A variable parameter bidirectional stage routing model for tidal rivers with lateral inflow

Water stage variations are complicated in tidal rivers due to the combination of upstream flood and downstream tide. This study proposed a bidirectional stage routing model (BSR) for tidal rivers by considering the stage hydrograph as the overlay of bidirectional propagation of upstream flood wave and downstream tidal wave. The basic stage routing equations derived from the storage equation and water balance equation are used to calculate the upstream flood wave routing and the downstream tidal wave propagation, respectively. The lateral inflow can be directly specified as an additive term in the basic equation of BSR model or indirectly estimated by a lateral flow rate reflecting the proportion of lateral inflow to the upstream water. Parts of the model parameters are time-variable to account for the effect of water surface slope on water stage. The model inputs include stage hydrographs at the upstream and downstream boundaries. The channel geometric characteristics data are not required when the average channel width is estimated as a parameter. The case study results in the tidal reaches of the Caoe River and Qiantang River in China demonstrate that the BSR model performs well in water stage simulation. With relatively simple structure, modest data requirement and stable performance, the physically-based BSR model has the potential for water stage routing in tidal rivers when channel geometry data are not available.