Course CT4460 Polders April 2015 Dr. O.A.C. Hoes Professor N.C. van de Giesen Delft University of Technology Artikelnummer 06917300084 These lecture notes are part of the course entitled ‘Polders’ given in the academic year 2014-2015 by the Water Resources Section of the faculty of Civil Engineering, Delft University of Technology. These lecture notes may contain some mistakes. If you have any comments or suggestions that would improve a reprinted version, please send an email to [email protected]. When writing these notes, reference was made to the lecture notes ‘Polders’ by Prof. ir. J.L. Klein (1966) and ‘Polders and flood control’ by Prof. ir. R. Brouwer (1998), and to the books ‘Polders en Dijken’ by J. van de Kley and H.J. Zuidweg (1969), ‘Water management in Dutch polder areas’ by Prof. dr. ir. B. Schulz (1992), and ‘Man-made Lowlands’ by G.P. van der Ven (2003). Moreover, many figures, photos and tables collected over the years from different reports by various water boards have been included. For several of these it was impossible to track down the original sources. Therefore, the references for these figures are missing and we apologise for this. We hope that with these lecture notes we have succeeded in producing an orderly and accessible overview about the genesis and management of polders. These notes will not be discussed page by page during the lectures, but will form part of the examination. March 2015 Olivier Hoes i Contents 1 Introduction 1 2 Geology and soils of the Netherlands 3 2.1 Geological sequence of soils 3 2.2 The Paleozoic, Mesozoic and Cenozoic (tertiary) eras 3 2.3 The Cenozoic (Quarternary) era 4 2.4 Human influence on soil formation 8 2.5 Sea level rise 9 3 Polders and belt canals 12 3.1 The creation of the Dutch polders 12 3.2 Types of polders 15 3.3 The belt canal system 18 3.4 Water boards today 18 4 Land acquisition, preparation and improvement 23 4.1 Introduction 23 4.2 Peat bogs and peat moors 25 4.3 Subsidence in peat polders 28 4.4 Reclaiming land and embanking 30 4.5 Accretion methods 32 4.6 Land improvement 34 5 The required groundwater level 36 5.1 Groundwater level 36 5.2 Water withdrawal by plant roots 36 5.3 Groundwater level categories 39 5.4 The most favourable groundwater level 40 6 Drainage 44 6.1 Groundwater flow 44 6.2 Hydraulic conductivity 50 6.3 Drainage methods 53 6.4 Determining the polder water level 55 7 Allotment layout 62 7.1 Allotment 62 7.2 Plot size in polders 64 7.3 Polders in the province of North Holland 66 7.4 Polders of Lake IJssel 69 7.5 Land consolidation 72 8 Watercourses and roads 74 8.1 Watercourses 74 8.2 Calculation of canal profiles 75 8.3 Head loss over culverts, bridges and weirs 77 8.4 Backwater curves 79 8.5 Layout of roads and canals 80 ii 9 Precipitation and evaporation 83 9.1 Precipitation 83 9.2 Evapotranspiration (ET) 87 9.3 Precipitation and evapotranspiration 91 10 Water balance and water surplus 93 10.1 Water balance 93 10.2 Water surplus in polders 97 10.3 Design discharge 103 10.4 Water surplus in urban areas 106 11 Discharge by gravity flow 109 11.1 General introduction 109 11.2 Discharge sluices 109 11.3 Shutter in a sluice 114 12 Pumping stations 115 12.1 Introduction 115 12.2 Pumping stations 117 12.3 Water-lifting devices 117 12.4 System characteristic 121 12.5 Pump characteristics 122 12.6 Additional remarks 127 13 Water supply 129 13.1 Water deficit 129 13.2 Water quality 133 13.3 Water supply 134 13.4 The Dutch national close-off sequence 136 14 Economic evaluation of measures 139 14.1 Points of particular interest for determining costs and benefits 139 14.2 Comparing costs and benefits 141 14.3 Determining the costs of improvement measures 144 14.4 Determining the benefits of improvement measures 144 iii 1 Introduction Water has always been a big issue in the Netherlands. Sometimes there is too much, sometimes too little, and now and then it leads to dangerous situations. On the one hand excess water has to be discharged as quickly as possible to prevent flooding; on the other hand freshwater has to be conserved. Freshwater is used to control water levels, as drinking water, for irrigation, and in preventing salinization (by flushing or dilution). In recent years a water shortage has led to an increased number of botulism and blue-green algae cases and to problems with cooling water capacity for electricity production. Water can also be a threat because of river discharge, sea or extreme rainfall events, for example: - RIVER DISCHARGE On 31 January 1995 the river Rhine discharged 12,000 m3/s, forcing 200,000 people to leave their homes; - SEA Large parts of the provinces of Zealand and South Holland were flooded due to a spring tide during the night of 31 January 1953. The storm caused over 90 dike breaches and approximately 1,835 people were drowned; - EXTREME RAINFALL The total precipitation in 1998 measured 1,240 mm, while the average annual precipitation over the period 1971-2000 was only 793 mm. Extreme rainfall events on 13 and 14 September and on 27 and 28 October 1998 flooded large parts of the Netherlands. The national government paid a total of € 290 million in compensation for the damage caused. These threats, combined with the fact that a quarter of the Netherlands is below mean sea level (Figure 1-1), make measures necessary to protect the country. Examples of measures are the construction of dikes, ditches and pumping stations. < 0 msl 0-5m+msl 5-10+msl 10-25+msl >25m+msl North Sea Germany Belgium Figure 1-1 Elevation map of the Netherlands; roughly 25% is located below Mean Sea Level (MSL) (source: AGI-RWS) 1 On the one hand, excess precipitation needs to be drained. On the other hand, water levels also need to be controlled, in order to secure the growth of agricultural crops. To do so, excess water is drained by ditches or drains into canals. These canals discharge into larger canals, which discharge their water into rivers, lakes or the sea by means of pumping stations. A belt canal system can be used as an intermediary. Water levels are controlled not only in polders and belt canal systems but also in urban areas, brooks and even whole river basins. Since river basins usually have a relatively large slope, the levels are controlled by weirs. Embankments and dikes also protect the land from large discharges into brooks and rivers. Ditches and canals in polders have practically no slope. Therefore the water level can be completely controlled, independent of the adjacent area. Pumping stations and drainage sluices discharge excess water and water can be let in during dry periods. This taking in of water can be problematic because of both the quality and quantity of the water. Dutch catchments with larger slopes can be found in the more elevated areas in the east and the inner dune area. Polders are catchments with hardly any slope and can be found in the low-lying areas and near the large rivers. Between the two is a transitional area with slopes deviating from relatively large to non-existent. In the Netherlands polder and dike management is one of the oldest forms of engineering. It is probable that dikes already existed in peat areas more than two thousand years ago. Though the Romans occasionally constructed a dike, the serious construction of dikes along the large rivers started around 800 AD. Dikes along the sea have an even later origin. Urban areas were first protected from sea flooding; rural areas did not follow until much later. In the Middle Ages people became more involved in flood prevention and control. More dikes were constructed, draining the area by drainage sluices, Dutch scoops and Archimedean screws. In the 14th century wind mills improved the drainage and made it possible to reclaim marshes and lakes. With the invention of steam-driven pumping stations at the end of the 18th century more and deeper lakes could be reclaimed. Later the diesel and electrical pumping stations followed. Nowadays almost all Dutch stations are driven by electricity, but in other areas of the world diesel is still used, especially in areas where electricity is insufficiently available. Dutch polder and dike engineering have led to some prestigious projects such as the Delta Works in the Province of Zealand and the Zuiderzee Works forming the current Lake IJssel. Both these projects were not only developed for flood protection, but also for freshwater retention, recreation, and nature conservancy and urban development. 2 2 Geology and soils of the Netherlands 2.1 Geological sequence of soils When thinking of geology one of the first questions that arises is how old the earth is. A few centuries ago man assumed that it was 6,000 years old. However, throughout history understanding of the earth has continued to improve; by 1800 its estimated age had increased to 80,000 years and nowadays it is assumed to be 5 billion years old with a 3 billion year-old crust. The thickness of the crust is generally thought to be 40 km; drillings have given us information about the outer 4 to 5km. Table 2-1 Sequence of geological eras Era Million of years ago (mya) Precambrian 4500 - 542 Paleozoic Primary 542 - 245 Mesozoic Secondary 245 - 65 Tertiary 65 - 2.4 Cenozoic Quarternary 2.4 - now However, the solid crust has not been static since its origin.