Flood prevention and water management plans in the low-lying and frequently flooded polders of the Netherlands have changed their defensive focus in an attempt to make room for water
IN 2000, an independent committee advised the Dutch government and the Dutch Association of Water Boards on the policy to be followed for water management in the Netherlands. Intensive rainfall on two occasions in 1998 and 1999 led to costly drainage problems in the western and northern parts of the low-lying Dutch polders. Combined with flood events on the rivers Rhine and Meuse in 1993 and 1995, there were grounds to raise questions on the future direction for water management in this densely populated, low-lying country that is vulnerable to flooding from the sea, from the large rivers that pass through it, and from local intensive rainfall. The government set up an independent Committee on Water Management in the 21st Century, chaired by Mr Tielrooy. The committee was given one year to evaluate current policies and advise on future courses of action. With a budget of US$356.7M, the committee embarked on a series of research projects, which were carried out by specialised institutes and consultants.
WL/Delft Hydraulics is one of the consultancy firms that advised on the policy to be followed. The company looked at:
• Future weather scenarios and associated river flows and sea levels.
• Interrelation between regional water systems and the national water system.
• National water management, in particular water distribution over the various Rhine branches.
The polders of the north and west of the Neth-erlands were reclaimed from the sea and are generally well below sea level. They are cris-crossed by small ditches and rivers and drain into higher boezems, or reservoirs, which comprise a network of canals and lakes filled by pumps.
Water management in polder landscapes is characterised by pumping water out into the boezems as quickly as possible during heavy rainfall, and letting boezem water back into the farmland of the polders in periods of drought. Any eutropic water from the Rhine will also supply the boezems, to compensate for deficits in periods of drought and to dilute brackish groundwater.
To manage the flooding of the polders, which is set to became worse every year with increased precipitation and land subsidence, it has become imperative to improve the winter water storage capacity of the polders themselves, through a system of wetlands and artificial lakes. If part of a polder can be flooded in heavy rainfall, it could avoid flooding of an entire polder or boezem system.
The problem is that the areas earmarked for temporary water storage are already claimed for other uses. The fertile polders are heavily used for agriculture, nature conservation and recreation. The western part of the Netherlands is also densely populated, home to major cities like Amsterdam, Rotterdam, Delft and The Hague. Multiple land use is the possible solution to the problem.
WL/Delft Hydraulics used a combination of numerical simulations, theoretical investigations, experimental investigations and field observations to examine the possibility of multiple land use in the polder areas. An initial itinerary identified possible combinations of temporary water storage and other land uses. The policy requires large amounts of space: for the main and regional water system until 2015 an additional 57,000ha, and by the year 2050 an extra 58,000ha.