Modelling compound flooding: A case study from Indonesia

Flood-prone areas can be vulnerable to a range of flooding mechanisms, with the associated hazards likely to increase due to the impacts of global climate change. Here, we investigate compound flooding from waves, sea surge and river flow in northern Jakarta, Indonesia, which is a global hotspot of flooding, by combining process-based coastal and river models. The coastal hydrodynamic modelling of Jakarta Bay shows that, coastal storms can lead to substantial increase in sea water level due to wind and wave setup in the nearshore areas. The compound flood hazard in the low-lying coastal areas surrounding the West Flood Canal in the northern Jakarta, which drains the Cilliwung river, from a range of statistically significant compound flood scenarios, was simulated and analysed. The results reveal that low-lying areas around the river estuary are prone to flooding even during regular, low-intensity storm events while rarer storms caused extensive floods. Floods were not caused by direct overwashing of sea defences but by overspill of riverbanks due to high sea water level caused by wind and wave setup and sea surge obstructing high river flows during compound events. Potential changes to future flooding in the northern Jakarta were investigated by taking into account the regional sea level rise and land subsidence in Jakarta in numerical flood simulations. It was found that the sea level rise combined with rapid land subsidence will inundate the existing coastal flood defences during storms. As vast areas of the city will be below mean sea level by 2100, overwashing of coastal defences will lead to extensive flooding in the northern, western, and eastern Jakarta even during more frequent low-intensity storms.