Introduction: The process of flow changes and the movement of water and sediment together and simultaneously in open erosion channels is a very complex phenomenon. Among other things, the channel bed may be eroded in place or along the route, and its grain size may be decomposed and changed. The entry of non-uniform and non-permanent flow of water and sediment into a system such as the area of the river path to the reservoir causes changes in the cross section and instability of the flow regime, especially during floods. Also, hydraulic constructions and implementation of techniques and methods of river organization, such as stabilizing the shore, direct routing of floods and adjusting the longitudinal profile of the channel, construction of walls, aqueducts, leveling and leveling and adjusting the slope of the floor are among the protective works necessary for the continuation of peace. The movement of water and sediment and regime stability in natural rivers. that the sedimentary materials that move along with the water flow may be suspended in the stream or that the sedimentary materials may slide or roll on the river bed.

Methodology: In this research, first, using HEC-RAS software, the condition and mode of sedimentation of the longitudinal and transverse sections of the Kajo River located in Sistan and Baluchistan province were calculated and the results obtained with the data of GSTARS software to estimate the volume of sediment input to Zirdan dam reservoir. In a period of 50 years, it was estimated with the help of the relationship between sediment discharge and water flow rate and combining it with the continuous flow curve. The appearance of the Kajo river from the Zirdan dam to the connection to the Bahuklat river, according to the field survey, the river course can be divided into the following three areas based on its physical characteristics and flow: the arterial area upstream of the Kajo river and the meandering area in The middle area of the Kajo river and the delta area (plain) is in the downstream of the Kajo river. Kajo watershed is located in the geographic coordinates of 60°19'' to 61°20'' east longitude and 25°30'' to 26°48'' north latitude. Zirdan Reservoir Dam is also located 140 km north of Chabahar city on the Kajo River in Sistan and Baluchistan province.

Results and Discussion: After calibrating the GSTARS model and HEC-RAS, determining some important parameters that the model is sensitive to, the model was used to predict the amount and distribution of Zirdan dam sediment in the long term. Most of the sediments, which are mostly coarse-grained, are deposited at the beginning of the reservoir, and the delta resulting from this sedimentation is clearly identifiable, and this is because the water level in the reservoir is fixed and equal to the normal level, i.e. 265, in the plan. . that in case of having the amount of water during the operation of the dam and predicting this amount according to the flows entering the reservoir and the water demand downstream of the dam, the program can obtain accurate information about the location of the resulting delta. Downstream, the accumulation of fine-grained sediments carried by thick currents can be observed. The results of the model calibration showed that by choosing Young''s 1984 function as the relation of sediment transport, the number of three flow pipes and the roughness coefficient of 0.03, the model results have a better match with the results of experimental methods. Most of the sediments are deposited at a distance of 5 kilometers from the dam by forming a delta. Also, the result of calculations shows the volume of sediments in the tank after 50 years, 17 million cubic meters.



Conclusion: Based on the calculations made by the numerical model, the volume of 50-year sediments is estimated to be 21 million cubic meters. According to the results of the numerical model, the useful life of the dam, i.e. the period when 80% of the initial volume of the dam is replaced by sediments, is estimated to be 123 years. Also, the annual drop in the reservoir volume Zirdan dam is about 23 percent. As a result, considering the above conditions, the best function of sediment transfer according to the hydraulic conditions of the area is considered to be the Yang-Ruby function.