Estimating Flood Potential and Determining the Priority of Areas in terms of Flooding by Physical Factors to Combat Erosion in Hajiler Watershed

Floods and their consequences, with the intensification of human exploitation of nature in the early twentieth century, have had negative effects on vital ecosystems. Also, adverse effects of erosion, while destroying the harvest site, lead to reduced production capacity and degradation of physical and chemical properties of soil in lands. To manage this phenomenon, the factors of production and flood must be identified and then areas with high potential in flood production must be identified to enable the possibility of executive and corrective operations at smaller and risky levels. The level of flood areas in the country is estimated at 91 million hectares, of which about 42 million hectares have moderate to very high flood intensity. Therefore, knowing the flood situation of the regions is a necessity to prepare strategic plans for sustainable management of basins. The purpose of this study is to estimate flood potential and determine the priority of flood areas by physical factors to combat erosion in Hajilar watershed by using a combination of data and information based on the CN-SCS method and also to determine critical flood areas.

In this study, in order to investigate the potential and zoning of flood risk in the basin, at first, by studying and examining the foundations and theoretical background of the subject, the physical factors affecting the occurrence of floods were identified.Then, the required information was collected and layers of each of the proposed factors were prepared in Arc GIS 10.7 software and Arc-Hydro and Arc CN-Runoff extensions. In this regard, the information layers of the waterway network, level lines, and elevation classes were prepared using a digital elevation model with a scale of 1: 120,000. Lithological information layers were obtained using the geological maps of Siah Rud, Tabriz, belonging to the Geological Survey of Iran. The precipitation map of the basin was prepared using data from meteorological stations within the study area and also adjacent stations using IDW interpolation method. Land use layers of the area were obtained using the area land use map and monitoring using Sentinel 2 satellite imagery. The soil map of the region has been prepared using studies of natural resources of Arasbaran basin prepared by the Forests, Rangelands and Watershed Management Organization of the country and controlled with lithological conditions and other environmental factors. Vegetation of the area was prepared through NDVI index. Then, by extracting the number of curves (CN) and the amount of penetration (S), the layers were combined and the runoff height of the basin was estimated by the runoff curve number (CN-SCS) method, the units were classified by SPSS software and the priority of areas in terms of floods in the basin was determined. In order to determine the peak discharge of flood through the obtained data, first based on the proposed Schwab relation, the water accumulation time of all sub-basins was prepared separately and, finally, the maximum peak discharge obtained from this runoff was obtained.

The results of the SCS-CN model which was intended to determine areas with different flood potential in the basin and compare it with environmental factors such as slope, lithology, land use, rainfall, etc in the basin indicate that this model is highly capable of estimating the flood potential in different areas of the basin. Results of changes in the number of curves based on effective environmental factors were also presented in Table 1.The potential for runoff production was high in high altitudes with poor pasture landuse or dryland agriculture with poor permeability soil, as well as in residential areas where the city surface consists of impermeable or low permeability surfaces.Also, there was a high flood potential in the  dense and medium pastures with hydrological group B. Irrigated and rainfed agriculture with hydrological group A has the lowest runoff potential and thus has the lowest amount of flooding in the basin.By obtaining the runoff height in different parts of the basin and determining the levels of changes in the values through quartering, it was found that mainly the central and lower parts of the basin are in the first priorities in terms of flood potential. Poor coverage, low permeability, and low rainfall in this area are some of the factors that increase flood potential. The southern and central regions are areas marked by low sensitivity to flooding. By prioritizing areas in terms of floods and mapping them, the results can be used in watershed management operations at the level of high-sensitivity units to reduce erosion and damage. Taking into account the values of runoff height relative to the sub-basins and water accumulation time obtained based on the Schwab estimation method, the results showed that the flood volume and maximum peak discharge in the basins have a good relationship with each other and the highest maximum peak discharge is related to H33 and H18 sub-basins with volumes of 51.44 and 48.67 and the lowest is related to H12 and H29 sub-basins with volumes of 3.77 and 3.86 m3/s.

Floods are among the most important environmental hazards that cause human and financial losses every year. In the meantime, using the SCS-CN experimental model, as a method for estimating floods in basins with different environmental conditions, and the inter-environmental approach in it can be a useful solution in watershed management studies. Considering that human intervention has caused an increase in floods in all areas, providing methods for accurate flood estimation is one of the basic needs of the relevant responsible organizations. Accordingly, the present study was conducted to determine the priority of areas in terms of flood potential and the results showed that 9% and 51% of the basin areas are in the very dangerous and high-risk flood categories, respectively. According to the final map obtained, Areas with very high risk and high risk are mainly located in residential areas and in the lower areas of the basin, which are the first priority in programs related to water resources, especially flood control and watershed management in the upstream units. Also, according to the results, areas with medium risk potential occupy 23% of the area and 17% of the area has low risk potential. The results of the study confirmed the high potential of the studied area in terms of flood risk, so lands with very high and high risk are lands that should be protected and appropriate watershed management measures must be conducted to control the speed of floods and reduce soil erosion.