Flood Disaster Analysis Using Landsat-8 and SPOT-6 Imagery for Determination of Flooded Areas in Sampang, Madura

Based on data of disaster which is defaced by Badan Penanggulangan Bencana Daerah (BPBD) of Sampang that in the period of 2015 - 2017 as many as 25 cases from 31 cases of disaster caused by flood disaster or 80.65% from total disaster. Therefore, the purpose of this research is to create a map of flood vulnerability in Sampang. From the vulnerability map, we can know the area with the impacted flood level in Sampang so that from the map of flood affected areas can be known the extent of the affected area in each class. In this study, two Landsat-8 and SPOT 6 data were used. For Landsat-8 imagery used for land cover on district level disaster level vulnerability maps, while high-resolution SPOT-6 images were used for land cover making maps of flood affected areas Sampang district. With the flood affected areas in this study, it is expected to be used as a determinant of flood affected areas in Sampang district. Based on data processing and analysis it is found that the highest impacted area is located in Sampang district with 12 cases of 17 cases of total flood disaster in Sampang district based on data from BPBD Kabupaten Sampang in 2016. There are 4 classes of flood affected areas in Sampang district i.e. not affected by 9039,540 ha, low impact 46262.881 ha, medium impact 43012.431 ha and high impact of 14009,760 ha.


Introduction Definition of Disaster according to Law Number 24
Year 2007 on Disaster Management mentions the definition of Disaster is an event or series of events that threaten and disrupt the life and livelihood of the community caused by both natural factors and / or non-alam factors or human factors resulting in the occurrence of human lives , Environmental damage, property loss, and psychological impact [1].
In Indonesia there are frequent disasters, one of which is the flood. Flood is an event or condition in which an area or land is inundated due to an increased water volume [2]. In general, floods occur in the rainy season. This is caused by the intensity of rain water is very high and lasted for a long time and the river cannot accommodate again so that the river has increased intensities water debit, consequently river water overflows and impacts on settlements or agricultural areas of citizens. Sampang Regency is a district with high flood vulnerability. Every year floods occur not only during the rainy season but during high tide. According to the Regional Disaster Management Agency, one of the flood-prone areas in East Java. Floods that occur can cause harm to the community and local government.
The purpose of this research is to utilize Remote Sensing and Geographic Information System in determining flood vulnerability in Sampang Regency with the method used is scoring and weighting method. With the map of flood prone levels, it can be known areas with high levels of vulnerability. Referring to the disaster data of BPBD Kabupaten Sampang as a booster data, information obtained from the area affected by flood is mostly located in Sampang district. Furthermore, the analysis of floodaffected areas using high-resolution SPOT-6 images, rainfall data, altitude, slope type of soil and river buffer as a determinant variable of flood, expected to obtain the results of detail and accurate impact areas so that it can be used as the basis for determining the region Affected by the flood in Sampang district.

Materials and Methods
The location of this research is in Sampang Regency. Astronomically, Sampang District is located between 70 10 "-70 20" South Latitude and 1130 13'-1130 23 'East Longitude.

2.2.3
Rainfall Data Processing Station. The rainfall map is obtained from the interpolation of Rainfall using the Inverse Distance Weighted Method (IDW) then classified and scored the classification.

2.2.4
Processing Altitude and Slope. Tilt Height and slope maps are obtained from the interpolation of contour RBI data and then classified and scored

2.2.5
Type of Soil. From georeferenced soil map was to classify type of soil with digitation, after that score was given from classification data.

River
Buffer. River Buffer map was obtained from buffer process, after that scoring is given.

2.2.7
Scoring. Scoring on each variable in this research, based on some references and previous research, but with many improvements like study area. There are table of scoring for each variable: Where :

RMS Error accuration
To calculate RMSeror and SoF value, a net design and GCP distribution was made.
From the calculation of SoF value is 0.2242. Where the smaller number of network strength factors mentioned above, the better the network configuration [3]. GCP coordinate from measurement converted to point SRGI by using calculation of Least Square Adjustment then that coordinate used for rectification process using image processing software until got 5

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The required RMSE <1,5 pixel, that is equal to 1,395 pixels. So, from these results, the image of SPOT-6 is acceptable.

Classification Accuration Test
From result of classification with Supervised classification, obtained accuracy test with confusion matrix at Landsat 8 equal to 74.31%, while in SPOT-6 equal to 87.54%. The accuracy test in medium resolution image is ≥70% while in high resolution image is ≥85%. Thus, from the results of accuracy tests on Landsat 8 and SPOT-6 imagery was acceptable.

Flood Vulnerability Mapping On Distric Sampang
The map of the flood vulnerability area in this study refers to research conducted by Primayuda, 2006 obtained from scoring, overlay and weighting of flood variables i.e. slope, altitude, rainfall, soil texture, river buffer and land cover. For the class used there are 4 classes. By using Landsat 8 image as land cover variable, we get the map of flood vulnerability as shown below:  According to disaster data of BPBD, the highest disaster-affected area is located in Sampang District with disaster data of 12 cases from 17 cases of flood disaster in 2016. In this study, the data is used as a reference to analyse the determination of flood affected areas.

Result of Flood Variable on Sampang District
In analysing the flood affected area in Sampang Sub district, 6 parameters of flooding area are determined as below.

Rainfall Station
Rainfall data used the rainfall station data located in Sampang district in September 2016. From the interpolation results it can be seen that Sampang District has a very high rainfall. Classification and scoring refers to BMKG, 2013 with the results as in the following table:

Hight and Slope on Sampang District
The height affects the process of flooding. The lower the height of the area the greater the potential for flooding. Classification and scoring on the class height refers to previous research by [7] with the results as in the table below: About the River where, for the river line flood exposure at least 50 meters from the edge of the river water will be classification refers to Purnama (2006) with the results shown in the following table:

Imapacted Flood Area Maps of Sampang
Sampang's Flood Affected Area Maps was Map Sampang flood affected area is obtained from the overlay of variable rainfall, altitude, slope, rainfall, soil texture of river buffer and land cover. Referring to Primayuda, 2006 obtained classification of flood affected areas into 4 classes shown with the image below: From the results of the classification that has been done got the area of each affected area as the following

Reflectant Variable Data Result
In this research, the test of reflectance value with correlation test on image, SPOT-6 image and Landsat -8 image by using ratio of NDVI value on each RGB value of image. The RGB value of the image is defined as the nearest wavelength in the two correlated images. It aims to find out how big the relation of reflectance value in the image data shown with the value of R that produced. Referring to Sugiyono, 2007 for interpretation of Correlation Coefficient (R) in the interval range 0.8 -1 has a very strong relationship level. So, the strongest correlation result is shown in figure (b) that is correlation of NDVI value between Landsat-8 with SPOT-6 equal to 0.8633. This is because the wavelength of the RGB between SPOT-6 and Landsat-8 has an approximate wavelength.

Conclusions
Based on the processing and data analysis that has been done, here are the conclusions that the authors get in the author of this thesis. 1.From the classification result, got 4 class area flood vulnerability level in Sampang Regency that is very low, low, medium and very high. 2.Sampang District is a sub-district that has high flood vulnerability and one of the flood affected areas in accordance with disaster data BPBD Kabupaten Sampang. From the classification of the affected areas, the total area was not affected by 9039,540 ha, the low impact was 46262.881 ha, medium impact 53012.431 ha, and the high impact was 14009.760 ha. 3.From the results of analysis of each parameter in the flood affected areas in Sampang district, it can be seen that the most dominant parameters are rainfall, altitude and slope.

Recomendation
The advice given from the results of this research are: 1. Preferably in rectifying high-resolution images using GCP coordinate data obtained from GPS Geodetic, this is done so that the RMSE results are smaller. 2. Need for further research in the form of flood risk modeling in Sampang Regency that can be used to anticipate in flood prevention.