عنوان مقاله [English]
As a developing system, the river always changes its location and morphological characteristics according to the time period, geomorphic factors, geology, water currents and sometimes due to human interventions (Rezai Moghadam et al., 2013). In fact, normally, the landscape is adjusted by internal management factors such as slope, direction of slope, topography and type of sediments and external management factors such as climate change, sediment change and vegetation changes. The form of erosion or sedimentation in the bed, removing the walls, changing the flow bed, changing and moving the rivers and changing the form of the river is revealed. (Schumm, 2005). In other words, these changes cause many changes around the river and create natural problems and human and financial losses. (Gregory, 2006) Khiauchai river is considered one of the most important rivers of Ardabil province, Khiauchai river from the junction point The rivers above the villages of Dizo and Moyil started in the south of Meshgin Shahr, and the streams flowing in the village of Moyil are one of the most important branches of Khiauchai. This river passes through a path about fifteen kilometers east of the tourist town of Meshginshahr. Many branches connect to Khiauchai River
Simulation of Khiauchai riverbed changes using CAESAR
In order to investigate the capability of cellular automation in simulating the morphological changes of the river system in the Khiauchai river basin, the Caesar model was chosen. Among the cellular models of landscape evolution, the Caesar model is a two-dimensional model of water and sediment transport, which is one of the most recent cellular river models. In this model, daily discharge is used as input for the hydrological model based on TOPMODEL. The size of the particles used in the model is 1 to 256 mm. After entering the data including: daily discharge and sediment size, the height model of the cells is updated simultaneously; Therefore, despite the complexity in practice, the Caesar model in interval mode only requires simple input data of topography
HEC-RAS hydrological model was used for the zoning of Khiauchai river flood channel. The HEC-RAS hydrodynamic model is a model developed and developed by the United States Army. In 1964, HEC developed a computer model, HEC-2, in order to provide appropriate solutions to increase efficiency and improve by helping hydraulic engineers in analyzing river channels and determining flood zones. HEC-2 was a model that quickly evolved into a full-featured program for analytical river calculations. Finally, its power and potentials were expanded in the following years to apply things such as bridge and embankment analyses. Due to the increasing popularity and efficiency of personal Windows-based analysis software, in the early 1990s, HEC introduced a Windows-compatible counterpart of HEC-2 called the River Analysis System (RAS).
Combined results of cellular automaton model (CAESAR) and hydrological model (HEC-RAS)
Based on this, first, the results of the cellular automaton model before and after the simulation were extracted, and then the amount of flood zones was extracted with the hydraulic model (HEC-RAS).
Calculating the return period of flood probability using Hyfran software
Floods cause many risks to human societies and human facilities, which cause significant geomorphological changes. Based on this, the data related to Khiauchai hydrometric station was prepared from the regional water company of Ardabil province. After preparing the annual maximum instantaneous discharge, Pearson type 3 distribution was fitted to the base 10 logarithm of the peak discharges using Hyfran software, and the maximum instantaneous discharge was calculated for different return periods. The flow of flood flow with return periods of 2, 3, 5, 10, 20, 50, 100, 200, 1000 years of Khiauchai River is shown in table (1), graph (1). Based on the results obtained from table (1), the lowest probability of flooding with a return period of 2 years is about 50%, and the highest probability of flooding with a return period of 1000 and 200 years is 99.9% and 99.5%, respectively.
The results before and after the simulation of the cellular automaton model (CAESAR)
According to the measurements that were done in the first section of the first interval and by drawing the transverse profile of the main channel (before simulation and after simulation), the changes of the two profiles were compared. These changes are both in the width of the channel and in the average height and in the geometric shape of the river. According to the transverse profiles, the changes of the channel were investigated. The average depth and width of the channel before the simulation is 1339 and 7 meters, respectively, and after the simulation, it is 1339.56 and 11 meters, respectively, diagram (2).
Figure (2) shows the values of the flood zone with a return period of 2 years. Based on the map obtained from the output of HEC-RAS software and referring to table (4) and the river bed changes before and after being simulated in the cellular automation model (CAESAR), the area of the Khiauchai river flood zone with a return period of 2 years is about It is 3.8 hectares. These floodplains mostly correspond to the morphologic changes of the river bed, which locally include the edge of the river channel. The width of the areas exposed to floods with a return period of 2 years is about 153 meters. In other words, with the return period of 2 years, the morphological bed of the Khiauchai River will not undergo such changes, and the floods created will not pose significant risks to urban and rural areas.
Finally, the largest flood zone with a return period of 2 years includes a part of the lower reaches of the Khiauchai River. However, due to their periodicity and high potential for forming the planform of the channel, these floods are of great importance in the formation of multiple forms and erosion and slow sedimentation and morphological changes of the Khiauchai river channel.
According to the flood zoning map (Figure 3), the effect range of floods with a return period of 50 years along the Khiauchai River increases by about 12.8 hectares. Also, the average flood width of 50-year floods reaches about 277 meters. Flood zones with a return period of 50 years include return periods of 2, 3, 5, 10, 25 years. These floods have flooded the agricultural lands around the Khiauchai river and even some residential areas have suffered damage and danger. Based on the results obtained from the output of CAESAR and HEC-RAS models, these floods rarely go out of their channels and lead to damage. Such floods lead to erosion and sediment production.
Based on form (4), the section containing floods with a return period of 1000 years along the Khiauchai river will increase by about 13.8 hectares. The average flood width of 1000-year floods reaches about 345 meters. These floods include all the floodplains studied in this research with different return periods. As a result of this increase in area and width, it can be seen in all the upstream, middle and downstream parts of Khiauchai River. In other words, during the return period of 1000 years, the floodplain of the Khiauchai River covers all parts of the river. Such floods can affect a major part of the area of the river flood plain due to the high discharge and participation of the discharges of different branches, and in addition to human and financial losses and destruction of agricultural lands, they have many morphological consequences such as changing short routes, shortcuts, etc. . Floods with a return period of more than 1000 years have affected the residential areas of the villages around the Khiauchai river and changed the height of the river bed in different periods and finally led to changes in the river bed and the amount of erosion, sedimentation and finally the changes in the shape of the river in be long-term
According to the simulation of Khiauchai River bed and flood changes using HEC-RAS hydraulic model and CAESAR cellular automation model, these results were obtained, which shows the very high spatial variability of flood risk and Khiauchai River bed changes. This variability originates from the variable geomorphological conditions along the river. The results of CAESAR model show that the simulation of the river changes before and after the simulation in section number 22 in the second period varied from 1518 to 1520, which shows the changes of the river bed in about 2 meters. In the simulation of cross-section number 44 of the third interval, the amount of changes in the river bed was about 1442.5 to 1443.5 meters, which shows the amount of changes of 1 meter. Transverse section No. 60 also shows about 2 meters, which in the third period accounted for the most changes in the river bed. The results of the HEC-RAS hydraulic model showed that the flood zone with a return period of 2 years covers about 8.3 hectares, a return period of 50 years covers 12.8 hectares, and a return period of 1000 years covers about 13.8 hectares of the area around Khiauchai River.