Mapping Vulnerability to Flood in Urbanizing Watershed: The Case of Surakarta Urban Region

The vulnerability of urban areas to flooding increased along with the rapid urbanization process. The study selected the Subosukawonosraten (Surakarta, Boyolali, Sukoharjo, Karanganyar, Wonogiri, Sragen, and Klaten) Region as a case study to assess the vulnerability. The ESA framework: exposure, sensitivity, and adaptability—is applied. Exposure is examined using the built-up index in 2020. Sensitivity is assessed based on population density in 2020. Additionally, precipitation and topography are also discussed. The study uses hot spot analysis to pinpoint the most populated and developed areas of the region, which are Surakarta and its peri-urban. Following that, the study investigates to what extent urban villages situated in this hot spot can adapt in light of the floods that often occur in the region. The findings imply that, primarily because of rapid urbanization, the strategy to deal with vulnerability should be considered an important regional-scale strategy that should integrate technical and community-based development approaches to establish a more sustainable strategy for resiliency.


Introduction
Discussion about vulnerability has been focusing more on urbanized areas and how it has higher vulnerability to flooding [1,2].Originally, the word "vulnerable" comes from the Latin "vulnerabilis" which means 'to wound' [3].During its development, in the 1960s the idea of vulnerability was focused on physical environmental problems and established in the domain of environmental science.Furthermore, in the 1970s sociologists developed the idea into a socially constructed vulnerabilitywhich argued that environmental problems are triggered by social causes and have a social impact [4].Vulnerability is also discussed in income and poverty alleviation framework [5,c].In urban studies, vulnerability is defined as the magnitude of responsiveness and the speed of recovery (resilience) to risk during a negative event [7].Being vulnerable is the state of being susceptible to harm from exposure to pressures brought on by environmental and societal change as well as from a lack of adaptability [8].
The Intergovernmental Panel on Climate Change Report (IPCC) of 2001 [9,6] defines vulnerability 1264 (2023) 012013 IOP Publishing doi:10.1088/1755-1315/1264/1/012013 2 "The degree to which a system is susceptible to, or unable to cope with, adverse effects of climate change, including climate variability and extremes.Vulnerability is a function of the character, magnitude, and rate of climate change and variation to which a system is exposed, its sensitivity, and its adaptive capacity".IPCC established the E-S-A framework to measure vulnerability: Exposure, Sensitivity, and Adaptability.The framework is about the level of exposure, sensitivity, and adaptability to stress or harmful events [10,11].It measures the sensitivity and ability of ecosystems to recover from disturbances [12].The disturbance can be influenced by social and economic factors such as population growth and industrialization [13].More specifically, exposure is understood as the conditions experienced by a system when experiencing external interference.Sensitivity is to what extent the interference that occurs in a system will affect the system.A purposeful shift made in response to or in anticipation of stress and external stimuli is called adaptation.Certain conditions are required to be adaptive, such as social and physical factors, as well as the capacity to mobilize these factors.So adaptability refers to the ability of a system to deal with the impact of disruptions that occur [14].
From a sociological perspective, watersheds are seen as places where vulnerability is a result of interactions between humans and ecosystems [15].The urbanization process is the anthropogenic activities affecting this interaction that then affect the functional capabilities of ecosystems in watersheds due to ecosystem changes [13].In this context, urbanization can have significant impacts on the hydrological cycle of water evaporation, precipitation, runoff, flood or inundation, and river flow [1].Changes from urbanization lead to increased vulnerability in several ways.One way is through the alteration of land use patterns, in which urbanization often involves the replacement of natural vegetation with impervious surfaces such as roads, buildings, and pavements, which can increase surface runoff and reduce infiltration capacity, leading to flooding, inundation, erosion, and sedimentation.
During rapid urbanization, vulnerability to floods increases.Rainstorms and flood disasters brought on by extreme precipitation occurrences have drawn major attention from the international community as a result of the quick growth of society and the economy [1].For the Asian Region, one of the key concerns of vulnerability is land use and land-cover change, population pressure, and low levels of adaptive capacity [9,14].According to Turner et al. [16] exposure to flood disaster is defined as the state before a system is disrupted by a flood.Land use is one of the components of exposure to flood disasters [17], because the higher the ability of land use to reduce inundation, the less vulnerable the area is to flood, and the other way around when the percentage of hardening ground is higher [18,19].The other components of flood exposure are topography (particularly elevation and slope, in which above 30% is considered steep), distance to a river, and river sedimentation [14,15,20].
The degree to which a system is impacted by climate-related stimuli-either negatively or favorably-is known as its sensitivity.Based on system features, a system's sensitivity to certain exposures can have both negative and positive consequences [9,19].Sensitivity describes the overall response of a system to disasters that occur, so sensitivity indicators represent several factors that alter (increase or decrease) the human and financial costs of extreme events [17].According to Hapsoro & Buchori [21], the high population density in an area illustrates the high chance of loss of life and property, so that population density affects the level of sensitivity of a system to disaster.
Moreover, adaptive capacity limits a system's ability to withstand negative effects.Adaptability in the context of floods is seen as the ability of a system to minimize potential damage and overcome the impact of floods [22].Access to resources like availability of shelter, health support, information, and technology, as well as the ability and knowledge to use them, active stakeholders and social organizations, or the mobilizing groups in society, governance institutions, determine the capacity to adapt [4,18,19,21,23].
Overall, examining vulnerability to flood in urbanizing regions becomes an important discussion in the anthropogenic era, especially from the perspective of a watershed.Therefore, this study aims at investigating the vulnerability of an urbanizing watershed, by taking the upstream Bengawan Solo IOP Publishing doi:10.1088/1755-1315/1264/1/0120133 River Basin in Central Java Province, Indonesia as a case study.The entire Bengawan Solo River lies 600 km along two provinces Central Java and East Java.It is the longest river in Java.The whole river basin of Bengawan Solo crosses across 14 local governments from upstream to downstream covering more than 16,000 km2.The study will apply the E-S-A Framework of vulnerability, and analyze the status of the three variables of vulnerability in the framework: exposure, sensitivity, and adaptability at the macro and micro level of the case study area.

Data & Methods
The study is conducted in two levels of analysis: macro and micro.The area for macro-level analysis is part of the upstream region of Bengawan Solo River Basin, which consists of 7 administrative units: Surakarta City, and Districts of Boyolali, Sukoharjo, Karanganyar, Wonogiri, Sragen, and Klaten, called Subosukawonosraten Region (Figure 1).The region covers 6,478 km 2 and consists of 1,313 villages.The Subosukawonosraten region is considered an urbanizing region from its level of urbanization and the economic perspective.Based on the Census 2010, the level of urbanization in the region is 51.4 percent [23].It is believed that the result of the Census 2020 will show an increased level of urbanization.From the economic development perspective, the largest sector that contributes to its GDRP is the industrial sector, which is 30% based on Central Bureau of Statistics Indonesia 2020 data.Therefore, as an urbanizing watershed, the region has the potential to experience a process of spatial change due to anthropogenic activities that raise the propensity to flood.Therefore, this region will be an important case for a discussion about vulnerability to flood in an urbanized river basin.

Figure 1. Case Study: Subosukawonosraten Region
The study applies a quantitative method with GIS (hotspot analysis) and is supported by descriptive analysis.The study is conducted in two stages: macro and micro level (see Figure 2).The macro-level region is Subosukowonosraten.The exposure and sensitivity indicators are applied in this macro-level analysis.Indicators for exposure is the built-up area in the year 2020.Using satellite imagery made available by the United States Geological Survey (USGS) through Earth Explorer, the built-up area is plotted for the Subosukawonosraten Region.Earth Explorer offers Landsat pictures from Landsat 7 (covers the years 1990s to 2020) and Landsat 8 (covers the years 2013 to 2020).The ArcGIS system's Map Algebra and/or Raster Calculator were then used to evaluate the image.The Normalized Difference Built-up Index (NDBI) and the Normalized Difference Vegetation Index (NDVI) were the formulas used to determine the amount of built-up and vegetated land, respectively.The Built-up Index (BUI) was then calculated from the calculation of the difference between the two indexes.The BUI is then calculated with the village as a unit analysis.The data is then combined with the population density (population to the built-up area), which is used as an indicator for sensitivity, to be analyzed using hotspot analysis.Hot spot analysis is applied to reveal the location where exposure and sensitivity are concentrated at the regional scale of Subosukawonosraten.The change of rainfall over time and topography are also applied to add information on the macro region's susceptibility to flood.
The second stage is the micro level.The micro-level region is first delineated based on the concentration of exposure and sensitivity indicators resulting from the hotspot analysis.Furthermore, the urban villages vulnerable to flood in the hotspot area are identified.The urban villages are selected based on those that are the most affected by the recent flood in February 2023.The number of villages was selected in a way so that they cover more than 50 percent of households affected by the February 2023 flood.Afterward, the adaptability or adaptive capacity indicators are assessed for the selected village by conducting semi-structured interviews.The adaptivity will be discussed based on the access of the community to essential resources during and after the flood, the role and inter-relations of institutions (social, private, government) in responding to flood, and pasca-flooding management.
Hotspot investigates clusters within a space.These groupings could reflect hot or cold locations in terms of high or low values of a particular variable, respectively.Each entity must be located within its immediate surroundings, so the neighborhood serves as one of the crucial components in forming the groupings.For the level of confidence of 95%, the statistically significant Z-score should be larger than 1.96 or lower than -1.96, while p-values should be lower than 0.05.Either a very high or a very low (negative) statistically significant Z score, shows that it is extremely improbable that the observed pattern is a spatial random pattern.Moreover, to examine the spatial autocorrelation pattern of the variables, Moran's index is used in hot spot analysis.Moran's index, which ranges from -1 to 1: It gauges how closely related one object is to those around it.A higher index indicates stronger spatial autocorrelations, whereas an index of zero indicates there is no spatial autocorrelations completely.
To assess the adaptability, this study selected 8 kelurahan as cases for structured interviews.Those kelurahan are Joyotakan, Semanggi, Sewu, Pasar Kliwon, and Sangkrah (Surakarta City), as well as Telukan, Gadingan, and Laban (Sukoharjo District).Those kelurahan are selected as cases because those kelurahan have the highest number of affected households compared to other locations.The total affected households in the eight selected kelurahan are more than 50 percent of the total flood victims.Respondents interviewed are kelurahan officers and heads of selected urban neighborhoods in several kelurahan.The total interviewees are 16 respondents.

Result and Analysis
The built-up area of the Subosukawonosraten Region covers 43 percent of the region or more than 2,800 km 2 .As previously mentioned, Li et al. [13] found that extreme precipitation, coupled with rapid urbanization, is a major factor contributing to increased vulnerability to flood disasters.In the Subosukawonosraten region, built-up area, rainfall, and topography variables are applied to examine the region's exposure to floods.As the Built Up Index of Subosukawonosraten in 2020 is approaching 50 percent, the area indicates a significant urban development process.The high percentage of builtup areas with high population density combined with the increased rainfall reinforced the vulnerability to flood [1].The data from the past six years (2017-2023) shows a substantial increase in precipitation.The amount of rainfall has nearly doubled compared to the previous seven years, from 1,266 mm to 2,500 mm.Out of the 27 observation posts, 23 recorded an increased number of rainfall events per year.This suggests a trend of rising precipitation levels in the overall region (see Figure 3).Furthermore, the data of built-up area 2020 is analyzed with hotspot analysis together with the population density data using the village as the unit of analysis.The Getis-Ord Gi* statistic is calculated using the Hot Spot Analysis tool.The resulting z-scores and p-values show that geographic clustering of characteristics with high values occurs.The z-score is 105.300874, with a p-value of less than 0.05, which is statistically significant.Index Moran is positive 0.5447, which shows spatial autocorrelation exists.The presence of high values for built-up areas and population density in one village still indicates the probability that the nearby villages will also have high values.The results reveal one considerably significant hot spot (with a 99% significance level), that is Surakarta City and its periurban area as the location of concentration of exposure (built-up area) and sensitivity (population density to the built-up area) indicators (Figure 3).The hot spot consists of 167 villages and kelurahan from the sub-districts in the city and those sub-districts of the peri-urban area surrounding the City (Figure 4 and Table 1).Source: Analysis The increased rainfall and the lower contour line of the hot spot area expose the area (Surakarta and its peri-urban) to a higher risk of flood.The height of the hotspot area is between 75 to 175 m, while the area of Subosukowonosraten outside the hot spot is from 300 to more than 3,000 m.A combination of the two exposure indicators and the hotspot can be seen in Figure 5.Following the macro-scale analysis for exposure and sensitivity, it will be discussed the adaptability to flood disasters.This will be conducted at the micro level.The cases for examining the adaptability are chosen based on the area impacted by the flood in 2023.All those locations are located in the hot spot from the macro analysis (see Figure 6).In total, the flood in February 2023 affected three sub-districts and 15 villages in Sukoharjo Districts as well as four sub-districts and 16 kelurahan in Surakarta City.
The number of households affected in Surakarta is more than 4,000 households, while in Sukohrjo District is about 3,200 households.The total number of households affected by the recent flood is about 7,300 households.The information about affected households is gathered from the broadcast from the Local Mitigation Agency that is released during the flood to villages/kelurahan and neighborhood/RT-RW through online group messages.Adaptability to flood is assessed by three main indicators: access to resources, social organization, and mitigation plan [4,18,19,21].This study evaluates access to resources, social organization, and mitigation plans through the stages carried out by the government and the community in overcoming floods.The interviews reveal that the flood in February 2023 started when heavy rain took place for several days in the Subosukawonoraten Region.There are four main stages related to effort for disaster management that can be identified.The first stage is the initial information: the warning that Gajah Mungkur Dam in Wonogiri would soon reach its maximum capacity and the water gate will be open to release the water.The other factor crucial to the warning situation is the heavy rain in the other elevation area: Klaten and Karanganyar Region which affects the debit of Dengkeng River in Klaten and Samin River in Karanganyar affecting the river flows to the hot spot area.This information comes from the Local Agency for Disaster Management (Badan Penanggulangan Bencana Daerah/BPBD) and the Water Authority (Balai Besar Wilayah Sungai/BBWS).The information is spread mostly via online group messaging to all kelurahan (the city) and to DESTANA-Desa Tanggap Bencana/Disaster Responsive Village Group (the Sukoharjo District), which all of the locations considered will be affected by the flood.The information about Wonogiri Dam is also posted on the social media of the institutions.All kelurahan and villages are passed through the information to the heads of urban neighborhoods (RT/RW) or heads of kampongs/village as the main actors for coordination with communities, and the other organizations like Civilian Neighborhood Guards (Perlindungan Masyarakat/Linmas) and the Territorial Defense Management (Badan Pembina Desa/Babinsa), which is the representative of the National Arm Forces.In addition to this warning, communities in several urban neighborhoods are also aware of the flood from their past experiences by seeing the river condition, as most of them live so close to the river, even some of them are in the riverbank area.
The second stage is the critical time when the flood happened: the evacuation phase.Most households both in kelurahan and in villages, raise their belongings to the second floor of their home or their neighbors' house, or to the top of cabinets or other high storage areas.Most of them are already affected by flood or inundation repeatedly, so many households adapted their homes to become two floors or made concrete floors as their rooftop.In some parts of neighborhoods, communities stay at their and in other locations where the intensity of the flood is higher, they are evacuated at the assigned locations.The Local Agency for Disaster Management takes a role together with the head of kelurahan/urban villages, head of urban neighborhood, or RT/RW for the evacuation stage.For villages in Sukoharjo District, the coordination is mostly done by DESTANA, because many elements from different organizations become part of DESTANA.The assigned locations for evacuation vary depending on the existing resources that the neighborhoods have.The facilities used for the location of evacuation is mostly school, worship facilities like a mosque or church, tents assembled on embankments, village office, or factory.In Joyotakan Village in Surakarta for example, the owner of a factory is always allowing their factory to be used as the points for evacuation for the closest neighborhoods.There is no formal agreement or written guidance on this or no formal mitigation plan for evacuation sites, only an informal agreement or initiative in the event of a disaster, or following what has been done before, that whenever there is a flood the community will use these facilities for evacuation sites.
The third stage is to open the emergency communal kitchen to cook and provide food for communities in flood-affected areas.This emergency kitchen is mostly managed by women in the neighborhood, and the community uses open space in the neighborhood.For example, in one of the neighborhoods in the Pasar Kliwon sub-district, they use a bridge over the James River as the location for the emergency kitchen.Parallel to this communal kitchen is the management of assistance distribution like clothes, dry food, mattresses, including medicine from various parties for flood victims.The assistance for the affected households come from different parties: government organization, communities, as well as private sectors, and political parties.The assistance is collected at the village office so that the assigned person from RT / RW can take and distribute it to the members who need it most.The fourth stage is considered the after-flood phase when people clean their houses and throw the goods damaged by flooding, and the recovery stage of health and psychological conditions of people affected by floods.During this phase, the Local Environmental Agency, Local Landscaping Agency, as well as Health Agency are intensively working with communities trying to bring back the condition as soon as possible.All of these initiatives are also carried out and decided directly at the time of a disaster, which is generally without formal guidance yet.
The last factor observed in this study related to adaptability is the overall mitigation plan.It is examined throughout the four stages of flood mitigation that also including the preventive plan.The interviews inform that the mitigation plan to address the flood susceptibility in the eight selected cases in the hotspot incorporates two main approaches: technological approaches and community-based approaches.The construction of retaining walls along riverbanks, water pumps, and water gates are just a few examples of the regional system infrastructure that is prioritized by a technological approach.Constructing retaining walls along riverbanks helps contain and control the flow of water during periods of heavy rainfall.This prevents the river from overflowing and flooding nearby urban areas.Water pumps are installed to remove excess water from flood-prone areas and pump it to safer locations.This helps in lowering the water level and mitigating flood hazards.Water gates are used to regulate the flow of water in river systems.By controlling the opening and closing of these gates, authorities can manage water levels and reduce flood risks.In addition to that installing an efficient drainage system is crucial for urban regions prone to flooding.The system should be designed to handle large volumes of water during heavy rainfall, diverting it away from populated areas to prevent inundation.This drainage system is a technological approach that also requires community awareness to manage it as the system is influenced by people's daily behavior.In some study sites, can be found drainage that is closed by concrete and filled by garbage which then significantly reduces its function to drain water.
The Community-Based Approach is the second strategy found in the observation for adaptability.Community-based approaches involve engaging and involving the local communities in flood mitigation efforts.These measures often focus on raising awareness, community preparedness, and enhancing resilience against floods.In the selected cases, it is found that locals frequently have insightful knowledge of historical occurrences, flood patterns, and customary coping techniques.This knowledge can be used to develop mitigation techniques that are more sustainable and suited for the situation at hand For example, in mapping the disaster hot spot or creating the disaster evacuation route.Interviews reveal that Local organizations like SIBAT (Siaga Bencana Berbasis Masyarakat/Community Based Disaster Mitigation) in certain kelurahan, can be an example of a community organization that works well incorporating different networks and stakeholders during not only the evacuation, or right-after disaster management, but also in educating the community to take care of the environment to avoid a flood.Programs like tree seeding on the river banks by the community are important as part of disaster prevention in the long term.Another potential program to be incorporated into the long-term mitigation plan that involves the community is the Program Kampung Iklim (Proklim).The Proklim is one of the government's programs to increase community participation in climate change adaptation.The program consists of sub-programs like river cleaning, environmental greening programs, composting, biopore making, and others.

Conclusion
This study's main goal is to examine and comprehend the flood risk in a rapidly urbanizing area located in the Bengawan Solo River Basin's upstream zone.The study's geographic scope includes seven different administrative regions, including Boyolali, Sukoharjo, Karanganyar, Wonogiri, Sragen, and Klaten.The framework applied in this study is the E-S-A (Exposure, Sensitivity, and Adaptability) Framework [9].The research findings indicate that the region's susceptibility to flood disasters is influenced by a combination of factors.Notably, the region's high percentage of built-up areas and dense population contribute to its vulnerability.This vulnerability is further exacerbated by an observed increase in rainfall patterns, and the proximity to the river.The concerning trend of escalating rainfall, in conjunction with rapid urbanization, underscores the area's heightened exposure and sensitivity to flooding.The core city, Surakarta, and its surrounding peri-urban areas are the most obvious areas where flooding susceptibility caused by urbanization is present.These places exhibit similar exposure and sensitivity patterns to broader geographic regions, which suggests a pervasive issue.The inherent flood risk is increased by the core urban area's geographic placement in a lower topographical region and by its proximity to the river.Together, these traits provide a setting that is disproportionately prone to flooding, especially during times of intense rainfall or other contributing variables.
In addition to the aforementioned elements, the study explores the adaptability factor in a few chosen urban villages/kelurahan and kampong/villages.The three main indicators of resource accessibility, social organization, and the existence of a mitigation strategy are used to evaluate adaptability.These indicators are tracked throughout the four stages of flood management in the selected cases of study.The results show that communities demonstrate a favorable capacity for adaptivity in the face of flooding difficulties.There is a noticeable presence of the resources that are required for effective evacuation activities.Additionally, there are existing informal agreements and partnerships with other parties that make it easier for these resources to be available and used.This collaboration aids in the efficient handling of flood conditions and streamlines evacuation efforts.The involvement of local organizations who actively coordinate the evacuation activities is a crucial component of this adaptability.
Overall, regarding adaptability, the finding shows that the examined areas have put in place flood mitigation plans that integrate both technology-hard infrastructure and community-based initiatives approaches.However, to manage the increased vulnerability to flood, the Surakarta Urban Region requires (1) a more deliberate integration of the two approaches; and (2) more preventive programs to raise awareness of the community to avoid flood.To effectively address the mounting flood risks in the Subosukawonosraten urban region, the integration and precaution strategies should be carefully structured and systematically executed.Therefore, a varied approach is advised to improve resilience and lessen the impact of flooding.This entails seamlessly integrating technology solutions.The technical methods must function in harmony with community-based initiatives that can take advantage of networks and knowledge in local communities and beyond.This integration aims to create a more resilient urban environment that is prepared to handle the difficulties presented by flooding.The vulnerable area needs to accomplish a complete flood management framework that not only strengthens physical infrastructure but also gives the communities the power to actively participate in flood prevention, response, and recovery operations by combining both technology and communityoriented solutions.

Figure 4 .
Figure 4. Hotspot Analysis: Concentration of Built-Up Area and Population in the Subosukawonosoraten Region (Analysis)