Flood Hazard Zonation by Combining SCS-CN and WLC Methods (Case study: Khiyave Chay Meshkinshahr Basin)

  One of the most striking natural hazards in the world is flood which generates a lot of financial and human losses every year. It can be said that in comparison with other natural hazards, it occurs with high abundance and in vast expanses. Some of its causes can be severe or prolonged rainfalls, melting, breaking the dam and landslide, high waves, channel closure, rainfall intensity, type of rainfall, time and volume of rainfall, previous river conditions, drainage basin, inappropriate use, and falling of forest trees in the sources of the rivers. Knowing susceptible areas to floods is one of the basic measures in natural resource management and development planning. One of the most important flood management methods is flood zoning. The zoning of potential flooding is to identify and describe areas with potential for surface runoff. The Khiyav Chay Watershed Basin, with an area of 318 km2, is located in Meshgin shahr. Due to the specific circumstances of the region, such as topography, slope, and climatic conditions (sudden precipitation and spring precipitation, melting, flooding of rivers in the spring), there is a high potential for flood occurrence. Therefore, the purpose of this research was to study the area's potential for flood occurrence.

In this study, ten factors of slope, height, rainfall, CN, runoff height, distance from the river, soil, lithology, vegetation, and user-use were identified as effective factors for flood formation in the region. Using Landsat 8 images including OLI and TIRS sensors and the Maximum Likelihood supervised classification method, in the ENVI 5.3 environment, the land use map was obtained. Then the user map was compared with the index table and integrated with the hydrologic group data, and the CN curve number was prepared. In the next stage, with mean precipitation and CN, and by using SCS method, ARC GIS software and Arc-Hydro and Arc CN-Runoff subtraction, the runoff height of the range was calculated. Also, the NDVI index, one of the most widely used indices for vegetation monitoring, was undertaken to prepare a vegetation map of the basin. Then, the other layers of information were provided in the GIS environment. The weights of the layers using the Critical method based on the correlation, interference, and standard deviation of the factors were determined. The final analysis and modeling was done using the WLC model as one of the methods of multi-criteria analysis techniques. By studying the zoning of the potential flood area of the study area and comparing it with each of the standard maps, it was concluded that the high risk areas were mainly in the hilly and mountainous areas of the area (slope over60%). Due to the slope and elevation of the area, the main role was with runoff, flood discharge, penetration, precipitation losses, and flow and water velocity. In these high risk areas, due to the fact that most of the formations belong to the formation of volcanic activity in the late third and early fourth centuries, the degree of permeability was very low but the runoff and CN amount were high. Secondly, areas with potential hazard were located within the urban boundaries of Meshkinshahr. In the city of Meshgin Shahr, on the east side, is the deep valley of khiyave chay, where the khiyave chay River flows. Two other radial valleys in the natural pathway formed the surface water stream, along which residential neighborhoods were developed that were subject to flood and extreme flow of surface water. Due to the fact that most of the city is made up of asphalt and residential surfaces, the permeability was very low, in contrast to the amount of runoff (99%) and CN (curve above 8). According to the results of weighing, height factors with weight coefficients (0.173), lithology with weight coefficients (0.163), slope with weight coefficient (0.139) and rainfall with weight (133/0) were the most important factors on flood formation in the region. The results of the study showed that 13.33% and 22.88% of the study area were in high risk and high class. According to the final map, high-risk areas, in the first priority, were mainly in the hilly and mountainous regions of the region, but in the second priority they were within the urban boundaries (especially in the central regions of the city due to lower construction and permeability). The results of the study also indicated that due to the high potential of the study area in terms of the risk of flood, water protection and protection measures at the basin level should be considered. In addition, the simultaneous use of remote-sensing and GIS and using the SCS-CN model could be useful in preparing a flood zoning map.