The problem of weighting variables in the occurrence of complex phenomena such as floods is a challenging issue. In previous research, several variables have been used to evaluate floods. The use of variables depends on the environmental conditions and the availability of data (Du et al. 2022). The criteria of flow accumulation, distance from the river, slope, and elevation respectively have obtained the most weight in the AHP method. In the AHPS method, the factors of slope, flow accumulation, elevation, and MFI have obtained the highest weight values and the weight of the factor of distance from the river has been greatly reduced. Previous studies carried out with similar environmental conditions in Iran, which were weighted by AHP method, show that the two factors of slope and distance from the river are the most important influencing variables in flooding in the region (Arabameri et al. 2019; Shahiri Tabarestani and Afzalimehr 2021; Mousavi et al. 2022).
In the SE method, which is objective and based on past floods, the factors of distance from the river, elevation, and slope were assigned a higher weight. The flood hazard assessment in the adjacent basin (Tajen basin) of the study area, which was carried out with the objective method and correlation matrix, shows that the variables of slope, distance from the river, and elevation are the most important factors affecting floods (Avand et al. 2021). The weight of the geological factor was the lowest in all the methods used in this study. Placing all flood points in one lithology has reduced the weight of this factor. The results of the work of Khosravi et al. (2016) in the Haraz basin (North of Iran), which was carried out using Shannon's entropy method, showed that the variable distance from the river and geology had the greatest and least influence on the flood hazard map.
In the SEA method, the order of the weights is the same as in the SE method, but their values have changed. Considering the average weight of these four methods, the weight of the variables of distance from the river, elevation, and slope had a greater effect on the flood hazard assessment in the Talar basin (Fig. 6). The studies conducted worldwide show the difference in weight of variables with environmental conditions.
Figure 6: Weights of effective criteria in floods with different MCDM methods
Most previous studies that have used the SE method for flood hazard assessment have used the frequency ratio (FR) method. This method was initially used in the present study, but it displayed an exaggerated weight for the accumulation flow variable, which practically made it impossible to use this method. Although, in other research related to this topic (Shannon Entropy), the flow accumulation variable was not used.
The spatial distribution of the very high class in the FHI map is mainly limited to the main bed of the river, and the high hazard zone is located at a maximum distance of 300 meters from the river. The effective factor in this map is the accumulation flow variable, which has a high weight; so, only the pixels of the flow path are placed in the very high susceptible class. This factor has caused some of the observed flood areas to be located further away from the river channel and in medium and low-class zones.
In the FHIS map, the weight of the slope factor was higher than other criteria. This factor caused the plain area, which has a slope of less than 5 degrees, lie in the high susceptible class, and even up to a few kilometers from the river, which according to reports and evidence has no floods, are also in this class. Therefore, it seems overestimated in identifying the highly susceptible class. The factor of distance from the river has attained more weight than other variables in the FHI-SE map. Therefore, the buffers of 300 meters and 300–1000 meters from the main channel of the river, are placed in the very high and high susceptible class. Generally, in the Shannon Entropy method, the greater the dispersion of the values of an attribute, the higher the weight. The factor of distance from the river in the FHI-SE method is given a higher weight due to the distribution of flood points at distances further from the river. The effect of the elevation factor is also seen as the second most effective variable in this map, so that the elevation classes of > 1500 meters and 200–1500 meters, except for the rivers, lie in the very low and low hazard classes, respectively. The weight of the distance from the river was increased in the FHI-SEA map compared to the FHI-SE map. This factor caused buffers < 100 and 100–400 meters to be placed in very high and high classes, respectively.
In this study, the variable distance from the river was considered different for mountainous and plain areas. To investigate the effect of this variable in mountainous areas, in another map, the classification of the distance from the river was considered the same for the entire basin (mountains and plains). Based on this new classification, the final flood hazard map was prepared with the FHI-SE method. In several places as examples, valley cross-sections are drawn in a very high and high flood zone (Fig. 7). The depth of the valley in these cross-sections varied from 50 to 150 meters and the width of the valley was 200 meters at most. Considering the depth of the valley, the area of the catchment area, and river discharge in floods, this depth will never be at risk of flooding in the current climatic conditions. However, in the flood hazard map, which is displayed in two dimensions, this area is placed in the high-risk class. This issue leads to the exaggeration of flood zones in the mountainous region. In the original FHI-SE map, the sum of two very high and high susceptible zones was 4.5%. While by applying the distance from the river in the same way in the mountains and plains, the area of these classes has increased to 7.1%. Therefore, by making a difference in the classes of the distance from the river according to the ratio of the depth to the width of the valley, it is possible to reduce the exaggerated areas in the very high and high-hazard classes.
Figure 7: In this map, the distance from the river (buffers) is considered the same for mountainous and plain areas. Sample cross-sections were drawn from very high and high-hazard zones.