In Korea, the flood defense level has been typically determined by the classification of river grade. Each river interval should, in principle, have different level necessary to consider the flood risk behind the river, which however is not currently applied in planning the river due to the lack of scientific methods. Therefore, to develop a reasonable method for zoning the flood defense levels of river intervals, the methodology of the QRA (Quantitative Risk Assessment) originated from the reliability engineering was introduced to a river interval with highly urbanized area. First, the authors presented the FN curve showing the numbers of fatalities at various flood frequency conditions. Then, they evaluated the adequacy of flood risk comparing with the societal risk tolerance criteria for the river basin. Finally, they adjusted the flood defense level according to the distributions of population, land use, and buildings in riparian and checked whether it satisfied the risk criteria or not.