Investigation of the distribution of major floods in the Atrak river basin

Flood is one of the most destructive natural hazards, especially in arid and semi-arid areas that experience great of rainfalls. This hazard is caused by the combination of several natural and human factors. Floods occur when the flow of the river increases to a certain extent. that the water crosses the banks of the river and inundates the nearby areas, as a result it affects the communities that live in the vicinity of the rivers. In addition to that, climate change increases the possibility of floods and their magnitude. Annual 75 world floods on average, affects millions of people worldwide. About 20,000 deaths per year, resulting in economic losses of $95 billion. In recent years, the frequency of floods has increased by more than 40%, as climate change significantly affects the intensity, pattern and magnitude of deadly floods. Especially Asian countries are exposed to floods and earthquakes with a relatively high impact. In Asia, more than 90% of human casualties are caused by natural disasters that are due to flooding. Recently, Iran has experienced catastrophic floods. In addition to the impact of climate change. The impact of human activities caused by the poor management of the watershed, including deforestation, excessive livestock grazing, the development of rural and urban settlements, have reduced the hydraulic space of rivers. In all these researches, the study of floodplains, different return periods, and inundation values in each application have been analyzed. Arid and semi-arid areas are definitely more prone to flooding due to heavy rains. In the present study, in order to investigate and distribute large-scale floods in Etrak river basin, the daily inundation data of 35 hydrometer stations were used over a period of 30 years. In order to extract large floods in the region, first, using the statistical method known as the inverse numerical filter method, the base runoff was separated from the total. The results of time series analysis extracted after applying the regression numerical filter method show that in 3 decades, different behavior of floods in the catchment area are observed. In the first decade, the floods had a higher intensity, and this high intensity also had a high frequency. In the second decade, the volume of floods decreased drastically, and in the third decade, the volume of floods was the same as in the first decade, but they occurred at longer intervals. The investigation of most severe floods showed that with the increase in the volume of the total runoff, there was a large difference between it and the base runoff, and the occurrence of this process is sudden and sometimes reaches more than 10 times the base runoff. The high volume of discharge in flood periods and on the other hand, the decade fluctuation in the occurrence of severe floods in the Atrak catchment area showed that for long-term forecasting, the inverse numerical filter method can be used.