Is Surabaya being planned as a low-risk city? A case study on the effect of urban spatial plans in the Kedurus catchment area on flash flood risk reduction in Surabaya

Spatial plans are key instruments in directing future developments and reducing a city’s flash flood risk. This study conducts a surface runoff simulation using SWAT analysis in the Kedurus catchment area. SWAT analysis is a hydrological analysis to measure surface runoff from precipitation with consideration of land uses, soil types, climatic data, topography and related infrastructure systems. Based on the simulation, four sub-catchment areas are currently experiencing flash flooding. Surabaya’s detailed spatial plan (RDTR) could reduce the total flood volume in the city by fifty-one per cent if all measures (drainage and other infrastructures) in the plan are implemented successfully. Nevertheless, the implementation of the measures is still questionable due to limited budget and land acquisition. In the case of plan failure, the planned developments will cause higher surface runoff, putting Surabaya is at higher risk of flooding. Therefore, Surabaya needs to diversify its flash flood risk reduction approach to ensure that the plan will achieve a low-risk city in the future