Projection of coastal floods in 2050 Jakarta

Urban Climate 17 (2016) 135–145

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Projection of coastal floods in 2050 Jakarta Hiroshi Takagi a,⁎, Miguel Esteban b, Takahito Mikami c, Daisuke Fujii a a b c

Tokyo Institute of Technology, School of Environment and Society, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8550, Japan The University of Tokyo, Graduate School of Frontier Sciences, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8563, Japan Waseda University, Department of Civil and Environmental Engineering, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan

a r t i c l e

i n f o

Article history: Received 9 February 2016 Received in revised form 20 May 2016 Accepted 22 May 2016 Available online xxxx Keywords: Jakarta Coastal flood Land subsidence Sea-level rise Oceanic tide Urbanization Abnormal high tides ENSO 18.6-year lunar nodal cycle

a b s t r a c t The present work projects the extent of coastal floods by the year 2050 in Jakarta, one of the fastest growing megacities in the world, using a coastal hydrodynamic model that considers abnormal high tides, sealevel rise, and land subsidence. The tidal constituents were adjusted in a manner to reproduce the observed tidal record during the extensive coastal floods that took place on November 2007, when it is likely that a La Niña event, the 18.6 year lunar nodal high-tide cycle, and other abnormal tide mechanisms took place simultaneously. The simulations demonstrate that by the middle of this century extensive floods could potentially reach several kilometers inland in Jakarta. Land subsidence is clearly one of the major challenges facing the city, as considering only the influence of sea-level rise indicates that such floods may be limited to within a few hundred meters of the coastline. From 2000 to 2050 the potential flood extent is estimated to increase by 110.5 km2. Land subsidence is responsible for 88% of this increase. The results also indicate that the rate of flood area expansion in the 2025–2050 period would be 3.4 times faster than during the 2000–2025 period, demonstrating a non-linear increase in flood risk with the passage of time. © 2016 Elsevier B.V. All rights reserved.

1. Introduction Jakarta, the capital of Indonesia, is one of the largest coastal megacities in the planet, with a resident population exceeding 9.6 million in 2010, plus approximately 2.5 million daily commuters from the adjacent cities, and a total land area of 662 km2 (Djaja et al., 2004; Firman et al., 2010). The population growth rate reached 1.39%/year over the period 2000–2010 (Central Board of Statistics, 2010), which has made Jakarta ⁎ Corresponding author. E-mail addresses: [email protected] (H. Takagi), [email protected] (M. Esteban), [email protected] (T. Mikami), [email protected] (D. Fujii)

http://dx.doi.org/10.1016/j.uclim.2016.05.003 2212-0955 © 2016 Elsevier B.V. All rights reserved.

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H. Takagi et al. / Urban Climate 17 (2016) 135–145

one of the most densely populated cities in the world, and is expected to reach 30 million inhabitants by 2030. Greater Jakarta has also been experiencing rapid economic growth over the last decades, offering its resident a variety of opportunities in terms of working and accommodation (Sagala et al., 2013). As a consequence of this rapid development Jakarta has been facing many urban development issues, though the issue of land subsidence appears to have become especially serious over the last couple of decades. In fact, the occurrence of land subsidence was clearly recognized as far back as 1978, when substantial cracks were found in buildings and a bridge in downtown Jakarta (Djaja et al., 2004). In Jakarta, the subsidence rates along the coast vary from 9.5 to 21.5 cm/year in the period between 2007 and 2009 (Chaussard et al., 2013). Land subsidence can be classified into four types, namely: subsidence due to groundwater extraction, subsidence induced by the load of buildings, subsidence caused by natural consolidation of alluvium soil, and geotectonic subsidence. For the case of Jakarta the first type, subsidence due to groundwater extraction, appears to be the main cause for the lowering of the land. In particular, the extraction of water for industrial uses is a widespread practice which can induce rapid rates of land subsidence. Such subsidence can lead to severe damage to buildings and infrastructures, increase the extent of flooded areas, destroy local ground water systems or increase seawater intrusion (Braadbaart and Braadbaart, 1997; Abidin et al., 2009; Ng et al., 2012). Jakarta has already experienced many severe river floods due to heavy monsoon rains, particularly in the years 1996, 2002, 2007, 2013, and 2014. Particularly, the floods that took place in January 2013 resulted in 40 deaths, 45,000 refugees, and substantial economic damage. Such extreme flooding events might become more frequent in the future due to the impacts of land use and climate change (Kure et al., 2014). The floods in February 2007 also caused extensive economic losses, mounting up to between 4.1 and 7.3 trillion IDR (450–800 million USD, see Sagala et al., 2013). In fact, Jakarta is considered to be one of the most vulnerable cities to coastal floods (Firman et al., 2010, Ningsih et al., 2011). Though Jakarta is not on the route that tropical cyclones take and thus is unlikely to face a significant typhoon storm surge event, coastal areas could experience the indirect minor impact of a major typhoon which travels near the Philippines or Vietnam in the western north Pacific (Ningsih et al., 2011; Takagi et al., 2014). As highlighted by the Intergovernmental Panel on Climate Change 5th Assessment Report (or IPCC AR5), coastal floods are likely to become more common in coastal areas around the world due to the consequences of future climate change, including sea-level rise (SLR) and tropical cyclone intensification, and other factors such as population increase, urbanization, and land subsidence. In this sense it is worth noting that at the present global losses due to floods and other natural disasters are generally increasing (Esteban et al., 2015a, 2015b; Jonkman and Dawson, 2012; Jongman et al., 2012; Nguyen et al., 2014). As at present CO2 emissions continue to increase, it appears logical that a significant amount of SLR due to thermal expansion of the oceans would be inevitable, unless significant actions are taken by the international community to reduce emissions. Aside from this constant and ongoing increase in sea levels due to the effects of climate change, interannual sea surface oscillations are also likely to increase the risk of floods in low-lying coasts. One such interannual variation is the El Niño Southern Oscillation (ENSO). During the 1997–1998

Fig. 1. Coastal town in Jakarta situated below sea level, which was extensively inundated during a high tide on November 26, 2007 (Left). The thin dyke protecting the settlement was raised by the local government by about a meter after the flood event, obstructing people's vision of the sea (Right). (Photo Left: courtesy of Brinkman J.J., Deltares - Jakarta flood hazard mapping 2007–2014, Right: taken by one of the authors on Sep. 2, 2015).


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ENSO event, a rise and subsequent fall of mean sea level was observed in many areas in the Pacific. These changes are well correlated with global mean sea surface temperature anomalies, which have exhibited a similar response for every major ENSO event since 1981, suggesting that the observed mean sea level change is mostly caused by thermal expansion (Nerem et al., 1999). Unexpected high sea levels could also take place due to other oceanic mechanisms. For instance, Shoji (1961) demonstrated (using the data from 22 tidal stations in Japan) that the daily mean sea levels move along the coast. Furthermore, Kitade and Matsuyama (2000) found that such traveling mean waters could be separated into two types of waves around the Boso Peninsula: one which travels in the form of an internal Kelvin type wave and the other being a continental shelf wave. An abnormal high tide with a maximum amplitude of 60 cm that caused inundation over a wide area of Yokohama Port in October 2006 is likely to have been generated by a shelf wave (Takagi et al., 2008). Finally, when considering coastal floods it is important also to consider the effect of oceanic tides, responsible for periodic sea surface oscillations. The tidal range in the Java Sea is around 1.2 m to 2 m, with the highest values found around Surabaya, Madura, and Bali (National Development Planning, 2010). Though eight short-period ocean tide constituents (O1, K1, M2, S2, P1, Q1, N2, and K2) explain about 83% of the total variance in observed sea levels at the coastal tide gauge in Jakarta Bay (Koropitan and Ikeda, 2008), a long-period tide constituent referred to as the “18.6-year lunar nodal cycle” is considered to have caused an amplification of tides in 2007, resulting in a magnification of the extent of coastal floods in Jakarta (Ministry of Infrastructure and the Environment, 2012; Yamashita et al., 2013) (Fig. 1). This tidal oscillation can be generated when the moon's orbital surface and the earth's equatorial surface incline by 5.2° and by 23.4°, respectively, for the earth orbital surface around the sun, and the cross nodal point between the moon's orbit and the earth equatorial surface rotates with a period of 18.613 years (Yasuda, 2009). Accounting for this nodal cycle is crucial to accurately estimate regional and extreme sea levels (Haigh et al., 2011; Baart et al., 2012). Despite such studies identifying the many factors that can combine to contribute to the flooding of coastal areas in Jakarta, to the authors' knowledge there has been no integrated study that has combined all of them to the simulation of future conditions in the city. The present work thus attempts to project the extent of coastal floods by the year 2050 in Jakarta, using a coastal hydrodynamic model that considers daily and long-term oceanic tides, the influence of unexpectedly high sea level such as ENSO, SLR, and land subsidence. Essentially the authors would like to understand what would be the effect if the abnormal high tide on November 26, 2007 would again occur sometime around the year 2050. 2. Jakarta's 2050 scenarios In order to attempt to understand what could happen to Jakarta in the future it is necessary to develop a number of possible scenarios that can provide some guidance for government authorities and policy makers.

Fig. 2. Measurements in Indonesian sea levels taken from the TOPEX and Jason series of satellite radar altimeters (reproduced by the authors using the Regional Sea Level Data of the University of Colorado).

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The year 2000 was chosen as the baseline for the study. As highlighted earlier, the main factors that need to be considered include SLR, ground subsidence, ocean tides and unexpectedly high sea levels caused by the ENSO event or coastal-trapped long waves. Each of these will be described below, in order to come up with possible future inundation scenario up to the year of 2050. 2.1. Sea-level rise scenarios The TOPEX/POSEIDON satellite has observed variations in global mean sea level with a precision of 4 mm at 10-day intervals since the late 1992. The satellite's data shows a raise in Indonesian sea levels of about 7 mm/year between 1992 and 2015 (Fig. 2). This rate is not necessarily in line with global average increases, as it is estimated that global sea level has been raising by approximately 3.1 ± 0.7 mm/year between 1993 and 2003 (IPCC, 2007). However, it is known that regional discrepancies exist in the patterns of SLR because of natural climate variability, such as differences in the rates of thermal expansion (Cabanes et al., 2001; Church et al., 2004; IPCC, 2013). Therefore, and given uncertainties in the patterns of future regional SLR, the authors simply assumed that SLR around Indonesia will continue at the rate of 7 mm/year and eventually amount up to a difference of 35 cm between 2000 and 2050. Such estimations clearly neglect any possible non-linear responses of SLR. While these non-linear responses are important, most scenarios highlighted in the IPCC show how such effects are more likely to become important in the second half of the 21st century, providing some support for the rather conservative and linear assumptions made by the authors in the present work. 2.2. Land subsidence scenario A GPS land subsidence monitoring network was established in Jakarta and observations were carried out during the period between December 1997 and October 2001 (Djaja et al., 2004). According to such research the rate of subsidence observed was 11.3 cm/year at Pantai Indah Kapuk (North Jakarta) in the period between June 1999 and June 2000. In the present work the authors used the vertical displacements at 14 locations (only 11 out of 14 points are shown in Fig. 3) monitored by this GPS network and estimated annual

Fig. 3. (a) Map of northern Java Island, Indonesia. Red continuous box shows the computational domain. Blue discontinuous line shows the area in which land subsidence was estimated, (b) estimated annual land subsidence rates in Jakarta (●: GPS land subsidence monitoring station presented by Djaja et al. (2004)).


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average subsidence rates over downtown Jakarta as well as surrounding areas by means of a Kringing interpolation (Fig. 3). Owing to the tendency that the subsidence rates increase towards Northwest Jakarta coast, it would naturally follow that the extent and depth of the expected coastal floods will increase in coastal areas, as these are experiencing the greatest subsidence. In a similar manner to the SLR scenario, the future subsidence scenario assumed that the ground surface would continue to sink at a linear rate similar to that at each location at present. Therefore, the total difference in land level between any given future year and the 2000 baseline can be calculated by simply multiplying the annual subsidence rate at a given location (Fig. 3) by the number of years that would pass (i.e., 50 years in the case of the 2050 projection). 2.3. Abnormal high tide scenario As explained earlier, tidal oscillations have been known to play an important part in coastal flooding in Jakarta, and their influence must also be carefully considered. The K1 tidal component is known to be the predominant influence among the eight principal tidal constituents, and propagates from the Flores Sea at the eastern end of the Java Sea (Koropitan and Ikeda, 2008). However, for the case of Jakarta it is also important to consider the influence of the 18.6-year lunar nodal period. This 18.6-year cycle causes long-term fluctuations in oceanic tides, especially for the diurnal components K1 and O1 (Yasuda, 2009). Since the K1 component is predominant in Jakarta, it is expected that tidal anomalies associated with the 18.6-year lunar nodal cycle can be reproduced by adjusting the K1 constituent in a way in which simulated water levels fit with observed water levels at various tidal stations. JCDS (2012) considers that the peak of the lunar nodal period coincided with an annual spring tide, bringing about the coastal flood on November 26, 2007 (Fig. 1). According to this, the next peaks would be expected to appear in the period of 2025–2027 and 2044–2046. On the other hand, sea levels are also known to be influenced by ENSO events (Woodworth, 2006). For instance, the National Development Planning (2010) presents a time series of sea level anomalies from the year 1993 to 2008 and demonstrates that sea levels were depressed by up to 20 cm below normal during El Niño phases, while they were elevated by 10 to 20 cm during La Niña. According to this report, the latest La Niña started in the middle of 2007 and continued till the middle of 2008, and thus the 2007 coastal flood took place in the midst of La Niña phase, which likely influencing the event. Coastal-trapped long waves could also have had an influence on the abnormal high tide in Jakarta. The wind anomalies over the Indian Ocean excite Kelvin waves that propagate eastward along the equator to the Indonesian coast and then on into the Indonesian Throughflow (ITF) as coastally trapped Kelvin waves (Potemra et al., 2002; Wijffels and Meyers, 2004). Although the mechanisms of the abnormal tides may not be fully explained, these influences should also be considered in any simulations of future flooding in addition to normal astronomical tides. 3. Coastal flood modeling In order to reveal the possible exacerbation of the risk of coastal floods in Jakarta around the year 2050 the authors used a hydrodynamics model that took into account SLR, local land subsidence, and abnormal high tide. This section will describe briefly the model used, together with its validation with data on observed water levels at a tide station at Jakarta. 3.1. Methodology Delft3D-FLOW (Deltares, 2011) was applied for the simulation of tides that travel from the deep sea to shallow water, and eventually flood over downtown Jakarta. Though this model is applicable for a 3D domain, the present study used a 2D horizontal grid, and thus the code is equivalent to a non-linear long wave model, which is that most commonly used to simulate tidal propagation (Takagi et al., 2016a, 2016b). Data regarding bathymetry in the target area was obtained from the General Bathymetric Chart of the Oceans (GEBCO) datasets, with topography data originating from the SRTM 90 m Digital Elevation Database. The simulation domain encompassed a wide area that included the entire Jakarta Bay between 5.9°S– 106.68°E and 6.2°S–107.03°E (see Fig. 3). The computational grid sizes gradually become smaller from the offshore to the onshore region, with the largest size being constituted of 200 m (Easting) × 440 m (Northing) and the smallest size of 50 m × 120 m grid cells. The main eight tidal constituents, which were derived

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from the TPXO7.1 Global Inverse Tide Model (Egbert and Erofeeva, 2002), were assigned along the offshore open boundary. After the model was validated it was then run for each of the future scenarios, as will be outlined later. 3.2. Validation In order to validate the model the simulation was run for 6 days, including November 26, 2007, when extensive coastal flooding took place in Jakarta (Ningsih et al., 2011). Fig. 4 presents the observed water levels and a range of simulated tides by the authors or other software at Jakarta Port (Tanjung Priok) for the 2007 flooding event. The calculated astronomical tide by software WXTide32 shows water levels very similar to the uncalibrated simulation result by the authors (the difference being only a few centimeters). However, a

Fig. 4. (a)Satellite image of Jakarta (©Google Earth) (b) inundation depths at AM10:00, November 26, 2007 (simulation with the adjusted K1), (c) comparison of water levels at Jakarta Port between observed and simulated water levels. Observed tide (the black broken line), computational results of Delft3D with the calibrated K1 (red line) and without any calibration (gray line), and predicted tide by WXTide32 (blue chain line).


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remarkable difference in water levels of up to 20 cm can be observed between the observed tide and that calculated by the WXTide32 software. Though the exact reasons behind this discrepancy are still not fully ascertained, the authors hypothesize that the difference in the peak of the heights could be explained by abnormal water level increases due to the lunar nodal tidal cycle, La Niña event or other abnormal high tide mechanisms, as mentioned in the preceding sections. Therefore, the authors attempted to adjust the tidal constituents, particularly K1, in a manner such that the simulated water levels agree with the observed water levels at the tidal station (Fig. 4). After a number of trials, it was found that the gap would become sufficiently small when the amplitude of K1 is multiplied by 1.6, and would result in a good agreement with the maximum water levels recorded on November 26, 2007. It should be noted that the baseline topographical data (that of the year 2000) was used in this calibration run without taking into account the land subsidence between 2000 and 2007. The justification for this resides in the fact that a long tidal wave would not noticeably change in amplitude due to a slight difference in water depth due to the subsidence of a coastal area. 4. Results and discussion The aim of the model developed was to attempt to estimate the increase in flood risk in Jakarta towards the middle of the 21st century, though it is important to keep in mind a number of things when interpreting the results of the present research. First of all, it is important to remember that the Shuttle Radar Topography Mission (SRTM) was flown in February 2000 (Farr et al., 2007), and thus the original SRTM data used in the current work reflects the ground elevations of the year of 2000 (baseline year), rather than the very latest data. Table 1 shows how the potential area that can be inundated by seawater will increase from 2000 to 2025 and 2050. The authors performed the simulations for two cases, namely, (1) only considering SLR and (2) considering SLR plus land subsidence. Since it is expected that high tides will further exacerbate flooding potential, abnormal high tide mechanisms such as the 18.6-year lunar nodal tidal cycle and a La Niña event were considered to be acting concurrently in both simulations, as described earlier. As Fig. 5 indicates, there is large-scale potential for inundation along the coastline of Jakarta, as evidenced by the historical flooding that has taken place in the early 21st century. SLR will clearly have an effect in the extent of the potentially inundation, and according to the simulations carried out the area that can experience floods deeper than 1 m is estimated to be 12.9 km2 by 2050, taking only SLR into account. However, the potential for inundation would expand to 25.7 km2 and 110.5 km2 by 2025 and 2050, respectively, if both SLR and subsidence are considered. The reason for calculating the extent of a flood that is 1 m or higher relates to physical and psychological limitations of human beings. According to an experimental study carried out on human subjects inside a large size water flume (Suga et al., 1995), participants felt intense fear and could not walk without supporting ropes when the flow reached a height of 1 m or higher. Thus, any area inundated to a depth of 1 m or higher can be considered as a hazardous, with substantial damage being expected to human settlements and their inhabitants. Although SLR is obviously an important factor to consider when looking at the increase in future flood risks, the potential maximum extent of flood areas by the middle of this century is far more influenced by land subsidence. Essentially, flooding may be limited to within a few hundred meters from the coastline when considering only SLR. However, the progress of land subsidence appears to drastically exacerbate the situation, and could potentially lead to seawater reaching several kilometers inland, resulting in it no longer being a local coastal issue but rather an extensive urban flood issue. The conclusion that land subsidence is far more important than SLR would appear obvious given that the annual rate of land subsidence exceeds 10 cm per year (Fig. 3), while sea levels are rising by only several millimeters per year, an order of magnitude Table 1 Expansion of potential flood area from 2000 to 2025, 2050. Time span

Factors considered

Flooded area deeper than 1.0 m

2000–2050 2000–2025 2000–2050

Sea-level rise Sea-level rise + land subsidence Sea-level rise + land subsidence

12.9 km2 25.7 km2 110.5 km2

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Fig. 5. Potential inundation areas in Jakarta are shown in black. The red circle (●) represents the National Monument (Monas), located in the city center of Jakarta.

lower. It is interesting to note that the rate of flood area expansion in the 2025–2050 period (3.4 km2/year) is much faster than in the 2000–2025 period (1.0 km2/year), highlighting how the increase in flood hazard appears to be non-linear. Land subsidence due to the extraction of groundwater is a problem that has been faced by many other megacities in Asia (Fig. 6). However, the current subsidence rate in Jakarta appears to be the fastest among the historical series of cities in the region, which have all substantially slowed-down in recent years. The present study assumed that land subsidence would continue at the constant rate estimated using the short-span GPS data between 1997 and 2001, a likely scenario unless groundwater extraction can be swiftly stopped, which would appear a remote possibility at the time of writing. Budiyono et al. (2016) made an assumption that land subsidence in Jakarta will stop by 2025 in their river flood modeling according to the “100-0-100” sanitation policy issued by the Ministry of Public Works (PU), in which the government aims to provide 100% of water supply needed for Jakarta Metropolitan area by 2019. If this action is realized and consequently the subsidence can be stopped, the present study suggests that the extent of coastal flood in Jakarta would be significantly decreased from its maximum possible extent, in the order of 80–90%. The increase in flood risk associated with SLR and land subsidence could be counteracted by the use of hard structures and/or soft measures. A coastal dyke, which is the most common type of countermeasure against coastal flooding, can be expected to prevent hinterlands from being flooded be retaining seawater in front


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Fig. 6. Land subsidence in Asian megacities (drawn by the authors using data from Kaneko and Toyota, 2011).

of it. However, many of dykes that have already been built in Jakarta look rather brittle, and it is dubious whether they could withstand a severe storm. Given that land subsidence looks set to continue into the future, this will require progressively bigger dykes, which will become increasingly more fragile unless their quality and size increases substantially. Such designs probably need to take into account the risk of overtopping, as typically poorly built dykes can quickly collapse. Once a part of the dyke system collapses, flood waters can quickly intrude through the openings and fill up the hinterlands in a very short time (e.g. Takagi and Bricker, 2014; Jayaratne et al., 2015; Takagi et al., 2016a, 2016b). As ground subsidence continues to take place, the possible flow velocity due to the hydraulic drop increases. Therefore, the damage due to a future flood is expected to increase as the land continues to subsidence. It is worth noting how seawater could also penetrate through sewage pipes and flood the ground even if coastal dykes work properly (Fig. 7). Again, as land subsidence continues it is likely that this problem will get progressively worse, and will start to require large numbers of pumping stations to be installed, which would evidently be rather costly. To make matters worse, subsidence can create an adverse slope in sewage systems discharging into the sea, which could further introduce seawater into urban areas. The results of the present research clearly indicate that coastal flood risk management in Jakarta and other developing megacities is a serious issue that needs to be carefully considered in the context of rapid urban development, the resulting land subsidence that can arise from it, and other climate change factors. 5. Conclusions The possible extent of coastal floods around the year 2050 in Jakarta was projected using a coastal hydrodynamic model which considers astronomical tides, abnormal high sea level, SLR, and land subsidence. The

Fig. 7. Inundated port areas in Jakarta (Left: Muara Angke, Right: Tanjung Priok), located below the sea level. Seawater frequently intrudes through the sewage system and inundates roads, not receding for hours (photo taken by the authors on Left: Sep. 3, 2015 and Right: Feb. 7, 2016).

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oceanic tides along the offshore boundary were adjusted according to the observed tides during the coastal flood on November 26, 2007, when La Niña or other abnormal tides and the 18.6 year lunar nodal tidal cycle likely occurred simultaneously and caused an abnormal high tide. While ENSO events may not be predictable, the lunar nodal cycle has been predicted to appear in the period of 2025–2027 and 2044–2046, likely causing another abnormal tidal event in the future, which could easily coincide with a La Niña year. The simulations demonstrate that land subsidence in Jakarta is likely to be the predominant driver that leads to the exacerbation of coastal floods, followed by the influence of SLR. The combination of these two factors could potentially induce extensive floods by the middle of this century, reaching several kilometers inside the city. It is also important to notice that the increase in the areas that could be potentially flooded expands in a nonlinear manner, and the potential flooded area in the 2025–2050 period is estimated to be 3.4 times wider than the 2000–2025 period. The projections shown in the present research suggests that coastal flood issues need to be discussed in the context of urbanization, particularly land subsidence, in addition to climate change. Essentially, it is important that both of these issues should not be treated separately. Adaptation strategies to both land subsidence and climate change will definitely be expensive. However, it is clear that land subsidence needs to be stopped as soon as possible, and the longer this problem is allowed to continue the more expensive that any future adaptation strategies will become. Acknowledgements Funding for this research was supported by the Environment Research and Technology Development Fund (S-14) of the Ministry of the Environment, Japan. 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