It is required to estimate reliable design floods for hydraulic structures in order to respond more effectively to recent climate change.
In this study, differences of design floods that were estimated the flood frequency analysis (FFA) and the design rainfall-runoff analysis
(DRRA) were analyzed. In Korea, due to lack of measured flood data, the DRRA method is used in practice to determine the design
floods. However, assuming the design floods estimated by the FFA as true values, the DRRA method over estimated the design floods
by 79%. Thus, this study proposed a practical method to estimated design flood in ungauaged watersheds through regressive adjustment
of flood quantiles estimated from the DRRA method. To this end, after investigating the differences between design floods acquired
from the FFA and the DRRA method, nonlinear regression analyses were performed to develop the adjustment formulas for 8 large-dam
watersheds. Applying the adjustment formula, the accuracy was improved by 65.0% on average over the DRRA method. In addition,
when considering the watershed size, the adjustment formula increases the accuracy by 2.1%p on average over when not considering
the watershed size.