Flood Hazards Susceptibility Map and its Occurrence Probability using Shannon Entropy Model (Case Study: Firoozabad River Basin)

Introduction
Among natural hazards, flood is very important. One of the main ways to reduce the risks of floods is to provide flood sensitivity maps. . Between 1988 and 1997, around 390000 people were killed by natural disasters in the world, 58% were flood victims, 26% were affected by a storm and another 16% by the earthquake. The total livelihoods in these 10 years was about 700 billion $. That is 33, 29, and 28 percent were respectively from storm, earthquake, and flood. In this regard, a worrying trend is the increasing trend of mortality and flood damage in the world in recent decades. Increasing population and assets in flood plains, and the changes in hydrosystems and the destructive effects of human activities have been a major cause of this trend. In this research, Shannon entropy model has been used to provide flood sensitivity maps.
Research and analysis
Firoozabad is a city with an area of ​​11917 km southwest of Fars province. The city of Firoozabad is located at 28 degrees and 50 minutes north latitude, 52 degrees and 34 minutes east, with an altitude of 1325 meters from sea level. The Firoozabad basin is one of the sub-basins of the Mordan watershed. The Mard watershed with an area of ​​4,785 and 400 hectares, extending to the territory of three provinces of Fars, Bushehr and Hormozgan. The area of ​​this basin is 3165 meters high from Khorramkouh in southern Zagros to the Persian Gulf coast. For the study of flood, 34 flood waters were first selected in the Firoozabad basin, and then these 34 points were classified into two groups. With 22 points, 65 percent of the points for training and modeling, and 12 points, 35 percent of the locations that were not used in modeling were used for validation. First, a map of the status of the floods was developed. Then, 10 factors, slope, tilt, lithology, land use, NDVI, SPI, TWI, altitudes, rainfall and distances from the river, were selected as flood factors in Firoozabad basin.
Discussion
Prioritizing the effective factors in the occurrence of flood by Shannon entropy index showed that NDVI layers (2.03), rainfall (2.00), distance from river (1.89), SPI (385.1), altitudes (0.999), gradient with weight (0.932), lithology (0.478), TWI (0.379), and land use (0.280), respectively (0.184) have the highest lowest impact on flood events. NDVI is the most important factor in the occurrence of flood in terms of Shannon entropy. In order to prepare a flood susceptibility map, 10 factors were prepared. After final weight of each factor by Shannon Entropy model and its multiplication in the classes, the weight maps were combined and the final flood sensitivity map was prepared. Then, the final map was classified into 5 classes of very low, moderate, high and very sensitive sensitivity. According to the central part of the basin, the map is more susceptible to flood sensitivity. To provide a distance map from the river, using the multiring buffer command was used in ArcGis software, and classified into five classes. To provide the effective factor of elevation classes, a digital elevation model was used in ArcGis software, and classified into five classes. Two important factors (SPI) and (TWI) are known as important factors in connection with water. TWI is the amount of flow accumulation in each location in the catchment area and flow trend downstream by gravity. To ensure the accuracy and validation of the maps prepared in this study, a regional attribute index or ROC curve was calculated from the area after the field survey. The basis of the results of the ROC curve for the Firoozabad Basin, for predicting the surface area under the curve with 35% of the validation data is 92.42%, and for the success rate with 65% of the education data, is equal to 93.53%. Therefore, Shannon entropy model has acceptable accuracy in preparing a flood susceptibility map in Firoozabad basin.
Conclusion
Shannon entropy model has acceptable accuracy in preparing flood susceptibility map in Firoozabad basin. Entropy is one of the management approaches used to deal with irregularities, instability of turmoil and uncertainties in a system. According to the final map, the sensitivity of the flood around the Firoozabad River is susceptible to a high flood event. Therefore, the construction of residential lands and agricultural lands and gardens around the river should be avoided. According to the results of the study, a natural disaster sensitivity map such as flood is important to manage and plan the future. Preparation of flood susceptibility map and its occurrence probability using Shannon entropy model (case study of Firoozabad river basin) is necessary to prevent financial and financial losses to these sectors.