Finding potential of flooding for Amameh Basin’s sub-basins

Although floods are natural phenomena, human activities and interventions into the processes of nature, such as alterations in the drainage patterns from urbanization, agricultural practices and deforestation, have considerably changed the situation in all rivers' basins. As the result of these changes, exposition to risk and vulnerability in flood-prone area have been growing constantly. Understanding of this damaging phenomenon and adapting human activities based on legality of volatility are among the most important and fundamental scientific and practical methods to deal with the flood damages to life and property. It is necessary to promote and harmonize changes in water policies and land use, as well as environmental protection and nature conservation, in order to improve flood management in the frame of integrated river basin management. Due to large extent of basins, and economic and administrative constraints, reclamation of basins not only is impossible from point of view of the flood control in an individual project, but may have inverse effects. As regards that flooding and its damages is increasing in the most basins of Iran, determining flood productive areas and prioritizing sub-basins for comprehensive management of basins and flood control projects is very important. Therefore, assessing influence of different return periods on the intensity of flooding and run-off producing, and also determining of areas with maximum role on flood production in Amameh Basin has been done, in this study. Although the runoff resulting from rainfall is a complex and non-linear process, its modeling which is not too easy to run was performed in this research. One of the most important aims in this study was checking an applied and systematic method for identifying and prioritizing regions, regarding flood controlling operations and determining its function for other sub-basins in a system of actions and reactions. In the current research, ArcGIS software and HEC-HMS model were combined and the influence of different return periods on the intensity of flooding and run-off producing were considered. The contribution rate of different sub-basins in outlet flood of Amameh basin has been determined based on 2, 5, 10, 20, 25, 50 and 100-year return periods. The study area is located in northeast of Tehran City and southern hillside of Central Alborz Mountains. Among six main basins of Iran, this area sits in The Central Basin. To achieve the objectives of this study that is Sub-basins flooding priority, the Single Successive Sub-watershed Elimination (SSSE) method from rainfall-runoff model on HEC-HMS model has been used. A digital elevation model (DEM) for study area was also prepared and used to estimate the region’s hydrologic and geomorphic properties (including: basin shape factor, circular rate, main stream length, mean slope of basin and sub-basins, equivalent rectangle length and width, area, and so on). Furthermore, extracting main drainage network and dividing Amameh basin into five sub-basins was done by using this DEM. Finally, Sub-basins flooding priority map was extracted based on the (f) factor. The results of this study showed that the contribution rate of sub-basins in total basin’s output flow was not only affected by area and peak flow of sub-basins, but the interaction of many different factors such as location of sub-basins, distance from outlet, curve number (CN), and flood routing role in main river, also had significant impact on the sub-basins flooding. This research indicated that it is possible to study the interaction of climatic conditions and physiographic factors on flooding potential of basins and sub-basins, by combining ArcGIS software and hydrological models. Moreover, influence of time changes and different return periods on various sub-basins’ run-off potential can be studied desirably. The results of this investigation illustrated that the contribution rate of different sub-basins in total basin’s output flow varied in different return periods. Therefore, to reduce the impact of sub-basins flooding in the basin’s outlet, a reasonable return period should be considered to carry out the basin management practices for flood mitigation with a higher certainty. In addition, flooding potential and run-off producing had an ascending trend with increasing of return period duration. Also, upstream sub-basins were more important in the output peak flow and run-off producing. So, it is recommended that corrective and flood controlling operations perform on the upstream instead of the downstream.