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Semarang polder

Flooding is commonplace in Java's northern coastal regions. To some extent, the floods are due to climate changes like increased precipitation and rising sea levels. But in the urban areas, there is the additional problem of large quantities of groundwater being extracted from the soil for use as drinking water. This lowers the soil level and increases flooding. The result is immediately visible: there is water in the streets much of the day as the tide pushes the groundwater up to the surface. Some houses are now permanently under water.

Floods threaten urban coastal regions in two ways: from the sea and from the hinterland through the drainage of rainwater via rivers and canals. Major floods damage houses and infrastructure, but also cause unhygienic conditions. This is because most of the towns have open sewers, so when floods occur, the contents of the sewers flow into the town. The poorest urban districts generally experience most inconvenience, as poor people are unable to take the required measures or to move house.

The only short-term solution to this water problem appears to be the construction of a polder. A polder is a low-lying area, surrounded by dikes, in which the water level is controlled artificially. Most people are unable to move to higher areas, so the problem has to be tackled locally. At the same time, soil subsidence will continue unabated until implementation of an alternative to extracting groundwater. For the time being, the only answer is to control the water level.

Witteveen+Bos is involved in this project as the result of a seminar that the company organised in Jakarta in 2001 for an audience that included representatives of the Indonesian government. The theme of the seminar was reducing water problems in Indonesian coastal cities by creating a polder system. The seminar attracted major interest and resulted in the initiation of a process to create a pilot polder. The city of Semarang was chosen for this polder, mainly because of the governmental feasibility. The Indonesian government and the authorities in Jakarta province, which faces similar water problems, are following developments in the polder project with great interest.

Water board

A number of matters must be organised before a polder can be created: besides the technical preparation, there is the institutional aspect. The Netherlands has water boards and district water boards that take care of water management. Indonesia does not have anything similar on a comparable scale. So a water board was established to manage the pilot polder. It is being overseen by the Dutch district water board of Schieland and Krimpenerwaard. The new water board consists of eight people, including three local residents, three professors, and two people representing the authorities. There was a groundbreaking ceremony in April 2010 at which the Mayor of Semarang, the Governor of the Central Java province, and the Secretary General of the Ministry of Public Works signed treaties for transferring powers to the water board – some of which are far-reaching by Indonesian standards. The water board will devise, write, and introduce all rules, procedures, and protocols necessary to ensure the proper working of the polder.


A pumping station in Indonesia currently has an average service life of 5 years, compared with 40 years in the Netherlands. This is due to the smaller amount of maintenance. This is clearly a task for the new water board. Besides its 35 % subsidy for the construction of the polder, the State of the Netherlands is also considering paying 35 % of the maintenance costs for the first 10 years to promote the sustainability of the project.


Creation of the polder is now on the eve of the call for tenders. The necessary activities include:

  • constructing dikes
  • building pumping stations
  • dredging rivers to create more drainage capacity
  • revitalising the drainage system in the city
  • controlling waste
  • constructing retention basins

These activities were prepared by Witteveen+Bos in cooperation with the Indonesian parties, from the plan phase and feasibility study all the way through to the design and specifications. We will also oversee the construction work.


Waste lies and floats everywhere in Semarang. People have no alternative for waste that they cannot use in some other way. There are unofficial circuits for the processing of waste, because for the poor, waste is a way of trading. This form of waste processing is obviously far from adequate. As with all other dirt in the drains, the waste mixes with the rising water and ends up everywhere. It is a hygienic disaster that occurs day in, day out. The waste problem needs to be controlled to make sure that the newly-built pumping station does not get blocked up. A systematic form of waste collection must be set up. Use of such a system needs to be explained to the local population to convince everybody to take part.

Drainage system

There are often obstacles in the drainage channels, such as fishing nets and small dams to enable fishing. Officially, this is not allowed. In principle, Indonesia has excellent water management legislation, but less is done in the way of enforcing it. The complete drainage system in the city needs to be cleaned and repaired.


Managing the polder will cost money, some of which will have to be provided by local residents. But they will want to see results before they part with any money. Support must be mustered for paying and a payment structure ('water rates') must be set up. Once everything is working, the population will be better off financially on balance, because there will be less damage to homes and infrastructure, land prices will go up, the area will be more attractive economically, and there will be less loss of income through damage and illness. However, there must be starting capital first. This is being raised by various parties, partly through donations and subsidies. Witteveen+Bos has also contributed to this effort.