Document Type : Original Article
Subjects
Rivers are active flows that face an increase in discharge after each heavy rainfall, and as a result, they become flooded. River flooding is a serious threat to farmlands and residential areas around it. Considering that after the construction of the Shahrchai Dam, numerous floods have occurred downstream of the dam every year and caused a lot of damage, especially to farmlands, the need to research to identify areas at hazard of flooding along the Shahrchai River is felt more than ever. Therefore, the present research attempts to prepare a flood hazard zoning map for periods with different returns along the Shahrchai River route of Urmia using the HEC-RAS model and geographic information system (GIS) technique. These maps can be useful in land management programs for sustainable development and dealing with the region's flood hazards.
Shahrchai River is considered one of the most important rivers in the Urmia Lake basin, which plays a significant role in supplying the water needed by the city of Urmia, the villages in the Shahr Chai basin, as well as supplying water to Lake Urmia. This river originates from the border heights of Iran and Turkey, such as the Zarineh Mountains, with a height of 3100 meters, and Kamal Mountain, with a height of 3386 meters. After passing through Urmia city and irrigating the agricultural lands downstream of the basin, Shahrchai enters Urmia Lake from the east (Keshtiban village). The length of this river is about 60 km, and its basin area is 705 Km2. Shahrchai basin is located in Urmia county in terms of political situation. According to data from meteorological and hydrometric stations, the average annual rainfall in the basin is about 360 mm, and the average temperature is about 11 degrees Celsius.
In the present study, the HEC-RAS hydrodynamic model and various data and tools were used to investigate the hydrological behavior of the Shahrchai River in different return periods. In this way, the data of the hydrometric station of Urmia band and Keshtiban were used to study daily and monthly discharge during long-term statistics. To extract the land use map of the region, the Landsat 8 image of the OLI-TIRS sensor was used for July 2021 with the pass and row 169-34 and ENVI software. Supervised classification method and maximum likelihood algorithm were used to identify each user. Digital elevation model (DEM) with a pixel size of 12.5 meters were used to prepare the TIN layer. Next, HEC-RAS hydrodynamic software and HEC-GeoRAS extension were used to simulate the flood along the Shahrchai River.
The study and the maps showed that the Shahrchai River did not witness floods with higher discharge in the first reach during the return periods of 2 and 5 years. As a result, the river mainly floods in its bed, and during these periods, there is no damage to the farmlands and gardens along the river. However, in the return period of 25 years, we see flooding of the riverbanks and the floodplain, which will lead to damages. The evaluation of flood zones for different return periods shows that in the 10-year return period, 44 hectares of farmland and about 2 hectares of residential areas will be flooded, and damages will be caused to them. The second reach, also known as the urban reach due to its location inside the city of Urmia, is the only reach where residential areas suffer the most damage compared to other sections. The third reach has completely different conditions compared to the second reach. In this reach, farmlands include the most flood-prone areas. Also, due to the low slope of the area, the speed of the water flow decreases, and as a result, the depth of the water causes the flooding of the banks and the floodplain of the Shahrchai River to increase. The evaluation of the flood zones for the return period of 10 years shows that about 50 hectares of farmlands and 0.2 hectares of residential areas are flooded.
In the present study, flood zone maps were prepared separately for the return periods of 2, 5, 10, 25, 50, and 100 years for the first, second, and third reaches. The maps showed that in the first reach, in the return periods of 25 years and above, the most damage is done to farmlands and gardens. So, in the return period of 25 years, about 57 hectares of farmlands and gardens face the hazard of flooding. In the second reach, residential areas and urban facilities are among the areas involved in the hazard of flooding in return periods of 25 years and above. During this period, about 8 hectares of urban areas are threatened by floods, with a return period of 25 years. In the third reach, farmlands will witness the most flooding. The area of farmlands threatened by floods with a return period of 25 years reaches 87 hectares. In this reach, the village of Keshtiban and the agricultural lands suffered much damage from floods, with a return period of 25 years and more due to its location on the riverbank.
