Mapping of flood in the Juana Watershed for risk assessment

This research intends to map floods in the Juana Watershed. However, the flood disaster in the Seluna River system started when the flood land was developed for residential areas, agriculture as well as industry, so the damage potency due to the flood will increase. Along with the increasing of population number and density, the social impact that is more serious than flood will follow – like the threat of loss of life. The impact is the management of flood risk becomes more complex and expensive. The methodology for flood mapping consists of the assessment of flood index, vulnerability, and capacity. Determination of flood hazard index is carried out based on the classification of inundation depth. The process is carried out by using spatial calculation, so it can produce the risk map and grid value that can be used in arranging the explanation of the risk map. The result of flood mapping is hoped to be able to support the model development of flood risk management towards the area layout.

The flood disaster in Juana starts when the flood flatland is developed for residential, agricultural, and industrial purposes, so the damage potency of the flood [1] will increase.Along with the increasing density of the population, a more serious social impact from flooding will follow, like the threat of loss of life [2].Therefore, the flood risk becomes more complex and expensive.
Generally, the regulation spatial is not designed by considering flood risk management [3], but for other social purposes like controlling the population location (by controlling the development of residences), determining the location of industry and trade, or protecting wild animals and agricultural areas from encroachment by urban areas [4].Therefore, the arrangement of spatial design is usually decided on the administrative level; it is often not based on the catchment area.The spatial design is an effective approach for preventing the increase in flood risk through active control of the development of area and property in the system area of the Juana River.
The condition of topography, geology, hydrogeology, land cover, climatology, and the availability of water resource infrastructure in the river area are the determinants that influence the pattern of flooding.The spatial study, by integrating the condition above into the Geographical Information System Map, will facilitate carrying out the area zone analysis regarding the surface flow hydraulics characteristic of the river area.The mapping of urban flood hazards in the big catch scale can give a comprehensive understanding of the areas prone to flooding in a city, and it is very needed for the design of cities and disaster mitigation [5], [6].The Geographical Information System is an effective tool for mapping the flood hazard on the big catch scale.
This research intends to map floods in the Juana Watershed to find out the flood hazards in Juana for various return periods.Long-term and short-term planning should be incorporated, and sustainable flood hazards should be practiced in urban environments.

Research Location
The research location is in the Juana watershed, which is located in Kudus and Pati and is in the subwatersheds of Gembong, Ju, Ju1, Ju2, Juana1, Juana2, Juana3, and Logung, as presented in Figure 1.The problem that is happening now is that when the Juwana River is still flowing, it serves as a branch of the Serang River that directs flood flows to Wilalung Gate and Juwana River tributaries with a flow time of 50 years (1540 m3/s).The slope of the Juwana River is relatively flat, i.e., approximately 0.00008.With a relatively small slope, This results in low flood water velocities, leading to surface elevation as the water rises.As a result, many areas on the right and left sides of the river were affected by flooding of embankments unable to receive floodwater into the depressions.The area was also flooded by floodwaters from the Juwana River colmata, which is now a settlement and rice fields.

Risk of Disaster
In the specific module of the facilitator; Management of Disaster Handling by PNPM-MP (2018) [7], the disaster risk is the loss of potency that is caused by a disaster in an area in a certain period which can be in the form of death, injury, sick, soul is threatened, loss of security, evacuate, damage or loss of property, and disturbance of society activity [7].The higher vulnerability causes a bigger risk; however, the bigger capacity causes a smaller disaster risk.The tabulation of research indicators from the reference source above is presented in the Table below.Based on the theory above, the assessment of disaster risk is necessary to attend only to the variable of hazard and vulnerability.Because the indicators of ability and capacity find the obstacle due to the data limitation in the field and need long enough time to research it, so there is assumed that the ability and capacity of every object in a category are the same.In addition, regarding the definition of danger risk (hazard) is the potency of loss that is caused by a disaster in an area in a certain period of time, so the loss that is caused by a disaster is formed from 2 unsure that related and completed each other that are hazard and vulnerability.Because the disaster danger will not become the loss if there is no vulnerability, the object is the interaction result of 3 components that are danger threat (hazard), capacity, and vulnerability.
Flood hazard maps are critical in detecting areas at risk of flooding disasters.Flood hazard maps indicate regions that are susceptible to flooding hazards.The interpretation of a typical flood hazard map should be able to provide information on the spatial extent, depth, and flood frequency.Flood hazard maps specification and ranking of flood-prone regions rely on the end users' requirement [9].
Accuracy in the information provided by flood hazard maps is crucial in flood mitigation and management-related practices.Validating and checking the predictive capability of flood hazard maps is necessary for flood hazard analysis through model validation and uncertainty analysis [10].

Methodology
The methodology for flood mapping consists of an assessment of flood index, vulnerability, and capacity.This research only maps the flood depth and the determination of flood hazard index is carried out based on the classification of inundation depth.The process is carried out by using spatial calculation, so it can produce the risk map and grid value that can be used in arranging the explanation of the risk map.
HEC-HMS is used for rainfall-runoff modeling and flood discharge.Flood modeling at Juana Watershed is using the MIKE Flood Integrating 1d-2d to the Q2, Q10, and Q25 design flood at the main river (Figure 4).Verification of flood modeling results is carried out by comparing the result from the satellite Sentinel-1 from the European Space Agency with the running result of modeling.
The flood event data is taken from the period from 20 th February 2020 until 27 th February 2020.The verification result shows that the running result of Q10 at the Juana watershed almost matches the Landsat map (Figure 5).Then, the modeling result of Q25 is also verified with the flood observation result at BBWS Pemali-Juana and it shows the match result.

Results and Discussion
The National Institution of Disaster Control (BNPB) has had the disaster risk map, Ina-Risk/ Classification Index of Flood Disaster made based on the flood depth from flood events [9]. Figure 2 presents the flood hazard map of Indonesia and Figure 3   The ratio of existing flood hazard areas based on the BNPB, map of Landsat Sentinel-1, and modeling of Q25 at each sub-watershed is presented in Table 2. Flood-prone areas based on flood modeling and the Landsat Sentinel almost match high-risk flood hazards; however, there are quite large differences in area size.But flood-prone areas based on BNPB data have a different method 1311 (2024) 012069 IOP Publishing doi:10.1088/1755-1315/1311/1/0120697 that about 96% of inundated areas have high risk; the modeling map is about 88%, and the BNPB map is about 68%.Further research is needed to obtain a more accurate verification by modeling the whole river system.

Figure 1 .
Figure 1.Research Location and River System of Juana [11] presents the flood hazard map of Juana Watershed based on the BNPB.There were two major flood events in Juwana on December 25, 2007, and February 20, 2023.In this study, flood extent from the flood events is compared with modeling flood results to verify the 1311 (2024) 012069 IOP Publishing doi:10.1088/1755-1315/1311/1/0120694 flood modeling (Figure6).Historical flood map data is only for flood events on December 25, 26, and 27, 2007; for flood events on February 20, 2020, we use data from Sentinel-1 Landsat.