**4. Conclusions**

This study attempted to conduct a rapid assessment of the flood on 1 January 2020 in Jakarta and several areas of the Ciliwung–Cisadane Watershed. This study was conducted to determine the return period of the flood that occurred. The rainfall–runoff–inundation (RRI) model is a distributed model that is able to simulate rainfall–runoff and flooding simultaneously. The model based on the inundation map from remote-sensing satellites was calibrated with ground-station rainfall data. The resolution of the Digital Elevation Model (DEM), which was too high, hindered the simulation process; it made it necessary for the resolution of the DEM to be decreased. The DEM was upscaled; then, the grid size was changed from 30 m to 100 m. After the model was calibrated, a return-period flood model was performed with a return-period rainfall input. There were six return periods of rainfall being simulated: 2, 5, 10, 25, 50 and 100 years.

The discharge and inundation areas of all return periods were compared with the simulation of the flood on 1 January. The comparison showed that the discharge extent on 1 January exceeded the discharge for a return period of 100 years. This result corresponds with the rain recorded at several rainfall stations, which surpassed the 100-year rainfall return period. Thus, there are several conclusions. First, the flood that occurred on 1 January 2020 was superior to a 100-year return period of flooding. Second, the rainfall, which was the most in history, was the main effect of this flood because it exceeded the 100-year return period of rainfall.

In this study, the DEM implemented in this model had a low resolution because of the limitation of data sources. In addition, the model needed to be compared with the existing discharge during the flood. These conditions need to be considered in future research.

**Author Contributions:** Conceptualization, M.F., N.F.J. and I.R.M.; methodology, M.F., N.F.J. and I.R.M.; software, Y.I.S. and N.F.J.; validation, Y.I.S. and A.R.; formal analysis, Y.I.S. and A.R; investigation, Y.I.S. and A.R.; resources, M.F. and I.R.M.; data curation, Y.I.S. and A.R.; writing—original draft preparation Y.I.S., A.R. and N.F.J.; writing—review and editing, M.F. and I.R.M.; visualization, Y.I.S. and A.R.; supervision, M.F. and I.R.M.; project administration, M.F. and I.R.M.; funding acquisition, M.F. and I.R.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Institute for Research and Community Services of Institut Teknologi Bandung (LPPM ITB) through ITB Excellent Research 2023 and by the Indonesia Endowment Funds for Education (LPDP) through gran<sup>t</sup> scheme PRJ-102/LPDP/2021.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not Applicable.

**Data Availability Statement:** Data is unavailable.

**Conflicts of Interest:** The authors declare no conflict of interest.
