How to deal with flooding Preparations for drafting the flood risk management plans for the river basins of the River Dijle upstream of Leuven and the River Woluwe in Flanders. Park Jourdain along Kleine Maalbeek Composition and editing Publisher ARCADIS Belgium NV Philippe D’Hondt, VMM D/2011/6871/037 Final editing Ivo Terrens, Annelies Haesevoets, More information Maureen Nuyts (VMM) Infoloket VMM [email protected] Photography +32 53 72 64 45 VMM www.vmm.be ARCADIS Belgium NV De Urbanisten (www.urbanisten.nl) Photo cover Conseil Général du Loiret Restructured Woluwe in the new Stadsarchief Leuven business park along the eastern Désiré Vanautgaerden Brussels ring road Stad Leuven Guido Lieben Carl Vandervoort Dienst Openbare Werken Zaventem Maps VMM ARCADIS Belgium NV Layout CIBE communicatie op maat van de publieke sector (www.cibecommunicatie.be) 2 Table of content Foreword 4 ❶ Two river basins with flood risk 6 ① Floods and measures in the basin of the River Dijle 13 ② Floods and measures in the basin of the River Woluwe 18 ③ Choice of project areas 23 ❷ Approach 24 ❸ Damage and impact caused by flooding 28 ① Methodology 31 ② Results 36 ❹ Research on types of measures 48 ① Alleviation: protection against flooding 50 ② Avoidance: prevention of flooding and damage 64 ③ Awareness and capacity to respond 74 ④ Assistance before, during and after a flood 78 ❺ Consultation with stakeholders 84 ❻ The future, how will we continue this work? 98 ① Alleviation: protection from flooding 100 ② Avoidance: prevention of flooding and damage 106 Park Jourdain along Kleine Maalbeek ③ Awareness 111 ④ Assistance before, during and after a flood 114 Epilogue 118 Table of content • 3 Foreword In the last few years Flanders repeatedly came into the news with reports on floods and flood damage. Flanders is susceptible to flooding. In Europe, and the rest of the world, more attention is being paid to problems with flooding. Floods occurred in the past and cannot be excluded in the future either. However, there is the impression that the impact of floods has increased. Therefore we should consider how we want to deal with these floods. How can we prevent or mitigate floods in Flanders and at what price? What is socially feasible and what can people do for themselves? Because floods can wreak a great deal of damage, there have always been attempts to prevent them. This has generally been done through technical constructions to protect people from floods, such as deepening and widening waterways, and constructing dikes and flood reservoirs. Because of the increase in hardened surface area in Flanders and the possible consequences of climate change, there may be more frequent and more extreme floods. Moreover, we are building more and more in the natural flood plains, so that the potential damage from flooding increases. This raises questions as for example, should we provide additional protection to avoid floods? And if so, what risk do we want to protect ourselves from? Do the costs and impact of a protective measure offset the risk of floods and damage? More and more, the view is gaining ground that we cannot protect ourselves 100% against floods, but that we should make more effort to mitigate the damage caused by flooding. There is also more attention for source-targeted measures to reduce the probability of floods. Recently the European ‘Flood Risks Directive’ came into force. Drawing up a flood risk man- agement plan (FRMP) is the last step laid down by the European directive. The emphasis is on protection, prevention and preparedness, taking into account the specific characteristics of the river basin. The Flemish Environment Agency (VMM) had this study done within the framework of the Euro- pean Interreg IVB NWE project ‘FloodResilienCity’ (FRC) as a preparation to drawing up a FRMP for the Dijle and for the Woluwe . The aim of this project is to ascertain to what extent we can protect urbanised centres along rivers in North-West Europe from floods. The starting point is that floods will occur more frequently and possibly more extremely because of climate change. At the moment a great deal of infrastructure, including city centres, is not protected and in practice such protection is also difficult to achieve. Therefore we have to develop an approach to deal in an appropriate and targeted way with the 'unavoidable' and become more resilient to floods. 4 What this document offers This document provides a summary of the project to prepare the drawing up of a flood risk management plan for the river basins of the Woluwe and the Dijle in Flanders. This summary contains six main sections: Chapter ❷ provides an ❶ overview of the working ❸ method and the approach of the project. A number of In Chapter ❶ 'Two river Chapter ❸ 'Damage and terms are explained, as well impact caused by flood- basins with flood risk' we as the assumptions made in ing' provides an explanation discuss the river basins of this project. the Dijle and the Woluwe about the methods we in Flanders and tell some- used to calculate economic thing about the floods that damage, social damage occurred in the past and and ecological impact. This the measures already taken ❷ chapter also shows the result against flooding. of those calculations for the river basins of the Dijle and the Woluwe in present conditions. ❹ In Chapter ❹ 'Research on types of measures' we look for various types of meas- ures. Alleviation measures are assessed on the basis of an extensive cost-benefit analy- sis. In addition an overview is given of possible measures ❻ for avoidance, awareness and assistance. We consider to Chapter ❻ 'The future, how what extent these measures will we continue this work?' ❺ can avoid or reduce the provides an overview of the damage. measures which seem to be Chapter ❺ 'Consultation of promising for execution or stakeholders' is the result further study in the context of a series of workshops, of the entire process. For in which we presented the each measure there is a list outcomes of the previous of action points still to be chapters and discussed them undertaken to implement with various stakeholders. the measure in question. Foreword • 5 ❶ Two river basins with flood risk 6 Two river basins with flood risk • 7 The Dijle and Woluwe rivers are both part of the Dijle-Zenne river basin, located in the centre of Flanders. The Woluwe rises in the Brussels Region and enters the Flemish Region at the commune of Zaventem. This river flows through the urbanised area of Brussels and its surrounding munici- palities. At many locations the river is embanked or covered over. The Dijle rises in Wallonia and flows into Flanders south of the city of Leu- ven. The Dijle flows through Leuven The Dijle and Woluwe river basin city and then further on in the direc- tion of the city Mechelen where it joins the Zenne river. Upstream of Leuven the Dijle valley remains fairly undevel- oped and agricultural lands and nature predominate. Once entering Leuven The basin of the River Dijle upstream of Leuven is char- the river is shut in by the city. acterised by fairly steep slopes. Over the years, the Dijle wore away a valley of around 50 metres depth. The river basin used to be virtually fully wooded. However, these woods had to largely give way to agricultural land. This leads to faster drainage of rain water, resulting in erosion. The Dijle as such was never straightened or embanked up- stream of Leuven. Up to the present, the Dijle retains its meandering pattern and is therefore unique in Flanders. 8 Legend Urban buildings Other buildings Industry and trading infrastructure Other infrastructure Sea port infrastructure Airport infrastructure Green urban zones Arable farming Pasture Alluvial pasture Orchard Deciduous forest Coniferous forest Mixed forest Heath land Beach and dunes Water Highway District road Use of land in the basin of the River Dijle upstream of Leuven Two river basins with flood risk • 9 Meandering Dijle upstream of Leuven Leuven is situated on the transition towards the low- lands of Belgium. At the level of Leuven a number of efflu- ents are formed that subsequently converge again. A large part of the Dijle, Dijle branches and tributaries (Voer, Vunt, Molenbeek) were vaulted over during the years. The vaults are a bottleneck for the flow of water through Leuven. The highly meandering pattern of the Dijle upstream of Leuven is unique in Flanders 10 Waterways in the centre of Leuven The Woluwe river flows in the Zenne valley. This valley Parallel to the four major waterways in the area (Wolu- is traditionally a marshy and alluvial one. In the past several we, Kleine Beek, Kleine Maalbeek and Vuilbeek) there are waterways were moved, excavated, or modified to improve large main sewers in many places. In addition various col- drainage of the area or for the construction of water mills lectors were built in the area. The collectors and main sew- and ponds. Pressure on the waterways rose due to intense ers discharge rain, ground water and waste water. There are urbanisation and industrialisation in the area. On the one many interaction points between the waterways, collec- hand there was the augmented discharge of rain water tors and main sewers. On the waterways there are several because of the increase in hardened surface area, which overflow spillways of these mains. On the other hand the resulted in floods. On the other hand there was a need waterways as such also frequently overflow into the collec- to discharge waste water from households and industry. tors and main sewers. The result is a highly meshed network In order to deal with these issues, the decision was taken between the scarce open waterway sections and tubed to construct a system of collectors and main sewers.