Flood vulnerability assessment trough overlay-scoring data method based on Geographical Information System (GIS) in Giriwoyo, Wonogiri, Indonesia

The Sub-District of Giriwoyo, located on the southern edge of Wonogiri District, is classified as an agricultural land. This subdistrict is located in the Keduang sub-watershed. This geographical condition makes this subdistrict extremely vulnerable to flooding. This research seeks to determine the distribution of flood-prone areas and categorize the level of flood vulnerability in the Wonogiri Regency, with Giriwoyo as a research site. Quantitative descriptive research employs a scoring procedure based on environmental conditions-related scoring parameters. These variables include slope, soil type, rainfall, altitude, land use, and buffers for river flow. The measured parameters are then layered in a geographic information system (GIS) application to generate a flood vulnerability map. The data analysis procedure employed a descriptive approach. The results indicate that the dispersed, areas of Giriwoyo are classified into 4 vulnerability catogeries such as not vulnerable, vulnerable enough, vulnerable and very vulnerable. Areas with vulnerable categories are scattered throughout the giriwoyo area which is dominated by rice fields and settlements with a flat slope. The very vulnerable area dominates the western part of the giriwoyo which has a lower elevation and flat slope. The parts of giriwoyo that are not prone to flooding are in Selomarto, Tukulrejo, and Girikikis.


Introduction
Floods are natural disasters that result in physical and socio-economic losses [1].Flooding often occurs in coastal areas, river streams, and dense residential areas with poor drainage.Climate change can exacerbate flood vulnerability in these areas [2].Some areas of the earth are experiencing an increase in the intensity and frequency of rain and others are experiencing droughts due to climate change [3].This makes areas affected by increased rainfall will be more prone to flooding.Vulnerability will also increase if environmental conditions inhibit the infiltration of runoff water into the soil.Environmental conditions that affect flooding include land use, slope, height, drainage and soil type [4].These components are important to consider in preparing flood vulnerability maps as a basis for mitigation.1314 (2024) 012109 IOP Publishing doi:10.1088/1755-1315/1314/1/012109 2 Giriwoyo Sub-District is the southern part of Wonogiri District which has experienced flooding.Giriwoyo Sub-District is part of the Keduang Sub-watershed and has several rivers.Watershed areas are catchment areas so that their areas are often the passage of water runoff and have the potential to experience severe flooding [5].The appearance of the area varies from plains to hills with a slope of 6-40% [6].Agricultural activities are the main occupation of the community in Giriwoyo.The community cultivates land on agricultural lands such as rice fields and fields.Excessive tillage causes damage to the soil [7].Soil that has been damaged is prone to erosion caused by water [8].Erosion will bring the top layer of soil into waterways and be deposited in water bodies [9].Soil deposition in water bodies silts rivers, reservoirs, or lakes so that water easily overflows and inundates the surrounding area [10].
The threat of flooding faced in Giriwoyo Sub-District makes mitigation efforts needed to reduce the impact caused.Appropriate initial mitigation efforts to minimize the impact of flooding are carried out by mapping flood-prone areas [11].Flood vulnerability mapping can be done by utilizing the Geographic Information System (GIS).GIS has been commonly used to manage floods in various regions [12].The use of GIS to assess flood vulnerability has been developed in various methods.Assessment of areas prone to flooding can be done by indexing through scoring environmental conditions in the study area.The flood vulnerability score given to environmental conditions in an area is based on past events and opinions from professional experts [13].The indexing results were used in this study to determine the category of vulnerability and distribution of areas prone to flooding in Giriwoyo Sub-District.

Research time and place
This study used a descriptive quantitative approach to describe flood vulnerability in Giriwoyo Sub-District, Wonogiri District.The study's observation area on Giriwoyo District, Wonogiri Regency, Central Java, Indonesia.This sub-district is located in the northern part of wonogiri district and is bordered by the Gajah Mungkur Reservoir.Giriwoyo consists of 16 villages and 2 wards.Almost all region of the Giriwoyo area have been affected by floods.Data shows that severe flooding events occurred later this year from 2021 until now [14].

Research materials and tools
The data needed is related to environmental conditions that contribute to flood vulnerability in an area.The preparation of flood vulnerability maps in this study requires data on rainfall, altitude, land slope, river buffer, soil type and land use in Giriwoyo Sub-District, Wonogiri District.Rainfall data was obtained from River Basin Organization (RBO) Bengawan Solo river in 2011-2021.Further analysis was carried out on rainfall data using the Thiessen polygon method to obtain an overview of the distribution of rainfall in the study area.Altitude data is obtained from the Digital Elevation Model Nasional (DEMNAS) by the Geospatial Information Agency (GIA).Land slope data is taken from the Digital Elevation Model (DEM) by the United States Geological Survey (USGS).River buffer and land use data are obtained from the Rupa Bumi Indonesia (RBI) map by GIA.Soil type data was obtained from Indonesian Center for Agricultural Land Resources Research and Development (ICALRRD).

Flood Vulnerability Assessment
Data from each parameter is scored to calculate the value of the flood vulnerability index.The score given is adjusted to the criteria of actual circumstances at the location.Scoring is based on table shown below.Very dry Lowest 1 Source : [15] Higher rainfall scores higher, while lower rainfall scores lower.The rainfall data that has been obtained is matched with the score in the table to be accumulated in the calculation of the flood vulnerability index.[16] Areas with lower elevation are given higher scores, while areas with higher elevations are given lower scores.Lowlands are more prone to flooding because water flows from high to lower areas.Areas with high elevation generally have a low chance of flooding.[17] Areas with a flat slope get a higher score, while areas with a steep slope get a lower score.Water easily moves in steep areas so it is less prone to flooding, but can easily gather in flat areas so it is more prone to flooding.Not vulnerable Low 1 Source: [18] The closer the distance, the higher the score.The farther the distance, the lower the score given.Areas with closer proximity to the river have a higher flood vulnerability compared to farther area.[20] The shape of land cover has a significant influence on flooding.The type of residential land use is given a high score because the land used has been paved so that it is unable to inhibit water runoff.Land with denser vegetation cover such as forests is given a low score because it is able to withstand water runoff optimally.[21] Each environmental parameter has its own influence on the occurrence of floods.The magnitude of the influence of environmental parameters is represented by the weight value given to each parameter [22].Table 7 contains information that the greater the weight value on a parameter, the higher the effect on flooding.Parameters given lower weight values are weak in influencing the occurrence of floods.
The next stage is to overlay all the thematic maps (sources of environmental diversity) so that they can be scored to calculate potential flood vulnerability in each area.Overlay is a step of combining two or more maps and the attributes contained into one through a computer application [23].The combination of several types of maps produces maps with new information.The flood vulnerability map in this study was made by overlaying maps of rainfall, elevation, slope, river buffer, soil type and land use.The combination of these maps produces new information that can visualize the category of flood vulnerability in an area.
Flood vulnerability can be determined through mathematical calculations by combining the use of score values and weights in each parameter.The score and weight values are multiplied to determine the vulnerability value.The accumulated results of mathematical calculations on each parameter produce the value of the flood vulnerability index.
Where; y = Flood Vulnerability Index, Wi = weight value for each i parameter; Xi = score value for each i parameter; n = amount of data [24].
The results of these mathematical calculations are then classified according to the level of vulnerability.The categories of flood vulnerability are presented in table 8

Rainfall classification
Giriwoyo Sub-District is affected by mixed rainfall.The rainfall in Giriwoyo consists of dry (1706 mm/year) and medium (2276 mm/year) (Figure 1.).The dry rainfall category predominates in the central region of Giriwoyo.The western part and a small part of the eastern part of Giriwoyo have a medium rainfall category.Similar rainfall was also found in Giritontro District bordering Giriwoyo on the southwest side with dry (1750 mm / year) and medium (2250 mm / year) categories.According to the classification of the rainfall, scores 3 and 2 are given for the calculation of the flood vulnerability index.

Altitude classification
The elevation at Giriwoyo ranges from 22 m-1210 m with an average of 330 m (Figure 2.).Most of the giriwoyo area is located at an altitude of 330 m which extends from the east to the west in the middle.The northern, southern, and a small part of the southeast are higher than the central region.The high area is part of the hills of the Sewu Mountain range.The hills flank from the north and south sides so that the height in the central part of Giriwoyo is lower.This low region is an area that collects water from the hills on its sides.There are also rivers in the low region that empty into the Gajah Mungkur Reservoir on the Northwest side.

Buffer distance classification
A river buffer, also known as a riparian buffer or stream buffer, is an undeveloped strip of land or vegetation along the margins of a river, stream, or other body of water.Giriwoyo subdistrict is dominated by river buffers at a distance of 0-200 m.This indicates that the Giriwoyo area is vulnerable to potential flooding.

Land uses classification
The Giriwoyo area has 4 types of land use in general.Those land uses include settlement, ricefield, field and moor, also shrubs.The sub-district is dominated by paddy fields, then followed by fields and moors.Even though they do not dominate, settlements also have the potential to be vulnerable to flooding.

Flood Vulnerability Index
The not vulnerbale category covers an area of 128.32 ha or 1.20% of the total area.Areas that are not prone to flooding are on the north and south sides.Both sides are hills with high and steep heights and slopes.High slope increases the water rate on the ground surface thereby minimizing flooding [26].The land cover is dominated by forests [27].Vegetation in forests can protect soil from erosion and optimize infiltration rates [28].The region also has no watershed network.The vulnerable enough category covers 3208.47 ha or 30.06% of the total area.This region is relatively safe from flooding because it has a fairly high altitude and a fairly steep slope.Not many settlements were built, but more were used as non-rice field agricultural land [29].Non-paddy agricultural land is able to provide optimal infiltration capabilities [30].The rest is a scrub area that is able to withstand the rate of soil erosion [31].The vulnerable category covers 5780.19 ha or 54.16% of all study areas spread across each village.This category has the widest coverage area because most of Giriwoyo has a high altitude, slope and land use score against flooding.This region is dominated by low elevations with a flat slope.Low regions become vulnerable areas because they can be estimated as floodplains [32].Almost the entire land is used as rice fields and settlements.The existence of settlements makes an area prone to flooding because it inhibits the rate of infiltration in addition to being a gathering place for a number of humans [33].The river network here is relatively tight so it is vulnerable to evaporation if the river capacity has been exceeded.The very vulnerable category covers 1555.71ha or 14.58% of the total study area.Some villages fall into this category such as: Pidekso, Bulurejo, Tukulrejo, Ngancar, Bumiharjo, Selomarto, Giriwoyo, Guwotirto, Sejati, Sendangagung, Platarejo, Tawangharjo, Sirnoboyo, and a small part of Gedongrejo and Tirtosuworo.This region is very prone to flooding with a high category.The density of vegetation is very less because the entire land is used as settlements and rice fields so that the rate of infiltration is very hampered.This is because the conditions are ideal for human activities because it is in a low and flat area.The condition of vulnerability is strengthened by the presence of rivers that cross this region.The condition of the area that is getting closer to the river increases its vulnerability to flooding related to the occurrence of floodplains if the river overflows [34].

Conclusions
Giriwoyo's potential flood vulnerability index is divided into 4 categories (not vulnerable, vulnerable enough, vulnerable, and very vulnerable).The vulnerable category dominates 54% of the total area spread in each village.The very vulnerable area dominates the western part of the giriwoyo which has a lower elevation and flat slope.Adding vegetation density as an effort to mitigate flood disasters.

Table 1 .
Rainfall score classification

Table 2 .
Altitude score classification

Table 3 .
Slope score classification

Table 4 .
Buffer distance score classification

Table 5 .
Soil types scoreSoil types with a low infiltration rate are given a high score, whereas soils with a high infiltration rate are given a low score.Areas that have soils with low infiltration capabilities are more prone to flooding because water cannot absorb into the soil and become runoff.

Table 6 .
Land uses score classification

Table 7 .
Weight Classification of Flood Vulnerability Parameters

Table 8 .
. Higher index values indicate that the area is increasingly prone to flooding.Conversely, lower index values indicate lower flood vulnerability.Flood vulnerability index classification

Table 10 .
Altitude index

Table 11 .
Slope index

Table 12 .
Buffer distance index Giriwoyo has two soil types, namely Inceptisols and Entisols.The inceptisols ordo was dominates this sub-district.The large number of inceptisols ordo also indicates that Giriwoyo is vulnerable to potential flooding.

Table 13 .
Soil types index

Table 14 .
Land uses index The results of a gradual overlay of rainfall, altitude, slope, river buffer, soil type and land use maps followed by flood vulnerability index calculations (scoring and weigthing) produce a flood vulnerability map of Giriwoyo Sub-District (Figure7.).The flood vulnerability map depicts categories of areas prone to flooding by taking into account various parameters.There are four categories of floods in Giriwoyo Sub-District, namely: not vulnerable, vulnerable enough, vulnerable and very vulnerable.

Table 15 .
Flood Vulnerability area and percentage