The development of the Dutch flood safety strategy technical report Mark Z. Voorendt THE DEVELOPMENT OF THE DUTCH FLOOD SAFETY STRATEGY - technical report, improved edition - Mark Z. Voorendt April 29, 2016 ISBN/EAN 978-90-74767-18-7 NUR-code 956 ©2015, 2016 M.Z. Voorendt, Delft University of Technology published by Bee’s Books, Amsterdam series: Delta Technology, Design & Governance Cover page: graph from the Delta report, part 1 (1960) PREFACE This report is part of my research on the ’structural evaluation of multifunctional flood defences’. The research is part of the programme on ’integral and sustainable design of multifunctional flood defences’ which is supported by the Dutch Technol- ogy Foundation STW, which is part of the Netherlands Organisation for Scientific Re- search (NWO), and is partly funded by the Ministry of Economic Affairs. This pro- gramme is one of the ’Perspective’ programmes that are organised within consortia of research institutes and users. The research programme consists of several projects in which various aspects of mul- tifunctional flood defences are studied. These include technical aspects (strengths and loads), safety philosophy, governance, architecture and financial aspects. For details of the programme, one is referred to the project proposal (see the information on www.flooddefences.org). The current project on structural evaluation is being carried out under supervision of promoter prof.drs.ir. Han Vrijling and with advice from ir. Wilfred Molenaar, dr.ir. Jarit de Gijt and dr.ir. Klaas Jan Bakker, all working at Delft University of Technol- ogy. The research project is externally supported by Witteveen+Bos (especially ir. Paul Ravenstijn and ir. Gerben Spaargaren), Arcadis (dr.ing. Marco Veendorp and dr.ir. Hessel Voortman), Deltares (dr.ir. Meindert Van, ir. Han Knoeff and ir. Harrie Schelfhout) and STOWA (ir. Henk van Hemert). I also got much support from many (other) employees of the Department of Hydraulic Engineering of Delft University of Technology, especially prof.dr.ir. Bas Jonkman, prof.dr.ir. Matthijs Kok, prof.dr.ir. Marcel Stive, ir. Ad van der Toorn, ir. Henk Jan Verhagen, dr.ir. Paul Visser. Their support is highly appreciated! This edition of the report contains some improvements of the report issued in De- cember 2015. Next to some minor corrections, additional relevant information has been included and the text has been better structured. Nevertheless, the author is open to any comments on this renewed edition. Mark Voorendt Delft, April 29, 2016 i CONTENTS 1 Early history1 2 Advances in the design of flood defences5 3 The first half of the twentieth century9 4 Philosophy of the Delta Committee 15 4.1 Reasoning way one: Historical study of water levels............. 16 4.2 Reasoning way two: Statistical analysis..................... 18 4.3 Reasoning way three: Econometric optimisation............... 22 4.4 Basic levels outside Hoek van Holland...................... 25 5 The second half of the twentieth century 27 6 Legalisation of the safety standard 35 7 The Veerman Committee 39 8 Other developments in the early 21th century 43 8.1 More developments in legalisation........................ 43 8.2 The assessment of Dutch flood defences.................... 46 8.3 The multi-layer flood safety approach...................... 48 8.4 Change towards a flood risk-based approach................. 52 References 57 iii 1 EARLY HISTORY The Netherlands are located in a delta area where the rivers Rhine, Meuse, Scheldt and Ems flow into the North Sea. Rivers, sea and land formed a dynamic system, which ever more interfered with the spatial and occupational ambitions of the in- habitants of the low countries. The inhabitants of the low countries had to cope with regular floods and resulting loss of goods and lives. The first inhabitants of the Frisian land (in the north of the Netherlands) therefore settled down on higher plains, but this came to an end when, due to climate change, these plains flooded ever more frequently. In the first century AD Pliny the Elder, a Roman author and natural philosopher vis- ited the Netherlands and characterised a pitiful country, where ... two times in each period of a day and a night, the ocean with a fast tide submerges an immense plain, thereby the hiding the secular fight of the Nature whether the area is sea or land. There this miserable race inhabits raised pieces ground or platforms, which they have moored by hand above the level of the highest known tide. Living in huts built on the chosen spots, they seem like sailors in ships if water covers the surrounding country, but like shipwrecked people when the tide has withdrawn itself, and around their huts they catch fish which tries to escape with the expiring tide. It is for them not possible to keep herds and live on milk such as the surround- ing tribes, they cannot even fight with wild animals, because all the bush country lies too far away. (Gaius Plinius Secundus,78) As a result of the floods, from the sixth century BC, many people moved to higher land areas like the Drents Plateau, or they started to create mounds, to elevate their dwellings to a height less prone to floods, usually not more than four or five me- tres above average sea level. Incidentally more rigorous measures were attempted: already in the first or second century BC, at the Frisian town of Peins (in the mu- nicipality Franeker), a dike was constructed of which a 40-meter section has been discovered lately. Around 1000 AD, dikes started to be constructed on a larger scale 1 2 1E ARLY HISTORY to protect larger areas of land. Monasteries, like the monastery of Aduard in Gronin- gen that was founded in 1192, often organised the construction of these first dikes to protect their estates (Bosker, 2008). The inhabitants of small towns and hamlets also started to cooperate to manage the water. The first known collaboration of this kind was in Utrecht, around 1122, where twenty towns worked together to dam the Kromme Rijn near Wijk bij Duurstede. This collective was later on institutionalised in the Hoogheemraadschap van den Lekdijk Bovendams (’Water board of the Lek dike upstream of the dam’). The first water board, the Hoogheemraadschap van Rijnland, was meanwhile founded in 1255 by count William II of Holland. Since about 1250, a growing number of water boards (waterschappen and hoogheem- raadschappen in Dutch) were set-up to supervise the farmers who were responsi- ble for the maintenance of the dikes and water courses. The regulations for farm- ers and landlords were established in farmstead systems (verhoefslagstelsels), by-laws (keuren) and ledgers (leggers). Farmstead systems contain regulations about the ap- portionment of dike maintenance responsibilities. By-laws are collections of legal regulations applying to rivers, brooks, ditches and flood defences that are adminis- tered by the water-board, but also by other parties. Ledgers are legal documents that contain information on the functional requirements and maintenance duties regards hydraulic works like water courses, flood defences, catchment areas and correspond- ing structures. They also contain specific information on the status of channels and flood defences, dimensions and shapes of hydraulic works, position and dimensions of maintenance strips and protecting zones along water courses and flood defences. These regulations were later on incorporated by the water-boards that were made responsible for the supervision of the flood defences1. The need to control water quantities did not only originate in high sea and river lev- els, but also in the exploitation of peatlands. Peat was namely excavated in large amounts for salt extrusion and fuel. Initially, back quays (achterkades) were con- structed to prevent water intrusion in these excavated parts of land from higher, not yet exploited, land, but good dewatering caused settling of the peat. Therefore, ditches and streams had to be dug deeper, but at the long term this did not help suf- ficiently. That is the reason why dikes and dams with sluices were constructed. In North Holland, many small dikes were constructed to protect against the intruding sea and bank erosion of lakes. This, amongst others, led to construction of the 100 km long West Frision Circle dike (Westfriese Omringdijk), which was completed around 1250. That dike did not only function to protect against floods, but it was also an im- portant road connection2. It was count Floris V who improved the organisation and coordination of dike maintenance in North Holland, but due to political unrest, this did not hold. He was more successful in the Alblasserwaard, which he decided to en- close by dikes in 1277. This was the start of the Hoogheemraadschap van de Alblasser- waard. Meanwhile, the situation in North Holland only improved when William III of Holland ruled this part of the Netherlands. He also gave orders to protect Staveren in Friesland with a dike (1325), followed by Albrecht of Bavaria who initiated construc- 1By-laws and ledgers are legal documents up to now and are complementary to the present Water Act. 2Therefore, the principle of multifunctional flood defences is already quite old. 3 tion of the sea dikes near Oostergo and Westergo (1398). (Beenakker, 1991), (van Buijtenen and Obreen, 1956). Despite all these efforts, flood defences failed regularly, like in 1164 (Saint Juliana’s Flood, about 20 000 fatalities), 1362 (Marcellus Flood, 25 000 to 40 000 or even more fatalities), 1421 (Saint Elisabeths’ Flood, one of the most well-know floods, but ’only’ about 2000 fatalities) and 1570 (All Saints’ Flood, more than 20 000 fatalities) and 1717 (Christmas Flood, more than 14 000 fatalities). These were hard times, when people had to fight the water persistently to survive3.(Buisman and van Engelen, 2000) The state of the profession of dike construction has been written down in a well- known, but unfinished, book by Andries Vierlingh (1578).