The Identification and Ranking of Flood-Prone Areas in the Alluvial Fans, North of Izeh, Khuzestan Province

  Flood is one of the most important hazards in the alluvial fans and its analysis is associated with many complications. The term alluvial fan flooding refers to only a specific type of flood hazards that occurs only on alluvial fans (NRC, 1996). These floods in alluvial fans are characterized as high flow velocity, different flow paths, very active erosion, transport, and deposition processes. Many of the rural and urban areas of Iran which are located in areas of alluvial fans are potentially at risk of flood. Therefore, the identification of high risk areas of flood at different scales can be useful in managing them. This research was conducted with the aims of estimating the flash flooding in the northern alluvial fan of the city of Izeh, identifying potentially hazardous areas in terms of flooding, and prioritizing them for management purposes. The study area with an area of ​​75 km2 is located in the north of the city of Izeh and the Lake Miangaran. The mean elevation is 1470 m. According to the Izeh meteorological station, the average rainfall in the region is 637 mm and the average annual temperature is 23 ° C.

The boundaries of the watersheds and alluvial fans were separated and mapped using Google Earth Images of 2016. The potential of flood hazards was studied in three main steps: (1) Identifying active and passive zones of alluvial fans; the geomorphic indices including intersect point of alluvial fan, braided drainage pattern, and alluvial fan topography profile were used to identify the active and passive zones of alluvial fans. (2) The estimation of runoff and discharge with SCS method; the Ghahreman and Abkhezr method (2004) was used to calculate the amount of rainfall during different return periods. The information layers of the soil, land use, and vegetation cover were prepared from maps of the Natural Resources Administration of Khuzestan province. By combining the information layers, the curve number (CN) values ​​for different basins and weighted average were calculated. Using the data such as catchment area, rainfall and its duration, number of the curve, length of stream and its slope, peak of flood discharge ​​and time to peak were calculated. (3) Ranking of the risk areas by TOPSIS method; in the TOPSIS method, the n×m matrix is ​​evaluated for a decision that has m option and n criterion. The basis of this technique is based on the notion that a selective choice should have the least distance with the positive ideal solution (best possible) and the greatest distance with the negative ideal solution (the worst possible condition). In the study area, twelve catchment- alluvial fan systems were identified. The active areas of the alluvial fans were 5 to 100% of their total areas. Based on soil characteristics and surface coverage, the average relative weight of CN varied from 78 to 90. The rainfall was calculated at 30, 60, 120, 180, and 360 minutes at the return periods of 2, 5, 10, 20, 50, and 100 years. Due to the small size of the basins and the short duration of concentration, the rainfall of 120 minutes with a return period of 10 years, which was 36.77 mm, was considered as the peak estimation criterion. To assess the ranking areas in the TOPSIS method, four criterion including flood peak due to precipitation of 120 minutes with a 10-year return period, time to peak of discharge, active alluvial fan area, and area of ​​villages located in active alluvial fan were used. Twelve studied alluvial fans also appeared as options in the matrix. The results of the flood hazards ranking in the alluvial fans of the study area showed that fans of 2 and 5 were the villages of Perchestan Gurii with a population of 1168 people and the village of Pershestan Ali Hossein Mola with a population of 317 which had respectively had proximity coefficients of 1 and 0.4481 in rankings 1 and 2. Within the studied area, the active areas of the alluvial fans are considered to be a major contributor to the flood hazard ranking so that the variables of the alluvial fans area and the area of ​​villages based on them account for 77% of the weight of the ranking. Hence, the determination of the active regions of the alluvial fans can be used on a regional scale using Google Earth satellite 3D images. Using multi-criteria decision-making methods such as TOPSIS can rank the flood hazards in the alluvial fans of north Izeh with regard to the influential variables. This regional-level ranking can show areas at risk and, if necessary, detailed geomorphological studies and field studies will be needed.