Hydraulic analysis of a flood channel from the to the Biesbosch and

Master thesis

[De Koning, 2012]

Author: Jeroen Winkelhorst

Graduation intern HKV LIJN IN WATER/student TU Delft Date: 29 January 2013

January 2013

January 2013 Hydraulic analysis of a flood channel

Final report Master Thesis Delft, 29 January 2013

Written by Jeroen Winkelhorst Student number: 1550365

The research is performed to fulfil the requirements of the Master of Science degree in Civil Engineering as set forth by Delft University of Technology, Faculty of Civil Engineering and Geosciences, Section of Hydraulic Engineering, Chair of River Engineering

Graduation committee Prof. dr. ir. J.K. Vrijling (Chairman committee) TU Delft Prof. dr. ir. Uijttewaal TU Delft

Prof. dr. ir. M. Kok TU Delft and HKV LIJN IN WATER

Ir. M. Duits HKV LIJN IN WATER dr. ir. J.W. Stijnen HKV LIJN IN WATER

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January 2013 Hydraulic analysis of a flood channel

Preface

After months of hard work I am pleased to present this report in which I investigated a flood channel. Unfortunately, the first months were a slow start, because the subject was unclear during the first two months. However, soon after the moment that the subject was defined, namely the hydraulic analysis of a flood channel, I started modelling, which I really enjoyed. Sometimes everything went smoothly, sometimes it was trial and error, but finally, with the help of some specialists at HKV LIJN IN WATER, the model worked perfectly. The next step was schematizing the flood channel, and finally simulations have been performed. When I look back, it is satisfying to see the process from beginning to end. It was a process in which I learned a lot about numerical models, geographical information systems, flood channels, the Room for the River and the Delta Program, etc. I also learned the benefits of working with a structured approach, the importance of a good planning and recognizing the moment that I should stop trying to solve problems by myself, and ask for help. All in all, The thesis was worth the effort, and a satisfying end of my study Hydraulic Engineering.

I would like to thank all the people who helped me making this thesis a success. First, I would like to thank the company where I did the thesis, HKV LIJN IN WATER, for giving me the opportunity to write my thesis there. I would like to thank Matthijs Duits, my mentor at HKV LIJN IN WATER, for all the time he invested in supporting me, and giving me critical feedback on my report. I would also like to thank Jan Stijnen for his involvement and network. Furthermore, I would like to thank all the other HKV LIJN IN WATER specialists that helped me during the process, of which the most important are Andries Paarlberg, Joana Viera da Silva, Ton Botterhuis, Anne Wijbenga and Vincent Vuik. Furthermore, I would like to thank Robert Vos (Delta Program Rijnmond- ), Ralph Schielen (Delta Program Riveren) Robbert de Koning (architect flood channel) and the professors in the committee for their time and wise input: Prof.dr.ir. J.K. Vrijling, Prof.dr.ir. Uijttewaal and Prof.dr.ir. M. Kok.

For my friends and family, especially the last months were a little bit boring, because I had little time for them. I would like to give special thanks to my lovely girlfriend Froukje, who supported me intensively when I needed it, and helped me to write this report in good English. Finally, I would like to thank the Lord God for his strength and wisdom.

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January 2013 Hydraulic analysis of a flood channel

Summary

Due to climate change, it is expected that the normative river discharge of the will increase from 16000 m3/s in 2015 to 18000 m3/s in 2100. In order to make the Dutch water system climate proof, dikes must be reinforced and raised and/or the normative water level must be lowered. In this study, a flood channel as a measure to decrease water levels has been investigated, and has been compared to dike reinforcements.

A flood channel is defined as a new river outside the existing winter bed that is restricted by dikes or higher grounds, and is up- and downstream connected to floodplains of the main river. A flood channel is only used at high river discharges, and therefore, the flood channel is usually dry at low discharges. A flood channel is especially effective when a local obstruction in the main river is bypassed, and it is an alternative for e.g. dike replacements.

In this study, a flood channel that runs from the Afgedamde Maas towards the Biesbosch and Bergse Maas has been analyzed. This flood channel has been designed by architect Robbert de Koning, and is part of a larger plan to make the Dutch lower rivers climate proof until the year 2100. The flood channel bypasses the hydraulic bottleneck Gorinchem, which should increase the effectiveness of the channel. The length of the flood channel is approximately 20 kilometers, and the flow width is 600 meters. The flood channel can roughly be divided in three parts:  Inlet: from to the inlet structure (3 kilometers, land use: flood plain)  Main channel: from inlet structure to outlet structure (9 kilometers, land use: agriculture)  Outlet channels: from outlet structure to the Biesbosch (west channel, length 9 kilometers) and from outlet structure to (south channel, length 8 kilometers; the land use in both channels is nature) The flood channel is designed for a use of once every hundred years, which means that the flood channel starts flowing at a Rhine discharge of approximately 12600 m3/s. The maximal expected water level decrease at the Waal is 1.30 meter.

An alternative for the flood channel are dike reinforcements. From the rivers nearby the flood channel, the dikes along the Beneden and Boven are most difficult to reinforce, followed by the Waal. Reinforcement is difficult due to structures with historical cultural value, protected townscapes and ribbon building. Reinforcements along the , Bergse Maas and Amer would not lead to large problems.

For the calculations of the flood channel, the reference situation has been chosen equal to the reference situation used in the calculations of Delta Program Rijnmond-Drechtsteden (DP-RD), which is the year 2015, when the Room for the River program will be finished. The flood channel has been added to this reference, and simulations have been performed for the situation with and without flood channel. The upstream boundary condition for reference year 2015 is a 16000 m3/s Rhine discharge, and the upstream boundary condition for the year 2100 is a 18000 m3/s Rhine discharge.

The flood channel has been analyzed first by a simple one-dimensional model, based on a stationary situation. The river system has been divided in sections with a geometry and roughness, and is calibrated on a 16000 m3/s Rhine discharge. After determining the roughness values, the flood channel has been added to the model, and the water level and corresponding

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discharge has been calculated for each river section by Bresse’s equation. At a 16000 m3/s Rhine discharge, the discharge through the flood channel is 1906 m3/s, which results in a water level decrease of 97 centimeters at the Waal, and a water level increase of 38 centimeters at the Bergse Maas. The results showed that the flow in the flood channel is obstructed by the outlet channels, because the discharge capacity of this channel is smaller than the discharge capacity of the inlet channel and main channel. Further, the influence of the inlet- and outlet structure on the water levels is small, and the discharge of the flood channel is most sensitive to changes in the outlet channels, according to the sensitivity analysis.

The second step was to model the flood channel in a two-dimensional model. This model is based on the WAQUA Rhine-Maas Estuary model, calibrated for the Hydraulic Boundary conditions 2011 (RMM-HR2011 model). This two-dimensional model has been updated to the reference situation by adding the Room for the River measures Flood plain excavation Avelingen, Ontpoldering Noordwaard and Ontpoldering Overdiepse polder. The Room for the River measure -Zoommeer has not been implemented because the lake lies outside the model boundary. In order to add the flood channel, a new numerical grid has been created, because the flood channel lies outside the existing numerical grid. During the modelling of the flood channel, the reference situation has been conserved where possible (bottom height, land use), and only the parts of the design that contribute to the flood channel’s discharge have been modelled. The inlet- and outlet structure have been modelled as pillars.

At a 18000 m3/s Rhine discharge, the flood channel’s discharge is 2707 m3/s (23% of the Waal’s discharge). As a result, the discharge through the Amer increases up to 32%, while the discharge through the Nieuwe Merwede decreases up to 28%. The discharge through the decreases with approximately the same value as the increase of the discharge through (+541 m3/s). The discharge through the Noordwaard decreases up to 52% due to the flood channel. The water level decrease at the Waal is 1.10 meter (of which 11 centimeters due to Room for the River measure Munnikenland), and the water level increase at the Maas is 0.44 meter. In the table below, the results for the other Rhine discharges are presented:

Discharge at Lobith Maximum water level Maximum water level Discharge flood

3 difference Waal difference Maas channel [m /s] [m] [m] [m3/s] 13000 -0.75 0.38 1511 16000 -0.96 0.44 2157 18000 -1.10 0.44 2707

Furthermore, the results showed that the water flow towards the inlet structure is obstructed by a high quay, that the bulges on the north side of the main channel (part of the design) do not have a hydraulic function and that there is a backwater curve in the outlet channels. Moreover, the discharge capacity of the west outlet channel is too small, because water that flows through the south outlet channel still flows to the Biesbosch, even after entering the flood plains of the Maas.

The water level decrease in the Waal due to the flood channel is lower than the expected value based on the IVB studies (0.96 instead of 1.30 meter at a 16000 m3/s Rhine discharge). According to the uniform Chezy approach, a water level decrease in the Waal of 103 centimeters is possible when the outlet channels have the same width and roughness as the main channel.

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When the results of the two-dimensional simulations are compared to the results of the one- dimensional simulation, it can be concluded that for a measure implemented in a complex water system, as is the case in this study, the strongly simplified 1-dimensional model gives a reasonable estimation of the order of magnitude of the effects.

Based on the water level results and flood channel design, the costs have been calculated. The calculated costs of the flood channel and dike reinforcements are based on the price level of 2003. For the costs calculation of the flood channel, the PRI-method has been used, which was also used in the conceptual phase of the Room for the River program. The calculated costs for the flood channel are 1567.6 million euros, and are expected to be an underestimation. The cost-efficiency, given the water level effect of 28100 m2 at the Waal and Merweden, is 17.9 m2/106 €, which is low for such a large measure. For the costs of the dike reinforcements, a method from the cost-benefits analysis of the Room for the River project has been used. The benefit of the flood channel due to the reduced dike reinforcements has been calculated on 266.3 million euros, which is only 17.0% of the investment. The costs of the dike reinforcements along the Bergse Maas and Amer have not been taken into account, thus the benefits are expected to be smaller. However, practical limits to dike reinforcements and values related to landscape, nature and culture are not taken into account, which is expected to change the cost-benefit balance.

The results showed that the performance of the flood channel is significantly lower than the expectations, and that the lower performance of the flood channel is mainly caused by the discharge capacity of the outlet channels. In order to improve the flood channel’s design, the discharge capacity of the outlet channels must be increased. Furthermore, it is advised to improve the discharge distribution between the outlet channels and to improve the flow in the inlet. The cost effectiveness of the flood channel is low, and the benefits of the flood channel due to reduced dike reinforcements are small compared to the investment. Based on the water level results and costs calculations, the investigated flood channel is not hydraulically optimal, and not a cost effective alternative for dike reinforcements. However, hydraulic optimalization of the channel is possible, and when values of landscape, nature and culture will be taken into account, the cost effectiveness could be more positive.

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January 2013 Hydraulic analysis of a flood channel

Contents

1 Introduction ...... 1 1.1 Development Dutch river system ...... 1 1.2 Study goal ...... 3 1.2.1 Area of interest ...... 3 1.2.2 Design water level increase due to climate change ...... 5 1.2.3 Flood channel as a water level lowering measure ...... 6 1.2.4 Research question ...... 7 1.3 Outline of this report ...... 8

2 Flood channel ...... 9 2.1 Introduction to flood channels ...... 9 2.1.1 Definition flood channel ...... 9 2.1.2 Alternatives flood channel ...... 9 2.1.3 Optimal location ...... 10 2.1.4 Flood channels in Holland ...... 11 2.1.5 Impact ...... 13 2.1.6 Costs ...... 14 2.2 Design parameters ...... 15 2.2.1 Location choice ...... 15 2.2.2 Main channel ...... 15 2.2.3 Inlet and outlet structure ...... 17 2.3 Case study ...... 19 2.3.1 Introduction ...... 19 2.3.2 Flood channel design ...... 19 2.3.3 Water level decrease Waal ...... 22 2.3.4 1/100 years water level ...... 23 2.3.5 From design to schematization ...... 23 2.4 Alternative: dike reinforcements ...... 23 2.4.1 Introduction to dike reinforcements ...... 24 2.4.2 Recent and current dike reinforcements programs ...... 25 2.4.3 Dike reinforcements in the study area ...... 26

3 One-dimensional approach ...... 31 3.1 Introduction ...... 31 3.2 Background ...... 31 3.2.1 River sections ...... 31 3.2.2 Solving approach ...... 32 3.3 Reference situation ...... 33 3.3.1 Schematization river system ...... 33 3.3.2 Geometry ...... 35 3.3.3 Tuning roughness values ...... 35 3.4 Flood channel ...... 39 3.4.1 Schematization flood channel...... 39 3.4.2 Geometry ...... 39 3.4.3 Roughness ...... 40 3.4.4 Inlet and outlets ...... 41

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3.4.5 Summary ...... 42 3.5 Results ...... 42 3.6 Sensitivity analysis ...... 44 3.7 Summary ...... 45

4 Two-dimensional approach ...... 47 4.1 Introduction...... 47 4.2 WAQUA RMM model ...... 47 4.2.1 Background ...... 47 4.2.2 Model boundaries ...... 48 4.2.3 Model schematization ...... 50 4.2.4 Grid and numerical parameters ...... 53 4.2.5 Model changes RMM-ref2015 ...... 54 4.2.6 Model calibration and validation ...... 57 4.3 Schematization flood channel ...... 59 4.3.1 Background ...... 60 4.3.2 Starting points schematization ...... 62 4.3.3 Inlet ...... 62 4.3.4 Main channel ...... 66 4.3.5 Outlet channels...... 68 4.3.6 Schematization summary ...... 70 4.4 Boundary Conditions ...... 71 4.4.1 Background ...... 71 4.4.2 Combination of boundary conditions ...... 73

5 Results ...... 75 5.1 Introduction...... 75 5.2 Discharge distribution ...... 76 5.2.1 Water balance ...... 76 5.2.2 Discharge distribution water system with flood channel ...... 77 5.3 Results river system ...... 80 5.3.1 Waal – – Nieuwe Merwede ...... 81 5.3.2 Maas – Amer ...... 84 5.3.3 Two-dimensional figures ...... 87 5.3.4 Summary ...... 88 5.4 Results flood channel ...... 89 5.4.1 Introduction ...... 89 5.4.2 Water level along axis flood channel ...... 89 5.4.3 Inlet ...... 92 5.4.4 Main channel ...... 96 5.4.5 Outlet ...... 99 5.4.6 Summary ...... 102 5.5 Two-dimensional versus one-dimensional approach ...... 102 5.6 Design review ...... 104 5.6.1 Model results versus expected results ...... 104 5.6.2 Review design parameters ...... 105 5.6.3 Proposed design improvements ...... 106 5.6.4 Summary ...... 108

6 Costs ...... 109

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6.1 Costs flood channel ...... 109 6.1.1 Method ...... 109 6.1.2 Results ...... 111 6.2 Costs dike reinforcements...... 112 6.2.1 Introduction ...... 112 6.2.2 Design water levels Waal – Boven Merwede – Nieuwe Merwede ...... 112 6.2.3 Design water level Bergse Maas and Amer...... 116 6.2.4 Method ...... 117 6.2.5 Results ...... 120 6.3 Cost efficiency ...... 120 6.4 Summary ...... 121

7 Conclusion and recommendations ...... 123 7.1 Summary: research question, approach and results ...... 123 7.2 Conclusions ...... 124 7.3 Recommendations for further research ...... 128 7.4 Epilogue ...... 128

8 References ...... 131

Appendix A: Chapter supplements………………………………………………………. 137 A.1 Chapter introduction; climate change...... 137 A.2 Chapter one-dimensional approach; figure description...... 139 A.3 Chapter one-dimensional approach; sensitivity analysis...... 140 A.4 Chapter two-dimensional approach; barriers...... 147 A.5 Chapter flood channel; dike reinforcements...... 153

Appendix B: Grid design and domain decomposition………………………………157 B.1 Introduction ...... 157 B.2 Grid ...... 159 B.3 Horizontal domain-decomposition ...... 167 B.4 Conclusion ...... 174

Appendix C: Background cost calculation...... 175 C.1 Cost calculation flood channel ...... 175 C.2 Cost calculation dike reinforcements...... 178

Appendix D: History Dutch river system………………………………………………. 187

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1 Introduction

Due to climate change, it is expected that the Rhine discharge will increase, and that the sea level will rise. As a result, the water levels on the Dutch rivers will rise, and action is necessary in order to keep the current safety levels. But what is the best solution against the water level increase, reinforcing the dikes or giving the river more space? In this study, a measure that gives the river more space will be investigated.

This chapter will introduce the subject of the study presented in this report. In paragraph 1.1, the future developments of the Dutch river system are placed in the context of the historical changes in the river system. The goal of this study, which is related to the future developments of the river system, will be presented in 1.2. This chapter will end with the outline of this report (paragraph 1.3).

1.1 Development Dutch river system

During the Golden Age (17th century), the Dutch evolved into a big and rich seafaring trade power, with the largest fleet of the world. Dutch cities became large trading centres due to their place at the rivers or coast, and therefore, the water system was the most important part of the Dutch infrastructure, and sailable rivers and connections to the sea became essential for the Dutch prosperity.

The rivers gave the Dutch prosperity, but also difficulties. Because the rivers were unregulated at that time, the country was frequently flooded, which caused many deaths and large material losses. The floods were in most cases caused by a combination of poor dikes and ice dams. For trade, navigability of the rivers was a big problem, which was frequently not possible due to islands and sandbanks in the river. In the 18th century, engineers started to create plans in order to control the river flow. At that time, each polder has its own water board, which was responsible for its own dikes. Sometimes, dikes on the other side of the river were demolished in order to prevent floods on their own lands. The engineers tried to control the river by improving dikes and create controlled floods in order to prevent uncontrolled floods. Unfortunately, this approach did not prevent the uncontrolled floods, and did also not improve the navigability.

A new approach to prevent the frequent floods was required. Halfway the 19th century, engineers Ferrand and Van der Kun proposed to normalize the rivers, and prevent floods instead of control them. Rivers were normalized by removing obstacles, by fixing the width of the river and by joining parallel streams to one main flow, see Figure 1-1. This new view resulted, after decades of improvements and three normalization rounds, in less floods and sailable rivers. Other essential improvements of the Dutch river system are the digging of the Nieuwe Merwede (1850 – 1885), and separation of the rivers Waal and Maas by the digging of the Bergse Maas (1904).

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Figure 1-1 River before and after normalization [Heezik, 2007]

The last large river floods took place in 1926, especially along the Maas. As a result, dikes along the Rhine branches and Maas were raised and strengthened. In 1953, a large part of South West Holland flooded due to a storm surge from sea. This was a start point for the Delta Plan, which focused on protection against storm surges, but also became a starting point for reinforcement of the river dikes. However, due to public protests the dike reinforcement process delayed, and in the end resulted in a lowered safety norm (normative Rhine discharge of 15000 m3/s). High river water levels in 1993 and 1995 made the necessity of strong dikes clear again, and led to an accelerated completion of the ongoing dike improvements. The high water levels also became the starting point for the Room for the River program. In contrast to the previous century, the program focussed on giving the river flow more space, instead of concentrating the river flow and raising the dikes. In 2015, the Room for the River program will be finished, and the water system will be ready for a 16000 m3/s Rhine discharge at Lobith. A more detailed description of the history of the Dutch river system can be found in appendix D.

In the beginning of the 21st century, a new problem arose. The Intergovernmental Panel on Climate Change (IPCC) report of 2007 shows scientific proof for the changing climate conditions. Consequently, the river discharge is expected to increase, and the sea level is expected to rise. Therefore, the Dutch government started up a committee to investigate the long-term measures that should be taken to make The ‘climate proof’. According to the report that was finished in 2008, the expected sea level rise will be between 0.65 and 1.30 meter until the year 2100, and the normative Rhine river discharge is expected to increase from 16000 m3/s in 2015 to 18000 m3/s in 2100 [Deltacommissie, 2008]. The current river system is not ready for the expected discharge increase, and therefore, the Delta Program is being established in order to investigate the measures needed to make the Dutch river system climate proof.

Just as in the Golden Age, nowadays the Dutch water system is still important for the Dutch prosperity, e.g. for transport from and towards the port of and as a sweet water source. In the 19th century, the bad condition of the rivers beds and dikes were the main problem, which was solved by normalizing the rivers and improving the dikes. In the 21st century, the changing boundary conditions due to climate change will be the main problem. The measures performed in the last century proved that it is possible to prevent floods by controlling the Dutch river system. However, to keep the current Dutch safety level in the future, new dike improvements or measures in the river system are required.

Current water system (2015) A part of the Dutch river system with discharge distribution for a Rhine discharge of 16000 m3/s at Lobith is presented in Figure 1-2. The largest river that enters The Netherlands is the Rhine. At Lobith, the Rhine crosses the Dutch border, and a few kilometers downstream, the Rhine bifurcates in the Waal and Pannerdensch Kanaal, which bifurcates in the IJssel and . The Waal flows without large confluences or bifurcations to the Afgedamde Maas, the old mound of the Maas, where the name of the Waal changes to Boven Merwede. At Werkendam, the Boven Merwede splits in the Beneden Merwede and Nieuwe Merwede. The water from the Beneden Merwede flows to the sea through the , while the water in the

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Nieuwe Merwede flows to Hollands Diep, where it confluences with the Bergse Maas. The Haringvlietsluizen separates Hollands Diep from the sea.

Figure 1-2 Discharge distribution at a normative Lobith discharge of 16000 m3/s

Most river discharge from the Rhine and Maas enters the North Sea through the Nieuwe Waterweg and Hollands Diep. The maximum water levels at the river mounds are determined by the maximum sea level, while the maximum water levels upstream of Werkendam (Boven Merwede and Waal) and Waalwijk (Bergse Maas) are fully determined by the river discharge (river area). In the area in between, the maximum water level is determined by a combination of river discharge and sea level (transition area), see Figure 1-3.

Figure 1-3 Sea dominated area (west), transition area (middle), and river dominated area (east) [Stijnen and Slootjes, 2010]

1.2 Study goal

In this paragraph, the part of the Dutch river system that is subject to this study will be presented first (1.2.1). Subsequently, the expected water level increase due to climate change in this area will be presented in paragraph 1.2.2, and in paragraph 1.2.3, a flood channel will be introduced as possible measure to lower the water levels. Finally, in paragraph 1.2.4, the research question is presented.

1.2.1 Area of interest

In Figure 1-4, a part of the Dutch river system is presented. This study will refer frequently to this part of the river system.

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Figure 1-4 Part Dutch river system with names

Notable locations:  The Biesbosch is a large protected natural environment with islands, sandbars and influence from the tide. The amplitude of the tide is approximately 10 centimeters.  The city of Gorinchem, located along the Boven Merwede, forms a hydraulic bottleneck because the width of the river is limited due to cities on both sides of the river. Another complication is the fact that dike reinforcements are difficult due to ribbon building [Spankracht, 2002].  At Werkendam, the transition from the river-dominated area to the transition area (water level determined by a combination of river discharge and sea level) is located.  The Afgedamde Maas is the old mouth of the Maas. In the Afgedamde Maas, a sluice is located, which separates the water level of the Waal from the Bergse Maas. There is no constant flow through the Afgedamde Maas.

Room for the River measures In the area of interest as presented in Figure 1-4, a few Room for the River projects are being executed. The most important projects are displayed in Figure 1-5. It is expected that the measures will be finished in the year 2015.

Figure 1-5 Room for the River projects in the study area

Along the Bergse Maas, the Overdiepse polder will be ontpolderd in order to lower the local water levels. Along the Boven Merwede, the flood plain excavation and dike replacement Munnikenland will be performed in order to lower the local water level. Further downstream near Gorinchem, flood plain excavation Avelingen will be performed in order to give the river more space. One of the largest Room for River projects is being executed along the Nieuwe Merwede, the ontpoldering of the Noordwaard. This measure is mainly performed to lower the water levels upstream at Gorinchem. The water level decrease due to the ontpoldering of the Noordwaard is 30 centimeters at Gorinchem. An alternative for this measure was a flood channel in order to bypass the bottleneck. This alternative was dropped due to a combination of high costs and spatial planning.

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1.2.2 Design water level increase due to climate change

Due to the climate change, it is possible that the river discharge will increase, and that the sea level will rise. As a result, the design water levels (MHW) will increase.

In the figures below, the MHW of the Waal and the Maas for the years 2015 (Rhine discharge of 16000 m3/s, sea level rise 8 cm compared to the level of 1990) and 2100 (Rhine discharge 18000 m3/s, sea level rise 85 cm compared to the level of 1990) are presented.

Figure 1-6 Design water level change Waal – Nieuwe Merwede. Data based on [Slootjes et al., 2011]

Figure 1-7 Design water level change Maas – Amer. Data based on [Slootjes et al., 2011]

In the figures above, the Room for the River measures mentioned before have already been assimilated. The figures still show a significant water level increase at the Waal and Maas. The design water level (MHW) increase at the Waal varies between 47 (961 km) and 77 centimeters (931 km). The MHW increase at the Maas varies between 41 (232 km) and 68 centimeters (261 km). The water level increase due near Gorinchem (955 km) is 51 centimeters.

To make the water system climate proof, the dikes along the Waal and Maas must be reinforced and raised, the design water level must be lowered due to measures, or a combination of both options must be executed.

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1.2.3 Flood channel as a water level lowering measure

In order to handle the rising discharges through the Waal and Nieuwe Merwede in the next century, measures should be taken. In theory, dike reinforcements are always possible at any location. However, the costs may be high, because many houses and historical buildings are located on or next to dikes. Therefore, also alternative measures are being investigated in order to minimize the amount of dike reinforcements.

The Delta Program is investigating possible solutions. They are investigating solutions on two scales, namely on the level of water system changes and on the level of more local solutions. Water system changes could be new rivers or a changed discharge distribution at a river bifurcation. In December 2012, the temporary results of the Delta Program showed that large water system changes, which could affect the discharge on the Waal, are not expected [Deltanieuws, 2012]. Therefore, the solution for the water level increase must be found in more local measures.

Previous studies, like Integrale Verkenning Benedenrivieren (IVB), Spankracht and Room for the River, already investigated many solutions to decrease the water levels. In Figure 1-8, possible measures according to the Spankracht study are presented.

Figure 1-8 Overview possible measures according to the Spankracht study [Spankracht, 2002]

The figure above shows that possible measures are flood channels (green lines), dike replacements (yellow/read lines) and retention areas (blue areas). Not mentioned in the figure, but also investigated, are flood plain excavations, summer bed lowering and removal of obstructions. The figure shows that flood channels are possible on multiple locations. The advantage of large measures as a flood channel is that one measure could replace multiple small measures.

In the IVB study, a flood channel from the Afgedamde Maas towards the Biesbosch is investigated (see Figure 1-9), and also in the conceptual phase of the Room for the River program, flood channels have been investigated (see Figure 1-10).

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Figure 1-9 Measure in IVB study; flood channel Figure 1-10 Flood channels (D) investigated within from Afgedamde Maas towards the the Room for the River project Biesbosch (blue line) [IVB, 2000]. [Bureau Benedenrivieren, 2004]

The maximum water level decrease at the Waal due to the flood channels is 40 centimeters for a 100 meters wide channel, and 2 meters for a 1600 meters wide channel [IVB, 2000]. However, due to the high costs of 800 to 2000 million euros, the flood channels from Figure 1-10 are not further investigated [Bureau Benedenrivieren, 2004]. The Spankracht and IVB study both indicate a flood channel in the area of interest suitable for a long term solution.

The Delta Program is investigating long term measures that could lower the water levels, and as a result avoid large dike reinforcements. In the studies Integrale Verkenning Benedenrivieren and Spankracht, it was already advised to investigate a flood channel as a long term measure to decrease the design water levels near the hydraulic bottlenecks Gorinchem and Werkendam. Because the Delta Program is investigating long term measures, and a flood channel could be a realistic long term measure, they started researching this option again. In this study, a flood channel from the Afgedamde Maas to the Biesbosch, that bypasses Gorinchem, will be subject to research.

1.2.4 Research question

Architect Robbert de Koning designed a flood channel from the Afgedamde Maas to the Biesbosch, as part of a total solution against the water level increase in the Boven and Nieuwe Merwede [De Koning, 2012]. The hydraulic effectiveness of this flood channel will be investigated in this study. A hydraulically optimal flood channel is defined as a flood channel of which the design (geometry and roughness) and effects on the river system could not be improved significantly.

The main research question is therefore as follows: Is the flood channel designed by Robbert the Koning hydraulically optimal, and a cost effective alternative for dike reinforcements?

To answer the main research question, the following sub questions will be investigated: 1. What is the influence of the flood channel on the water system? 2. Does the flood channel perform according to the expectations? Why or why not? 3. Is there room for improvement in the design of the flood channel? 4. What are the costs of the flood channel compared to dike reinforcements? 5. Does a one-dimensional model of a flood channel give a reliable estimation of the results (compared to a two-dimensional model)? And are those results reliable enough in a conceptual phase?

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1.3 Outline of this report

To answer the research question and the sub questions of paragraph 1.2.4, this report has the following structure: A general introduction to flood channels will be given in chapter 2, where also the case study will be presented in detail. In chapter 3, the flood channel will be schematized in a one- dimensional model, and in chapter 4, the flood channel will be schematized in a two-dimensional model. The results of the two-dimensional model are presented in chapter 5, where they will also be compared to the results of the one-dimensional model. In chapter 6, the costs of the flood channel are estimated, and will be compared to the costs of dike reinforcement. Finally, in chapter 7, the conclusions of the research will be presented.

8 January 2013 Hydraulic analysis of a flood channel

2 Flood channel

In this chapter, a flood channel will be introduced as a measure to lower water levels in a river. The chapter starts with a general introduction in paragraph 2.1. Subsequently, the design parameters of the flood channel will be described in paragraph 2.2, and in paragraph 2.3, a flood channel design is introduced that will be elaborated on in the following chapters. An alternative for the flood channel, dike reinforcements, will be presented in paragraph 2.4.

2.1 Introduction to flood channels

In order to introduce a flood channel, first the definition will be given in paragraph 2.1.1. Secondly, a few alternatives for flood channels will be described briefly in paragraph 2.1.2, and in paragraph 2.1.3, effective locations will be discussed. Subsequently, a few flood channels in The Netherlands will be presented in paragraph 2.1.4, followed by a description of the impact of a flood channel on the environment in paragraph 2.1.5. Finally, some attention will be paid to costs in paragraph 2.1.6.

2.1.1 Definition flood channel

A flood channel, also called green river or bypass, is defined as a new river outside the existing winter bed that is restricted by dikes or higher grounds, and up- and downstream are connected to floodplains of the main river. A flood channel is only used at high discharges, and therefore, at low river discharges the flood channel is usually dry. In that case, the flood channel is also called ‘green river’, because in a dry situation the channel is usually green due to vegetation, and can be used for e.g. nature, agricultural or recreational purposes. Hence, it follows that a flood channel with a small channel or pools that is also wet at low river discharges is called a ‘blue river’ [Klijn et al., 2001].

Figure 2-1 Secondary channel and two types of flood channels: blue river and green river

A measure similar to a flood channel is a secondary channel. In contrast to a flood channel, a secondary channel is located inside the flood plains, and is therefore more frequently in use than a flood channel (more than once a year). In Figure 2-1, a secondary channel, a green and a blue flood channel are schematized.

2.1.2 Alternatives flood channel

Goal of a flood channel is discharging water at high river discharges to lower the water level in the main river. In history, the usual measure against an increased river water level was raising the dikes. Since the start of the Room for the River program the primary solution is being found

9 Hydraulic analysis of a flood channel January 2013

in lowering the water level by giving the river more space, e.g. by constructing a flood channel. Other measures to give the river more space are presented in Figure 2-2.

Dike replacement Flood plain excavation Summer bed lowering

Groin lowering Obstacle removal Temporary water storage Figure 2-2 Room for the river alternatives for a flood channel [Ruimte voor de Rivier, 2012]

The options presented in Figure 2-2 are not always possible, or do not give the desired result. A flood channel is worth considering when the width of the river is (locally) limited, and becomes an obstruction at high discharges. Due to the backwater curve of such a resistance, the water level upstream is also raised. An example of such a resistance is presented in Figure 2-3.

Figure 2-3 Flood channel around bottleneck

In the figure above, the width between the dikes is locally narrow, and is therefore a hydraulic bottleneck. Alternatives for a flood channel like dike replacement or winter bed measures are not always possible, e.g. when:  flood plain width is limited by a city (dike replacement difficult)  there are houses on a dike (dike replacement difficult)  landscape is protected (flood plain measures difficult)

When the problems as described above apply, a flood channel is a realistic option, especially when the flood plains are small. In that case, the houses do not have to be removed and the river landscape does not change radically. Sometimes it is also possible to implement measures downstream of the resistance to lower the water level by the backwater curve of the measure, but then the resistance is not removed.

2.1.3 Optimal location

The hydraulic effectiveness of a flood channel depends for a large part on its location. Flood channels are effective at places where the flood plains are small, e.g. due to cities or other obstructions (Figure 2-3). On these locations, the water level is pushed up. Due to a flood

10 January 2013 Hydraulic analysis of a flood channel

channel, the flow could bypass the local narrowing, resulting in a large water level decrease [Klijn et al., 2001].

Figure 2-4 Schematized water level decrease due to a flood channel at a narrowed part

In Figure 2-4, the water level decrease due to a flood channel at a narrowed river section is schematized. Due to the narrowing, the water level increases. When the narrowed part is bypassed by a flood channel, the water level will decrease more than in a situation without a narrowing, because in addition to the lower discharge through the river section, the water level increase due to the obstruction has disappeared.

Flood channels are also effective when the bottom level slope of the flood channel is larger than the main river bottom level slope, e.g. old river arms, natural lows and bend cutoffs [Klijn et al., 2001]. In areas where the surrounding area is higher than the flood plains, a flood channel is not applicable

2.1.4 Flood channels in Holland

At this moment, there are only a few flood channels in The Netherlands. However, due to the Room for the River program, several new flood channels will be created. A few flood channels that will be created are discussed briefly below.

Arnhem Meinerswijk

Figure 2-5 Flood channel/flood plain lowering Arnhem [RWS, 2012]

In Figure 2-5, the Room for the River project Arnhem Meinerswijk is displayed. A combination of a flood channel and a flood plain lowering will result in a water level decrease of 7 centimeters. Next to the water level decrease, another goal is to change the discharge distribution at Pannerdensche Kop. The width of the channel, between the John Frostbrug and Mandela brug, is 200 meters, and downstream of the Mandela brug the width is much wider. The total length of the channel is approximately 3 kilometers. The land use will be grassland and nature. The sill height of the inlet is 0.5 meter above the ground, and as a result, the channel will be used several times a year [RWS, 2012].

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Veessen-Wapenveld Between the cities of Veessen and Wapenveld, the flood plains are narrow, and to lower the water level at high discharges, a large flood channel will be constructed. A flood channel was preferred above dike replacement because of the high cultural value of the flood plains. The water level decrease at the normative discharge can be 71 centimeters at most. A few kilometers upstream, at the hydraulic bottleneck due to the city Deventer, the water level decrease is still 20 centimeters.

Figure 2-6 Flood channel Veessen-Wapenveld.

The width of the channel varies between 550 and 1500 meters, and the length of the channel is approximately 7 kilometers. The inlet is located at 5.65 meters above NAP and will flow in average once every 100 years. At the normative discharge, 45% of the IJssel’s discharge flows through the channel [Barneveld and Vieira da Silva, 2011].

Bypass Kampen The city of Kampen is located just before the mouth of the IJssel in the IJsselmeer. Near the city, the IJssel is locally narrow due to buildings on both sides of the river. To bypass this hydraulic bottleneck, a new channel will be constructed that connects the IJssel to the Drontermeer. In Figure 2-7, the bypass Kampen is presented.

Figure 2-7 Bypass Kampen

The bypass is designed to discharge 700 m3/s, resulting in a water level decrease in the main river of 60 centimeters at a normative discharge. The length of the bypass is approximately 6 kilometers. The channel in the bypass is always wet, but due to the inlet structure the bypass is only hydraulically effective at high IJssel discharges. The land use will be a combination of nature and recreation. [Project IJsseldelta, 2010].

12 January 2013 Hydraulic analysis of a flood channel

2.1.5 Impact

The construction of a flood channel impacts several fields, of which the most important ones are mentioned below.

Landscape Because flood channels lay outside the flood plains, the impact on the landscape is large. The channel that is embedded by two (new) dikes, crosses the landscape, and therefore changes the view. This changed view, which in most cases is experienced as negative, could be minimized by making the channel not too narrow. In that case, the dikes will be experienced as two separate dikes instead of a small channel. A disadvantage is that people living in the area are less aware of the flood channel.

The land of the main channel could be used for several purposes. This is dependent of the flooding frequency. When the flooding frequency is high, the flood channels could be used as natural area. A negative consequence of this option is the relative large resistance of the corresponding vegetation. Therefore, the cross section of the flood channel should be larger to discharge the same amount of water. When the flooding frequency of the channel is low, the land could be used as grassland, agricultural area or for recreational purposes.

Society The large impact of water safety measures on society became clear due to protests in 1974- 1978. During the dike reinforcements along the Waal, ongoing protests from the society against the changes finally led to a lower safety norm [Heezik, 2007]. In addition, in the design phase of the flood channel from Veessen to Wapenveld, a lot of resistance from the stakeholders has been experienced (see Figure 2-8).

Figure 2-8 Protest against flood channel [http://toinevanbergen.web-log.nl/]

Due to the large impact of a flood channel on the landscape, buildings that have to move and mandatory property selling, the resistance from society against a flood channel is expected to be strong.

Flood risk When a flood channel is constructed, the area in between the flood channel and the flood plain is surrounded by water at high discharges, and forms a small dike ring. When a dike breaks, the consequences for a small dike ring are larger than for a large dike ring, because the water level in the small ring increases more rapidly. A faster water level increase results in a larger probability of fatalities, because there is less time to flee.

Further impact of a flood channel on flood risk:  Lower hydraulic forces on dikes due to lower water levels  Because the dikes of a flood channel could be dry for a long time, the permeability of the clay could increase. This must be monitored.

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 The increased total dike length results in a theoretical increase in probability of failure, due to the increase of dike sections [Roovers and Barneveld, 2006].  Inlet and outlet construction increases the probability of failure  River morphology at high discharges is unknown. Therefore, also the influence of this morphology on the discharge distribution between river and flood channel is unknown  Roughness in the channel should not be higher than the roughness used in the design.

Morphology The influence from the flood channel on the morphology of the main river is dependent on the flooding frequency of the flood channel. Because flood channels are in general used with a low frequency (<1 time a year), flood channels do not have a permanent impact on the river morphology [Gerritsen and Schropp, 2010].

For the flood channel Veessen-Wapenveld, the sedimentation during a discharge wave in the main river is 2 meters at the most, but the impact on the water level is only 6 millimeters [Barneveld et Vieira da Silva, 2011]. Hence, the influence of the morphological change in the main river on the water level is negligible, but should be taken into account for navigation.

In the flood channel, the morphological impact is maximal at the inlet construction due to the high velocities [Spankracht, 2002].

2.1.6 Costs

Flood channels have approximately the same cost-effectiveness as dike replacement, groins lowering and summer bed lowering, and even a better hydraulic effectiveness than removing hydraulic bottlenecks and flood plain lowering [Silva et al., 2001]. In Table 2-1, a general description of costs and benefits of a flood channel is given.

Costs Benefits Design and research Preventing flood damage downstream Moving buildings Preventing measures at other locations Adapting or demolishing buildings and Municipal benefits infrastructure Earthworks Recreational benefits Damage due to seepage / water quality effects Nature benefits Land acquisition Improving environmental quality Value devaluation land Construction dikes and structures

Table 2-1 Costs and benefits flood channels [Spankracht, 2002]

However, flood channels are expensive in an absolute sense, with costs in the range of hundreds of millions to a billion euros. The costs for the Veessen-Wapenveld flood channel, with a length of 8 kilometers, are estimated at 194 million euros, and the costs for Bypass Kampen (including the summer bed lowering) are estimated at 330 million euros. Despite these high costs, the measures at the urban bottlenecks are efficient due to the substantial reduction in water levels that are expected to be obtained by their implementation [Silva et al., 2001].

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2.2 Design parameters

A flood channel is designed to discharge a part of the flow from the main river. In this paragraph, the parameters that determine the design discharge will be described.

2.2.1 Location choice

When the choice has been made to create a flood channel, the location has influence on the effectiveness, as described in paragraph 2.1.3.

If you look for example to Figure 2-6, the flood channel from Veessen to Wapenveld, the location of the channel fixes the length and the bottom level slope of the channel. Furthermore, the borders of the channel are located as much as possible along existing land boundaries, thus the width (variation) is not totally free. In the case of Figure 2-6, the land use in a part of the channel does not change, so also the roughness is not a free design parameter anymore.

Concluding, the location choice has a lot of influence on the total efficiency of the channel because it partly fixes the geometry and roughness.

2.2.2 Main channel

The discharge through the main channel is determined by the length, width, bottom level and roughness of the channel.

In Figure 2-11, a flood channel is schematized. Water flows through the inlet (A) into the main channel (B) and leaves at the outlet (C).

Figure 2-9 Schematization of a flood channel

The discharge through a flood channel is influenced by:  Water level difference between the water level before inlet and after outlet  Channel geometry and roughness  Type of inlet and outlet structure (sill height)

The discharge through the main channel, based on roughness and water level difference is described by the following Chezy equation:

Q bhC R i Where: Q Discharge main channel [m3/s] b Width channel [m] h Water depth [m] C Chezy roughness [m1/2/s] R Hydraulic radius [m] i bottom level slope [-]

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The Chezy equation above describes a relation between the bottom slope and Chezy roughness. For uniform flow, the bottom slope is equal to the water level slope. For flood channels, the bottom level slope is not always equal to the water level slope due to local geometry variations. In that case, it is better to use the water level slope, see Figure 2-10.

Figure 2-10 Schematization of flood channel with varying bottom level slope

Length The length of a flood channel is an important design parameter because it determines the average bottom level slope of the channel. When the length of the channel is shorter than the main river section between inflow and outflow of the channel, the average bottom slope of the channel is steeper than the slope of the main river, and vice versa. A steeper slope has a positive influence on the flood channels discharge, hence its effectiveness.

When the route of the flood channel is much shorter than the route of the main river, a strong flow could cause much erosion, resulting in sedimentation in the main river. Regulating the inlet discharge, thus flow velocity, could prevent this sedimentation.

Width The width and water depth determines the wet cross section of the channel. An optimal width is described by the following guidelines:  The width of the channel should not narrow just after the inlet, because it could lead to a higher water level, thus to a smaller flow into the channel.  It is preferred to oversize the channel width, to ensure the channel’s design discharge when the roughness increases due to other land use [Klijn et al., 2001].  It is also preferred that the width of the channel widens near the outlet, to advance the venture effect [Klijn et al., 2001].

Bottom level The bottom level is dependent of location choice of the channel. The flood channels discharge is a function of the water depth, bottom level slope and roughness, and all those parameters are dependent on the bottom level. Thus, the bottom level is an important characteristic of the flood channel, and a large part of the bottom level is fixed by the location choice.

Roughness The roughness of the channel has influence on the discharge. Especially near the inlet, the roughness has significant influence on the discharge flowing into the channel. When the width, especially near the inlet, is limited, it is smart to prescribe the maximal vegetation height in order to control the maximum roughness. When for example the vegetation changes from grassland to nature, the water level in the river due to a (large) flood channel decreases with approximately 15% [Klijn et al., 2001]. In Figure 2-11, the roughness values of three types of land use are displayed.

16 January 2013 Hydraulic analysis of a flood channel

Productiegrasland (Production Verruigd grasland (rough Droge ruigte grass) C=42 m1/2/s grassland) C=33 m1/2/s C=27 m1/2/s Figure 2-11 Chezy roughness for different land uses at a water depth of 4 meters [Van Velzen, 2003]

For a uniform stationary flow, the discharge is directly proportional to the roughness, so the discharge difference between productiegrasland and droge ruigte is equal to the relative difference in roughness. Due to the large effect of the roughness on the flood channel’s efficiency, it is wise to prescribe the maximum allowed roughness. This is especially important when the flooding frequency is low (less than one time a year).

2.2.3 Inlet and outlet structure

General The water enters the flood channel through the inlet construction, and leaves the channel through the outlet construction. The type of construction determines the flow frequency of the channel, and has influence on the total discharge.

There are several types of inlet types. In general, four types of inlet constructions can be distinguished:  Open (no construction)  Fixed sill  Movable sill  Erodible sill

The ideal type of structure depends on the land use in the flood channel and flooding frequency. Before and after the inlet construction, bottom protection is necessary, because the velocities are usually high, especially when a sill is applied.

Figure 2-12 Water flowing over a fixed inlet of a flood channel [Silva et al., 2001]

At the end of the flood channel, an outlet structure could be used to prevent flooding from downstream direction. This is especially useful when the flood channel is used with a low

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frequency or when the water level downstream the outlet is influenced by the sea. For the outlet structure, the same structure types as the inlet structures could be used.

Types of structures The structures than can be used for the in- or outlet of a flood channel are briefly described in the table below. Which type of structure is the best depends on the land use in the flood channel and flooding frequency.

Fixed sill Description: Crest height is based on a certain water level exceedence frequency. Water flows over the crest. Advantages: - flow frequency determined by crest height Disadvantages: - discharge is limited by crest height Erodible crest Description: Flow frequency is determined by crest height. When the crest overflows, the dike erodes to sill height

Advantages: - flow frequency determined by crest height - discharge not dependent of crest height Disadvantages: - erosion process takes some time - crest has to be restored after use Movable sill or gates Description: Discharge into channel can be precisely determined by moving the sill or gates Advantages: - flow frequency manually controlled - discharge not limited by crest height Disadvantages: - most expensive type of structure Bres Description: Removal of dike is manually initiated. Dike erodes away due to flow Advantages: - flow frequency manually controlled - discharge not limited by crest height Disadvantages: - erosion process takes some time - crest has to be restored after use Table 2-2 Type of inlet structures flood channel [Lam, 2004]

Discharge An open inlet, which is an inlet without construction, has the least hydraulically resistance. With an open inlet, the water level is equal to the water level in the main river. A disadvantage is that the channel is flooded several times a year, and that the moment of flooding is uncontrolled. With the other inlet types, the flooding frequency can be influenced. The sill of those types can be schematized as an imperfect weir [DHV, 2010b], see Figure 2-13:

Figure 2-13 Schematization weir

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An imperfect weir is described by the following formula:

Qweir bm( h2  z sill ) 2 g ( h 1  h 2 )

Where: 3 Qweir weir discharge [m /s] b width weir [m] m weir coefficient (0.9

h1 water level upstream sill [m]

h2 water level downstream sill [m]

zsill sill height [m] 2 g gravity [m/s ]

In the equation above, the driving force is the water level difference over the weir and the sill height is the resistance. The type of the weir, which determines the weir coefficient, has influence on the efficiency.

2.3 Case study

In this paragraph, the flood channel designed by Robbert de Koning will be discussed briefly. The information is based on [De Koning, 2012].

2.3.1 Introduction

In the previous paragraphs, some background information about flood channels has been presented. Delta-Program Rijnmond-Drechtsteden (DP-RD) is investigating long term measures to make the river system climate proof, and one of the concepts is a flood channel. This flood channel design will be schematized and investigated.

Goal of this study is to investigate the influence of this flood channel on the water system, and to analyze the design of the channel proposed by the architect. The results of the study should make clear if a flood channel at this location is hydraulically effective, and the study should propose recommendations for a better flood channel design.

2.3.2 Flood channel design

At request of Delta Program Rijnmond-Drechtsteden, architect Robbert de Koning designed a plan to make the Dutch lower rivers climate proof. A part of that design is a flood channel that runs from the Afgedamde Maas to the Biesbosch, as presented in Figure 2-14. This flood channel is designed for a water level that occurs in average once every hundred year.

At high discharges, the flood channel relieves the Boven- and Nieuwe Merwede, resulting in lower water levels upstream (due to the backwater effect) and downstream of the inlet (less discharge).

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Figure 2-14 Flood channel design Robbert de Koning [De Koning, 2012]

The flood channel can roughly be divided in three parts:  Inlet: Waal to the inlet structure  Main channel: inlet structure to outlet structure  Outlet channels: outlet structure to the Biesbosch and Amer

Inlet design The distance between inlet structure of the flood channel and the Waal is 3 kilometers. The Afgedamde Maas has a sharp angle with the Waal, so it is expected that the water will flow mainly through Munnikenland (Figure 2-15). Therefore, Munnikenland should be adapted to ensure that the water flows also through this area to the inlet structure. In addition to the Room for the River project, which will be finished in 2015, a farm near the inlet structure has to be removed (indicated in Figure 2-14 with a red cross). The land use in Munnikenland will be nature. The bottom level of Munnikenland is approximately 1 meter above NAP.

Figure 2-15 Area between Waal and inlet flood channel [De Koning, 2012]

According to the architects design, the inlet and outlet structure of the flood channel are vertical moving gates between piles, like an old structure in Arnhem (Figure 2-16). When the flood channel is in use, the bottom level of the gates is above the maximum water level. The inlet structure is located under an elevated road, and consists of moving gates. The total width of the inlet structure is 800 meters. In the design, the width of a gate is estimated on 10 to 15 meters.

20 January 2013 Hydraulic analysis of a flood channel

Figure 2-16 Example Inlet/outlet structure with vertical moving gates, Arnhem Meinerswijk [RWS Beeldbank]

Main channel design The main part of the flood channel is located between the inlet and outlet structure. This part has a length of 9 kilometers.

In Figure 2-17 the main channel design is displayed. On the south side of the channel, the channel is limited by a dike with a 1:3 slope. On the north side, the slope of the dike is between 1:3 and 1:6. The dike height is approximately equal the dike height of the Waal. The flood channel is supposed to flood every 100 year.

Figure 2-17 Main channel between inlet and outlet structure [De Koning, 2012]

Currently, there is a business park located in the channel. This park has to be removed, to ensure good flow conditions. Also all other buildings such as houses and farms have to be removed or replaced. Further, there is an old dike south of Almkerk, which must be removed. At the moment, the land is mainly used for agricultural purposes. It is assumed that the land use does not change.

The width after the inlet structure is 750 meters, and before the outlet structure 500 meters. In the other parts of the channel, the width varies between 500 and 1500 meters. The bottom level at the inlet structure is 1 meter above NAP, and the outlet structure is at NAP level.

The water level in the flood channel is approximately equal to the water level in the main river. This means that the dike height from the flood channel is approximately equal to the dike height of the main river.

Outlet design The outlet structure is similar to the inlet structure and is located under the A27 highway. This structure also has vertical moving gates. The structure has a total width of 500 meters. The

21 Hydraulic analysis of a flood channel January 2013

gates have a width of approximately 15 meters each. The structure is located under the elevated highway A27.

Figure 2-18 Outlet channels [De Koning, 2012]

From the outlet structure, water flows away through two channels (west and south channel) to the Biesbosch and Amer, see Figure 2-18. Both channels are located near small streams (West kil and Bleeke kil). The dikes of the small streams are set back to make space for a flood plain. The distance between the dikes is approximately 350 meters for the south channel and 250 meters for the west channel. The area between the dikes is a combination of dry and wet nature. The west channel, which flows through the West Kil, first cuts through a polder near Nieuwendijk, and then follows the creek. The length of the west channel is 7.5 kilometers, and the length of the south channel is 7 kilometer.

2.3.3 Water level decrease Waal

The design water level decrease due to the flood channel is not calculated for this specific design. The estimated effect is based on calculations for Integrale Verkenning Benedenrivieren studies [IVB, 2000].

In Figure 2-19, the flood channel is indicated with the blue line. The geometry is somewhat different from the design of Robbert de Koning, but the geometry is roughly the same. The inlet is north of the inlet of Robbert de Koning’s channel, and the channel has one outlet instead of two. The IVB channel follows existing creeks and the lower areas.

Figure 2-19 Flood channel from Afgedamde Maas to the Biesbosch [IVB, 2000]

In the figure below, the water level change at certain locations due to the flood channel with several widths is presented.

22 January 2013 Hydraulic analysis of a flood channel

Figure 2-20 Design water level decrease due to flood channel from Afgedamde Maas to the Biesbosch (measure 21) [IVB, 2000]

The width of the flood channel designed by Robbert de Koning is in average 600 meters. Thus, a water level decrease of 1.30 meters at Gorinchem is expected for a 16000 m3/s discharge at Lobith, based on the information in Figure 2-20. At the Maas, a discharge increase of 0.54 centimeters is expected. The corresponding discharge through the channel is unfortunately unknown.

2.3.4 1/100 years water level

The flood channel is designed for an average use of once every hundred years. In Figure 2-21, the water level return period for a river location near the inlet structure is presented for the current climate conditions.

Figure 2-21 Return period water level Afgedamde Maas kilometer 244 [Hydra zoet versie 1.3.1]

The Rhine discharge that occurs in average once every hundred years is approximately 12600 m3/s and the corresponding water level near the inlet structure is 5.27 meters above NAP. With the changing climate conditions, the water level that occurs once every hundred years is expected to rise, and therefore the corresponding discharge is also expected to rise.

2.3.5 From design to schematization

In the previous paragraphs, the case study has been presented. In order to do calculations, the design has to be schematized. In the next chapters, the flood channel will be schematized for a one-dimensional model, and for a two dimensional model. The results of those schematizations will finally lead to answers about the hydraulic effectiveness and influence on the water system.

2.4 Alternative: dike reinforcements

In the previous paragraph, the case study of a flood channel has been presented, which is a measure to decrease the amount of dike reinforcements. Dike reinforcements, an alternative for the flood channel, will be qualitatively described in this paragraph. First, a short introduction to

23 Hydraulic analysis of a flood channel January 2013

dike reinforcements will be presented in paragraph 2.4.1. Subsequently, some information about recent dike reinforcements will be presented (2.4.2), and finally, the reinforcement possibilities of the dikes nearby the flood channel will be discussed in more detail (2.4.3).

2.4.1 Introduction to dike reinforcements

In Holland, the maximal probability of failure of a dike is fixed by law. In the case that the calculated probability of failure is larger than the maximal accepted probability of failure, the dike must be reinforced. In general, a too large probability of failure could be caused by a too low crest height or by a stability problem. Some limit states of dikes are presented in Figure 2-22.

Figure 2-22 Dike limit states [Weijers and Tonneijck, 2009],

A dike can be reinforced by increasing the crest height (against e.g. overflow and overtopping), by adding berms (against e.g. sliding and uplifting), by decreasing the slope or by adding special structures (against e.g. piping).

When a dike is reinforced with soil and clay, only increasing the crest height is often not enough. The dike must be widened due to a wider crest, a gentler slope or a wider berm. It is preferred to extend the dike body on the inner side (land side), because extending on the outer side (river side) decreases the flow area of the river, which is undesirable. However, due to e.g. buildings, reinforcement is not always possible on the inner side of the dike, see Figure 2-23.

Figure 2-23 Dike reinforcement on the outer side Figure 2-24 Dike reinforcements by special [Beeldbank RWS] constructions [Dijkversterkingbas, 2013]

24 January 2013 Hydraulic analysis of a flood channel

When reinforcements on the inner- and outer slope of a dike are not possible, e.g. due to valuable buildings, special constructions could be used to strengthen the dike. Possible solutions are sheet piles (damwand), a berm (steunberm) or a diaphragm wall (Diepwand), as presented in Figure 2-24. The space needed for a special structure is less than needed for a traditional reinforcement. However, disadvantage is the relatively high costs.

Landscape, Nature and Culture When reinforcing a dike, construction costs are important, but also the value of landscape, nature and culture (LNC values) should be taken into account. The impact of dike reinforcements could be large when an old city center lays adjacent to the river (Figure 2-25) or when there are buildings on or next to the dike (Figure 2-26). In addition, reinforcements at locations with protected landscapes and cultural historical buildings are also undesirable. Smart solutions are required to minimize the impact.

Figure 2-25 High water Deventer [Beeldbank RWS] Figure 2-26 Dike reinforcement [Beeldbank RWS]

Traditional dike reinforcements are often not possible when an area has high LNC values. In that case, a solution could be found in (expensive) structures to minimize the impact, but sometimes that is also undesirable, e.g. when an old fortress wall with high cultural value must be reinforced. In that case, a solution could be found in lowering the design water level by measures downstream. However, in an area with high LNC values, dike reinforcements are difficult and often undesirable, but sometimes they cannot be avoided.

2.4.2 Recent and current dike reinforcements programs

Many dike sections along the Waal and Merweden were reinforced during the eighties and nineties, with, in most cases, a dike lifetime of 50 years, and a structural lifetime of 100 years. The sections reinforced before 1992 are designed for a design Rhine discharge of 16000 m3/s, while the sections reinforced after 1992 are designed for a Rhine discharge of 15000 m3/s [Arcadis, 2003]. In 2015, after completion of the Room for the River program, the river system will be adapted to a 16000 m3/s Rhine discharge, for a large part due to measures that lowered the design water level [Room for the River, 2006].

In 2007, the 2e hoogwaterbeschermingsprogramma (HWBP2) started. In this program, the dike sections that did not satisfy the Dutch safety norms in the second examination round (2006) are reinforced. In the area nearby the flood channel (which lies in dike ring 24), the most important dike reinforcement is the Merwededijk, near Werkendam. It is expected that the HWBP3, the program with reinforcements based on the results of the third examination round (2011), will start in 2014. The results of the third examination round are presented in Figure 2-27 and Figure 2-28.

25 Hydraulic analysis of a flood channel January 2013

Figure 2-27 Results dikes third examination round [RWS ,2011a]

Figure 2-28 Results structures third examination round [RWS, 2011b]

The figures above show that some dikes sections along the Waal and Boven Merwede, especially on the north side, are disapproved. However, most structures suffice according to the norms. The reason that some dikes do not meet the safety norms could be:  Crest height of the dike is too low  Dike is not strong enough (stability)

The first plans to reinforce disapproved dike sections and structures will be created in 2013 [helpdeskwater]. From the year 2014, HWBP3 is part of the Delta Program.

2.4.3 Dike reinforcements in the study area

Due to the expected climate change, as described in paragraph 1.2.2, the Rhine discharge increases from 16000 m3/s in the year 2015 to 18000 m3/s in the year 2100. To keep the current safety standards, the dikes should be reinforced or the design water level should be lowered by measures. In this paragraph, the consequences of dike reinforcements in the study area will be presented.

The analysis of the dikes is based on a study performed as part of the Room for the River program. In that study, the consequences of dikes reinforcements are described for many dike sections [Arcadis, 2003]. Based on that study, the consequences of dikes reinforcements in the study area will be discussed. Background information about the dikes relevant for this study can be found in appendix A.5.

Figure 2-29 Dike rings with corresponding numbers [helpdeskwater, 2013]

26 January 2013 Hydraulic analysis of a flood channel

The flood channel, as described in paragraph 2.3, lies in dike ring 24 (see Figure 6-7). The dikes along the Waal (downstream of Zaltbommel), Boven Merwede, Nieuwe Merwede, Bergse Maas and Amer will be discussed.

Waal; Zaltbommel to Afgedamde Maas On the north side of the Waal (dike ring 43), the last dike reinforcements were performed between 1980 and 2001. On most places, the reinforcements took place on the inner side of the dike, but at a few locations on the outer side. Near Opijnen-Neerijnen, a protected townscape, and Fort Vuren (Figure 2-30), special constructions were used. When the water level rises, the main problems of the dikes will be the height and piping. There are some houses with a high cultural value, and historical villages on or next to the dikes, that could make new reinforcements complex.

Figure 2-30 Fort Vuren and Slot Loevestein along the Waal [Google maps]

On the south side of the Waal (dike ring 38), between Zaltbommel and Brakel, dike reinforcements on the inner side of the dike are possible on most locations. Only near the cities Zaltbommel (protected townscape) and Zuilinchem, some (light) constructions are required. During dike reinforcements in the 1980s, a lot of houses on the river side of the dike were removed (160 houses at Brakel), and therefore, there is still space for new dike reinforcements on most locations.

Boven Merwede On the north side of the Boven Merwede (dike ring 16), there is ribbon building along the dike, especially between Gorinchem and Hardinxveld (see figure Figure 2-31a). Furthermore, the city center of Gorinchem is protected against the water by old fortress banks, which have a high cultural value. A part of the dikes can be reinforced on the inner side of the dike; however, at the locations with ribbon building strengthening must take place on the outer side of the dike. Furthermore, special structures are required at a few locations. Dike reinforcements along the north side of the Boven Merwede will have a severe impact on the landscape and society, and large measures are required to make the dikes ready for an 18000 m3/s Rhine discharge [Arcadis, 2003].

27 Hydraulic analysis of a flood channel January 2013

a. Historical walls b. Ribbon building c. Walls d. Loevestein, Nieuwe Gorinchem Boven-Hardinxveld Hollandse Waterlinie [Google maps] [Google maps] [fotovlieger.nl] Figure 2-31 Impression Dikes Boven Merwede

The fortress city Woudrichem is located on the south side of the Boven Merwede. The city has historical walls, including two coupures, which have a high cultural value, and therefore, adaptation is undesirable. Between Woudrichem and Sleeuwijk, there are some buildings on the inner side of the dike, but between Sleeuwijk and Werkendam, there is intensive ribbon building. Therefore, reinforcements must be performed partly on the outer side of the dike, and locally, special constructions will be needed. The area on the north and south side of the Boven Merwede are both part of the Belvedere area; there are several parts of the Nieuwe Hollandse Waterlinie and tracks from the Middle ages and the Roman empire.

Beneden Merwede On the north side of the Beneden Merwede (dike ring 16), the cities Hardinxveld-Giessendam, and are located. The dike on the north side consists of adjacent buildings, including the historical protected townscapes of Sliedrecht and Papendrecht. A large part of the dike consists of special structures, and therefore, dike reinforcements are difficult and expensive.

Figure 2-32 Beneden Merwede [Google maps]

On the south side of the Beneden Merwede, there is a lot of nature and one city, , with the old town adjacent to the water. To reinforce this area, a lot of structures are required, and the impact on the townscape of this city with a high cultural historical value is severe.

Nieuwe Merwede On the east side of the Nieuwe Merwede, the Noordwaard (dike ring 23) and natural area the Biesbosch are located. The Noordwaard is part of a large Room for the River project, which is currently being executed. There is enough space for dike reinforcements, if necessary. On the west side of the Nieuwe Merwede, the island of Dordrecht is located. A part of the dike is currently being reinforced. There is enough space for further dike reinforcements.

Along Steurgat, the creek south of Werkendam, some special structures are required due to ribbon building near Werkendam. Along the southern part of Steurgat, dike reinforcements are easier, because there is enough space.

28 January 2013 Hydraulic analysis of a flood channel

Bergse Maas and Amer Dike reinforcement along the Bergse Maas and Amer is relatively easy because there is a lot of space near the dikes, and there are not many buildings. On most locations, the dike could be reinforced on the inner side. The main disadvantage could be view for some houses. However, near Raamsdonkerveer and , some special constructions would be necessary. It is expected that the landscape, cultural historical or archaeological values will not be affected severely.

Summary From the rivers discussed above, the dikes along the Beneden and Boven Merwede are most difficult to reinforce, followed by those along the Waal. Reinforcement is difficult due to structures with historical cultural value, protected townscapes and ribbon building. Reinforcements along the Nieuwe Merwede, Bergse Maas and Amer will not lead to large problems.

29

January 2013 Hydraulic analysis of a flood channel

3 One-dimensional approach

3.1 Introduction

The flood channel designed by Robbert de Koning is presented in paragraph 2.3. In this chapter, a simple one-dimensional model of this channel will be created. Goal is to estimate the water level effect on the Waal and Maas due to the flood channel, and the corresponding discharge through the channel.

The one-dimensional model will be created in order to have a reference for the results of the two-dimensional model, which will be created later. The model will be one-dimensional and stationary, in order to keep the model simple. Therefore, it is possible to solve the system manually. However, because the flood channel connects the Waal with the Bergse Maas and the Biesbosch, the system can only be solved by iteration, which is time consuming. Therefore, the river system will be modelled by a set of scripts in MATLAB, a numerical computing environment. To model the river system, it will be divided in several sections; each section is limited by a bifurcation or confluence. If for each section a geometry and roughness have been defined, and the boundary conditions are known, the water levels and discharges in the sections can be calculated.

The reference situation will equal to the reference situation that is used in the calculations of Delta Program Rijnmond-Drechtsteden (DP-RD), which is the year 2015, when the Room for the River program will be finished [Slootjes and De Waal, 2010]. Due to SOBEK calculations made for DP-RD, the water levels in the reference situation are known for several Rhine discharges, which will be used to calibrate and verify the model.

In paragraph 3.2, some background about the calculation method will be given. Subsequently, the schematization of the reference situation (3.3) and the schematization of the flood channel (3.4) will be presented. Those schematizations in combination with the boundary conditions give the results (3.5). The results of the sensitivity analysis are discussed in paragraph 3.6, and finally, a summary of all the results will be presented (3.7).

3.2 Background

In this paragraph, the theory behind the simplified model will be explained. 3.2.1 River sections

The river system and flood channel will be divided in several river sections, as already mentioned in the introduction. The river sections exchange water level and discharge information to each other. For each section, the water level will be calculated as described below.

Each river section will be modelled as a rectangular shaped cross section with an average slope. In Figure 3-1, the cross section (left) and longitudinal section (right) are presented. The parameters water level (h) and bottom level (bl) are absolute values above a reference level, in this case NAP. Parameters at the beginning of a section have subscript 1 and parameters at the end of a section have subscript 2.

31 Hydraulic analysis of a flood channel January 2013

Figure 3-1 Schematised river cross section and longitudinal section

For each river section, a backwater level calculation will be performed by Bresses equation (derivation can be found in [Vriend et al., 2007]). This equation is a simplification of the mass and momentum equation for a canal with a rectangular cross section. The equation is valid for a stationary situation with a small Froude number:

L 1 L1/ 2 d12 dee () d  d  2 where:

q2 3 de  2 Cib 4 3 0.24de d2 L1/2   idbe

In this formula:

d1 [m] upstream water depth river section

d2 [m] downstream water depth river section

de [m] equilibrium depth L [m] length river section

L1/2 [m] half-length river section q [m2/s] discharge C [m1/2/s]Chezy smoothness coefficient

ib [-] bed level slope

The formulation above calculates the upstream water level of a river section, based on the geometry, roughness, section discharge and downstream water level. In the model, the upstream water level of each section will be determined by this formulation.

3.2.2 Solving approach

The river system will be modelled by using the mathematical software MATLAB. Even through the basics of the model are simple, due to the several bifurcations, it is complicated to solve the system.

When a river splits into two branches, the discharge distribution between the two branches must be determined. At a bifurcation point, the water levels of both sections are equal, and the total discharge is known. Therefore, the water level equations can be written as a function of

32 January 2013 Hydraulic analysis of a flood channel

the discharge of one of the sections, and as a result, the discharge distribution between the branches can be calculated.

In the MATLAB script, each section is modelled by the formulation that has been described in paragraph 3.2.1. The adjacent river sections exchange water level and discharge information, and at the bifurcation points, the discharge distribution will be determined based on the method described above. When the boundary conditions and section parameters are known, the system can be solved.

In MATLAB, the system has been split in two scripts. The first script (REF-script) describes the river sections of the whole system, except the sections of the flood channel. Those sections are described in a second script (FC-script). The interaction between the two scripts is as follows: 1. REF-script: Given the upstream and downstream boundary conditions, the water levels of the reference situation (river system without flood channel) will be solved, assuming a discharge through the new flood channel. 2. FC-script: Based on the calculated water levels at the inlet and outlets of the channel, the script calculates the corresponding flood channel discharge by iteration. The script also determines the discharge distribution between the outlet channels. In most cases, the solution is constant after three iterations. 3. REF-script: Based on the new water levels at the inlet and outlets of the flood channel, the calculated flood channel discharge, and the up- and downstream boundary conditions, the water levels of the reference are solved again. This results in new water levels at the inlet and outlets of the flood channel. 4. FC-script: Based on the new calculated water levels at the inlet and outlets of the channel, the script calculates the new discharge through the flood channel. 5. Etc.

After six iterations, and in the flood channel subroutine 3 iterations, the solution of the system is constant. The model’s outputs are water levels and discharges for all the river sections.

3.3 Reference situation

In this paragraph, the model of the river system will be presented, to which the flood channel will be added later. The schematization is presented in paragraph 3.3.1, and subsequently, the geometry (3.3.2) will be determined. Furthermore, the roughness of the model will be tuned (3.3.3), and finally, the tuned model will be validated.

3.3.1 Schematization river system

As already mentioned in the introduction, the river system in the one-dimensional model will be schematized by river sections. Most of the sections start and end at a river confluence or bifurcation, and each river section has a geometry and roughness.

Only the part of Dutch river system that is relevant for the flood channel will be schematized. The upstream boundaries of the model are the Waal and Maas, and the downstream boundaries of the model are the Beneden Merwede and Hollands Diep. The reference situation will be the year 2015, after completion of the Room for the River project. The most important Room for the River measure with respect to the flood channel is the ontpoldering of the Noordwaard, a new

33 Hydraulic analysis of a flood channel January 2013

flood channel that discharges water from the Nieuwe Merwede to the Biesbosch at high river discharges.

Description schematized river system In Figure 3-2, a schematization of the river system, including the Noordwaard is presented. The flood channel (red lines) is presented in the figure, but is not part of the reference situation and will be described later, in paragraph 3.4.

Figure 3-2 Schematization river system with flood channel

The water system in the reference situation is schematized as follows: The Waal is an upstream model boundary where a discharge will be imposed. At river kilometer 950, the name of the Waal changes into Boven Merwede, and at river kilometer 961, the Boven Merwede splits into the Nieuwe Merwede and Beneden Merwede. The Beneden Merwede is a downstream model boundary, where a discharge will be imposed. Four kilometers downstream along the Nieuwe Merwede, the inlet of the Noordwaard is located, which is a flood channel that only functions at high river discharges. The remaining discharge in the Nieuwe Merwede flows towards Hollands Diep, where it confluences with the Amer and Gat van Kampen, which includes the discharge from the Noordwaard. Hollands Diep is the downstream water level boundary of the model.

The Bergse Maas is an upstream boundary where a discharge will be imposed. The name of the Maas changes into Amer at river kilometer 250, and flows towards Hollands Diep.

Noordwaard After completion of the Room for the River project, the Noordwaard is part of the river system. At high river discharges, the Noordwaard discharges water from the Nieuwe Merwede towards the Biesbosch.

Because the Noordwaard functions only at high river discharges, modelling of this section is more complex than the other river sections. Instead of modelling the branch with a river section, the Noordwaard has been modelled by a discharge-water level relation (Q-h relation). Based on SOBEK water level calculations performed for Delta Program Rijnmond-Drechtsteden [Slootjes et al., 2011], the discharge of the Noordwaard has been related to the water level in the Nieuwe Merwede. This relation has been linearly interpolated (see Figure 3-3).

34 January 2013 Hydraulic analysis of a flood channel

Figure 3-3 Q-h relationship Noordwaard

Due to the Q-h relationship, the discharge decrease in the Nieuwe Merwede due to the Noordwaard can be estimated. The discharge that is extracted from the Nieuwe Merwede, is added to the section Gat van Kampen, where as a result the water level increases.

3.3.2 Geometry

The schematized model, as described in the previous paragraph, has been implemented in MATLAB, as described in paragraph 3.2. For each river section, the width, length and bottom level slope must be defined.

For the width of each section, the average flow width at high discharges has been determined, based on aerial photos. The bed level slope for each section has been based on the Actueel Hoogtebestand Nederland (AHN). The length of the sections is based on GIS information. In Table 3-1, the input parameters for the river sections (see Figure 3-2) are presented.

Section L B I [km] [m] [-] Waal 38 320 1.0*10-4 Boven Merwede 11 400 1.2*10-4 Nieuwe Merwede (1) 4 350 0.4*10-4

Nieuwe Merwede (2) 17 550 0.5*10-4 Hollands Diep 39 1550 0.3*10-4 Noordwaard 3 2000 0.7*10-4 Gat van Kampen 10 350 2.3*10-4 Maas 38 225 0.4*10-4 Amer 13 400 0.5*10-4

Table 3-1 Geometry river sections

3.3.3 Tuning roughness values

In this paragraph, the roughness of the schematized model will be tuned based on a reference model. For each river section, a roughness value will be determined. After tuning, the model will be validated. However, the boundary conditions must be specified first.

Boundary conditions In order to tune the roughness values of the model, the boundary conditions must be defined. The normative Rhine discharge in 2100 is 18000 m3/s, and the highest discharges available for tuning are the 13000 m3/s, 16000 m3/s and 18000 m3/s Rhine discharges. In order validate the model for a lower and upper boundary, the model will be tuned on the 16000 m3/s Rhine

35 Hydraulic analysis of a flood channel January 2013

discharge instead of 18000 m3/s Rhine discharge. The 16000 m3/s discharge will also be used for the analysis of the flood channel. In all cases, the sea level is equal to the average of a normal astronomical tide. The boundary conditions used for tuning of the roughness and validation are presented in Table 3-2.

Rhine [m3/s] 13000 16000 18000 Waal [m3/s] 8285 10165 11759 Maas [m3/s] 2800 3504 3974 Beneden Merwede [m3/s] 3118 3852 4441 Hollands Diep [m+NAP] 1.19 1.30 1.35

Table 3-2 Tuning of the roughness and verification boundary conditions, according to [Silva and Van der Linden, 2007] and [Slootjes et al., 2011].

Tuning roughness values With the geometry and boundary conditions known, the river system (without the new flood channel) has been tuned by adapting the roughness of the river sections. The model has been tuned on SOBEK water levels that describe the reference situation 2015 (model results of [Slootjes et al., 2011]). In Figure 3-4 and Figure 3-5, the water levels after tuning the roughness are displayed, and more details, including the roughness values, are given in Table 3-3.

Figure 3-4 Water level after tuning the roughness values by a 16000 m3/s Rhine discharge (Waal – Merwede – Hollands Diep)

36 January 2013 Hydraulic analysis of a flood channel

Figure 3-5 Water level after tuning the roughness values (Maas – Amer)

The water level of the model is indicated with a dark blue line, the water level of the reference with a light blue line. The water level difference is indicated by the green area.

Each river section should represent the water levels well, but especially at the places where the model will be connected to the flood channel, a good fit is important. Therefore, special attention has been paid to Waal kilometer 950, and to the flood channel’s outlets at the Bergse Maas (250 km) and the Biesbosch (Gat van Kampen). The roughness of each section has been optimized, based on the Root Mean Square Error (RMSE) and visual inspection. In addition to the figures above, more details about the water levels after tuning the roughness is presented in Table 3-3.

Q = 16000 m3/s Start End C Discharge [m3/s] Water level fit [m]

[km] [km] [m1/2/s] Reference Model RMSE [m]

Sections Waal 912 950 54 10165 10165 0.034

Boven Merwede 950 961 46 10165 0.070

Nieuwe Merwede (1) 961 964 43 6313 0.046

Noordwaard 964 1820 1785

Nieuwe Merwede (2) 964 981 44 3982-4840 4527 0.062

Hollands Diep 981 990 50 9817 0.038

Maas 212 250 50 3504 3504 0.100

Amer 250 263 55 3504 0.087

Water level [m+NAP]

Water level diff. Reference Model [m]

Stations Gat van Kampen 58 2.56 2.56 0.00

Maas 250 km 250 2.63 2.58 -0.05

Waal 950 km 950 6.08 6.12 +0.04

Table 3-3 Results after tuning the roughness

The results for the Waal, Boven- and Nieuwe Merwede are satisfying, with a RMSE between 3.4 and 7 centimeters, and a Noordwaard discharge deviation of -1.9%. The Maas and Amer have somewhat larger RMSE values of 10 and 8.7 centimeters. Due to the large length of the sections, the local geometry and roughness are not represented well. Given the model

37 Hydraulic analysis of a flood channel January 2013

limitations, a better fit between the water levels of the model and the water levels of the RMM- model was not possible. The water level stations are accurate up to 5 centimeters.

Verification After tuning the roughness, this value has been verified. The model has been verified with results from the same model as used for tuning, for a lower (13000 m3/s) and higher Rhine discharge (18000 m3/s). The model is not validated for storm surges, because the flood channel is only effective at high discharges. However, during storm surges the flood channel could be used as storage, but that aspect is not taken into account in this stage. In the table below, the results of the validation are presented:

Q=16000 m3/s Q=13000 m3/s Q=18000 m3/s Q [m3/s] RMSE[m] Q [m3/s] RMSE[m] Q [m3/s] RMSE[m] Section

Waal 0.034 0.095 0.150 Boven Merwede 0.070 0.048 0.066 Nieuwe Merwede (1) 0.046 0.031 0.047 Noordwaard 1785 1101 2336 (compared to reference) (-1.9%) (-7.5%) (+2.5%) Nieuwe Merwede (2) 0.062 0.057 0.059 Hollands Diep 0.038 0.045 0.050 Maas 0.100 0.100 0.083 Amer 0.087 0.060 0.130

Water level [m] Station Reference Model Reference Model Reference Model [m] [m] [m] [m] [m] [m] Gat van Kampen 2.56 2.56 2.04 1.89 2.87 3.06 Maas 250 km 2.63 2.58 2.14 2.07 2.93 2.92 Waal 950 km 6.08 6.12 5.26 5.22 6.72 6.82

Table 3-4 Verification results

The verifications results show that the accuracy of water level representation of the Boven Merwede, Nieuwe Merwede (1) and Nieuwe Merwede (2) and Hollands Diep are satisfying, with a similar accuracy as with a 16000 m3/s Rhine discharge. The Waal has a larger deviation up to 15 centimeters. The Noordwaard discharge is at a 13000 m3/s Rhine discharge 7.5 percent too low, while the discharge at an 18000 m3/s Rhine discharge is 2.5 percent too high. This discharge deviation is in line with the water level deviations at Gat van Kampen. The validation of the Maas and Amer show a similar accuracy as the results with a 16000 m3/s Rhine discharge.

From the verification, it can be concluded that the model functions reasonably. The main purpose of the model is a qualitative analysis of the flood channel design. Over the river sections, most of the RMSE values are smaller than 10 centimeters. Locally, higher deviations could occur, but they are in many cases caused by local aspects that are not implemented in the model. The Noordwaard functions also reasonably, only the water level at station Gat van Kampen deviated up to 19 centimeters.

38 January 2013 Hydraulic analysis of a flood channel

3.4 Flood channel

In this paragraph, the schematization of the flood channel will be presented. The sections will be described in paragraph 3.4.1. For these sections, the geometry (3.4.2) and roughness (3.4.3) will be determined. The inlet- and outlet structures will be described in paragraph 3.4.4, and finally a short summary of this paragraph will be presented (3.4.5)

3.4.1 Schematization flood channel

The flood channel design by Robbert de Koning has been introduced in paragraph 2.3. In this paragraph, the schematization for the one-dimensional model will be presented.

In Figure 3-2, the flood channel is indicated by the red river sections, and in Figure 3-6, the schematization of the flood channel is presented in more detail. The channel has been divided in three parts, inlet channel (1), main flood channel (2-6) and outlet channels; the west outlet channel connects the outlet structure with the Biesbosch, and the south outlet channel connects the outlet structure with the Bergse Maas. Between the inlet channel and the main channel, the inlet structure is located. Between the main channel and outlet channels, the outlet structure is located.

Figure 3-6 Sections flood channel, background image by [De Koning, 2012]

The sections are chosen so that they represent the variation in (flow) width, length and depth accurately.

3.4.2 Geometry

The flood channel has been divided in several sections, as presented Figure 3-6. For each section, the bottom level slope, width and length must be determined.

The bottom level slopes are determined by AHN data. Just before and after the inlet structure, the bottom level is higher than the surroundings. In line with the design, the bottom level around the inlet structure is lowered to 1 meter above NAP, which is equal to the bottom level at the flood plain on the east side of the Afgedamde Maas. For the main channel (sections 2-6), the average bottom level slope between the inlet and outlet structure has been used, instead of a separate slope for each river section.

The length and flow width of the sections are based on the design and aerial photos. In Table 3-5, the length, width and bottom level slope of each river section is presented.

39 Hydraulic analysis of a flood channel January 2013

Section L B I [km] [m] [-] Section 1 3 1200 2.6*10-4 Section 2 2 750 1.1*10-4 Section 3 2 700 1.1*10-4

Section 4 2 725 1.1*10-4 Section 5 2 675 1.1*10-4 Section 6 1 600 1.1*10-4 Section west 10 250 1.0*10-4 Section south 7 350 1.4*10-4

Table 3-5 Geometry and roughness flood channel

The table shows that the inlet has a large width compared to the other sections. In the main channel, the width varies slightly, and narrows in section 5 and 6. The sum of the width of the outlet channels is equal to the width of section 6.

3.4.3 Roughness

In contrast to the existing river system, the roughness for the flood channel cannot be determined by tuning based on an other model. Therefore, the roughness will be determined based on the land use.

The roughness is dependent on the vegetation and land use. For the roughness, distinction is made between the first section (from Waal to inlet structure), section 2 until 6 (inlet tot outlet structure) and section west and south. The roughness coefficients have been based on a RWS report [Van Velzen et al., 2003]. In Table 3-6, a list of vegetation types and corresponding roughness values is presented.

Description Chezy roughness Chezy roughness depth = 3m depth = 5m [m1/2/s] [m1/2/s] Productiegrasland 38 43 Natuurlijk gras en hooiland 35 40 Verruigd grasland 30 35 Natte ruigte 33 36 Droge ruigte 24 30 Riet ruigte 8 14 Natte ruigte 33 38 Hardhout ooibos 13 10 Zachthout ooibos 12 9

Table 3-6 Chezy roughness values for different vegetation pieces [Van Velzen et al., 2003]

Inlet In the first section, from the Waal to the inlet structure, it is expected that the main part of the water flows from the Waal through the flood plains towards the inlet structure, and does not follow the riverbed of the Afgedamde Maas. The flood plain on the east side of the Afgedamde Maas is a combination of rietruigte (height 1-2m), natuurlijk gras en hooiland, zacht/hardhout

40 January 2013 Hydraulic analysis of a flood channel

and open water. Based on this information, the estimated roughness will be somewhere between 10 m1/2/s and 40 m1/2/s. The average value of this range is 25 m1/2/s. Because of the open water, the roughness will be somewhat lower, so the Chezy value estimated on 28 m1/2/s (Chezy is actually a smoothness coefficient).

Main channel The land use in the main flood channel will stay productie grasland and natuurlijk gras and hooiland, so the Chezy value is between 35 m1/2/s and 43 m1/2/s. There are also some trees and other obstructions, so the expected roughness is estimated on the lower limit, 35 m1/2/s.

Outlet channels The west and south outlet channel are existing channels. The existing channels are 15 – 20 meters wide, while the distance between the replaced dikes is 250 – 350 meters. The widened part will be, according to the architect, nature. It is estimated that this nature will be a combination of droge ruigte and natte ruigte. An averaged Chezy value of 30 m1/2/s will be used.

3.4.4 Inlet and outlets

In the design, the inlet and outlet structures are gate structures with a sill height equal to the bottom level. Therefore, the inlet and outlet structures have been modelled as bridge piers (energy loss), and not as weirs. The bottom levels of the structures are equal to the bottom level of the surrounding area.

The in- and outlets are vertical moving gates between piles. The resistance of the piles will be calculated with the d’Abuisson formula, which gives, according to Chow [Chow, 1959], a good indication of the water level change due to the structure:

2 Q KA b2 d 32 g ( h 3  h 1 )  U 1

In order to use d’Abuissons formula in the model, it has been rewritten to the following form:

2 2 Q Q  b23 d KA  bd11  hh   13 22gg

In this formula:

KA [-] coefficient of contraction

b1 [m] flow width before structure

b2 [m] flow width at structure

h1 [m] water level before structure

h3 [m] water level after structure

d1 [m] water depth before structure

d3 [m] water depth after structure g [m/s2] gravitational acceleration

U1 [m/s] velocity before the structure Q [m3/s] discharge

41 Hydraulic analysis of a flood channel January 2013

The formula is based on energy loss due to the structure. The coefficient of contraction can be found in tables [Chow, 1959].

The equation has been implemented in the model and will be solved by iteration. The formula for the inlet and outlet structure is equal. In the schematization, it has been assumed that the piles have semicircular noses and tails, and have a conveyance ratio of 0.8. This results in a coefficient of contraction of 1.13 [Chow, 1959].

3.4.5 Summary

In the previous paragraph, the flood channel has been schematized, and the geometry and roughness values have been determined. In Table 3-7, a summary of the results of the previous paragraph is presented.

Section L B I C Remarks [km] [m] [-] [m1/2/s] Section 1 3 1200 2.6*10-4 28

Inlet structure 0.8*800 Coefficient of contraction KA=1.13 Section 2 2 750 1.1*10-4 35 Section 3 2 700 1.1*10-4 35

Section 4 2 725 1.1*10-4 35 Section 5 2 675 1.1*10-4 35 Section 6 1 600 1.1*10-4 35

Outlet structure 0.8*500 Coefficient of contraction KA = 1.13 Section west 10 250 1.0*10-4 30 Section south 7 350 1.4*10-4 30

Table 3-7 Geometry and roughness flood channel

3.5 Results

The water levels for the river system including the flood channel, as schematized in the previous paragraphs, have been calculated for a Rhine discharge of 16000 m3/s. In order to understand the figures in this paragraph, some additional information is presented in appendix A.2.

Flood channel In the figures below, the water levels of the system including the flood channel are presented:

42 January 2013 Hydraulic analysis of a flood channel

Figure 3-7 Water level flood channel for a 16000 m3/s Rhine discharge

In the flood channel, the water depth is approximately 4 meters, and at both outlet channels, a backwater curve is visible. Not clearly visible, but present, are water level steps at the inlet structure (3km) and outlet structure (12km). The discharge through the flood channel is 1957 m3/s.

The red line in the figure represents the equilibrium height for uniform flow. In the outlet channels, the water level is below the equilibrium height, with a backwater curve as a result. In sections 2 to 6, between inlet and outlet structure, the equilibrium depth is below the actual water level, because the water is pushed up due to the water level of the outlet channels. The equilibrium depth of section 1 is far below the equilibrium level.

Based on the observations above, it can be concluded that the discharge capacity of the outlet channels is too low compared that of the inlet and main channel. The inlet channel does not form an obstruction, and the influence of the inlet- and outlet structure on the water level is small.

Waal – Boven Merwede – Nieuwe Merwede In the figure below, the water level decrease due to the flood channel on the Waal is visible:

Figure 3-8 Water level along Waal-Merwede-Hollands Diep for a 16000 m3/s Rhine discharge

43 Hydraulic analysis of a flood channel January 2013

In Figure 3-8, the effect of the flood channel is clearly visible at kilometer 950. The maximum water level difference is 0.97 meter, and thirty kilometers upstream, the effect is still 20 centimeters. At the confluence with Amer, the water level difference is zero, as expected.

Maas – Amer In the figure below, the water level increase in the Bergse Maas due to the flood channel is visible:

Figure 3-9 Water level along Maas and Amer for a 16000 m3/s Rhine discharge

The figure above shows the water level difference due to the flood channel in Bergse Maas and Amer. The southern outlet channel ends in the Maas at river kilometer 250, causing a water level increase of 38 centimeters. Thirty kilometers upstream, the effect is still 14 centimeters. At the confluence with the Nieuwe Merwede, the effect is zero.

Summary The results show that the flow in the flood channel is obstructed by the outlet channels, because the discharge capacity of this channel is smaller than the discharge capacity of the inlet channel and main channel. Further, the influence of the inlet- and outlet structure on the water levels is small. In the table below, some results are summarized:

Discharge at Lobith Maximum water level Maximum water level Discharge flood

3 difference Waal difference Maas channel [m /s] [m] [m] [m3/s] 16000 -0.97 0.38 1957

Table 3-8 Summary results 1D-approach

3.6 Sensitivity analysis

A sensitivity analysis has been performed with the schematized one-dimensional model. Goal of the sensitivity analysis is to determine the influence of the flood channel’s design parameters on the flood channel’s discharge. For each river section, the width, slope and roughness have been varied. The detailed results of the sensitivity analysis are presented in appendix A.3. The most important results are summarized below: 

44 January 2013 Hydraulic analysis of a flood channel

 Increasing the width of narrowed sections 5 and 6 to the same width of section 4 (725 meters), results in a small discharge increase, and a water level decrease at the Waal of 1 centimeter.  The influence of the inlet and outlet structure on the system is small. A wider in- or outlet structure results in a smaller water level step at the structure, but the influence on discharge and water levels is negligible. A smaller structure has more influence on the channel than a wider channel; a width decrease of 50% results in a water level increase at the Waal of 2 centimeters.  The main channel and the outlet channels have approximately the same sensitivity to a roughness increase, the inlet channel a little bit less. A lowering of the Chezy coefficient with 5 m1/2/s (actually a roughness increase) results in a water level increase of 5 centimeters at the Waal. The system is less sensitive to a roughness decrease than to an increase.

3.7 Summary

In this chapter, the flood channel and surrounding river system has been schematized in a simple one-dimensional stationary model. After tuning, the system shows an accuracy from 3.4 to 10 centimeters. The verification shows an average accuracy of 5 to 15 centimeters. From the tuning and validation accuracy, it can be concluded that the model functions reasonably, given the simplifications, and gives a good first approximation of discharges and water levels. The model can be used for a qualitative analysis and estimation of the order of magnitude of the results. However, for a more detailed quantitative analysis, a more advanced model is required.

At a 16000 m3/s Rhine discharge at Lobith, the flood channel discharges 1957 m3/s, resulting in a water level decrease at the Waal of 97 centimeters and a water level increase at the Bergse Maas of 38 centimeters. This is below the expected values based on the study Integrale Verkenning Benedenrivieren [IVB, 2000] (Waal: -1.30 m, Maas: +0.54), see paragraph 2.3.3. The largest obstructions of the flood channel are the outlet channels, because the results show that the discharge capacity of these channels is smaller than the other sections. However, removing this obstruction results in a water level decrease at the Waal of approximately 104 centimeters (+7 cm), which is still 26 centimeters less than 1.30 meters.

The sensitivity analysis showed that the flood channel is in general more sensitive to an increasing resistance than to decreasing resistance. A parameter that has a lot of influence in all the modelled section is the roughness. Further, widening of section west and south channel has, compared to the widths of other sections, most influence on the flood channel’s discharge. The influence of the inlet and outlet structure on the flood channel’s discharge is small.

45

January 2013 Hydraulic analysis of a flood channel

4 Two-dimensional approach

4.1 Introduction

The flood channel designed by Robbert de Koning is presented in paragraph 2.3. In the previous chapter, the flood channel has been modelled in a one-dimensional way. In this chapter, the flood channel will be modelled in a more detailed 2-dimensional model.

To model the flood channel in 2D, a 2-dimensional reference model is necessary. The WAQUA Rhine-Maas Estuary model (RMM model) is a two-dimensional model, which has been updated to a recent situation in order to use the model for the Dutch Hydraulic Boundary Conditions 2011 (HR2011). This model will be used as a base, and it will be adapted to represent the reference year 2015; the year that the river system should be ready for a normative Rhine discharge of 16000 m3/s.

The flood channel will be schematized and implemented in the reference model. First, the RMM model will be described in paragraph 4.2, and in paragraph 4.3 the schematization of the flood channel will be explained. The boundary conditions that are imposed on the model will be listed in paragraph 4.4.

4.2 WAQUA RMM model

In this paragraph the WAQUA Rhine-Maas Estuary model (RMM model) will be described, and the steps to create the reference situation in 2015 will be explained.

The structure of this paragraph is as follows: 1.2.1 Background information related to the RMM model 1.2.2 Description of the boundary conditions 1.2.3 Brief explanation of the schematization of the RMM-HR2011 1.2.4 Some information about the numerical grid and corresponding parameters 1.2.5 Modifications of the RMM-HR2011 model in order to create the RMM-ref2015 model 1.2.6 Model tuning and validation

4.2.1 Background

History The WAQUA Rhine-Maas Estuary model (RMM model) is a two-dimensional model of the Dutch lower rivers (, Rhine, Maas) and Northern Delta (). At request of Rijkswaterstaat Waterdienst, the existing WAQUA Rhine-Maas Estuary 1998 model is updated to the situation of 2009 (RMM-HR2011). This two-dimensional model is primarily designed to calculate water levels of the Dutch Rhine-Maas estuary for the Hydraulic Boundary Conditions 2011 (HR2011). Presumably, the model will replace the existing one-dimensional SOBEK NDB-model (Noordelijk Delta Bekken model) in the future. The WAQUA RMM-HR2011 model was finished in 2011.

47 Hydraulic analysis of a flood channel January 2013

Database The schematization of the RMM model is described by a large amount of data: bottom level, land use (roughness), dikes, groins, weirs, barriers, etc. The data is stored in a geo-database, a database where the information is related to a certain location. The database can be viewed and edited by the ArcGIS extension Baseline, developed on behalf of Rijkswaterstaat. Therefore, the model database is called a Baseline-tree. The Baseline tree of the RMM-HR2011 model is updated until the year 2009, and is stored in a Baseline 4 (RMM-HR2011_4) and Baseline 5 (RMM-HR2011_5) database.

From the Baseline database that describes the schematization, input files for WAQUA can be derived, which is two-dimensional hydraulic simulation software part of the SIMONA package. For this study, Baseline version 5.2.0.435 and SIMONA 2011 will be used.

River and sea domain In general, the schematization of a river is much more detailed than a schematization of a sea, because the bathymetry and roughness of a river varies much more than the bathymetry and roughness of a sea. The RMM model describes a part of the Dutch river system, and a part of the North Sea. In order to optimize the simulation time of the RMM model, the river and sea part have been split, and both have a separate numerical grid with an optimal resolution. Those grids are adjacent, see Figure 4-1.

Figure 4-1 Grid sea (yellow) and river (red) domain [Zijl et al., 2010]

The river and sea grid are connected by domain decomposition. Due to domain decomposition, the river and sea grids can exchange information. The river and sea part of the RMM model are simulated separately, yet simultaneously. At each time step, water level and velocity information is exchanged at the boundary between the grids.

In this study, only the river domain is important. The sea domain is only required to impose the downstream boundary conditions.

4.2.2 Model boundaries

Model domain The model domain of the RMM model is roughly indicated in Figure 4-2. The upper boundaries are Hagestein (Lek), Tiel (Waal) and Lith (Maas). The lower boundary is the North Sea, where the Nieuwe Waterweg and Haringvliet estuary discharge their water.

48 January 2013 Hydraulic analysis of a flood channel

Figure 4-2 Model domain RMM-model

The model extend is indicated in Figure 4-2 by the green and blue areas. In general, the model area consists of riverbeds, flood plains, large lakes and a part of the North Sea. Most areas behind dikes are not part of the model. In addition, the Volkerak-Zoommeer, which is connected to the Haringvliet by sluices, is also not part of the model.

Upstream boundaries For the upper boundaries (Lek, Waal and Maas) and a few lateral inflows, a discharge (constant or time series) must be defined. The defined discharge at the boundaries of the Lek, Waal and Maas is distributed over the width of the summer bed (not over the winter bed).

Downstream boundaries The downstream boundary of the model is the North Sea. The downstream boundary condition is defined by a (varying) water level.

The RMM-HR2011 model is designed for Dutch Hydraulic Boundary condition 2011 calculations, and should replace a SOBEK 1-dimensional model in the future. For consistent results, the downstream boundary conditions in the WAQUA RMM-HR2011 model are imposed on the same points as in the SOBEK model; one point near the Nieuwe Waterweg, and two points near Haringvliet (Figure 4-3). The water level between the two water level points near Haringvliet is linearly interpolated. In order to prevent instabilities near the boundary point in the Nieuwe Waterweg, the viscosity of the water is locally increased from 1 m2/s to 100 m2/s [Zijl et al., 2011].

Figure 4-3 Sea domain (red). Boundary conditions imposed on places indicated with black bullets [Zijl et al., 2011]

Wind The wind direction and velocity must be separately imposed on the river and sea domain. For accurate model results, the velocity above land should be approximately 70% of the velocity above sea [Zijl et al., 2011]. It is possible to impose a global wind direction and velocity, but also a local varying wind direction and velocity are possible.

49 Hydraulic analysis of a flood channel January 2013

4.2.3 Model schematization

As already mentioned in paragraph 4.2.1, the model schematization of the RMM-HR2011 model is stored in a Baseline tree. The schematization of the RMM-HR2011 model is detailed to the level that even separate trees are modelled. In the baseline tree, two types of data could be distinguished: input data and processed data. The processed data are derived from a combination of the input data. In Figure 4-4, some input data (a) and processed data (b, c) are presented. The derived data for export to WAQUA are a combination of input data and processed data.

a. Dikes (red), quays (thick black), b. Bottom level c. Land use height difference lines (thin black), break lines (dotted orange) Figure 4-4 Information stored in baseline database

Bottom level In Baseline, the bathymetry is described by a combination of bottom level points and lines with height information (Dikes, height difference lines and break lines, see Figure 4-4a). Break lines and height difference lines indicate a sharp transition in the bottom level. In the RMM model, the resolution of the bottom level points is approximately 5 meters in the summer bed, but in the flood plains, the resolution is lower. The bottom level in WAQUA, which is based on height points and lines, of a part of the RMM-HR2011 model is displayed in Figure 4-5.

Figure 4-5 Bottom level part RMM-HR2011 model in WAQUA [m+NAP]

The resolution of the bottom level in the WAQUA simulation is not equal to the processed bottom level file in Baseline. For use in WAQUA, the bottom level file is discretized to the numerical grid.

50 January 2013 Hydraulic analysis of a flood channel

As a result, the resolution of the bottom level is equal to the resolution of the grid. The average resolution of the RMM model is 40 m2.

Weirs Not all the available height information from the input files is processed to the bottom level file, because some information could be lost during conversion to a WAQUA input file. Objects that are smaller than the grid resolution are visible in the bottom level file in Baseline, but are too small to project on the WAQUA grid. Therefore, those objects are modelled as weirs, and are calculated in WAQUA as an energy loss. Small dikes for example, are modelled in the RMM model as a quay, and not by a height difference line or as a dike. Those small dikes are not visible in the bottom level file. In the figure below, the weirs in a part of the RMM-HR2011 model are presented.

Figure 4-6 Weir height in RMM-HR2011 model [m+NAP]

Roughness In Baseline, the model roughness is processed based on several input parameters. All the input files are converted to a point, line and polygon shape. Those shapes are combined and converted to two roughness files as WAQUA input, one for the m-direction and one for the n- direction of the grid. In the roughness files, each grid point has a separate roughness value.

The roughness of the summer bed is determined by calibration. The summer bed is therefore divided in compartments, each with a different roughness value. In the model, the summer bed roughness in areas east of Gorinchem and Geertruidenberg are not determined by calibration, but are copied from other WAQUA Rhine and Maas models [Zijl et al., 2011].

The roughness of the flood plains is determined by the land use, e.g. agriculture, nature or a lake. The land use is coupled to a roughness coefficient, based on e.g. vegetation types [Van Velzen, 2003]. An example of land use schematization in the RMM model is presented in Figure 4-4c. In addition to the land use, also avenues of trees and separate trees are modelled and in Baseline converted to a roughness value.

Due to the grid size, some objects are too small to model physically in the bottom level file. Those objects, e.g. small water free areas or bridge pillars, are also modelled as a roughness value.

At the end, all the separate roughness values defined as points (tree), lines (avenues of trees) and polygons (land use, summer bed compartments etc.) are combined to a representative roughness value.

51 Hydraulic analysis of a flood channel January 2013

Figure 4-7 Chezy roughness [m1/2/s] (dependent of water depth, so image is illustrative)

Dividers Areas where water could never flow, e.g. tall buildings, are schematized as high water free areas. If the area is larger than a grid cell, this area is described in WAQUA by dividers (in Dutch: schotjes), which blocks the discharge through a cell face. If a high water free area is small compared to a grid cell, this area is described by a roughness value. In Figure 4-8, an area with dividers is presented.

Figure 4-8 Numerical grid with dividers, which are cell faces without discharge

Barriers There are five barriers in the model (see Figure 4-9). The Maeslantkering, Haringvlietsluizen and Hartelkering are sea barriers. The Hollandsche IJssel kering and Kromme Nolkering are river barriers. The barriers are discussed in detail in appendix A.4.

Figure 4-9 Barrier locations [Zijl et al., 2011]

52 January 2013 Hydraulic analysis of a flood channel

4.2.4 Grid and numerical parameters

Grid As discussed in paragraph 4.2.1, the RMM-HR2011 model is split in two domains, a river and a sea domain, that are coupled in WAQUA by domain decomposition. In Figure 4-10, the two domains are presented. Both domains have an orthogonal curvilinear grid, see Figure 4-11.

Figure 4-10 River (red) and sea (yellow) domain Figure 4-11 Detail curvilinear grid RMM-RH2011 model [Zijl et al., 2010]

The river domain, with 1080*3023 cells, has a grid resolution of approximately 40*40m2. The grid size is adapted to local geometry so that each river cross section has at least 8 cells.

The grid of the sea domain used in the RMM-HR2011 model has already been presented in Figure 4-3. This grid is smaller than the grid of the sea domain that is presented in Figure 4-10. In Figure 4-3, the part of the grid that is inactive is not displayed, where in Figure 4-10 the whole grid, including the inactive part, is displayed. The large inactive grid part is caused by the location where the downstream boundaries are imposed, see paragraph 4.2.2. In previous models, the downstream boundary conditions were imposed on different locations.

Time step In WAQUA, an implicit numerical scheme is applied. Therefore, the time step for a stable calculation is not limited by the Courant-Friedrichs-Lewy condition. In WAQUA, the time is numerically integrated by the Alternating-Direction-Implicit scheme. This method becomes inaccurate for large time steps in combination with a complex topography, e.g. closed boundaries or 90o channels. For those cases, the time step should be limited by the Courant number for wave propagation in order to retain accurate results. For the RMM model, a step of 15 seconds gives accurate results [Zijl et al., 2011].

Viscosity Because the horizontal turbulence is not solved in the model, the turbulence is modelled by adding a horizontal viscosity of 1 m2/s. This value is similar to other Dutch river models [Zijl et al., 2011].

Dispersion coefficients Differences in salt concentration lead to pressure differences, thus to flows. In the depth averaged two-dimensional WAQUA model, the three-dimensional effects are not well represented. To ensure a good representation, salt intrusion in the model is calibrated by a space varying dispersion coefficient, with values between 50 m2/s and 2000 m2/s [Zijl et al., 2011].

53 Hydraulic analysis of a flood channel January 2013

Dry cells The dry-cell criterion is set to 0.1 meter. Cells with a water level lower than this value are assumed dry, and are not participating in the calculation [Zijl et al., 2011].

4.2.5 Model changes RMM-ref2015

The RMM-HR2011 model describes the situation in 2009. The flood channel, as described in paragraph 2.3, is a concept within the Delta Program Rijnmond-Drechtsteden (DP-RD). DP-RD use the year 2015 as a reference situation, when the Room for the River works will be completed and the river system is ready for a normative Rhine discharge of 16000 m3/s. In order to be consistent with DP-RD, the RMM-HR2011 model will be adapted to the reference situation (RMM-ref2015).

The difference between the situation in 2009 and the reference situation 2015 are the following measures [Slootjes and De Waal, 2011]:  Ontpoldering Noordwaard  Ontpoldering Overdiepse polder  Flood plain excavation Avelingen  Water storage Volkerak-Zoommeer (outside model boundaries)

For the implementation of the flood channel, also implementation of the Room for the River measure Munnikenland is required. This measure will not be part of the RMM-ref2015 model, but will be added together with the flood channel. The Room for the River Measures that will be implemented, including Munnikenland (which is not part of the reference model), are displayed in Figure 4-12.

Figure 4-12 Room for the River measures that will be implemented in the RMM-HR2011 model

Ontpoldering Noordwaard The Noordwaard polder is located along the river Nieuwe Merwede, between the city of Gorinchem and nature reserve the Biesbosch. After ontpoldering of the Noordwaard, the water level reduction is maximal 60 centimeters near Werkendam and 30 centimeters near Gorinchem. The Noordwaard is designed for a discharge up to 2400 m3/s. This project is one of the largest measures in the Room for the River program, and will be completed in 2015. In Figure 4-13, the schematization is presented.

54 January 2013 Hydraulic analysis of a flood channel

Figure 4-13 Project plan Noordwaard with several Figure 4-14 Flow direction after ontpoldering polders [RvdR, 2010b] Noordwaard [RvdR, 2010a]

Near Gorinchem, the river Boven Merwede is relatively small and at high discharges the water level is pushed up. Because widening the flood plains was not possible due to cities on both sides, the water levels will be lowered by ontpoldering the Noordwaard. Downstream of Werkendam, a two-kilometer wide inlet structure will be located with a sill height of 2 meters above NAP. At high discharges, water will flow from the Nieuwe Merwede through the inlet structure, into the Noordwaard (Figure 4-14). From the Noordwaard, the water flows into Gat van Kampen and Gat van den Kleinen Hil, which are both connected to Hollands Diep.

After ontpoldering, the Noordwaard will be a combination of restored old creeks and polders with different average flooding frequencies: 100 days a year, 25 days a year, once every 100 years and once every 1000 years. The part of the polder that is flooded several times a year will be transformed to nature. Other parts will be used for recreational and agricultural purposes. The houses that remain in the Noordwaard will be built on mounds [RvdR, 2010a].

Overdiepse Polder The Overdiepse Polder is located in the Bergse Maas. The ontpoldering of the Overdiepse Polder is performed to lower the local and upstream water levels. At high discharges, water flows through the polder, resulting in a maximum water level decrease of 27 centimeters. The measure is displayed in Figure 4-15.

Figure 4-15 Room for the river measure Overdiepse polder [Provincie NB, 2009]

The existing dike between the polder and the Bergse Maas will be lowered and the houses and farms in the polder will be moved to mounds. These mounds will be build beside a new dike, on the north side of the . According to the plan, the land use will not change. The polder will be flooded in average once every 25 years. The project will be ready in 2015 [Provincie NB, 2009].

55 Hydraulic analysis of a flood channel January 2013

Flood plain excavation Avelingen Part of the Room for the River program is the flood plain excavation, with a new secondary channel, of business park Avelingen, near the city of Gorinchem. An impression is visible in Figure 4-16. This excavation results in a local water level reduction of 10 centimeters. Due to the sill at the inlet, the gully only flows at high discharges. The project will be ready in 2013.

Figure 4-16 Impression flood plain excavation Avelingen

Water storage Volkerak-Zoommeer In storm surge situations, when the Haringvliet barrier, Hartel barrier and Maeslant barrier are closed, water cannot flow into the sea. Once every 1400 years a storm surge and a high river discharge coincide. In that rare case, the river discharge must be stored, until the barriers open again. As a part of the Room for the River project, extra storage is added to the Volkerak- Zoommeer to enlarge the total storage capacity of the delta, see Figure 4-17. This results in a maximum water level decrease of 50 centimeters [DHV, 2010a].

Figure 4-17 Water storage Volkerak-zoommeer [Slootjes et al., 2010]

The Volkerak-Zoommeer is not included in the model domain of WAQUA RMM-HR2011 model, and is therefore not implemented in the RMM-ref2015. Without the lake in the model, the measure cannot be implemented in an ordinary way. However, the water storage has a significant influence on the results if a storm surge coincided with a high river discharge [Jeuken et al., 2010].

Flood plain excavation and dike relocation Munnikenland The flood plain excavation and dike relocation Munnikenland lies between the Waal and Afgedamde Maas, north of the inlet structure of the new flood channel. Munnikenland is not implemented in the reference of the model used by DP-RD, and will therefore also not be implemented in the RMM-ref2015 model. However, this measure will be added together with the new flood channel, which will be described in paragraph 4.3.3.

56 January 2013 Hydraulic analysis of a flood channel

The dike on the east side will be relocated to give the river more space. Furthermore, the dike will be lowered, resulting in a yearly flooding of the flood plains and polder. Goal of this measure is to lower the water level at normative discharges, and to increase the quality of the landscape. When the measure has been implemented, the water level at high discharges will be lowered with 11 centimeters. The measure is presented in Figure 4-18.

Figure 4-18 Flood plain excavation and dike relocation Munnikenland

4.2.6 Model calibration and validation

Calibration RMM-HR2011 model The RMM HR-2011 model is mainly calibrated on water levels. Furthermore, the parameters tide, discharge, wind and salt concentration are taken into account during the calibration. The overall water level and tide representation of the RMM-model is comparable with the SOBEK TMR2006 model, and the tide representation is even better (Correctness of fit criteria between 2 and 5 cm) [Zijl et al., 2011].

For the calibration of the RMM-HR2011 model, water level stations downstream of Werkendam and Keizersveer are used. Upstream of these locations, the roughness coefficients from the Maas and Rijntakken model were adopted, which are probably too high, resulting in lower quality of upstream water level representation [Zijl et al., 2011].

At high river discharges, the water levels between Hellevoetsluis and Keizersveer are too high. This is probably caused by the way the Haringvliet sluices are modelled [Zijl et al., 2011].

Validation The RMM-ref2015 model is, except the Room for the river measure Volkerak-Zoommeer, equal to the 1-dimensional SOBEK model used by DP-RD. The results of both models will be compared.

In Figure 4-19 and Figure 4-20, the results for the Waal and Maas are presented for a 16000 m3/s Rhine discharge. In this situation, the sea barriers are all open, and the Volkerak- Zoommeer measure is not in use.

57 Hydraulic analysis of a flood channel January 2013

Figure 4-19 Waal; water level comparison WAQUA and SOBEK reference 2015 at 16000 m3/s Lobith discharge

Near the inlet of the Noordwaard (kilometer 964), the water level difference changes from negative to positive. Upstream of this point, the water level in the WAQUA model is less pushed up than the water level in the SOBEK model. Downstream of this point, the water level in WAQUA is higher, which implicates that the discharge through the Noordwaard in the WAQUA model is less than in the SOBEK model.

When looking to the figure above, the water level difference could be divided in three parts, see Table 4-1.

River section [km] Average difference [m] Maximum negative Maximum positive difference [m] difference [m] 915 – 951 0.018 -0.111 0.127 951 - 965 -0.159 -0.244 -0.014 965 - 983 0.101 -0.014 0.142 Total (915 – 984) 0.005 -0.244 0.142

Table 4-1 Water level difference Waal per river section

Upstream of river kilometer 951, the WAQUA water level is in average 1.8 centimeters higher than the SOBEK water level. Between kilometer 951 and 983, the differences are much larger. Especially in section 951 – 965, the water level difference is much lower when compared to the SOBEK model (24.4 centimeters). Because the water level difference changes sign at the inlet of the Noordwaard, the reason for the differences is probably found in the schematization of the Noordwaard.

58 January 2013 Hydraulic analysis of a flood channel

Figure 4-20 Maas; water level comparison WAQUA and SOBEK reference 2015 at 16000 m3/s Lobith discharge (3504 m3/s Lith discharge)

The water level difference at the Maas shows a sign change at kilometer 249. Upstream of kilometer 249, the WAQUA water level is in average 2.9 centimeters lower than the SOBEK model. However, downstream of this point, the average water level difference increases to nearly 11 centimeters. As mentioned before, the water levels at high river discharges between Keizersveer and Hellevoetsluis are too high, probably due to a wrong modelling of the Haringvliet sluizen [Zijl et al., 2011]. This could explain the water levels downstream of kilometer 249.

River section [km] Average difference [m] Maximum negative Maximum positive difference [m] difference [m] 210-249 -0.029 -0.113 0.098 249-259 0.109 -0.008 0.143 Total (210 – 259) -0.002 -0.113 0.143

Table 4-2 Water level difference Maas per river section

The comparison of the WAQUA and SOBEK model in a storm surge situation will not be discussed here, because in this study the models will only be used for situations with high discharges.

In summary, the results of the SOBEK and WAQUA models that describe the reference 2015 situation show some clear differences, locally to 24 centimeters. The differences at the Waal and Nieuwe Merwede are probably caused by the schematization of the Noordwaard. The difference at the Maas and Amer is probably caused by modelling of the Haringvliet sluizen. Despite these differences, the WAQUA RMM-HR2011 model has the same accuracy as the SOBEK TMR2006 model [Zijl et al., 2011]. The SOBEK reference 2015 model is based on the SOBEK TMR2006 model, and the WAQUA RMM-ref2015 model is based on the RMM-HR2011 model. Therefore, the RMM-ref2015 model is expected to be accurate enough for the conceptual study of the flood channel.

4.3 Schematization flood channel

In the previous paragraph, the schematization of the RMM model has been described. The schematization of the flood channel, as introduced in chapter 2, will be described in this chapter.

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The structure of this paragraph is as follows: 1.3.1 Some background information related to the flood channel schematization 1.3.2 Starting points of the schematization 1.3.3 Schematization inlet (Waal to inlet structure) 1.3.4 Schematization Main channel (inlet to outlet structure) 1.3.5 Schematization outlet channels (outlet structure to the Biesbosch and Bergse Maas) 1.3.6 Review design with schematization

4.3.1 Background

Baseline The function of the Baseline software is already introduced in paragraph 4.2. The RMM model is schematized in Baseline, and this model can be changed by implementing measures. Therefore, the flood channel will be schematized as a Baseline measure. This measure is a schematization of the flood channel, but also describes the parts of the reference that have to be removed. By implementing this measure, information will be added and removed from the reference schematization, which results in a changed schematization that includes the flood channel, see Figure 4-21.

a. Part reference model b. Measure c. Reference including measure Figure 4-21 RMM model (a), measure (b) and the RMM model including the measure (c=a+b)

Due to the separate schematization of the measure, it is easy to exchange the schematization. Another advantage is that all the properties of the schematization, including the changes to the reference, are documented.

Grid creation and domain decomposition In a two-dimensional model like WAQUA, all the spatial information of a schematization is related to numerical grid coordinates. Therefore, a simulation in WAQUA is only possible when there is a numerical grid that covers the whole model area. For this project, a new flood channel will be designed, but unfortunately, the largest part of the flood channel lies outside the numerical grid of the Rijn-Maasmonding (RMM) model (see Figure 4-22). As a result, the flood channel cannot be schematized using this RMM grid.

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Figure 4-22 Gap in the RMM-grid at location of the flood channel. Red lines are dikes for the flood channel.

In order to model the flood channel, a new grid has been designed. This grid has three connections with the RMM grid, East, West and South (see Figure 4-23), and covers the whole area of the new flood channel. The new grid is connected to the RMM grid by domain decomposition.

Figure 4-23 Couple interfaces between new grid (blue) and RMM grid (grey). Connections between the grids are indicated with the red lines.

More information about the creation of the grid and domain decomposition can be found in appendix B.

Software The flood channel has been modelled by using the ArcGIS plug-in Baseline 5.2.435. The model will be converted to a WAQUA schematization, and simulated in a Simona 2011 environment. As a reference, RMM-ref2015 schematization is used. This reference is based on the schematization mentioned in Table 4-3.

Name Official name Description RMM WTI2011 model RMM-HR2011_4 RMM model updated until 2009 RvdR Avelingen AvSNIP3hvschl Room for the river measure RvdR Noordwaard ip_scenario2 Room for the river measure RvdR Overdiepse Polder bm_ovediep_a2 Room for the river measure RvdR Munnikenland wl_mun_vka3d Room for the river measure wl_mun_ruwh RMM-ref2015 model RMM-ref2015 RMM-HR2011_4 converted to baseline 5, including the RvdR measures Avelingen, Noordwaard and Overdiepse Polder

Table 4-3 Baseline model and measure versions

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4.3.2 Starting points schematization

In Figure 4-24, the design of the flood channel by Robbert de Koning, and the boundary of the RMM-ref2015 model, including Room for the River measure Munnikenland, is presented. The design of this flood channel will be schematized and implemented in the RMM model. After implementing the flood channel in the RMM-ref2015 model, the schematization including the flood channel will be called RMM-ref2015+fc.

Figure 4-24 RMM-ref2015 model (including Room for the river measure Munnikenland) with the design of the flood channel by Robbert de Koning [De Koning, 2012] as background image

The flood channel designed by Robbert de Koning is schematized based on the following starting points:  Conservation of the reference situation when possible (bottom height, land use)  Only the parts of the design that contribute to the flood channel’s discharge will be modelled  Reference is the RMM-ref2015 model as described in paragraph 4.2, which is, except the Room for the River measure Volkerak-Zoommeer, equal to the reference used in the Delta Program [Slootjes and De Waal, 2010]  The Room for the River measure Munnikenland is not part of the RMM-ref2015, but is implemented with the flood channel schematization

The schematization of the flood channel will be discussed in three parts according to Figure 4-24:  Inlet (Waal to inlet structure)  Main channel (inlet structure to outlet structure)  Oulet (outlet structure to Biesbosch/Maas)

4.3.3 Inlet

In this paragraph, the changes due to implementation of the flood channel in the area from the Waal to the inlet structure will be discussed. The inlet structure of the flood channel is located next to the sluices that separate the northern part of the Afgedamde Maas from the southern part. The area in the reference situation, after completion of Room for the River measure Munnikenland, is displayed in Figure 4-25.

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Figure 4-25 Munnikenland after completion Figure 4-26 Changes in Munnikenland and of the Room for the River project Afgedamde Maas due to implementation flood channel

The inlet design of the flood channel by Robbert de Koning is presented in Figure 4-26. In this figure, the red lines are the dikes before implementation of the flood channel. The most important differences between the reference and the design are marked with capital letters, and will be described below.

A. Dike replacement and bottom level change The area indicated with A in Figure 4-26 is showed with more detail in the figure below.

Figure 4-27 Bottom height and dike (red line) before (left) and after (right) implementation flood channel

Due to implementation of the flood channel, the following has been changed compared to the reference situation:  The dike marked with A in Figure 4-26 is set back in order to make space for the inlet structure (D). The dike is replaced by a summer dike (black in Figure 4-27) with a height of 2 meters above NAP, which will overflow several times a year. Thus, this area is a new flood plain.  There are riverbanks located in the new flood plain, between the Afgedamde Maas and the replaced dike. Those riverbanks have a height of at the most 3 meters above NAP, and block the flow towards the inlet structure (D). Therefore, the bottom level in this area has been lowered to 1 meter above NAP, which is approximately equal to the bottom level on the east side of the Afgedamde Maas.  Between the dike and the Afgedamde Maas some nature is located (right of the ‘i’ sign in Figure 4-27 left). This nature has been subject to small changes. The summer dike on the east side of the area has been lowered from 3-5 meters to 2 meters above NAP. The

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dike between the nature and new flood plain has been removed. The roughness did not change.  In the reference, a part of the area in the new flood plain is used for agricultural purposes. The roughness of these areas has been set to grass production, because it is assumed that agriculture is not possible when an area is flooded several times a year.

B. Dike replacement and buildings removal The dike on the east side of the Afgedamde Maas is placed around some buildings, marked with B in Figure 4-26. This could hinder the water flowing from the Waal to the inlet structure, and therefore, this dike has been replaced. The adjacent nature area has been extended to this new area on the outer side of the dikes; see Figure 4-28 and Figure 4-29.

Figure 4-28 Bottom height and dikes before (left) and after dike replacement (right)

Figure 4-29 Land use before (left) and after dike replacement (right)

C. Bottom level lowering near sluices Near the inlet structure, the sluices of the Afgedamde Maas are located. On the north side of the sluice, some quays are located on a small peninsula in the Afgedamde Maas, with bottom heights from 2 to 5 meters above NAP. Because this island is located near the inlet structure, the high bottom levels could hinder water flowing towards the inlet structure. Therefore, the bottom levels of this island and quays have been lowered up to 2 meters above NAP, see Figure 4-30.

64 January 2013 Hydraulic analysis of a flood channel

Figure 4-30 Island in Afgedamde Maas before and after bottom level lowering

D. Inlet structure According to the design, the inlet structure has moving gates and a road on top. The bottom level of the inlet structure is equal to the surrounding ground level. In the schematization, only the roughness due to the inlet structure has been schematized, not the moving gates. The dimensions of the schematized inlet structure have been based on the inlet structure applied to the flood channel between Veessen-Wapenveld [Arcadis, 2012]. The inlet structure has been modelled by pillars of 4 by 20 meters with a spacing of 25 meters, see Figure 4-31. The foundation is located 10 centimeters above the ground. The width of the inlet structure is 875 meters.

Figure 4-31 Schematized inlet structure

Summary In Figure 4-32, the changed bottom level, weirs and roughness of the inlet area are presented, and in Figure 4-33, the land use is presented, both before and after implementation of the flood channel.

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Figure 4-32 Bottom level and weirs in reference situation (left) and after implementation flood channel (right)

Figure 4-33 Land use in reference situation (left) and after implementation flood channel (right)

4.3.4 Main channel

The area between the inlet and outlet structure of the flood channel is the ‘main channel’. In Figure 4-34, the design is displayed, including some capital letters used to explain the schematization in more detail. The details are explained below.

Figure 4-34 Main channel design

General comments  On both sides of the flood channel, dikes have been placed, with a height equal to the dike height in the Afgedamde Maas (6.82 meters above NAP).  The bottom level of the main channel is based on the reference situation (AHN), except the area just behind the inlet structure, which is lowered to 1 meter above NAP.

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 In the reference situation, the land in the main channel is mainly used for agricultural purposes. There are some farms and houses, there is a small lake and there are a few wood parcels. For the flood channel, the buildings that are located in the flood channel have been removed, and the land use has been changed to grass production. The land use of other parts did not change, and therefore remain grass production, lake or wood parcel, see Figure 4-38.

A. Area after inlet structure In the figure below, the area after the inlet structure is displayed before and after implementation of the flood channel.

Figure 4-35 Area after inlet structure before (left) and after (right) implementation flood channel

 In the reference situation, the dike on the west side of the Afgedamde Maas, just after the inlet structure, lies in the area of the new flood channel. This dike has been relocated to the border of the flood channel.  In the reference situation, the grey area in Figure 4-35 has a bottom level between 1 and 3.5 meters above NAP. Because the bottom level west of this area is lower than 1 meter above NAP, the bottom level has been lowered to 1 meter above NAP. It is expected that this lowering is necessary for a good inflow into the channel.

B. Removal business park  In the reference situation, there is a business park located in the flood channel, see Figure 4-36. This business park has been removed to increase the flow capacity. The land use has been changed into grass production.  The road directly south of the business park has been lowered to the surrounding bottom level.

Figure 4-36 Business area inside flood channel

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C. Gentle slopes north side channel According to the design of the flood channel, the slopes on the north side of the channel are gentle. The slope of the gentlest parts is approximately 1:6, indicated with C in Figure 4-34. The slopes do not have a hydraulic function, but have been created to increase the quality of the landscape. The gentle slope is clearly visible in Figure 4-37.

D. Removal old dike An old dike crosses the flood channel (E in Figure 4-34). This dike has been removed.

Summary In Figure 4-37, the bottom level of the flood channel is displayed, and in Figure 4-38, the land use is displayed.

Figure 4-37 Bottom level flood channel

Figure 4-38 Land use flood channel

4.3.5 Outlet channels

From the outlet structure, the channel splits into two channels. One channel flows to the Biesbosch, and one channel flows to the Bergse Maas. In Figure 4-39, the design of the outlet channels is presented. The capital letters in this figure will be described in more detail below.

Figure 4-39 Outlet channel design

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General comments  The outlet channels follow the routes of small existing creeks. The existing dikes in the area have been removed, and have been replaced by new dikes, according to the design of Robbert de Koning.  The bottom level of the outlet channels is based on the bottom level in the reference situation (AHN). Old dikes and other obstacles have been removed.  In the outlet channels, the land use in the reference situation is a combination of buildings, agricultural area, grass production, creeks, wood parcels and nature. In the situation with the flood channel, the land use of forest, creeks and lakes did not change. The remaining parts have been set to droge ruigte.

A. Outlet structure  The outlet structure is similar to the inlet structure, as described in paragraph 4.3.3. The only difference is the width, which is 550 meters.  The highway A27, that crosses the flood channel, will be located on top of the outlet structure.

B. Nieuwendijk  In the design of Robbert de Koning, the creek northwest of Nieuwendijk flows in northern and southern direction. Because the flow can only be discharged to the south, the creek will not be schematized in northern direction. See Figure 4-40.  In the design of Robbert de Koning, the channel in front of Nieuwendijk is designed as a wet sailing channel. In order to keep the schematization straight forward, this area is modelled similar to the other parts of the outlet channels (bottom level equal to reference situation, land use nature (droge ruigte)).

Figure 4-40 Creek near Nieuwendijk

C. Bottom level and roughness channels  The depth of the creeks in the outlet channels is unknown. Therefore, the depth has been estimated at 1.5 to 3 meters below NAP.

D/E. Connection Biesbosch and Maas  The West channel, including the creek, has been connected to the Steurgat, which is connected to the Biesbosch.  The South channel, including the creek, has been connected to Spijkerboor, which is connected to the Bergse Maas.  At the connection between flood channel and Biesbosch/Maas, the dike in the reference has been removed.

Summary In Figure 4-41 and Figure 4-42 the bottom level and land use before and after the implementation of the flood channel are presented.

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Figure 4-41 Bottom level and weirs outlet channels before (left) and after (right) implementation flood channel

Figure 4-42 Land use before (left) and after (right) implementation flood channel

4.3.6 Schematization summary

In this paragraph, the flood channel design by Robbert de Koning, as described in chapter 2, has been schematized. The most important choices and assumptions are listed below:

 In the design, the width between the pillars of the inlet and outlet structure is estimated at 10 to 15 meters. However, the inlet structure of the flood channel between Veessen and Wapenveld shows that a pillar spacing of 25 meters is economically possible, which results in a lower flow resistance. Therefore, this spacing has been used in the schematization of the inlet- and outlet channel.  The farm near the inlet structure, B in Figure 4-26, has been removed according to the architects design, because it is expected that it will be an obstacle for the flow towards the inlet structure. Because the new roughness of this area has not been defined in the design, it is assumed that this is equal to the surrounding flood plain. At the location of the replaced dike, a new quay has been created. The height of this new quay is based on the height of the adjacent quay.  The height of the quay between the new flood plain before the inlet structure and the Afgedamde Maas is not specified in the design. The height is estimated at 2 meters above NAP. It is expected that this height will not hinder the flow towards the inlet structure significantly, and that the flood plain can be used for agricultural purposes in summer.

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 On the north side of the sluice in the Afgedamde Maas, some quays are located on a small peninsula. Because this island is located near the inlet structure, it is expected that the high bottom levels could hinder water flowing towards the inlet structure. In the design, the height after lowering has not been defined. The bottom level of this island and quays have been lowered up to 2 meters above NAP, which is equal to the quay height of the flood plain before the inlet structure.  In the design, the land use of the flood channel is specified, but not the roughness. Therefore, the roughness corresponding to the land use has been estimated based on Handboek vegetatie [Van Velzen, 2003].  In the design of Robbert de Koning, the creek northwest of Nieuwendijk flows in northern and southern direction. Because the flow can only be discharged to the south, the creek has not been schematized in northern direction.  In the design, the land use of the flood channel south of Nieuwendijk, between the outlet structure and the previous mentioned creek, is a sailing channel. In the schematization, this part of the channel is modelled as nature, similar to the other parts of the outlet channels.  In the design, the removal of a few obstacles is specifically mentioned. The removal of the buildings is mentioned in general. Therefore, all the buildings in between the dikes of the flood channel have been removed. In addition, old dikes in the outlet channels have been removed (not mentioned in the design).

4.4 Boundary Conditions

The RMM-ref2015 model and the flood channel schematization have been discussed in the previous paragraphs. In order to do calculations with those schematizations, the boundary conditions have to be defined. In paragraph 4.4.1, some background information related to the boundary conditions is presented, and in paragraph 4.4.2, the boundary conditions that will be used are presented.

4.4.1 Background

River and sea dominated areas In rivers far from the sea, the river discharge wave determines the governing water level (river dominated area). In the river delta close to the sea, storm surges are governing the water level (sea dominated area). In the transition area between river dominated and sea dominated area, the river and sea both have significant influence on the governing conditions (transition area). The borders between those areas are presented in Figure 4-43. However, peak discharges and storm surges are in most cases independent events.

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Figure 4-43 Transition from dominant river influence to dominant sea influence) [Slootjes et al., 2011]

In paragraph 4.3, a flood channel has been schematized. The inlet of the flood channel is located in the river area, while the outlet of the flood channel is located in the transition area. In average, the flood channel will be used once every hundred years during high river discharges, as discussed in paragraph 2.3.4. In the situation of an extreme river discharge, the probability of a simultaneous storm surge at sea is marginal. Figure 4-44 presents the combination of river discharge and sea water level that could cause the 1/2000 water level near one of the outlets of the flood channel.

Figure 4-44 Origin 1/2000 water level at Maas km 251 [Hydra Zoet, WTI 2011]

The flood channel will be used from a Rhine discharge of approximately 12600 m3/s (see paragraph 2.3.4). At those and higher Rhine discharges, the expected maximum sea level near the outlet of the channel is less than 2.15 meters above NAP, according to Figure 4-44. The average high tide is 1.14 meters above NAP (amplitude 1.08 meters), thus this maximum sea level is up to a meter above the average high tide. The tide amplitude at Maas kilometer 251 due to a normal astronomical tide is 0.06 meter.

Because the flood channel only has a function at high river discharges, and the maximum expected wind set-up at the coast during those discharges is at the most 1 meter above normal tide, the influence of a storm surge will not be not taken into account in this study.

Constant discharge The highest water levels in rivers are reached when the top of a high water wave passes. In the river dominated area, the governing water levels are usually calculated with a discharge wave as boundary condition.

In this model, the decision has been made to calculate the water levels with a constant river discharge. Advantage of the choice for a constant river discharge is the easier interpretation of the results, and the consistency with the results from the DP-RD SOBEK model. Disadvantage of

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the simplification is that the water level decreasing effect of the storage capacity will be ignored. However, a part of the study area is in the transition area, where the influence of the storage capacity on the water level decreases. Taking the river discharge constant is a conservative approach.

Climate change Due to the climate change, the normative Rhine discharge will increase from 16000 m3/s in 2015 to 18000 m3/s in the year 2100. More background information about these discharges can be found in appendix A.1. In addition, the sea level is expected to rise in the next century. The expected sea level rise is presented in Table 4-4.

Reference year Sea level rise from 1990 [m] 2015 0.08 2050 0.35 2100 0.85

Table 4-4 Sea level rise for reference years [Bruggeman et al., 2011]

Discharge distribution In PKB Room for the River, it is agreed that the Nederrijn/Lek is saved for discharges higher than 16000 m3/s at Lobith. The discharge through the Nederrijn is not allowed to be higher than the discharge that occurs at a Rhine discharge of 16000 m3/s. At higher Rhine discharges, the discharge through the Nederrijn does not increase, and the extra water will be divided proportionally between the IJssel and the Waal [Slootjes and De Waal, 2010]. This changed discharge distribution is presented in Table 4-5, and has been implemented in the RMM model.

River branch Reference 2015 KNMI W2050/G-2100 KNMI W-2100 (16000 m3/s) (17000 m3/s) (18000 m3/s) Waal 10165 10970 11758 Neder-Rijn 3376 3376 3376 IJssel 2459 2654 2866 Table 4-5 Discharge distribution Rhine branches [Silva and van der Linden, 2007]

4.4.2 Combination of boundary conditions

In the transition area, the design water level could be caused by several combinations of boundary conditions. Therefore, a standardized set of boundary conditions has been defined, see Table 4-6. In this set, the Maas discharge is coupled to the Rhine discharge by its median, given the Rhine discharge. These combinations are taken into account by doing a probabilistic calculation with the several stochasts.

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Rhine dominate,RMM, reference year 2015. Total 3384 sums (western dir. 3132, eastern dir 252) Rhine/Maas 50% Western wind directions Eastern wind directions Rhine Maas Sea Windsp. Winddir Barrier Sea Windsp. Winddir [m3/s] [m3/s] [m+NAP] [m/s] [-] [m+NAP] [m/s] [-] 600 55 1.14 0 SW closed 1.30 0 NNE 2000 217 2.00 10 WSW open 10 NE 4000 687 3.00 20 W 20 ENE 6000 1156 4.00 30 WNW 30 E 8000 1626 5.00 42 NW ESE 10000 2095 6.00 NNW SE 13000 2800 N SSE 16000 3504 S 18000 3974 SSW Number of stochastic values: 9 6 1+4 7 2 1 1+3 9

Table 4-6 Set boundary conditions probabilistic calculations [Geerse and Duits, 2012]

Because the flood channel is only effective at high discharges, and the influence from the sea level on the flood channel is expected to be small, the flood channel will be calculated deterministically. To be consistent with the Delta-program, a few combinations from the probabilistic stochasts have been chosen. The combinations of boundary conditions that will be imposed on the model boundaries are presented in Table 4-7.

Rhine Wind Wind Wind Simulation ID discharge setup speed direction 2015 [m3/s] [m] [m/s] (wl sea +0.08m NAP) 13000 0 3 WNW 0007 16000 0 3 WNW 0008 18000 0 3 WNW 0009

Table 4-7 Boundary condition combinations

The boundary conditions summarized:  Constant river discharges (13000 m3/s, 16000 m3/s, 18000 m3/s); they describe the range of discharges when the flood channel is in use (>12600 m3/s)  Changed discharge distribution at Pannerdensche Kop due to sparing of the Nederrijn- Lek, see paragraph 4.4.1.  Sea level with normal astronomical tide (also the situation without tide will be calculated)  Sea level rise of 8 centimeters for the year 2015 compared to 1990  Storm surges are not taken into account  Influence of the wind is negligible  Lateral discharges are coupled to the Rhine discharge at Lobith. The lateral discharges are based on Hydraulic Boundary conditions 2006 [Van der Veen, 2005].

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5 Results

In this chapter, the results of the two-dimensional schematization of the flood channel will be presented. The structure of this chapter is as follows: 5.1 Introduction and background about figures 5.2 Discharge distribution in the water system 5.3 Water levels and velocities in the water system 5.4 Results flood channel 5.5 Comparison results one-dimensional and two dimensional approach 5.6 Design review of the flood channel 5.7 Summary

5.1 Introduction

In the previous chapters, the flood channel as a possible solution for the design water level increase due to the climate change has been elaborated. The flood channel has been implemented in a one-dimensional model, and in this chapter, the results of the more detailed two-dimensional schematization of the flood channel will be presented.

Figures The results will be presented in figures. First, some background information about the figures is given in order to interpret the figures well.

The boundary conditions have already been discussed in paragraph 4.4. As stated there, the model has been simulated with three different constant Rhine discharges (13000 m3/s, 16000 m3/s and 18000 m3/s), where the Maas discharge is coupled to the Rhine discharge. At the downstream boundary condition, there is no wind set-up, but calculations have been performed with and without tide. In the situation without tide, the water levels and velocities of the model are constant at each time step, while in the situation with tide, the water levels and velocities vary.

In the figures in this chapter, the type of downstream boundary condition differs per parameter:  Water level (1D-plot): In order to be consistent with the water level results of Delta Program Rijnmond-Drechtsteden, the water levels in the figure are equal to the maximum water levels (varying tide as downstream boundary condition)  Velocity (1D-plot): the velocity results are based on the situation with a constant water level as downstream boundary condition  All parameters in the two-dimensional plots are based on the situation with a constant water level as downstream boundary condition

In order to get an idea of the differences in the results, the water level range and the velocity range for the situation with and without tide for a 16000 m3/s Rhine discharge are displayed in Figure 5-1.

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Figure 5-1 Water level range and velocity range (Dark blue line is maximum water level, black line is velocity with constant water level) for a 16000 m3/s Rhine discharge

From the figure, it follows that the water level range due to the tide is maximal 64 cm at Hollands Diep (984 km) and 10 centimeters at Werkendam (962 km). The maximum velocity range is 0.5 m/s. Therefore, it is sufficient to represent only the maximum water level and velocity for the situation without tide. The dark blue line (WL upper boundary) is the water level line and the black line is the velocity line that will be presented in the figures in this chapter.

5.2 Discharge distribution

The water balance of a river system must be closed. Therefore, the discharge distribution over the branches with and without the flood channel will be presented in this chapter.

5.2.1 Water balance

Water system In the figure below, a part of the Dutch river system is presented. Water is flowing from east to west. The names of the several branches are displayed in the figure; the red lines are the transitions between the names of branches. The colours represent the flow velocities larger than 0.05 m/s, thus the flowing parts. The white arrows show the positive flow direction.

Figure 5-2 Part of Dutch water system with names. Colours indicate a velocity of at least 0.05 m/s.

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Schematization water system The water system between the Nieuwe Merwede and Amer is complex due to several in- and outflow locations (see Figure 5-2). Water from the Nieuwe Merwede is flowing into the Biesbosch through the Noordwaard and Natuur Ontwikkelingsproject Noordwaard (NOP). In addition, water from the Bergse Maas is flowing into the Biesbosch, but in much lower quantities than water from the Nieuwe Merwede. The main outflow from the Biesbosch is flowing into the last kilometer of the Amer, but the Biesbosch also discharges water more upstream into the Amer. In the schematization of the water system, presented in Figure 5-3, this interaction is schematized by taking the main flow from the Biesbosch to Hollands Diep as a separate flow, and by including the small flows from the Biesbosch to the Amer into the discharge of the Amer. In the schematization, the flow from the Nieuwe Merwede towards the Biesbosch consists out of the Noordwaard and NOP discharge (see Figure 5-2).

The part of the water system as presented in Figure 5-2, is schematized in Figure 5-3 for a Rhine discharge of 16000 m3/s. There are two inflows (Waal and Maas), and two outflows (Beneden Merwede and Hollands Diep).

Figure 5-3 Discharge distribution at river discharge of 16000 m3/s at Lobith

In the water balance, all the significant flows are taken into account. The accuracy is approximately 5 m3/s. Therefore, small deviations could occur when calculating the water balance for separate parts. However, the deviations are negligible compared to the total discharge.

5.2.2 Discharge distribution water system with flood channel

In this paragraph, the discharge distribution of the water system after implementation of the flood channel will be compared to the reference. In addition, the sensitivity to different Rhine discharges will be analyzed.

16000 m3/s Rhine discharge In Figure 5-4, the discharge distribution, after implementation of the flood channel, for a discharge of 16000 m3/s at Lobith is presented. The deviation compared to the reference without flood channel (see Figure 5-3) is given in percentages.

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Figure 5-4 Discharge distribution water system with flood channel compared to the reference at a 16000 m3/s Rhine discharge

The observations based on the figure above will be discussed per river branch.

Waal/Boven Merwede/Nieuwe Merwede:  21% of the Waal discharge is flowing into the flood channel (2157 m3/s).  At the bifurcation between the Beneden- and Nieuwe Merwede, relatively more water is flowing towards the Beneden-Merwede than to the Nieuwe Merwede (36% of the Boven Merwede discharge in reference, 39% in situation with flood channel).  The water that is flowing from the Nieuwe Merwede to the Biesbosch through Noordwaard and NOP is 47.6 % percent lower (1073 m3/s versus 2046 m3/s). As a result, the discharge decrease in the Nieuwe Merwede before the inlet of the Noordwaard is larger than after the inlet.

Maas/Amer:  The outflow flood channel discharges 1283 m3/s towards the Bergse Maas. A part of this discharge flows to the Biesbosch (162 m3/s), but the main part flows towards the Amer (1119 m3/s).  The inflow from the Biesbosch into the Amer has been increased, but is in percentage nearly equal when compared to the total Amer discharge (6.3% versus 6.1% of the total inflow).

Flood channel:  The largest part (1283 m3/s, 59%) of flood channel discharge is flowing towards the Maas, and the other part (874 m3/s, 41%) is flowing towards the Biesbosch.

Biesbosch:  The total inflow in the Biesbosch is 2480 m3/s, which is 2.6% higher. The discharge increase is small, but the composition totally changed: - Noordwaard and NOP: 43.3% versus 84.6% in reference - Bergse Maas 21.5% versus 15.4% in reference - Flood channel 35.2% (not in reference)  Despite the increased inflow, the outflow from the Biesbosch to Hollands Diep decreased with 0.6%. The extra inflow into the Biesbosch flows to the Amer through the smaller channels.

78 January 2013 Hydraulic analysis of a flood channel

Hollands Diep:  The total inflow in Hollands Diep is 10672 m3/s. Where in the reference situation the major contribution came from the Nieuwe Merwede, in the situation with flood channel the Amer is responsible for the major contribution. The changed composition of the flow into Hollands Diep is as follows: - Nieuwe Merwede 35.6% versus 43.8% in reference - Amer 43.7% versus 35.6% in reference - Biesbosch 20.5% versus 21.8% in reference  The inflow into Hollands Diep is 5.4% more (551 m3/s). This discharge increase is equal to the discharge lowering at the Beneden Merwede.

13000 and 18000 m3/s Rhine discharge In Figure 5-5 and Figure 5-6, the discharge distribution for a 13000 m3/s and 18000 m3/s Lobith discharges are presented. The results will be compared with the observations for the 16000 m3/s Lobith discharge.

Figure 5-5 Discharge distribution water system with flood channel compared to the reference at a 13000 m3/s Rhine discharge

Figure 5-6 Discharge distribution water system with flood channel compared to the reference at a 18000 m3/s Rhine discharge

Observations based on the figures above:  A higher Waal discharge leads to a proportional higher flood channel inflow. At a 13000 m3/s Lobith discharge, 18% of the Waal discharge flows into the flood channel, and at a 18000 m3/s Lobith discharge 23% of the Waal discharge flows into the flood channel

79 Hydraulic analysis of a flood channel January 2013

 The Noordwaard discharge decreases with increasing Waal discharge. At a 13000 m3/s Lobith discharge, the decrease in discharge compared to the reference is 44%, while for an 18000 m3/s Lobith discharge the decrease is nearly 52%.  The flow from the flood channel that flows through the southern outlet channel, and still flows to the Biesbosch increases with the increasing Rhine discharge. At a 13000 m3/s Rhine discharge, all the water from the southern channel flows towards the Amer, while at an 18000 m3/s Rhine discharge, 26% of the flow from the southern channel still flows towards the Biesbosch.  At increasing Rhine discharge, the relative inflow of the Nieuwe Merwede into Hollands Diep and Amer decreases, while the relative inflow from the Biesbosch increases. See graph below.

Figure 5-7 Discharge fraction inflow Hollands Diep

Summary The most important observations from this paragraph are mentioned below:  The flood channel’s discharge is 18% of the Waal’s discharge at a Rhine discharge of 13000 m3/s, and 23% of the Waal’s discharge at an 18000 m3/s Rhine discharge.  The largest part (59%) of flood channel discharge is flowing towards the Maas, and the other part (41%) is flowing towards the Biesbosch. However, a part of the flow towards the Maas through the southern channel still flows towards the Biesbosch at high Rhine discharges (with a maximum of 26% of the discharge in the southern channel).  The discharge through the Noordwaard decreases significantly due to the flood channel. This decrease becomes larger at increasing Rhine discharge, and varies between 44% and 52%.  The total inflow into the Biesbosch increases slightly due to the flood channel, but the composition totally changed. The inflow from the Noordwaard decreases, while the inflow from the Bergse Maas increases. Furthermore, the flood channel has a significant contribution.  The composition of the inflow into Hollands Diep changes after implementation of the flood channel. The inflow from the Nieuwe Merwede decreases, while the inflow from the Amer increases. The inflow from the Biesbosch decreases slightly.

5.3 Results river system

In this paragraph, the results of the river system with the flood channel will be compared to the reference situation.

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This paragraph has the following structure: 2.3.1 Results Waal – Boven Merwede – Nieuwe Merwede 2.3.2 Results Maas – Amer 2.3.3 Two-dimensional results river system 2.3.4 Short summary of the results

5.3.1 Waal – Boven Merwede – Nieuwe Merwede

In this paragraph, the water levels and velocities along the river axis of the Waal, Boven Merwede and Nieuwe Merwede will be discussed. The results for a 16000 m3/s Rhine discharge will be presented first. At the end, these results will be compared to the results of a 13000 m3/s and 18000 m3/s Rhine discharge. Note that the water level decrease at the Waal also includes the Room for the River measure Munnikenland.

Observations In Figure 5-8, the maximum water level with and without flood channel is presented for a 16000 m3/s discharge at Lobith. The difference between the two water levels is indicated by the green area plot. The corresponding velocities are presented in Figure 5-9.

Figure 5-8 Maximum water level along the Waal at Lobith discharge of 16000 m3/s

Figure 5-9 Velocity along the Waal at Lobith discharge of 16000 m3/s

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The inlet of the flood channel is located at Waal river kilometer 949/950. The start of the Nieuwe Merwede and Beneden Merwede is located at river kilometer 962. The inlet of the Noordwaard is located at river kilometer 964. At river kilometer 980, the Nieuwe Merwede confluences with the Amer into the Hollands Diep.

From Figure 5-8 and Figure 5-9, the following can be observed (situation with flood channel compared to the reference): 914 – 949 km From kilometer 914 in downstream direction, the water level difference increases gradually to –0.96 meters at kilometer 949, which is the start of the flood channel. In the same river section, the flow velocity increases to a maximum difference of 0.25 m/s at kilometer 949. 950 – 963 km Directly downstream of the inlet of the flood channel, the velocity in the Waal drops with 0.8 m/s, and becomes 0.2 m/s lower than the reference. From there, the velocity difference increases to -0.45 m/s. In the same section, the water level difference increases from –0.96 meter to –0.26 meter at kilometer 963. 964 – 980 km At kilometer 962, the Boven Merwede splits in the Nieuwe Merwede, and at kilometer 964, the inlet of the Noordwaard is located. From kilometer 963 to 966, the velocity drops with 0.6 m/s, and at kilometer 967, the velocity is 0.23 m/s lower than the reference (instead of 0.45 m/s at kilometer 963). Downstream of this point until kilometer 980, the velocity difference stays constant. The water level difference changes from –0.26 meters at kilometer 963 to +0.04 meters at kilometer 980.

In the figure below, the water level difference along the river axis of the Waal for three different discharges is presented.

Figure 5-10 Water level difference Waal due to the flood channel for three discharges at Lobith

The results from Figure 5-10 confirm the previous water level observations based on Figure 5-8 and Figure 5-9, only the maximum water level difference is different (0.75 meters for a 13000 m3/s Rhine discharge, and 1.10 meters for a 18000 m3/s Rhine discharge). Between kilometer 950 and 963, the flood channel has relatively more effect on the water level slope than on other locations.

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Explanation The observations are explained below: 914 – 949 km Due to the flood channel, the water level decreases, resulting in a backwater curve upstream of kilometer 949. Due to this curve, the water level is lower, but the Waal discharge is equal, which results in a velocity increase. For a constant discharge and river width, and a decreasing water level, the flow velocity must increase, because Q bhU (Q=discharge, b= river width, h = water depth, U=flow velocity). 950 – 963 km The Waal discharge downstream of kilometer 950 is lower than in the reference due to the flood channel, which results in a lower water level and lower flow velocity. The flow velocity has been analyzed in more detail, see NOTE1.

At high discharges, the water level in section 950-963 is lowered by the backwater effect of the Noordwaard. This is partly the reason for the increased flow velocity in the reference. Due to the less effective Noordwaard (see paragraph 5.2.2) in the situation with flood channel, the backwater effect is smaller, and the flow velocity increase is smaller.

The water level differences decrease between kilometer 950 and 963, while the river discharge difference with the reference is equal. The reason for the water level difference decrease is the decreased backwater effect from the Noordwaard, and larger influence from the sea (Hollands Diep), because the section lies in the transition area (see paragraph 4.4.1).

The water level at river kilometer 950-963 is more sensitive to discharge changes than the river downstream of this point. This is mainly caused by the hydraulic bottleneck between Gorinchem and Werkendam. See NOTE2 for more details. 964 – 980 km After the inlet of the Noordwaard, the river discharge is lower than in the reference situation, but the water level difference is small compared to the reference due to the influence of Hollands Diep. Therefore, the flow velocity is lower than in the reference. The water level increase at kilometer 980 is caused by the larger total discharge into Hollands Diep (less discharge into the Beneden-Merwede).

NOTE1: Flow velocity decrease kilometer 950-963 The lower velocity compared to the reference in section 950-963 can be explained by the Chezy formulation for uniform flow:

Q Q bhC Ri&() U   U  C Ri  C hi alsb  h bbh b b Where: Q = discharge [m3/s] U = flow velocity [m/s] h = water depth [m] b = flow width [m] C = Chezy coefficient [m1/2/s] R = Hydraulic radius [m]

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ib = Bottom level slope [-]

The formulation above is valid for uniform flow, when the bottom level slope is equal to the water level slope. In river section 950-963, the water level slope is steeper than the bottom 0.5 level slope. If uniform flow is assumed, than U ≈ (hib ) . Because h has decreased and ib is assumed to be constant, U must decrease (U1 in Table 5-1). If the bottom level slope is replaced by the (varying) water level slope, U increases even more (U2 in Table 5-1).

C h ib iwl U1 U2 [m0.5/s] [m] [-] [-] [m/s] [m/s] Reference 46 11.03 1.2*10-4 2.25*10-4 1.67 2.29 Incl. flood 46 10.27 1.2*10-4 1.84*10-4 1.61 1.99 channel Difference 0.76 0.06 0.30

Table 5-1 Approximation velocity based on a Chezy equation at river kilometer 955. U1 is flow velocity based a (constant) bed level slope, U2 is velocity based on the water level slope

The velocity difference for the approximation including the varying water level slope is approximately equal to the observed velocity difference.

NOTE2: Sensitivity discharge changes kilometer 950-963 The water level between river kilometers 950 and 963 seems to be more sensitive to discharge changes than the water level downstream of kilometer 963 (see Figure 5-10). When the Q-h relation of kilometer 956 is compared to Q-h relation of kilometer 964, this observation is confirmed by the difference in slope.

Figure 5-11 Q-H relation river kilometer Waal 956 and Nieuwe Merwede 964. At high discharges, slope Q- H relation of Waal 956 is steeper than Nieuwe Merwede 964, thus more sensitive to discharge changes.

The steeper slope of the Q-h relation at kilometer 956 compared to kilometer 964 is mainly caused by the hydraulic bottleneck between Gorinchem and Werkendam.

5.3.2 Maas – Amer

In this paragraph, the water levels and velocities along the river axis of the Maas and Amer will be discussed. The results for a 16000 m3/s Rhine discharge (3504 m3/s Maas discharge) will be presented first. At the end, these results will be compared to the results of a 13000 m3/s and 18000 m3/s Rhine discharge.

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Observations In Figure 5-12, the maximum water level with and without flood channel is presented for a 16000 m3/s discharge at Lobith. The difference between the two water levels is indicated by the green area plot. The corresponding velocities are presented in Figure 5-13.

Figure 5-12 Maximum water level along the Maas at Lobith discharge of 16000 m3/s

Figure 5-13 Velocity along the Maas at Lobith discharge of 16000 m3/s

The flood channel ends in the Maas at river kilometer 249/250. At kilometer 262, the Amer and Nieuwe Merwede confluences in Hollands Diep.

From Figure 5-12 and Figure 5-13, the following can be observed (situation with flood channel compared to the reference): 212 - 249 km From kilometer 212, the water level difference increases to a maximum of 0.44 meter, and the velocity difference increases to a maximum of –0.2 m/s, both maximums at kilometer 249. 250 - 260 km Downstream of the confluence with the flood channel, the water level differences decrease until zero at Hollands Diep. The velocity difference between kilometer 252 and 260 is constant and approximately 0.3 m/s.

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In Figure 5-14, the water level differences along the axis of the Maas and Amer are presented for different discharges at Lobith.

Figure 5-14 Water level difference Maas-Amer due to the flood channel for three discharges at Lobith

The results from Figure 5-14 confirm the earlier observations. The unexpected difference is that the water level difference for the 18000 m3/s Lobith discharge is approximately equal to the 16000 m3/s Lobith discharge (both 44 centimeters).

Explanation The observations are explained below: 212 - 249 km The water level at kilometer 249 is increased due to the extra discharge from the flood channel. Upstream of this point, the water level difference and velocity differences are caused by the backwater curve. 250 - 260 km Downstream of kilometer 250, the water level difference at the Amer decreases fast due to the relative constant water level at Hollands Diep. The raised flow velocity is caused by the combination of the increased discharge and fixed water level downstream.

Why is the water level increase equal for the 16000 m3/s and 18000 m3/s discharge at Lobith? The maximum water level increase at the Maas is approximately equal for the 16000 and 18000 m3/s Rhine discharge as showed in Figure 5-14. The discharge results in paragraph 5.2 already showed that the discharge from the Bergse Maas to the Biesbosch increased with increasing Rhine discharge. In the table below, some water levels and discharges are presented.

Discharge Including flood channel Reference Difference at Lobith Q [m3/s] h [m+NAP] Q [m3/s] h [m+NAP] Q [m3/s] h [m] [m3/s] 13000 3585 2.649 2680 2.271 905 0.38 16000 4379 3.202 3260 2.765 1119 0.44 18000 4838 3.536 3648 3.099 1190 0.44

Table 5-2 Discharge Amer and water levels at river kilometer 249

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At the Amer, the discharge increase compared to the reference for a 16000 m3/s Rhine discharge is 1119 m3/s, while the increase for an 18000 m3/s Rhine discharge is 1190 m3/s. The difference is 71 m3/s. However, the discharge difference at the Amer between the 13000 m3/s and 16000 m3/s Rhine discharge is 214 m3/s, which is twice as large (scaled to difference in Rhine discharge). The relatively small discharge increase, in combination with an increased flow velocity, could explain why the water level difference is the same for the 16000 m3/s and 18000 m3/s Rhine discharge.

5.3.3 Two-dimensional figures

In the figures below, the water level and flow velocity for a 16000 m3/s discharge at Lobith is presented. The water flows from east to west

Figure 5-15 Water level at a 16000 m3/s discharge at Lobith

Figure 5-16 Flow velocity at a 16000 m3/s discharge at Lobith

The figures above confirm the earlier observations based on the plots along the axis. The differences of the water level and flow velocity with the reference situation are presented in the figures below.

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Figure 5-17 Water level difference (situation with flood channel minus reference, but flood channel is not plotted)

Figure 5-18 Velocity difference(situation with flood channel minus reference, but flood channel is not plotted)

The water level difference plot shows decreased water levels along the Waal and Merwede, and increased water levels along the Maas and Amer. In the north side of the Biesbosch, the water level has decreased while the water level on the south side of the Biesbosch has increased. This is the case for the flow velocities as well. Also, note the increased flow velocities in the Waal upstream of the inlet of the flood channel, and the increased flow velocities downstream of the outlet of the flood channel.

5.3.4 Summary

The water level effects due to the flood channel are summarized in Table 5-3. Furthermore, results show that:  Along the Waal, downstream of the inlet of the flood channel, the flood channel is most effective between kilometer 950 and 963. Downstream of kilometer 963, the water level difference compared to the reference decreases due to the increasing influence of the sea (Hollands Diep).  Along the Maas, the water level increase due to the flood channel is influenced by the water level at Hollands Diep. Therefore, the water level decreases relatively fast.

Discharge at Lobith Maximum water level Maximum water level Discharge flood

3 difference Waal difference Maas channel [m /s] [m] [m] [m3/s] 13000 -0.75 0.38 1511 16000 -0.96 0.44 2157 18000 -1.10 0.44 2707

Table 5-3 Summarized results in the situation with flood channel compared to reference

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5.4 Results flood channel

The schematization of the flood channel has been implemented in the RMM-ref2015 schematization. In this paragraph, the results of the flood channel will be presented.

First, a short introduction will be given with respect to the results (5.4.1), and the water levels along the axis of the flood channel will be presented in paragraph 5.4.2. Subsequently, the results of the inlet (5.4.3), main channel (5.4.4) and outlet (5.4.5) will be presented. Finally, a summary of the results will be given (5.4.6).

5.4.1 Introduction

The flood channel is connected to the Waal, Biesbosch and Maas. The results of the flood channel will be presented in three parts, the inlet, main channel and the outlet, see the figure below.

Figure 5-19 RMM-ref2015 model (including Room for the river measure Munnikenland) with the design of the flood channel by Robbert de Koning as background image

The inlet structure is located next to the Afgedamde Maas. The outlet structure is located south of Nieuwendijk. The inlet is the area between the Waal and the inlet structure. The main channel is the area between the inlet structure and the outlet structure. The outlet is the area between the outlet structure and the Biesbosch/Maas.

In the two-dimensional figures, an optimal scale is chosen in order to see small differences. Therefore, it is possible that some parts have a value outside the value range of the legend. In that case, the area is in the figure is white.

Only the results for the 16000 m3/s Rhine discharge will be discussed, because the 13000 m3/s and 18000 m3/s Rhine discharge show similar results (same patterns, other values).

5.4.2 Water level along axis flood channel

In this paragraph, the water level and flow velocities along the axis of the flood will be presented for 16000 m3/s Rhine discharge. Because other Rhine discharges show the same patterns, they are not mentioned.

Observations The water level and flow velocity for the flood channel at a Lobith discharge of 16000 m3/s are presented in Figure 5-20.

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Figure 5-20 Maximum water level along the flood channel at Lobith discharge of 16000 m3/s

The water flows from the Waal (river kilometer 0) to the inlet construction (river kilometer 3). The outlet construction is located at 12.3 km. From there, the channel splits into two channels, the West and South channel. The West channel flows to the Biesbosch, and ends at kilometer 21. The South channel flows to the Maas, and ends at kilometer 19.7.

From the figure above, the following can be observed: 0 – 3 km Between the Waal and the inlet structure, the water level decrease is 26 (Inlet) centimeters (water level slope 0.87*10-4). The flow velocity decreases from 2 m/s in the Waal to 0.4 m/s in the flood channel and increases at the inlet construction to 0.8 m/s. 3 – 12 km Between the inlet and outlet structure, the water level decrease is 35 (Main channel) centimeters (water level slope 0.37*10-4). The velocity increases gradually from 0.30 m/s just after the inlet structure to 0.78 m/s at the outlet structure. The velocity peak at the outlet structure is approximately equal to the peak at the inlet structure. However, the peak is relatively low compared to the flow velocity before and after the inlet structure. 12 – 21 km In the West channel, the water level decrease is 1.81 meter (water level (West channel) slope 2.0*10-4). The water level line changes direction at kilometers 17.3 and 18.8. The first channel, at kilometer 17.3, is at the location of the connection channel between the west and south channel. The second channel, at 18.8 kilometer, is at the location where the west channel widens. The flow velocity increases from 0.65 m/s just after the outlet structure to 0.97 m/s at kilometer 17.3. From there, the flow velocity decreases, until the water flows into the Biesbosch, where the flow velocity increases again. 12 – 20 km In the South channel, the water level decrease is 1.45 meter (water level (South channel) slope 1.9*10-4). The water level line changes direction at kilometer 17.8, where the channel flows into the flood plains of the Maas. From there, the water level is quite constant. The flow velocity increases from 0.59 m/s just after the outlet structure to 0.96 m/s at kilometer 17.3. From there, the velocity drops 0.5 m/s, and increases again when the water flows into the Maas.

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Explanation The observations are explained below: 0 – 3 km A part of the water from the Waal flows calmly towards the inlet structure, (Inlet) where the flow converges and accelerates due to the inlet structure. 3 – 12 km In the main channel, the acceleration is probably caused by the decreasing (Main channel) flow width slightly increasing bottom level. The relatively low velocity peak compared the velocity before and after the structure could be partly explained by the geometry. The flow width before and after the outlet structure is nearly equal to the flow width at the outlet structure, while this difference at the inlet structure is larger. 12 – 21 km The water level line of the west and south channel has the shape of a (West channel) backwater curve, which is confirmed by the, in downstream direction, & increasing flow velocity and steep water level slope. The reason for the 12 – 20 km backwater curve is expected to be a combination of a (too) small channel (South channel) width and relatively high roughness. This backwater curve is expected to have a negative effect on the flood channel’s discharge.

2D water level and flow velocity plot In the figures below, the water level and flow velocities of the flood channel and surrounded water system are presented for a 16000 m3/s Rhine discharge.

Figure 5-21 Water level in flood channel at a 16000 m3/s Rhine discharge

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Figure 5-22 Velocity in flood channel at a 16000 m3/s Rhine discharge

The figures confirm the observations made by the analysis of the water levels along the axis. Note the water level slope of the flood channel compared to the water level slope in Waal, Maas and Biesbosch. The water level in the main channel is quite constant, while the water level slope in the outlet channels is steep, with relatively high velocities. Figure 5-22 makes clear the difference in flow velocities between the Waal and Maas. In the following paragraphs, the two- dimensional plots are described in more detail.

5.4.3 Inlet

The results of the inlet will be discussed below. The observations are based on figures, which are presented at the next pages.

Most important observations  In Figure 5-23, the upstream water level in the Waal is 5.4 meters above NAP, while the downstream water level in the Waal is 4.8 meters above NAP. The water level at the inlet structure is 4.88 meters above NAP. Due to the water level difference, a part of the Waal discharge, mainly the part flowing in the Waal’s flood plains, flows southwards into Munnikenland (Figure 5-24).  In Munnikenland, which is the area between the Afgedamde Maas and the Waal, the main flow is in southwest direction towards the Afgedamde Maas. At the Afgedamde Maas, the water level has a local maximum, and the flow divides in two parts, a flow in northern direction that flows back to the Waal, and a flow in southern direction that flows towards the inlet structure. The water that flows back to the Waal enters the Waal east of fort Loevestein (white area in Figure 5-24)  The effect of the quay at the east side of the Afgedamde Maas is clearly visible in Figure 5-23, where the water level iso-line is located at the location of this quay. The height of this quay varies between 2.5 and 4 meters above NAP, and at the lowest point of the quay, the water level of the Afgedamde Maas has a maximum.  The flow in the flood plain before the inlet structure comes mainly from the north, the direction of he local maximum water level in the Afgedamde Maas. The flow from the eastern direction is much smaller.  The natural area just before the inlet structure has a relatively high roughness, and the flow velocities in that area are small (see Figure 5-25 and Figure 5-28).  At the location of the removed farm and changed dike location, east of the inlet structure (see B and C in Figure 4-26), the flow velocities are negligible. This also the case for the flow velocities at the ‘island’ before the sluices.

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Explanation The observations are explained below:  At the location in the Afgedamde Maas where the flow divides, the water level is maximal, and the quay between Munnikenland and the Afgedamde Maas is at its lowest point. This quay has a significant influence on the flow, and hinders the flow from Munnikenland towards the inlet structure.  The high roughness due to the natural area just before the inlet structure hinders the flow from the Afgedamde Maas towards the inlet structure. This is probably a part of the reason for the disproportional velocity distribution at the inlet structure (Figure 5-14).  Due to the low flow velocities, the removed farm and changed dike location east of the inlet structure are expected to have an insignificant influence on the flow towards the inlet structure. The lowered ‘island’ before the sluices could have probably more influence, but because the main flow is coming from the north, the influence is expected to be small.

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Figure 5-23 Inlet; water level with iso-lines and weir locations

Figure 5-24 Inlet; water level with iso-lines and flow direction

Figure 5-25 Inlet; flow velocity and direction

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Figure 5-26 Inlet; bottom level with iso-lines

Figure 5-27 Inlet; bottom level and weir heights

Figure 5-28 Inlet; Chezy roughness and flow direction

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5.4.4 Main channel

The results regarding the main channel will be discussed below. The observations are based on figures, which are presented at the next pages.

Most important observations  The water level difference between the inlet structure and the outlet structure is approximately 35 centimeters. The water level slope has maxima after the inlet structure and before the outlet structure. In the area in between, the water level slope is gentle.  The flow velocity in the channel varies between 0.3 m/s and 0.8 m/s. The maximum velocity at the inlet structure is 1 m/s, while the maximum velocity at the outlet structure is 0.9 m/s. The flow velocity decreases at the places where the channel becomes wider, and vice versa.  In the last kilometers before the outlet structure, the velocity increases from 0.4 m/s to 0.7 m/s (Figure 5-31).  In the last kilometers of the channel, the width becomes smaller, and the bottom level increases slightly (Figure 5-32).  After the inlet structure, a wood parcel is located with a higher roughness than the surrounding area (see Figure 5-34). This parcel has a significant negative influence on the discharge. The local flow velocity is 0.41 m/s instead of the 0.66 m/s.  The two bulges on the north side of the channel and the bulge on the southeast side are not part of the main flow.  The flow width at the outlet structure is approximately equal to the flow width before the outlet structure. The velocity increase at the outlet structure is approximately 0.2 m/s. The water level difference before and after the outlet structure is approximately 5 centimeters.

Explanation The observations are explained below:  In downstream direction, the flow velocity increases. The expected reason for this increase is the decreasing flow width and slightly increasing bottom height.  Because the flow velocity at the wider parts of the channel is lower than the smaller parts, a channel with a width equal to the width at the end would probably have the same discharge capacity. The bulges do not have any effect on the discharge; they only decrease the flow velocity.  The wood parcel just after the inlet structure forms a significant obstruction; it would be better for the flow to remove this parcel.  Based on the observations above the outlet structure is a small obstruction, but is not the main reason for the gentle water level slope of the main channel. This is because the water level decrease due to the structure (5 cm) is small compared to the total water level decrease in the channel (35 cm).

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Figure 5-29 Main channel; water level with iso-lines and weir locations

Figure 5-30 Main channel; water level with iso-lines and flow direction

Figure 5-31 Main channel; flow direction and velocity

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Figure 5-32 Main channel; bottom level with iso-lines

Figure 5-33 Main channel; bottom level and weir heights

Figure 5-34 Main channel; Chezy roughness and flow direction

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5.4.5 Outlet

The results with respect to the outlet channels will be discussed below. The observations are based on figures presented at the next pages.

Most important observations  The water level decrease in the south channel is 1.45 meters, and 1.80 meters in the west channel. The water level in the channel that connects the west and south channel does not have a water level slope.  After the outlet structure, the flow splits, and a part of the water flows into the south channel, and a part into the west channel. The flow in the west channel is more concentrated to the creek, while the flow in the south channel is spread over the whole width, see Figure 5-37. Compared to the main channel, the flow velocities are relatively high.  In the west channel, the water level slope is gentle until the sharp turn west of Nieuwendijk, and becomes steep after this sharp turn. The bottom level after the turn is higher than before the turn.  A part of the flow through the south channel enters the flood plain of the Maas, and subsequently flows northwards through Spijkerboor  There is no flow in the area between the outlet of the south channel and the A27 highway (east of Aakvlaai).

Explanation The observations are explained below:  The channel that connects the west channel with the south channel does not have a hydraulic function, because the discharge through the canal is approximately zero. This is also the case for the area east of Aakvlaai.  The increasing flow velocities and steep water level slope in the west and south canal indicate a backwater curve. This could be caused by a too small flow area or high roughness of the channels.  In the west channel, the flow is more concentrated to the creeks. This is probably caused by the higher bottom level, compared to the south channel. Lowering the bottom level could probably increase the flow through the west channel.  A part of the flow that flows through the south channel still flows northwards towards the Biesbosch as well. This implicates that the discharge capacity of the west channel is too small.

99 Hydraulic analysis of a flood channel January 2013

Figure 5-35 Outlet channels; water level with iso-lines and weirs

Figure 5-36 Outlet channels; water level with iso-lines and flow direction

Figure 5-37 Outlet channels; flow velocity and direction

100 January 2013 Hydraulic analysis of a flood channel

Figure 5-38 Outlet channels; bottom level with iso-lines

Figure 5-39 Outlet channels; bottom level and weir heights

Figure 5-40 Outlet channels; Chezy roughness and flow direction

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5.4.6 Summary

In the previous paragraph, the flow through the flood channel has been discussed. The most important results are:  The flow towards the inlet structure is hindered by the quay on the east side of the Afgedamde Maas and the natural area in front of the structure. The dike replacement east of the Afgedamde Maas and the lowered ‘island’ before the sluices do not have a significant hydraulic function.  In the main channel, the wood parcel just behind the inlet is a significant flow obstruction. Furthermore, the bulges do not have an effect on the flood channel’s discharge. Due to the high water level at the outlet structure, the water level slope in the main channel is small.  In the outlet channels, there is a backwater curve, and therefore, they have a large water level slope. The cause of the backwater curve is expected to be a combination of geometry and roughness. This backwater curve is expected to have a significant negative influence on the flood channel’s discharge. Furthermore, the discharge capacity of the west channel is too small. Therefore, water that flows through the south channel still flows to the Biesbosch, after entering the flood plains of the Maas. The area east of Aakvlaai does not have a hydraulic function.

5.5 Two-dimensional versus one-dimensional approach

In chapter 3, the flood channel has been schematized in a one-dimensional approach. In this paragraph, the results of the WAQUA model (two-dimensional approach) are compared to the results of the one-dimensional approach. The results are described here briefly.

In the figure below, the water level along the river axis of the Waal of the one-dimensional approach and the two-dimensional approach are presented.

Figure 5-41 Water level Waal-Nieuwe Merwede after implementation flood channel; WAQUA model versus model one-dimensional approach

Between river kilometers 932 and 980, the water level fits quite well, with differences between -0.11 and 0.25 meter. The average water level of the WAQUA model is a few centimeters higher. The maximum water level decrease in the Waal due to the flood channel is 0.96 meter in the WAQUA model, and 0.97 meter in the one-dimensional model, a negligible deviation.

102 January 2013 Hydraulic analysis of a flood channel

Figure 5-42 Water level Maas-Amer after implementation flood channel; WAQUA model versus model one-dimensional approach

In the figure above, the water level differences between the one- and two-dimensional approach for the Maas and Amer are presented. The water level difference is approximately equal to the water level difference at the Waal. The water level increase due to the flood channel in the WAQUA model is 44 centimeters, and the increase according to the one-dimensional model is 38 centimeters (-14%).

In Figure 5-43, the water levels of the flood channel are compared.

Figure 5-43 Water flood channel; WAQUA model versus model one-dimensional approach

The water levels fit quite reasonable, and the shapes are similar, with the gentle water level slope in the main channel, and the steep water level slope in the outlet channels. At the inlet structure, and in the outlet channels, there are some deviations, probably caused by the difference in detail of the schematizations. The discharge through the flood channel shows similar results. At a Rhine discharge of 16000 m3/s, the flood channel in the one-dimensional approach discharges 1957 m3/s, and in the WAQUA model 2157 m3/s, a difference of 200 m3/s, which is 9.3% of the WAQUA flood channels discharge.

In summary, the results of the one-dimensional approach give a reasonable estimation of the expected changes due to the flood channel. The reason for the difference can be found in the

103 Hydraulic analysis of a flood channel January 2013

simplified one-dimensional schematization with many assumptions. Based on the comparison above, it can be concluded that the one-dimensional approach gives a good estimation of the order of magnitude of the results, with deviations in the order of 10-15%. Therefore, the results of the sensitivity analysis performed in the one-dimensional approach could be useful by analyzing the two-dimensional results.

5.6 Design review

5.6.1 Model results versus expected results

Study Integrale Verkenning Benedenrivieren The expected water level decrease due to the flood channel design by Robbert de Koning is based on the Study Integrale Verkenning Benedenrivieren (IVB). As already mentioned in chapter 2, the maximum expected water level decrease at the Waal due to the flood channel is 1.30 meter for a flood channel width of 600 meters [IVB, 2000]. This water level decrease is calculated for a Rhine discharge of 16000 m3/s, and for a reference situation without the Room for the River measures. At the Maas, the expected water level increase is 0.54 meter. The results from the RMM-model show a maximum water level decrease of 0.96 meter at the Waal for a 16000 m3/s Rhine discharge, which is much lower (-26%) than the expected results based on the IVB study. This decrease is including the Room for the River measure Munnikenland (11 centimeters at 18000 m3/s Rhine discharge). The water level increase at the Maas is 0.44 meters, 18.5% lower than expected. The smaller water level differences indicate a lower flood channel discharge.

There could be various reasons for the difference in expected water level, but unfortunately, the way the flood channel is schematized in the IVB study is unknown. Reasons for the water level difference could be the different reference of the models, the different schematization of the flood channel in the IVB study, the different start and end locations of the flood channel, the different model type (IVB is one-dimensional) etc. However, it can be concluded that the water level decrease due to the modelled flood channel is significantly lower than the expected results based on the IVB study, and that the exact reason is unknown.

Uniform flow equation Based on the dimensions of the main channel, and bottom/water level slope of the total flood channel, the flood channel’s discharge at uniform flow could be estimated by the Chezy equation:

Q bhC Rib Where: Q = discharge [m3/s] h = water depth [m] b = flow width [m] C = Chezy coefficient [m1/2/s] R = Hydraulic radius [m]

ib = Bottom level slope [-]

For the width (b), the estimated flow width of the main channel is used. The water depth varies between 4 meters at the inlet construction, 6 meters in the main channel to 3 meters in the

104 January 2013 Hydraulic analysis of a flood channel

outlet channels. An averaged value of 4.5 meters has been used. The roughness is based on the observations in the main channel for a 16000 m3/s Rhine discharge.

Two calculations have been performed, one based on the bottom level slope, and one based on the water level slope between the Waal and Maas at a 16000 m3/s Rhine discharge.

b [m] h [m] C [m1/2/s] I [-] Q [m3/s]

1 Based on average bottom level 600 4.5 42 0.000107 2476 slope main channel

2 Based on water level difference 600 4.5 42 0.000097 2359 between Waal and Bergse Maas

Table 5-4 Discharge calculation flood channel by Chezy equation

The results show a discharge of 2476 m3/s for the calculation based on the bottom level slope in the main channel, and 2359 m3/s based on the water level slope. The bottom level slope of the main channel is steeper than the bottom level slope of the outlet channels. Hence, the calculated discharge based on the bottom level slope of the main channel is only possible if the width of the outlet channels increases in order to compensate the lower bottom slope. Without those adaptations, the expected discharge through the flood channel in the case that the flood channels have the same roughness and width as the main channel is 2359 m3/s, based on the water level slope. So theoretically, if the outlet channels have the same width and roughness of the main channel, and the influence of the inlet- and outlet structures on the discharge is negligible, a flood channel discharge of 2359 m3/s should be possible. This is approximately 200 m3/s more than calculated with the WAQUA simulation. With an adapted width of the outlet channels, the discharge increase could be more than 100 m3/s extra.

Figure 5-44 Q-h relation Waal kilometer 949

In the figure above, the discharge-water level relationship of the water level in the Waal near the inlet of the flood channel is presented. Based on the Q-h relation above, the water level decrease due to the extra flood channel discharge could be calculated. The water level decrease due to the extra flood channel discharge of 200 m3/s is 6.5 centimeters. If this extra decrease could be achieved with small design changes, it could be worth the extra investment.

5.6.2 Review design parameters

In paragraph 2.2, the design parameters for a flood channel have been presented. They will be used to verify the parameters of the schematized flood channel.

Location and bottom level The bottom level of the flood channel is determined by the location choice. In an ideal situation, the bottom level slope of the flood channel is larger than the slope of the main river. In the

105 Hydraulic analysis of a flood channel January 2013

description of the design guidelines, it is assumed that the flood channel starts and ends at the same river. In that case, the water level of the river is related to the bed level (approximately equal water depth). However, the flood channel discussed for this study forms a connection between two rivers. Therefore, there is no relation between the water depth upstream and downstream of the flood channel, and the main force behind the flood channel’s discharge is the water level difference and not the bed level difference. The bed level slope of the flood channel is approximately 0.5*10-4, and for a 16000 m3/s, the water level slope is 1.0*10-4. The bed level slope in the Merwede and Maas is approximately 0.5*10-4. Because the average bed level slope of the flood channel is equal to the slope in the main rivers, and the water level slope is even larger, the flood channel is located at a good location, according to the guidelines.

Geometry and roughness In an ideal situation, the width of the channel would be wider than necessary, and the channel would widen near the outlet. The channel should not narrow just behind the inlet structure, and also the roughness behind the inlet structure should not be too high. When the roughness increases, the channel width should also increase. In the modelled flood channel, the width of the main channel increases slightly after the inlet structure. However, there is a rough area located behind the inlet. The width of a large part of the main channel is too wide, which is in accordance with the guidelines. In the outlet channels, the total width is approximately equal to the width in the main channel, but the roughness is higher. This is not according to the design rules, because the width should increase when the roughness increases. Furthermore, the outlet channels widen near the connection with the Maas and Biesbosch. In summary, the design is not completely in accordance with the design rules, as the results have already shown.

5.6.3 Proposed design improvements

Based on the results from the previous paragraphs, some proposals to improve the design of the flood channel, from a hydraulical point of view, will be presented.

Figure 5-45 Design inlet flood channel

Proposed design improvements inlet flood channel:  The dike replacement, indicated with B in Figure 5-45, and ground level lowering, indicated with C in the same figure, do not have a significant contribution to the flood channel’s discharge, and therefore it is advised to skip the execution of those measures.  The natural area just before the inlet structure hinders the inflow due to the high roughness. Therefore, it is advised to perform measures to lower the roughness of this area.  The quay east of the Afgedamde Maas hinders the flow towards the inlet structure. Therefore, it is advised to lower this quay.

106 January 2013 Hydraulic analysis of a flood channel

Figure 5-46 Design ‘main’ part flood channel

Proposed design improvements main part flood channel:  The two bulges on the north side of the main channel, and the bulge south west of the main channel, do not contribute to the flood channel’s discharge. Therefore, it is not necessary to create them.  The wood parcel just behind the inlet structure hinders the flow significantly. In order to improve the flow and increase the flood channel’s discharge, it is advised to lower the roughness of this parcel.

Figure 5-47 Design outlet channels flood channel

Proposed design improvements outlet flood channel:  The backwater curve in the outlet channels indicates too much flow resistance. According to the sensitivity analysis performed at the one-dimensional approach, widening the channels could lower the water level in the Waal with 7 centimeters (see paragraph 3.6). Therefore, it is advised to increase the width and/or lower the roughness of the channels.  The outlet channels do not distribute the flood channels discharge according to the capacity of the Biesbosch and Maas. The channels should be redesigned in order to distribute the flood channel’s discharge according to the capacity of the Biesbosch and Maas, or the two channels should be replaced by one channel. In that case, the discharge distribution is not a problem anymore.  The connection channel between the west and south channel and the flood plain east of E in Figure 5-47, do not flow, and therefore do not have a hydraulic function related to the flood channel. It is advised to remove them from the design.

As the proposed improvements show, the inlet and outlet structure do not have to be adapted. The results do not show an unexpected large water level drop at the structures. This is confirmed by the sensitivity analysis performed at the one-dimensional approach.

107 Hydraulic analysis of a flood channel January 2013

Also widening the narrowed last kilometers of the main channel (roughly west of E in Figure 5-46) has not been proposed. The sensitivity analysis performed at the one-dimensional approach shows that widening this part has a small influence on the flood channels discharge (1 centimeter water level decrease extra at the Waal), hence, widening this part has not been advised.

5.6.4 Summary

In this paragraph, the design of the flood channel has been reviewed. The water level decrease in the Waal due to the flood channel is lower than the expected value based on the IVB studies (0.96 instead of 1.30 meter at a 16000 m3/s Rhine discharge). According to the uniform Chezy approach, a water level decrease of 103 centimeters in the Waal is possible when the outlet channels have the same width and roughness as the main channel.

The review of the design parameters showed that especially the discharge capacity of the outlet channels is not in accordance with the guidelines. The most important proposed design improvements are the lowering of a quay East of the Afgedamde Maas, the decreasing of the roughness of a natural area before the inlet structure, and measures to increase the flow capacity of the outlet channels.

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6 Costs

The flood channel can only be realistically compared to other alternatives when the costs are known. Goal of the cost calculations performed in this chapter is to compare the investments in the flood channel with the costs of dike reinforcements. The costs of the construction of the flood channel will be described first (paragraph 6.1) and the cost of the alternative, dike reinforcements, will be described subsequently (paragraph 6.2). The cost efficiency will be discussed in paragraph 6.3, and finally, a short summary of the cost calculations will be given in paragraph 6.4.

6.1 Costs flood channel

In this paragraph, the investment costs of the flood channel will determined. First, the method will be described, and secondly the results will be presented.

6.1.1 Method

PRI-method Rijkswaterstaat created a method to determine the costs of infrastructural projects, the PRI method (Project Ramingen Infrastructuur). For the Room for the River project, this method was used to estimate the costs of more than 700 measures. The background of the cost estimation of those measures was published in a report [Van der Linde et al., 2004]. Based on that report, the costs of the flood channel construction will be determined.

The PRI-method described in the report is based on PRI-2003, and the costs are based on the price level of July 2002. The PRI-method distinguishes several types of costs:  Construction costs  Property costs  Engineering costs  Other costs (e.g. permits, research costs etc)  Unexpected costs

Each type of the costs mentioned above can be divided in direct and indirect, expected and unexpected costs. The direct expected costs are directly related to the design, and are the most easy to determine. If in the conceptual phase all types of costs should be elaborated separately, the cost estimation would take too much time for the phase of the design. Therefore, in the PRI method, many costs are described as a percentage of the direct construction costs. Those percentages are converted to a multiplying factor that is different for each type of measure. For a flood channel, the multiplying factor is 2.76 [Van der Linde et al., 2004]. In the multiplying factor, the following costs are assimilated:  Indirect construction costs, unexpected construction costs and taxes  Expected and unexpected engineering costs and taxes  Costs for research, permits, taxes, damage, climate change and unexpected costs  General unexpected costs

109 Hydraulic analysis of a flood channel January 2013

The costs for buying properties (and related costs) are not included in this multiplying factor, and will be calculated separately. Also costs of disposal of contaminated soil and some other not standard costs are not included. The construction and property cost can be determined by using standardized prices. A list with standardized prices can be found in the Room for the River report [Van der Linde et al., 2004].

Application of the method According to the method as described above, the costs of the flood channel will be determined. To determine the total investment of the flood channel, the following steps will be taken:  Calculate the direct construction costs, and multiply this with a factor 2.76  Calculate the property costs, and add a percentage for unexpected property costs (11%) and indirect property costs (16%) [Van der Linde et al., 2004]

The sum of these investments is an estimation of the total investment in the flood channel. Maintenance of the channel will not be included, and the costs will not be scaled to a longer investment period.

The construction costs are e.g. the construction of new dikes, inlet- and outlet structures, demolishing buildings and excavation work. Property costs are e.g. buying houses, companies, farms and corresponding properties. The costs are based on the schematization of the flood channel as described in paragraph 4.3. Deviations and supplements related to costs are mentioned below. More information about the cost structure can be found in appendix C.1.

Assumptions related to the construction costs:  Length of dikes, quays, roads and structures are based on GIS information.  For the new dikes in the flood channel, it has been assumed that the costs per kilometer are equal to those of new dikes at the Waal between Zaltbommel and Vuren.  The cost of replacing pipes and cables has not been taken into account.  All the excavated ground is assumed clean, and reused or dumped in the nearby area.  In the design, the ground level in bulges on the north side of the channel raises with a gentle slope. This raised ground level has not been taken into account.  On top of the inlet structure, a local road is located, and on top of the outlet structure, the highway A27 is located.  The outlet channels will be transformed to a natural area.

Assumptions related to the property costs:  Number of houses, companies and farms are based on aerial photos. Therefore, the numbers will deviate from the real numbers. However, they give an estimation of the order of magnitude of the numbers. The areas of the parcels are partly determined by GIS, and partly based on average values.  All the houses, companies and farms within the area of the flood channel will be demolished, and have therefore been bought. Two types of houses are distinguished, medium and large (>200 m2) and two types of companies are distinguished, medium and large (>10000 m2).  The agricultural land use in the main channel will not change, and therefore, this land has not been bought. The land in the outlet channel, where the land uses changes to natural area, has been bought.  Transition costs of land due to a changed land use have not been taken into account.

110 January 2013 Hydraulic analysis of a flood channel

The large amount of assumptions shows that the costs are more an estimation of the order of magnitude of the costs than a detailed estimation of the costs.

6.1.2 Results

The method described in the previous paragraph has been applied on the flood channel. The results are summarized in Table 6-1.

Main Outlet Inlet channel channels Total [106 euro] [106 euro] [106 euro] [106 euro] Known direct construction costs Demolish buildings 0.1 6.6 1.5 8.3 In- and outlet construction 78.7 0.0 71.4 150.1 Clearing areas 0.2 0.7 5.9 6.9 Lowering quays 1.4 0.0 0.0 1.4 New dikes 11.7 105.8 137.3 254.9 Creating natural areas 0.2 0.0 4.1 4.3 Ground excavations 7.3 0.7 4.0 11.9 Total direct construction costs 99.6 113.9 266.3 437.8 Multiplying factor construction costs 2.76 Total investments costs (except property costs) 1208.4

Known direct property costs Buying properties 8.6 83.3 48.0 139.9 Buying buildings 1.5 101.3 34.8 137.6 Damage restitution agricultural area 1.5 0.0 0 1.5 Total direct property costs 11.6 184.6 82.8 279.0 Multiplying factor property costs 1.2876 Total property costs 359.2

TOTAL INVESTMENT COSTS 1567.6 Table 6-1 Summary investment costs flood channel

The main costs of the flood channels are the inlet- and outlet construction (including the road), the new dikes and the property costs. The other costs are relatively small. The large property costs in the main channel are mainly caused by the significant amount of houses (174) and the business park (60 companies).

A lot of money could be saved when there would be one instead of two outlet channels, and the land use in the area remains agricultural. In that case, the dike costs would be more than 50% lower (68 million euros) and the property costs would be 41 million euros lower. Further, optimizing the size and type of the inlet- and outlet structure could save a significant amount of money.

The results show that the total investment costs to construct the flood channel is ca. 1.6 billion euros. In the Room for the River program, the costs of similar measures are in the order of 1800-2300 million euros. Therefore, the calculated costs are probably somewhat underestimated, probably due to the large assumptions that have been made. The building and property costs are probably underestimated because changes in the local water system and

111 Hydraulic analysis of a flood channel January 2013

changes and adaptations in infrastructure have not been taken into account. Furthermore, the amount and area of houses and companies is probably underestimated due to the rough estimation.

6.2 Costs dike reinforcements

Due to the flood channel in the Waal and Merweden, the costs of the dike reinforcements will have been reduced. In this paragraph, these reduced costs will be calculated. After the introduction in paragraph 6.2.1, the design water levels along the Waal (6.2.2) and Bergse Maas (6.2.3) will be calculated. The method to calculate the costs will be described in paragraph 6.2.4, and finally the results will be presented (paragraph 6.2.5).

6.2.1 Introduction

Due to the climate change, the normative Rhine discharge is expected to increase from 16000 m3/s in the year 2015 to 18000 m3/s in the year 2100, and the sea level is expected to rise. When the current water system is not going to be changed, the dikes along the rivers should be raised in order to compensate the water level increase. A qualitative description of dike reinforcements in the study area has already been presented in paragraph 2.4.

In this chapter, the costs of the required dike reinforcements will be calculated. Reference is the year 2015, with a Rhine discharge of 16000 m3/s. The costs will be calculated for both the situation without flood channel, and the situation with flood channel.

In order to calculate the costs of dike reinforcements, the following assumptions have been made:  The dike height in 2015 is equal to the design water level  The design water level increase is equal to the required increase in dike height

In the chapter, only the construction costs will be calculated. Other aspects, like the value of landscape, nature and cultural historical buildings will not be taken into account. They have been qualitatively described in paragraph 2.4.

6.2.2 Design water levels Waal – Boven Merwede – Nieuwe Merwede

In this paragraph, the design water level change along the Waal, Boven and Nieuwe Merwede for the situation with and without flood channel will be determined.

Design water levels (without flood channel) The expected design water level increase due to the climate change has already been presented in paragraph 1.2.2. However, the figure is presented again in Figure 6-1:

112 January 2013 Hydraulic analysis of a flood channel

Figure 6-1 Design water level increase Waal, Boven Merwede and Nieuwe Merwede. Based on [Slootjes et al., 2011]

The design water levels are based on probabilistic SOBEK calculations with reference situation 2015 (equal as described in paragraph 4.2.5, except Room for the River measure Volkerak- Zoommeer) [Slootjes et al., 2011].

The change in dike height is assumed to be equal to the change in design water level. The costs for the dike reinforcements in the situation without flood channel are therefore based on the water level increase presented in the figure above.

Design water levels (with flood channel) The dike reinforcements are assumed equal to the design water level increase between 2015 and 2100. The calculations with the flood channel are performed for a 13000 m3/s, 16000 m3/s and 18000 m3/s discharge. Unfortunately, the design water level has not been calculated, because it is performed in a probabilistic environment where up to 3400 simulations are required. The design water level will be estimated based on the results of the calculated discharges; the normative Rhine discharge in 2015 is 16000 m3/s and the normative Rhine discharge in 2100 is 18000 m3/s.

First, the relation between the design water levels (see Figure 6-1), and the calculated water levels for the 16000 m3/s and 18000 m3/s Rhine discharge in the situation without flood channel will be made clear, see Figure 6-2:

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Figure 6-2 Water level due to river discharges and design water levels along the Waal, Boven Merwede and Nieuwe Merwede; situation without flood channel. Design water levels are based on [Slootjes et al., 2011]

As described in paragraph 4.4.1, the design water level in the river dominated area is mainly determined by the river discharge, and the design water level in the transition area is mainly determined by a combination of the river discharge and sea level.

The figure shows that between river kilometer 914 and 958, the water level of the 16000 m3/s Rhine discharge is approximately equal to the design water level in 2015, and that the 18000 m3/s Rhine discharge is approximately equal to the design water level in 2100. Therefore, the design water level increase (MHW2100 minus MHW2015) is approximately equal to the water level difference between 18000 m3/s and 16000 m3/s discharges. The water level step of the normative discharge at kilometer 953 is caused by a changed safety level. Downstream of kilometer 958, the water levels due to the 16000 m3/s and 18000 m3/s discharge decrease more than the design water levels. This is caused by the, in downstream direction, increasing influence of the sea on the design water level. A part of the downstream water level difference between the design water levels is caused by the expected sea level rise.

For the flood channel calculations, the design water level has not been calculated. However, based on Figure 6-2, the calculated water levels are directly related to the design water levels. Therefore, the calculated water level change due to the flood channel is assumed equal to the design water level change.

Downstream of river kilometer 958, the influence from the sea is increasing, and calculated water levels are not directly related to the design water level. Therefore, the design water level difference in the transition area due to the flood channel will be estimated by the following equation:

hhQQ18000 16000 MHW  f  hFC With f  hhMHW2100 MHW 2015 Where: ΔMHW [m] Estimated design water level difference f [-] Fraction; calculated water level difference between 16000 and 18000 m3/s Rhine discharge (in situation without flood channel) divided by the design water level difference between 2015 and 2100

114 January 2013 Hydraulic analysis of a flood channel

ΔhFC [m] Water level difference due to flood channel compared to

hQ=18000 (not displayed in Figure 6-2)

hQ=18000 [m+NAP] Calculated water level at a Rhine discharge of 18000 m3/s in a situation without flood channel

hQ=16000 [m+NAP] Calculated water level at a Rhine discharge of 16000 m3/s in a situation without flood channel

hMHW2100 [m+NAP] Design water level 2100

hMHW2015 [m+NAP] Design water level 2015

Downstream of kilometer 958, this fraction will be multiplied with the calculated water level difference due to the flood channel (at a 18000 m3/s discharge). Upstream of kilometer 958, the correction factor is equal to 1. In the figure below, the correction factor and estimated design water level difference due to the flood channel are presented.

Figure 6-3 Estimated normative discharge difference due to flood channel. Estimated design water level is the water level difference flood channel times the correction factor (f)

The corrected water level difference is expected to give a reasonable estimation of the design water level change, thus the change in dike height. The correction factor (f) is 0.71 at kilometer 962 and is 0.36 at kilometer 980. The estimated design water level in 2100 with the flood channel is presented in Figuur 6-4.

Figuur 6-4 Estimated design water level in 2100 with flood channel along the Waal

115 Hydraulic analysis of a flood channel January 2013

The figure above shows that the design water level in 2100 with flood channel between river kilometer 943 and 960 is below the design water level in the reference situation. River kilometer 943 is located between Herwijnen and Brakel, and kilometer 960 is located just upstream of Werkendam and Boven-Hardinxveld.

6.2.3 Design water level Bergse Maas and Amer

In order to estimate the required dike reinforcements along the Bergse Maas and Amer due to the flood channel, the change in design water level due to flood channel must be determined.

Design water level Maas and Amer The calculations in this study are performed with a dominant Rhine discharge, and for the Maas discharge, the median has been used. Therefore, the calculated water levels in the Maas are not directly related to the design water levels, as is the case with the Waal discharge.

In Table 6-2, the relation between the Rhine discharge, Maas discharge, sea level and wind is presented for Bergse Maas kilometer 251, near the outlet of the flood channel.

Table 6-2 Illustration points WTI2011 model Bergse Maas km 251, return period of 2000. Only illustration points with an exceeding frequency of more than 1% are displayed. [Hydra Zoet version 1.3.1]

The table shows that the water level near the outlet of the flood channel is strongly influenced by the sea. The design water level is in most of the cases a combination of a storm surge and Rhine discharge between 8000 m3/s and 10500 m3/s. The combination of the strong sea influence and the missing relation between the Maas discharge and the design water level makes it impossible to relate the calculated water levels along the Maas to the design water levels.

Expected design water level increase Bergse Maas and Amer Table 6-2 shows that at governing circumstances, the Rhine discharge is in 95% in of the cases lower than 10548 m3/s. Given the climate conditions in 2015, the flood channel is expected to start flowing at a Rhine discharge of 12600 m3/s (see paragraph 2.3.4). Based on the table above, it can be concluded that at high Rhine discharges, there is a small probability (less than 5%) that the water level at Bergse Maas kilometer 251 is equal to the design water level. Hence, it is likely that the Bergse Maas has discharge capacity available during high Rhine discharges. Therefore, the design water level increase due to the flood channel is expected to be

116 January 2013 Hydraulic analysis of a flood channel

smaller than the calculated water level increase. This effect is expected to be larger downstream of kilometer 251, and smaller upstream of kilometer 251.

The preliminary conclusion that the Bergse Maas has discharge capacity available is based on calculations with a dominant Rhine discharge. However, calculations along the Maas performed with a dominant Maas discharge show different results. In the figure below, the medians of the Rhine and Maas are presented for several discharges.

Figure 6-5 Medians Rhine and Maas [Fioole, 1999]

The figure shows that for a Maas discharge of approximately 3200 m3/s, the Rhine discharge is 12600 m3/s, which is the Rhine discharge when the flood channel starts flowing. The normative Maas discharge is 3800 m3/s in 2015, and 4600 m3/s in 2100 [Silva and Van der Linden, 2007], which means that at the normative Maas discharge, the flood channel is in use. In that case, it is expected that the available extra discharge capacity of the Bergse Maas is negligible, and that dike reinforcements along the Bergse Maas and Amer due to the flood channel are inevitable.

Conclusion Based on the calculated water levels, the design water levels cannot be estimated, because the relation with the calculated water levels is not clear. A probabilistic calculation should be performed in order to estimate the influence of the flood channel on the design water level of the Bergse Maas and Amer. Therefore, the costs of the extra dike reinforcements due to water level increase along the Bergse Maas and Amer will not be calculated in this study. It is expected that extra dike reinforcements along the Bergse Maas and Amer will be necessary.

6.2.4 Method

Investment function For the cost-benefit analysis of the Room for the River project, the costs of dike reinforcements were determined for many dike sections based on an investment function [Eigenraam, 2005]. The investment function is a combination of a linear and exponential function, based on empirical data:

117 Hydraulic analysis of a flood channel January 2013

u I()() u c  bu  e

Where: I Costs dike reinforcements per kilometer [106 euro] u Dike raise [cm] λ Constant [cm-1] b Constant [106 €] c Constants [106 €/cm]

Figure 6-6 Investment function dike ring 43 (λ=0.0043 cm-1, b=0.02*106 €, c= 1.5847*106 €/cm)

For each dike ring, specific constants are available [Eigenraam, 2005]. The function starts with a dike ring dependent base cost, and increases with costs dependent of the dike raise, see Figure 6-6. The costs function estimates all the investment costs related to dike reinforcements, and could therefore be compared to the investment costs of the flood channel. The costs are based on the price level of 2003 (which is approximately equal to the price level of the flood channel, July 2002).

Application of the method For this study, especially the cost reduction of the dike reinforcements due to the flood channel is relevant. Therefore, the costs of the dike reinforcements are determined with and without flood channel.

At the start of the costs calculation, the following information is known:  Design water level increase between 2015 and 2100  Water levels at a 16000 m3/s and 18000 m3/s Rhine discharge and median Maas discharge  Estimated relation between the river discharge and design water level (see paragraph 6.2.2)  Water level decrease due to flood channel  Investment function for dike increase with parameters per dike ring [Eigenraam, 2005]

In Figure 6-7, the dike rings and numbers along the Waal and Merwedes are presented:

Figure 6-7 Dike rings with corresponding numbers [helpdeskwater, 2013]

118 January 2013 Hydraulic analysis of a flood channel

The flood channel is located in dike ring number 24. The upstream model boundary for the Waal is located at a river section between dike rings 41 and 43. In downstream direction towards Hollands Diep, the Waal river passes dike rings 40, 38, 24, 16, 23 and 22. The dikes along the Waal, Boven Merwede, Beneden Merwede and Nieuwe Merwede will be taken into account in this calculation.

The benefits that will be calculated are schematized in the figure below:

Figure 6-8 Schematization dike reinforcements and benefits

The figure shows the design water level increase due to the climate change, and the water level reduction due to the flood channel. In the situation without flood channel, the increase in dike height would be equal to the design water level increase. In the situation with flood channel, the design water level increase is reduced; hence, the dike height increase is less. The cost differences between those calculations are the benefits due to the flood channel. Note that downstream of kilometer 958, as mentioned in 6.2.2, the water level difference due to the flood channel will be corrected for the influence of the sea.

The following points should be taken into account:  This method of cost calculation implies that the (corrected) water level decrease due to the flood channel is equal to the design water level decrease. This is not completely true because the influence of the wind and sea barriers have not been taken into account, and the influence of the sea level has been implemented in a simplified way.  At the upstream model boundary, the water level decrease due to the flood channel is still significant. The upstream water level decrease is taken into account by multiplying the benefit of the most upstream river section with the 0.5 times the length that is necessary to double this water level decrease.  The landscape, nature and cultural historical (LNC) values are implicitly taken into account by reinforcing with more expensive methods. However, damage to and loss of LNC values is not taken into account.  The water levels are determined at the river axis. However, the river axis does not have the same length as the dikes. Therefore, the dike reinforcement costs are first calculated at the river axis, and afterwards corrected for the difference between axis length and dike length.  At the dikes between dike ring 23 and 24, the water levels of Steurgat are not known. Based on the results, it is estimated that the water level will increase slightly due to the flood channel. The water level difference has been set to zero, so no dike height changes have been taken into account.

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6.2.5 Results

The results of the method as described in the previous paragraph are presented in Table 6-3. Note that the costs are based on the price level of 2003.

Dike ring Costs dike raise at design Costs dike raise at design Benefits flood channel number water level increase water level increase incl. [106 euro] (reference) flood channel [106 euro] [106 euro] 43 184.1 97.4 (-47.1%) 86.7 16 177.7 111.2 (-37.4%) 66.5 22 154.3 149.5 (-3.1%) 4.8 41 24.4 20.4 (-16.4%) 4.0 40 21.9 17.2 (-18.9%) 4.7 38 79.2 29.9 (-62.2%) 49.3 24 41.6 3.5 (-91.5%) 38.1 23 33.5 29.4 (-12.2%) 4.1 43 (upstream) 4.5* 41 (upstream) 3.7* Total 716.7 458.6 266.3

Table 6-3 Costs dike increase and benefits due to flood channel. More details can be found in appendix C.2. *Estimated benefits upstream of model boundary

The costs of the required dike reinforcements due to the climate change along the Waal and Merweden (from kilometer 914 to 976, Beneden Merwede untill kilometer 975) are estimated on 716.7 million euros. In the situation with the flood channel, the costs would be 458.6 million euros. Hence, due to the flood channel the costs for dike reinforcements decrease with 266.3 million euros. The highest cost reduction can be found at dike rings 43 and 16, while the highest relative cost reduction can be found at dike rings 38 and 24, both relatively small dike rings.

The cost reduction could be relatively increased by performing more water level decreasing measures. When due to the flood channel the necessary dike reinforcement decreases from 50 centimeters to 10 centimeters, this is still relatively expensive due to the initial costs (see Figure 6-6), but if dike reinforcements are not required anymore, the initial costs would also be saved.

As already described in paragraph 2.4, dike reinforcements along the Waal, Boven-Merwede and Beneden-Merwede are more difficult than along the Nieuwe-Merwede, Bergse Maas and Amer. Therefore, the cost-benefits balance could change when e.g. LNC values are taken into account. However, these values are not quantified in this study, thus have not been be taken into account quantitatively. 6.3 Cost efficiency

In paragraph 6.1, the costs to construct a flood channel have been presented, and in paragraph 6.2, the financial benefits of the flood channel became clear. Based on the calculated values, the cost-effectiveness can be calculated.

The cost-efficiency is described as follows [Spankracht, 2002]:

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normativewaterleveleffect(m2 ) E  costsmeasure(106 € )

The water level effect is the estimated design water level reduction in squared meter (red area in Figure 6-8) due to the flood channel. The costs are the total investment costs of the flood channel.

Description Costs Effectiveness Costs construction flood channel 1567.6 million euros 17.9 m2/106 € Benefits flood channel (Waal-Merweden) 266.3 million euros Description Water level effect [m2] Water level decrease Waal-Merweden1 30535 m2 (without correction) Water level decrease Waal-Merweden1 28100 m2 (including correction) Water level increase Waal-Merweden2 852 m2 (without correction) Water level increase Waal-Merweden2 155 m2 (including correction) Water level increase Maas-Amer3 11739 m2

Table 6-4 Overview results costs. The water level effect is presented for the situation with and without the correction to estimate the design water level in the transition area. The correction is explained in paragraph 6.2.2. 1 Includes Waal from kilometer 914 + estimation effects, Boven Merwede, Nieuwe Merwede until kilometer 976, Beneden Merwede until kilometer 975 2 Includes Waal from kilometer 976 until 1029 (Hollands Diep and Haringvliet) 3 Maas from kilometer 203 until Amer kilometer 262

The cost-effectiveness of the flood channel has been calculated by dividing the total investment of the flood channel by the corrected water level effect of the Waal and Merweden, and is 17.9 m2/106 €. According to the Spankracht study [Spankracht, 2002], a measure with a cost efficiency of 10-30 m2/106 € is moderately effective. They advice a minimum cost efficiency of 100 m2/106 € for measures with a water level effect of more than 27000 m3/s. Flood channels investigated for the Room for the River study had a cost-efficiency between 4 and 12 m2/106 €, while the executed Room for the River measures ontpoldering of the Noordwaard and flood channel Veessen-Wapenveld show a cost effectiveness of 40 m2/106 € [Blokkendoos, 2006]. Therefore, the cost efficiency of the flood channel is higher than the flood channels investigated in the Room for the River project, but is still low compared to the measures that are being executed. If the costs are underestimated, as expected, and if the cost of the dike reinforcements along the Bergse Maas and Amer are taken into account, the cost effectiveness will decrease even more.

6.4 Summary

In this chapter, the costs of the flood channel and the dike reinforcements benefits due to the flood channel have been calculated. For the costs of the flood channel, the PRI-method has been used, but with many assumptions. For the costs of the dike reinforcements, a method used for the cost-benefits analysis of the Room for the River project has been used. The calculated costs of the flood channel and dike reinforcements are both general cost estimations. The costs are based on the price level of 2003.

The calculated costs for the flood channel are 1567.6 million euros, and are expected to be an underestimation. The cost-efficiency, given the water level effect of 28100 m2 at the Waal and

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Merweden, is 17.9 m2/106 €, which is low for such a large measure. The calculated costs for dike reinforcements, without flood channel, are 716.7 million euros. Due the flood channel, the reductions in dike reinforcement costs is 266.3 million euros. The costs of the dike reinforcements along the Bergse Maas and Amer have not been taken into account, thus the benefits are expected to be smaller.

Due to the flood channel, the design water level between kilometer 944 and 960 does not increase compared to 2015. That area has a high LNC value due to historical buildings and ribbon building. The costs calculated in this chapter are only the costs related to construction, and loss of LNC values, public protests and other non-construction related values are not taken into account. Therefore, it is expected that the cost-benefit balance could change in a more positive direction, with respect to the flood channel, when LNC values are taken into account.

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7 Conclusion and recommendations

In this chapter, the conclusions of the research will be presented. First, a short summary of the research question, approach and results will be presented in paragraph 7.1. In paragraph 7.2, the conclusions will be presented, and subsequently in paragraph 7.3, the recommendations for further research will be listed. This chapter is finished with an epilogue where an alternative route for an flood channel will be presented (7.4).

7.1 Summary: research question, approach and results

In this paragraph, a brief summary of the research question, approach and results are presented. They are useful background for interpreting the conclusions.

Research question The main research question was: Is the flood channel designed by Robbert the Koning hydraulically optimal, and a cost effective alternative for dike reinforcements?

The sub questions were: 1. What is the influence of the flood channel on the water system? 2. Does the flood channel perform according to the expectations? Why or why not? 3. Is there room for improvement in the design of the flood channel? 4. What are the costs of the flood channel compared to dike reinforcements? 5. Does a one-dimensional model of a flood channel give a reliable estimation of the results (compared to a two-dimensional model)? And are those results reliable enough in a conceptual phase?

Approach The research question has defined the problem. In order to find the answer, the followings steps have been taken: 1. Literature research on flood channels and the Dutch water system 2. Analysis of the flood channel design. Part of this analysis was contact with the architect in order to make the design more clear. 3. One-dimensional modelling of the flood channel and water system in order to make the problem as simple as possible. 4. Two-dimensional modelling of the flood channel in order to get more detailed information about the flow in the flood channel and influence on the water system. 5. Analysis of the two-dimensional results, and comparison with the results of the one- dimensional model and the expected results. 6. Analysis of the costs of the flood channel, also compared to the costs of dike reinforcements.

Results In the table below, the results of the two-dimensional modelling are summarized:

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Rhine discharge Maximum water level Maximum water level Discharge flood at Lobith difference Waal difference Maas channel [m3/s] [m] [m] [m3/s] 13000 -0.75* 0.38 1511 16000 -0.96* 0.44 2157 18000 -1.10* 0.44 2707

Table 7-1 Summarized results water level changes and flood channel’s discharge *including Room for the River measure Munnikenland (-0.11 meter at 18000 m3/s Rhine discharge)

At an 18000 m3/s Rhine discharge, the water level decrease at the Waal is approximately equal to the expected design water level decrease, while the water level increase along the Bergse Maas is expected to be lower than the design water level increase.

In the table below, the costs and benefits of the flood channel are summarized:

Description Costs Effectiveness Costs construction flood channel 1567.6 million euros 17.9 m2/ 106 euro Costs dike reinforcements 716.7 million euros Benefits flood channel (Waal-Merweden) 266.3 million euros

Table 7-2 Summary costs and benefits flood channel

The effectiveness is calculated by dividing the water level effect in squared meter by the costs in million euros.

7.2 Conclusions

In this paragraph, the conclusions of the research will be presented. The sub questions of the research are answered first, and finally the main research question will be answered. The numbers mentioned in the conclusions below are based on an 18000 m3/s Rhine discharge at Lobith (unless stated otherwise). The flood channel consist of three parts, the inlet (Waal to Inlet structure), main channel (inlet structure to outlet structure) and two outlet channels (outlet structure to Biesbosch and outlet structure to Bergse Maas).

Influence flood channel on water system The flood channel connects the Waal with the Bergse Maas and Biesbosch. The influence from the flood channel on the water system is mainly investigated by analyzing the changed discharges of the river branches, which are of course directly related to the changed water levels. From the results, the following can be concluded:  The flood channel changes the discharge distribution of the investigated water system significantly. Up to 23% of the Waal’s discharge (2707 m3/s) is extracted due to the flood channel and transported to the Amer or Biesbosch. As a result, the discharge through the Amer increases up to 32%, while the discharge through the Nieuwe Merwede decreases up to 28%. The discharge through the Beneden Merwede decreases with approximately the same value as the discharge through Hollands Diep increases (+541 m3/s).  The design water level in 2100 along a part of the Waal and Boven Merwede (river kilometer 944 to 960) does not increase after implementation of the flood channel, compared to the year 2015. The estimated design water level does not increase

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between Brakel/Herwijnen and Werkendam. Reinforcements along this river part, especially between Woudrichem and Boven-Hardinxveld are difficult due to cultural historical buildings and ribbon building. Along the Beneden Merwede, where reinforcements are also difficult, the design water level still raises 24 to 60 centimeters after implementation of the flood channel.  The flood channel has a negative effect on the effectiveness of Room for the River measure Noordwaard; the discharge through the Noordwaard decreases up to 52% (-1303 m3/s). This is caused by the lower discharge along the Nieuwe Merwede and higher water levels in the Biesbosch due to the outlet of the flood channel. Due to the lower Noordwaard discharge, the water level reduction due to the flood channel is small downstream of the inlet of the Noordwaard.  The main discharge from the Biesbosch (Gat van Kampen) towards Hollands Diep is less affected by the flood channel than the discharges from the Nieuwe Merwede and Amer into Hollands Diep. The discharge from the Biesbosch to the Amer increases slightly (+0.6%) for an 18000 m3/s Rhine discharge, and decreases for a 13000 m3/s discharge (-2.3%). At an 18000 m3/s Rhine discharge, the flow from the Nieuwe Merwede towards Hollands Diep decreases 15%, while the flow through the Amer increases up to 32%. The main reason for the small discharge change from the Biesbosch is the lower discharge through the Noordwaard.

In summary, the flood channel has a significant effect on the water system because discharge changes up to 32% occur in the river branches, compared to the situation without flood channel. The discharge through the Noordwaard is significantly lower, and as a result, the discharge from the Biesbosch towards Hollands Diep is slightly affected by the extra flood channel discharge. Along a part of the Waal and Boven Merwede, where dike reinforcements are difficult, the design water level with the flood channel in 2100 is lower than the design water level in the reference 2015.

Performance flood channel The performance of the flood channel has been determined by comparing the discharge of the flood channel and water level changes along the Waal and Maas with the expectations according to the study Integrale Verkenning Benedenrivieren (IVB). From the results, the following can be concluded:  The performance of the flood channel is significantly lower than the expectations. The water level decrease at the Waal and increase at the Bergse Maas are both smaller than the estimated values in the IVB study. At a 16000 m3/s Rhine discharge, water level decrease at the Waal is 0.96 instead of 1.30 meter (-26%), and the water level increase at the Maas is 0.44 instead of 0.54 meter (-18%). The discharge through the flood channel in the IVB study is not known, but based on the water level results, it is expected to be higher than the calculated discharge of 2157 m3/s.  The lower performance of the flood channel is mainly caused by the discharge capacity of the outlet channels. The width of the outlet channels in combination with the roughness and bottom level results in a lower discharge capacity than the main and inlet channel. In the results, this is visible due to a steep backwater curve. A raw schematization showed that a flood channel discharge of 2359 m3/s (+9.3%) should be possible if the roughness of the outlet channels would be equal to the roughness of the main channels.

In summary, the performance of the flood channel is lower than expected. The main reason for the lower performance is the discharge capacity of the outlet channels.

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Costs The conclusions regarding the costs are based on the cost analysis in combination with the water level results. From the results, the following can be concluded:  The cost effectiveness of the flood channel is low. The calculated costs of the flood channel are 1567.6 million euros. The cost effectiveness, which is the water level effect (m2) divided by the costs, is 17.9 m2/106 euros, while similar large Room for the River measures like the Ontpoldering of the Noordwaard and flood channel Veessen- Wapenveld show a cost effectiveness of 40 m2/106 euros. In this calculation, the negative water level effect along the Bergse Maas and Amer are not taken into account.  The calculated benefits of the flood channel due to reduced dike reinforcements are small compared to the calculated investment. The benefits of the flood channel are 266.3 million euros, only 17% of the investment into the flood channel. Negative influences like dike reinforcements along the Maas and Amer are not taken into account. However, practical limits to dike reinforcements and values related to landscape, nature and culture are not taken into account, which could change the cost-benefit balance.

In summary, the investment necessary to construct the flood channel is more than 5 times larger than the benefits due to reduced dike reinforcements, and the cost-effectiveness is low compared to large Room for the River measures. The effectiveness could improve when LNC values and practical limits to dike reinforcements are taken into account.

Reliability one-dimensional approach The conclusions regarding the one-dimensional model are based on the results of the one and two-dimensional model.

For a measure implemented in a complex water system, as is the case in this study, the strongly simplified 1-dimensional model gives a reasonable estimation of the order of magnitude of the effects. The maximum water level difference calculated with the one-dimensional model deviates 1 cm (+1%) at the Waal and 6 centimeters (-14%) at the Bergse Maas from the water level difference calculated with the two-dimensional model. The estimated flood channel discharge deviates 251 m3/s (-12%). Hence, for a first order estimation of the effects of a measure implemented in a complex water system, a well calibrated stationary one-dimensional model can be used. If a higher accuracy is required, a more advanced one-dimensional model or a two-dimensional model should be used.

Suggested design improvements The suggested design improvements of the flood channel design are based on analysis of the water levels and flow pattern. The suggested improvements are divided in three categories, and are presented below:

Required design improvement:  The discharge capacity of the outlet channels must be increased. The outlet channels limit the discharge of the flood channel because the discharge capacity of the outlet channels is lower than the capacity of the inlet- and main channel. The results show a steep backwater curve in the outlet channels. Increasing the flow width, decreasing the roughness or lowering the bottom could improve the discharge capacity.

Advised design improvements:  Improve the distribution between the west and south outlet channel, because the discharge distribution between the west and south outlet channel is not equal to the

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discharge capacity of the Bergse Maas compared to the Biesbosch. The results show an additional flow from the Bergse Maas towards the Biesbosch due to the flood channel.  Improve the flow towards the inlet structure by decreasing the roughness of the natural area before the inlet and lower the quay between Munnikenland and the Afgedamde Maas. The results show that the natural area and the quay are an obstruction for the flow, and therefore hinder the flow towards the inlet structure.  Increase the width of the main channel from Almkerk to the outlet structure. In the last kilometers, the flow width decreases compared to the other parts of the channel. By removing the narrowest parts, the flow width would approximately equal in the whole channel.

The areas mentioned below do not contribute to the flow, because the flow velocities are negligible. Therefore, they are hydraulically not effective parts of the design:  The two bulges north and the bulge south-east of the main channel  Connection channel between west and south outlet channel  Area south-east of the mouth of the southern outlet

In summary, the design of the flood channel could be improved by increasing the discharge capacity of the outlet channels, by changing the discharge distribution between the outlet channels and by adapting the area before the inlet structure.

Main research question: Is the flood channel designed by Robbert the Koning hydraulically optimal, and a cost effective alternative for dike reinforcements? A hydraulically optimal flood channel is defined as a flood channel of which the design (geometry and roughness) and effects on the river system could not be improved significantly.

The results show that the discharge capacity of the outlet channels is lower than the discharge capacity of the inlet and main channel. By adapting the outlet channels a discharge increase through the flood channel of 9% could be possible, and more optimalizations are possible. Hence, the flood channel design is not hydraulically optimal.

The flood channel changes the discharge distribution between the Waal and Maas significantly. In general, a flood channel is used to bypass a river part, and has only a water level lowering effect on the main river. However, the flood channel designed by Robbert de Koning results in a water decrease at the Waal, and a water level increase at the Maas, where dike reinforcements will probably be necessary. Therefore, the flood channel solves problems along the Waal and Merweden, but also creates new problems along the Bergse Maas and Amer. However, dike reinforcements along the Maas and Amer are easier than along the Waal and Merwede. Furthermore, the positive water level effect due to the lower river discharge decrease downstream of the inlet disappears fast downstream of Werkendam (Nieuwe Merwede). This is caused by less effective Noordwaard and increasing influence from the sea. Hence, a flood channel that connects two rivers and lies partly in the transition area is not hydraulically optimal.

The cost analysis shows that the costs of the flood channel are high (1567.6 million euros), and the cost effectiveness is low for such a large measure (17.9 m2/106 euros, where >100 m2/106 euros is advised [Spankracht, 2002]). Furthermore, the benefits due to the reduced dike reinforcements are only 17% of the investment. Hence, the flood channel is not a cost effective alternative for dike reinforcements. It must be noted that LNC values are not taken into account, which could change this conclusion.

127 Hydraulic analysis of a flood channel January 2013

Concluding, the flood channel designed by Robbert de Koning is not hydraulically optimal, and not a cost effective alternative for dike reinforcements. Hydraulic optimalization of the channel is possible, and when LNC values will be taken into account, the cost effectiveness could be more positive with respect to the flood channel.

7.3 Recommendations for further research

General recommendations:  It is recommend to compare the costs and benefits of the flood channel and dike reinforcements in more detail, including the LNC values and practical limits regarding the dike reinforcements.  It is recommend to investigate other water lowering measures that could lower the design water level, especially along the Boven- and Beneden Merwede, and compare them with the flood channel.  It is recommend to investigate an other route for the flood channel, as indicated in Figure 7-1 and described in paragraph 7.4. Along this route, the dike length is shorter, and less buildings have to be removed, which will result in significantly lower construction costs. The water level effect is expected to be similar.

If the flood channel will be subject to more research, the following is recommend:  It is recommended to improve the design of the flood channel by implementing the proposed improvements in order optimize the flow through the channel. Especially the outlet channels are in need of improvements. In these channel, it is advised to make the discharge distribution between the west and south channel in accordance with the discharge capacity of the Bergse Maas and Biesbosch, and to investigate if it is possible to solve the discharge distribution problem by using one outlet channel instead of two.  It is recommended to perform a probabilistic water level calculation in order to calculate the design water levels. Furthermore, the behavior of the flood channel at several combinations of Rhine and Maas discharges could be investigated with these results. It is further advised to investigate the influence from the flooding frequency on the design water level.  It is recommended to investigate the behavior of the flood channel for a discharge wave at the Rhine instead of a constant discharge. Also the interaction with discharge waves along the Bergse Maas is worth investigating.  It is recommended to improve the costs calculation by adding more details. The size of the properties could be analyzed in more detail by using detailed GIS information, and the costs of the properties could be more precisely determined by using the WOZ value of the buildings. The costs of ground excavation could be calculated in more detail by distinguishing clean and unclean ground. Further improvement is possible by taking benefits into account.  It is recommended to investigate which cost reductions are possible. E.g. by using one instead of two outlet channels, by changing the land use in the outlet channels from nature to agriculture, by optimizing the size of the inlet- and outlet structures etc.

7.4 Epilogue

One of the main reasons to investigate the flood channel is the practical limit to dike reinforcements due the large amount of (historical) buildings along the Boven and Beneden

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Merwede. In this study, the flood channel designed by Robbert de Koning has been analyzed. Based on the knowledge earned in that analysis, another route for the channel has been proposed to investigate, see Figure 7-1.

Figure 7-1 Proposed flood channel to investigate (blue), from Woudrichem to the Noordwaard (finished in 2015). In red, the contours of the flood channel designed by Robbert the Koning are displayed [De Koning, 2012]

The proposed channel still bypasses the Boven Merwede, but the route is located more in the northern part of the polder. The flood channel starts between Woudrichem and Rijswijk, crosses the highway A27 between De Schans and Uppel and ends at the Noordwaard. The outlet could be connected to the Noordwaard (Ontpolderd in 2015), or to a (widened) Steurgat. An optimal solution for the connection must be found in order to prevent water flowing back to the Nieuwe Merwede.

The proposed flood channel has approximately the same width and bottom level as the investigated flood channel (700 meters), and also the water level slope is expected to be approximately equal. Therefore, the flood channel should have approximately the same discharge capacity as the investigated flood channel.

The advantages of the proposed flood channel, when compared with the investigated flood channel, are:  A problem of the investigated channel is the business park south of Giessen and the large amount of houses near Almkerk en Nieuwendijk. The proposed route crosses less houses (42 instead of 275), farms (11 instead of 32) and business buildings (2 instead of 72) on the route of the channel (numbers are estimated based on aerial photos).  The proposed flood channel is shorter, and has one instead of two outlet channels, which results in shorter dikes and less parcels on the route of the channel, thus less construction and property costs.  In the proposed channel, the water is transported from the Waal towards Hollands Diep mainly through the Biesbosch, and less water is transported towards the Bergse Maas and Amer. Hopefully, this will result in a smaller design water level increase at the Bergse Maas and Amer.

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 This social impact of the flood channel on the society is expected to be smaller, because less people are directly affected by it.

A disadvantage of the proposed flood channel could be a decreasing efficiency of the Noordwaard. However, with the flood channel designed by Robbert de Koning, the efficiency already decreased up to 50%. Another disadvantage is large impact on the village Uppel, where many houses would have to move. A concern is the connection between the outlet of the channel and the Biesbosch, which must be designed in such a way that the water from the flood channel cannot flow back towards the Nieuwe Merwede.

130 January 2013 Hydraulic analysis of a flood channel

8 References

[Arcadis, 2003] Referentie alternatief dijkversterking (RAD). Arcadis, Royal Haskoning and Fugro. June 2003.

[Arcadis, 2012] Bijlage 14. Grote kunstwerken. Hoogwatergeul Veessen-Wapenveld. Toelichting Rijksinpassingsplan. SNIP 4. VW TM Grote kunstwerken. Arcadis, 5 July 2012.

[Barneveld et Vieira da Silva, 2011] Veessen-Wapenveld Hoogwatergeul SNIP 3. VW TM Rivierkunde. H. Barneveld, J. Vieira da

Silva. HKV LIJN IN WATER PR1448.30, June 2011.

[Blokkendoos, 2006] BlokkendoosPKB, Versie 2.00.0011

[Bruggeman et al., 2011] Deltascenario’s: Verkenning van mogelijke fysieke en sociaaleconomische ontwikkelingen in de 21e eeuw op basis van KNMI ’06 en WLO scenario’s. W. Bruggeman, M. Haasnoot, S. Hommes, A. Linde, R. Brugge, B. van der Rijken, E. Dammers, G.J. van den Born. Deltares, 2011.

[Bureau Benedenrivieren, 2004] Benedenrivieren in samenhang. Stand van zaken regionale voorkeuren Ruimte voor de Rivier. Projectorganisatie Ruimte voor de Rivier, Bureau Benedenrivieren, April 2004.

[Chow, 1959] Open channel hydraulics. Ven te Chow. 1959.

[De Koning, 2012] IABR. Internationale architectuur biennale Rotterdam 2012, making city: Rijn-Maas delta, deelgebied Biesbosch en omstreken. Robbert de Koning. April, 2012.

[Deltacommissie, 2008] Samen werken met water. Een land dat leeft, bouwt aan zijn toekomst. Bevindingen van de Deltacommissie 2008.

[Deltanieuws, 2012] Deltanieuws. Delta Program nieuwsbrief. Jaargang 2, nummer 6, December 2012

[DHV, 2010a] Waterberging Volkerak-Zoommeer. Hydraulische analyses en modelberekeningen. DHV/RWS PDR, March 2010.

[DHV, 2010b] Voorspellen afvoer Nevengeulen. DHV/Waterdienst, February 2010.

[Dijkversterkingbas, 2013] http://www.dijkversterkingbas.nl/project/dijk-versterkt. 23 january 2013.

[Eigenraam, 2005] Veiligheid tegen overstromen. Kosten-baten analyse voor Ruimte voor de Rivier deel 1. C.J.J. Eigenraam. CPB document no. 82, April 2005.

131 Hydraulic analysis of a flood channel January 2013

[Fioole, 1999] De 50%-lijnen van Bovenrijn en Maas. A.Fioole. RIZA memo WST 98.113. RIZA Dordrecht, July 1998.

[Geerse and Duits, 2012] Sommensets Deltamodel waterveiligheid. Bepalen van de ideale sets van sommen per

watersysteem. C. Geerse and M. Duits. HKV LIJN IN WATER , February 2012.

[Heezik, 2007] Strijd om de rivieren. 200 jaar rivierenbeleid in Nederland. A.A.S. van Heezik. HNT Historische Producties/Rijkswaterstaat. 2007.

[Helpdeskwater, 2013] http://www.helpdeskwater.nl/publish/pages/27049/1008_406_krt4_dijkringen.pdf. 6 January 2013.

[Gerritsen and Schropp, 2010] Handreiking sedimentbeheer nevengeulen. H. Gerritsen en M. Schropp. Rijkswaterstaat, February 2010.

[IVB, 2000] Advies Integrale Verkenning Benedenrivieren. Vergroting van de afvoercapaciteit in de benedenloop van de Rijn en Maas. Bestuurlijk advies aangeboden aan de staatssecretaris van Verkeer en Waterstaat door de Stuurgroep Integrale Verkenning Benedenrivieren. February, 2000.

[Jeuken et al., 2010] Klimaatbestendigheid en opties voor adaptatie in de regio Rijnmond-Drechtsteden. Analyse van recente resultaten uit Klimaatbestendig NL Waterland en Kennis voor klimaat. Ad Jeuken, Nadine Slootjes, Niels van Oostrom. Deltares – HKV rapport 1202138-000, May 2010.

[Klein, 2001] Spankrachtstudie deelrapport 11. Groene rivieren: mogelijkheden voor toepassing. F. Klein, R. Maaten, R. van Buren. WL | Delft Hydraulics rapport. In opdracht van DG Rijkswaterstaat, RIZA, November 2001.

[Lam, 2004] Inlaatconstructies voor noodoverloopgebieden. Mogelijkheid, haalbaarheid en effecten. K.S Lam. Afstudeerrapport TU Delft, 2004.

[Project IJsseldelta, 2010] Project IJsseldelta 2010. Factsheet June 2010. http://www.ijsseldeltazuid.nl/bibliotheek/actuele-documenten/

[Provincie NB, 2009] Rivierverruiming Overdiepse polder. Inpassingsplan Overdiepse polder. Provincie - Brabant, 5 June 2009

[Roovers and Barneveld, 2006] Second opinion. Hoogwatergeul Veessen-Wapenveld, maatregel in het kader van de PKB

Ruimte voor de Rivier. G. Roovers (Oranjewoud) and H. Barneveld (HKV LIJN IN WATER). February 2006.

132 January 2013 Hydraulic analysis of a flood channel

[Ruimte voor de Rivier, 2006] Planologische kernbeslissing Ruimte voor de Rivier deel 4. Vastgesteld besluit. Ruimte voor de Rivier, December 2006.

[RvdR, 2010a] Ontpoldering Noordwaard op hoofdlijnen. Brochure Ruimte voor de Rivier.

[RvdR, 2010b] Inrichtingsplan ontpoldering Noordwaard. Ruimte voor de Rivier, October 2010.

[Ruimte voor de Rivier, 2012] http://www.ruimtevoorderivier.nl/hoe-lossen-we-dit-op/hoe-lossen-we-dit-op/. 18 December 2012.

[RWS, 2011a] Landelijke rapportage derde toetsronde. Kaart dijken en duinen. Rijkswaterstaat, augustus 2011.

[RWS, 2011b] Landelijke rapportage derde toetsronde. Kaart kunstwerken. Rijkswaterstaat, augustus 2011.

[RWS, 2012] http://www.rijkswaterstaat.nl/images/Infographic%20uiterwaardvergraving%20Meinerswij k_tcm174-327706.pdf. August, 2012

[Silva et al., 2001] Room for the Rhine branches in The Netherlands. What the research has taught us. W. Silva, F. Klijn, J. Dijkman. WL Delft Hydraulics. October 2001.

[Silva and Van der Linden, 2007] Van Lobith en Eijsden naar zee. Aanspraak op ruimte en afvoercapaciteit in het rivierbed op de lange termijn vanuit de veiligheid tegen overstroming. W. Silva and T. van der Linden. In opdracht van ministerie van Verkeer en Waterstaat. May 2007.

[Slootjes and De Waal, 2010] Uitgangspunten en randvoorwaarden berekeningen waterveiligheid – keuzenotitie. Nadine

Slootjes (HKV), Hans de Waal (Deltares). HKV LIJN IN WATER and Deltares, 2010.

[Slootjes et al., 2010] Gevoeligheidsanalyse waterberging Zuidwestelijke Delta. Hoofdrapport. N. Slootjes, M.K. Karelse, Y.J.G. van Kruchten, T. Louters, J. Bulthuis, S. de Goederen, J.W. Slager, R.

Slomp. RWS/HKV LIJN IN WATER/DHV, June 2010.

[Slootjes et al., 2011] Resultaten MHW berekeningen t.b.v. probleemanalyse en verkenning hoekpunten voor het Delta Program Rijnmond-Drechtsteden. N. Slootjes, A. Jeuken, T. Botterhuis and Q. Gao.

Deltares and HKV LIJN IN WATER , 2011.

[Spankracht, 2002] Spankracht bouwstenennota. Een overzicht van beschikbare ruimtelijke en technische maatregelen voor veilige verwerking van toekomstige maatgevende Rijnwaterafvoeren. Project Spankrachtstudie. December 2002.

[Stijnen and Slootjes, 2010] Eerste verkenning Waterveiligheid Rijnmond-Drechtsteden. J.W. Stijnen and N. Slootjes. Kennis voor Klimaat – Hotspot Regio Rotterdam. HKV, rapport PR1555.20.

133 Hydraulic analysis of a flood channel January 2013

[Van der Linde et al., 2004] Kostenschattingen PKB/MER Ruimte voor de Rivier. M. van der Linde, R. van Lier and S.R. Prins. Projectorganisatie ‘Ruimte voor de Rivier’. Referentie L966, versie 2.02, 23 March 2004.

[Van der Veen, 2005] Laterale toestroming voor NDB-model voor HR2006. R. van der Veen, 2005. Memo ADV*2005-013A. Rijkswaterstaat RIZA, Arnhem.

[Van Velzen, 2003] Stromingsweerstand Vegetatie in uiterwaarden. Deel 1 handboek. E.H. van Velzen, P. Jesse, P. Cornelissen, H. Coops. versie 1-2003. RIZA rapport 2003.028. Arnhem, November 2003.

[Vriend et al., 2007] Rivierwaterbouwkunde CT3340. TU Delft college dictaat. Prof. dr. ir. H.J. de Vriend, Ir. H. Havinga, Dr. ir. P.J. Visser, Dr. ir. Z. B. Wang. TU Delft, 2007.

[Vuik et al., 2011] WAQUA-productieberekeningen Benedenrivierengebied voor WTI-2011: rapportage fase 1.

V. Vuik, E. Collard, M. Rotsaert, J. Vieira da Silva. Svasek Hydraulics and HKV LIJN IN WATER, December 2011.

[Weijers and Tonneijck, 2009] Flood defenses. Lecture notes CT5314. J. Weijers and M. Tonneijck. February 2009.

[Zijl et al., 2011] WAQUA-model Rijn-Maasmonding. Modelopzet, calibratie en verificatie. F. Zijl, D. Kerkhoven, A.Z. Visser, T. van der Kaai. Deltares, 2011.

134

Appendices

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A Chapter supplements

A.1 Introduction: Climate change

At request of the Delta Program, the Delta scenarios have been developed. These scenarios describe the bandwidth of the expected climate change until the year 2100. There are four Delta scenarios, based on four climate scenarios (KNMI) and four social-economical scenario’s (WLO) [Bruggeman et al., 2011]. The bandwidth of the climate scenarios is presented in Table A-1.

2050 2100 Precipitation winter +4 % – + 14 % +8 % – +28 % Precipitation summer -19 % – + 6 % -38 % – + 12 % Sea level rise 15 cm – 35 cm (KNMI’06) 35 cm – 85 cm (KNMI’06)1 65 cm – 130 cm (Veerman2) Governing discharge Rhine 16500 m3/s – 19000 m3/s 17000 m3/s – 22000 m3/s (without upstream floods) (without upstream floods) 3 3 15500 m /s – 17000 m /s 3 3 16000 m /s – 17500 m /s (with upstream floods) (with upstream floods) Governing discharge Maas 4200 m3/s 4600 m3/s

Table A-1 Bandwidth climate scenario’s [Bruggeman et al., 2011]

In Table A-2, the most important climate scenarios for water safety are presented. The governing scenarios are the scenarios with the least and most change, based on Table A-1. Because of floods upstream in Germany during high river discharges, the Delta committee argues that it is physically impossible that a discharge of more than 18000 m3/s will reach The Netherlands. From discharges of 12000 m3/s floods in Germany will occur. [Deltacommissie, 2008] Therefore, an upper discharge limit due to floods in Germany is taken into account.

Scenario Governing Rhine Maas discharge Sea level rise from discharge [m3/s] (coupled to Rhine) 1990 [m] [m3/s] Reference 2015 16000 3504 0.08 W2050 / G2100 17000 3739 0.35 W2100 18000 3974 0.85 Veerman 18000 3974 1.30

Table A-2 Deltascenarios [Slootjes en De Waal, 2010]

The sea level rise of 1.30 meters, introduced by the Veerman Committee [Deltaprogramma, 2008], is a plausible upper limit, but has a lower change of occurring than the other KNMI scenario’s [Bruggeman et al., 2011].

1 absolute, reference is NAP 2 relative, reference is bottom level

137 Hydraulic analysis of a flood channel January 2013

References [Bruggeman et al., 2011] Deltascenario’s: Verkenning van mogelijke fysieke en sociaaleconomische ontwikkelingen in de 21e eeuw op basis van KNMI ’06 en WLO scenario’s. W. Bruggeman, M. Haasnoot, S. Hommes, A. Linde, R. Brugge, B. van der Rijken, E. Dammers, G.J. van den Born. Deltares, 2011.

[Deltacommissie, 2008] Samen werken met water. Een land dat leeft, bouwt aan zijn toekomst. Bevindingen van de Deltacommissie 2008.

[Slootjes and De Waal, 2010] Uitgangspunten en randvoorwaarden berekeningen waterveiligheid – keuzenotitie. Nadine

Slootjes (HKV), Hans de Waal (Deltares). HKV LIJN IN WATER en Deltares, 2010.

138 January 2013 Hydraulic analysis of a flood channel

A.2 One-dimensional approach: figure description

In the table below information is presented that could be useful by interpreting the figures in chapter 3 (One-dimensional approach). Unfortunately, the table below is not completely consistent with the two-dimensional approach.

General background information Blue line Water level [m] Light blue line Reference water level [m] Grey thin line Bottom level [m] Green area Water level difference compared to reference [m] Flood channel 0 – 3 km Section 1 (Waal to inlet structure) 3 km Inlet structure 3 – 5 km Section 2 5 – 7 km Section 3 7 – 9 km Section 4 9 – 11 km Section 5 11- 12 km Section 6 12 km Outlet structure, bifurcation outlet channels west and south 12 – 17 km (dotted line) South channel (outlet structure to Amer) 12 – 22 km West channel (outlet structure to Biesbosch) Waal – Merwede – Hollands diep 912 - 950 km Waal 950 km Inlet flood channel 950 – 961 km Boven Merwede 961 – 964 km Nieuwe Merwede to inlet Noordwaard 964 – 981 km Nieuwe Merwede dowstream inlet Noordwaard 981 km Confluence with Amer, start Hollands Diep 981 – 1020 km Hollands Diep Maas - Amer 212 – 250 km Maas 250 km Outlet flood channel 250 – 262 km Amer 262 km Confluence with Nieuwe Merwede at Hollands Diep

139 Hydraulic analysis of a flood channel January 2013

A.3 One-dimensional approach: 1D sensitivity analysis

Goal of the sensitivity analysis is to determine the influence of the flood channel’s design parameters on the flood channel’s discharge. The sensitivity analysis is performed per river section. For each section, the width, slope and roughness have been varied.

The sensitivity analysis is discussed in four parts, section 1, section 2-6, section west and south and the structures. The results are presented in a table, which shows the absolute discharge and water level changes, and in a figure, that shows the water level difference compared to the reference over the longitudinal axis of the flood channel.

Section 1 In Table A-3 and Figure A-1, the results of the sensitivity analysis performed on section 1 of the flood channel are presented. Section 1 starts at river kilometer 0 and ends at river kilometer 3, where the inlet structure is located.

OBJECT CHANGE DISCHARGE [m3/s] WATERLEVEL [m+NAP]

Inlet [%] West [%] South [%] Waal Maas

Reference 1906 740 1165 5.15 2.96 Section 1

-4 Section 1 h1 = 1.3m (slope = 10 ) 1904 -0.10 742 +0.27 1161 -0.34 5.15 2.96 Section 1 Chezy = 33 m1/2/s 1949 +2.26 753 +1.76 1196 +2.66 5.13 2.97 Section 1 Chezy = 23 m1/2/s 1845 -3.20 720 -2.70 1125 -3.43 5.18 2.95 Section 1 Width x 1.5 (1800m) 1997 +4.77 768.5 +3.85 1229 +5.50 5.10 2.98 Section 1 Width x 0.5 (600m) 1637 -14.11 646 -12.70 992 -14.85 5.29 2.91

Table A-3 Sensitivity analyses Flood channel section 1

Figure A-1 Water level differences flood channel due to measures in section 1 (Downstream of km 12, the thin line is channel south and the thick line is channel west).

In section 1, the roughness and channel width are varied. Further, the influence of a more gentle bottom level slope has been investigated.

140 January 2013 Hydraulic analysis of a flood channel

Observations:  The steep water level difference slope in section 1 is caused by the difference in discharge through the flood channel. A lower discharge through the flood channel results in higher water level at the Waal, and a lower water level in the flood channel, and vice versa.  A narrowing in section 1 effects the flood channel discharge more than a widening. A width reduction to 600 meter (-50%) causes a discharge reduction of 14%, and a water level increase in the Waal of 14 centimeters. A widening of section 1 to 1800 meter (+50%) results in a discharge increase of 4.7%, and a water level decrease at the Waal of 5 centimeters. Narrowing section 1 has approximately 3 times more influence than widening.  Changing the slope of section 1 (from 2.7*10-4 to 1.0*10-4) does not affect the discharge through the flood channel; it only has some influence on the water level in the section itself, approximately a 4 centimeters increase. The reason for the low influence can be clarified by the low equilibrium depth in section 1. The water level is determined by the downstream water level and upstream ‘boundary’, not by the bottom slope.  The discharge is 40% more sensitive to a roughness increase than to a decrease. A 5 m1/2/s lower Chezy value results in a 3.2% lower discharge through the channel, and a water level increase in the Waal of 3 centimeters.

The influence of the tested parameter on the water level of the Waal is maximal 14 centimeters. The maximal water level change on the Maas is 5 centimeters. The water level change in the flood channel due to the varied parameters is maximal at the end of section 1.

The flood channels discharge is 3 times more sensitive to an extreme narrowing than to widening of section 1. The flood channels discharge is not sensitive to small bottom level slope changes of section 1. Further, the discharge is more sensitive to a raised roughness than to a lowered roughness in section 1.

Section 2-6 In Table A-4 and Figure A-2 the results of the sensitivity analysis performed on sections 2 to 6 of the flood channel are presented. Section 2 starts at river kilometer 3 (at inlet structure) and section 6 ends at river kilometer 12, where the outlet structure is located.

OBJECT CHANGE DISCHARGE WATERLEVEL

Inlet [%] West [%] South [%] Waal [%] Maas [%]

Reference 1906 740 1165 5.15 2.96 Section 2 - 6 Section 2 - 6 Chezy= 30 m1/2/s 1813 -4.88 708 -4.32 1105 -5.15 5.20 +0.97 2.94 -0.68 Section 2 - 6 Chezy= 40 m1/2/s 1983 +4.04 765 +3.38 1218 +4.55 5.11 -0.78 2.98 +0.68 Section 3 - 6 B = 725m 1925 +1.00 749 +1.22 1177 +1.03 5.14 -0.19 2.97 +0.34 Section 5 and 6 B = 725m 1920 +0.73 747 + 0.95 1173 +0.69 5.14 -0.19 2.97 +0.34 (eq to sect. 4) Section 2 I x 2 1944 +1.99 751 +1.49 1193 +2.40 5.13 -0.39 2.97 +0.34 Section 2 I / 2 1899 -0.37 738 -0.27 1161 -0.34 5.16 +0.19 2.96 0.00

Table A-4 Sensitivity analysis section 2 – 6

141 Hydraulic analysis of a flood channel January 2013

Figure A-2 Water level differences flood channel due to measures in section 2 - 6. (Downstream of kilometer 12, the thin line is channel south and the thick line is channel west)

In section 2-6 the roughness, that is equal for section 2-6, varies. Further, the influence of the narrowed sections 3, 5 and 6 is investigated by changing the width of the narrowed sections to the width of section 4 (725 meters). In order to investigate the influence of the slope, the bottom level slope of section 2 are doubled and halved. The bottom level of the remaining sections (3-6) has been linearly interpolated between the bottom level at the end of section 2 and the bottom level at the outlet structure (0 m).

Observations:  Widening of sections 3 – 6 to the width of section 4 (725 meter) results in flood channels discharge increase of 1.00%. When only section 5 and 6 are widened to 725 meter, the flood channels discharge increases with 0.73%. Thus, the influence of section 5 and 6 on the flood channels discharge is somewhat larger than the influence of section 3.  Increasing the bottom level slope in section 2 has 5 times more influence than decreasing the slope. In addition, the influence on the water level at the Waal is larger. The water level in the channel rises where the slope in section 2 is decreased, and lowers where this slope increases.  Varying the roughness shows that the discharge is 20% more sensitive to a roughness increase than to a decrease. A 5 m1/2/s lower Chezy value results in a 4.8% lower discharge through the channel, and a water level increase in the Waal of 5 centimeters.

The flood channel’s discharge is more sensitive to an increasing roughness, than to a decreasing roughness. However, in section 2, the discharge is also more sensitive to a steeper bottom level slope then to a more gentle slope. The narrowing in section 5&6 has a little bit more influence on the flood channels discharge than the narrowing in section 3.

The influence of the tested parameter on the water level of the Waal is maximal 5 centimeters (second parameter 5 centimeters). The maximal water level change on the Maas is 2 centimeters. The water level change in section 2-6 is maximal at section 6 by removing resistance and maximal at section 2 by adding more resistance.

Section West and South In Table A-5 and Figure A-3 the results of the sensitivity analysis performed on the West and South outlet channels of the flood channel are presented. Both sections West and South start at

142 January 2013 Hydraulic analysis of a flood channel

river kilometer 12, from the outlet structure. Section west ends at river kilometer 22 and section south ends at river kilometer 19.

OBJECT CHANGE DISCHARGE [m3/s] WATERLEVEL [m]

Inlet [%] West [%] South [%] Waal Maas

Reference 1906 740 1165 5.15 2.96 West and South Section Width x 1.5 West & South (375 and 525 m) 2038 +6.93 832 +12.43 1206 +3.52 5.08 2.91

Section Chezy = 25 West & South m1/2/s 1826 -4.20 695 -6.08 1131 -2.92 5.19 2.95 Section Chezy = 35 West & South m1/2/s 1965 +3.10 777 +5.00 1188 +1.97 5.12 2.97 Section h2 = -1.5 m West & South (larger slope) 1934 +1.47 760 +2.70 1174 +0.77 5.14 2.97 Section h2 = -0.5m West & South (smaller slope) 1887 -1.00 725 -2.03 1161 -0.34 5.16 2.96 Section Width = 350 m West (equal to South) 1963 +2.99 920 +24.32 1044 -10.39 5.12 2.92 Section i = 10-4 South (equal to West) 1898 -0.42 750 +1.35 1148 -1.46 5.15 2.96

Table A-5 Sensitivity analysis section West and South

Figure A-3 Water level differences flood channel due to measures in section west and south (Downstream of kilometer 12, the thin line is channel south and the thick line is channel west).

In the West and South channel, the roughness has been varied, but is kept equal for West and South channel. Further, the influence of the bottom level slope is investigated and the width of both sections has been widened. In addition, one calculation is done with an equal width for section West and South.

Observations:  Widening of both channels width 50% results in discharge increase of 6.9% to 2038 m3/s.  When the width of section West is set equal to the width of section South (350 meter), the total discharge increases with 3%. More interesting is the changing discharge

143 Hydraulic analysis of a flood channel January 2013

distribution, with 24% more water flowing through the West channel, and 10% less through the South channel.  Changing the bottom level slope of both branches causes small changes in the total discharge. The model is more sensitive to a steeper slope than to a gentler slope.  When the bottom level slope of section South is set equal to the slope of section west, the total discharge through the flood channel lowers with 0.42%. The discharge distribution between the South and West channel moves slightly to the west.  The model’s discharge is 35% more sensitive to a roughness increase than to a roughness decrease.  In general, when the total discharge through the flood channel changes, the discharge change is the largest in channel West.  The water level change in the flood channel is maximal at the start of the outlet channels. The water level at the end of the outlet channels varies only when the discharge (distribution) changes.

Widening of the West and South channel has a significant impact on the total discharge. When the width or slope of one of the channel changes, the discharge distribution changes. Changes in the discharge distribution have relatively more impact on the West than on the South channel. The model is more sensitive to a steeper slope than to a gentler slope. The model is also more sensitive to a roughness increase than to a decrease.

The influence of the tested parameter changes on the water level of the Waal is maximal 8 centimeters. The maximal water level change on the Maas is 5 centimeters.

Structures In Table A-6 and Figure A-4 the influence of the structures on the water level is presented. The inlet structure is located at river kilometre 3; the outlet structure is located at river kilometre 12.

OBJECT CHANGE DISCHARGE [m3/s] WATERLEVEL [m+NAP] Inlet [%] West [%] South [%] Waal [%] Maas [%] Reference 1906 740 1165 5.15 2.96 Structures Inlet structure Bx0.5 (400m) 1877 -1.52 729 -1.49 1148 -1.46 5.17 +0.39 2,96 +0.00 Inlet structure Bx1.5 (1200m) 1913 +0.37 742 +0.27 1170 +0.43 5.15 +0.00 2,97 +0.34 Outlet structure Bx0.5 (250) 1870 -1.89 725 -2.03 1146 -1.63 5.17 +0.39 2,96 +0.00 Outlet structure Bx2 (1000) 1916 +0.52 744 +0.54 1172 +0.60 5.15 +0.00 2,97 +0.34 Inlet and outlet Contraction 1903 -0.16 739 -0.14 1164 -0.09 5.15 +0.00 2,96 +0.00 structure 0.7; Ka = 1.20 Table A-6 Performed analysis at inlet and outlet structure

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Figure A-4 Water level difference caused by structures

The width of the inlet and outlet structure has been varied to investigate the influence. Further, the inlet and outlet construction is increased.

Observations:  A 50% smaller inlet structure causes a discharge decrease of 1.52%. A 50% wider inlet structure causes a discharge increase of 0.37%. The corresponding water level differences are maximal 3.5 centimeters (smaller inlet) and 0.7 centimeters (wider inlet).  Widening the outlet structure with a factor two increased the discharge with 0.5%. The maximal water level change is 3 centimeters.  Changing the contraction coefficient from 0.8 to 0.7 (more contraction) results in a discharge decrease of 0.16 %.

From the observations above, it can be concluded that the influence of the inlet and outlet structure on the system is small. A wider in- or outlet structure makes the water level difference between before and after the structure smaller, but the influence on discharge and water levels is negligible.

Summary A sensitivity analysis has been performed with the schematized one-dimensional model. Goal of the sensitivity analysis is to determine the influence of the flood channel’s design parameters on the flood channel’s discharge. For each river section, the width, slope and roughness have been varied. The detailed results of the sensitivity analysis are presented in appendix A.3. The most important results are summarized below:  The discharge of the flood channel is most sensitive to a widening of the outlet channels. A widening with 50% results in a discharge increase of 132 m3/s (+6.9%), and an extra water level decrease at the Waal of 7 centimeters.  Increasing the width of narrowed sections 5 and 6 to the same width of section 4 (725 meters), results in a small discharge increase, and a water level decrease at the Waal of 1 centimeter.  The influence of the inlet and outlet structure on the system is small. A wider in- or outlet structure results in a smaller water level step at the structure, but the influence on discharge and water levels is negligible. A smaller structure has more influence on the channel than a wider channel; a width decrease of 50% results in a water level increase at the Waal of 2 centimeters.

145 Hydraulic analysis of a flood channel January 2013

 The main channel and the outlet channels have approximately the same sensitivity to a roughness increase, the inlet channel a little bit less. A lowering of the Chezy coefficient with 5 m1/2/s (actually a roughness increase) results in a water level increase of 5 centimeters at the Waal. The system is less sensitive to a roughness decrease than to an increase.

146 January 2013 Hydraulic analysis of a flood channel

A.4 Two-dimensional approach: Barriers

There are five barriers in the Rijn-Maasmonding model (RMM-model). Each model is described separately in the following paragraphs.

Figure A-5 Barrier locations [Zijl et al., 2011]

Maeslant barrier The Maeslant barrier is a movable barrier with two large doors located in the New Waterway. When the barrier is closed, the waterway to the port of Rotterdam is blocked. Therefore, the barrier is designed to close once every 5 to 10 year. The barrier closes only when high sea water levels are expected.

Figure A-6 Maeslant barrier (Google earth)

When the expected water level at Rotterdam is NAP +3.00 m or at Dordrecht is NAP +2.90, the barrier is prepared for closure. In the preparation phase, water is pumped out of both doors to make them float. When the upper-Rhine discharge at Lobith is less than 6000 m3/s, the closure operations starts when water level near the barrier exceeds NAP +2.00 meter. When the upper- Rhine discharge at Lobith is more than 6000 m3/s, the closure starts when the tide flow reverses to a flow from sea into the New Waterway.

The closing procedure is as follows. First both doors, which are floating, float to the right position and start sinking. Near the bottom there is a short stop to remove sediment from the sill, and subsequently the gates sink to the bottom. When the water at the riverside of the barrier is higher than at the seaside of the barrier, the barrier starts inflating to floating position. In this floating position there is a discharge under the doors. Depending of the

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expected water levels in the next 24 hours, the doors sink again or open. The opening and closing process is automated.

Maeslant barrier Model Sill height NAP –17.00 m NAP –17.00 m Height gates when closed NAP + 5.00 m +∞ m Bottom height gates when closed NAP –16.72 m Draft gates approx. 7 m Height gates when floating approx. NAP +7.00 m +∞ m Bottom height gates when floating NAP –7 m Sink speed gates Closure time approx. 90 m approx. 75 min Time from closed to floating approx. 90 min approx 75 min position Discharge coefficient culvert 0.85 Discharge coefficient weir 1 Leak opening with gates closed 100 m2 100 m2 Table A-7 Facts about Maeslant barrier

In WAQUA, the Maeslant barrier is schematized with two vertical moving barriers instead of horizontal moving doors. Each barrier consists out of a culvert and a weir. The culvert represents the (closing) gate; the weir represents the height of the gates when they are closed. For stability reasons, the culvert and weir are not active at the same moment, so when the doors are floating or closed, waves cannot overtop the barrier. The barrier is controlled outside WAQUA, with a MATLAB script.

Opening and closing procedure in the model is approximately the same as described before. In the model, the sediment stop is not included to save calculation time. However, influence on water level is negligible.

The most important differences between the model and reality are:  The doors are closing vertically instead of horizontally. Closure and opening times are adapted to this schematization, so the effect on the water level is negligible.  The leak opening in the schematization is only below the gates, while in reality water leaks also between the doors. The total leak area is equal.  Overflow is not possible in the model due to problems with numerical stability.

Hartel barrier The Hartel barrier is located in the Hartel canal, a canal approximately parallel to the New Waterway, and also a connection between inland waters and the sea. The Hartel barrier together with the Maeslant barrier protects the Old and New Maas against high sea levels. The barrier is designed to close every 5 to 10 year.

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Figure A-7 Hartel barrier (Google earth)

The barrier has two, vertical moving, gates that close at high sea water levels. The widths of the gates are 95.5 and 46.8 meter.

The barrier will close when the expected water level at Rotterdam is NAP +3.00 meter or NAP +2.90 meter at Dordrecht. When the upper-Rhine discharge at Lobith is less than 6000 m3/s the closure operations starts when the water level near the barrier exceeds NAP +2.00 meter. When the upper-Rhine discharge at Lobith is more than 6000 m3/s, the closure starts when the tide flow near the barrier reverses, resulting in an inland water flow.

The closing procedure is as follows. In the preparation before closure, the gates are lowered to NAP +3.50 meter, with a velocity of 1 m/s. Closure starts when the water level near the barrier exceeds NAP +2.00 meter of tide flow near the barrier reverses (based on computer calculations), or starts when the Maeslant barrier closes. When the water level at the riverside is higher than at the seaside of the barrier during closure or when closed, the barrier goes to sluice state (spuitoestand). From these positions the gates will close again or open, depending on the expected water levels.

Hartel barrier Model Sill height NAP –6.50 m NAP –6.50 m Height gates when closed NAP +3.00 m NAP +3.00 m Bottom height gates when open NAP + 13.50 m NAP + 7.00 m Sink speed gates South gate: 1 m/min South gate: 0.0044 m/s North gate: 0.2 m/min North gate: 0.0255 m/s Lift speed gates South gate: 0.7 m/min South gate: 0.0044 m/s North: 0.2 m/min North gate: 0.0255 m/s Discharge coefficient culvert 0.64 Discharge coefficient weir 1 Leak opening with gates closed 47 m2 0 m2 Table A-8 Facts about Hartel barrier

The gates are modelled with a double barrier, a culvert and a weir. The culvert represents the (closing) gate. The weir represents the height of the gates when they are closed. In WAQUA, overflow and underflow at the same time is not possible due to numerical instabilities. For the Hartelkering, overflow has more influence on the water level then underflow, so leak opening is set to zero. The barrier is controlled outside WAQUA by a MATLAB script.

The most important differences between the model and reality are:

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 Leak opening is not schematized, in reality it is 47 m2 (effect is only a few millimetres);  In rest, the bottom height of the gates is NAP +7 meter instead of NAP +13.50 meter;  Sink/lift speeds differ in the model from reality;

Haringvliet sluices The Haringvliet sluices control the discharge between the Haringvliet estuary and the North Sea. In general, the sluices are open at low tide and closed at high tide, but in special occasions, e.g. a storm surge, the sluices are also closed at low tide.

The Haringvliet sluices have 17 openings, and each opening has a vertical movable gate. The gates open when the water level on the inner side of the sluices is higher than the water level on the outer side of the sluices, and the other way around. This usually takes place when tide is changing from high to low (and vice versa). When the sluices are open, the size of an opening is dependent on the upper-Rhine discharge at Lobith. The gates stay closed for discharges lower than 1100 m3/s and are completely open for discharges of more than 9000 m3/s.

Under normal conditions, the sluices have much influence on the discharge distribution between Haringvliet and the New Waterway. In extreme conditions also the Maeslant- and Hartel barrier have significant influence.

Haringvliet sluices Model Sill height NAP -5.5 m NAP -5.5 m Height gates when closed NAP +5.0 m NAP +5.0 m Bottom height gates when open NAP +6.0 m NAP +5.0 m Sink speed gates 0.009583 m/s 0.0088 m/s Discharge coefficient culvert 0.83 Discharge coefficient weir 1 Table A-9 Facts about Haringvlietsluices [Zijl et al., 2011]

In the RMM model, each of the 17 gates lies in a separate grid cell. The sluices are modelled with a double barrier, a culvert and a weir. The culvert represents the (closing) gate. The weir represents the height of the gates when they are closed. The gates close/open when the water level difference over the sluices is more than 5 centimeters.

The most important differences between the model and reality are:  The sink/inflate speed in the model is different from reality. This is mainly caused by the lower bottom height of the gates in the model. The closing time is the same (20 minutes). The opening time in reality is 25 minutes, because it is not possible to open all the gates at the same time. However, in WAQUA the sink and inflate speed cannot be defined separately so the opening time is overestimated.  The opening size of a gate is dependent on the upper-Rhine discharge at Lobith. Because Lobith is not included in the model, the discharge near Tiel is used.

Hollandsche IJssel Barrier In the Hollandsche IJssel near the city of Krimpen aan de IJssel the Hollandsche IJssel barrier is located. The barrier has two movable gates, placed after each other, and a sluice. The barrier protects the area behind the barrier against high water levels.

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The barrier will close when the expected water level at Hoek van Holland is more than NAP +2.0 meter. Dependent on discharge of the Rijnlandse boezem and the water level at Hoek van Holland, start of closure is triggered by a water level of NAP+2.25 meter at the barrier or through low water tide reversal. The gate will open when the water level outside the barrier is equal to or lower than the water level on the inner side of barrier.

Hollandsche IJssel barrier Model Sill height NAP –6.50 m NAP –6.50 m Bottom height gates when open NAP +12.00 m NAP +12.00 m Height gates when closed NAP +5.00 m NAP +∞ m Closing time 50-65 minutes 60 minutes Opening time ca. 30 minutes 30 minutes Discharge coefficient culvert 1 Discharge coefficient weir 1 Table A-10 Facts about Hollandsche IJssel barrier

In WAQUA, the barrier is modelled as one gate instead of two. The barrier is modelled as a culvert and a weir. The culvert represents the (closing) gate. The weir represents the height of the gates when they are closed. In the model, the height of the weir is infinite. The sluice is permanently closed with a gate height of NAP +5.00 meter.

In the model, the gate closes when the water level of approximately 300 m downstream the barrier is more than NAP +1.80 meter and there is a positive water level slope over the barrier. The gate opens when the water level approximately 300 m downstream the barrier is lower than NAP +1.80 meter and there is a negative water level slope over the barrier [Zijl et al., 2011].

The most important differences between the model and reality are:  Only the storm surge barrier function is modelled. Other functionality, e.g. shipping, is not included;  Sluice is permanently closed in the model;  The opening and closing trigger is based on a local water level instead of an expected water level near Hoek van Holland and an expected discharge on the Hollandsche IJssel;  In WAQUA, start of closure is based on a water level, not on low water tide reversal;

Krommen Nolkering The Krommen Nolkering is a barrier in the Heusdensch Kanaal, near the bifurcation between the Heusdensch Kanaal and the Maas. The Heusdensch Kanaal connects the Afgedamde Maas with the Maas. The barrier will be closed when there are high Maas discharges.

Closure of the barrier starts when the water level near Heesbeen exceeds NAP +3.50 meter, and the opening of the barrier starts when the water level is below NAP +3.50 meter (in model NAP +3.49 m).

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Krommen Nol barrier Model Sill height NAP -4.00 m NAP -4.00 m Bottom height gate when open NAP + 11.40 m Height gate when closed NAP +6.00 m NAP +∞ m Closing time approx. 45 min 45 min (0.0057 m/s) Opening time approx. 45 min 45 min (0.0057 m/s) Discharge coefficient culvert 1 Discharge coefficient weir 1

Table A-11 Facts about Krommen Nol barrier

The barrier is modelled as a culvert and a weir. The culvert represents the (closing) gate. The weir represents the height of the gate when it is closed. In the model, the height of the weir is infinite. The most important difference between the model and reality is the sink/inflate speed. In the model, this speed is linear, constant and equal for sinking and inflating. In reality, this is not the case.

References [Zijl et al., 2011] WAQUA-model Rijn-Maasmonding. Modelopzet, calibratie en verificatie. F. Zijl, D. Kerkhoven, A.Z. Visser, T. van der Kaai. Deltares, 2011.

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A.5 Flood channel: dike reinforcements

For the Room of the River program, a study is performed to investigate the dike reinforcements that are required without Room for the River measures. Some results are presented in Figure A-8. The corresponding profiles of the dike reinforcements are presented in Figure A-9, and more details can be found in Table A-12.

Figure A-8 Proposed type of dike reinforcement for several dike sections [Arcadis, 2003]

Legend

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Figure A-9 Types of dike reinforcement investigated [Arcadis, 2003]

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Table A-12 Background information dike reinforcements [Arcadis, 2003]

References [Arcadis, 2003] Referentie alternatief dijkversterking (RAD). Arcadis, Royal Haskoning and Fugro. June 2003.

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B Grid design and domain decomposition

For the flood channel, a new grid has been designed which is connected to the grid of the Rijn- Maasmonding-model by domain decomposition. This chapter starts in paragraph B.1 with a short introduction to the opportunities to create a numerical grid for the flood channel. In paragraph B.2, the design of the new grid for the flood channel is described, and in paragraph B.3 the steps that were taken to connect this new grid (FC-grid) to the grid of the RMM model (RMM-grid) were described. In the last paragraph (B.4), some conclusions are drawn regarding the grid and domain decomposition.

B.1 Introduction

In a two-dimensional model like WAQUA, all the spatial information of a hydraulically model is related to numerical grid coordinates. Therefore, a simulation in WAQUA is only possible when there is a numerical grid that covers the whole model area. For this project, a new flood channel has been designed, but unfortunately, the largest part of the flood channel lies outside the numerical grid of the Rijn-Maasmonding (RMM) model (see Figure B-1). As a result, the flood channel cannot be simulated using this RMM grid.

Figure B-1 Gap in the RMM-grid at location of the new flood channel. Red lines are dikes for the flood channel.

In order to create a numerical grid that covers the flood channel area, there are two options: 1. Extending the RMM grid 2. Create a new grid and connect this one to the RMM grid by domain decomposition

Both options are discussed below.

Extending the RMM-grid Extending the RMM-grid means that the lines of the RMM grid are extended through the gap to cover the area of the flood channel. Extending the grid lines has some limitations:  When a line with a certain index value is extended through the gap, it can only be connected to the same line (same m or n indices) on the other side of the gap. This limits the amount of lines, but also the general direction of the lines. See Figure B-2 (A).  Because the flood channel is located in a gap that is surrounded by grid points, it is only possible to extend existing lines. No new horizontal and vertical lines can be added

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between the existing lines, and therefore, the amount of grid lines is limited. See Figure B-2 (B).

A possible grid in the gap is presented in Figure B-2 (C). The grid cells are limited by the fixed boundaries.

Figure B-2 Limitations (A and B) and freedom (C) in extending grid in a’ gap’

Due to the limitations on the amount and direction of the extended gridlines, it is difficult, or maybe even impossible, to make a smooth grid with a reasonable resolution for the flood channel.

New grid, connected the RMM-grid by domain decomposition Instead of extending the RMM grid, it is also possible to make a new grid and connect this one to the RMM grid by domain decomposition. This option has more freedom in grid design than extending the RMM-grid, the advantages and disadvantages:  The number of lines in horizontal and vertical direction are unlimited, but should be ‘n’ of ‘1/n’ times the amount of gridlines at the RMM grid at the connection, see Figure B-3 (A)  There is no fixed relation between the line indices of the new grid and the indices of the original grid, so the orientation of the lines is free (B).  There is a limitation, it is not possible to connect m-lines on one side of the gap to n- lines on the other side of the gap if they are both connected to the same grid (C).

Figure B-3 Limitation (C) and freedom (A and B) in using domain decomposition

Conclusion Extending the RMM grid has many limitations, and it is questionable if it is possible to make a numerical stable grid with this option. Based on the observations above, creating a new grid for the flood channel, and connect this one to the RMM-grid by domain decomposition, is the best option, because there are less restrictions.

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B.2 Grid

In this paragraph, the steps taken to design the new FC-grid will be presented. First, some backgrounds about grids will be discussed in paragraph B.2.1. Based on that knowledge and the design of the flood channel, the design requirements are determined in paragraph B.2.2. The steps to create the grid is described in paragraph B.2.3 and the results are discussed in paragraph B.2.4.

B.2.1 Background

The numerical grid is the foundation for all the spatial model input of a hydraulical 2D-model. The model input (e.g. bottom level, roughness, boundary conditions) and model output (e.g. water levels, currents) are determined for each grid cell.

Basically, a numerical grid is nothing more than a set of coordinates with m and n indices. Each grid point has a (m,n) coordinate. The grid points are connected to each other by line segments. Lines connecting points with a constant m-indice are called m-lines, and lines connecting points with a constant n-indice are called n-lines. The area between four grid points (and thus four line segments) is a grid cell.

Grid types The simplest grid is an equidistant rectangular grid, as presented in Figure B-4. The distance between the m and n lines is equal in both directions, and the angle between the m and n lines at each grid point is 90o. Advantages of this type of grid are the simplicity, easy creation and perfect smoothness and orthogonality. On the other hand, disadvantages of the equidistant rectangular grid are the poor representation of river bends, navigation channels and coastlines; this is especially important for morphology calculations. In addition, local refinement of the grid is not possible. Another type of a simple grid is a circular grid. An example of a circular grid is presented in Figure B-4 (middle).

Figure B-4 Equidistant rectangular grid (left), circular grid (middle) and curvilinear grid (right)

A more complicated type of grid is a curvilinear grid, in which the grid lines can be curved, and distance between gridlines is free. A curvilinear grid can follow horizontal geometry smoothly and makes it possible to refine the grid locally, e.g. at the winterbed of a river. Disadvantage of this freedom is the risk of inaccuracy due to non-smooth or non-orthogonal grid cells.

Staggered grid A grid cell is defined by grid points, which are the vertices of a cell. The vertices are connected by straight lines; the cell faces. In a collocated grid, the knowns and unknowns variables of each cell are located at the vertices. WAQUA arranges the variables differently, and uses a staggered grid to solve the discretized equations. The arrangement of variables in a staggered grid is presented in Figure B-5.

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Figure B-5 Arrangement of unknowns of a staggered grid (Arakawa C-grid) [Simona, 2012]

In a staggered grid, the arrangement of the variable is as follows:  Vertices: bottom level and other model input  Cell center (+): scalar variables (e.g. density, water level, salinity)  Centre of a cell face (- & |): u- and v-velocity components Thus the unknowns are shifted half a cell width compared to the grid points (vertices).

For a staggered grid, the number of computational values per grid cell is a factor four lower compared to a collocated grid, while the accuracy is maintained. Furthermore, implementation of boundary conditions is simplified [Simona, 2012].

Accuracy In a hydraulic simulation, the water levels and velocities are calculated for each grid cell by discretized mass and momentum equations. Due to simplifications of the discretized equations in WAQUA, a numerical grid must satisfy some conditions to ensure accurate results. There are three grid parameters that have influence on the accuracy of a (curvilinear) grid:  Smoothness  Orthogonality  Aspect ratio

In WAQUA, it is assumed that the grid points are smoothly distributed in space. This is the case when the ratio between the grid sizes of two adjacent grid cells is not too large. A grid is smooth when the deviation in cell size for each grid cell is not more than 20% compared to the adjacent grid cell in the same m- or n-direction. Therefore the smoothness must be smaller than 1.2 [Simona, 2012].

Another assumption in WAQUA is an orthogonality of zero. The orthogonality is the angle between the m and n lines at the grid points. For a curved curvilinear grid, it is theoretically impossible to have an orthogonality of zero. Therefore, the guideline for steady flow is an orthogonality smaller than 0.5 [Simona, 2012].

The guideline for the aspect ratio between the cell-size in m and n direction is 1 to 2 [Deltares, 2011].

Prerequisites domain decomposition The domain decomposition will be discussed in detail in paragraph B.3. However, to connect two grids the following aspects should be taken in mind while designing the grid:

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 The grid cells at the connection should have at least 2 cells overlap to make horizontal domain decomposition possible  The grid administration of both grids should have the same positive orientation for m and n indices at the connection  The number of lines at the connection should be ‘n’ of ‘1/n’ times the amount of gridlines at the existing grid (see Figure B-3 A)

B.2.2 Design requirements

In order to create a new grid the requirements have to be defined. The RMM grid at the inlet and outlet of the flood channel is presented in Figure B-1. The requirements for the new grid are:  The grid should (at least) be connected with the RMM grid at the inlet and at the outlet  The grid should cover the whole flood channel area (see Figure B-1), including a small part north east of the inlet that lays outside the RMM grid (see Figure B-6, left image)  The grid must be numerically stable at a time step of 0.25 minutes

Figure B-6 RMM grid at flood channels inlet (left) and outlets (right)

B.2.3 Approach

In the previous paragraph, the requirements for the new grid have been presented. Based on those requirements the approach to create a new grid is described below.

Grid type First, a type of grid must be chosen. The options are a (equidistant) rectangular grid and a curvilinear grid. As discussed in paragraph B.2.1, a curvilinear grid has advantages over a rectangular grid. Because the RMM grid is curvilinear, the new grid should at least be curvilinear at the connection with the RMM grid. Because of the advantages of a curvilinear grid over the rectangular grid, and because the new grid should at least be curvilinear at the connection with the RMM grid, the new grid will be curvilinear.

Grid size The next step is to determine the grid size that is numerically stable. For a numerically stable calculation, the Courant number should be below a certain value. In an explicit calculation scheme, the Courant number should be smaller than 1. In WAQUA, an implicit method is used to solve the mass and momentum equation. Therefore, the Courant number should be smaller than 4*20.5 for a stable calculation [Simona, 2012].

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Where: Δt (min) Timestep g (m/s2) Gravitational acceleration H (m) Water level Δx (m) Grid size x-direction Δy (m) Grid size y-direction The cell size of the RMM grid is in average 40*40 m2. This cell size in combination with a time step of 15 seconds leads to a courant number that meets the requirement. Therefore, the cell size of the new grid will also be 40*40 m2 (in average).

Grid orientation As already noted in paragraph B.2.1, the grid administration of two grids should have the same positive orientation for m and n indices at the couple boundary.

Figure B-7 Turned grid administration RMM grid

In Figure B-7, the grid orientation of the RMM-grid is displayed in black, and two options for a new FC-grid are drawn in green and blue. The orientation of the RMM-grid at the end of the flood channel is rotated a quarter compared to the begin of the flood channel. Therefore, the grid indicated with green is not possible, because the grid orientation at the west side of the green grid differs 90o from the orientation of the RMM-grid. The only solution is the grid indicated with blue.

Connection with RMM-grid With the grid orientation known, the couple boundaries for domain decomposition can be defined. There will be three connections with the RMM grid, East, West and South (see Figure B-8). In order to have more freedom in grid design, there will be two connections at the west side, because with one connection much more cells and lines are fixed than with two connections.

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Figure B-8 Couple boundaries between new grid and RMM-grid

If the total overlap between the two grids at each connection is two grid cells, the couple interface can be located between those two cells.

Rough design With the type of grid, grid size and global design known, the rough design can be created. For the grid design, the software RGF Grid is used.

Starting points:  Curvilinear grid  In average, the grid size should be equal to the RMM grid (40*40 m2). This means that there will be no refinement at the connection of the new grid and the RMM grid  Grid should cover the whole flood channel  At the connection, the overlap between the two grids must be 4 water level points  The grid orientation at the connection with the new grid must be equal to the orientation of the RMM grid. Therefore, the new grid must turn equally in grid orientation as the RMM grid.

First, the grid is roughly designed by splines. Later, these splines can be converted to a grid. The splines follow the land boundaries as much as possible, but at the west side this is not possible due to the turning grid orientation (Figure B-9).

Figure B-9 RMM grid (blue) with land boundaries Figure B-10 Improved rough design (brown) and the rough design with splines (green)

In Figure B-10 the design is further improved. The rough design covers the land boundaries. In addition, the locations where the new grid will be connected to the RMM-grid are determined. The splines at the connection are chosen in such a way that the amount of cells at the RMM grid between the splines is a multiple of 8. This makes refinement in a later stage easier.

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In Figure B-11, the rough design is further improved by dividing the design displayed in Figure B-10 in approximately equal areas. This is necessary to let the initial grid follow the splines smoothly.

Figure B-11 Rough design, divided in ‘equal’ areas Figure B-12 Initial grid

The splines are converted to a grid with a refinement factor of 4. The result is visible in Figure B-12. Unfortunately, a few splines were not converted to the grid. These grid cells were added manually. Therefore, the initial grid deviates somewhat from the rough design in splines.

The initial grid design is fixed at the connections with the RMM grid; the grid cells that are not part of the couple boundaries can be adapted freely, as long as they cover the land boundaries.

Global optimalisation This initial grid is globally smoothed and orthogonalised. After executing the smoothening option in RGF-grid, a large part of the grid meets the smoothness requirement of 1.2. This is also the case for the orthogonality, as you can see in Figure B-13, where only the blue parts have an orthogonality larger than 0.05.

Figure B-13 Initial grid before and after orthogonalisation

After smoothening and orthogonalisation, the grid is refined to a factor 1:2. The result is visible in Figure B-14. The locations where the grid is extended are sensitive for the non-smooth and non-orthogonal grid cells.

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Figure B-14 Grid refinement to 1:2

Local optimalisation The grid meets the smoothness and orthogonality requirements at most places, but locally they are still violated. Before the grid is refined to the definitive resolution, the local deviations were reduced. At some places, large deviations are acceptable because they are outside the land boundaries.

Figure B-15 Orthogonality before (left, dark blue 0.26) and after (right, dark blue is 0.20) local optimalisation

In Figure B-15, the difference in orthogonality before and after local smoothening and orthogonalizing is presented. Note that the scale of both images is different. After optimalisation, only at the North East part the requirements have not been met. The fixed points due to the connection with the RMM grid makes it impossible to make that area smooth and orthogonal. The area is presented in detail in Figure B-16.

Figure B-16 Local orthogonality northeast part of the grid, before (left, dark blue 0.36) and after (right, dark blue is 0.17) optimalisation

The inlet of the flood channel is located below the blue area. In the area where the orthogonality is too high, the expected velocity and discharges are low. Therefore, it is expected that an error in that area does not have significant influence on the results, and is therefore tolerated.

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B.2.4 Results and analysis

After local optimalisation of refinement, the grid is ready for use. The final grid is displayed in Figure B-17. Analyses of different parameters of the grid are presented in the several figures in this paragraph.

Locally, especially near the East connection with the RMM grid, the smoothness, orthogonality and aspect ratio violates the criteria. Those violations are caused by the fixed cells that are necessary for the connection with the RMM grid. Fortunately, the cells that violate the criteria are north of the inlet. It is expected that the flow velocities are low, and therefore the effect on the results will be negligible.

Figure B-17 Final grid Figure B-18 Orthogonality (white = 0.04, maximum value = 0.26)

Figure B-19 M-smoothness (light blue = 1.2, Figure B-20 N-smoothness (light yellow is 1.2, maximum value = 1.59) max 2.53)

Figure B-21 Resolution (Green is 20 m2, white = Figure B-22 Aspect ratio (light blue = 2, max = 5) 40m2, max = 130m2)

In Figure B-18 until Figure B-22 the orthogonality, smoothness, resolution and aspect ratio of the new grid is presented. With exception of the Northeast part of the grid, most results are satisfying:  The orthogonality lies in general within the range of 0.05, except near some boundaries where no water flows.

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 In general, the m- and n-smoothness values are everywhere lower than 20%. Only near the couple boundaries, a few cells have somewhat larger values, caused by the fixed cells. Most of the smoothness peak values are outside the land boundaries.  The average resolution varies between 25 m2 and 60 m2, and is in accordance with the requirement. Near the borders of the grid, the resolution at some locations raises to 80 m2. The peak resolution is Northeast of the grid, outside the land boundaries.  Guideline for the aspect ratio is a value of 2. There are two areas where the aspect ratio is more than two: near the east connection and on the inner side of the curve to the south. The reason for the first area has already been discussed. The reason for the second area is the large curvature.

In general, the grid meets all the requirements as stated in paragraph B.2.2. The grid covers the whole flood channel, and at the connection, the new grid has two grids cells overlap with the RMM grid. It is expected that the grid will be numerically stable, but some extra attention is needed at the Northeastern part of the grid.

Recommendations  Avoid too many ‘grid extensions’ and grid corners near places where the water will flow, because smoothness and orthogonality could be a problem there. It would be better to create a large rough grid without extensions that covers the whole area, and delete unnecessary parts later.  If extensions to the grid are necessary, design them in ‘blocks’  If the grid will be connected to another grid, count the lines at the connection well before starting with the design  Work with line distances that are easy to refine

B.3 Horizontal domain-decomposition

The grid designed in paragraph B.2 must be connected to the RMM grid. This is possible with horizontal domain decomposition (ddhor). First, the background necessary to understand the domain decomposition is explained in paragraph B.3.1. Secondly, the approach to couple the grids is presented in paragraph B.3.2. Finally, the results are presented in paragraph B.3.3.

B.3.1 Background

Horizontal versus vertical domain decomposition By applying horizontal domain decomposition (ddhor), it is possible to couple different numerical grids to each other. This is for example useful when a river is connected to a sea. A river needs a higher grid resolution than a sea to be able to describe the water movement well. If the sea is modelled with the same resolution as the river, running the model will be time consuming and ineffective. By connecting the grids with horizontal domain decomposition, separate grids for the sea and the river can communicate during a simulation, each grid with its optimal resolution.

Where with horizontal domain decomposition two-dimensional grids are connected, with vertical domain decomposition vertical layers are connected. This is for example useful when a river is connected to a lake, where the stratification is being researched. For the river, a depth- averaged velocity is detailed enough, while for the lake the depth dependent velocity is

167 Hydraulic analysis of a flood channel January 2013

important. However, vertical domain decomposition has not been used in this project, so it will not be discussed in more detail.

Couple interface Grids that are coupled with horizontal domain decomposition exchange information about e.g. water levels and velocities. This information is exchanged at the couple interface. This interface defines the transition between the two grids, and has the same absolute coordinates at both grids.

Figure B-23 Simple example horizontal domain decomposition of two grids in WAQUA

In Figure B-23, the couple interface is indicated with the red line. When the two grids on the left (A and B) are coupled by ddhor, the resulting grid is the grid on the right (A & B). By using ddhor, the grids are not merged to a new grid in a physical way, but the separate grids behave like they are glued together, because information exchange between the grids is possible. The results of grid A are displayed left of the couple interface, and the results of grid B are displayed right of the couple interface.

This couple interface is defined through depth and velocity points of the grid [Simona, 2011]. In order to make the information exchange between both grids possible, both grids overlay the other grid with one cell (blue area). This area is defined by the area (green dotted lines) and enclosure lines (blue dotted lines). Although the area and enclosure lines are defined through the water level points, the overlay is defined through the depth points. Therefore, the overlay area is staggered half a grid cell compared to the area and boundary lines.

In order to make coupling of two grids possible, the grids should have a total overlap of two cells (vertices in depth points), see Figure B-23. Those cells should have the same absolute coordinates.

COPPRE file In order to perform a WAQUA simulation, the software must know which grid cells should be taken into account, and which not. This information is written in a so called COPPRE file. This file assigns a value to each grid cell.  A grid cell that lies outside the enclosure, and should not be taken into account, has a value of 0.  An active cell has a positive integer value. When the active part inside the enclosure is one partition, the COPPRE value for all active cells is 1, but when the active part is

168 January 2013 Hydraulic analysis of a flood channel

divided in several partitions, e.g. for parallel computing, the value assigned to a cell is equal to its partition number.  When ddhor is applied, both grids should overlay the active part of the other grid with one cell. To those cells that overlay the other grid, a COPPRE value of –1 is assigned. Those cells are only used for information exchange between the grids.

The cell values in the COPPRE file are based on polygons in the grid ENCLOSURE file and polygons in the AREA file. In the AREA file, active grid cells are grouped by one or more polygons. To each polygon, a subdomain number (positive integer value) is assigned, which is equal to the cell value in the COPPRE file. The ungrouped cells have a default value of –1. The polygon in the ENCLOSURE file describes the outer model boundary and the area outside this boundary is excluded from the calculation by setting the COPPRE value to 0.

The interaction between the AREA and ENCLOSURE file is further explained with three examples:

Figure B-24 Relation Area, enclosure and COPPRE file; one partition, no ddhor

If only one polygon is defined in the AREA file with subdomain number 1, and no ddhor, the polygon in the AREA file is equal to the polygon in the ENCLOSURE file (Figure B-24). In the COPPRE file, the grid cells within the area/enclosure polygon are set to 1, and the grid cells outside the enclosure are set to 0.

Figure B-25 Relation Area, Enclosure and COPPRE file, 4 partitions, no ddhor

If the active area is described by four polygons with different subdomain numbers, the ENCLOSURE file describes the outer borders of all the polygons (Figure B-25). In the COPPRE file, the area outside the enclosure file is set to 0 and the cells inside the polygons as defined in the AREA file, are set to a value 1, 2, 3 or 4. Each group of cells with a unique positive integer COPPRE value is a separate partition, which will run in a separate core.

169 Hydraulic analysis of a flood channel January 2013

Figure B-26 Relation Area, Enclosure and COPPRE file, 1 partition, ddhor (couple edge in red)

In the case that a grid is connected by ddhor, the cells that overlay the other grid should have a COPPRE value of –1. If there is one partition, the active cells are grouped by a polygon with subdomain 1 in the AREA file (Figure B-26). The ENCLOSURE file describes the outer border of the AREA file, and the overlapping cells. In the COPPRE file, the polygon defined in the AREA file is set to 1, the area outside the enclosure is set to 0, and the area between the polygons defined in the ENCLOSURE and AREA file is set to –1. The border between the active cells and overlapping cells is the couple edge (red in Figure B-26).

Limitations ddhor Coupling of two domains in WAQUA has some limitations. The most important limitations are summarized below [Simona, 2011]:  Time step and time frame should be the same in all domains  The grid orientations at the coupling interface must be the same for both domains  It is possible to use different roughness formulations in the different domains. But if k- Nikuradse roughness formulation is used in one domain, is should be used in all the domains  The mutual interface of two domains should have the same refinement everywhere. Variation is not possible in WAQUA  All couple interfaces (cell face between an active cell and a cell with COPPRE value –1) must be coupled

B.3.2 Approach

In the paragraph B.2.3, the coupling interface is already determined. In this paragraph, the new grid will be coupled to the RMM-grid by horizontal domain decomposition (ddhor).

Couple interfaces The first step in coupling two grids is to determine the location of the couple interfaces. Those interfaces should be chosen in such a way that at both ends of the interface no water is flowing. In Figure B-27 the coupling interfaces, projected on the new grid are presented. They are equal to Figure B-8.

Figure B-27 Couple interfaces between new grid (blue) and RMM grid (grey)

170 January 2013 Hydraulic analysis of a flood channel

When water is flowing through the flood channel, it enters the new grid through the East interface, and leaves the new grid through the West and South interface. The East interface is connected with the northern part of the Afgedamde Maas, the West interface is connected with the Biesbosch, and the South interface is connected with the Bergse Maas. There is no connection between the new grid and the southern part of the Afgedamde Maas because it is separated from the Northern part by dikes and a sluice. All coupling interfaces start and end at dry parts (behind dikes).

Area and enclosure files With the couple interfaces determined, the AREA and ENCLOSURE files can be created. The principle of the placement of the area and enclosure lines near the couple interface is explained in paragraph B.3.1. The AREA and ENCLOSURE files are created in accordance to the theory.

The position of the Area and Enclosure polygons of couple interface East is described below. Because the approaches for the West and South polygons are similar, they are not described in this report. In the figure below, the ‘East’ couple interface is presented, including the lines from the Area and Enclosure polygon of both grids.

Figure B-28 Couple interface East. Enclosure, Area and Couple lines of RMM and New (FC) grid. Area and enclosure boundary are through water level points, thus do not match with the depth grid

The couple interface is displayed in red. The dotted lines describe the lines on the RMM-grid and the solid lines describe the lines on the new FC-grid. The overlapping cells (not marked in the figure) are described by the Area and Enclosure polygon. Both grids overlap the other grid with one active and one inactive row (COPPRE value –1). Because the Area and Enclosure polygons are defined by the water level points, they do not match with the depth point grid. The Area lines are only visible near the couple interface. At the other locations, the line has the same coordinates as the enclosure polygon, and is therefore not visible.

The corresponding values in the COPPRE file for the coupling of the east connection are displayed in Figure B-29. In this figure, the active cells with a COPPRE value of 1 are within the boundaries of the Area file. The cells with a COPPRE value of 0 are outside the enclosure boundary. The cells between the Area and Enclosure boundary are the cells with value –1, which are the cells that overlap the other grid.

171 Hydraulic analysis of a flood channel January 2013

Figure B-29 COPPRE values for the East couple Figure B-30 Subdomain or partition numbers flood interface (upper grid is RMM grid, channel’s grid lower grid is new FC grid)

The COPPRE values of the whole new grid are presented in Figure B-30. In addition to the East connection, also the West and South connection are visible.

Partitioning RMM model The RMM model is divided in several partitions in order to compute the results of the different partitions in parallel. Originally, the RMM model is splitted in 12 domains, 1 sea partition, and 11 river partitions, so it can run on 12 cores. The partitions are defined by polygons in the AREA file.

Figure B-31 Partitioning river part of RMM model in 10 partitions (dark blue cells have a COPPRE value of 0 or –1)

Because the new flood channel runs in a separate domain, it uses an extra core. Therefore, if the partitioning of the river domain is not adapted, the computation runs on 13 cores, which is inefficient because a node has 12 cores. Therefore, the partitioning of the river part has been changed to 10 partitions, see Figure B-31. The partitions of the river domain are redefined in such a way that the extra simulation time is small. With this new partitioning, the river domain uses 10 cores, the sea domain uses 1 core and the new Flood channel domain also uses 1 core during a simulation.

Model input files After defining the enclosure and area files, some changes in the model input files are required.

172 January 2013 Hydraulic analysis of a flood channel

In the file that is used to start the WAQUA calculation, the new domain must be added: echo "start waqpre flch" waqpre.pl -runid flch."$RUNID1" -input siminp.flch."$RUNID1" -bufsize 200 -back n - isddh y -decomp Area-flch_fc3 -npart 1 -buf_prt 200 -partit strip_row

In the command for the ‘real’ processing, the domains are described in the file condig_DD. To the config_DD file the following lines are added:

DOM 3 GLOBAL NAME = 'flch.' EXECUTABLE = 'waqpro_double.exe' RUNID = 'flch.001' EXPERIMENT = 'flch'

B.3.3 Results and analysis

In Waqview, a viewer of WAQUA output files, it is possible to visualize the grid enclosure after domain decomposition. In the viewer, the results of the flood channel domain (new grid) and river domain (RMM grid) are viewed as one grid.

In Figure B-32, the output grid is presented. Note that near the East connection, the new flood channel grid lies above the RMM grid. Therefore, the viewer displays the enclosure lines of the underlying RMM grid through the new grid. This does not have any effect on the results because the information is stored related to m,n coordinates, and they are different for both grids.

Figure B-32 Connected grids with grid enclosure (black) and velocity enclosure (pink)

In Figure B-33, the flow pattern at the inlet is presented, after connection of the two grids. The transition between the two grids is smooth. The domain decomposition has succeeded.

173 Hydraulic analysis of a flood channel January 2013

Figure B-33 Flow field at east couple interface

B.4 Conclusion

This chapter shows how a new grid has been created and how it was successfully connected to the RMM grid by domain decomposition.

The grid is curvilinear with an average grid size between 25 and 60 m2. The smoothness and orthogonality criteria meet the requirements at most cells. The locations where the criteria are violated are dry area’s, or area’s with low flow velocities. The most critical part of the grid, namely the northeast part, does not show abnormal results (Figure B-33).

The new grid and the RMM grid have been successfully connected by horizontal domain decomposition. In calculations, the model shows smooth transitions in the results at the couple boundaries.

B.5 References

[Deltares, 2011] Delft3D-RGFGRID. Generation and manipulation of curvilinear grids for Delft3D-FLOW and Delft3D-wave. User manual. Version 4.00. Deltares, May 2011.

[Simona, 2012] WAQUA/TRIWAQ - two- and three-dimensional shallow water flow model. Technical documentation. Version 3.16, March 2012.

[Simona, 2011] User’s guide for Parallel WAQUA/TRIWAQ and for domain decomposition. Version 2.20, June 2011.

174 January 2013 Hydraulic analysis of a flood channel

C Background costs

C.1 Cost structure flood channel

In the table below, the calculation of the construction and property costs of the flood channel is presented. The costs are categorised in the inlet, main channel and outlet channels. The tables are in Dutch.

In the table, the omschrijving, code and eenheidsprijs are based on a standardised list [Van der Linde et al., 2004]. The price level is July 2002.

EENHEIDS Totale

PRIJS kosten

OMSCHRIJVING CODE AANTAL [106 €] [106 €] TOELICHTING

Inlet

Dijkverplaatsing Aankoop bedrijventerrein Z-rietdijk 56319.2 m2 0.000125 7.0 Aankoop grond bedrijf oostzijde AM

Aankoop Landbouwgrond Type1 Q-001010 256082 m2 0.000006 1.5 Aankoop grond natuurgebied

Aankoop bedrijf algemeen type 9 Q-001045 1 st 0.97 1.0

Sloop bedrijf algemeen type 9 Q-001075 1 st 0.088 0.1

Dijkaanleg Waal Zaltbommel tot Vuren Q-000025 0.3785 km 4.64017738 1.8

Opruimen en opschonen terreinen Q-007020 312401 m2 0.00000073 0.2

Inrichting natuurterrein Q-007025 312401 m2 0.00000059 0.2

Grond ontgraving totaal scenario 1 Z-grond12 17880 m3 0.00000726 0.1 Verlagen dijk met 2 meter

klasse 1/2 (kruinbreedte 6m. helling 1:3)

Dijkverplaatsing Dijkaanleg Waal Zaltbommel tot Vuren Q-000025 2.15 km 4.64017738 10.0 westzijde AM

Aankoop commercieel dienstverl. bedrijf Q-001055 1 st 0.48 0.5 Kerk

Sloop comm. Dienstverl. Bedrijf Q-001085 1 st 0.044 0.0 Sloop kerk

Grond ontgraving totaal scenario 1 Z-grond12 8500 m3 0.00000726 0.1 Terrein kerk

klasse 1/2

Schadevergoeding landbouw (1/3 grond) Q-006010 765000 m2 0.000002 1.5 Schadevergoeding voor

onderlopen gebied (nieuwe

uiterwaarden)

Grond ontgraving totaal scenario 1 Z-grond12 168696 m3 0.00000726 1.2 Verlagen kade met 4 meter

klasse 1/2 (kruinbreedte 6m. helling 1:3)

Grond ontgraving totaal scenario 1 Z-grond12 969105 m3 0.00000726 7.0 Ontgraving oeverwallen etc

klasse 1/2

Grondverlaging Grond ontgraving totaal scenario 1 Z-grond12 23500 m3 0.00000726 0.2 Verlagen bodemhoogte sluis klasse 1/2

Opruimen en opschonen terreinen Q-007020 19000 m2 0.00000073 0.0

Grond ontgraving totaal scenario 1 Z-grond12 4037.5 m3 0.00000726 0.0 Kadeverlaging van 4.5 naar 2

klasse 1/2 meter

Inlaat Beweegbare in/uitlaat constr (2*2.5m) Q-blok05 875 m 0.08055959 70.5 constructie

175 Hydraulic analysis of a flood channel January 2013

Waterdoorlatende constr.. lokale wegen F-007015 875 m 0.00934847 8.2

Main channel

Dijkaanleg Waal Zaltbommel tot Vuren Q-000025 22.8 km 4.64017738 105.8 Noord en zuidkant main

Dijken channel

Opruimen en opschonen terreinen Q-007020 1003200 m2 0.00000073 0.7

Bedrijventerrein Aankoop commercieel dienstverl. bedrijf Q-001055 52 st 0.48 25.0

Aankoop bedrijf algemeen type 9 Q-001045 8 st 0.97 7.8

Sloop comm. Dienstverl. Bedrijf Q-001085 60 st 0.044 2.6

Aankoop bedrijventerrein Z-rietdijk 560000 m2 0.000125 70.0

Vastgoed Aankoop vrijstaande woning type 2 Q-002015 174 st 0.29 50.5 Woningen

Sloop vrijstaande woning type 2 Q-002040 174 st 0.01439992 2.5 Woningen

Aankoop villa of woonboerderij type 3 Q-002020 9 st 0.48 4.3 Woningen

Sloop villa of woonboerderij type 3 Q-002045 9 st 0.03180802 0.3 Woningen

Aankoop woonbouwgebied Q-001025 61200 m2 0.00017 10.4 Woningen

Aankoop fruitteelt/glastuinbouwbedrijf Q-002025 2 st 0.9 1.8 Glastuinbouw

Sloop fruitteelt/glastuinbouwbedr. Q-002050 2 st 0.05964813 0.1 Glastuinbouw

Aankoop Landbouwgrond Type1 Q-001010 25625 m2 0.000006 0.2 Glastuinbouw

Aankoop boerenbedrijf type 15 Q-001071 20 st 0.48 9.6 Boerderij

Sloop boerenbedrijf type 15 Q-090020 20 st 0.03180802 0.6 Boerderij

Aankoop Landbouwgrond Type1 Q-001010 200000 m2 0.000006 1.2 Boerderij

Aankoop commercieel dienstverl. bedrijf Q-001055 5 st 0.48 2.4 Bedrijven

Sloop comm. Dienstverl. Bedrijf Q-001085 10 st 0.044 0.4 Bedrijven

Aankoop bedrijventerrein Z-rietdijk 12500 m2 0.000125 1.6 Bedrijven

Grond ontgraving totaal scenario 1 Z-grond12 56332.5 m3 0.00000726 0.4 Oude dijk (b=8m. h=3.5m.

Obstakels klasse 1/2 L=870m)

Grond ontgraving totaal scenario 1 Z-grond12 28125 m3 0.00000726 0.2 Weg ten zuiden

klasse 1/2 bedrijventerrein (b=8m.

h=1.5m. L=1500m)

Grond ontgraving totaal scenario 1 Z-grond12 9253.13 m3 0.00000726 0.1 Weg ten zuiden van Almkerk

klasse 1/2 (b=8m. h=1.25m. L=630m)

Outlet channels

Natuurgebied Inrichting natuurterrein Q-007025 6992396 m2 0.00000059 4.1

Opruimen en opschonen terreinen Q-007020 6992396 m2 0.00000073 5.1

Vastgoed Aankoop vrijstaande woning type 2 Q-002015 92 st 0.29 26.7

Sloop vrijstaande woning type 2 Q-002040 92 st 0.01439992 1.3

Aankoop woonbouwgebied Q-001025 27600 m2 0.00017 4.7

Aankoop commercieel dienstverl. bedrijf Q-001055 5 st 0.48 2.4

Sloop comm. Dienstverl. Bedrijf Q-001085 5 st 0.044 0.2

Aankoop bedrijventerrein Z-rietdijk 12500 m2 0.000125 1.6

Aankoop boerenbedrijf type 15 Q-001071 12 st 0.48 5.8

Aankoop Landbouwgrond Type1 Q-001010 6952296 m2 0.000006 41.7

Dijken Dijkaanleg Waal Zaltbommel tot Vuren Q-000025 29.6 km 4.64017738 137.3

176 January 2013 Hydraulic analysis of a flood channel

Opruimen en opschonen terreinen Q-007020 1124800 m2 0.00000073 0.8

Grond ontgraving totaal scenario 1 Z-grond12 91800 m3 0.00000726 0.7 Verhoogde weg Nieuwendijk

Obstakels klasse 1/2 (b=8m. L=1800m. h=3m)

Grond ontgraving totaal scenario 1 Z-grond12 12187.5 m3 0.00000726 0.1 Verhoogde weg Nieuwendijk

klasse 1/2 (b=8m. L=650m. h=1.5m)

Grond ontgraving totaal scenario 1 Z-grond12 23250 m3 0.00000726 0.2 Verhoogde weg Hank (b=8m.

klasse 1/2 L=600m. h=2.5m)

Grond ontgraving totaal scenario 1 Z-grond12 323750 m3 0.00000726 2.4 Dijk verwijderen (b=8m.

klasse 1/2 L=5000m. h=3.5m)

Grond ontgraving totaal scenario 1 Z-grond12 96750 m3 0.00000726 0.7 Dijk verwijderen (b=8m.

klasse 1/2 L=1000m. h=4.5m)

Uitlaat Beweegbare in/uitlaat constr (2*2.5m) Q-blok05 550 m 0.08055959 44.3

Waterdoorlatende constr.. hoofd- en F-007010 550 m 0.04934313 27.1

spoorwegen

References [Van der Linde et al., 2004] Kostenschattingen PKB/MER Ruimte voor de Rivier. M. van der Linde, R. van Lier en S.R. Prins. Projectorganisatie ‘Ruimte voor de Rivier’. Referentie L966, versie 2.02, March 2004.

177 Hydraulic analysis of a flood channel January 2013

C.2 Costs dike reinforcements

In the table below, the costs of the dike reinforcements with and without the flood channel are presented for several dike rings.

The investment costs are based on the following function [Eigenraam, 2005]:

u I()() u c  bu  e

Where: I Costs dike reinforcements per kilometer [106 euro] u Dike raise [cm] λ Constant [cm-1] b Constant [106 €] c Constants [106 €/cm]

The values of λ, b and c are specified for each dike ring the KBA of Room for the River [Eigenraam, 2005]. The dike rings with corresponding number are presented in the figure below.

Figure C-1 Dike rings with corresponding numbers [helpdeskwater, 2013]

The following costs have been calculated per dike ring:  Cost dike reinforcement between 2015 and 2100 for the river system without additional measures (costs dh_MHW)  Cost dike reinforcement between 2015 and 2100 for the river system including the flood channel (costs MHW-FC)  Difference between the costs dike reinforcements with and without flood channel (cost_diff)

The calculated costs per dike ring are based on the length of the river axis, not on the length of the actual dike. Therefore, the actual dike length has been divided by the length of the river axis, which results in a correction factor for each dike ring. The final costs are the calculated costs multiplied with this correction factor. In the table below, the correction factor and corrected costs are presented for each relevant dike ring.

178 January 2013 Hydraulic analysis of a flood channel

Dike ring Length Length Correction Costs dike raise Costs dike Benefits number river dikes factor (reference) raise incl. flood axis [km] dike [106 euro] flood channel channel [km] length [-] [106 euro] [106 euro] 43 41 45.3 1.10 184.1 97.4 (-47.1%) 86.7 16 21 25.5 1.21 177.7 111.2 (-37.4%) 66.5 22 17 22.5 1.32 154.3 149.5 (-3.1%) 4.8 41 7 7.0 1.00 24.4 20.4 (-16.4%) 4.0 40 4 6.9 1.72 21.9 17.2 (-18.9%) 4.7 38 25 28.1 1.12 79.2 29.9 (-62.2%) 49.3 24 19.5 19.5 1.00 41.6 3.5 (-91.5%) 38.1 23 21 21.0 1.00 33.5 29.4 (-12.2%) 4.1 43 4.5* (upstream) 41 3.7* (upstream) Total 716.7 458.6 266.3

Table C-1 Costs dike increase and benefits due to flood channel. *Estimated benefits upstream of model boundary

In the tables below, the costs for dike reinforcement per dike ring are specified. dh_MHW Design water level changes between 2015 and 2100 dh_FC Water level change due to flood channel (corrected for decreasing influence river in transition area) dh_MHW+dh_FC Estimated design water level change due to flood channel costs dh_MHW Investment calculation based on dh_MHW costs MHW-FC Investment calculation based on dh_MHW+dh_FC costs diff. Difference between the investment calculations (benefits due to flood channel)

179 Hydraulic analysis of a flood channel January 2013

Dike ring 43

Lambda b c 0.0043 0.02 1.5847

Sum: 166.7 88.168 78.58 WATER LEVELS COSTS dh_MHW costs costs costs Location dh_MHW dh_FC +dh_FC dh_MHW MHW-FC diff. [km] [m+NAP] [m+NAP] [m] [106 €] [106 €] [106 €] Waal 914 0.700 -0.141 0.559 4.033 3.437 0.596 Waal 915 0.741 -0.147 0.594 4.217 3.580 0.638 Waal 916 0.762 -0.153 0.609 4.314 3.642 0.672 Waal 917 0.772 -0.154 0.618 4.360 3.679 0.681 Waal 918 0.698 -0.165 0.533 4.024 3.333 0.691 Waal 919 0.718 -0.175 0.543 4.113 3.373 0.740 Waal 920 0.737 -0.184 0.553 4.199 3.413 0.786 Waal 921 0.710 -0.194 0.516 4.077 3.267 0.811 Waal 922 0.710 -0.201 0.509 4.077 3.240 0.838 Waal 923 0.742 -0.217 0.525 4.222 3.302 0.920 Waal 924 0.723 -0.228 0.495 4.136 3.185 0.950 Waal 925 0.757 -0.238 0.519 4.291 3.278 1.012 Waal 926 0.710 -0.246 0.464 4.077 3.068 1.010 Waal 927 0.710 -0.256 0.454 4.077 3.030 1.047 Waal 928 0.774 -0.275 0.499 4.370 3.201 1.169 Waal 929 0.744 -0.288 0.456 4.231 3.038 1.194 Waal 930 0.722 -0.295 0.427 4.131 2.930 1.201 Waal 931 0.774 -0.305 0.469 4.370 3.086 1.283 Waal 932 0.769 -0.318 0.451 4.346 3.019 1.328 Waal 933 0.760 -0.338 0.422 4.305 2.912 1.393 Waal 934 0.768 -0.363 0.405 4.342 2.850 1.492 Waal 935 0.740 -0.395 0.345 4.213 2.638 1.574 Waal 936 0.740 -0.410 0.330 4.213 2.587 1.626 Waal 937 0.739 -0.437 0.302 4.208 2.492 1.716 Waal 938 0.719 -0.454 0.265 4.118 2.370 1.748 Waal 939 0.729 -0.479 0.250 4.163 2.321 1.842 Waal 940 0.714 -0.502 0.212 4.095 2.200 1.895 Waal 941 0.733 -0.543 0.190 4.181 2.132 2.049 Waal 942 0.683 -0.593 0.090 3.958 1.834 2.124 Waal 943 0.702 -0.650 0.052 4.042 1.727 2.315 Waal 944 0.667 -0.712 0.000 3.888 0.000 3.888 Waal 945 0.684 -0.764 0.000 3.962 0.000 3.962 Waal 946 0.683 -0.843 0.000 3.958 0.000 3.958 Waal 947 0.671 -0.935 0.000 3.906 0.000 3.906 Waal 948 0.627 -1.026 0.000 3.717 0.000 3.717 Waal 949 0.642 -1.104 0.000 3.781 0.000 3.781 Waal 950 0.645 -1.055 0.000 3.794 0.000 3.794

180 January 2013 Hydraulic analysis of a flood channel

Waal 951 0.643 -1.018 0.000 3.785 0.000 3.785 Waal 952 0.638 -0.982 0.000 3.764 0.000 3.764 Boven Merwede 953 0.541 -0.939 0.000 3.365 0.000 3.365 Boven Merwede 954 0.529 -0.929 0.000 3.318 0.000 3.318

Dike ring 16

Lambda b c 0.01 0.02 3.0476

Sum: 146.46 91.69 54.78 WATER LEVELS COSTS dh_MHW costs costs costs Location dh_MHW dh_FC +dh_FC dh_MHW MHW-FC diff. [km] [m+NAP] [m+NAP] [m] [106 €] [106 €] [106 €] Boven Merwede 955 0.515 -0.870 0.000 6.824 0.000 6.824 Boven Merwede 956 0.489 -0.792 0.000 6.564 0.000 6.564 Boven Merwede 957 0.496 -0.732 0.000 6.634 0.000 6.634 Boven Merwede 958 0.516 -0.643 0.000 6.835 0.000 6.835 Boven Merwede 959 0.481 -0.521 0.000 6.486 0.000 6.486 Boven Merwede 960 0.492 -0.393 0.099 6.594 3.582 3.012 Nieuwe Merwede 961 0.469 -0.286 0.183 6.371 4.100 2.270 Beneden Merwede 962 0.483 0.241 0.242 6.506 4.498 2.008 Beneden Merwede 963 0.493 0.233 0.260 6.604 4.630 1.974 Beneden Merwede 964 0.502 0.195 0.307 6.693 4.975 1.718 Beneden Merwede 965 0.522 0.191 0.331 6.896 5.168 1.728 Beneden Merwede 966 0.528 0.186 0.342 6.958 5.249 1.708 Beneden Merwede 967 0.532 0.151 0.381 6.999 5.579 1.420 Beneden Merwede 968 0.538 0.125 0.413 7.062 5.852 1.210 Beneden Merwede 969 0.555 0.114 0.441 7.242 6.106 1.136 Beneden Merwede 970 0.563 0.078 0.485 7.329 6.522 0.807 Beneden Merwede 971 0.571 0.072 0.499 7.416 6.666 0.750 Beneden Merwede 972 0.577 0.061 0.516 7.482 6.834 0.648 Beneden Merwede 973 0.583 0.047 0.536 7.548 7.042 0.506 Beneden Merwede 974 0.592 0.036 0.556 7.649 7.258 0.391 Beneden Merwede 975 0.603 0.013 0.590 7.774 7.627 0.147

* Corrected for the influence of the sea downstream of Boven Merwede kilometer 958

181 Hydraulic analysis of a flood channel January 2013

Dike ring 22

Lambda b c 0.0066 0.02 3.3124

Sum: 116.7741 113.109 3.67 WATER LEVELS COSTS dh_MHW costs costs costs Location dh_MHW dh_FC +dh_FC dh_MHW MHW-FC diff. [km] [m+NAP] [m+NAP] [m] [106 €] [106 €] [106 €]

Nieuwe Merwede 971 0.599 -0.055 0.544 6.697 6.302 0.395 Nieuwe Merwede 972 0.616 -0.043 0.573 6.824 6.511 0.313 Nieuwe Merwede 973 0.629 -0.031 0.598 6.922 6.690 0.232 Nieuwe Merwede 974 0.644 -0.018 0.626 7.037 6.898 0.139 Nieuwe Merwede 975 0.653 -0.009 0.644 7.107 7.036 0.070 Nieuwe Merwede 976 0.663 0.001 0.664 7.185 7.191 -0.007 Nieuwe Merwede 977 0.672 0.011 0.683 7.256 7.341 -0.085 Nieuwe Merwede 978 0.674 0.013 0.687 7.271 7.377 -0.106 Nieuwe Merwede 979 0.681 0.013 0.694 7.327 7.428 -0.101 Hollandsch Diep 980 0.684 0.014 0.698 7.351 7.461 -0.110 Wantij km* 1 0.555 0.114 0.441 6.379 5.611 0.768 Wantij km* 2 0.563 0.078 0.485 6.436 5.896 0.540 Wantij km* 3 0.571 0.072 0.499 6.493 5.993 0.500 Wantij km* 4 0.577 0.061 0.516 6.537 6.107 0.430 Wantij km* 5 0.583 0.047 0.536 6.580 6.246 0.334 Wantij km* 6 0.592 0.036 0.556 6.646 6.389 0.257 Wantij km* 7 0.603 0.013 0.590 6.727 6.632 0.096

* The water levels of the Beneden Merwede are used for the water levels of Wantij. The water levels have been corrected for the influence of the sea

182 January 2013 Hydraulic analysis of a flood channel

Dike ring 41

Lambda b c 0.0033 0.02 1.2716

Sum: 24.40 20.45 3.96 WATER LEVELS COSTS dh_MHW costs costs costs Location dh_MHW dh_FC +dh_FC dh_MHW MHW-FC diff. [km] [m+NAP] [m+NAP] [m] [106 €] [106 €] [106 €]

Waal 914 0.700 -0.141 0.559 3.366 2.874 0.492 Waal 915 0.741 -0.147 0.594 3.516 2.992 0.524 Waal 916 0.762 -0.153 0.609 3.595 3.044 0.551 Waal 917 0.772 -0.154 0.618 3.633 3.075 0.558 Waal 918 0.698 -0.165 0.533 3.359 2.787 0.571 Waal 919 0.718 -0.175 0.543 3.432 2.820 0.611 Waal 920 0.737 -0.184 0.553 3.502 2.854 0.648

Dike ring 40

Lambda b c 0.0026 0.02 1.172

Sum: 12.77 10.04 2.73 WATER LEVELS COSTS dh_MHW costs costs costs Location dh_MHW dh_FC +dh_FC dh_MHW MHW-FC diff. [km] [m+NAP] [m+NAP] [m] [106 €] [106 €] [106 €]

Waal 922 0.710 -0.201 0.509 3.117 2.500 0.618 Waal 923 0.742 -0.217 0.525 3.221 2.547 0.674 Waal 924 0.723 -0.228 0.495 3.159 2.459 0.700 Waal 925 0.757 -0.238 0.519 3.270 2.529 0.741

183 Hydraulic analysis of a flood channel January 2013

Dike ring 38

Lambda b c 0.004 0.02 0.6954

Sum: 70.44 26.62 43.81 WATER LEVELS COSTS dh_MHW costs costs costs Location dh_MHW dh_FC +dh_FC dh_MHW MHW-FC diff. [km] [m+NAP] [m+NAP] [m] [106 €] [106 €] [106 €]

Waal 927 0.710 -0.256 0.454 2.810 1.923 0.887 Waal 928 0.774 -0.275 0.499 3.057 2.067 0.990 Waal 929 0.744 -0.288 0.456 2.940 1.929 1.011 Waal 930 0.722 -0.295 0.427 2.856 1.838 1.018 Waal 931 0.774 -0.305 0.469 3.057 1.970 1.087 Waal 932 0.769 -0.318 0.451 3.038 1.913 1.125 Waal 933 0.760 -0.338 0.422 3.002 1.822 1.180 Waal 934 0.768 -0.363 0.405 3.034 1.770 1.264 Waal 935 0.740 -0.395 0.345 2.925 1.590 1.334 Waal 936 0.740 -0.410 0.330 2.925 1.547 1.378 Waal 937 0.739 -0.437 0.302 2.921 1.466 1.455 Waal 938 0.719 -0.454 0.265 2.844 1.362 1.482 Waal 939 0.729 -0.479 0.250 2.882 1.321 1.561 Waal 940 0.714 -0.502 0.212 2.825 1.218 1.607 Waal 941 0.733 -0.543 0.190 2.898 1.160 1.737 Waal 942 0.683 -0.593 0.090 2.709 0.907 1.802 Waal 943 0.702 -0.650 0.052 2.780 0.816 1.964 Waal 944 0.667 -0.712 0.000 2.650 0.000 2.650 Waal 945 0.684 -0.764 0.000 2.713 0.000 2.713 Waal 946 0.683 -0.843 0.000 2.709 0.000 2.709 Waal 947 0.671 -0.935 0.000 2.665 0.000 2.665 Waal 948 0.627 -1.026 0.000 2.505 0.000 2.505 Waal 949 0.642 -1.104 0.000 2.559 0.000 2.559 Waal 950 0.645 -1.055 0.000 2.570 0.000 2.570 Waal 951 0.643 -1.018 0.000 2.563 0.000 2.563

184 January 2013 Hydraulic analysis of a flood channel

Dike ring 24

Lambda b c 0.0059 0.02 1.3338

Sum: 41.64 3.52 38.13 WATER LEVELS COSTS dh_MHW costs costs costs Location dh_MHW dh_FC +dh_FC dh_MHW MHW-FC diff. [km] [m+NAP] [m+NAP] [m] [106 €] [106 €] [106 €]

Boven Merwede 953 0.541 -0.939 0.000 3.324 0.000 3.324 Boven Merwede 954 0.529 -0.929 0.000 3.268 0.000 3.268 Boven Merwede 955 0.515 -0.870 0.000 3.203 0.000 3.203 Boven Merwede 956 0.489 -0.792 0.000 3.085 0.000 3.085 Boven Merwede 957 0.496 -0.732 0.000 3.116 0.000 3.116 Boven Merwede 958 0.516 -0.643 0.000 3.208 0.000 3.208 Boven Merwede 959 0.481 -0.521 0.000 3.049 0.000 3.049 Boven Merwede 960 0.492 -0.393 0.099 3.098 1.623 1.475 Nieuwe Merwede 961 0.469 -0.286 0.183 2.996 1.894 1.102 Afgedamde Maas 244 0.541 -0.939 0.000 3.324 0.000 3.324 Afgedamde Maas 245 0.541 -0.939 0.000 3.324 0.000 3.324 Afgedamde Maas 246 0.541 -0.939 0.000 3.324 0.000 3.324 Afgedamde Maas 247 0.541 -0.939 0.000 3.324 0.000 3.324 Werkendam Binnen 963 0 Steurgat 964 0 Steurgat 965 0 Steurgat 966 0 Steurgat 967 0 Steurgat 968 0 Steurgat Ruigt 969 0 Gat van het Zand 970 0 Gat van het Zand 971 0 Gat van het Zand 972 0 Spijkerboor 973

* The water level of the Afgedamde Maas is assumed to be equal to the water level at Boven Merwede 953 km * Benefits Steurgat and Biesbosch are assumed to be 0. The water levels are equal or higher than the reference

185 Hydraulic analysis of a flood channel January 2013

Dike ring 23

Length Lambda b c 21 0.0034 0.02 1.0082

33.49 29.43 4.06 WATER LEVELS COSTS dh_MHW costs costs costs Location dh_MHW dh_FC +dh_FC dh_MHW MHW-FC diff. [km] [m+NAP] [m+NAP] [m] [106 €] [106 €] [106 €]

Nieuwe Merwede 963 0.474 -0.205 0.269 2.298 1.694 0.604 Nieuwe Merwede 964 0.494 -0.179 0.315 2.361 1.822 0.539 Nieuwe Merwede 965 0.492 -0.175 0.317 2.355 1.829 0.526 Nieuwe Merwede 966 0.497 -0.170 0.327 2.371 1.858 0.513 Nieuwe Merwede 967 0.513 -0.139 0.374 2.422 1.993 0.429 Nieuwe Merwede 968 0.533 -0.116 0.417 2.486 2.122 0.365 Nieuwe Merwede 969 0.552 -0.093 0.459 2.548 2.251 0.297 Nieuwe Merwede 970 0.577 -0.082 0.495 2.631 2.364 0.267 Nieuwe Merwede 971 0.599 -0.055 0.544 2.705 2.523 0.182 Nieuwe Merwede 972 0.616 -0.043 0.573 2.762 2.619 0.143 Nieuwe Merwede 973 0.629 -0.031 0.598 2.807 2.701 0.105 Nieuwe Merwede 974 0.644 -0.018 0.626 2.858 2.796 0.063 Nieuwe Merwede 975 0.653 -0.009 0.644 2.890 2.858 0.031 Werkendam Binnen 963 0 Steurgat 964 0 Steurgat 965 0 Steurgat 966 0 Steurgat 967 0 Steurgat 968 0 Steurgat Ruigt 969 0 Ruigt 970 0 Ruigt 971 0 Gat van Nooderklip 972 0 Gat van Nooderklip 973 0 Gat van Kampen 974 0 Gat van Kampen 975 0

References

[Eigenraam, 2005] Veiligheid tegen overstromen. Kosten-baten analyse voor Ruimte voor de Rivier deel 1. C.J.J. Eigenraam. CPB document no. 82, april 2005.

[Helpdeskwater, 2013] http://www.helpdeskwater.nl/publish/pages/27049/1008_406_krt4_Dike ringen.pdf. 6 January 2013.

186 January 2013 Hydraulic analysis of a flood channel

D History river system

NOTE: This appendix is about the history of the Dutch southwest river Delta. Because the information is not directly used in the report, the appendix has not been translated into English.

In dit hoofdstuk wordt een overzicht gegeven van de ontwikkeling die het zuidwestelijke rivierengebied de afgelopen eeuwen heeft doorgemaakt. Het overzicht is gefocust op het gebied dat voor de onderzoeksvraag relevant is, oftewel de Waal, (Nieuwe) Merwede en Biesbosch. De geschiedenis is ingedeeld in een aantal perioden. De perioden worden begrensd door wijzigingen in het rivierenbeleid, meestal veroorzaakt door (bijna) overstromingen.

Voor 1850

Situatieschets

In de 17e en 18e eeuw werd het rivierengebied met regelmaat geteisterd door overstromingen. De rivieren werden nauwelijks gereguleerd met het gevolg dat het water haar eigen weg ging en de mens in de strijd tegen het water vaak aan het kortste eind trok.

De Rijn kwam bij Lobith Nederland binnen en splitste al snel in de Oude-Rijn en de Waal. Doordat de Oude Rijn verzandde stroomde het meeste water via de Waal. De Waal was op twee plaatsen met de Maas verstrengeld waarna beide rivieren uiteindelijk samenvloeiden in de Merwede, die vervolgens uitmondde in een doolhof van langzaam dichtslibbende killen. In tegenstelling tot de hedendaagse rivieren bestonden deze een paar eeuwen terug nog uit veel ondiepten, zand- en grindplaten en eilandjes die vaak met riet of houtgewas waren beplant. Er waren geen ononderbroken stroomgeulen en in het winterbed waren ruime, maar ook hele nauwe stukken. In de rivieren waren op sommige plaatsen kribben geplaatst die bedoeld waren om de oever vast te leggen en het land tegen het water te beschermen. Een nadelig bijeffect van deze kribben was dat ze de stroming belemmerden en daarmee de kans op overstromingen vergrootte. In het algemeen waren de rivierdijken in slechte staat. De kruinen waren vaak te smal en de taluds hadden een steile helling.

Figure D-1 Maas, Waal, Nederrijn en Lek in begin 19e eeuw [Bron: Kraayenhoff, 1835]

Wateroverlast en overstromingen waren niet uitzonderlijk in de 17e en 18e eeuw. De overstromingen werden soms veroorzaakt door te lage of slechte dijken bij hoogwater, maar vaker nog door het ontstaan van ijsdammen. Door deze dammen kon de dijk lokaal breken of

187 Hydraulic analysis of a flood channel January 2013

overstromen, maar kon ook de afvoerverdeling beïnvloeden waardoor andere Rijntakken zwaarder werden belast en daar dijken doorbraken. IJsdammen ontstonden voornamelijk op plaatsen waar de stroming belemmerd werd, bijvoorbeeld bij onregelmatigheden in rivierbreedte of ondiepten.

Beleid

De afvoerverdeling tussen Waal en Nederrijn/IJssel was in de 18e eeuw al een thema dat waterstaatkundigen bezig hield. De Noorder Lekdijk was slecht en omdat er bij hoge afvoeren veel water werd afgevoerd via de Nederrijn en IJssel was men in Holland en Utrecht bang dat de dijk zou breken met overstromingen tot Leiden en Amsterdam. Om de afvoer via de Nederrijn en Lek te verkleinen werd in 1745 besloten de (instabiele) afvoerverdeling als volgt vast te leggen: 2/3 van de Rijnafvoer naar de Waal, 2/9 naar de Neder-Rijn en 1/9 naar de IJssel. Er werden diverse waterwerken uitgevoerd waardoor deze afvoerverdeling vanaf 1784 ook daadwerkelijk werd bereikt.

De realisatie van de afgesproken afvoerverdeling betekende niet het einde van overstromingen. Bij de ingenieurs was er veel verdeeldheid over de juiste aanpak. Er waren grofweg twee groepen. De ene groep was voorstander van afleidingen of zijdelingse overlaten om het teveel aan water tijdelijk af te voeren en zo dijkdoorbraken te voorkomen. De andere groep geloofde niet dat dit de oplossing voor het overstromingsprobleem was en zette in op het verbeteren van de afvoer door verwijdering van obstakels en normalisatie (stroomverbetering).

In 1809 vond een van de ergste rampen in het rivierengebied plaats doordat in de rivieren ijsdammen ontstonden en vele dijken braken. Naar aanleiding van deze ramp werd ‘Comite Central du Waterstaat’ bijeen geroepen met de opdracht om zo spoedig mogelijk een plan te maken voor de verbetering van de rivieren. De commissie was voorstander van het afleiden van rivieren ter voorkoming van overstromingen, en er werd besloten de Liemerse overlaat en de Lingewerken uit te voeren. De Lingewerken was ook een verkapte vorm van een afleiding. Bij hoog water op de Lek zou de Zuider Lekdijk doorgestoken worden (om doorbraak van de Noorder Lekdijk te voorkomen) en dat water zou dan via de Lingewerken afgevoerd worden.

Figure D-2 Ondergelopen gebied na dijkdoorbraken in 1809 [Bron: Zurcher,1809]

In 1820 vond er een ramp plaats die vergelijkbaar was met die van 1809. De rivierwaterstanden overtroffen de tot dan toe hoogst bekende. Er werd weer een rivierencommissie ingesteld die tot de algemene conclusie kwam dat het rivierenvraagstuk onoplosbaar was en dat overstromingen niet te voorkomen waren. De commissie deed wel enkele voorstellen, onder

188 January 2013 Hydraulic analysis of a flood channel

andere de aanleg van verschillende afleidingen, enige dijken te verbeteren en aanleg van de Nieuwe Merwede. De Nieuwe Merwede werd aanbevolen omdat de doorstroming door de killen bij de Biesbosch steeds meer belemmerd werd door aanwas van slib. Dit was niet alleen een probleem bij hoogwater en ijsvorming, ook de economieën van Rotterdam en Dordrecht leden eronder doordat de steden steeds moeilijker per schip bereikbaar waren.

Figure D-3 Opslibbing Biesbosch van 1699 tot 1869 [Bron: Van der Ham, 2002, p12]

Het laatstgenoemde rapport leidde tot dermate veel maatschappelijke discussie dat er in 1828 een nieuwe rivierencommissie werd ingesteld die de opdracht kreeg om het commentaar op het rapport van de eerste commissie te verwerken. Het duurde, mede door politieke omstandigheden, twintig jaar voordat de commissie haar rapport gereed had. Opnieuw werd geconcludeerd dat er afleidingen moesten komen, dijken moesten worden versterkt en dat de Nieuwe Merwede er moest komen. In het rapport van 1820 ging de aanleg van de Nieuwe Merwede samen met het sluiten van de beneden Merwede, maar volgens de commissie van 1828 was deze sluiting niet nodig.

Biesbosch

Het gebied dat nu bekend staat als de Biesbosch was voor 1421 onderdeel van een ingepolderd gebied, de Groote of Zuid Hollandsche Waard (Figure D-4). De Biesbosch, het eiland van Dordrecht, het oosten van de Hoekse waard en delen van Noord Brabant waren onderdeel van deze waard. Deze waard, die ontstaan is in 1283, bevatte tientallen dorpen. In het jaar 1421 was er de St. Elisabethsvloed die de dijk van de Groote Waard deed breken waardoor een groot deel van de waard onder liep. De gaten werden provosorisch gerepareerd, maar bij de stormvloed van 1424 liep de waard weer onder en werd het verlies van een deel van de waard definitief. De hoger gelegen plaatsen, zoals Dordrecht, bleven behouden, maar een groot aantal dorpen is verdronken. Het schijnt dat tientallen jaren na het verlies van de waard vele kerktorens nog boven het wateroppervlak uitstaken.

Bij de vloed van 1424 hebben dijken aan de zee- en rivierkant het begeven waardoor de Groote Waard een open verbinding is tussen zee en Merwede. Door de werking van het getij schuurt het land uit wat uiteindelijk leidt tot het ontstaan van het Hollands Diep en een binnenzee. Doordat de Merwede uitmondt in de binnenzee slaat er veel sediment neer in de binnenzee waardoor zandplaten ontstaan die uiteindelijk vele eilandjes vormen. De binnenzee verandert langzaam in een netwerk van kreken.

189 Hydraulic analysis of a flood channel January 2013

Figure D-4 Groote of Zuid-Hollandsche Waard in 1420 (voor de St. Elisabethsvloed) [Bron: de Nijs, 2008, p53]

1850 – 1953

Beleid

Waterstaatsinspecteurs Ferrand en Van der Kun waren belast met de uitvoering van het rapport van de rivierencommissie van 1828. In 1850 presenteerde ze een kort rapport waarin ze voorstelden om de rivieren regelmatiger te maken, de afvoerverdeling tussen de verschillende riviertakken te behouden, verstrengeling van rivieren op te heffen en de benedenmonden van rivieren te verbeteren. Ze zagen het gebruik van zijdelingse afleidingen als laatste redmiddel, en zetten daarmee een nieuwe koers in voor het rivierenbeleid.

Het rapport was volledig gericht op het voorkomen van overstromingen. Ferrand en Van der Kun stellen in het rapport dat de werken die overstromingen voorkomen mogelijk ook zorgen voor een goede bevaarbaarheid. Dit was een grote zorg vanwege de slechte bereikbaarheid van de havensteden. Het plan was om na het voltooien van de geplande werken aanpassingen te doen om de bevaarbaarheid te verbeteren.

Het rapport van Ferrand en Van der Kun belandde niet in het archief, maar werd de start van grote veranderingen in het rivierengebied. Er werd een normaalbreedte voor rivier vastgesteld, er kwamen regels voor dijkhoogten en bekadingen, er werd begonnen met het scheiden van Maas en Waal en gestart met het graven van de Nieuwe Merwede. Verder werd de doorstroming van rivieren verbeterd door het opruimen van obstakels, weghalen van ondiepten en afsnijden van bochten. De orde van grootte van de rivierverbeteringsplannen is vergelijkbaar met de die van de Deltawerken in de 20e eeuw.

In 1861 vonden weer grote overstromingen plaats in het rivierengebied. In dat jaar gaf de regering de opdracht om de rivierensituatie aan een nieuw onderzoek te onderwerpen, mede vanwege de maatschappelijke weerstand tegen de plannen die werden uitgevoerd. Dit rapport gaf echter een onderbouwing van de al verworven inzichten en zorgde niet voor een grote verandering ten opzichte van de rivierverbeteringen die vanaf 1850 waren ingezet. Een belangrijke aanvulling was de aanleg van de Bergsche Maas zodat de Maas weer een eigen monding zou krijgen. In het rapport wordt verder veel aandacht besteed aan het gebruik van stoomijsbrekers om ijsdammen te voorkomen. Sinds het rapport van 1861 zijn de doelstellingen

190 January 2013 Hydraulic analysis of a flood channel

van de riviercommissie om overstromingen te voorkomen niet meer op grote schaal ter discussie gesteld.

In het begin van de 20e eeuw was situatie sterk verbeterd, maar nog niet helemaal bevredigend. De rivieren waren nog steeds niet allemaal evengoed bevaarbaar en de situatie van de Waal ging zelfs achteruit. Door de steeds groter worden schepen werd dit een probleem dat er toe leidde dat er een derde normalisatie ronde werd uitgevoerd waarbij de normaalbreedte soms met 100 meter werd verkleind.

In 1926 werd de hoogste bekende stand van de Rijn bij open water (zonder ijs) bereikt. Onder andere bij de Lekdijken ontstonden wellen en in veel dijken ontstonden scheuren. De grootste overstromingen vonden plaats langs de Maas. Na het hoogwater van 1926 werden de dijken langs de Rijntakken verhoogd en versterkt. Sindsdien hebben er geen grootschalige rivieroverstromingen in Nederland plaatsgevonden. De rivierverbetering die is gestart in 1850 heeft de veiligheid langs de rivieren duidelijk verhoogd.

Uitgelichte Projecten

Nieuwe Merwede In 1850 is gestart met de aanleg van Nieuwe Merwede. Bij de start van de werkzaamheden waren er nog geen baggerschepen en was baggeren alleen met de hand mogelijk. In het ontwerp was er daarom vanuit gegaan dat de natuur het werk moest doen. Het idee was om de meeste killen (kreken) te sluiten en een nieuwe rivier te vormen door het verwijden en uitschuren van een aantal kreken die in een lijn lagen. De nieuwe rivier zou een normaalbreedte krijgen van 400 meter. Tijdens de aanleg van de Nieuwe Merwede bleek dat er geen uitschuring plaatsvond op plaatsen waar taaie klei of veenophopingen aanwezig waren waardoor de waterstand bovenstrooms steeg. Ondertussen was de stoombaggermolen ontwikkeld waardoor er ook machinaal gebaggerd kon worden en het proces kon worden versneld. In 1866 werd de rivierbreedte van 400 meter bereikt. In datzelfde jaar braken de dijken langs de linkeroever, en als gevolg daarvan werd besloten de dat de rivier naar beneden toe verbreed zou worden tot 600 meter. In 1875 werd besloten de bovenmond te verbreden tot 450 meter en in 1885 was de Nieuwe Merwede voltooid. Sinds de aanleg van de Nieuwe Merwede zijn er geen ijsdammen meer geweest rondom Gorinchem en Werkendam. Door de aanleg van de Nieuwe Merwede werd de Biesbosch in tweeën gesplitst, de Hollandse en Brabantse Biesbosch. Door latere inpolderingen werd de Dordtse Biesbosch van de Sliedrechtse Biesbosch gescheiden.

191 Hydraulic analysis of a flood channel January 2013

Figure D-5 Tracé Nieuwe Merwede [Bron: De Nijs, 2008, p83]

Normalisatie In 1850 werden door Ferrand en Van der Kun normaalbreedten voor rivieren opgesteld, die later enige keren door voortschrijdend inzicht zijn gewijzigd. Door de aanleg van kribben en het verbinden van eilanden met de oever werd stapje voor stapje toegewerkt naar één hoofdgeul die ontstond door uitschuring. Door het kiezen van de juiste normaalbreedte werd gestreefd naar de optimale rivier die veel water af kon voeren, goed bevaarbaar was en ijsdammen voorkwam.

Figure D-6 Fasen in riviernormalisatie [Bron: Heezik, 2007, p32]

Maas en Waal De Maas en de Waal verstrengelden bij hoogwater bij de Heerewaardense overlaten en vloeiden uiteindelijk samen in de Boven Merwede. De Waal had een groot debiet tot juni, waardoor het water in de lentemaanden opstuwde bij de samenvloeiing en de afvoer van het Maaswater werd belemmerd. Hierdoor werden de dijken langs de Maas over een grote lengte verzadigd en begaven het daardoor regelmatig. In de winter had de samenvloeiing regelmatig ijsdammen tot gevolg waardoor het water werd opgestuwd en dijksecties overstroomden.

192 January 2013 Hydraulic analysis of a flood channel

Na de voltooiing van de Nieuwe Merwede kon de Boven-Merwede nog steeds niet in een bevaarbare conditie voor navigatie worden gebracht omdat de rivier te breed was. Het smaller maken van de Waal was alleen mogelijk wanneer de afvoer van Maas en Waal zouden worden gescheiden.

Dit alles leidde ertoe dat de Maas en Waal volledig gescheiden werden, en dat de Maas een eigen monding kreeg. De scheiding van Maas en Waal was voltooid in 1904 na aanleg van de Bergsche Maas. Hierna konden beide rivieren genormaliseerd worden. De normalisatie van de Boven Merwede was in 1916 voltooid.

Figure D-7 Tracé Bergsche Maas [Bron: de Nijs, 2008, p85]

1953 – 1995

Beleid

Na de watersnoodramp van 1953 was naast het rampgebied ook in het rivierengebied roep om meer veiligheid. Deze roep was niet ongegrond want de rivierdijken waren versterkt tot het niveau van het laatste hoog water. Daarom werd in 1956 besloten om naast de zeedijken ook de rivierdijken te versterken. In het rapport dat Rijkswaterstaat in 1956 publiceerde wordt geadviseerd om rekening te houden met en maximale Rijnafvoer van 20000 m3/s vanwege de diverse onzekerheden. Onder andere op basis van dit onderzoek stelde de minister dat voor het ontwerp van rivierdijken uitgegaan moest worden van een afvoer van 18000 m3/s bij Lobith (overschrijdingskans 1/3000 jaar). Dit betekende dat bijna alle rivierdijken versterkt zouden moeten worden. Volgens planning zou de versterking in 1998 gereed zijn.

Rond 1965 werd er gestart met de dijkversterkingen. Er ontstond echter grote maatschappelijke weerstand die er toe leidde dat in 1975 de commissie Rivierdijken (commissie Becht) werd ingesteld. Volgens de commissie was een overstromingskans van eens per 1250 jaar voldoende, wat een maatgevende afvoer van 16500 m3/s betekende. Met deze norm werden dijkversterkingen nog steeds nodig geacht, en daarom stelde de commissie dat de dijken versterkt moesten worden volgens uitgekiend ontwerp waarbij landschappelijke en culturele waarden zoveel mogelijk behouden moesten worden.

In 1984 heeft Rijkswaterstaat de waterstanden behorend bij een maatgevende afvoer herberekend en kwam tot de conclusie dat deze hoger was dan eerder aangenomen omdat de wisselende ruwheid van het zomerbed niet was meegnomen. Dit leidde tot nieuwe protesten

193 Hydraulic analysis of a flood channel January 2013

met een hoogtepunt in 1991. In 1992 wordt Commissie Toetsing Uitgangspunten Rivierdijkversterkingen (commissie Boertien) ingesteld. Zij concludeerde dat een overstromingskans van eenmaal per 1250 jaar overeenkomst met een afvoer van 15000 m3/s. Voor de nog uit te voeren dijkversterkingen pleitte de commissie voor het zogenaamde uitgekiend ontwerpen, zoals aanbevolen door de commissie Becht. De commissie wees net als de commissie Becht overlaten, groene rivieren, opvangbekkens, zomerbedverlaging en kribaanpassing van de hand en wees erop dat dit geen alternatieven waren voor dijkverzwaring.

In december 1993 trad de Maas op veel plaatsen in Limburg buiten haar oevers, waardoor 7% van de provincie Zuid-Limburg overstroomde. Er werd een commissie Watersnood Maas (commissie Boertien II) in het leven geroepen. Boertien II richtte zich in tegenstelling tot Boertien I meer op ruimtelijke en natuurlijke oplossingen voor het probleem. De commissie adviseerde verbreding en verdieping van de Maas.

Biesbosch

Ook de Biesbosch werd getroffen door de watersnoodramp van 1953. De maatregelen die na de watersnoodramp werden getroffen hadden grote invloed op de Biesbosch. Door de deltawerken werden de zeearmen afgesloten en is de invloed van het getij grotendeels verdwenen. Het getij ging van 2 meter terug naar gemiddeld 20 centimeter. De Sliedrechtse Biesbosch heeft met een getijdenverschil van 70 centimeter het grootste getij. Door de afname van het getij werden de geulen ondieper en de platen lager. Sinds het verdwijnen van het getij zijn de wilgenbossen sterk in aantal toegenomen.

1995 – 2015

Beleid

Nadat in 1995 Nederland weer was getroffen door hoogwater, ditmaal met een grote evacuatie tot gevolg, besloot het kabinet tot het versneld uitvoeren van de al geplande dijkversterkingen. Door de Deltawet Grote rivieren werden veel procedures, waaronder de MER, buiten werking gezet zodat de dijkversterkingen zonder veel vertraging plaats konden vinden.

Na het hoogwater gingen de ontwikkelingen snel. Mede door de steeds bekender wordende klimaatverandering werd door steeds meer deskundigen onmogelijk geacht om in de toekomst de dijken maar te blijven verhogen. In februari 1995 werd door Frankrijk, Duitsland, Belgie, Luxemburg en Nederland in de Verklaring van Arles vastgelegd dat voor de Rijn en Maas snel concrete maatregelen getroffen moesten worden. De maatregelen moesten gericht zijn op het vergroten van waterbergend vermogen. Vervolgens kwam al snel de notitie ‘Ruimte voor Water’ waarin Rijkswaterstaat het toenmalig huidige beleid ter discussie stelt en pleit voor ruimte voor het bergen van water. Ook de Technische Adviescommissie voor Water, die zich jarenlang heeft ingespannen om het waterkerend vermogen van dijken te optimaliseren, pleitte opeens voor het vergroten van de ruimte voor de rivier.

In 1996 werd met de beleidslijn Ruimte voor de Rivier afscheid genomen van de dijkversterkingstrategie omdat dit geen duurzame oplossing voor de toekomst zou bieden. In 2000 werd dit bekrachtigd met het kabinetsstandpunt Ruimte voor de Rivier. Naar aanleiding van dit besluit werd PKB Ruimte voor de Rivier gestart, een Spankrachtstudie en commissie Noodoverloop gebieden (Commissie Luteijn).

194 January 2013 Hydraulic analysis of a flood channel

In de spankrachtstudie werd onderzoek verricht naar de benodigde ruimte voor de rivier op lange termijn. Het PKB Ruimte voor de rivieren had het doel om de wettelijk vereiste veiligheid tegen hoogwater tot 2015 zo spoedig mogelijk in overeenstemming te brengen met de per 2001 geldende maatgevende afvoer van 16000 m3/s. Naast veiligheid focust het Ruimte voor de rivier programma ook op ruimtelijke kwaliteit. In 2006 zijn de definitieve Ruimte voor de rivier plannen gepresenteerd, waarvan er al een aantal zijn afgerond en een groot deel momenteel in uitvoering is.

Referenties

[De Nijs, 2008] De delta in wording. Overzicht van het benedenrivierengebied door de eeuwen heen. T. de Nijs. Rijkswaterstaat waterdienst. 2008

[Heezik, 2007] Strijd om de rivieren. 200 jaar rivierenbeleid in Nederland. A.A.S. van Heezik. HNT Historische Producties/Rijkswaterstaat. 2007.

[Krayenhoff, 1835] Verzameling van hydrographische en topographische waarnemingen in Holland

[Van de Ven, 1993] Man-made lowlands. History of watermanagement and land reclamation in The Netherlands. G.P. Van de Ven. Stichting Matrijs, 1993.

[Van der Ham, 2002] Afleiden of opruimen. De strijd om de beste aanpak tegen rivierbederf. Een beschouwing van 300 jaar rivierverbetering in het kader van de spankrachtstudie. Dr. W van der Ham. In opdracht van Rijkswaterstaat. 2002.

[biesbosch.nu] http://www.biesbosch.nu/historiepagina.php?code=4. 30 maart 2012.

[Wikipedia/Nationaal_Park_De_Biesbosch] http://nl.wikipedia.org/wiki/Nationaal_Park_De_Biesbosch. 30 maart 2012

[Zurcher, 1809] Figurative kaart, dienende ter aanwijzing van de voornaamste dijkbreuken enz.: langs de rivieren; voorgevallen in de Louwmaand MDCCCIX (1809)

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