The Role of Land Use Changing in Uncertainty of Design Flood of Hydraulic Structures (The Case Study about Madarsoo Watershed Basin)

To design hydraulic structures, predicting the discharge of flood is necessary, the main cause lies in failure that may occur in downstream. Most hydraulic structures were destructed because of the lack of resistance against floods with flow rate more than design flood, so selecting design flood is one of important decisions should be made as an effective factor accompany with other hydrologic factors in developing hydraulic structure projects.Selecting design flood depends on hydrologic and environmental factors of water shed basin, so changing each environmental factor affects on the discharge of design flood directly or indirectly and consists of the uncertainty in evaluating design flood. Therefore, protecting and controlling environmental factors and conditions affecting on watershed basin after performing great hydraulic structures projects are very important. In this paper it is tried to study the role of land use changing and the lack of consistency between present land uses and the land capacity that is one important environmental factors affecting on the numbers and amounts of regional flood and uncertainly of design flood.Therefore, after environmental and basic studies, the map of land uses and capacity were prepared and the flow rate of design flood was evaluated by the method of (SCS) and the model of Curve Number (CN) in present land use and land capacity. Comparing obtained results showed that the flow rate of design flood of hydraulic structures in field study at land capacity condition were 30 to 70 percent less than the discharge of design flood in the present land use condition and considering the change of land using during passing the time, the evaluation of design flood’s flow rate with respect to useful life time and stability of structures is not certain.