Analysis of Flood-Prone Areas on Bekasi River Watershed

Das Kali Bekasi is an area that has physical conditions that support the occurrence of local and natural flood disasters during the rainy season. This research aims to: (1) Determine the distribution of flood-prone areas in the Bekasi River Watershed, (2) Find out the most dominant factors of flood vulnerability that occur in the Bekasi River Watershed. The method used in the research is quantitative descriptive analysis. Analyze the data to determine the distribution area prone to flooding by weighting scoring and overlaying rainfall, land use, slope, soil type, and river density. To find out the dominant factors that cause flood vulnerability, a correlation test is used. The results of the research are (1) Flood vulnerability in the Bekasi river basin, based on the results of this research, is divided into 3 classes, namely the not vulnerable class with an area of 82.61 Km² (5.93%), the moderately vulnerable class with an area of 323.85 Km² (23, 26%), very vulnerable class with an area of 985.96 Km² (70.81%). (2) Correlation test to determine the dominant factor causing flooding in the Bekasi River basin and it is known that the dominant factor is the slope parameter, followed by the river density parameter, soil type parameter, land use parameter, and rainfall parameter with the lowest correlation value.


Introduction
Natural disasters are events caused by natural phenomena such as tsunamis, volcanic eruptions, earthquakes, and floods that threaten and disrupt people's lives.The impact of these disasters can be devastating, leading to casualties, material losses, environmental damage, and psychological effects.
Floods are one of the most common disasters that occur at a national level.They tend to happen seasonally and often hit large cities, causing significant damage and losses, both material and nonmaterial.

International Journal for Disaster and Development Interface
Volume 3, Issue 2, October 2023, pp.68 -79 DOI: https://doi.org/10.53824/ijddi.v3i2.57 The Bekasi River basin area, particularly in Bogor Regency, Bekasi City, and Bekasi Regency, is highly susceptible to flooding.Several physical factors contribute to the vulnerability of this area, including the slope from upstream to downstream, average rainfall of 2000-2500 mm per year, changes in land use that lead to poor absorption of rainwater and runoff, soil types that cannot hold water well, and high river flow density.These parameters are used to assess flood vulnerability and identify the primary factors that trigger it.The level of flood vulnerability is determined through various analysis methods, including overlay analysis and correlation tests between the parameters used and the level of flood vulnerability.Natural disasters are events caused by natural phenomena such as tsunamis, volcanic eruptions, earthquakes, and floods that threaten and disrupt people's lives.The impact of these disasters can be devastating, leading to casualties, material losses, environmental damage, and psychological effects.Floods are one of the most common disasters that occur at a national level.They tend to happen seasonally and often hit large cities, causing significant damage and losses, both material and non-material.

Method
The research methodology used quantitative descriptive techniques, which included collecting secondary data, processing it, and analyzing it.The method involved using Geographic Information System (GIS) analysis with overlay techniques.Additionally, correlation tests were used to evaluate the relationship between each parameter used in the research and flood vulnerability.

Flood-Prone Areas in Das Kali Bekasi
Das Kali Bekasi is an area that is prone to flooding.To determine the flood vulnerability, several parameters such as rainfall, slope, land use, soil type, and river density are taken into account.Each parameter is assigned a specific value weight based on its contribution to flood events.The vulnerability assessment is classified into different aspects, and each aspect is given a different weight assessment of flood vulnerability.
The flood vulnerability level map shows the extent of the area and the level of flood vulnerability.
There are three classes of vulnerability levels.The level I class covers an area of 82.61 Km², which is about 5.93% of the total area of the Bekasi river watershed and is not prone to flooding.The level II class covers an area of 323.85 Km², which is about 23.26% of the total area and is quite vulnerable.The level III class covers an area of 985.96Km², which is about 70.81% of the total area and is very vulnerable.
In summary, the flood vulnerability map is a visual representation that depicts information about the area and its level of vulnerability to flooding.Class III is very vulnerable to be found in most areas of the Bekasi river watershed including Gunung Putri District, Cileungsi, Cibarusah, Serang Baru, Setu, Bantar Gebang, Jati Sampurna, Jatiasih, Mustika Jaya, West Cikarang, South Cikarang, North Cikarang, East Cikarang, Central Cikarang, Karang Bahagia, Cibitung, South Tambun, Rawalumbu, South Bekasi, West Bekasi, North Bekasi, Medan Satria, East Bekasi, North Tambun, Tambelang, Sukatani, Babelan, Sukawangi, Sukakarya, Tarumajaya, Muara Gembong and Bungin Branch, with a total area of 985.96Km The Bekasi River basin includes flat areas and estuaries in the downstream watershed.There are many rice fields and settlements in this area that have moderate and light rainfall.The type of soil in this region is fluvisol, and the river density has a flow density of less than 1.44 Km².
To determine flood vulnerability in the Bekasi River basin, a scoring and overlay process is used.
The parameters involved in the process are rainfall, slope slope, land use, soil type, and river density.A scoring method is used to assign a value to each flood vulnerability parameter, and the vulnerability assessment is classified into aspects that contribute to flood events.Each parameter is assessed based on relevant aspects, and each aspect has a different weight assessment of flood vulnerability.
The flood vulnerability level map shows the extent of the area and the level of flood vulnerability that occurs.The vulnerability level is classified into three classes: The level I class is not prone to flooding and covers an area of 82.61 Km², which is about 5.93% of the total area of the Bekasi River watershed.The level II class is quite vulnerable and covers an area of 323.85 Km², which is about 23.26%.and The level III class is very vulnerable and covers an area of 985.96Km², which is about 70.81% of the total area.
A flood vulnerability map is a visual representation that shows the area and the level of vulnerability to flooding that occurs.Class III is very vulnerable to be found in most areas of the Bekasi river watershed including Gunung Putri District, Cileungsi, Cibarusah, Serang Baru, Setu, Bantar Gebang, Jati Sampurna, Jatiasih, Mustika Jaya, West Cikarang, South Cikarang, North Cikarang, East Cikarang, Central Cikarang, Karang Bahagia, Cibitung, South Tambun, Rawalumbu, South Bekasi, West Bekasi, North Bekasi, Medan Satria, East Bekasi, North Tambun, Tambelang, Sukatani, Babelan, Sukawangi, Sukakarya, Tarumajaya, Muara Gembong and Bungin Branch, with a total area of 985.96Km².This region comprises of level grounds and estuaries in the downstream watershed.There are numerous rice fields and settlements that are densely populated with moderate and light rainfall, with fluvisol being the predominant soil type.Moreover, the river has a flow density of less than 1.44 Km².The Bekasi River basin is characterized by a type C climate, which is typically humid and rather wet with values ranging from 0.333 to 0.6.This climate type covers all sub-districts within the basin.
Over the past 5 years, rainfall data in the area has shown that each rainy season is marked by moderate to very heavy rainfall, ranging from 2000 to 3000 mm or more per year.The slope of a landform can be evaluated using topographic data or Digital Elevation Model (DEM).In this study, contour data was initially used and later converted to raster data format, such as DEM, for processing using ArcGIS software.The results of data processing are presented in the following.classified based on the slope.Based on the soil type classification table, it is evident that the soil in the Bekasi River basin has a high sensitivity level towards water infiltration.This results in the soil's inability to efficiently absorb rainwater, which eventually leads to runoff, one of the primary causes of local flooding.Furthermore, the rise in development activities has further complicated the process of water absorption into the soil.

d. River Density
The Bekasi River Basin has several rivers and tributaries distributed throughout the area, which allows for smooth water flow from upstream to downstream.This indicates a relatively high river density in the region.The Bekasi River Basin has a generally good river density, though some areas lack sufficient rivers and tributaries to effectively drain water from upstream to downstream.For example, several areas in the District such as Babelan, Sukawangi, Tambelang, Sukakarya, Rawalumbu, Jatiasih, Jatisampurna, Gunung Putri, Cileungsi, Setu, Mustika Jaya, Cibarusah, Bantar Gebang, Cikarang Barat, Cikarang Utara, Cikarang Selatan, and Serang Baru.In these sub-districts, the density of the river is less than 1.44 Km², so these areas have the potential to experience flooding.

The Dominant Factor of Flood Vulnerability
Testing using correlation tests is important in determining the factors that have the greatest impact on flood events.This analysis process utilizes the correlation method to measure the relationship between the independent and dependent variables.In this study, the independent variable comprises of five parameters, namely rainfall, land use, slope, soil type, and river density, while the dependent variable is the level of vulnerability to flooding.
A correlation test measures the strength of the relationship between two variables.The correlation coefficient ranges between -1 to +1.A value close to -1 denotes a negative relationship and a lower correlation.On the other hand, if it is close to +1, the relationship is positive, and the correlation is higher.A value of 0 indicates that no relationship exists between the two variables.Sample Correlation Value (r) Interpretasinya 0,00 -0,09 The correlation relationship is ignored 0,10 -0,29 Low correlation relationship 0,30 -0,49 Moderate correlation relationship 0,50 -0,70 Medium correlation relationship >0,70 The correlation relationship is very strong Sumber : SPSS Indonesia Based on the correlation test results on rainfall parameters, it can be concluded that rainfall has a small influence of 32.2% on the causes of flooding in the Bekasi River Basin, as the rainfall data falls under the light category of around 1500-2000 mm/year.On the other hand, the land use parameter has a higher correlation value of 0.375, indicating that it has a greater impact of 37.5% on the cause of flooding.This can be attributed to the fact that most of the land use in the Bekasi River watershed is already occupied by buildings such as settlements, service facilities, companies, and industries.
Additionally, the slope parameter has a strong correlation value of 0.682, which means that the slope has a significant impact of 68.2% on the cause of flooding.This is due to the fact that the majority of the area in the Bekasi River watershed is flat and sloping, leading to a higher vulnerability to floods.
The soil type parameter shows a correlation test result of 0.478 which means that the soil type parameter contributes 47.8% to the cause of flooding.It is known that the level of soil concentration in Das Kali Bekasi gets a high score which causes rainwater not to be absorbed properly.
The correlation test shows that the river density parameter has a 65.5% impact on flooding.
Although the Bekasi River Basin has good river density, some areas have poor river density data which is less than 1.44 km/km2.Therefore, when scoring the river density parameter, it receives a maximum score.
Each parameter has different results due to two factors, the difference in the parameter's condition and the used scoring results.Among the five parameters, the slope has the most significant impact on flooding due to its high correlation value.In contrast, rainfall, land use, soil type, and river density have lower correlation values due to slight variations in conditions and their respective scoring values.

Flood Prone Map Validation
Flood risk map validation testing is carried out by comparing the results of flood risk maps made from overlaid parameters that affect flooding, with locations validated by the Regional Disaster Management Agency (BPBD) in Bogor Regency, Bekasi City, and Bekasi Regency.
Validation is conducted by directly collecting samples and coordinates, as well as utilizing data from the Regional Disaster Management Agency (BPBD) in Bogor Regency, Bekasi City, and Bekasi Regency related to flooded areas.
Based on this, the validation process has a fairly high level of accuracy, and the results of the spatial analysis can be trusted in mapping the level of flood vulnerability in the Bekasi River basin.The areas classified as non-vulnerable still have some level of vulnerability, and the moderately and very vulnerable areas are spread throughout the Bekasi River watershed, covering a total of 40 districts.The physical conditions in these areas are characterized by heavy rainfall, land use dominated by fields and rice field gardens, steep slopes, and a high river density with a flow density of 2.28-3.10km².However, there are four districts with physical conditions that are not conducive to flooding, such as the type of soil (andosol and nitosol) with a low level of water infiltration sensitivity, namely Megamendung, Babakan Madang, Sukaraja Sukamakmur and for the class level is quite vulnerable there are 4 districts, namely Jonggol, Klapanunggal, Cibinong, Citeureup.
The class with a very vulnerable level is more likely than part of the Bekasi River watershed area with a total of 32 districts, namely, Gunung Putri, Cileungsi, Cibarusah, Serang Baru, Setu, Bantar Gebang, Jati Sampurna, Jatiasih, Mustika Jaya, West Cikarang, South Cikarang, North Cikarang, East Cikarang, Central Cikarang, Karang Bahagia, Cibitung, South Tambun, Rawalumbu, South Bekasi, West Bekasi, North Bekasi, Medan Satria East Bekasi, North Tambun, Tambelang, Sukatani, Babelan, Sukawangi, Sukakarya, Tarumajaya, Muara Gembong and Bungin Branch.Physical conditions in the IJDDI 2023, 3, 1, 77 http://ijddi.netclass are very vulnerable, namely light to moderate rainfall, soil types in the form of fluvisol which are not sensitive to water infiltration, flat slopes, land use in the form of rice fields and dense settlements, and for river density less than 1.44 Km².These physical parameters are the cause of local flooding and not infrequently also floods of shipments that occur seasonally, namely the rainy season.
An analysis of the causes of flooding in the Bekasi River basin indicates that rainfall, land use, slope, soil type, and river density all play a significant role in increasing the risk of flooding, especially during the rainy season.After conducting correlation tests on several parameters, it was found that the slope was the most influential factor in causing flooding, followed by river density, soil type, land use, and rainfall with lower correlation values.

Recommendations
The local government in the Bekasi River basin and the central government through institutions such as BPBD and Bappeda West Java need to take more steps to prevent and handle floods that frequently happen in the area.The primary focus should be on repairing and controlling parameters that are prone to flooding.Some factors such as slope, rainfall, and soil type are natural factors that cannot be changed.Therefore, preventive measures are crucial, such as river normalization activities that involve the regular cleaning of mud to reduce high water discharge rates.Increasing greenery in floodprone areas can also be helpful.
Further research is necessary to address the limitations of this study, such as the use of methods that only use weights scoring, and analysis based on what the researcher obtained.To determine the impact of losses experienced by residents in the flood disaster, future research could incorporate field survey research and interviews.

Acknowledgments
As an expression of gratitude, I dedicate my research and publication manuscripts to God Almighty for His guidance and to my beloved parents who have always been my source of encouragement and motivation.Without their prayers and support, this achievement would not have been possible.This is my small offering to honor and thank my parents for everything they have done for me.

Figure 1 .
Figure 1.Map of Bekasi River Watershed Flood Vulnerability Level

Table 1 .
Rainfall Map Processing ResultsAfter performing an isohyet analysis of the Bekasi River watershed, which spans over an area of 526.49Km², the rainfall patterns can be classified into four main categories.The first category is light rainfall, which occurs in an area of 175.85 Km² and ranges from 1500 to 2000 mm/year.The second category is moderate rainfall, which covers an area of 486.82 Km² and ranges from 2000 to 2500 mm/year.The third category is heavy rainfall, occurring in an area of 204.26 Km² and ranging from 2500 to 3000 mm/year.The fourth and final category is very heavy rainfall, which covers an area of 175.85 Km² and exceeds 3000 mm/year.Land UseThe Bekasi River Basin has undergone significant land conversion, with most of the land now used for building settlements, service facilities, offices, and industrial estates.Although there are still vacant lands such as moors and rice fields, their distribution is uneven.The downstream part of the region has many rice fields, especially in Bekasi Regency, which still actively engage in rice farming.
Source: Theml, S. 2008: Methodological Catalog of Geo Hazard Map Preparation with GIS (Data processing by author) b.Slope

Table 3 .
Slope Map Processing ResultsBased on the calculations, it can be inferred that a significant portion of the Bekasi River basin has a flat terrain.The section classified as relatively flat covers 1051.66Km², which is a considerable part of the Bekasi River watershed area.c.Soil TypeThe dominant soil type in the Bekasi River basin has high water infiltration capacity and is less water-sensitive.

Table 6 .
Flood Vulnerability Parameter Correlation Test