Comparative Analysis of HEC-RAS, SWMM, and THDH Approaches in Highway Culvert Design

In this study, Hydrologic Engineering Center River Analysis System (HEC-RAS), Storm Water Management Model (SWMM) and Turkish Highway Design Handbook (THDH) approaches were examined in highway culvert design and the methods were compared on a sample highway culvert to show the differences in design calculations. A box type culvert, made of reinforced concrete and with dimensions of 3 meters in width and 2 meters in height, will be constructed at the intersection of the Kocaçay stream and the Nevşehir Avanos D302 highway at KM:800m. The culvert is designed to carry the 10-year and 100-year design flow rates calculated using the Rational method. The design flow conditions, and water levels of the culvert is originally designed according to THDH. The culvert along with the flow route up to a certain distance, upstream and downstream is modelled by using HEC-RAS and SWMM. The water levels obtained from the models were compared with the THDH design results. The hydraulic design of the culvert is conducted under inlet control conditions in all methods, however, with the discrepancies in the calculated headwater and tailwater depths. The nomogram method suggested in the THDH provides a practical means for determining headwater depth, tailwater depth and culvert dimensions. However, it does not adequately address the channel sections upstream and downstream of the culvert and the relevant flow conditions. To evaluate the environmental effects of the design flow rates of culverts, especially those located near residential areas, it is recommended to use HEC-RAS and similar GIS-supported modeling tools in hydraulic calculations.