Bart Schultz
Most rain falls in February with an average monthly rainfall of 400 millimetres. The driest month is September with an average rainfall of 22 millimetres. The land use in the polder in 2007 was 89% residential, 3% industrial, 5% offices and warehouses, 2% parks and gardens and 1% other.
In the preceding years, the Kelapa Gading Polder had undergone rapid development, making it a highly urbanised polder. This development had also led to a significant reduction in the area of open water and insufficient drainage capacity. In addition, most of the drainage system in the Kelapa Gading Polder was not working properly due to lack of maintenance.
The flooding could be caused by high water inside and from outside the area. Flooding from inside was caused by insufficient storage capacity in the urban drainage system and insufficient pumping capacity. From outside, the flooding could be caused by high river discharges and high sea water. Another factor was that the watercourses in the polder were not completely closed off from the outside water. The drainage systems within the Kelapa Gading Polder were also not well connected to each other. For example, some drainage systems were not connected to the main drainage system. In addition, a lot of solid waste and aquatic plants came into the drainage system. This had led to regular flooding in the area during the rainy season.
Subsidence is a well-known phenomenon in Jakarta and also occurs in the Kelapa Gading Polder. The subsidence in this polder was estimated at 3 to 6 centimetres per year. The consequences of the subsidence could be seen in various ways, such as cracks in buildings and roads, changes in the course of rivers and canals, expansion of flood-prone areas, poorly functioning drainage systems, increased seawater intrusion and increased tidal influence. By taking into account a subsidence of 5 centimetres per year over 25 years, the subsidence would amount to approximately 1.25 meters in total.
One of my Indonesian students had chosen the water management of this polder as the subject of her master thesis. She then found that the percentage of open water was only 0.3% of the polder area and the drainage capacity was 67 mm/day. The storage capacity and/or the drainage capacity had to be increased in order to be able to properly store and discharge the design rainfall with a probability of occurrence of 4% per year, which could cause the area to be flooded. She then used a computer program to investigate how this could best be done.
We also visited the area together during one of my visits to the country to look at the critical points, such as the places where good connections between the water management systems within the polder were lacking and where open connections with the outside water were present. The attached photos were taken during this trip. Everything is neatly mapped out in her master thesis. I hope that this has also contributed to the fact that the water management of this polder has also been improved, because this is an essential condition for being able to live and work here well.
