Regional frequency analysis of extreme rainfall events in Jakarta

Jakarta is vulnerable to flooding and extreme rainfall events are always the main cause of the occurrence of heavy flood events with loss of life and property. The flood in February 2007 is one of the most devastating events with the total rainfall during the 2-week period amounted to almost one-fourth of the total rainfall in 2007 at some stations. The huge scale of the loss due to this event as well as a series of big consecutive events the following years draw the attention to understand the flood risk in detail for Jakarta region. The study aimed to (1) estimate the return period for the February 2007 event in Jakarta, (2) develop intensity–duration–frequency (IDF) curves as well as the corresponding confidence intervals for Jakarta region up to 7-day duration and (3) understand the uncertainty of developed IDF curves induced with at-site and regional approaches. Daily rainfall data from 11 stations within Jakarta region with record lengths ranging from 15 to 22 years were collected. As the reliability of the frequency analysis is influenced by record length available, the frequencies and magnitudes of extreme rainfall events in the Jakarta region were analyzed by regional frequency analysis (RFA) based on the method of L-moments. One station (Darmaga) was removed from 11 stations after the data quality and homogenization check. The annual maximum rainfall series of duration of 1, 2, 3, 4, 5, 6, and 7 day at the 10 stations in Jakarta were then aggregated for the investigation. Because the value of discordancy measure exceeded the critical value, station Tangerang was subsequently removed from the group. Annual maximum series at these 9 stations were finally combined as the study region and passed the discordancy measure and heterogeneity measures of RFA. The accuracy of at-site rainfall quantile estimates by at-site and RFA was assessed and compared by error bounds at the confidence level of 90 %, based on Monte Carlo simulations by fitting generalized extreme value probability distributions to the regional average sample L-moment ratios. The results showed that the return period of the February 2007 extreme event in Jakarta is over 300 year for 3-day duration at Halim station; close to 200 and 150-year return periods for 7-day duration at Halim and Pdk Betung stations, respectively. For the first time, the Jakarta regional multi-day IDF curves together with the corresponding 90 % confidence intervals for 2, 5, 10, 30, 50, 100, 200, and 300 year were developed. For effective flood risk mapping/mitigation in Jakarta, duration of longer than 1-day design storm should be considered. Also frequent re-visit of the IDF curves is indeed important if severe storms occurred after the IDF curves are established. Finally, the study suggested that more stations/data should be acquired for the region so that the estimation is more robust and reliable.