Identification and separation of flooding source regions and investigating the impact of watershed management operations on the peak discharge (Case study: Bar watershed, Neyshabour, Iran)

Background and Objectives: The increasing flooding trend in recent years suggests that most of the country's regions are vulnerable to invasions of periodic and destructive floods. In this aspect, many cities, villages, industrial and agricultural facilities and residential areas are prone to flood occurrence, as well. Therefore, the basic identification of flood process within the catchment area is one of the most important measures in flood control and the mitigation of damages. The main objective of this research is to investigate and identify the flood areas and the effect of watershed management on flood peak discharge in the outlet of the Bar watershed, Neyshabour, located in Razavi Khorasan province.
Material and Methods: For this purpose, the basin was divided into 20 subbasins and the physical properties of the whole basin and subbasins were determined using the geographical information system oodand in a digital format. Then, by using the HEC-HMS hydrologic model, the corresponding flow discharges were calculated for each subbasin. Then, by successively deleting subbasins at each model runtime, i.e. Single Successive Subwatershed Elimination method (SSSE), the whole basin water discharge was calculated after the flood routing in the main streams without the subbasin by using the kinematic wave routing approach, Thus the effect/share of each subbasin in the production of flood is identified. Also, the flood discharge of the basin was calculated in the basin area unit and the flood index (f) was the basis for the priority of the basin.
Results: In calibration process, two parameters of curve number and manning coefficient were selected as the most effective parameters on flood discharge. The high Nash-Sutcliffe coefficient in flood events showed that calibration of the model in the watershed basin was appropriately done. The results showed that the subbasin B1 (in the northern part of the watershed) in the return periods of 50 and 100 years had the hieghst peak discharge of 38.9 and 44.1 cubic meters per second at the outlet of the subbasin, and the subbasins B11, B13 and B19 (in the western parts of the watershed) showed the minimum peak discharge. Also, according to the index (f), in flood plains with return periods of 50 and 100 years, the subbasins B4 and B3 (in the northern half of the watershed) ranked first and second, respectively and the subbasins B6, B11, B12, B13, B14 and B19 (in the southern part of the watershed and in the eastern and western parts of the watershed) showed the lowest priority in terms of their participation in basin flood. In subbasin B1, the highest level of peak discharge has been observed in the highest level of biological operations, ranging from 41.27 to 44.73 percent. On the other hand, the results showed that the higher the proportion of the biological activity to the subbasin area, the more obvious the role of these projects in reducing peak discharge. According to the study, the role of structural activities in reducing the flood peak is lower than biological activities, and increasing the number of structures along the river route will reduce the peak peak area of the subbasin.
Conclusion: By investigating the effect of biological activities and the construction of gabion check dams on the flood discharges, it can be said that the role of biological activities in reducing peak flow and flood volume is much more effective than structural activities (construction of gabion). Therefore, the CN factor is an effective and controllable factor for flood discharge of the basin, on reducing peak flow.