Efficiency of urban water detention methods in downstream cities

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Jakarta is a city which suffers from the problem of urban flooding and inundation. It is urgent to figure out solutions on how to manage the storm water in its urban area. Jakarta locates at the downstream of the catchment; the peak runoff generated in the city can occur earlier than the peak discharge coming from upstream, potentially leading to a ‘collision’ of both peaks. Since the storm water detention methods have both the runoff reduction and delay functions, whether adopting storm water detention methods in Jakarta is able to limiting the discharge magnitude because of its discharge reduction function or even increase the discharge because of the peak delay function is studied. In this article, there are 4 storm water detention methods, green roofs, household storage tanks, pervious pavements and on-site detention tanks, tested with a precipitation event with 100-year return period in a segment in the Jakarta City. The result of the example in this research shows that the delay functions of these 4 solutions are negligible, and the runoff reduction functions of all the solutions are almost lost in this extreme rainfall event when the discharge from the upstream reaches the peak value. So these 4 storm water detention methods in the example have no influence on the peak discharge in the downstream in this precipitation event. However, because the fast runoff is still contributing to the runoff at the exit of the segment when the peak discharge from the upstream comes, these storm water management methods are still possible to reduce the peak discharge during other precipitation events with smaller total precipitation amount or earlier peak intensity. An off-line detention tanks could have positive effects on reducing the peak discharge in the river to the downstream if they start to collect storm water certain period later so that they are capable to store the runoff when the peak discharge from the upstream comes.