Abstract: An unsteady flow simulation of tidal reach based on HEC-RAS is established to calculate the backwater of Jiangsu-Nantong Bridge. Furthermore, the standard equation from Survey and Design on Hydrology of Railway Engineering, the energy equation, momentum balance method and Yarnell equation of HEC-RAS software are compared and respectively applied to the calculation. The calculated results of energy equation and Yarnell equation agree well with the standard equation, while the momentum balance method has a big deviation. Moreover, the sensitivities of HEC-RAS methods are analysed. The energy method is affected significantly by the bridge cross-section and expansion (contraction) coefficient, while Yarnell equation is dependent on the experience factor, and momentum method is related with the ineffective area which need more details for research. The results could be taken as reference for the unsteady flow simulation of bridge backwater.

Key words: HEC-RAS, tidal reach, backwater, Yarnell equation, sensitivity analysis