In this paper the methodology of the coincident frequency analysis is presented. Using the HEC-SSP program the coincident frequency analysis of flood waves at the confluence of the Soča and Vipava rivers was performed. The probability distribution of water level of the Vipava River in Sovodnje was assessed on the basis of measured discharge data of the Vipava River in Miren and calculated discharge data of the Soča River at the confluence with the Vipava River. In order to obtain the response curves of the water level of the Vipava River at Sovodnje, simulations using a one-dimensional hydraulic model HEC-RAS were performed. The water level of the Vipava River in its lower reach near the confluence is highly influenced by increased discharge of the Soča River at the confluence with the Vipava River, which causes backwater of the Vipava River. In order to reduce the probability of flooding of the Vipava River at its lower reach near the confluence, the backwater effect should be reduced first.
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