Flood Prioritization of Dehbar Watersheds in Khorasan Razavi Province using TOPSIS Model, Morphometric Analysis and Regional Flood Frequency Analysis

The watershed is an ideal unit for natural resources management and for adjusting the influence of natural hazards on sustainable development. The prioritization of watersheds is a classification of sub watersheds based on the conditions of existing resources and the intensity of erosions and floods, which ultimately leads to the conservation and management of watershed operations in sensitive sub watersheds. Flood is one of the natural disasters that generate a lot of damages every year in different points of the world. Identifying potential flooding in sub watersheds is fundamental to decreasing the effects of natural hazards. The physical characteristics of a watershed are some of the most effective factors on natural hazards occurrence, and meteorological, hydrological, and water and soil conservation issues are directly and indirectly related to it. The morphometric characteristics of a watershed can be used to describe its hydrologic behavior. Compared to other natural hazards, this phenomenon occurred with great frequency and in a wide range. Investigation of effective parameters in flood occurrence in sub-basins through models such as multi-criteria decision making system and morphometric analysis can be helpful in determining the role of each sub-basin in flood occurrence

In this study, Dehbar watershed in Khorasan Razavi province was divided into 10 sub-watersheds. Dehbar watershed is located in Torqabeh-Shandiz city, 10 km west of Mashhad and in the south of Torqabeh city. The area of this area was estimated at 115.73 square kilometers. In order to evaluate more accurately and identify as many existing talents as possible, the study watershed has been divided into smaller hydrological units that have been studied separately. Therefore, Dehbar watershed was divided into 6 hydrological sub-basins and 4 non-hydrological sub-basins. And then 13 indicators and criteria including Area, Gravelius coefficient, Drainage density, Roundness coefficient, Form coefficient, Curve number, Branch ratio, main waterway length, Average slope, Average height, Concentration time, rainfall and runoff coefficient were selected. Analytic Hierarchy Process (AHP) was employed to calculate weights of Parameters for each sub-basin. TOPSIS model, Morphometric Analysis and Regional Analysis were used for prioritization. Finally, the findings of these three methods were combined using the mean ranking method.

The results of pairwise comparisons between the criteria showed that the criterion of runoff coefficient with a weight of 0.219 has the highest weight and importance among the criteria. And Rainfall indices, concentration time and curve number with relative weights of 0.148, 0.116 and 0.109 are in the next ranks, respectively. The results based on TOPSIS method showed that Sub-Watershed number (1) had the highest flood potential and Sub-watershed (3) and (2) are ranked second and third, respectively. Also, sub-area (7) has the lowest flood potential. The results of morphometric analysis also showed that sub-basins number 3, 1 and 8 have the highest potential for flooding. Also, sub-basin number 7 and 10 are in the final priority. The results of regional flood analysis method also showed that sub-basins number1, 3, 8 and 2 have higher peak discharges, respectively. Finally, these three methods were combined by the ranking average technique to achieve the final prioritization. The results of the integration of techniques showed that sub-watersheds number 1, 2 and 3are in the primary ranks. And sub-watersheds number 5, 10 and 4 are in the last ranks.

It is recommended that in watershed management measures aimed at combating and controlling floods in watersheds, in order to save costs, high-risk areas that have the highest flood watershed priority be given priority. Therefore, in these areas, flood control watershed management operations such as construction of small structures, strengthening of vegetation and land use management should be done.