شناسایی رابطه شدت-مدت بارش‌ها با دبی جریان در رودخانة دز تحت تاثیر بارش‌های فوق سنگین (سیلاب سال های 2016و 2019)

Introduction

Precipitation is the most important and effective source of water in a watershed, which changes with uneven spatial and temporal distribution in the basin. In this research, two heavy rains that led to floods in 2016 and 2019 were investigated in order to know and understand the mechanisms affecting the occurrence of these rains and to identify the relationship between the intensity of the rainfall and the flow rate. In order to analyze these relationships, an environmental approach was chosen. After drawing the maps of the upper atmospheric levels and identifying the effective system in causing super heavy rains, the flood discharge caused by the mentioned systems was extracted from the recorded data. The most effective system in causing heavy rains is the Sudanese low pressure system. The peak flow of floods has been delayed by 12 to 24 hours from the day of peak rainfall and the increasing trend of runoff is closely related to the increase in intensity and continuity of rainfall. Due to heavy rains with total rains of more than 160 to 120 mm in some stations, the discharge values have been recorded geometrically exponentially in hydrometric stations. The importance of investigating the phenomenon of precipitation is more obvious when a place witnesses a small or significant fall or suddenly a large amount of precipitation. In recent years, several studies have been conducted in relation to heavy rains, heavy rains and flood potential in the synoptic method and with the help of maps of different levels of the atmosphere in the world and in Iran. Considering the importance of rainfall and its impact on various economic, agricultural and social sectors, the need to investigate and identify the synergistic patterns of rainfall in the study area for use in planning and optimal management of water resources is evident. Ecosystems and physical structures of human societies are adjusted to normal climatic conditions, and when extreme climatic phenomena occur, they can rarely equip and prepare themselves. Therefore, the temporal variability of precipitation, especially extreme precipitations, has many economic effects.

materials and methods

The study method in this research is a combination of field method and statistical-analytical method. Research is of applied type. Obtaining atmospheric data of pressure, omega, specific humidity and flow for the indicated atmospheric levels during a 33-year period (1990-2017) from the website (NCEP/NCAR) and European Center for Medium-Range Forecasting (ECMWF) Determining the heaviest precipitations that occurred in the Dez River basin with the index method 98% percentile base and then determining the most durable rain waves (indicator waves) in each year and taking into account the duration of rainy days during the 33-year statistical period (1990-2017) and drawing weather maps using GRADS and SURFER software and performing factor analysis method and Clustering and visual analysis to identify the atmospheric patterns of heavy and long-term rainfall occurrences and synoptic analysis of the atmospheric patterns of heavy and long-lasting cloud precipitations and the investigation and analysis of the historical course of heavy and long-term cloud precipitations and the investigation and analysis of the degree of correlation or the relationship between the intensity of rainfall and the intensity of surface runoff and choosing the factor that expresses the highest intensity of rainfall.Results and discussion

The findings of this research are presented in two separate sections. In the first part, synoptic patterns leading to selected super heavy rains will be presented. In the second part, the flood discharge resulting from these extremely heavy rains has been analyzed. Based on the spatial arrangement and the functioning of the systems in the lower and middle layers of the atmosphere, synoptic patterns leading to the formation of these super heavy rains were identified. In the analysis of the flood hydrograph, while showing the flow of the flood caused by this rainfall during the flooding of the river in the water year, the hourly hydrograph of the flood caused by this rainfall has also been drawn and analyzed.

Conclusion

In the investigation of the middle layer of the Wardspehr in all the systems that led to the flood of 2019, it was observed that the Saudi Wacharkhand cell has been established over the Arabian and Oman seas and the east of the Arabian Peninsula by moving eastward. And the formation of a low-altitude center over Iraq is the most suitable pattern for heavy cloud precipitation in the Dez basin. On the synoptic maps of the lower and middle levels of the 2016 Wardspehr system, the African Watcher Khand has blocked the west of Iran with its significant northward expansion over eastern Europe. As a result, the activity period of the system has increased by more than 4 days. Therefore, the African cycle and its stop in Eastern Europe is the main reason for prolonging the activity period of rainfall systems leading to flooding in the area. Therefore, the Azureverse system has not played a significant role in prolonging the activity period of the systems on the field. In the investigation of the flood discharge of super-heavy rainfall systems, it was observed that in all the selected rainfall systems of Dubai, the peak floods have caused very severe floods with a delay of 12 to 24 hours from the day of the peak rainfall of the Dubai system. So that in the floods of 2019 with a peak discharge of 4503 cubic meters per second, the flood of 2016 with a peak discharge of 10510 cubic meters per second, while they were among the very heavy floods of the study period. Flow values due to short-term super heavy rains with intensities of more than 160 to 120 mm in some stations have been recorded in geometrical exponential form in hydrometric stations. Therefore, the increasing trend of surface runoff is an exponential trend compared to the increasing intensity of rainfall.