Floods are very serious natural hazards that occur frequently, causing huge damage to infrastructure, life, and the economy. Management officials need timely information about flood conditions in sunken areas to effectively organize emergency responses. An estimated 21 million people worldwide are affected by river floods each year, which may increase to 54 million by 2030.Climate change and socio-economic growth are the main causes of these events )Gorelick et al. 2017). Various climatic factors affect floods mainly through rainfall, including its intensity, amount and duration. Rapid estimation of flood space range in large areas provides a key data source for disaster assessment. Satellite RS is increasingly important for flood mapping. Optical RS is essential for monitoring flood dynamics based on low water reflection in infrared and high reflection in blue/green strips Dumitru et al. 2015. RS technologies make it easier to map large flooded areas, provide early alerts quickly, and help provide support in critical situations and minimize damage. Space sensors are more effective at providing real-time data for flood extent monitoring than ground-based techniques. The satellite images are very valuable for emergency response because they provide constant observations about it (Tarpanelli et.al.2022). Choosing the right sensors to effectively produce flood outbreaks in large geographical areas is a major challenge. Satellite platforms and other advances in RS have provided a wide range of satellite data applications (Cohen et al. 2019). GEE is widely used for better and faster data processing and analysis. GEE is a cloud platform for geospatial analysis. It is an integrated platform designed to empower a wide audience, including RS scientists who lack the technical knowledge of supercomputers (Gorelick et al. 2017). SAR is of particular interest in observing flood situations, actively emitting electromagnetic waves that are not affected by weather and time of day and can be used to detect floods in vegetation or urban areas. Therefore, the use of SAR radar data has been investigated by many researchers Landuyt et al. 2019.
It is necessary to use RS techniques for zoning the recent floods of Iran in different regions and to inform the authorities about the amount of agricultural, social and economic damages. Ganji et al. 2021estimated the flood area and time series changes occurred in April 2019. Calculating the MNDWI and NDWI indices by pre-processing the images of S2 and Landsat8 depicted that the largest area of flood zones was related to the MNDWI index, which is equal to 262.453 and 51.991 km2 for the city and town of AqQala, respectively. Bigham Sereshkeh et al. 2020 used two algorithms of SVR and RF for accuracy of classification of S2 imagery by pixel-based and object-based methods in zoning flood areas in Taleghan. To validate the extracted zones, hydraulic zoning method with return periods of 2, 5, 10, 25 and 100 years was used. The results showed that both RF and SVM methods in object-based approach often had the highest overlap with different return periods. The highest overlap with flooded areas was obtained in the two-year return period with 68% and 66% for RF and SVM. Mehrabi (2021) used S1 time series imagery, threshold and SBAS techniques to determine ground displacement and flood effects in Pol-e-Dokhtar, Iran. The results showed that the nature of the spatial extent of the flood was sinusoidal, so that the behavior of the flood corresponded with the rainfall periods. In addition, SBAS-In SAR findings showed how a severe flood can cause ground displacement and SAR data are effectively used for flood monitoring, water mapping, and flood-induced land displacement. Despite advances in RS, there is a clear gap between the natural disaster community and technology resources leverage in real time which hinders timely ground response efforts.
Other studies in the world using RS for flood detection are containing: Alexandre et al. 2020 introduced an automated processing chain for S1 (SAR) radar data. This processing chain is based on the S1-Tiling algorithm and the NDR, which can load and clip S1 images on S2 pixels multiple times. Tripathy and Malladi (2022) has introduced a new web application called GFM that generates flood maps quickly without getting into technical complexities. To extract flood extent from S1 satellite data, before and during flood is considered. Mountainous, urban and arid areas are highly vulnerable to floods due to severe storms and sudden rains, which cause flash floods to affect infrastructure and the ecological and biophysical environment. Elhag and Abdurahman (2020) used the S1TBX to support the display and analysis of the large archive of ESA SAR mission products. The results showed that the area of the flooded surface is approximately 9 km2, which mainly covers agricultural lands and urban areas of Tabuk city in Saudi Arabia. Hutan et al. 2019 were compared flooded areas identified using HEC-RAS and NDWI index obtained from Landsat 7-ETM + satellite imagery processing. The study area, the upper part of the Jijia River on the Moldovan Plateau, was affected by the July 2010 floods. During that event, the level of the River reached 579 CM at the Dingeni hydrometric station. By performing different flood simulations by applying the NDWI index and HEC RAS model, the flooded area is 15.8 and 16.26 km2 respectively. Reliable and rapid flood maps are vital parameters in the preparation of disaster management plans. Vanama et al. 2021 revealed an effective flood in the framework of mapping using multi-temporal images, including C-band S1A & 1B, SAR images and optical WorldView-3 images, to analyze the 2018 Kerala flood event, India. To identify floods, CD techniques, RI and NCI combined with semi-automatic thresholding were implemented on temporary descending pass SAR images. Mehmood et al. 2021 used Landsat 5, 7 and 8 to map flood outbreak areas. GEE was used to implement the FMA and Landsat image processing. The FMA relies on the development of a "data cube" that overlaps the pixels of Landsat images over a period of time. Temporary and permanent water zones were evaluated using MNDWI index, which estimated FMA accuracy in the range of 71–90% and overall accuracy in the range of 74–89%. In general, FMA is an accurate and efficient method for zoning flood points. Zhang et al. 2020 presents a new method for rapidly determining the extent of flooding in large, semi-arid regions with challenging environmental conditions, according to S1 data. First, a preprocessing scheme is applied to perform geometric correction and estimate the image intensity. Second, an automated threshold method is used to establish the initial classification of land and water by merging probability density distributions. Tarpanelli et al. 2022 evaluated a synthetic study of the S1 and 2 in the systematic assessment of floods in Europe. They collected ten years of river discharge data over almost 2000 sites in Europe and analyzed flood events over some established thresholds. Results show that assuming the configuration of a constellation of two satellites for each mission and considering the ascending and descending orbit, on average the 58% of flood events are potentially observable by S1 and only the 28% by S2 due to the cloud coverage. Konapala et al.2021 examined the various S2 bands by calculating water indices and images derived from S1 to assess their ability to create accurate flood maps. The performance of combinations of S-1 and S-2 bands was evaluated using 446 flood images. Pandey et al.2022 was monitored the large-scale flooding in 2020 in the Ganga-Brahmaputra basin using SAR data in cloud platform GEE, and estimated the impact of floods on the agriculture and population in the basin. The results showed that agricultural lands and settlements, affected by flood were 23.68–28.47% and 5.66–9.15%, respectively.
In short, the RS flood zoning method should be able to record floods in real time, meaning that instead of training and analyzing the parameters of the time-consuming model, it can work at high speeds and low computational costs, especially when using multiple images to observe flood evolution. In addition, flood detection method should be widely used for large, semi-arid areas with complex flood conditions and a variety of different land cover. Finally, flood process detection operations must be independent of reference image and specific satellite sensors. Accordingly, this paper uses an approach to automatically detect floods in semi-arid large scale region using S1 and 2 multi-temporal images of land range detection. Contrary to the methods presented above, the proposed approach has less calculations and higher detection speed. In this method, the flooded areas in different landuse areas are well separated according to different filters including slop, detection of permanent water areas, MODIS farm land maps and JRC global surface water and JRC golbal human settelement population density dataset, although CD method with thresholding 1.25 were applied and the results were validated and compared with two indices of NDWI and MNDWI in the Kermanshah province.