**1. Introduction**

Flooding is a natural process of a body of water, which rises to overflow land that is not normally submerged, due to high flow of runoff or sea-surge water [1–3]. There are several types of floods, including fluvial floods, pluvial floods and coastal floods [4–6]. Pluvial floods could be caused by the rain process, which produces an amount of water flow, on the surface, called runoff. Runoff is simply determined by rainfall, area of catchment and catchment response, which is represented by a runoff coefficient [7]. Many factors can cause flooding, including meteorological, geomorphological and anthropogenic factors [8–15]. Due to meandering river shape conditions, especially if the riverbed narrows throughout, a large discharge of water can derive an overflow [16]. Some places that previously did not experience floods have become areas affected by flooding due to changes in land use in the upstream areas [17,18]. In urban areas, flooding occurs due to illegal settlement and sedimentation, which reduce the capacities of rivers and channels [19–21]. In cold-climate regions, early spring snowmelt combined with heavy rainfall can also cause flooding [22].

**Citation:** Sihombing, Y.I.; Rizaldi, A.; Farid, M.; Januriyadi, N.F.; Moe, I.R. Jakarta's 2020 New Year Flood Assessment with a Rainfall– Runoff–Inundation (RRI) Model. *Environ. Sci. Proc.* **2023**, *25*, 100. https://doi.org/10.3390/ ECWS-7-14317

Academic Editor: Athanasios Loukas

Published: 3 April 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Seawater levels rising and high rainfall, which are effects of climate change, are also causes of flooding in some areas [23–26]. Social behavior that occurs in the community can also indirectly have an impact on runoff: for example, urbanization. Urbanization will affect the land use of some areas, making changes in land cover that will affect runoff [27,28].

Jakarta was hit by flooding again in early 2020, on January 1st. One hundred and three locations were submerged by the flood around the provinces of DKI Jakarta and West Java, which are located in the Ciliwung–Cisadane Watershed [29]. Flooding occurred due to high rainfall intensity on 31 December 2019, exactly 1 day before. At least two of the eight stations around the DKI Jakarta area recorded rainfall of more than 300 mm; this is the highest rainfall of the last 25 years [30].

Flooding in the Ciliwung–Cisadane Watershed is not a new issue. It has happened since the 1660s [31]. An increase in discharge in the Ciliwung river was allegedly triggered by a change in the land use in the upstream area to become a tea plantation [32]. Some countermeasures to deal with increased runoff in the Ciliwung River have been carried out since the 1970s, and some have been implemented to date [33]. However, over time, as some areas began to develop, the problem of flooding in the Ciliwung–Cisadane Watershed became increasingly complex, and flooding still occurred.

The floods, which have not ye<sup>t</sup> been solved, have caused several areas in the Ciliwung– Cisadane Watershed to still suffer from flooding. This has caused material and nonmaterial losses. These losses were not only caused by the size of the affected area but also due to the unpreparedness of the area in the face of disaster. The more prepared an area is in dealing with disasters, the more it will reduce the losses suffered [34]. This preparedness is not only related to the protection of an area against flooding but all the effort in that area in dealing with the flood disaster itself. This preparedness includes structural and nonstructural efforts [35]. Integration and sustainability of preparation, protection and the ability to respond effectively are the keys to resilience to flood disaster [36].

Many studies about the floods in the Ciliwung–Cisadane Watershed have been carried out, with different considerations. Climate change, land subsidence, land-use change and even social phenomena such as urbanization have been considered in several studies about flooding in the Ciliwung–Cisadane Watershed [37,38]. Future projection of flooding in the Ciliwung basin has been discussed in several studies as well. Moe et al. conducted a study related to the possibility of flooding in Jakarta, with scenarios caused by land subsidence combined with land-use changes [39,40]. Emam et al. showed how climate change and land-use change influence the flood behavior in Jakarta; it increased the peak flow of a 50-year return period in 2030 by 130% [41]. Januriyadi et al. not only conducted a study about future flooding but also analyzed the flood risk, due to climate change and urban development, for 2050 and showed that the risk has multiplied extremely [42].

Although many studies have been conducted about flooding in the Ciliwung–Cisadane Watershed, the occurrence of floods almost every year makes flooding still of interest to be investigated. To predict future changes is of grea<sup>t</sup> interest, but projecting current events to past predictions is also beneficial. The news related to Jakarta floods in the early 2020s, and moreover, the information that the rain that occurred was the most in history, startled Indonesia. The question is how large the flood discharge was due to that rain event, and to be more specific, which return period would have been equivalent to the coming flood on 1 January 2020. Today, it is not impossible to assess the flood after it happened. In 2018, Moe et al. conducted a rapid assessment to predict the affected area due to flooding in the Upper Citarum River Basin [43]. The aim of this study is to know the characteristics of the flood on 2020 New Year's Eve using rapid assessment.

### **2. Materials and Methods**

### *2.1. Study Area*

The study area was located at the Ciliwung–Cisadane Watershed, as shown in Figure 1. The watershed consists of 15 river basins, which accumulatively have an area of 5269.84 km2. It administratively covers 3 provinces and 9 cities/municipalities, as follows: the Province

of DKI Jakarta (5 cities), Bogor, Depok, Bekasi and Tangerang. The study area was located between latitude 5◦5928 S and latitude 6◦4718 S and between longitude 106◦2445 E and longitude 107◦1254 E.

**Figure 1.** Study area.
