Assessment and Zoning of the Risk of Flash Flooding Based on Physiographic Factors and Morphometric Indices (Case Study of Qasr-e Shirin Basin)

Sudden flash floods are generated by severe storms with high peak discharge (Abraham, 1984, p. 163) and are generally due to complex interactions between topographical, geological, geomorphological, and hydrological conditions (Abu Zaydou et al., 2016, 56). The flash flood is a complex phenomenon, whose prediction is very difficult (Cao et al., 2016, p. 2). The flash flood results in severe material damage and even human casualties and extreme erosion (Farhan & Iid, 2017, p. 718). It is the result of the activity of two groups of different parameters. The first group has meteorological features that vary in space and time, and the second group includes constant parameters including geomorphological and geological conditions (Josef et al., 2011, p. 755). The morphometric characteristics of drainage basins are significantly correlated with hydrological parameters (Maysa 2006, p. 1238) and the possibility of estimating their hydrologic behavior. Physiographic factors such as gradient, soil texture, land use, and rock permeability have different hydrological responses to precipitation occurrences in different basins. This affects the formation and characteristics of a sudden flood (Tinco et al., 2018, 595). Qasr-e Shirin Basin, due to the outcrops of Marne and Chile formations, geomorphologically, is an eroded area with a drainage network that is relatively dense and is susceptible to flash flood events due to heavy rainfalls. So far, there has not been any study to assess the risk of flash flood events in this basin since the assessment and zoning of the flash flood event in this basin is necessary. The purpose of this study is to assess and categorize the risk of flash flooding based on the morphometric and physiographic characteristics of Qasr-e Shirin Basin.
 

In this study, two methods of standardization of morphometric parameters and the FFPI model have been used. In the first method, 11 morphometric parameters were used to calculate the degree of risk. These parameters are calculated according to Equations (1) and (2).Equation1 :              HD =
Equation2 :              HD =
The MFFPI model uses six physiographic parameters to capture the potential hazard of a sudden flood. Each of these parameters has its weight and is classified into five classes. The weight of each parameter is multiplied in each of the five sub-parameters and the final score of each layer is calculated (Tinco et al., 2018, p. 596). In the next step, the six-layer layers are assembled in the Raster Calculator and the final map of the potential flood event is calculated (Tinco et al., 2018, p. 507). The layers of the topographic slope (S), flow accumulation (Fa), and amplitude curvature (Pc) are extracted from a 10-meter DEM. Lithology layer (L) from Geological map 1: 250,000 Qasr-e Shirin sheet, Land Use layer (LU) from modified land-use plan of Kermanshah province with 1: 100000 scale, and soil texture layer from 1: 250000 map of Kermanshah province. 

The studied basin has six sub-basins and the drainage network model in Qasr-e Shirin basin and sub-basins is dendritic. In the standardization method, the total sum of the degree values of the eleven morphometric parameters showed that Qasr-e Shirin Basin and sub-basins 1, 2, and 3 have a high potential hazard. Sub-basin 4 has a high potential hazard and sub-basins 5 and 6 have a potential low risk of flash floods. According to the final map, the Falling Flood Potential (FFPI) of Qasr-e Shirin Basin has the extremes of high, medium, low, and very low levels of flash floods. The highest and the lowest risky areas of flash floods have 33.63% and 9.86% of the basin area, respectively. Areas with very low and low risk of flood occurrences correspond to the highlands of the basin, high mountain ranges, and river valleys prevailing on the river bed. Areas with high potential risk and a large number of flash floods are in line with the erosion plain and hill.

Calculating the risk according to eleven parameters showed that 83.3% of the area of Qasr-e Shirin basin had a high risk, 9.5% had a potential hazard, 7.2% had a potentially hazardous risk. In fact, 93% of the area of the Qasr-e-Shirin Basin had a potential high and severe risk of flash floods. According to the second method, about 60% of the area of Qasr-e Shirin Basin had a high potential hazard, about 20% had had a moderate potential, and about 20% of the basin area had a potentially hazardous and very low potential. A review of the map from the MFFPI model showed that the high heterogeneity of this map was influenced by the heterogeneity of slope parameters, directional direction, and flow density. The comparison of the results of the two models suggested that most of the area of the Qasr-e Shirin basin had a potentially high risk of occurrence of a flash flood. The degree of risk method, which is based on the measurement of eleven linear, shape, and ergonomic morphometric parameters, presented the potential risk of a flash flood event for the entire basin. Since the drainage network is responsible for the discharge of the flood, the results had a high degree of accuracy in assessing the risk of a flash flood event in the whole basin. But the MFFPI model used the effective physiographic parameters for creating floods in flood risk zoning and it identified high-risk areas within the basin. Finally, it can be admitted that the results of the two methods, despite differences like the parameters used, are complementary to each other. Based on the results of these models, the Qasr-e Shirin Basin had a high potential hazard in the event of a sudden flood event and the city of Qasr-e Shirin is in a very high-risk zone. Therefore, the Qasr-e Shirin Basin requires the implementation of protective projects and flood control.