FLASH FLOOD (Δ) RISK AND DAMAGE ASSESSMENT IN BATU, EAST JAVA

Hydrometeorological disasters are showing an increasing trend in Indonesia. Flash floods are part of a hydrometeorological disaster that has a significant livelihood impact. Flash Flood is triggered by the intensity of extreme rain, several actions of mitigation can be taken by early warning systems, hazard and risk mapping, community preparedness


INTRODUCTION
Climate change that occurs, can affect the high increase in rainfall [1]which is in northeastern China. From 1965 to 2014, there were twenty-four TCs ranging from severe tropical storm to super typhoon over Hadahe River basin in twenty years. General Circulation Models (GCMs. The increase in rainfall, which is the impact of climate change, can lead to disasters, one of which is flooding and flash floods [2]. A flash flood is a natural disaster phenomenon that has high destructive power and can cause damage to settlements and infrastructure, loss of property, and loss of life [3]. Flash floods are caused by high rainfall in a short time [4]in particular to buildings and infrastructure due to its fast occurrence and high magnitude in financial loss. Risk assessment of the flash flood identify the critically flood-prone areas and provide assistance in improving the resiliency of mitigation plans. In this study, we developed an assessment of flood susceptibility, vulnerability, the impact of socio-economic, and integrated flash flood index based on the historical data of flood events recorded in Kuala Lumpur. Data of rainfall characteristics, inundated location, and areas are extracted from the reports of flood events from the year 2005-2015. Each event is then segregated according to the place of incidence, providing point-based recurrence of flood at each identified location. Indicators of assessment include frequency and month of occurrence, rainfall characteristics (of intensity, duration, and depth which in increases water levels or water discharge in rivers. Flash floods are prone to occur in areas that are mountain water catchment areas [3]. Batu City is located at an average altitude of 862 mdpl, so most areas in Batu City have hilly/slope contours. Batu City is an area that has the potential to develop tourism, agriculture, plantations, settlements, services, and industry. The Batu City area is an area that has a role as a conservation area and a protected function area. The economy of Batu City is largely supported by the tourism and agriculture sectors. The residents of Batu City mostly work as farmers; the main agricultural products of Batu City are fruit, flowers, and vegetables. The mainstay plantation product, which is the main commodity of Batu City, is apples. Flash floods in Batu City occurred on November 4, 2021. Flash floods in Batu City occurred due to climate change factors and land conversion that occurred in Batu City where the water catchment area was decreasing, which resulted in insufficient rainwater being absorbed into the ground.
Research on flash floods has been carried out using hazard data based on topography, Aster DEM, meteorology, density river system and socio-economic vulnerability. After that, to know the weight of the multi-criteria hazard and socio-economic vulnerability, it is necessary to carry out an AHP analysis, which will then be overlayed and will produce a risk layer from the Bandung Flood disaster. The formulation of the problem in this research is to know the risk of the flash flood disaster in Batu City, East Java [5].
To assess the risk of flash floods in Yunan Province, using time series data obtained based on crawling satellite image data based on the time of the disaster, which is then compared with existing images. Then to identify the risk of flash floods, using the XGBoost Modebnadal by employing machine learning based on several variables, namely precision, recall, accuracy, F-Score and Kappa. Then later, the location of the risk of flash flood disaster will be obtained from Yunnan Province, China [6]a widely applied technology in preventing catastrophic flash flood disasters, has become the current research hotspot. However, most existing machine learning methods for assessing flash flood risk rely on a single classifier, which is suitable for processing small sets of sample data, but the resulting prediction accuracy and generalization ability are insufficient. Meanwhile, machine learning methods that integrate multiple classifiers are thus far unknown. Extreme Gradient Boosting (XGBoost. Because flash floods have high destructive power, a damage assessment is needed to assess how much damage is caused by flash floods. Damage Assessment is a process to assess losses and damage caused by disaster events that have occurred [7]. Damage assessment can also be used as a consideration in making decisions. Damage Assessment contains information on the losses generated by disasters [8].
This research was conducted based on the research problem that has been formulated, namely the occurrence of the phenomenon of climate change in Batu City. In addition, several research questions remain, such as how to describe the risk of flash floods in Batu City based on hazard conditions, the sensitivity and exposure, the vulnerability of flash floods that occur in Batu City, and the risk of flash floods in Batu City itself and how much loss will result from the flash flood disaster in Batu City.

METHOD
This research uses the risk assessment method or risk analysis method. This risk assessment is a method that combines several characteristics and frequency of danger from a certain place or potential losses caused by flash floods. Hazard is a variable that tends to be constant, therefore vulnerability and capacity are variables that can be managed so that the risk value can be small. The calculation of risk assessment can be assessed using the following equation: = ( + ) R = Risk H = Flood Disaster Hazard V = Vulnerability (Exposure + Sensitivity) AC = Adaptive Capacity [9].
This research began with the collection of secondary data in the form of land use map data for the Spatial Planning [10] in 2021 and flash flood hazard data obtained from InaRisk. The required land use data is in the form of shapefiles while flood hazard data is in the form of maps in .tiff format. The primary data collection is carried out by interviewing local experts regarding the weight of vulnerability.
In calculating (Δ) risk, using projected vulnerability data, the vulnerability data in this study is taken from the 2030 Batu City spatial planning, which is used to predict the increase in risk and impact of flash flood disasters. If it is not studied, the risk of flash floods in Batu City will be able to make the impact of flash floods higher in the future.
The obtained primary and secondary data will then be subjected to several stages of analysis: Table 1. Workflow

Hazard
In the hazard analysis, flash flood hazard data from InaRisk will be classified into three classes ranging from high, low, and medium. Each of these classifications will be given weight and will produce a Flash Flood Hazard Map in Batu City.

Vulnerability
Vulnerability is the level of insecurity or inability to cope with climate change due to climate variability and climate extremes [11]. The variable components and vulnerability analysis indicators will be assessed.

Adaptive Capacity
Capacity Analysis describes adaptive capacity as the ability of communities, locations, and infrastructure to adapt to climate change [12]. Adaptive capacity in Batu City as a form of risk reduction for flash flood disasters in the study will be assessed based on the availability of drainage and the availability of clean water.

Risk Assessment
Risk analysis is the last analysis of a series of analyses that have been carried out, including Hazard, Vulnerability, and Capacity analysis. The data that has been generated from the three analyses are then calculated and overlayed to see the classification of the level of risk caused by the Flash Flood disaster in Batu City in 2022.

Damage Assessment
The last is calculating the damage and loss assessment. At this stage, it is to calculate the number of buildings and land, as well as the prediction of losses caused by flash flood risk and (Δ) flood risk. The calculation in the damage assessment uses the overlay method from ArcGIS to find out how much impact it has, and then the value of the loss is calculated based on the standard selling price of land in Batu City in 2022. Damage and Loss Assessment (DaLA) is a methodology for obtaining accurate data on damage and loss caused by a disaster [7]. The position of the DaLA is very important in post-disaster management. The success of a post-disaster recovery program is largely determined by good recovery planning. Good planning begins with accurate data and information. The main objective of post-disaster needs assessment is to measure the scale of the impact of the disaster as soon as possible [13].

RESULT AND DISCUSSION
Batu City is one of the newly formed cities in 2001 as a part of Malang Regency. Previously, the Batu City area was part of the Subunit for Development Area 1, North Malang. The beautiful natural panorama and cool air make Batu City an attractive tourist destination so that the tourism sector is relied on to increase regional income. Batu City residents mostly work as farmers work as farmers and the main agricultural products from Batu City are fruit, flowers, and vegetables. The mainstay plantation product, which is the main commodity of Batu City, is apples. The economy of Batu City is largely supported by the tourism and agriculture sectors. Most of Batu City's GDP growth is supported by the location, which is in a mountainous area, and the rapid development of tourism.

Land Use 2022
Since the city of Batu has been designated as an administrative area and has developed very rapidly since 2021, many agricultural and forest lands are now developed into builtup areas, settlements, and tourism, and have undergone very rapid changes to date.

Spatial Planning 2030
The rapid development of Batu City is also stated and planned in the spatial planning of 2030. Many have experienced changes in land use from agriculture and plantations to residential areas and the centre of activities of Batu City.
Climate hazards in an area will be different due to differences in geographical conditions, types of settlements, demographics, and types of infrastructure. The first option is to conduct hazard analysis based on secondary data in the form of disaster maps from authorized agencies, and the second option is to conduct qualitative hazard analysis based on primary data. Based on the results of taking data from InaRisk as a baseline for the Danger of Flash Floods in Batu City, while to get projections of the dangers of flash floods in Batu City, elaboration was carried out by combining the historical data of the last flash flood, namely on November 4, 2021. The extent and classification of flash flood hazards.

Vulnerability Exposure
Vulnerability is a picture of urban internal conditions in the face of the impacts of climate change. This analysis is carried out by determining indicators of indicators and components of regional vulnerability, namely exposure indicators (E), sensitivity (S), and adaptive capacity (AC). Due to data limitations, this analysis uses baseline statistical data in 2022 and uses assumptions from plans made by the Batu City Government as a reference in the 2030 projections.
Based on the location of the hazard and the use of land within the inundation area, it can be seen in the use of land in 2022 if it has a lot of impact on the agricultural sector. This is the basis for studies in determining the sectors that impact climate change most. The indicators used to identify vulnerabilities are exposure and sensitivity. Exposure consists of Agricultural Land Area, Number of Farmers. As for the sensitivity of the type of agriculture, the number of workers, and agricultural production. Exposure is a component that depends on the physical conditions of the location or geography based on climatic variations that can be the cause of disaster. The classification of total exposure to the agricultural sector to flash flood disasters in Batu City, for areas with a high classification, consists of Gunungsari and Sumbergondo; for areas with a medium classification, consists of Bulukerto and Giripurno; the rest are in the medium exposure classification.
Meanwhile, the projection of total exposure to flash flood hazards is based on the planned spatial pattern in 2030, which is relatively reduced agricultural land and is projected to become a settlement. The classification of the total projected exposure of the agricultural sector to flash flood disasters in Batu City, for areas with a high classification is not identified, while the medium classification consists of Giripurno, Gunungsari, and Sumbergond, and the rest are in the low class.

Sensitivity
Calculations are carried out to determine the level of climate change sensitivity in the agricultural sector in Batu City using agricultural labor indicators and the dominance of agricultural land types.   The classification of total sensitivity of the agricultural sector to flash flood disasters in Batu City consists of Sumbergondo, the medium classification of Oro-Oro Ombo, and the rest are in the low classification. Meanwhile, the projection of total exposure to flash flood hazards based on the spatial pattern plan for 2030, obtained the total sensitivity of the projection with a reduced sensitivity classification compared to the baseline sensitivity. The classification of the total sensitivity of the projections to the agricultural sector to flash flood disasters in Batu City is not available for areas with a high classification, while areas with a medium classification are in Sumbergondo and the rest are in the low classification.

Risk
The risk of climate change is the result of a study that considers physical, social, cultural, economic, and environmental factors. The risk study is a basis used to mitigate climate change and flash flood disasters in Batu City.    The risk classification for high vulnerability is in Sidomulyo, Sumberejo, Sumbergondo, Bulukerto, and Gunungsari. Meanwhile, the medium classification consisted of Oro-Oro Ombo, Ngaglik, Temas, Bumiaji, Pandanrejo, and Punten, while the rest were in the low classification. (Δ) The risk of climate change is the result of a study that considers the 2030 spatial pattern plan in Batu City to determine the prediction of the risk of flash flood disaster in Batu City until 2030.
Based on the analysis, there is an increase in the projected risk of flash flood in Batu City, climate change that occurs can exacerbate the impact of damage due to flash floods if it occurs in the future and in the long term will threaten the environmental sustainability of the socio-economic status of the community.

Damage and Loss Assessment
Damage and Loss Assessment (DaLA) is a methodology for obtaining accurate data on damage and loss, an indicator of calculations based on available 2022 secondary data. In this calculation, the damage assessment is carried out based on the dominance of the affected land use in the 2022 baseline and the 2030 projection.  The flash flood that occurred in Batu City in 2021 resulted in considerable losses, especially to the agricultural sector. In this case, the Batu City government needs to take serious action to minimize the possibility of flash floods that can occur in the future.