How to protect East Jakarta against flooding

Abstract

Jakarta is heavily subjected to land subsidence. Due to this subsidence, the city is sinking further to under sea level. This has influence on the flood safety, both from an extreme sea event as an extreme rainfall event. The major cause of the subsidence is assumed to be the groundwater extraction, which takes place due to a lack of piped water. To reduce subsidence, the groundwater extraction must stop. It is concluded that this would not be feasible in the short term and scenarios are made on how subsidence will continue in the next years.

To ensure flood safety, measurements have to be taken. Research has already been carried out for West Jakarta, but this report focuses on solutions for East Jakarta. Four different solutions are developed to ensure flood safety. The first is to heighten the coastal dike and the flood defences along the river with the same level as the relative subsidence. To accomplish this, high flood defences should be constructed in the densely populated areas along the rivers. A spatial analysis is performed to come to a cost estimation model for the necessary land acquisition for three types of flood defences. These designs are combined to come to a cost efficient design. Another way to ensure flood safety is to close off the rivers and to pump the water into sea. In this case heightening of the flood defences along the rivers is not necessary. To reduce the peak discharges and thus the needed pumping capacity, a retention lake should be built. This can be done inland, but it is concluded that this will not be a cost efficient solution. A more cost efficient solution is to construct an offshore retention lake. This can be done by building an outer sea dike. In this case, the rivers will flow into the retention lake, which is maintained at a given water level. The pumping capacity needed to ensure flood safety depends on the size of the lake. An optimum has to be found to come to the most cost efficient design. In this study it was concluded that the most cost efficient solution is to not make a retention lake at all, but install pumps with sufficient capacity instead to handle the peak discharge. To reduce the pumping capacity, tidal gates can be constructed at the river mouths. A great advantage of this solution is that an amount of pumps can be constructed to deal with mild subsidence rates and more pumps can be built when concluded that subsidence rates turn out to be larger. In this way an adaptive solution is created.