation is the basis of the schematisation used in this study. Boundary conditions differ from those used in VNK. A framework is developed to create different spatial layouts of the dike- ring area (figure 4-8). Most layouts are obtained by the raising or lowering of existing spatial line elements, such as roads, railroads and dikes. For some layouts new elements are added. Simulations are made for 4 breach locations and a total of 15 different spatial layouts. The Digital Elevation Model is adjusted to obtain the desired spatial layout. The effect of the canal system is tested by schematising the emergency barriers in the canals (noodboezemkeringen). These barriers are physically present in the canals, but were not yet schematised in the model.

The simulations show the flood patterns for 10 days. The patterns differ in total area of flooding, water depth, flow velocity and the rise rate of the water. The discharge through the breach in the dike is also

7 Compartmentalisation in Central Holland

influenced in some cases. For each simulation the total damage and the number of casualties are calculated by the program HIS-SSM.

Several conclusions can be drawn from the simulation results. Firstly, the canal system functions as a network through which the floodwater can spread very quickly. Floodwater enters the canals near the breach location and is transported quickly to more distant parts of the canal system. Flooding will occur at locations where the canal embankments are relatively low. This effect can be countered by closing the emergency barriers that are present in the canals. The flooding at distant locations is prevented and more floodwater will remain near the breach location.

Secondly, the use of small compartments (<10 km2) near the breach location effectively reduces the inflow of floodwater in the polder. The water level rises quickly in the compartment, approaches the water level in the river (or sea) and restrains further inflow of floodwater. The peak discharge in the polder can be reduced by 30%. When the water level in river decreases again, water flows from the polder back into the river. The floodwater volume that eventually has to be pumped out of the area is small because of this effect. On the other hand, a large number of casualties can be caused due to the quick rise rate of the water in the compartment. The use of small compartments therefore should only be used in sparsely populated areas, or measures should be taken to reduce the number of casualties, like an excellent evacuation plan and the presence of many evacuation routes.

Thirdly, the redirection of floodwater can prevent damage and casualties if a high rise rate of the water level is not desirable. In this case, the inflow of floodwater remains the same, but the floodwater that enters the polder is redirected to a location with a relatively low value. Casualties because of a high rise rate are prevented. The effectiveness of the redirection depends on the route available. More casualties can be expected along this route because the floodwater arrives earlier than in the current situation. The number of casualties due to high flow velocities does not increase because the maximum flow velocities near the breach (where flow velocities are generally highest) remain approximately the same.

The concepts developed in this study do not make an integral layout for Central Holland. They must be considered as building blocks that can be used to create a polder layout that reduces damage and casualties optimally for more than one breach location. These building blocks can also be used for areas similar to Central Holland: large flat areas below sea level with high concentrations of value and inhabitants. But compartmentalisation or redirection should be implemented with care. The impact of a measure on damage and casualties depends on local water depth, flow velocity and rise rate. A hydrodynamic calculation is therefore necessary to determine these effects.

8 Compartmentalisation in Central Holland

Table of contents

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Preface 5

Summary 7

1. Introduction 11 Flooding 11 Towards a new safety approach 11 Problem description and goals 12

2. Area description 17 The Rhine-Meuse Delta 17 Dike-ring area 14: Central Holland 18

3. Model development 23 Model schematisation 23 Boundary conditions: approach 24 Flood scenarios 26 Damage and casualties 31

4. Spatial design of the dike-ring area 33 Framework 33 Selection process 39 Designing the layouts 41

5. Simulation results 43 Ter Heijde 43 Maeslant barrier 47 Rotterdam Maasboulevard 51 North Sea Channel 60

6. Sensitivity analysis: Maasboulevard 65 Breach dimensions 65 Land roughness 70

7. Reflection 73 Model results 73 Sensitivity analysis 78

8. Conclusions and recommendations 81 Main conclusion 81 Conclusions by subject 83 Recommendations 85

Abbreviations and terms 87

References 90

9 Compartmentalisation in Central Holland

10 Compartmentalisation in Central Holland

1. Introduction

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Flooding

The Dutch have a long history in relation to water. The water is a blessing on the one hand, supplying drinking water and transport possibilities. On the other hand, there is always the threat of flooding, causing damage and taking lives of civilians. The Dutch have experienced several big flood events. Well known is the sea flood of 1953 when almost 200.000 acres of land flooded, more than 1800 people died and approximately 680 million euros of damage was caused (source: www.delta2003.nl, October 2005). Also the rivers cause trouble, for instance the big flooding of the river Rhine (1926) and the more recent nearly floods in the rivers Rhine and Meuse (1993 and 1995). Despite the occurrence of these events, the Dutch civilians have good faith in the protection offered against flooding. Economic valuable areas are located in polders next to the river. Many of these polders are growing in economic value and in number of inhabitants [Rijkswaterstaat, 2004b].

A polder is an area with a land elevation below sea level. The polder level usually is several meters lower than the water level in sea or river. Large primary dikes surrounds one or more polders to protect them from flooding. An area protected by these primary dikes is called a dike-ring area. Within the dike-ring area, smaller dikes are present for water management purposes. A primary dike has to meet higher safety standards than these secondary dikes. Appendix A shows the location of the dike-ring areas in the Netherlands.

Towards a new safety approach

The safety policy against flooding in the Netherlands is twofold: first there is a safety standard for the dikes to prevent flooding and secondly measures are taken to mitigate damage when the dikes fail. The second part is also known as dealing with the residual risk. The measures taken are mostly non-structural (calamity management). Examples of these measures are evacuation plans and plans and means for quick dike breach closure. Examples of structural measures are compartment- talisation and building on high grounds (figure 1-1).

The safety standards in the Netherlands are based on the probability of a certain water table being exceeded. The dikes are often higher than this water level. However, the Delta Committee (1960) advised to base the safety approach on the risk of flooding. This means that the probability of failure and the consequences of failure should be taken in

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account. (Risk = probability of failure * consequences of failure). Consequences of failure (flooding) are direct and indirect economic damage, casualties, ecological damage, landscape damage, social uncertainty and emotional damage. Note that not all of these damages can be monetary acknowledged [Rijkswaterstaat, 2004b].

Before 1992 the necessary knowledge to follow this risk approach was not available. The Technical Advisory Committee on Water Defences (TAW) analysed the risk approach in the recent project Towards a new safety approach [TAW, 2000]. This study focused on the probability of flooding instead of the existing probability of water level exceedance. For four pilot dike-ring areas the probability of flooding could successfully be calculated. The Secretary of State advised to implement this method at all dike-ring areas. This resulted in the project Veiligheid Nederland in Kaart (VNK) carried out at the Road and Hydraulic Engineering Institute of Rijkswaterstaat (DWW) [VNK, 2005a]. Besides calculating the chance of flooding for several dike-ring areas, the institute also calculates the expected damage for each area when flooding occurs. With this information, the risk of flooding can be calculated [VNK, 2005]. The Secretary of State announced that these results are used to start a discussion to determine whether or not the Dutch safety approach needs to be changed. The project Waterveiligheid 21ste eeuw (WV21) facilitates this discussion. In 2008 the (new) policy concerning safety against flooding will be determined [Schultz van Haegen, 2005].

Problem description and goals

Probability versus Consequences

To facilitate a discussion about the Dutch safety policy, information is required about both the Probability (prevention) as the Consequences (mitigation) of a flood event (figure 1-1).

...... Figure 1-1 Flood risk Measures to reduce flood risk by flood prevention or mitigation of the Prevention Mitigation consequences of a flooding. Higher dikes Calamity management Retention evacuation plans 'Room for the River' means for breach closure etc Structural measures compartmentalisation high value, high elevation

The last years, much attention has been given to the probability of flooding. An example is the project Room for the river, in which “river cross sections are widened by situating the dikes further away from the river, or by lowering the river forelands”(source: www.ruimtevoorderivier.nl, March 2006). Dealing with the residual risk was often considered a side issue that consists of evacuation plans. Only a few studies focused on designing structural measures to prevent

12 Compartmentalisation in Central Holland

casualties and damage in a polder (e.g. Alkema and Middelkoop (2005), Knot and Zijlma (2005), Verwijmeren (2002)). The result is that relatively much information is available on the probability of a flood event, while the impact of measures to reduce flood consequences is not extensively studied.

Only focussing on the prevention of a flooding event by heightening the dikes can have negative effects. The probability of a flooding event is reduced in this case, but consequently the effect of a flooding is more catastrophic. The flooding will be more violent because of the bigger difference between land and water level. Also the resulting water depth in the flooded polder will be higher, potentially causing more casualties and damage. Furthermore, the economic values in the polders have grown with an average of 600% in the last 40 years and are expected to keep growing [Rijkswaterstaat, 2004b]. The land level in most of the polders is lowering. Especially the soft soils (peat) are lowering relatively fast. This results in a bigger difference between water- and land level, so in more dangerous flooding events. The safety standards as they are presently stated are based on a limited economic analysis dating back from the Delta Committee (1960) and are no longer corresponding with the current economic activities in the polders [Rijkswaterstaat 2004b].

Structural mitigation measures

Hesselink et al. (2003) has shown that elevated land elements, such as old dikes and roads, within a polder can have a large effect on the flooding behaviour. In the polders in the eastern part of the Netherlands there is a slight land slope, but enough to redirect the water in a preferred direction. An example simulation exists of dike-ring 41, ‘Land van Maas and Waal’, where the water clearly flows from the high eastern part of the polder to the lower west. On the way, secondary dikes influence the flooding behaviour [Hesselink et al. 2003]. For (most) dike-ring areas in the western part of the Netherlands, such research does not exist. Results of earlier studies in the eastern part of the Netherlands cannot just be extrapolated to the western part because there are several different circumstances:

• Tidal influence from the sea. • More water available for flooding, coming from the sea. • More possibilities to influence the river water flow due to the sea flood defences, the Deltaworks. • A different type of polder: the polder is deeper, the land slope is smaller and the vegetation is different. • Presence of canal system, which can transport floodwater.

These circumstances are expected to cause more severe flood events, due to a continuing input of water from the sea.

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Compartmentalisation

In the safety against flooding, compartmentalisation stands for dividing the flooded area into smaller units by the construction of internal dikes. The flooding will be limited to one or several of these compartments. The consequences in these areas will be large, but flooding is prevented in the rest of the area.

In the Netherlands, compartmentalisation is already a fact. Each dike- ring area is a large compartment. The sole purpose of these dike-ring areas is to prevent a large part of the Netherlands to be flooded in case of a flood event. When a dike breaches, only one dike-ring area is supposed to be flooded. However, the area of this dike-ring area is large, so consequences will also be large when one of these areas becomes flooded. Further compartmentalisation of these areas by creating smaller compartments could decrease the impact of a flood event in the dike-ring area.

Canal system

In the low polder areas an extensive canal system is generally present. This system is mainly used to drain the polder in case of rainfall. These canals often have a higher water level than the surrounding polder and are therefore embanked. In case of a large flooding event, these canals could function as a route of low resistance for the floodwater. Little information is available on the effect of a canal system on the flood characteristics.

Problem

For some dike-ring areas in the western part of the Netherlands only little information is available about:

• The hydraulic effects of elevated land elements that redirect or retain floodwater (for instance as compartmentalisation dikes) on the flood pattern and to what extend these elements can mitigate flood consequences.

• The influence of the canal system on the flood pattern.

Information about the hydraulic effectiveness of these structural measures in a dike-ring area can be valuable in the discussion about the Dutch safety policy against flooding.

14 Compartmentalisation in Central Holland

Goal

More information is needed on the effect of the canal system and elevated land elements on the hydraulic flood characteristics of a dike- ring area in the western part of the Netherlands. The main question is to what extent elevated land elements can mitigate flood consequences (damage and casualties). This report mainly studies the hydraulic effect of three structural measures:

• Retention of floodwater by compartments in the dike-ring area.

• Redirection of floodwater towards a location of relatively low value by elevated land elements.

• Closure of barriers in the canal system.

The goal of the research is to derive general concepts (or ‘building blocks’) that can be used by spatial planners to construct structural measures in a dike-ring area to mitigate flood consequences optimally.

Study area

Central Holland (dike-ring area 14) is selected as a study area (figure 1- 2). It is the largest dike-ring area in the Rhine-Meuse Delta, hosts the most inhabitants and has the largest economic value of all dike-ring areas. For these reasons, further compartmentalisation of this area is expected to have a considerable effect on the consequences of a flood event. A more detailed consideration of all dike-ring areas and an explanation of the selection of Central Holland can be found in Appendix A.

...... Figure 1-2 The most downstream part of the Rhine-Meuse Delta (green). The enlarged section is dike-ring area 14: Central Holland. Edited from source: Meetkundige Dienst RWS.

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Report structure

The study area is described in more detail in chapter 2: Area description. A hydrodynamic model is developed in order to determine the influence of spatial adjustments to the dike-ring area on the flood pattern and flood consequences. The model schematisation and boundary conditions used for the simulations are presented in chapter 3: Model development. To determine what spatial adjustments are to be used, a framework is derived and presented in chapter 4: Spatial design of the dike-ring area. The effects of the spatial adjustments on the flood characteristics and consequences are presented in chapter 5: Simulation results. A sensitivity analysis is performed on breach dimensions and land roughness coefficients. This is described in chapter 6: Sensitivity analysis: Maasboulevard. Finally, the results are discussed and conclusions are drawn in chapter 7: Reflection and chapter 8: Conclusions and recommendations.

16 Compartmentalisation in Central Holland

2. Area description

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The Rhine-Meuse Delta

The Rhine-Meuse Delta is the most important Delta of the Netherlands. The two largest rivers, the river Rhine and Meuse, flow into the estuarine area and eventually into the North Sea (figures 2-1).

The average discharge of the River Rhine is 2200 m3/s, with peak values of 12.000 m3/s. The discharge of the river Meuse is smaller: average of 230 m3/s and peaks of 3.000 m3/s (source: www.knmi.nl, February 2006).

The rivers are embanked with primary dikes. These dikes prevent the hinterland from flooding in case of large discharges in the rivers. Furthermore, the low Dutch land areas are divided into compartments called dike-ring areas. The dike-ring areas have a low elevation and are surrounded entirely by primary dikes or high grounds. In case of a flooding event, only one dike-ring area is flooded in this way. Canals drain and irrigate the area. Drained water is pumped out of the dike- ring areas into the rivers or sea. There are 58 dike-ring areas in the Netherlands, 27 of them are located next to the river Rhine and Meuse.

The primary dikes along the rivers are designed for discharges that occur once every 1250 year. These discharges are statistically derived from historic discharge data. The design discharge for the river Rhine is 16.000 m3/s and for the river Meuse 3800 m3/s. [Rijkswaterstaat, 2001] (source: www.nederlandleeftmetwater.nl, February 2006)...... Figure 2-1 River basins of the rivers Rhine and Meuse. Source: www.natuurdichtbij.nl, March 2006.

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Dike-ring area 14: Central Holland

Dike-ring area 14 is called Central Holland. It covers 223.000 ha of land and has 3.2 million inhabitants [VNK, 2005b]. The safety level for the area is 10-4 per year. This means that the probability of the water level in river or sea exceeding the design water level of the dikes, in accordance with the safety standard, is 10-4 per year (Wet op de Waterkering).

Infrastructure

...... Figure 2-2 Central Holland: main cities, roads and railroads

The three largest cities in the Netherlands, The Hague, Rotterdam and Amsterdam are situated in Central Holland (figure 2-2). Together with the smaller cities in the area and the city of Utrecht (situated outside the dike-ring area) these cities form the Randstad. Approximately 60% of the Annual National Income of the Netherlands is produced in the Randstad (source: www.vrom.nl and www.rivm.nl, October 2006).

The area between Amsterdam, Rotterdam, The Hague and Utrecht is called het groene hart. It is Dutch policy to avoid building activities in this area in order to maintain the nature that is present.

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Area borders

...... Figure 2-3 Central Holland: bordering water bodies.

The dike-ring is surrounded by water (figure 2-3). The western border is the North Sea, where large dunes protect the area from flooding from the sea. The Southern border is the Nieuwe Maas and the Nieuwe Waterweg. These are branches of the river Rhine. The Maeslant barrier, a storm surge barrier, is present in the Nieuwe Waterweg (figure 2-3, number 1). The Hollandsche IJssel is an old branch of the river Rhine and is a draining channel. The Hollandsche IJsselkering is present in this channel (number 2). In the north and east, two channels border the area. The North Sea Channel and the Amsterdam-Rhine Channel (ARC) are connected and are mainly used for ship traffic. There is a strip of land between the North Sea Channel and the primary dikes of the dike ring area, caused by land reclamation years after the completion of the dike (see Appendix B: History of land reclamation for more information). The channels can be separated from all other water bodies by closing the sea locks at IJmuiden (number 3), the Oranjesluices at Amsterdam (number 4), the Beatrixsluices at the interception of the ARK and the Lek and the Irenesluices at the interception with the Nederrijn, a branch of the river Rhine (not in the figure). Several small sluices separate the Hollandsche IJssel and the ARC.

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Secondary embankments

...... Figure 2-4 (left) Central Holland: primary canals. [VNK, 2005a]

...... Figure 2-5 (right) Central Holland: other secondary dikes. [VNK, 2005a]

A system of primary and secondary canals is present in the area. The secondary canals collect the surplus of water in the area and transport in to the primary canals, where it is pumped into. The primary canals transport the water to larger pumping stations that pump the water out of the dike-ring area, into river, lake or sea.

The embankments of the canals are secondary dikes. They are relatively weak, because the water levels in the canals do not change much and if they change, it is only slowly. The water level in the canals is usually only a few decimetres below the top of the embankment. The canals also irrigate the area in dry periods. The larger canals are also used for ship traffic. In Central Holland many canals are present (figure 2-4).

Besides the embankments of the canals only a few secondary dikes are present (figure 2-5). In the middle of the area there is a dike along the Oude Rijn, an old branch of the river Rhine. In the south of the area, parallel with the Nieuwe Maas and Nieuwe Waterweg, there are remains of the old primary dikes. In the north of the area (not in the figure) some remains can be found of the Dutch Waterlinie. Several hundreds of years ago, strips of land used to be flooded as a military defence line. The dikes that were used to embank this water are still partly present. The dikes themselves formed the defence line, many fortifications can still be found in or on top of the dikes.

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Land elevation

...... Figure 2-6 Central Holland: land elevation level.

The land elevation in Central Holland varies between –6 m NAP (Dutch ordnance level) for some very deep polders and +40 m NAP for some dune areas. Most of the area is below sea level, between –1 and –4 m NAP. In general the cities are situated on higher grounds. Haarlem and The Hague are built near the dunes and are situated above sea level. Some other cities are below sea level, e.g. Zoetermeer, Rotterdam, Delft and Hoofddorp. Residential areas with a very low land elevation are the Prins Alexanderpolder in Rotterdam and the Zuidplaspolder in Gouda. These polders have a land elevation level of –6 m NAP.

Institutions

...... Figure 2-7 (left) Central Holland: Provinces.

...... Figure 2-8 (right) Central Holland: Water boards (right).

Central Holland contains several administrative units. The dike-ring area is located in the Provinces of Zuid-Holland, Noord-Holland and Utrecht. Five waterboards are present in the area: Rijnland, Delfland, Schieland en Krimpenerwaard, Stichtse Rijnlanden and Amstel, Gooi en Vecht. A water board is a government institute responsible for water issues only. The inhabitants of the water board area elect the members of the board. Each water board is also responsible for the maintenance of the dikes in their area.

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Selected breach locations

...... Figure 2-9 Central Holland: breach locations.

Four breach locations are selected in the primary dike (figure 2-9). These are not necessarily weak links in the dike. The probability of Assumption 3 failure is not taken into account. This study focuses on the consequences of a flood event. For this reason, the selection of the breach locations is made to create a large variety in flood circumstances.

The breach at Ter Heijde creates a flooding from the North Sea, the hinterland contains a large amount of canals. The breaches at Maeslant barrier and Maasboulevard are situated next to a tidal river. The influence of the sea is expected to be larger at the Maeslant barrier because it is very close to the sea. The hinterland of breach location Maeslant barrier contains a large secondary dike and a large amount of canals, while the hinterland at Maasboulevard is a residential area with low land elevation levels. The final breach location is next to the North Sea Channel. Flooding will originate from the canal into the hinterland, where a polder of low elevation is present. The breach locations are described in more detail in appendix E: Excursions and at the beginning of each paragraph in chapter 5: Simulation results.

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3. Model development

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Model schematisation

In the project Veiligheid Nederland in Kaart (VNK) a model schematisation for Central Holland is developed (figure 3-1). The schematisation is made in Sobek rural, version 2.06.000.39zzo. Sobek is a hydrodynamic software package developed by WL⏐ Delft Hydraulics. It solves De Saint-Venant equations (conservation of Mass and Energy) and can combine 1D channel flow with 2D overland flow, which makes it suitable for flood simulations. The 2D-module is also known as Delft Flooding System (Delft-FLS).

The grid cells of the digital elevation model (DEM) are 100 m x 100 m. This grid is based on the data in Actueel Hoogtebestand Nederland (AHN). The grid with land roughness values, the friction layer, is based on the data in Landelijk Grondgebruik Nederland (LGN4). In the schematisation are present: - Large canals and lakes (small water bodies are neglected). - (Rail)roads - Primary and secondary dikes (also canal embankments).

More detailed information on the model schematisation used in VNK can be found in [Melisie, 2005].

...... Figure 3-1 Model schematisation Central Holland, developed by VNK.

23 Compartmentalisation in Central Holland

Use is made of the digital elevation model (DEM); the friction layer and the schematisation of the canals as made in the project VNK. Changes are made to the DEM in order to create alternative spatial layouts of the polder. Changes are also made in the canal system: at some locations emergency flood barriers and sluices are schematised that were omitted in the original schematisation. Also, a different set of boundary conditions is developed for this study.

For breach development the formula of Van der Knaap is used, because it makes a distinctions between clay and sand dikes [Van der Knaap, 2000]. This formula calculates the growth of the breach width in time with a logarithmic function. The user must assign values to the depth of the breach and the period of time to reach this depth. The decrease of depth is linear over this period.

Calculations in Sobek are made with a constant water level in the canals. These water levels are obtained from the water boards in the area (Rijnland, Delfland and Schieland en Krimpenerwaard). An exception is made for the calculations for breach location Maasboulevard. Due to an error in the model schematisation, flooding would occur from the canals into the polder. To solve this issue, calculations are made with a dry canal system. This gives an error of 6 to 8% of the flood volume entering the polder during peak discharge. This value is considered acceptable.

Boundary conditions: approach

Hydraulic boundary conditions

Hydraulic boundary conditions for each scenario are determined by the methodology described below. The numeric values of the boundary conditions can be found in the next paragraph.

Step 1: The normative high water level for the primary dike is taken from the Hydraulic boundary conditions, 2001 [RWS, 2001].

These are the water levels that are used to test the dikes to the safety norm. The levels have a probability of occurrence of 10-4 Assumption 6 per year. The dikes are built to resist these water levels. In this study, however, the dikes are assumed to fail at this level. Recent research shows that the probability of a dike failure is higher than expected, mainly due to other failure mechanisms of the dike (piping) [VNK, 2005].

For breach location Ter Heijde and North Sea Channel, only step 1 has to be performed. For North Sea Channel the normative high water level is set to the same level as the Water Board Rijnland uses [Grontmij, 2001], because it is not in Hydraulic boundary conditions, 2001. The other two breaches are situated next to the tidal river, where both sea and river have influence, making the situation more complex. For these two cases the following extra steps have to be taken.

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Step 2: The most appropriate combination of hydraulic loads fitting these water levels is taken from the statistic software package Hydra-B.

More than one combination of hydraulic loads can cause the same normative high water level in the tidal river. Hydra-B is a database, linking the probability of exceedance of water levels to combinations of hydraulic loads, which are defined as stochastic functions. In this case, these loads are the storm at sea, closure of the storm surge barrier and the river discharge. The most determining combination of hydraulic loads can be derived from this database: - Discharge of the river Rhine at Lobith. - Discharge of the river Meuse at Lith. - Water level at sea. - Wind speed and direction.

The most determining combination of hydraulic loads is used. Hydra is developed by Rijkswaterstaat RIZA. More information about Hydra can be found in Van Nieuwenhuizen, Jacobse and Arnold (2005).

Step 3: The output of Hydra-B is used as input for the model Noordelijk Delta Bekken (NDB) in Sobek (figure 3-2). This is done to calculate the water level over time at the breach location.

The water level in the river is slightly underestimated because the density of the water is not taken into consideration. The heavy salt water pushes the lighter fresh water up. Corrections had to be made to come to the desired normative high water levels.

By creating a breach in the primary dike in the simulation, the drawdown of the water level due to the discharge into the polder is taken into account. The drawdown can amount to 20- 40 centimetres in case of a large flooding. More information about the NDB-model schematisation can be found in Van der Linden and Van Zetten (2001)...... Figure 3-2 Noordelijk Delta bekken-model schematisation in Sobek.

25 Compartmentalisation in Central Holland

Step 4: The output of the NDB-model is used as boundary condition for the model schematisation of Central Holland.

The drawdown of the river water level is correct as long as the flood volume entering the polder is similar to the case used in the NDB-model. If the floodwater discharge through the breach is reduced by measures taken in the polder, the drawdown will be less. Because a lower water level is used as boundary condition, the discharge into the polder is underestimated.

Breach development

Breach dimensions are estimated as accurately as possible. Information on dike levels and build-up (material) is obtained from the water boards (Rijnland, Delfland and Schieland en Krimpenerwaard) for each breach location. Subsequently, use is made of Visser (1998) to determine the Assumption 7 growth process. All breaches are supposed to be Type-B breaches (figure 3-3). The maximum breach width is 200 meters for clay dikes [Visser, 1998]...... Figure 3-3 Type B dike breach as defined in Visser (1998).

Flood scenarios

Assumption 1 During all flood events, meteorological influences are assumed to be Assumption 2 neglectable. Groundwater effects are also neglected. Human errors are not taken into consideration, e.g. at the closure of structures. The text below gives an overview of boundary conditions for each breach location and the used scenario that leads to the flooding.

26 Compartmentalisation in Central Holland

Ter Heijde

The normative high water at Ter Heijde is +5.65 m NAP. This level has a probability of exceedance of 10-5 per year. The high water level has to be reduced by 50 cm to obtain a probability of 10-4 per year [Melisie 2005]. The high water level used is +5.15 m NAP, caused by a 35-hour water rise due to a storm. The peak of the water rise is 4.5 hours after Assumption 8 high tide (figure 3-4).

...... Figure 3-4 Water level against time at breach location Ter Heijde.

Assumption 9 The dunes are assumed to fail at the beginning of the water rise, when the water reaches a level of +1.5 meter NAP. The maximum water level Assumption 10 is reached 6 hours later. The initial breach width is 100 meter, the maximum width is 200 meters. The breach crest level decreases from +14.2 meter to +0.2 meter NAP in one hour. The discharge through the breach does not influence the water level at sea.

The times of failure of the dunes and the period to reach maximum breach depth represent a worst-case scenario. The breach depth however is not very deep. The surrounding area has an elevation slightly above NAP, it is not expected that the breach will scour out much deeper because a large amount of sand from the collapsed dune will have to be transported away first.

Maasboulevard

The normative high water level at breach location Rotterdam Maasboulevard is +3.4 m NAP. This water level can be reached in case of a large storm at sea with the Maeslant barrier opened (probability 0.95 in Hydra-B) or in case the Maeslant barrier is closed with a high river discharge (probability 0.05). Both scenarios are taken into consideration. For both cases, a 29-hour water level rise due to a storm is used.

Maasboulevard, scenario 1: Maeslant barrier open

The normative high water level is reached for 95% of all combinations due to a large storm at sea in combination with the Maeslant barrier

27 Compartmentalisation in Central Holland

opened and low river discharge. The most determining combination contributes for 34% to the normative high water level. This means that if the normative high water level is reached, there is a chance of 0.34 that it is caused by this specific combination of hydraulic loads. The combination of loads in that case is:

Water level sea: +3.47 m NAP. Wind: 22 m/s, northwest. Discharge Rhine: 1580 m3/s. Discharge Meuse: 168 m3/s.

The drawdown of the water level at the breach location is 40 centimetres. The resulting water level is shown in figure 3-5.

...... Figure 3-5 3 Water level against time for breach location Rotterdam, Maasboulevard. Scenario Maeslant barrier open.

2 ) m l ( e v le r e t a W 1

0

13 14 15 16 17 18 19 20 21 22 23 24 Time (day)

Assumption 11 The dike breaches when the water level reaches a value of +1.5 meter NAP. The peak water level is reached 4 hours later. The initial breach Assumption 12 growth is 50 meters, the maximum width 200 meters. The initial breach is relatively small because it is a dike of sandy clay with a paved boulevard on top (source: Water board Schieland en Krimpenerwaard). The breach crest level decreases from +5 m to –10 meter NAP. Because the polder elevation level is also several meters below NAP, a large scouring effect on the breach is expected.

Maasboulevard, scenario 2: Maeslant barrier closed.

This scenario occurs when there is a large storm at sea, which forces the closure of the Maeslant barrier, and a high river discharge. These are two independent events (stochastic functions) that have to coincide. The chance that the normative high water level is reached by this combination is only 0.05 (or 5%). The most determining combination in this case contributes for 2.6%. This combination is:

Water level sea: +4.10 m NAP. Wind: 27 m/s, northwest. Discharge Rhine: 10.000 m3/s. Discharge Meuse: 2095 m3/s.

28 Compartmentalisation in Central Holland

The Maeslant barrier closes as the water level near Rotterdam reaches a level of +3 m NAP (source: www.keringhuis.nl, February 2006). The barrier opens again when the water level in the river is higher than the Assumption 19 level at sea. It is closed for 18 hours in total. It is assumed that the barrier closes or opens in one hour. The drawdown of the water level is 25 centimetres. The resulting water level peak is +3.12 m NAP. Note the difference in shape of the curves shown in figure 3-5 and 3-6, especially the shape of the peak is different...... Figure 3-6 Water level against time for breach 3 location Rotterdam, Maasboulevard. Scenario Maeslant barrier closed.

) 2 m l ( e v e l r e t Wa

1

13 14 15 16 17 18 19 20 21 22 23 24 Time (day)

The primary dike is assumed to fail at the same time as in scenario 1 (Maeslant barrier open). The water level is +2.0 m NAP at this moment. The maximum water level is reached 13 hours later. Breach dimensions and growth are the same as in scenario 1. Initial breach width is 50 meters, maximum breach depth –10 m NAP.

Maeslant Barrier

The normative high water just behind the Maeslant barrier is +3.4 m NAP. The combination of hydraulic loads is only determined for the case with the Maeslant barrier open. The combination as used for Maasboulevard with the Maeslant barrier open is most determining for this case as well:

Water level sea: +3.47 m NAP. Wind: 22 m/s, northwest. Discharge Rhine: 1580 m3/s. Discharge Meuse: 168 m3/s.

The drawdown in the river is smaller, because the breach location is closer to the sea (figure 3-7). The peak water level is +3.28 m NAP. The water levels after the peak are lower than at Maasboulevard because the influence of the sea is larger.

29 Compartmentalisation in Central Holland

...... Figure 3-7 Water level in the river against time 3 for breach location Maeslant barrier.

2 ) m l ( e v le r e t a

W 1

0

14 15 16 17 18 19 20 21 22 23 24 Time (day)

Assumption 13 The primary dike is assumed to fail when the water level reaches a level of +1.5 m NAP. The maximum water level is reached 4 hours Assumption 14 later. The initial breach width is 100 meters, the maximum width 200 meters. The crest level of the breach decreases from +4 to –4 m NAP. This value is higher than for the case Maasboulevard because the polder elevation level is several meters higher at this location.

North Sea Channel

Two cases are calculated for this case: 1) a breach in the primary dike with high water in the canal and 2) a storm at sea with failure of the sluices at IJmuiden.

Case 1

The normative high water level at the primary dike along the North Sea Channel is +1.25 m NAP (source: Water board Rijnland). It is assumed Assumption 20 that the sluices at the beginning and end of the canal are closed and no leakage occurs. So the canal is separated completely from all other water bodies. The time of failure of the dike is not important because the water level is assumed to have a constant level until the dike breaches. After dike failure the water level draws down. This effect is Assumption 15 calculated in the flood simulation. The initial breach is 50 meters wide, the maximum breach width 200 meters. The breach crest level deceases from +2.5 m NAP to –5 m NAP.

Case 2

The second case is more complex. The same storm at sea is simulated as for breach location Ter Heijde (figure 3-4). It is assumed that the Assumption 16 sluices at IJmuiden fail and do not hinder the flow from the North Sea into the North Sea Channel. The sluices at Spaarndam are also assumed to fail, so water can flow from the North Sea Channel into the

30 Compartmentalisation in Central Holland

canal system of the polder. Finally, the embankment of the belt canal Assumption 17 around the polder Haarlemmermeer is assumed to fail 3 hours after Assumption 18 the failure of the sluices at Spaarndam. The initial breach width is 20 meters, the maximum width 100 meters. The crest level decreases from 0 to –3 m NAP.

Damage and casualties

Two indicators measure the impact of a flood event: damage and the number of casualties caused by the flooding. These are calculated by the computer program Hoogwater Informatie Systeem – Schade en Slachtoffermodule (HIS-SSM) version 2.1, developed by HKV consultants and the Road and Hydraulic Engineering Institute of Rijkswaterstaat. The program contains land use information of the Netherlands and damage functions for each land use. Damage is calculated when the Sobek-output ‘water depth’ is put into the program. The number of casualties is determined by the water depth (h [m]), the flow velocity (v [m/s]) and the rise rate (w [m/hour]) of the floodwater. f(h) is the casualty factor, which is multiplied by the number of inhabitants in a grid cell.

Casualties caused by high flow rates:

f(h) = 1 if h•v ≥ 7 and v ≥ 2

Casualties caused by high rise rates:

f(h) = 1.45•10-3• e1.39h if w ≥ 0.5 and 1.5 ≤ h ≤ 4.7 f(h) = 1 if w ≤ 0.5 and h > 4.7

Casualties in remaining areas:

f(h) = 1.34•10-3• e0.59h if w < 0.5 and h > 0 f(h) = 1.34•10-3• e0.59h if w ≥ 0.5 and h < 1.5

[Jonkman, 2004] [Kok et al, 2004]

Assumption 22 It is assumed that no evacuation was performed preceding the flooding, because the dike is assumed to fail below the normative high water level. The selected method in HIS-SSM is that of 2004 (newest method), the selected dataset is that of 2002. More information about HIS-SSM can be found in Huizinga, Dijkman, Barendregt and Waterman (2005).

31 Compartmentalisation in Central Holland

32 Compartmentalisation in Central Holland

4. Spatial design of the dike-ring area

......

A smart spatial planning can reduce flood consequences, by redirecting or retaining floodwater in such a way that valuable areas will not become flooded. In this chapter a framework of strategies and spatial measures is presented, which can be used to create a spatial design of a polder. The first paragraph shows the different possibilities that can be selected. The second paragraph focuses on the selection process: which choices should be made within the framework?

Framework

Strategy

The base of a spatial layout is the selection of the strategy. The main choice that has to be made is whether to retain the water near the breach location or to let it flow through this area and redirect the water to a location of a relative low value. The choice will mainly depend on the number of inhabitants and value of the area and their spatial distribution. In case the area near the breach is of low value and the hinterland is of high value, retention is probably the best option (figure 4-1).

...... Figure 4-1 Concept of retention in compartments.

Retention In case of redirection or flow through, the floodwater will spread out over the area. Two strategies are distinguished. The first is the protection of valuable areas, regardless of where the floodwater will flow to instead (figure 4-2). This can be done by building dikes around the area. The other option is to redirect the water to a target location, where it is retained (figure 4-3). It has to be noted that it is possible to combine these two strategies, by redirecting the floodwater to a target location while valuable areas in the flow path are protected. The flow to a target location can be done as quickly as possible or the water can be slowed down. Casualties due to high flow velocities are prevented in this way and inhabitants have more time to escape the area.

33 Compartmentalisation in Central Holland

...... Figure 4-2 Concept of redirection.

Redirection or flow through ...... Figure 4-3 Concept of protection.

Protection

...... Figure 4-4 Framework component showing possible inland strategies to reduce flood consequences.

flood

retain flow through (compartments)

protect valuable to target location area

slow fast

34 Compartmentalisation in Central Holland

Overland flow and channel flow

Two types of flow can occur during a flood event: channel flow (modelled as 1D flow) and overland flow (2D flow). Channel flow in a well-maintained, open channel is characterised by small friction forces and water depths of several meters. Overland flow is characterised by large friction forces, caused by the many obstacles on land, and water depths of several decimetres (for the floodwater front). The water loses energy directly to objects in the water, like buildings or vegetation and indirectly by turbulence and changed streamlines. As a result of the higher resistance force, overland flow will propagate much slower than channel flow.

In most flood events, overland flow will be dominant. The channel flow can become more important in case a large amount of channels is present. Water levels in the channels will rise quickly (a high water wave runs through the channels) and the embankments at a different location can be overtopped or collapse, bringing floods to distant locations in a relatively short period. This problem can be countered by heightening the embankments along the channels or by using sluices or emergency barriers. These are structures in the water that can be closed to prevent the floodwater entering the channel any further. In mountainous areas, the slope of the canal is also of importance. In the Netherlands most canals have relatively small slopes, 10-4 to 10-5. Changing the slope of a canal is not feasible. This characteristic is therefore not taken into account in this study...... Figure 4-5 Framework components: canal characteristics. canals

sluices/barriers height of dikes

In case overland flow is dominant, the slope and roughness as well as the line elements present in the flooded area are of importance. Line elements are ground bodies with an elevation higher than the polder plain. Floodwater can be embanked by these elements (compartmentalisation) or only redirected towards a different location. These line elements can be old dikes, special compartmentalisation dikes or even roads and railroads [Knoeff, 2001a-d]. A difficulty in using elements that are not designed as water barriers is that many cuts are present in these elements. Road crossings are the main cause of Assumption 5 these cuts. For this study, it is assumed that the cuts can be closed off.

If the selected strategy is redirection, these line elements can redirect the floodwater into the desired direction. Other elements can be placed perpendicular to the flow direction to decrease the flow velocity. Complementary dikes can be placed around crucial infrastructure or high-density residential areas to protect it from flooding.

35 Compartmentalisation in Central Holland

If the desired strategy is retain, compartments are necessary. Five compartment characteristics are distinguished:

- Area - Height of the dikes - Shape - Land slope - Roughness

The Area of a compartment and the height of the surrounding dikes determine the volume of water that can be retained in the compartment. The size also influences the velocity of water rise in the compartment. This is shown by the formula for retention in a basin:

Q dh = A dt

with: Q = discharge (m3/s) A = compartment area (m2) h = water level in the polder (m) t = time (s)

The compartment area is present in this formula; the height of the dikes indirectly because it determines the maximum h.

The Shape of the compartment determines the time of arrival of the floodwater for each location in the compartment. A round shaped compartment is flooded quicker than a rectangular shaped one (figure 4-6)...... Figure 4-6 Shapes of compartments.

The Land slope determines the velocity of the floodwater in the compartment, together with the water pressure caused by the water level difference at the breach location. Compartments can be located parallel with or perpendicular to the isolines of land level. The land slope will be zero in the first case and maximal in the second case.

The roughness of the land determines the resistance the floodwater experiences. It is a parameter that can easily be changed in a model, but not in reality. Obstacles and vegetation are factors that influence the roughness of the land surface.

36 Compartmentalisation in Central Holland

...... Figure 4-7 Framework component: compartment characteristics. compartment

comp. size comp. shape

land gradient height of dikes land roughness

In figure 4-8, the framework is presented in its full extent.

37 Compartmentalisation in Central Holland

...... Figure 4-8 Framework for designing a spatial layout in preparation of a flooding event.

38 Compartmentalisation in Central Holland

Selection process

The choices to be made in the framework are in this stage still subjective. Not much information is available on when to choose what strategy. This report intends to gain more insight in the selection process. However, for the design of the spatial layouts a first approach is necessary. This approach is explained in this paragraph. At the start of the design process no information is available on the flood pattern. Three aspects of flooding and land characteristics are taken into consideration:

1. Volume of floodwater.

In case the volume of floodwater is small and can be retained easily, then it is likely that compartments bordering the flooding source (river or channel) are most effective. But if the flood volume is large and casualties are expected in this small compartment, larger compartments are required or redirection has to be selected as strategy. High water levels in the area are avoided. On the other hand, retention in small compartments near the source can postpone flooding of the hinterland, giving inhabitants more time to escape the area.

2. Spatial distribution of values and inhabitants.

If the area just behind the dikes is a large residential area, it is probably a good solution to let the water flow through the city and retain it at another location. This will avoid large water depths and therefore casualties in the city.

3. Hydraulic layout of the flooded area.

The elements that are already present in the area should be used in the advantage of the designer. If channel flow is expected to be dominant during the flooding, one should focus on the canals. If overland flow is expected to be dominant, the line elements in the land area are of importance.

Considering these aspects gives an indication of what strategy to follow. Redirection can avoid large water depths, but it also can create large flow velocities and a larger affected area. The dominant effect will have to be determined. Hydrodynamic calculations are necessary to determine the dominant characteristic(s) of a flooding. Several characteristics qualify for being the dominant effect, causing the most casualties and/or damage: - Large flooding depths (damage and casualties). - High water velocity (damage and casualties). - Large flooding area (damage). - Quick rise of the water table (casualties). - Short time available for escape, or - Unavailability of escape routes (casualties).

39 Compartmentalisation in Central Holland

...... Figure 4-9 Main questions and following strategy in strategy selection process. flooding volume

large small

large / small small compartments compartments near source

city near breach?

yes no

flow through retain near breach

determining characteristic

large water depth larger retaining area

high flow velocity lower gradient

obstructions in flow path

large area small compartments

quick water rise larger compartments

short evacuation time more smaller compartments

escape routes unavailable no retention near main routes

highten or strengthen main routes

If small compartments are used, the water depth and rise rate of the water will be high. On the other hand, the affected area and flow velocity will probably be low.

If redirection is selected as a strategy, the affected area and flow velocity are expected to be high and water depth and rise rate of the floodwater is expected to be low.

Redirection Small compartments High flow velocity ⇔ Large water depth Large affected area ⇔ High rise rate

Which effect has the largest impact on casualties and damage should be assessed by simulation with a hydrodynamic model-schematisation.

40 Compartmentalisation in Central Holland

Designing the layouts

In this study, several alternatives are designed for each breach location. The alternatives are based on different approaches:

1. Compartmentalisation in the canal-system, by closing barriers in the canals. 2. Lowering or heightening the existing line elements, such as (rail)roads, dikes and canal embankments. 3. Adding new line elements: new dikes for compartmentalisation purpose only.

The canal-system does not have a large influence in every flooding scenario and therefore is not taken into account for each breach location. When using the line elements, the first approach is to use the existing dikes and (rail)roads. This will enlarge the feasibility of the alternative. If necessary, new line elements will be added, trying to optimise the layout of the polder for the simulated flooding. The framework (figure 4-8) is used to design the spatial layouts of the polder. Figure 4-10 shows which approach is followed for each spatial layout. For example, the notation MB 2 under the red box land gradient means that the second layout for breach location MaasBoulevard differs from the other spatial layouts by a different land slope, in this case caused by the different route the floodwater is forced into. MB 2 is also noted under the protection strategy, meaning that some areas are also protected from flooding by heightening the dikes.

41 Compartmentalisation in Central Holland

...... Figure 4-10 The location of the designed spatial layouts in the framework.

42 Compartmentalisation in Central Holland

5. Simulation results

......

Ter Heijde

In case of failure of the dunes at Ter Heijde, the Westland area is of interest. The area is characterised by high land elevations and a land slope from the sea to the hinterland. Many primary and secondary canals are situated in the area. About 20 emergency barriers are present in these canals. These barriers can be closed when a canal embankment fails. Spatial line elements other than the embankments of the canals are scarce. Only a secondary dike (former primary dike) is present, a road is situated on top of this dike (figure 5-2 and 5-2, also see appendix E: Excursions).

...... Figure 5-1 Area of interest: Westland.

Figure 5-2 Breach location at Ter Heijde.

Ter Heijde Cause Storm at sea. High water level +5.15 m NAP. Breach Width: 200 meters; bottom level 0 m NAP.

see chapter 3: model development, Flood scenario for more information.

Present situation The simulation for the present layout of the polder shows a flood wave propagation from the sea to the lower polders in the hinterland (figure 5-3). The embankments of the canals function as compartment dikes at these locations. Most of the area is flooded with water depths between 0 and 1 meter. Some distant locations are flooded due to quick transport of the floodwater through the canal system. All sluices and emergency barriers are assumed to be open. Flooding occurs at locations with low canal embankments. The southern part of The Hague is also flooded. Maximum flow velocity at the breach is 1,5 m/s. When the storm is over floodwater no longer flows into the area because the relative high elevation of the hinterland. The peak inflow is 1700 m3/s.

43 Compartmentalisation in Central Holland

...... figure 5-3 Flood pattern, damage and casualties after 10 days. Ter Heijde, present situation.

...... Figure 5-4 Flood pattern and canal system.

The flood damage is calculated after 10 days; after this period the Assumption 23 breach is assumed to be closed off and the floodwater in the polder controlled. Total damage in this case is 5 billion euros, mainly caused in the urban areas. The number of casualties is calculated after 10 hours; Assumption 24 all surviving inhabitants are expected to have escaped the area after this period. The number of casualties in this case is 300, almost all of them inflicted near the breach location and in the south of The Hague. Both high rise rate of the water and high flow velocities are the cause of the casualties.

Spatial adjustments

The spatial layout of the area offers opportunities for canal compartmentalisation. There are many canals with emergency barriers. The first layout tests this concept. All emergency barriers are closed. Another option is to use the canals to get the floodwater to a target location. In layout 2 emergency barriers in the north of the area are closed and the eastern barriers are left open. In layout 3 a different approach is followed. A heightened road south of The Hague protects the valuable urban area from flooding (figures 5-5 to 5-7).

44 Compartmentalisation in Central Holland

Figure 5-5 Spatial layout 1 and corresponding flood pattern. Compartmentalisation of the canals.

Figure 5-6 Spatial layout 2 and corresponding flood pattern. Redirect by using canals.

Figure 5-7 Spatial layout 3 and corresponding flood pattern. Protect The Hague.

45 Compartmentalisation in Central Holland

Simulation results

The inflow of floodwater (discharge and flow velocities) for all three new layouts does not differ from the present situation. The differences between the four flood patterns are subtle.

Figure 5-5: By closing all barriers in the canals, floods are prevented in the east and south of the area. The floodwater in the canals reroutes to locations where it can flow without resistance. This results in a flooding north of The Hague with water depths up to 0.5 meter. The number of casualties stays the same, because they are mainly caused near the breach and in the south of The Hague. Damage increases from 5.0 to 5.3 billion euros.

Figure 5-6: Closing only some of the emergency barriers in layout 2 prevents flooding in the east, but flooding in the south is not prevented. The flooding north of The Hague, which occurs in layout 1 is much smaller in this case. Damage increases from 5.0 to 5.1 billion euros. The number of casualties stays the same.

Figure 5-7: In the third layout the city of The Hague is protected by a line element. The canal that runs through the city is still flooded. Water is stored just south of the road. The resulting water depth is 3 meters. Flooding from the canals in the east is reduced, leaving a smaller affected area. Casualties are reduced to 150, a reduction of 50%. Damage is reduced to 3.3 billion euros, a reduction of 1.7 billion or 33%. These reductions are established by the prevention of flooding of The Hague.

46 Compartmentalisation in Central Holland

Maeslant barrier

The Westland is also of interest in case the primary dike fails near the Maeslant barrier (figures 5-8 and 5-9). A secondary embankment is present a few kilometres inland. One primary canal is present in the area between the primary and secondary dike. A sluice is located at the location where the canal crosses the secondary dike. Behind the secondary dike the area mainly consists of greenhouses and agricultural area, so many canals are located in this area. The land levels vary from 0 m NAP near the sea to –2.5 m NAP more to the east...... Figure 5-8 Area of interest: Westland.

Figure 5-9 Breach location at Maeslant- barrier (red arrow).

Maeslant barrier Cause Storm at sea. High water level +3.49 m NAP. Breach Width: 100 meters; bottom level -4 m NAP.

see chapter 3: model development, Flood scenario for more information.

The breach does not grow wider than 100 meters. The breach erodes only during the peak inflow: after the storm is over and water levels drop, no more inflow occurs through the breach (figure 5-11). So not enough time is available to create a breach of 200 meters wide.

Present situation For the flood simulation with the present layout, the flood pattern resembles the flood pattern for the case Ter Heijde (figure 5-10). The secondary dike has a compartmentalisation effect: floodwater is retained between this dike and the primary dike. It is assumed that the Assumption 21 sluice in the secondary dike is opened. Flow occurs through this sluice and over the land surface in the west, near the sea. As a result, the hinterland area is also flooded. Floodwater enters other canals and distant locations are also flooded. Flood characteristics: • Maximum flow velocity in breach: 2 m/s. • Maximum flood depth: 3 m. • Peak discharge: 1650 m3/s. • Drawdown river: 20 cm. • Casualties: 140 • Damage: 2.7 billion euros.

47 Compartmentalisation in Central Holland

...... figure 5-10 Flood pattern, damage and casualties after 10 days. Maeslant barrier, present situation.

1.800

...... 1.600 Figure 5-11 Discharge through the breach. 1.400 1.200

) 1.000 3/s m ( 800 ge r a

h 600 c s i

h d 400 eac r 200 B

0

-200

-400

15 16 Time (day)

Spatial adjustments The secondary dike is not optimally used in the current layout. Water flows out of the compartment in two ways. The first alternative layout closed the first way: the sluice in the secondary dike. The second layout also closes the route over land, in the west near the sea, by extending the secondary dike to the sea. In the third alternative layout, the compartment is split up into two compartments. The final layout tests the concept of redirection by removing the secondary dike (figure 5-12 to 5-15).

48 Compartmentalisation in Central Holland

Figure 5-12 Spatial layout 1 and corresponding flood pattern. Sluice closed.

Figure 5-13 Spatial layout 2 and corresponding flood pattern. Sluice closed, dike extended.

Figure 5-14 Spatial layout 3 and corresponding flood pattern. Two compartments.

49 Compartmentalisation in Central Holland

Figure 5-15 Spatial layout 4 and corresponding flood pattern. Secondary dike removed.

Simulation results

Figure 5-12: In the first layout, the flooding of the middle of the area is largely prevented by the closure of the sluice. Floodwater is rerouted to the west of the compartment and eventually into the hinterland. Water depths are restricted to 1 meter. The maximum flow velocity and peak discharge in the breach remains the same, but the total inflow volume diminishes. After the peak, a return flow occurs of 300 m3/s. Floodwater still enters the canals and floods distant locations from these canals. This effect however is smaller than for the present situation. Damage is reduced to 1,4 billion euros, a reduction of 1,3 billion. The number of casualties decreases to 100 (-40).

Figure 5-13: The second spatial layout shows that by also extending the secondary dike a compartment is created that retains all of the floodwater. Flow velocities or flood depths do not increase. The peak inflow decreases slightly. Sixty casualties and 1,5 billion euro of damage are prevented, bringing the total to 80 casualties and 1,1 billion euro of damage.

Figure 5-14: The third layout shows the effect of a smaller compart- ment near the breach location. The peak inflow of floodwater is reduced and after this peak a return flow occurs of 500 m3/s. The result is a smaller flooded area. Compared to the second layout (one big compartment) no extra casualties are prevented. Two hundred million euro extra damage is prevented. Total damage is 0.9 billion euros. Total number of casualties is 80.

Figure 5-15: The final layout shows the effect of the secondary dike by removing it. Large parts of the hinterland are flooded. Compared to the present situation (sluice opened) 10 more casualties and 100 million euros of damage are caused. Apparently, the effect of the gap in the secondary dike on the flood pattern is almost similar to removing the entire secondary dike.

50 Compartmentalisation in Central Holland

Rotterdam Maasboulevard

In case the breach location is at Rotterdam, the low elevated hinterland is of interest. The greater part of the area is below sea level, containing two locations with the lowest land levels in the Netherlands: The polders Prins Alexander and Zuidplas. These are residential areas with land levels 6 meter below sea level. Several spatial elements are present that can influence the flood pattern. These are roads, railroads and two embanked channels (figures 5-16 and 5-17).

...... Figure 5-16 Area of interest: Rotterdam.

Figure 5-17 Breach location and low elevation polders (red circles).

The breach location is selected at Maasboulevard in Rotterdam, where failure of the primary embankment causes the residential area to be flooded. As pointed out earlier in this report, the breach location is not necessarily a weak link in the primary dike. Two scenarios are simulated: Maeslant barrier open or closed.

Scenario 1: Maeslant barrier open

Maasboulevard, scenario 1 Cause Storm at sea. High water level +3.40 m NAP. Breach Width: 200 meters; bottom level -4 m NAP.

see chapter 3: model development, Flood scenario for more information.

The present spatial layout of the polder already has a compartmentalisation effect. Especially the embankments of the canals function as compartmentalisation dikes because no cuts are present in these dikes. The polders Prins Alexander and Zuidplas become flooded. Near the breach location, water depths are moderate because land elevation levels are relatively high. The area south of the polder Prins Alexander, Capelle a/d IJssel, is not flooded: the secondary embankment protects this area. Flood characteristics: • Maximum flow velocity in breach: 1,7 m/s. • Maximum flood depth: 6,6 m. • Peak discharge: 1500 m3/s. • Drawdown river: 30 cm. • Casualties: 2300 • Damage: 15.4 billion euros.

51 Compartmentalisation in Central Holland

...... Figure 5-18 Flood pattern, damage and casualties after 10 days. Scenario: Maasboulevard, open storm surge barrier, present situation.

...... Figure 5-19 1.600 Inflow of floodwater through the 1.500 breach. 1.400 1.300 1.200 1.100 1.000 )

s 900 3/

m 800 (

e 700 g 600 har c s 500

h di 400 300 eac r

B 200 100 0 -100 -200 -300 -400 13 14 15 16 17 18 19 20 21 22 23 24 Time (day) Spatial adjustments The flooded area contains only two channels, which are not connected, so canal compartmentalisation is not an option. Line elements are present which can be raised, lowered or removed. Three spatial layouts are developed. The first and second layout test the concept of redirection, the third one focuses on improving compartmentalisation.

The first layout (figure 5-20) makes use of existing line elements. Two roads are raised and one road, a railroad and a channel embankment are lowered. The flood pattern is influenced in such a way that the floodwater flows quickly to the northeast part of the area where it is retained. The second layout (figure 5-21) is also constructed using existing elements only. Several railroads, roads and channel embankments are heightened in this case. Several compartments are created to retain the water. The third layout (figure 5-22) expands these adjustments by adding three new elements, compartmentalisation dikes, to the spatial layout of the polder. These elements are added as a continuation of existing elements.

52 Compartmentalisation in Central Holland

Figure 5-20 Spatial layout 1 and corresponding flood pattern. Redirection, east.

Figure 5-21 Spatial layout 2 and corresponding flood pattern. Redirection, west.

Figure 5-22 Spatial layout 3 and corresponding flood pattern. Compartmentalisation.

53 Compartmentalisation in Central Holland

Simulation results

Figure 5-20: The first flood pattern shows that the concept of redirection can be implemented. Flow velocities remain the same. No flooding occurs in the polders Prins Alexander and Zuidplas. The volume of floodwater that enters the area stays the same. The flooded area is expanded in the northwest, with flood depths between 0.5 and 2.5 meters. Other flood depths are similar or slightly smaller compared to the flood simulation for the present layout. Despite the larger flooded area damage reduces to 9.1 billion euros, a reduction of 6.3 billion because the polder Prins Alexander is not flooded. Casualties are also prevented in this polder, but the total number of casualties is increased because the water reaches the north of Rotterdam faster. High rise rates of the water cause most of the casualties. The result is an increase of casualties to 2400 (+100).

Figure 5-20: In the second flood pattern the area east of the canal Rotte is not flooded. Flow velocities remain the same near the breach. The remaining water in the west is not floodwater, but an existing lake (Zevenhuizer plas). More and larger adjustments are needed in the spatial layout to redirect the water to the west, because it is against the natural flow pattern (land gradients). The inflow of floodwater through the breach remains (roughly) the same. The water is retained in several compartments; only a small amount of water reaches the last compartment. In the middle of the flooded area, flood depths increase from 0.8 meter to 2-2.5 meter. The total flooded area increases somewhat. Flow velocities near the breach do not change. Damage is reduced to 8.8 billion euros (-6.6 billion). The high rise rate of the water level causes most of the casualties. The number of casualties increases to 3800 (+1600).

Figure 5-22: The last flood pattern uses compartments. The compartment near the breach has relatively high embankments: between +2.5 and +3 m NAP. The peak inflow remains 1500 m3/s, but the inflow volume decreases because the second inflow peak is much smaller and a negative discharge occurs after the peaks (figure 5-23). Floodwater is flowing back in the river. Large flood depths can be seen in the first compartment, but the rest of the area does not become flooded. The construction of all compartments north of the first compartments appears to be superfluous. The maximum flow velocity in the compartment remains unchanged. Damage is reduced to 2.8 billion euros, a reduction of 12.6 billion. The number of casualties doubles to 4500.

54 Compartmentalisation in Central Holland

...... 1.600 Figure 5-23 1.500 Discharge through the breach for layout 1.400 3: compartments. 1.300 1.200 1.100 1.000 ) 900 3/s

m 800 (

e 700 g r 600 ha c

s 500 di

h 400 c

a 300 e r

B 200 100 0 -100 -200 -300 -400 13 14 15 16 17 18 19 20 21 22 23 24 Time (day)

Scenario 2: Maeslant barrier closed

Maasboulevard, scenario 2 Cause Storm at sea + high river discharge High water level +3.40 m NAP. Breach Width: 200 meters; bottom level -4 m NAP.

see chapter 3: model development, Flood scenario for more information.

In case the Maeslant barrier is closed, the estuary will be filled up by the river discharge.The flooding resembles the situation of scenario 1, but with an increased discharge into the polder (figures 5-24 and 5- 25).

The total flooded area is larger than for scenario 1 (Maeslant barrier open). Large areas in the north and west are flooded with water depths between 0.5 and 1,5 meter. The compartmentalisation effect of the present layout can also be noticed for this case. Furthermore, no polders are flooded from a canal. Compared to scenario 1 (Maeslant barrier opened) damage increases with 20% and casualties with 55%. Flood characteristics: • Maximum flow velocity in breach: 2 m/s. • Maximum flood depth: 6,6 m. • Peak discharge: 1700 m3/s. • Drawdown river: 30 cm. • Casualties: 3600 • Damage: 18.7 billion euros.

55 Compartmentalisation in Central Holland

...... Figure 5-24 Inflow of floodwater in the polder through the breach. Scenario: Maasboulevard with storm surge barrier closed, present situation.

...... 1.800 Figure 5-25 1.600 Flood pattern, damage and casualties after 10 days. 1.400

Scenario: Maasboulevard with storm 1.200 surge barrier closed, present situation. ) 1.000 3/s m

( 800 ge r a

h 600 c s di

h 400 eac r

B 200

0

-200

-400

14 15 16 17 18 19 20 21 22 23 24 Time (day)

56 Compartmentalisation in Central Holland

Spatial adjustments The same spatial layouts are used as in the scenario with the Maeslant barrier opened. Figure 5-26 Spatial layout 1 and corresponding flood pattern. Redirection, east.

Figure 5-27 Spatial layout 2 and corresponding flood pattern. Redirection, west.

Figure 5-28 Spatial layout 3 and corresponding flood pattern. Compartmentalisation.

57 Compartmentalisation in Central Holland

Simulation results

The flood volume is much larger in this scenario. Flood patterns differ from those shown in scenario 1. The results will be reflected to the present situation and to the same layout in scenario 1.

Figure 5-26: The concept of redirection is not changed for the first layout. The inflow of floodwater in the polder is the same as in the present situation. No flooding occurs in the polders Prins Alexander and Zuidplas. Large areas in the north and west of the area become flooded instead. Compared to the first scenario additional flooding occurs west of Zoetermeer. Compared to the present layout, the water depths increase in the north of the area from 1-1.5 meter to 2.5-3 meter. In the northeast of Rotterdam, water depths of 2.5 meter are reached within 1.5 day. This is one day earlier than in the present situation. On the other hand, water depths will not continue to rise in this area, because water can flow through. So 2.5 meter is the maximum water depth, in contrast with almost 4 meter for the present layout. The maximum flow velocity near the breach still is 2 m/s, but these velocities occur in a larger area around the breach (approximately 1 km). The number of casualties increases to 3600 (+100), mainly caused by a shorter time of arrival of the floodwater. Damage is reduced by 5.5 billion euros because some residential areas are not flooded, bringing the total damage to 13.3 billion euro.

Figure 5-27: The second layout focuses on redirection the area west of the canal Rotte. A larger area is flooded than in the first scenario. The northeast of the area is flooded with water depths of approximately 1 meter. The average flooding depth at other locations is 2 meter. The inflow of floodwater is smaller than for the present situation. The peak discharge is 1400 m3/s instead of 1700 m3/s. This is caused by a quick water rise just downstream the breach. The natural flow of the floodwater is towards the northeast, but in this case it is stopped and redirected to the northwest. The water level rises quickly until the level is high enough to flow over the higher grounds in the west. The water level difference between river and polder diminishes and so does the discharge through the breach. Flow velocity near the breach decreases slightly. At locations where the floodwater overtops the embankments, flow velocities up to 2 m/s occur. At these locations the high rise rate of the water causes a larger number of casualties: 7600 (+4000). Damage is 16.9 billion euros, a reduction of 7.9 billion.

Figure 5-29: The last flood pattern uses compartments. Just as in the first scenario, only the first compartment becomes fully flooded. The other compartments are unused. Apparently, the first compartment is also for this scenario very effective. Total inflow of water is reduced, due to the fast water rise in the compartment. The peak inflow is reduced to 1000 m3/s (figure 5-29). Moreover, if the water level in the river drops again the flow direction in the breach changes. In that case, water is flowing out of the polder back into the river. This effect is significant, as is shown in figure 5-29, the peak return flow is 500 m3/s. The maximum flow velocity near the breach is 1.7 m/s. High flow velocities are reached especially where the high embankments become

58 Compartmentalisation in Central Holland

overtopped. Maximum flow velocities of 4-5 m/s occur at these locations. A smaller area is affected by the flooding, preventing 13.5 billion euros of damage. Remaining damage is 5.3 billion euro. The number of casualties is 7500 (+3900) caused by the high rise rate of the water and large water depths in the first compartment.

...... 1.800 Figure 5-29 Discharge through the breach for layout 1.600 3: compartments. Peak inflow is only 1.400 1000 m3/s and a return flow occurs of 500 m3/s. 1.200

) 1.000 3/s m

( 800 e g

har 600 c s i

h d 400 eac r 200 B

0

-200

-400

13 14 15 16 17 18 19 20 21 22 23 24 Time (day)

59 Compartmentalisation in Central Holland

North Sea Channel

The final breach location is selected at the north side of the dike-ring area (figures 5-30 and 5-31). A breach in this part of the primary dike only has flooding consequences in case the North Sea Channel has already flooded the area between the canal and the primary dike. The low elevated polder Haarlemmermeer is of special interest when flooding occurs. A belt canal is present around this polder. No primary canals can be found in the polder, but outside the polder a network of these canals exists (figure 5-32)...... Figure 5-30 Area of interest: Haarlemmermeer.

Figure 5-31 Breach location at Spaarndam.

When calculations are made with the normative high water level, +1,25 m NAP, only small areas become flooded (figure 5-32). A compartmentalisation effect of the canal embankments can be seen for these small flooding events. The floodwater is retained in an area of 8 or 10 km2. Because the area functions as agricultural area, no casualties and little damage is caused. In order to test the area on a larger flooding, a larger and more unlikely flood event is selected for further calculations.

North Sea Channel Cause 1. Storm at sea 2. Sluices IJmuiden and Spaarndam open 3. Breach in embankment belt canal. High water level +5.15 m NAP. Breach Width: 100 m; bottom level: -3 m NAP.

see chapter 3: model development, Flood scenario for more information.

This scenario is a large storm at sea (+5.15 m NAP) together with open sluices at IJmuiden and Spaarndam and a breach in the embankment of the belt canal of the polder Haarlemmermeer. This breach is smaller than the other cases because a canal embankment fails instead of a primary dike. The breach is 100 meter wide and has a bottom level of –3 m NAP. This is considered a worst-case scenario.

60 Compartmentalisation in Central Holland

...... Figure 5-32 Two flood patterns north of the polder Haarlemmermeer with high water level +1,25 m NAP.

Present situation

The present layout of the polder shows flooding of the greater part of the polder Haarlemmermeer, including parts of Schiphol Airport (figure 5-33). There is also transport of floodwater through the canal system, creating small flooded areas south of the polder Haarlemmermeer. Note that most of the water bodies outside the Haarlemmermeerpolder are existing lakes (see also figure 5-31). The city of Haarlem is flooded at some locations with several decimetres of water. The lakes connected to the canal system cause secondary inflow: they are slowly drained because they are connected to the canal system. Flood characteristics: • Maximum flow velocity in breach: 0.5 m/s. • Maximum flood depth: 2,5 m. • Peak discharge: 280 m3/s. • Casualties: 110 • Damage: 6.4 billion euros (Schiphol 3.2 billion euros).

...... Figure 5-33 10-day flood pattern, damage and casualties for breach location Spaarndam, worst case scenario.

61 Compartmentalisation in Central Holland

Spatial adjustments

This case is used to look into the flexibility of a compartmentalisation solution. The polder Haarlemmermeer is split up into two compartments (figure 5-34). Subsequently, a breach in the belt canal is simulated in the northern and southern part of the compartment. The compartmentalisation dike is the geniedijk, a remnant of the Dutch Waterlinie. This dike is already present and is waterproof. Barriers that have to be closed in case of a flooding event are present at five locations in the dike.

Note that in the case for the present situation this dike was omitted: it was not present in the original Digital Elevation Model. The following two layout alternatives represent the actual situation better than the case called ‘present situation’.

...... Figure 5-34 Location of breaches and the geniedijk in the polder Haarlemmermeer.

Figure 5-35 and 5-36 Left: 10-day flood pattern, damage and casualties for southern breach location. Right: 10-day flood pattern, damage and casualties for northern breach location.

62 Compartmentalisation in Central Holland

Simulation results

Figure 5-35: If the breach is located south of the geniedijk, the southern compartment is almost entirely flooded. Parts of the cities remain dry because of the higher elevation level. Flood depths in the compartment stay the same. Also the discharge and the maximal flow velocity through the breach remain equal. The northern compartment is flooded by overland flow from the north (which is caused by the failing of the sluices at Spaarndam). The number of casualties increases to 130 (+20). Damage is reduced by 3.4 billion, leaving a total damage of 3.0 billion euros. Most of the damage is prevented at Schiphol Airport.

Figure 5-36: If the breach location north of the geniedijk, the northern compartment becomes flooded entirely. Part of the southern compartment is also flooded with several decimetres of water. This flooding is caused by high water levels in the belt canal, which overtop the canal embankments. The discharge through the breach in the northern canal embankment is 640 m3/s instead of 280 m3/s. The flow velocity in the breach increases to 2 m/s. Because the entire northern compartment is flooded, total damage almost doubles to 12.4 billion euros. The number of casualties decreases to 100, meaning 10 casualties are prevented.

63 Compartmentalisation in Central Holland

64 Compartmentalisation in Central Holland

6. Sensitivity analysis: Maasboulevard

......

For flood modelling there are two parameters that have a large effect on the outcome of the simulation. These are the dimensions of the breach in the dike and the roughness of the land surface. A sensitivity analysis is performed on these parameters for one breach location: Maasboulevard, the scenario used is the one with closed flood defences (see chapter 3: model development, Flood scenarios).

Breach dimensions

The breach is modelled in Sobek as a sharp crested weir with decreasing crest height. Simulations are made with Sobek for different breach dimensions, the generated output is analysed.

...... Figure 6-1 Breach location: Rotterdam, Maasboulevard.

Three cases are analysed: 1) Breach width is varied while the bottom level is fixed at +2 m NAP. This is the level of the paved boulevard in front of the dike (source: Water board Schieland and Krimpenerwaard). 2) Breach width is varied while the bottom level is fixed at –4 m NAP. This is the level of a clay layer in the dike (source: Water board Schieland and Krimpenerwaard). 3) Breach depth is varied while the breach width is fixed at 300 meters.

In reality breach width and depth increase together, but in this analysis the two dimensions are dealt with separately. The used values of the breach width are 15, 100, 200, 250, 275, 300, 325, 350, 500 and 1000 meters. These are the estimated minimum and maximum value for a breach in a dike of sand and clay [Visser, 1998].

65 Compartmentalisation in Central Holland

The breach depth is taken at +2, +1, 0, -1, -2, -3, -4, -7, -10, -13 and –16 meter NAP. A breach of +2 m NAP occurs when the paved boulevard does not flush away in the flooding. If the breach is extremely scoured out during the flooding, a deep pool of –16 m NAP can be the result.

The different breach dimensions results in different inflow patterns, visualised in peak discharge (figure 6-4 and 6-5), maximal water levels in the river and the polder (figure 6-6 and 6-7), occurring at the time of the peak discharge, and the total volume of the floodwater measured over 10 days (figure 6-8 to 6-10).

Figure 6-2 shows an example discharge-time graph to show the importance of the peak discharge in relation to the total volume.

...... Figure 6-2 Discharge at the breach. Breach dimensions 200 m wide and bottom level at – 4 m NAP.

The discharge through the breach is dependent on the water level in the river and the polder and on the size of the breach (figure 6-3). The breach is modelled as a broad, sharp crested weir. When the water level in the polder is low enough, critical flow will occur above the weir crest (Fr ≥1). The discharge over the weir in that case is:

3 / 2 Q = 1.7 ⋅ mko ⋅ B ⋅ h1 (6.1)

with: Q = discharge [m3/s] 1/2 mko = empirical discharge coefficient [m /s] B = breach width [m]

h1 = water level above the weir crest [m]

[d’Angremond et al, 2000.]

66 Compartmentalisation in Central Holland

In case of high water levels in the polder, the free inflow of the floodwater is restrained. In that case, there will be no critical flow over the weir and the discharge over the weir is no longer described with equation 6.1.

...... Figure 6-3 Critical flow over a broad, sharp crested weir. [d’Angremond et al, 2000].

Figures 6-4 to 6-7 show the influence of the breach dimensions on the peak discharge from the river into the polder and the water levels in polder and river, just upstream the breach. These figures are constructed from output from Sobek, so each point in the graphs resembles a hydrodynamic simulation in Sobek.

Following equation 6.1, the discharge Q increases proportional to the width B. This can be seen in figure 6-4, where the first part of the lower graph increases linear. When the breach becomes larger than 500 meters, this is no longer the case. For the higher graph, the discharge does not increase linearly, but seems to approach a maximum value of 4000 m3/s. Figure 6-6 shows why a maximum value is reached. The water level draws down in the river, while the water level in the polder increases. The two levels converge to +2.80 m NAP. A lower water level in the river decreases the discharge over the weir to the power 3/2 (formula 6.1). When the water level difference is very small, the flow above the weir ceases to be critical and the discharge per meter will decrease. The same phenomenon occurs for the case with the breach level at +2 m NAP (lower line in figure 6-4), although the equilibrium in this case is at +3.20 m NAP (not shown in figures).

In varying the breach depth the water levels in polder and river also reach equilibrium (figure 6-7). This is the same level as in figure 6-6. In figure 6-5 is shown that with small breach depths, the increase in 3/2 discharge is more than proportional, explained again by h1 . The increase becomes less and less and finally approaches a maximum level of 4000 m3/s. Figure 6-7 shows that water levels converges to a value of +2.80 m NAP.

67 Compartmentalisation in Central Holland

...... Figure 6-4 Figure 6-6 Peak discharge against breach width, using two different breach Maximal water level in the river and polder against the breach width. depths. Breach bottom level = -4 m NAP.

4500 3.2

) 4000 3 /s 3 3500 )

m 2.8 (m (

3000 l e e g v

r 2500

e 2.6 l r ha 2000 c te 2.4 1500 a W dis

x 1000 2.2 a

m 500 2 0 0 200 400 600 800 1000 1200 0 200 400 600 800 1000 1200 Breach width (m) Breach width (m) River Polder depth + 2 NAP depth -4 NAP

...... Figure 6-5 Figure 6-7 Peak discharge against breach depth, using breach width = 300 meter. Maximal water level in the river and polder against the breach depth. The reference level is at +5 m NAP. A breach at +2 m NAP therefore Breach width = 300 m. has a breach depth of 3 meter.

4500 3.4 4000 3.2 3 )

/s) 3500 3 m 2.8 l ( m 3000 e 2.6 ( e lev g 2500 2.4 r e ar t 2.2

2000 a sch W

i 2 1500 d

x 1.8 a 1000

m 1.6 500 0 5 10 15 20 25 0 Breach depth (m below +5m NAP) 0 5 10 15 20 25 Breach depth (m below +5m NAP) River Polder

68 Compartmentalisation in Central Holland

When the flood volumes over ten days of flooding are considered (figure 6-8 to 6-10), a difference can be made between an increasing breach width and an increasing breach depth. An increasing breach width results in a continuing increase in floodwater volume. By increasing the breach depth, however, the total floodwater volume reaches a maximum value. Apparently, the breach depth has no influence on the volume of floodwater once it is below a certain value (-10 m NAP). The maximal inflow is reached at this depth. A maximal inflow is not reached for the breach width of 1000 meters (figure 6-8 and 6-9), probably due to the larger cross-section area of the breach allowing a higher inflow volume.

...... 30 Figure 6-8 10-day inflow volume against breach width, 25

) breach bottom = +2 m NAP. 3 m 6 20 (10

e 15 m u l o 10 v ow 5 Infl

0 0 200 400 600 800 1000 1200 Breach width (m)

...... Figure 6-9 1000 10-day inflow volume against breach width, breach bottom = -4 m NAP.

800 )

3 m 6 600 (10

e m u

l 400 o v

ow 200 Infl

0 0 200 400 600 800 1000 1200 Breach width (m)

...... Figure 6-10 900 10-day inflow volume against breach depth, 800 breach width = 300 m.

) 700 3

m 600 6

(10 500

e 400 m u l

o 300 v 200 ow 100 Infl 0 0 5 10 15 20 25 Breach depth (m below +5m NAP)

69 Compartmentalisation in Central Holland

Land roughness

The influence of land roughness on the floodwater inflow and flood pattern is analysed for the same breach location (Maasboulevard, scenario storm surge barrier closed). The land roughness in the polder is changed.

The breach used is 100 m wide and has a bottom level of –4 m NAP. The breach is modelled as a ‘dam break’ according to the breach growth formula of Van der Knaap [Van der Knaap, 2000]. The effect of the land roughness is tested by changing the global roughness coefficient. So each grid cell has the same roughness. The results are compared to a grid with distinct values for each grid cell. This Nikuradse-grid is used in this study (figure 6-11)...... Figure 6-11 Nikuradse grid used in this study.

The Manning coefficient is used (Appendix D: Roughness coefficients). The values selected are 0.03 (resembles gravel); 0.04 (well-maintained natural river); 0.05 (meandering natural river); 0.07 (dense bushes) and 0.10 (dense forest).

The effect of the roughness is tested on maximum water levels in the river and polder (near the breach), the discharge peak through the breach and the inflow volume for 3.5 days (figures 6-12 to 6-14). Note that in these figures the outcome for the Nikuradse-grid is shown separately, with label N.

The effect of roughness on the peak discharge is large. The discharge is almost 3000 m3/s if the area would be entirely made of gravel. Would the area consist out of a dense forest the discharge is only 1850 m3/s, a decrease of 38%. This can be explained by the restraining effect of the high roughness (forest) on the floodwater inflow. As can be seen in figure 6-12, the water level in the polder near the breach rises 70 cm between Manning roughness 0.03 and 0.10 m/s1/3. The water level rise is linear. In the river the drawdown of the water level decreases due to the lower discharge in the polder. This effect is only 20 cm between the same Manning values, so the water level difference decreases with 50

70 Compartmentalisation in Central Holland

cm by changing the roughness coefficient from n = 0.010 to 0.100 m/s1/3.

The effect of a higher roughness coefficient on the 3.5 day inflow is similar. The volume decreases from 370 Mm3 for a Manning roughness value 0.03 m/s1/3 to 200 Mm3 for 0.10 m/s1/3. This is a decrease of 46%.

...... Figure 6-12 3.5 Water levels against the Manning 3 roughness coefficient. The separate values ) 2.5 with label N on the right resembles the m N ( outcome for the Nikuradse-grid and not the 2 1/3 outcome for manning = 0.11 m/s . evel l

r 1.5 e

t a 1 W 0.5

0

0 0.02 0.04 0.06 0.08 0.1 0.12

Manning coefficient (s/m 1/3)

Polder River

...... 3500 Figure 6-13

Peak discharge against the Manning ) 3000 /s roughness coefficient. The separate values 3

on the right resembles the outcome for the m (

Nikuradse-grid and not the outcome for e 2500 manning = 0.11 m/s1/3. g ar

sch 2000 i d N

eak 1500 P

1000 0 0.02 0.04 0.06 0.08 0.1 0.12 Manning coefficient (s/m1/3)

...... 400 Figure 6-14 350 )

3.5-day inflow volume against the 3

Manning roughness coefficient. The m 300 6

separate values on the right resembles the 0 1 250 outcome for the Nikuradse-grid and not the ( 1/3 e outcome for manning = 0.11 m/s . m 200 lu o 150 v N w

o

l 100 f n I 50 0 0 0.02 0.04 0.06 0.08 0.1 0.12 Manning coefficient (s/m1/3)

71 Compartmentalisation in Central Holland

In figure 6-15, the time of arrival of the floodwater is shown for the different roughness values. The flood front reaches the north of the low elevation area after 96 hours in case the area is smooth (Manning value is 0.05 m/s1/3 or lower, but is closer to the breach location in case of higher roughness values. For example, the eastern part of the area (near Gouda) is flooded between 25 and 36 hours when Manning is 0.03 m/s1/3. The inhabitants have approximately one day to flee the area. If the manning value is 0.07 m/s1/3 this increases to two days (49 hours) and if manning is 0.10 m/s1/3 it increases to 60 or even 70 hours. chapter 7: ...... The results of the sensitivity analysis are discussed in Figure 6-15 Reflection. Time before flooding for various roughness coefficients. Flood duration = 3.5 day.

72 Compartmentalisation in Central Holland

7. Reflection

......

Model results

Compartmentalisation

Compartmentalisation of the polder is an effective measure in reducing flood consequences (figure 7-1). This is shown in the simulations for the cases Maeslant Barrier 3 and Maasboulevard 3. Especially the use of small compartments (10 km2) near the breach location is promising.

...... Figure 7-1 Concept of retention in compartments.

Retention

This has three positive effects on the amount of water entering the polder.

1. The inflow of floodwater into the polder is reduced.

The discharge into the polder is driven by the water level difference over the breach (polder-river/sea). The flow through the breach reduces until the water level in the polder equals the water level in the river. The flow velocity in the breach is 0 at this moment. By creating small compartments near the breach location, a high water level in the polder is reached quicker. This will restrain the inflow into the polder during the period of high water in river or sea. The effect is larger in case of large floods, because the high water level in the compartment is reached earlier than in case of a smaller flooding. This can be seen in the two selected cases: the effect is largest for Maasboulevard 3(closed), in which peak discharge is reduced by 40%.

73 Compartmentalisation in Central Holland

2. After high water, a large part of the floodwater will flow from the polder into the river or sea under gravitational flow.

The second advantage of a high water level in the compartment becomes clear when the water level in the river or sea drops. The water level in the polder will be high because of the small compartment, which retains the water near the breach. A part of the floodwater volume can flow back into the river under gravitational flow. The reverse discharge can be large, e.g. in the case for Maasboulevard 3(closed) it amounts to 500 m3/s.

3. Low flow velocities when the flow direction changes, gives opportunities to close the breach quickly.

When the flow direction in the breach changes, there is a period of low flow velocity in the breach. This period can be used to close off the breach and stop the flooding in an early stage. The pumping out of the floodwater and repairing of damage can begin directly after the closure.

Implementation of these small compartments should be done with care. First, the forces on the compartmentalisation dikes are expected to be large, these dikes must be able to resist these forces. Secondly, the case Maasboulevard 3 shows that the high rise rate of the water in the compartment has a strong negative effect on the number of casualties. So, the measure is most effective if a low number of inhabitants is present in the small compartment while the hinterland is densely populated. If this is not the case, other measures are necessary to prevent a large number of casualties in this area. Good evacuation plans and routes are possibilities, together with a well-functioning warning system.

74 Compartmentalisation in Central Holland

Redirection

If the strategy of redirection is selected, floodwater is redirected away from valuable areas, regardless of what other area is flooded (figure 7-3), or to a specific target location where the water is retained (figure 7-2). The two strategies can also be combined (e.g. Maas- boulevard 1 & 2).

To target location

The expected effects of redirection were high flow velocities and a short time of arrival of the floodwater (chapter 4: Spatial design of the dike-ring area). The short time of arrival is indeed noticed. For example in Maasboulevard 1 closed the time of arrival for the north of Rotterdam decreases from 2.5 to 1.5 day compared with the present situation. This causes more casualties in the hinterland because the flooding will surprise more inhabitants; they have too little time to escape the area.

The flow velocities in the breach, however, do not increase (Maasboulevard 1 & 2) or only marginal. In the case Maeslant barrier 4 an increase of maximal velocity in the breach is calculated from 2 to 2.15 m/s. The maximum velocity does not increase much because the velocity occurs just after the dike breaches. The water level difference between the dry polder and river is maximal at that moment. Water can flow freely into the polder. Later, when the water level in the polder has risen, the inflow into the polder restrains. The flow velocity decreases especially when small compartments are used. In the early stage of the flooding, however, there is no difference between the flow velocities in case of redirection or compartmentalisation.

...... Figure 7-2 Concept of redirection.

Redirection

In general, using redirection to a target location as a strategy reduces damage. The amount of damage prevented depends on local economic values. The reduction is optimal if a route of low economic value can be constructed towards the target location. Reductions up to 80% are calculated in this study.

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When a flood event occurs, the strategy of redirection can reduce damage and casualties compared to the present situation especially if the area near the breach is densely populated while the hinterland is not. In that case it is expected that the number of casualties in the densely inhabited area will decrease because the rise rate of the floodwater is low and the maximum flow velocity remains the same; the number of casualties in the hinterland will not increase (shorter time of arrival, but no inhabitants present) and damage is reduced by redirecting the flow into the desired direction.

If both the area near the breach and the hinterland are of low value, it is advised to use small compartments to reduce the volume of floodwater flowing into the polder and not the strategy of redirection.

Protection

Protection of valuable areas as a strategy is used in the cases Maasboulevard 1&2 and Ter Heijde 3. Placing dikes (partly) around residential areas prevents them from flooding (figure 7-3). The cases Maasboulevard 1 & 2 show that the protection of large areas on one location causes large flooding at a different location. The protection of only a district of The Hague in the case Ter Heijde 3 does not have a large impact on the flood pattern. It can be concluded that protection of an area is successful if it suffices two conditions: the protected area has to be small and of high value (economic and high number of inhabitants). In that case, the caused damaged in the area that is flooded instead is lower than the prevented damage in the area that is protected against flooding.

...... Figure 7-3 Concept of protection.

Protection

Canal system

If large amounts of floodwater enter the canal system, the water is transported quickly through these canals and cause flooding at locations where the canal embankments are relatively low. This effect appears in the cases Ter Heijde present situation and 1-4, Maeslant

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Barrier 1 and North Sea Channel 2. In all cases it is only a moderate effect, but if one of the embankments fails consequences will be large. This can be seen in the cases North Sea Channel 1 and 2, where the polder Haarlemmermeer is flooded because of a dike failure.

Placing barriers into the canals can reduce the flooding of distant locations from the canals. The quick transport of floodwater through the canals is stopped, preventing flooding at locations far away from the breach (Ter Heijde 1). The effect of these barriers near the breach location is small: the floodwater will just flow around these structures. The effect is largest at locations where overland flow does not occur.

The canals are also tested on the possibility of redirecting floodwater to a target location in the case Ter Heijde 2. The effect on the flood pattern is small. The amount of water that can be redirected is limited to the capacity of the canals, which is small compared to the total flood volume.

Maeslant barrier open or closed

For breach location Rotterdam, Maasboulevard two scenarios are simulated: 1) Maeslant barrier open with a storm at sea and 2) Maeslantbarrier closed for several hours with a storm at sea and a large river discharge. The discharge into the polder is much larger in case of closure of the Maeslant barrier. This is mainly caused by the time of dike failure. For both scenarios the dikes fail in an early stage (before peak water level is reached). In the scenario with the open barrier, the peak water level is reached 4 hours later than in the scenario with the closed barrier. In the scenario with the closed barrier, the water level raises slowly because of the river inflow and the peak level is reached 13 hours later. A large inflow into the polder will occur over a longer period. Would the dike failure occur in both cases 4 hours before peak water level is reached, the difference in inflow would be smaller. The remaining difference is caused by a different water level at that time (open: + 1.5 m NAP; closed: +2.9 m NAP) and the inflow after the peak water level. The water level in the river is higher in the scenario with the barrier closed because of the higher river discharge.

Flexibility and reliability

Flexibility

The model simulations show the impact of different spatial layouts for a single breach location. An optimal layout for the entire polder is not obtained. More simulations for different breach locations are necessary to draw conclusions on this optimal layout. It could be that a single optimal layout does not exist, because a positive effect for one breach location could mean a negative effect for a different breach location. This can clearly be noticed in the cases North Sea Channel 1&2. If the breach is located north of the compartmentalisation dike (geniedijk) instead of south, damage is 12.5 billion euros instead of 3 billion euros. The challenge is to find spatial adjustments that have a positive effect in all or most cases. In case of the geniedijk the dike should be able to

77 Compartmentalisation in Central Holland

let water flow through from north to south but must be a barrier for flow from south to east to have a positive effect on both floodings. The effect of multiple breaches also has to be taken into account.

Reliability

Assumption 4 An important assumption in this study is that all elements used in the spatial layouts have enough strength to resist the water forces. If the compartmentalisation dikes fail during a flooding, the consequences are large. This is shown in the Maeslant barrier cases. In layout 1 the compartmentalisation dike functions properly, while in the present layout two cuts are present in the dike. Layout 4 shows the flood pattern in case the compartmentalisation dike is not present. The differences between the flood pattern for the case without the dike and the case with the two cuts in the dike are small. The number of casualties and damage caused by both flood events are comparable. It seems that in case the flow direction is perpendicular to the dike, the consequences of a dike failure are of such magnitude that the influence of the remaining dike is small.

Sensitivity analysis

Land Roughness

The sensitivity analysis on the roughness of the land has a remarkable outcome: the Nikuradse roughness grid that is used in this study is to be considered as a grid with very high roughness values. Compared to constant Manning roughness values it is even rougher than a floodplain planted with a dense forest (Manning n = 0.10 m/s1/3).

The explanation of this effect has to be found in the way residential

areas are modelled. Nikuradse defined his coefficient ks as the grain size of the sand particles, which he glued to the sidewalls of the tubes in his

experiments. The dimension of ks therefore is [m]. The value of ks is between 1 and 10-5 for channel flow [d’Angremond et al, 2000] In the Nikuradse- roughness grid, however, values are used of more than 1 meter. The maximum grid cell value is 10 meter, which corresponds with an obstacle of 10 meters high (high buildings). Floodwater depths are much smaller than this value, so overland flow is subject to a very

high resistance. If a Nikuradse ks –value of 7 m is converted to a Manning or Chézy value, using a flood depth of 1 meter; the outcome is a Manning value of 0.24 m/s1/3 and a Chézy value of 4.2 m1/2/s. These values represent an extremely high roughness. The formulas in Appendix D: Roughness coefficients are used for this conversion.

Furthermore, the grid cell size is 100 meters, meaning that an area of 100 x 100 meters has the same high roughness value. This is a wrong representation of reality. If some high-rise buildings are situated in an area of 100 x 100 meters, there are also some locations of low

78 Compartmentalisation in Central Holland

elevation and low resistance in this area. Water will flow easily around the buildings over these areas of low resistance.

Apparently, the way residential areas are modelled in the schematisation is not correct. Low water levels in combination with high roughness values and a large grid cell size result in flow velocities of the floodwater that are too small. Two solutions are possible for this problem. A detailed grid of the area (e.g. 25 x 25 meters) where the roads in the residential area can be modelled into gives a more accurate small-scale flow pattern. The flow around high-rise buildings is taken into account. The construction of such a detailed grid is not feasible with the current model practice. The method would be time consuming and expensive. The second solution is to develop better average roughness coefficients for these areas. These values will have to be lower than the values in the Nikuradse –roughness grid used in this study. Difficulty with this solution is that not much information about flow velocities and water levels in case of flood events in residential area exist. More research on the roughness values is necessary to improve model schematisations of residential areas.

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80 Compartmentalisation in Central Holland

8. Conclusions and recommendations

......

Main conclusion

Local flood characteristics, such as water depth, flow velocity and rise rate, determine the damage and casualties caused by a flooding. The design of a spatial layout for a dike-ring area to reduce damage and casualties during flood events can therefore only be optimised by making hydrodynamic model simulations for these flood events. In this study four ‘building blocks’ are derived that seem to have general validity.

1. Compartmentalisation is a promising measure. The hydraulic effect of compartmentalisation is optimal in case a small compartment (10 km2) with high dikes is located near the breach location. The inflow of floodwater into the compartment is restrained by the quick rise of floodwater in this compartment. The floodwater volume entering the polder diminishes, resulting in a smaller total flooded area. The effect on flood consequences is large if a low number of inhabitants is present in the small compartment while the hinterland is densely populated.

2. Redirection as a strategy against flooding can reduce damage and casualties. A high rise rate of the water in the polder is prevented by redirecting the water instead of retaining it. The strategy can be used at locations where compartmentalisation is not possible, because high economic values or a large number of inhabitants are present near the breach location. The strategy is most effective if the area near the breach is of high value while the hinterland is not.

3. a. Floodwater that enters the canals will travel fast through the canal system in the polder and will flood locations where the canal embankments are relatively low. b. Compartmentalisation of the canals, by closing emergency barriers in these canals, is an effective measure to prevent the floodwater from spreading out over the canal system. Flooding at distant locations caused by overtopping of relative low canal embankments is prevented.

4. The protection of valuable areas by placing dikes around the area is effective as long as the protected area is small and of high value. In this case, the water volume of the flooding that is prevented is also small and will probably flood an area of lower value.

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Flexibility and reliability are important issues in designing spatial layouts: The designed layout must have a positive effect on flooding from different breach locations and all compartmen- talisation dikes will have to be strong enough. The impact of a failing compartmentalisation dike is large.

The structural measures in the polder that are studied on hydraulic effectiveness in this report should be compared to other measures to reduce flood risks, such as upgrading the primary dikes and calamity management. Cost-effectiveness is an important parameter in this comparison.

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Conclusions by subject

Flooding in Central Holland

[C1] Central Holland is a large area that is not likely to be entirely flooded in case of a single breach in the primary dikes. The volume of the floodwater is not large enough, even if a worst-case scenario is simulated.

[C2] The higher land levels near the Oude Rijn function as a natural barrier, preventing floodwater flowing from the north to the south and vice versa.

[C3] The impact of a flooding originating from the North Sea Channel and Amsterdam-Rhine Channel is small. In case of water levels that occur once in the 10.000 years, the expected flooded area still is smaller than 10 km2.

[C4] The impact of a flooding originating from the North Sea or Nieuwe Maas/Nieuwe Waterweg is large: peak discharge into the polder is calculated at 1700 m3/s in this study. The flooded area is approximately 100 km2 for a breach at sea and 150 km2 for a breach next to the river.

[C5] The inflow of floodwater into the polder will stop when the water level drops to the normal level after a storm. Continuation of the inflow will only occur if the polder level is lower than the water level. This is only the case in the area next to the Nieuwe Maas.

Channel flow

[C6] Quick transport of floodwater through the system of canals occurs in case of a large flooding. Locations at 10 kilometres or more from the breach become flooded within a few hours.

[C7] Compartmentalisation of the canals by closing barriers in these canals has a small effect on the flood pattern near the breach location. Floodwater will just flow around these structures at these locations.

[C8] Closing barriers in canals stops the quick transport of floodwater through the canals at more distant locations (approximately 5 kilometres or more). Floodwater cannot spread out over the canal system. Floods originating from the canals due to relatively low embankments are prevented.

[C9] The canals can be used to redirect floodwater to a target location. The effect is limited to the canal capacity, which is much smaller than the water inflow into the polder.

[C10] Failure of a canal embankment has a large effect on the flood pattern.

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Overland flow

Retention

[C11] Compartmentalisation of the polder has a positive effect on the damage caused by a flooding, because a smaller area is affected by the flooding. The reduction depends on the breach location and local area layout. In this study damage reductions of 50-80% are found.

[C12] Compartmentalisation has a positive effect on the number of casualties caused by the flooding if the first compartment that becomes flooded has a low number of inhabitants. The reduction depends on the breach location and local area layout. In this study a maximal reduction of 40% is found for the Maeslant-case. In case the compartment is densely inhabited a negative effect occurs. The number of casualties can increase with more than 100%.

[C13] The effect of compartmentalisation is optimal in case of small compartments with high dikes near the breach location. The high rise rate of the water in the compartment restrains the inflow into the polder. When the outside water level decreases the flow reverses from the polder into the river or sea.

[C14] Compartmentalisation of the polder Haarlemmermeer by the geniedijk is a good option to prevent a large amount of damage at Schiphol Airport, in case of a large flooding in the south compartment. If a large flooding occurs in the north compartment, the geniedijk should let the water flow through from north to south to prevent damage.

Redirection

[C15] Redirection of floodwater instead of retention shortens the time of arrival of the floodwater, causing an increase of the number of casualties if the hinterland is populated.

[C16] Redirection of floodwater does not cause more casualties due to high flow velocities. The maximal flow velocity occurs just after dike breach and is not much influenced by the selected strategy (redirect or retain).

[C17] Redirection of floodwater reduces the damage caused by the flooding, provided that the flow is redirected through areas of low value to areas of low value.

Flexibility and reliability

[C18] Failure of a compartmentalisation dike has large consequences on flood pattern, damage and number of casualties.

84 Compartmentalisation in Central Holland

[C19] Flexibility of a spatial layout is an important issue. Modelling results are very sensitive to the location of the breach.

Roughness overland flow

[C20] The Nikuradse-roughness grid used in this study is not suitable for calculations with (low) flood water levels. The schematised roughness values are too large. Modelling results are biased: rise rates of floodwater near the breach are too high and the time of arrival of the floodwater is too long.

Recommendations

Spatial design polder

In this study, no optimal spatial layout for the entire study area is found. Instead, some concepts are tested resulting in building blocks that can be used to design such an optimal layout. More flood simulations are necessary to get more insight in the effects of the building blocks on flooding from different breach locations and the influence of the building blocks on each other. A layout can have a positive effect on a flooding from one breach location, but might have a negative effect if the dike breaches at a different location.

[R1] It is recommended to research whether an optimal dike-ring layout is possible, which has a positive effect on a flooding from each breach location in the primary dike. Such a layout is expected to be unique for each dike-ring area.

[R2] The influence of failure of a compartmentalisation dike on the flood pattern should be further investigated to determine the reliability of a compartmentalisation solution.

[R3] The use of secondary dikes, roads and railroads as compartmentalisation dikes is questionable. The strength of these elements and the feasibility of closing cuts in these elements should be investigated.

[R4] The cost-effectiveness of the use of structural measures in the polder to reduce the flood risk in an area should be compared to other measures for reducing flood consequences (calamity management), such as evacuation or breach closure, and compared to measures for reducing the probability of flooding, like raising the height of primary dikes.

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Modelling issues Sobek

[R5] It is recommended to investigate the flow of floodwater through residential area in order to determine the best way to model this flow. Adjustments of land roughness values can be a possibility.

[R6] It is recommended to add some Sobek-functionalities, which improve the modelling of overland flow and the spatial adjustment of the flooded area: - Schematisation of cuts in line elements, such as viaducts. In the current version of Sobek, it is not possible to schematise these cuts correctly. - A user-friendly interface to adjust the digital elevation model (DEM) of the area. Allowing tracking of changes made to the DEM and a visual interface would be great improvements.

Modelling issues HIS-SSM

[R7] It is recommended to study and make agreements on the time after which the number of casualties should be calculated. The current calculation is based on maximum values of water depth, flow velocity and rise rate. These values are influenced by the run time of a simulation. Inhabitants are likely to have escaped the area after a certain period, before the flood front reaches them.

[R8] It would be an enhancement if HIS-SSM can indicate the cause of damage or casualties. At present, multiple runs are necessary to determine whether high flow velocities; large water levels or high rise rates are responsible for the damage and casualties.

Further research

In addition to the proposed research in [R1]-[R8], two recommen- dations are made that do not follow from the research done in this study, but are expected to have a large influence on the calculations of damage and casualties in case of a flooding event.

[R9] It is recommended to investigate the impact of water quality issues in case of a flooding event. These issues could have a large influence on flood damage and the number of casualties.

[R10] It is recommended to make investigations into the drying process of a flooded polder. Large indirect damage is expected if a polder is flooded for a long period. The use of pumping stations and canals that are present in a polder should be taken into account.

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Abbreviations and terms

......

Compartmentalisation Division into smaller units. In the context of this report: The division of flood-threatened land in small units (compartments) to limit the flooded area to one or several of these compartments. Dikes separate the compartments from each other.

DEM Digital Elevation Model. Grid of a land area containing land elevation levels.

Dike-ring area Large area of low land elevation, protected from flooding by primary dikes or higher grounds. One dike-ring area can contain multiple polders. (source: Wet op de Waterkering)

DWW Dutch: Dienst Weg- en Waterbouwkunde. Eng: Road and Hydraulic Engineering Institute. Part of Rijkswaterstaat.

Equilibrium State of balance.

Groene hart (dutch) Area of low population within the Randstad, where nature is preserved as much as possible. The area is bordered by the cities of Rotterdam, The Hague, Leiden, Haarlem, Amsterdam and Utrecht.

HIS-SSM Dutch: Hoogwater Informatie Systeem – Schade en Slachtoffermodule. Eng: High water information system – module damage and casualties.

Computer program to calculate damage and casualties in case of a flood event. Developed by DWW and HKV consultants.

Hydra-B Dutch: Hydra-Benedenrivieren Eng: Hydra for the downstream part of the Dutch rivers.

Statistical computer program, which links the probability of exceedance of water levels in the river to combinations of hydraulic loads that can cause these water levels.

NAP Dutch: Normaal Amsterdams Peil. Eng: Amsterdam Ordnance Datum. (source: NHV, 2002)

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Normative high water level Dutch: Maatgevende hoogwaterstand

Water level belonging to a certain probability of exceedance, used for designing dikes and structures. (source: Hydrologische woordenlijst, NHV 2002) e.g. the normative high water level of +3,4 m NAP at Rotterdam is statistically exceeded once per 10.000 years. NDB-model Dutch: Noordelijk Delta Bekken Eng: North Delta Basin

Hydrodynamic model of the North Delta Basin, the delta of the rivers Rhine and Meuse.

Piping Fail mechanism of a dike. The action of water passing through or under an embankment and carrying some of the finer material with it to the surface at the downstream face. (source: www.ieca.org, March 2006)

Polder Area of low elevation, which is protected by a primary or secondary dike against flooding and in which the water stage can be controlled. (source: NHV, 2002)

Primary dike Most important type of embankment in the Netherlands. A primary dike protects a dike-ring area from flooding and must meet higher safety demands than a secondary dike.

Primary canal Man-made watercourse, which is used to drain water to from lower polders. The surplus of water can be temporarily stored in the primary canal and is finally discharged to river, lake or sea. (source: NHV, 2002)

Randstad (dutch) Delta-metropolis. Area of urbanisation around the groene hart, in the western part of the Netherlands.

RIKZ Dutch: Rijksinstituut voor Kust en Zee. Eng: National Institute for Coastal and Marine Management.

Part of Rijkswaterstaat.

RIVM Dutch: Rijksinstituut voor Volksgezondheid en Milieu. Eng: National Institute for Public Health and the Environment.

Centre of expertise in the fields of health, nutrition and environmental protection. Works mainly for the Dutch government. (source: www.rivm.nl, March 2006)

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RIZA Dutch: Rijksinstituut voor Integraal Zoetwaterbeheer en Afvalwaterbehandeling. Eng: National Institute for Inland Water Management and Waste Water Treatment. Part of Rijkswaterstaat.

Rijkswaterstaat (dutch) Public Works department of the Netherlands.

Secondary dike All dikes that are not primary dikes. e.g. canal embankments, historic dikes or compartmentalisation dikes.

Secondary canal Small collector canals, which are used to collect the surplus of water in a polder and which discharges to the higher elevated primary canals. (source: NHV, 2002)

TAW Dutch: Technische Adviescommissie Water. Eng: Technical Advisory Committee on Water Defences.

VNK Dutch: Veiligheid Nederland in Kaart. Eng: Flood Risk and Safety in the Netherlands (FLORIS).

Project of DWW. (source: www.projectvnk.nl, March 2006)

Water board Administrative unit for water management issues only. In the Netherlands 27 water boards are present. (source: www.uvw.nl, March 2006)

Waterlinie (dutch) Fortifications linked together with strips of inundated land, which used to function as a military defence line in the Netherlands (from 1600 to 1940).

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