Bart Schultz
The Draft Structural Plan for the Province of Jakarta 2010-2030 from 2010 mentions 31 existing and 26 proposed polders. Based on recent publications, I have now arrived at 50 existing polders with a combined area of at least 25,000 hectares and 34 proposed polders. Given the land subsidence in Jakarta, it can be expected that the number of polders and the need for flood protection will further increase.
In 1917, Hendrik van Breen presented a plan for the development of the drainage system of Batavia, as Jakarta was called at the time. This plan included a map proposing polders in the coastal zone, among other things, and two discharge canals to divert water from upstream around Jakarta and contribute to discharge from the city itself. As far as I know, only parts of this plan were implemented at the time, specifically the Western Banjir Canal (banjir means flood), which was made in 1918 and originally had a capacity of 300 cubic metres per second. The Eastern Banjir Canal has only recently been fully completed.
The area enclosed by these two canals and the coastline consists largely of the aforementioned polders. Kop et al. (1983) provided values for the maximum short-term rainfall that can be expected in Jakarta. With a probability of once every 25 years, one should expect a day with precipitation of about 220 millimetres.
The elevation in the polders now varies from approximately two to ten metres below mean sea level. Particularly as a result of the extraction of groundwater beneath the thick top layer of clay, significant land subsidence is occurring at various locations. Abidin et al. (2011) report values ranging from 1 to 25 centimetres per year. This is considerably more than the expected sea level rise of a maximum of 1 centimetre per year.
The tidal range at the coast of Jakarta has an amplitude of approximately 0.5 metres. It is important to note that additional rises in water levels due to storms or tsunamis were virtually non-existent until recently. The risk of flooding was therefore primarily associated with extreme rainfall and river discharges. Wikipedia mentions that in the past, Jakarta has been struck by floods multiple times, usually caused by heavy rainfall between December and March, including in 1621, 1654, 1918, 1942, 1976, 1996, 2002, 2007, 2013, 2015, 2020, and in March and November in 2025. From these data, it can already be deduced that the frequency of flooding has increased.
For several polders, I was able to find values for flood protection. These ranged from once every 50 to 100 years. It should be borne in mind that protection is provided partly by the dikes, but partly by the combination of storage in the polders and pumping station capacity. Given that most floods are caused by extreme rainfall, the combination of storage and pumping station capacity plays a particularly important role. A point of particular importance in this regard is that the storage initially present in the polder may have been reduced over time by additional construction and infrastructure due to urbanization, with all the associated consequences.
The idea existed that Jakarta would be relocated to the new capital Nusantara in Kalimantan. Currently, Jakarta and its surrounding suburbs have 42 million inhabitants, and the new capital is projected to have 2 million inhabitants by 2040. It should therefore be clear that only a small part of Jakarta is being relocated. Consequently, it will be of great importance for Jakarta that land subsidence will be reduced as much as possible and that flood protection - both within the polders and through dikes and associated provisions – will be improved, managed, and maintained where necessary. That is a very major undertaking.
