Flood risk assessment of Kemang Area as a central business in South Jakarta

. This paper presents a flood risk assessment of Kemang area which around 80 Ha as central business at South Jakarta, DKI Jakarta, Indonesia. It is crucial to asses this area as it well-known as central business that has high damaged due to affected by flooding. The objective of this study to know the current level flood risk in the area and then how structural interventions through improving drainage pumps capacity with 2 m3/s contribute to the its level. The rational method, HEC-RAS 2D, and GIS have applied for methods flood risk analysis in two years return period of rainfall (138 mm/day). Flood risks calculated based on hazard index, vulnerability index and capacity index in scale of 20 RT (neighbourhood associations). As the result, flood risk map show that the Kemang area has 65 % highly risk of flooding, 10 % in moderate level and the rest is low level risk of flooding. The current planning of structural intervention not significant to reduce risk level. Therefore, it needs more actions to reduce the flood risk level through both structural and non-structural measures such as normalization of drainage system, policy, social and private engagement.


Introduction
Jakarta city is natural flood plain with around 40% land area below sea level.Urban development and rapid urbanization lead the city into high risk of flooding [1][2][3][4][5][6][7][8].Also, Climate change contribute to more frequent extreme rainfall events [9][10][11][12].Kemang area is one of central business in Jakarta which is located in Mampang Prapatan districts, South Jakarta, Jakarta capital region of Indonesia.It is also well-known as residential area for local and international inhabitants.However, there is crucial problem in this area that need to be addressed which is flood.In 2020, the depth of flood reached 1.5 m.Flood is caused by lack of drainage capacity, it is high density of population and human intervention to land.This intervention contributes to high of surface runoff at the urban area [13,14].
Therefore, lack of green space is counted less than 5% and land covered by building more than 87% of total area.Moreover, there is no water storage, neither detention nor retention pond.Also, changed in land cover into building or impermeable surface increased urban runoff [15][16][17].Kemang area is part of Krukut river catchment area, the downstream in this river has high of flood hazard [18].
Currently, local Government of DKI Jakarta applied structural measures to decrease flood risk level in Kemang area through improving drainage capacity with building storage and increasing pump capacity with 2 m 3 /s.Therefore, it is interested to check the contribution its intervention towards the flood risk level in Kemang Area.Also, other possibilities to decrease the flood risk level in Kemang.

Study area
Kemang area is located in Mampang Prapatan district, South Jakarta (figure 1).It covers around 80 ha and hydrologically is downstream area of Krukut Watershed, and Krukut watershed itself part of Ciliwung-Cisadane river basin.Therefore, the area definitely affected by Krukut river.The outlet of the area is Krukut river.

Methods
This study used data of digital elevation model with spatial resolution 2 m, rainfall in the last eighteen years 2000 to 2021, land use land cover, drainage network, pump capacity and historical events of inundation.
Analysis hydrology was carried out with rational method and for hydraulics simulation generated with HEC-RAS 2-Dimensional model.Then results of hydrology and hydraulics would be applied to be hazard index.

𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹𝐹 𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟
Flood risk is defined as hazard index multiple by vulnerability and divided by capacity [19]  Capacity index is calculated based on drainage capacity, zoning role, disaster education and flood early warning system criterion.Flood risk analysis conducted with two scenarios, first real existing condition without intervention and then with intervention of structural measures.Hazard index was extracted from hydrology and hydraulics model and historical of inundation.
The surface runoff was calculated by rational method: Where: Rainfall intensity was calculated with Mononobe method: Where: I : Rainfall intensity (mm/hour)

Data and software used
This study used hydrological data such as rainfall, inundation, geometry of drainage, digital elevation model to deliver hazard index.Runoff of each catchment area calculated with rational method and hydraulics simulation in channel used HEC-RAS 2D.It calculates inflow and outflow, and the difference between inflow and outflow defined as change in storage.Then socio-economic, physical and environment to generate vulnerability index.Then used drainage capacity, regulation and flood early warning system to calculate capacity index.The result of each index then figures out by mapping in ArcGIS software.

Hazard index
Hazard index in Kemang area is extracted from hydrologic and hydraulic model based on table 1 above.In hydrologic part, it was used long history of daily rainfall data from 2000 to 2021 of rain gage of Tangerang Selatan.Based on polygon Thiessen, Kemang area affected by it.For HEC-RAS 2D, Kemang area is divided into 25 catchment area to generate runoff in each catchment using rational method.Based on analysis frequency distribution of extreme rainfall and Kolmogorov Smirnov assessment.Log normal approaches was selected as it the most appropriate method.The result of simulation of 2D model was validation used inundation data.Also, the result of model was overlayed with 20 Rukun Tetangga (RT) boundary (Neighbourhood Associations) to know the level of index over 20 RT and three Rukun Warga (Community Associations) (figure 2).Model was generated in two scenarios, first with existing and then intervention with structural measure by improving drainage pump capacity around 2 m 3 /s in outlet of the catchment area.As a results, based on two years of precipitation period which around 138 mm/day show inundation area around 24,18 ha and then 23,92 ha after scenarios with added 2 m 3 /s drainage pump.Therefore, the level the hazard index remains the same.Based on hazard index map, over 20 of RT, 65% in high level, 25% in moderate and the rest 10 % in low level (table 7).

Vulnerability index
Vulnerability index is calculated based on social, economics, physical and environment aspects, as mentioned in table 2 till table 5 above.Each aspect is calculated based on matrix then vulnerability index at Kemang area is defined as the total score of each parameter which is divided into three classes namely low, moderate and high (table 8).Based on table 8 above, it can be seen that around 35% in high level, 35% in moderate and the rest is 30% in low level vulnerable to be flooded.Physical and environment aspects are two variable that determined the level of vulnerability index in Neighbourhood Associations.

Capacity index
Capacity index is calculated based on four parameter such drainage capacity, land zoning regulation, disaster education, and flood early warning system (table 9).In general, capacity index Kemang Area relatively low.The proportion level can be divided into (65%), medium (10%) and the rest 25% for low level.The main dynamics contributor in capacity index is drainage capacity, the others parameter is remaining the same condition.

Flood risk
Flood risk of Kemang area is defined based on three parameters hazard index, vulnerability index and capacity index.The spatial analysis of flood risk was generated using numerical methods by ArcGIS software which can be seen in the figure 3.  Overall, Flood risk in Kemang area relatively high.Over 20 RT, high risk 65% (13 RT), moderate 10% (2 RT) and the rest 25% (5 RT) for low level.Planning of structural intervention by adding 2 m 3 /s drainage pump in outlet of the catchment area not enough yet to reduce flood risk level in Kemang Area.

Conclusion
Based on the calculation, food risk level in Kemang area categorized in high level even with structural intervention scenario by adding drainage pump around 2 m 3 /s in outlet of the catchment area.It reduces just slightly area of inundation.According to flood risk map, 13 RT (Neighbourhood Associations) are in high risk level of flooding, following by two RT (10%) are in moderate level and the rest five RT (25) in low level.Therefore, it needs not only other structural such as drainage normalization, applied retention or detention pond in building catchment scale to retain runoff during peak hour before flow to drainage system which as part of sustainable drainage system which support sustainable water resources [4,20,21].Also, it needs non-structural intervention related socio-economic instruments such as policy and community engagement to reduce the flood risk level in Kemang area South Jakarta.For further research, it needs to consider five and 10-years rainfall return periods scenarios, and also with fluvial flood of Krukut River.

Fig. 3 .
Fig. 3. Flood risk map of Kemang area of South Jakarta.
Authors acknowledge the support given by P2MI of ITB for providing financial assistance for research needs, field surveys and publications.Thank you very much to the Chair of the FTSL ITB Water Resources Engineering Expertise Group and the ITB Civil Engineering Postgraduate Study Program for the support and permission to carry out this research, The author's gratitude also to Head of Water Resources Agency of Jakarta city, The Indonesian Agency for Meteorological, Climatological and Geophysics (BMKG), Authority for Ciliwung and Cisadane River Watersheds (BBWSCC), and National Planning and Development Agency-Directorate (BAPPENAS) for providing datasets used in this study , 03003 (2024) E3S Web of Conferences https://doi.org/10.1051/e3sconf/202447903003479 ISSAT 2023

Table 1 .
. It is based on national standard which Perka BNPB No 2/2012.Hazard index was set up based on hydrologic criteria.Detail criteria based on table 1: Hazard index.

Table 7 .
Hazard index analysis of Kemang area, South Jakarta.

Table 8 .
Vulnerability index analysis of Kemang area, South Jakarta

Table 9 .
Capacity index analysis of Kemang area of South Jakarta.