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Abstract: Flood is a hydrologic process whose dimensions change under different natural and artificial conditions of the earth surfaces as well as climatic conditions. Flood can be taken as a natural disaster and will bring about damages and casualties, if this phenomenon is not well recognized enough, or if it is not controlled or stopped. Appropriate planning can be taken into account to reduce the financial damages and civilian casualties caused by floods, by recognizing flood hazard and its spatial distribution. In this research, a method was suggested to determine flood hazards, based on main factors affecting hazard, and then applied for Hendijan-Jarrahi basin in the southwest of Iran. The results show that seven factors, including the intensity of flood potential, flood frequency, flood damage, flood casualties, the population exposed to the flood, residential places and lands exposed to the flood, were recognized as the main parameters to control flood hazard. Afterwards, the matric of hazard factors were identified by weighting to each of them and determining their domains based on data of floods during the last 50 years in second order basins of the country. Flooding areas also were recognized by using the Landsat TM+ 2002. In addition, the intensity of flooding potential were estimated with different return periods by applying the data of hydrometery stations and using regional analysis of daily and instantaneous peak flow for applying to ungaged basin. the layer of flood hazard were prepared based on 5 factors including flood frequency, flood damage, civilian casualties, the population and residential areas exposed to flooding. Then after, the flood hazard zoning were recognized and mapping, by combining and overlaying the layer of flood hazard with the layers of flooding areas and flooding intensity. By applying the model in Hendijan-Jarrahi basin, it was inferred that in this basin the maximum flooding took place in a fourth order Khairabad basin, following by Zohreh basin in Hendijan, otherwise, downstream Hendijan basin had the order of sever flood hazard. Moreover, Allah basin located in the upstream of Jarrahi had the maximum specific flood discharge yet the flood hazard in this basin was low risk. Also, the flood hazard is in the mean level of risk in Maroon and downstream of Jarrahi. Therefore, flood intensity and flood hazard do not much in one area, so it should be into account the basins which are flood prone to control the flood and reduce the flood risk.

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Soil erosion in the watersheds is a function of some factors such as climatology, topography, soil type and land use. Within these factors, the importance of precipitation because its high variation and land use because the role of humans on community development is very clear. In this research, the precipitation, discharge and sediment data in a statistical period from 1967 to 2009 were used. Drought situation was calculated by standard precipitation index with 3 months’ time scale. Land use maps were prepared in three periods based on satellite images, and the changes of land use between these periods were compared. Results showed that the area of rangelands and farms from initial to final section of study period varied from 14% and 21% to 41% and 11% respectively. Significant difference in sedimentation rate is visible between initial and final sections of study period. Percentage of drought occurrence increased to triple from initial to final period. The result of Mann-Kendall test showed a negative trend for watershed discharge at 95% significant confidence level. Therefore, it seems that increasing drought occurrences and therefor decreasing runoff and change of farms to rangeland, caused to the decreasing suspended sediment load in Senobar watershed.

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Hydraulic simulation of dam failure and estimation of damages From it, it will be of great help in the field of relief activities and proper planning in the downstream areas. In the present study, the hazardous areas due to the failure of the Ghare Aghach dam of piping type have been investigated using satellite images. Parameters due to dam failure were calculated using the BREACH model and its information was entered into HEC-RAS software. The results showed that using the BREACH model, the peak flow due to dam failure is 3318.8 cubic meters per second. The time due to dam failure was estimated to be 3.31 hours. According to HEC-RAS software, the results show that the maximum output flow due to the failure of the dam occurred 165 minutes after the simulation and is 3160.1 cubic meters per second. The flooding area of the downstream areas was estimated to be approximately 18.6 square kilometers. The area of endangered residential areas in Mehrgerd and Hosseinabad villages was estimated to be approximately 66995 and 12543 square meters, respectively. The radius of flooding relative to the coastline of the flow in the mentioned areas was estimated to be 443 and 3362 meters, respectively.