1 Introduction

Despite the high risk of natural disasters, people prefer to reside in coastal regions globally. Approximately 40% of the world’s population is estimated to live within 100 km of the coast (United Nations 2017). In Indonesia, 65% of people live in coastal areas (Ilman et al. 2011), where rapid population increase and economic development have been observed. The special capital region of Jakarta (hereafter referred to as Jakarta) is Southeast Asia’s most populous city, and its population size tripled in the past 50 years (Fig. 21.1). According to the Indonesian Disaster Database (DIBI), the majority of natural disasters are hydro-meteorological events (Badan Nasional Penanggulangan Bencana [BNPB] 2021). Based on the DIBI, flooding accounts for a large proportion of all disaster-related deaths (28.0%), injuries (37.6%), and houses destroyed (35.2%) and evacuated (95.5%) in Jakarta after 2000.

Fig. 21.1
figure 1

Maps of Jakarta (left) and mangrove areas in Angke Kapuk (right)

Natural disaster risk is the product of natural hazards, exposure, and vulnerability (United Nations Development Programme 2004). While the occurrence of natural hazards such as tsunami is uncontrollable, exposure is controllable but may not be in the short-term. A large number of poor and marginalized people settle in coastal areas of Asian countries (Pomeroy et al. 2006; Winsemius et al. 2018). In Jakarta, the poorest and most vulnerable people live in coastal areas illegally, and those under the most severe conditions suffer tidal flooding every day. They are reluctant to change settlements because of high land prices in safer places and good accessibility to their workplaces; they frequently return to these locales if they are forced into involuntary resettlement (Marschiavelli 2008; Padawangi 2012; Marfai et al. 2015). Vulnerability is the most controllable factor in present-day Jakarta, and hard and soft measures are applicable. Hard measures comprise artificial measures, including seawalls (gray infrastructures), and natural measures, including mangroves, which possess inherent coastal-disaster prevention functions (green infrastructures, GI) (Tanaka 2009; Nurhidayah and McIlgorm 2019), while soft measures minimize vulnerability through, for example, disaster prevention education.

Because many developing countries are located in tropical/subtropical regions, it is feasible to utilize GI, such as mangroves, coral reefs, and seagrass meadows, as ecological measures for disaster mitigation; this is referred to as Ecosystem-based Disaster Risk Reduction (Eco-DRR). By so doing, developing countries can ease the financial difficulty of constructing gray infrastructures (Esteban et al. 2017; Takagi 2019). Other advantages of GI include self-maintenance (Gedan et al. 2011) and self-repairing functions when damaged (Ferrario et al. 2014). Mangroves exert and enhance ecological resilience in coastal areas (Alongi 2008; Gilman et al. 2008; Takagi 2019). For example, mangroves can migrate in response to sea-level rise (SLR) and sustain coastal ecological resilience (Nyström and Folke 2001; Alongi 2008).

This chapter selects Jakarta as a coastal, highly-populated city with high vulnerability and reviews the current status and possibilities of urban mangroves in Jakarta Bay for natural disaster reduction. Currently, the range of mangrove habitat in urban Jakarta Bay is limited, and enlargement of habitat may be difficult. Therefore, this chapter also briefly discusses the potential role of mangroves as an icon in disaster prevention, that is, a kind of soft measure, utilizing mangroves as a visual cue for reminding the population of natural hazard risks.

2 Floods in Jakarta

2.1 Most Vulnerable People

Jakarta is located in the northwest of Java Island (Fig. 21.1). Population size has exceeded ten million. Thirteen rivers discharge into Jakarta Bay. The biggest are the Ciliwung and Cisadane rivers, and nine rivers are considered to be associated with floods (Marfai et al. 2015). Yusuf and Francisco (2009) provide a list of 132 vulnerable provinces in Southeast Asian countries, of which part of Jakarta’s municipalities are ranked 1st (Central Jakarta), 2nd (North Jakarta), 3rd (West Jakarta), 5th (East Jakarta), and 8th (South Jakarta).

Globally, the poorest people are the most vulnerable to natural hazards (Esteban et al. 2013; Tauhid and Zawani 2018). In Jakarta, the poor gather in informal urban settlements called kampung,Footnote 1 developed along, for example, riverbanks (Padawangi 2012), where people suffer from higher vulnerability than other urban residents (Firman et al. 2011; Arkema et al. 2013; Colven 2020). Jakarta’s poor population was 0.373 million (3.57%), and the percentage is the highest in North Jakarta (5.35%) (Badan Pusat Statistik [BPS] 2018).

2.2 Causes of Floods

The frequency and magnitude of floods have been higher since the 2000s, and the 2007 flood was the worst in the past three centuries (Marfai et al. 2015; Colven 2020) (Fig. 21.2). The factors influencing the severity of floods are (1) heavy precipitation in the wet season, (2) SLR, (3) El Niño–Southern Oscillation (ENSO), (4) urbanization, and (5) accumulation of garbage in streams (Steinberg 2007; Marfai et al. 2015; Budiyono et al. 2016; Nurhidayah and McIlgorm 2019). The SLR in Jakarta coasts is 0.57 cm/year (Firman et al. 2011), which exacerbates flood damage in association with subsidence. The ENSO may also influence precipitation and inter-annual oscillations in sea surface height, causing more severe floods (Budiyono et al. 2016; Esteban et al. 2017; Takagi 2017).

Fig. 21.2
figure 2

Submerged roads near the Citraland Mall by the 2020 Jakarta Floods (taken by the author on January 1, 2020)

Urbanization includes (4a) losses and degradations of urban lakes, called setu or situ, in the Ciliwung and Cisadane river watersheds (Henny and Meutia 2014), (4b) decrease of green areas and agricultural lands (Permatasari et al. 2016; Purwoko et al. 2016), and (4c) land subsidence. Approximately 10–20% of urban lakes have been lost (Henny and Meutia 2014), and the green space ratio reduced from 40% in 1985 to 9% in 2002 (Steinberg 2007), which may increase surface runoff, facilitate land subsidence, and cause downstream inundation (Firman 2009). Subsidence is mainly attributed to groundwater extraction and is considered to be the most important factor affecting future flood events (Budiyono et al. 2016; Takagi et al. 2016b). The highest subsidence has observed near the coast at a rate up to 15–20 cm/year between 1995 and 2005 (Chaussard et al. 2013). Moreover, (4d) sedimentation of silt in the downstream has worsened the flood events (Colven 2020).

Finally, (5) Jakarta has a long history of garbage dumping in canals and rivers from the age of Batavia (1619–1942) (Steinberg 2007), which reduces river flows (Fig. 21.3). Currently, 57.0–85.4%, 8.1–31.4%, and 0.4–19.9% of wastes are from houses, industry, and livestock, respectively (Purwoko et al. 2016). The total volume of garbage discharged is 23,400 m3/day, of which 14,700 are disposed by the City Sanitation Office, but some portion of the rest may be thrown into streams (Steinberg 2007). Furthermore, many do not receive garbage collection services (Colven 2020). Because riverbank settlers throw garbage into the river, the width of the Angke river was diminished from 40–60 m to 5–10 m (Steinberg 2007). A large proportion of rainwater from upstream contains garbage and may worsen downstream floods (Purwoko et al. 2016).

Fig. 21.3
figure 3

Accumulation of garbage in Waduk Pluit (Pluit Reservoir, downstream of Ciliwung river) (taken by Dr. Hajime Matsushima on March 17, 2015)

2.3 Current Countermeasures

Some countermeasures have been implemented. First, river dredging and expansion have been conducted as river normalization projects (Colven 2017, 2020). A flood canal, called Western Banjir Canal (WBC), was constructed and connected to the downstream of the Ciliwung river in the early twentieth century. The Eastern Banjir Canal (EBC) was developed to further mitigate floods and was utilized after 2011, but the plan to connect the WBC and EBC remains unrealized (the part indicated by a red line in the left-hand chart of Fig. 21.1). Second, the central government plans to make Ciawi and Sukamahi retention basins in Bogor district to reduce the water volume of the Ciliwung river. Third, the National Capital Integrated Coastal Development program is ongoing, which includes the construction of the Great Garuda Sea Wall (GGSW, 32 km) in the gulf of Jakarta and the construction of artificial islands, including one with the shape of GarudaFootnote 2 (more than 1000 ha) (Colven 2017; Nurhidayah and McIlgorm 2019). The construction of GGSW is controversial because it may cause damage to coastal fisheries and mangroves and does not mitigate subsidence (Colven 2017).

Fig. 21.4
figure 4

River cleaning activities by local government in Western Banjir Canal (downstream of the Ciliwung river) near the Seasons City Trade Mall (taken by the author on November 30, 2019)

Other activities include the removal of waste and trash from rivers and canals by Jakarta’s Public Facility Maintenance Agency (Figs. 21.4 and 21.5). Local communities also conduct river and canal cleaning voluntarily (Marschiavelli 2008; Marfai et al. 2015; Purwoko et al. 2016). In coastal and riverside kampung, people living in single-storied houses add a second floor as a place of refuge (Marschiavelli 2008; Marfai et al. 2015; Budiyono et al. 2016; Wijayanti et al. 2017). Takagi et al. (2016a) recommend escaping to the second floor rather than moving long distances to higher places when dykes break. Some people in Kali Adem, a coastal village, elect to live in fishing boats to avoid floods (Padawangi 2012). Some local communities utilize mosques as evacuation sites (Marfai et al. 2015).

Fig. 21.5
figure 5

River cleaning activities by pasukan orange (orange troops) of the Jakarta’s Public Facility Maintenance Agency at Saluran Cideng (Cideng waterway) (taken by the author on October 4, 2019)

3 Mangroves in the Gulf of Jakarta

3.1 Recognition of Mangroves as GI in Indonesia

The advantage of GI is gradually being acknowledged in Indonesia. The 1992 Flores Island Earthquake Tsunami highlighted the bio-shield function of mangroves in Indonesia (Karminarsih 2007). After the 2004 Aceh Tsunami, Indonesians recognized the term tsunami much more (Baeda et al. 2015) and became more aware of mangrove’s Eco-DRR function (Karminarsih 2007). The Indonesian government tried to create a green belt on the Banda Aceh coast after the 2004 Aceh Tsunami, although it remains incomplete (Dalimunthe 2018). Simultaneously, the Minister of Forestry launched a plan to reforest 600 thousand hectares of depleted mangroves (Barbier 2008). The government proclaimed the National Strategy for Mangrove Ecosystem Management (Indonesian Government 2012). Mangrove rehabilitation program Ayo Tanam Mangrove (Let’s Plant Mangrove) has been implemented nationwide by the Ministry of Fisheries and Marine Affairs for more than a decade, and mangrove afforestation is regarded as a prospective way of improving Eco-DRR in Indonesia (Dalimunthe 2018).

3.2 Functions and Status of Mangroves

Mangroves are a group of halophytic plants inhabiting the intertidal zone of tropical and subtropical regions. Giri et al. (2011) estimate the world’s mangrove area is 137,760 km2 in 2000, of which 22.6% (31,130 km2) is in Indonesia. Indonesian mangroves are distinguished by having the largest number of species and the largest area (Ilman et al. 2011), but about 30% of their area has been lost since 1980 (Giesen et al. 2006; Murdiyarso et al. 2015) mainly because of transformation to shrimp ponds (63%) and agricultural lands (32%) (Giri et al. 2008).

Mangroves may protect coastal areas from tsunamis, tidal surges, floods, and cyclones (or typhoons, hurricanes) by mitigating waves and winds and minimize damage to property and human life (e.g., Danielsen et al. 2005; Kathiresan 2012; Unnikrishnan et al. 2013; Sandilyan and Kathiresan 2015). Some types of mangroves have prop roots or pneumatophores, which strengthen their durability against winds and facilitate the accretion of sediment, resulting in wave attenuation (Alongi 2008; Ostling et al. 2009). Accumulation of sediment preserves or even expands the coastline (Marois and Mitsch 2015). The sturdy trunks and extensive root complexes of mangroves, along with accumulated sediments, mitigate the speed of flood, and sediments also absorb floodwater (Ilman et al. 2011). Moreover, mangroves mitigate the impact of dyke-breaking tsunamis, a kind of flooding “caused by the rupture of a coastal dyke” (Takagi et al. 2016a, p. 1630). People may save their lives from tidal waves by climbing, clinging to, or hanging in mangrove trees (Forbes and Broadhead 2007; Sandilyan and Kathiresan 2015).

3.3 Mangroves in North Jakarta

The most famous and extensively examined mangroves in Jakarta Bay are Mangrove Angke Kapuk (MAK) in Kelurahan Kamal Muara (Kamal Muara village) and Kelurahan Kapuk Muara (Kapuk Muara village) of Kecamatan Penjaringan (Penjaringan district) (Fig. 21.6; right-hand chart of Fig. 21.1). The MAK consists of mangrove forests in (1) Hutan Lindung Angke Kapuk (HLAK, Angke Kapuk protected forest, 44.76 ha), (2) Taman Wisata Alam (TWA, Angke Kapuk national park, 99.82 ha), (3) Arboretum (10.51 ha), (4) Suaka Margasatwa Muara Angke (SMMA, Muara Angke Wildlife Reserve, 25.02 ha), and (5) Ekowista Mangrove (95.50 ha) (Kusmana et al. 2013). There are 11 true (exclusive) mangroves in MAK, and the dominant species are Avicennia spp. and Rhizophora spp. (Pambudi et al. 2018). Some residential areas including Pantai Indah Kapuk (PIK), a relatively wealthy area, and golf courses are also located near the MAK. The east side of the MAK is Kelurahan Pluit (Pluit village), which is another village of Penjaringan district and contains mangrove forest (6) Kali Adem.

Fig. 21.6
figure 6

Riparian forests in HLAK (taken by Dr. Emma Colven on May 25, 2019)

The total mangrove area in Jakarta has decreased from 1165.33 ha in 1980 to 165.28 ha in 2016 (Hilmi et al. 2017). Mangroves in Angke Kapuk and adjacent areas have been transformed into residential and industrial areas, highways, an airport, and fish ponds since the late 1970s (Ambinari et al. 2016; Kusmana 2017). Wastes and pollutants from upstream and residential areas in MAK are also significant factors of mangrove degradation (Hilmi et al. 2017; Sofian et al. 2020). One of the main wastes is plastic debris in MAK (Cordova and Nurhati 2019; Sofian et al. 2020), mostly polystyrene (44.62%), a polymer used for food packaging (Purwoko et al. 2016; Cordova et al. 2021). Plastic debris prevents air uptake by pneumatophores (Purwoko et al. 2016) and reduces the strength of mangrove roots (Sofian et al. 2020) and interferes with mangrove regeneration by inhibiting the growth of seeds (Sasongko et al. 2014; Putri et al. 2015). Other pollutants, including heavy metals, nitrogen, and phosphorus, affect some sorts of mangroves (Hamzah and Setiawan 2010; Ambinari et al. 2016; Purwoko et al. 2016; Pambudi et al. 2018). Furthermore, groundwater extraction, land subsidence, and associated changes of hydrological conditions may result in the loss of mangroves (Gilman et al. 2008; Ilman et al. 2016). Because of waste accumulation and pollution, mangrove diversity diminishes (Sasongko et al. 2014; Putri et al. 2015), and those tolerant to pollution such as Avicennia marina and Rhizophora mucronata dominate in the MAK (Wibowo 2006; Pambudi et al. 2018).

Several mangrove rehabilitation projects have been implemented in HLAK (Penyusun 2009; Sasongko et al. 2014), Jakarta Fishing Port in Pluit village (Takagi et al. 2016a), and Kali Adem (Rahadian et al. 2019). The rehabilitation at HLAK is conducted by a group Sahabat Bakau (Friends of Mangrove). They have constructed 1780-m-long stone breakwater, 100 m from the coast, and 8000 mangroves were planted (Penyusun 2009).

Mangrove degradation causes significant CO2 emissions (Kathiresan 2012; Murdiyarso et al. 2015; Sandilyan and Kathiresan 2015) and the loss of natural disaster mitigation (Gilman et al. 2008; Giri et al. 2008). For example, the PIK was formerly a mangrove forest able to store excess rainfall and high tides, but this water storage function was lost, and the number of floods increased; thus, the toll road surface (roads go through the Ekowista Mangrove in Fig. 21.1) was raised by 1.2 m to prevent flood impacts (Ilman et al. 2011). Removal of mangroves also caused the loss of coastal sediments (Penyusun 2009).

4 Possibilities of Mangroves for Flood Damage Mitigation

To address Jakarta’s current issues, including floods, subsidence, and loss of greenery (Sasongko et al. 2014), rehabilitation of mangroves is recommended, but suitable areas are limited. The most realistic option might be to conserve current mangroves in an appropriate state while emphasizing soft measures more. In this section, requirements and limitations of rehabilitation are summarized, and potential mangrove functions as soft measures are discussed.

4.1 Hard Measures: Rehabilitation of Mangroves for GI

Several points should be noted in promoting the restoration of mangroves. A minimum width or area is required to sustain mangroves, to maintain species richness, and to support the provision of ecosystem services (Duke et al. 2007; Ilman et al. 2011; McIvor et al. 2012). There is controversy as to whether urban and aquacultural areas can be restored to mangroves (Giri et al. 2008; Biswas et al. 2009; van Oudenhoven et al. 2015), and if a suitable area is not fully secured, mangroves may not exert tidal attenuation function (Takagi 2017). Locations of restored mangroves are critical for disaster reduction. Mangroves reduce the rate of fatalities of a tsunami when they are located in front of residential areas (Danielsen et al. 2005) but escalate damage when settled areas are located between the coast and mangrove forest (Bayas et al. 2011). Another restriction is coastal squeeze, which happens because of SLR and urbanization. Belize’s example demonstrates that mangroves need to move landward 500 m for 1 m SLR (Guannel et al. 2016), but expansion inland is limited by many factors, including the presence of a developed land surface (Gilman et al. 2008; Pontee 2013; Marois and Mitsch 2015). Other conditions, including water depth and contamination level, may also be restrictive (Sasongko et al. 2014; Putri et al. 2015; Takagi et al. 2016a; Takagi 2019). Mangroves located in creeks, estuaries, and inlets may increase the height of a tsunami and accelerate the flow, resulting in more severe damage (Forbes and Broadhead 2007; Tanaka 2009; Power 2013).

Existing studies highlight the importance of considering biological diversity (Tanaka 2009; Takagi 2019), original vegetation (Karminarsih 2007; Purwoko et al. 2016), species that have a high tolerance of floods (Alongi 2008) and tsunamis (Kathiresan and Rajendran 2005; Tanaka 2009; Yanagisawa et al. 2009), and a mangrove landscaping strategy that effectively reduces the impacts of tsunamis and high waves (Hilmi 2018). Because polluted sediments and floating garbage may hinder the growth of mangroves, species with high tolerance to these factors are recommended (Putri et al. 2015; Pambudi et al. 2018). The most promising species for disaster prevention are Avicennia marina for floods (Alongi 2008) and Rhizophora spp. for tsunami (Kathiresan and Rajendran 2005; Yanagisawa et al. 2009), which dominate the MAK.

The current status of contamination and urbanization may be restrictive to any substantial improvement in mangrove diversity and area. A more promising idea may be to combine mangrove improvements with other remedies. Groundwater regulations contribute to diminishing subsidence and flood events (Budiyono et al. 2016; Esteban et al. 2017), although similar regulations have already been implemented in Jakarta, but in an unsatisfactory manner. Multiple lines of defense (Lopez 2009), whereby the coast is protected by a combination of seascape elements, including coral reefs, seagrass meadows, and mangroves (Guannel et al. 2016), attracts the attention of practitioners (Arkema et al. 2017) because multiple lines of defense may exert more disaster prevention functions (Barbier 2016; Guannel et al. 2016), along with the provision of varied ecological services, and establish a solid environmental complex.

4.2 Soft Measures: Mangroves as a Disaster Prevention Icon

Mangroves can be utilized more when applying soft measures for disaster reduction. The MAK may serve as an icon, similar to the panda used by the World Wide Fund for Nature as its icon or logo. In multiple activities involving citizens, authorities can utilize mangroves as an iconic cue to make citizens think of vulnerability to natural disasters and the necessity to stop littering.

First, risk-mitigating actions, including early-warning systems and disaster prevention education, should be emphasized more (Bayas et al. 2011; Marois and Mitsch 2015), and designs with mangroves could be adopted in documents of such disaster prevention initiatives. Some victims of the 2006 Java Tsunami did not flee to safer places when they saw the telltale drawdown because of a lack of disaster prevention education and early-warning systems (Kerr and Andrew 2007). Regardless of the existence of gray infrastructures, including concrete dikes, too much expectation for GI may lead to loss of life (Dahdouh-Guebas et al. 2005). Multiple malls scattered in Jakarta (Steinberg 2007) may serve as evacuation areas. Special consideration for those in kampung is necessary because they suffer floods every week (Marfai et al. 2015; Esteban et al. 2017; Colven 2020) and tend to underestimate their vulnerability. One feasible way of enhancing kampung residents’ disaster prevention activities includes participation in mangrove rehabilitation programs, which may also improve their knowledge of disaster prevention.Footnote 3 Tanaka (2009) states that vegetated areas such as mangroves can be used in disaster prevention education by local temples or local authorities. Because Indonesians regularly hold religious meetings, reduction of littering and improvement of disaster prevention knowledge through education are feasible. The iconic mangroves, printed in educational materials, may enhance participants’ recognition when used repeatedly.

Second, because deforestation of mangroves causes sedimentation of creeks, canals, and estuaries (Wolanski et al. 1992) and increases flood events, mangroves can be planted along upstream riverbanks. Riparian mangrove forests can also be utilized as an icon of disaster prevention for local residents.