Computationally efficient modelling of tidal rivers using conceptual reservoir-type models

•A surrogate modelling approach for tidally influenced river reaches is developed.•The approach is modular, flexible and compatible with other reservoir-type models.•A discrete and explicit calculation scheme that allows for very short calculation times is used.•A case study of the rivers Zenne and Rupel in Belgium was used to demonstrate the accuracy of the modelling approach.

Conceptual reservoir-type models of river systems, that mimic the results of detailed hydrodynamic models, provide a powerful tool for numerous river management applications. Thanks to their computationally efficient model structure they are very well suited for applications that require long term simulations or a large number of model iterations. However, one well-known problem is that conceptual models have difficulties to account for backwater effects. For this reason, their application to tidally influenced river reaches so far was almost non-existent. This paper introduces an extension of an existing grey-box reservoir modelling technique to incorporate tidally influenced river reaches. The methodology is demonstrated for the downstream part of the rivers Zenne and Rupel in Belgium. Results show a minor loss of accuracy when the detailed model is replaced by the surrogate conceptual model. Also, the tidal effects are well represented.