Effect of morphometric factors in prioritizing flooding of sub-watersheds in the north of Birjand Plain

Flood is one of the most important threats to human society, which has increased in recent decades with the increase in population and climate change. Therefore, studying the features of the basins, which are related to the level of flooding, can help in the correct management of this riskWatersheds are different in morphological characteristics, so they have different hydrological reactions in the same climatic and environmental conditions. The stability of the morphometric characteristics of the sub-basins has made them used in flood studies. So that the investigation of the morphometric factors of the basins can be of great help to the management and prioritization of flood sub-basins.In this research, an attempt has been made to introduce and prioritize flood-prone sub-basin using morphometric data in the environment of the geographic information system in the north of Birjand Plain, using the maximum entropy method and VIKOR decision-making model.

Location of Birjand plain This plain is located in longitude 58 degrees 45 minutes to 59 degrees 30 seconds and latitude 32 degrees 30 seconds to 33 degrees. The area of Birjand Plain watershed is 3155 square kilometers, of which 1045 square kilometers are plains (33%) and the rest are highlands.In this research, morphometric factors (13 factors), entropy method and VIKOR decision making method were used to prioritize 22 watershed sub-basins in the north of Birjand Plain. Since the morphological factors of the aquifer basin have different effects on the processes of runoff formation, there is a need to determine the effects of the parameters, which Shannon's entropy method is used in this research. In the following, using the VIKOR method, sub-areas are prioritized.

Morphometric factors are important in identifying and determining flood risk areas. The effect of morphometric factors on flooding is not the same, so it is necessary to determine the importance of each parameter. Shannon's entropy method was used to determine the impact of different morphometric factors (14 parameters) on flooding in the northern Birjand plain watershed (Table 2, row 2). The results showed that the index of the Vf parameter with a relative importance of 0.51 had the greatest impact on flooding and prioritization, which is consistent with the results of Shirani and Chavoshi (2017), Rahmati et al. (2014) and Amiri et all (2017) does not match, and it is consistent with the results of Beheshti Javid et al. (2017) that different areas are different in terms of flooding and are influenced by morphometric factors and land cover. The indicators of channel maintenance and meandering index of the river had the least impact on the flooding of the sub-basins of the studied area.In prioritizing the flooding of sub-basins, all factors are not the same in terms of influence, so some factors have a positive effect and some have a negative effect on flooding, for example, the more the amount of drainage density increases, the more it has a positive effect and causes an increase in flooding. It is possible that if the parameter of the channel maintenance index is lower, the flooding will be more. This attitude and point of view has not been seen in many studies, but in this research, the nature of the parameters was determined based on the opinions of experts and previous studies. In this study, the increase of basin asymmetry indices, tissue ratio index, channel maintenance index, and valley width to height ratio index have a negative effect, that is, with their increase, there is less flooding, and other factors have a positive effect, that is, with their increase, flooding and runoff production also increases, which is consistent with the research of Esmaili et al (2016).

Watershed projects are one of the most important strategies for flood control. The implementation of watershed projects for flood control requires the determination of flood producing areas and their prioritization. Considering the lack of sufficient hydrometric stations at the level of the sub-basins and the lack of recorded statistics and information about floods in the country's watersheds, it is practically impossible to infer the severity of sub-basin floods from the available data alone. Therefore, it is appropriate to use methods such as morphometric factors that require less hydrometric and quantitative data. There are various methods for determining the amount of runoff and prioritizing sub-basins in terms of flooding, and most of these methods are based on graphical methods and the use of empirical formulas, statistical analysis of flood data, and basin separation. In this research, 14 morphometric parameters were used to prioritize floods in the north of Birjand Plain, because the morphometric characteristics of each watershed sub-basin is like its fingerprint, and it is possible to prioritize sub-basins for flood control based on that. In this research, subareas 9, 22, and 14 were prioritized 1 to 3, respectively, based on morphometric criteria.