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.