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SADEWA rainfall and environmental aspects during the flash flood incident in Cicurug Sukabumi on September 21, 2020
23 Sep 2020

Generally, the distribution of rain in Java for several months is influenced by the east wind that blows with varying speed between moderate to strong. When the wind with moderate speed crosses Java Island and succeeds in creating convergence on the mainland of Java, it will generate convection and cloud / rain formation, but if the wind blows very hard the mainland of Java Island becomes clear without clouds or cloudy with light rain. In this condition, the water vapor content in Java island will decrease because it is carried to the north (Karimata Strait), while the surface temperature often shows no deviation. Sometimes clouds / rain grows in the highlands of the eastern part of Java Island (Batu / Lumajang / Bondowoso / Banyuwangi) when the east wind weakens a little and there is enough time to form clouds / rain in the area. Cloud / rain in the western part of Java Island is sometimes related to the meso scale cloud / rain system in the Indian Ocean which develops extending itself to the mainland of the western part of Java Island or clouds / rain occurs in the highlands as a result of air lifting due to topography.

Figure 1 Rainfall in Sukabumi and its intensity on September 20, 2020

The intensity of rainfall that was blamed for the flash flood in Cicurug Sukabumi on September 21 at around 16:30 WIB, we traced it using the simulation results of the WRF model integrated in SADEWA-PSTA. On September 20, 2020, daytime rain started at 12:00 WIB in eastern Sukabumi, then the rain extended to the west at 13:00 WIB including in the Curug Kembar area. Rain over Sukabumi lasted until 18:00 WIB, as observed in the simulation results, high rain intensity was observed over northwest Sukabumi while the other Sukabumi areas rained with light to moderate intensity (Figure 1).

Figure 2 Rainfall in Sukabumi and its intensity on September 21, 2020

On September 21, 2020, rain in Sukabumi Regency began to fall at 13:00 WIB with light to moderate intensity, to be precise in eastern Sukabumi Regency, at 14:00 WIB the rain in the western part of Sukabumi Regency began to appear with light intensity. Clouds and rain in the western part of the northwest, to be precise, of Sukabumi district, develop into thick clouds and bring down heavy rain. This rain is part of a cloud / rain system that extends from Lebak Regency which is monitored until 16:00 WIB (Figure 2).

The floods in Cicurug occurred as a result of overflowing of the Cibuntu, Citarik and Cipeuncit rivers at around 16:30 WIB on September 21. The condition of some of the affected areas is shown in Figure 3.

Figure 3 Part of the area affected by flash floods in Cicurug-Sukabumi

The relation between WRF-SADEWA simulation results and floods in Sukabumi shows that rain occurred in the Sukabumi Regency area from September 20, 2020 with light to moderate intensity until September 21, 2020 at 14:00 WIB. Rain with heavy intensity occurs north of Cicurug from 15:00 WIB to 16:00 WIB, however this heavy rain only ranges from 2 to 11 mm / hour or is not classified as extreme, based on GSMAP satellite data (Figure 4). So the trigger for flash floods is not due to extreme rainfall. Another aspect that needs to be considered is the condition of the surrounding environment (land cover and soil type). The drastic reduction in land cover causes the flow of rainwater on the surface to be high. Certain types of soil are very susceptible to landslides, such as Litosol and Andosol. It is known that the land in the northern part of Sukabumi is the type that is prone to landslides, namely Litosol and Andosol. If these two aspects are fulfilled, even moderate rain can cause flooding. At the time of the flash flood on September 21, atmospheric phenomena such as MJO, El Nio and IOD were in normal condition, meaning that there was no trigger for a surplus of rainfall. If MJO or La Ni is active or IOD is negative, then areas like North Sukabumi should get more attention because the potential for landslides and flash floods is getting higher.

Figure 4 Rainfall intensity for three days in Cicurug-Sukabumi based on GSMAP satellite

Compiled by: Ina Juaeni, Risyanto and the TReAK Team.

Cover image source: Tribunnews.

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