ANALYSIS OF MAPPING OF THE LEVEL OF FLOOD PRONECTION IN BOJONEGORO REGENCY BASED ON GEOGRAPHIC INFORMATION SYSTEM

Floods are natural disasters that often occur in various parts of the world, especially in flood-prone areas. Floods occur when water from rivers, lakes or seas overflows beyond its holding capacity. Floods can cause great damage to infrastructure, plants, animals and people. To overcome floods that occur frequently every year, it is necessary to map areas prone to flooding accurately and quickly using GIS which can be used to accurately monitor the location and area of the area that is experiencing flooding. This study aims to analyze flood-prone areas and compile maps of flood-prone areas and describe the conditions of flood-prone areas in Bojonegoro Regency based on Geographic Information Systems (GIS). Data processed using GIS include: RBI Map of Bojonegoro Regency, River Map, Soil Type Map, Rainfall Data, Landsat Imagery Data, Flood incident data in Bojonegoro Regency. From the results of data processing, the results of the analysis are as follows: The level of flood-prone areas in Bojonegoro Regency is divided into 3 classes, namely: very vulnerable class of 49,963.671 Ha or around 21.66% of the sub-districts in Bojonegoro including the District: Margomulyo , Ngraho, Padangan, Kasiman, Malo, Purwosari, Trucuk, Kalitidu, Ngasem, Bojonegoro, Kapas, Dander, Balen, Kanor, Baureno, Sumberrejo, Kepohbaru and Gondang. The vulnerable class of 173,162.171 Ha or around 75.05% is in sub-districts including: Margomulyo, Ngraho, Tambakrejo, Kedewan District, Malo, Trucuk, Sekar, Ngambon, Ngasem, Gondang, Bubulan, Dander, Temayang, Sugihwaras and Kedungadem. The non-prone class is 7,580.157 ha or 3.29% in the sub-districts including: Margomulyo, Ngraho, Tambakrejo, Kedewan, Malo, Trucuk, Sekar, Ngambon, Gondang, Temayang, Sugihwaras and Kedungadem. Areas that have a high level of vulnerability are around the Bengawan Solo River which crosses Bojonegoro Regency. This can be caused because the area around the Bengawan Solo River has a height of <25 m and frequent flooding from the upstream and tributaries of the Bengawan Solo River


INTRODUCTION
Floods are part of natural disasters that often occur throughout the world, especially in flood-prone areas such as lowlands, river valleys, and coastal areas. Floods occur when water from rivers [1], lakes, or seas overflows beyond its holding capacity, which can be caused by heavy rain, melting snow, or high tides. Floods can cause great damage to infrastructure, plants, animals and people. Therefore, flood risk mitigation is very important to reduce losses caused by floods. Flood risk mitigation efforts can include proper spatial planning, development of early warning systems, construction of flood-resistant buildings [2], [3] and infrastructure, land use regulation, and public education on how to deal with floods and how to reduce their impact. Floods often occur in Bojonegoro Regency every year [4]. The impact of the floods that occurred in Bojonegoro Regency included damage to infrastructure and casualties, so that it became a serious problem that required speed in handling. Subur Tjahyono, 2009 in his research stated that Bojonegoro Regency is the area that has been traversed by the Bengawan Solo for about 100 kilometers and 24,753 hectares of area is watershed [5]. Floods that have occurred in Bojonegoro Regency almost every year. Floods in Bojonegoro Regency can be seen in 1968 which flooded an area of 12,000 hectares and 152,000 with losses of up to Rp. 8 [6]. In order to deal with floods that occur frequently every year, it is necessary to map areas that are prone to flood disasters so that the community and the government can easily carry out disaster mitigation appropriately and quickly using GIS which can be used to monitor the location and area of the area that floods occur precisely. This study aims to analyze flood-prone areas and compile maps of flood-prone areas and describe the conditions of flood-prone areas in Bojonegoro Regency based on GIS. Floods occur when heavy rainfall [7] or extreme weather such as storms, typhoons, or blizzards cause excess water that cannot be absorbed by soil or waterways [8]. Floods can occur in urban or rural areas and can cause damage to buildings, roads and other infrastructure [9]. Floods can also cause loss of life [10] and cause negative impacts on public health. Therefore, flood mitigation and good infrastructure development are very important to minimize the impact of floods.

METHODOLOGY
These data can then be analyzed and manipulated in GIS to obtain more detailed and easy-to-understand information. There are two very important methods of data analysis, namely scoring and weighting. After carrying out these two processes, the next step is to analyze the level of flood vulnerability. 1. Scoring 2. Weighting 3. Analysis of Flood Vulnerability and Risk Levels [11], Analysis of the level of vulnerability and risk of flooding aims to determine which areas are more vulnerable to flooding and identify the factors that cause flooding. The value of the vulnerability can be calculated by the following formula: The potential value of an area for Threat is determined from the overall score of each parameter [12].
Areas that are very potentially dangerous have a very large total number and conversely areas that are not potentially dangerous have a small total number. Processing [10] Geographic Information System (GIS) is a process of collecting, storing, analyzing, visualizing, and processing geographic or spatial data using information technology. GIS enables users to integrate geographic data from a variety of sources, including maps, satellite imagery, and other numerical data. GIS processing involves several

Topographic Maps
Topographic data is used in analyzing flood-prone areas because it can determine the height of the land from the highest to the lowest according to the contour lines. The topographic map can be seen in Figure  2 below: Problem Identification

Litelatur Review
Spatial Data : The southern area is dominated by southern [13] limestone hills with altitudes ranging from 100 -> 900 m and in the northern area with low elevations and sloping around 25-> 100 m. Based on the map above, the altitude data in Bojonegoro Regency can be classified into 4 classes with details as in the following table: From the contour map above, it can be seen that there are variations in elevation in Bojonegoro Regency. The southern area is dominated by southern limestone hills with altitudes ranging from 100 -> 900 m and in the northern area with low elevations and sloping around 25-> 100 m. Based on the map above, the altitude data in Bojonegoro Regency can be classified into 4 classes with details as in the following

Soil Type Map
The Ministry of Agriculture's WEBGis provides a map of soil types which is the result of direct measurements in the field and then processed using processing methods or by digitizing to obtain the results. Figure 3 shows that there are 4 types of soil in Bojonegoro Regency, namely alluvial, gromusol, litosol, and Mediterranean soil types.

River Buffer Map
The river buffer map in Figure 4 and Figure 5 shows the distribution of rivers in Bojonegoro Regency which consists of 2 rivers and tributaries. Many tributaries are found in sub-districts in Bojonegoro including: Kanor, Purwosari, Gayam, Kalitidu, Sukosewu, Balen, Sumberrejo, Gondang and Kepohbaru. This can cause several areas that are crossed by these tributaries to be prone to flooding, if there is a high intensity of rain and the delivery of water from the upstream of the tributary cannot be accommodated and can become a flood of shipments to Bojonegoro City which is close to the downstream of the Bengawan Solo river. On the river buffer map this is the result of dividing into several classes, including a radius of 0-50 m which is the area closest to the river and tributaries so it is classified as prone to flooding, while the radius of 50-150m is an area that is quite vulnerable because it is still close to rivers and streams, and at a radius of 150-300m is an area that is safe from flooding. Buffers are focused on rivers and streams that frequently flood.

Landuse Map
The land cover map for Bojonegoro Regency was made based on Landsat 8 imagery data on December 30 2019. To get a land cover map, the latest image data is needed, then it is cut using polygons for Bojonegoro Regency. Areas with land use that is overgrown with vegetation will be able to absorb runoff water, so that the area can hold runoff water flowing and the possibility of flooding is less likely compared to land that is not planted with vegetation. Classification of satellite imagery can be divided into several classes as shown in table 5, namely:  The data used is multi-temporal data for land use parameters to support knowledge about the changes that have occurred so that they can find out the causes of their occurrence in Bojonegoro Regency.

Flood Intensity Map
This flood intensity map was obtained from data previously obtained from the relevant agencies in jpeg, excel format, then processed using Arcgis 10.8 to obtain a file in shp form to facilitate analysis using a Geographic Information System (GIS) so that it can be processed into a flood intensity map. Bojonegoro Regency.  c) Not Prone The condition of the non-prone class area has an area of 7,580.157 ha with relatively high topography > 500m. This area is located in the northern and southern parts of Bojonegoro Regency and includes sub-districts including: Margomulyo, Ngraho, Tambakrejo, Kedewan, Malo, Trucuk, Sekar, Ngambon, Gondang, Temayang, Sugihwaras and Kedungadem. The level of flood vulnerability will continue to increase if a certain area of land is used for residential areas for a wider population of certain vulnerability classes. This means that if the use of land that has never been inhabited by humans is associated with a particular flood-prone vulnerability class, then the flood hazard level will be low.