Estimating the Peak Discharge and Analyzing the Hydrological Changes of the River Case Study: The Mountainous Section of the Qaranqoo Chay River

ABSTRACT
Knowing the maximum flow rate is necessary to deal with floods, and one of the main problems in watershed planning is the lack or absence of necessary statistics, especially hydrological statistics. Therefore, the absence or lack of hydrometric information in watersheds requires appropriate experimental methods to estimate the maximum discharge. Thus, this research aims to estimate the maximum discharge in the mountainous part of Qaranqoo Chay and analyze the hydrological changes of the river. This research relies on fieldwork, using satellite images and topographic and geological maps. In this research, the water velocity was first measured using the Costa and Jarrett method, and then the flood discharge was estimated based on that. The hydrological conditions of the river were also calculated according to the data of the upstream and downstream stations of the range and using 18 indicators. The research results show that the maximum flood estimated by the Costa method (89.60 m3/s) is closer to the observed values, which indicates the efficiency of this method. Based on the research findings, the connection of Saraskandchay, Shorchay, and Khatonabadchay branches, high slope and impervious formations, low cross-section and channelization of the flow, local blockage due to the fall of large stone fragments and the filling of the section can be considered as the reason for the high maximum flood values in the mountainous part of Qaranqoo Chay. In addition, the results of the study showed that the construction of the Sahand dam caused a fundamental change in the downstream flow regime, and these changes affected the hydrological characteristics.
Extended abstract
Introduction
One of the most vital topics in hydrology studies is the topic of flood and flooding, after which the necessity of investigating the maximum discharge of the river regime is raised. The absence or lack of hydrometric information in watersheds requires appropriate experimental methods to estimate the maximum discharge. Considering that there is no hydrometric station along the mountainous part of the Qaranqoo Chay River and considering the importance of the subject, the purpose of this research is to use the morphometric characteristics of the channel and the size of the rubble of the river bed to estimate the amount of flood discharge in this area.
 
Methodology
This research is based on fieldwork, using satellite images, hydrometric data, and topographical and geological maps, and it is descriptive-analytical in terms of its purpose and method. ENVI, Ecognition, Arc GIS, and Excel software have been used for image processing and data analysis. First, to estimate the maximum output flood, the data related to the morphometry of the channel, including the cross-sectional area, wet environment, hydraulic radius, and channel slope, were calculated according to field surveys and existing information layers. In the next step, the water speed was calculated. In this research, two Costa methods (based on particle diameter) and Jarrett method (based on the characteristics and morphometric conditions of the flow channel) were used to estimate the water flow speed. Then, the instantaneous peak discharge rate was calculated according to the obtained speed values. It has been obtained considering the area of the cross-sections, and finally, by comparing the discharge values obtained from the application of the method used in the current research with the observed and recorded discharge values from the studied area, the accuracy of the results has been verified. To investigate the hydrological changes in the study area, the data from Chapini and Mianeh stations have been analyzed using 18 hydrological indicators.
 
Results and Discussion
Using the Costa method, the maximum flow rate was calculated at 89.60 (m3/s), and based on the results of the Jarrett method, the flow rate was estimated at 137.87 (m3/s). Considering the flow return periods in the studied stations, it can be stated that the flow rate calculated by the Costa method at the Chapini station and the Haft-Mianeh tunnel has a return period of 10 and 5 years. However, the amount of discharge estimated by Jarrett's method at Chapini station and Haft-Mianeh tunnel, respectively, Flowhas 25 and 10-year returns. The comparison of the results generally shows that the data calculated by the Casta method is closer to the observed data. The results show that the main course of the river in the study area passes through Eocene rhyolitic, basaltic, and trachyandesite rocks. Due to the proper strength of the rocks in this part of the river path, the valley-river is full of twists and turns, and the eroded slopes are often rocks that have turned into bedrock due to alteration caused by igneous intrusions. Therefore, the areas of the study route with geologically and lithologically resistant surface formations and their permeability coefficient are very low, vegetation cover is insignificant, and they have a high peak flow rate and a more significant potential for flooding. In addition, many ravines overlook the valleys in the study area, and the valleys are V-shaped and deep. The rivers excavate and deepen their bed to achieve balance, which indicates that the topography of the basin is young. Therefore, erosion and transport of alluvial and sedimentary materials is very high in this basin.
 
Conclusion
Based on the sample of stone pieces taken from the river bed and its effect on the flood speed, it was determined that the strength and speed of the flood depended on the size of these stone pieces. The larger average of the b-axis of the rubble in the river bed indicates the high power and speed of the water. Therefore, the results of this research show that the maximum flood estimated by the Costa method is closer to the observed value. According to the research findings, geological formation conditions or rock types are related to the production of runoff and floods in the studied area of Qaranqoo Chay. In addition, due to the joining of Saraskandchay, Shorchay, and Khatonabadchay branches, the slope is high, the cross-section is low, and the channelization of the flow, the local blockage due to the fall of large stone pieces and the filling of the section, the speed and power of the river are high in this area. Also, the investigation of hydrological indicators shows that with the construction of Sahand Dam, the annual flood discharge of the river and the peak point of the flow downstream have decreased.
 
Funding
This article is derived from a research project funded by the Research and Technology Vice-Chancellor of University of Mohaghegh Ardabili.
 
Authors’ Contribution
All of the authors approved the content of the manuscript and agreed on all aspects of the work.
 
Conflict of Interest
Authors declared no conflict of interest.
 
Acknowledgments
This article is derived from a research project funded by the Research and Technology Vice-Chancellor of University of Mohaghegh Ardabili. Therefore, we express our gratitude.