studying risk of flood in hydrologycal and water resources engineering projects

studying risk and frequency analysis of flood is done in order to design structures located in floody lands. in hydrological and water resources engineering projects, first of all, we should determine frequency of this phenomenon- their return period. in this research, the method of planning, the cumulative freguency curve is explained and probability and return period and errors concerning small quantities of probability are discussed. in addition, the application of probability theory about risk of flood in hydrological and water resources projects will be investigated. a table and diagram is designed and presented to describe return period according to expected life of the plan for different levels of probability.