Design of drainage culverts considering critical storm duration

A method for designing road-crossing drainage culverts taking into the account critical storm duration is proposed. Based on estimated design floods, a hydraulic design approach is also proposed to optimise the dimensions and hydraulic variables of the culverts. Two small watersheds in Korea (Baran and Banweol) and thirty-five culvert design zones were used in this study. Critical storm durations were determined by applying Huff curves and unit hydrograph models such as the Nakayasu model, the United States Department of Agriculture Soil Conservation Service (USDA SCS) Curve Number model, the Clark model, and the WFRpaddy model. The American Association of State Highway and Transportation Officials (AASHTO) equation was applied to the thirty-five drainage culvert watersheds along segments of Highway No. 39 to define the optimum dimensions and hydraulic variables of the drainage culverts. The selected unit hydrograph models accurately estimated unit hydrographs for the study watersheds. Thus, the models can be considered a useful tool for computing peak runoff rates in Korean watersheds. For the other major watersheds studied, the design floods obtained by standard design were 50% greater than those calculated by the unit hydrograph models, indicating that the rational method, which is often used in culvert designs in Korea, predicts lower values than those of the standard design method. The design floods determined by the unit hydrograph models were higher than those predicted by the rational method applied in this study. Each unit hydrograph model calculated different values for double or triple rectangular-shaped culverts. With the exception of one watershed, the differences compared with the results from the standard design and AASHTO methods ranged from 0 to 10%. Consequently, implementing a design flood based on the critical storm duration is more appropriate than a design flood calculated using only the rational method. Thus, the design method using the concept of critical storm duration can be recommended to estimate a design flood. If the design method for drainage culverts developed in this research is applied to the standard design in Korea, a variety of subjective methods of estimating the design floods can be objectively achieved. Also, through incorporation of a Geographical Information System (GIS), a design which considers anticipated land use changes could be achieved.