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Abstract. One of the mechanisms that greatly affect the pollutant transport in rivers, especially in mountain streams, is the effect of transient storage zones. The main effect of these zones is to retain pollutants temporarily and then release them gradually. Transient storage zones indirectly influence all phenomena related to mass transport in rivers. This paper presents the TOASTS (third-order accuracy simulation of transient storage) model to simulate 1-D pollutant transport in rivers with irregular cross-sections under unsteady flow and transient storage zones. The proposed model was verified versus some analytical solutions and a 2-D hydrodynamic model. In addition, in order to demonstrate the model applicability, two hypothetical examples were designed and four sets of well-established frequently cited tracer study data were used. These cases cover different processes governing transport, cross-section types and flow regimes. The results of the TOASTS model, in comparison with two common contaminant transport models, shows better accuracy and numerical stability.

Abstract. Interactions between physical and chemical mechanisms involved in pollutant transport in rivers occur with varying degrees, depending on flow discharge and physical conditions. One of the issues that greatly affect the transport, especially in small mountain streams, is transient storage zones. The main effects include temporary retention of pollutants and reduce its concentration at the downstream and indirect impact on sorption process in the streambed. This paper proposes a one-dimensional model to simulate the pollutant transport in rivers with irregular cross-sections under unsteady flow with transient storage zones. The proposed model verified with analytical solution and comparison with 2-D model. The model application shown by two hypothetical examples and four set of real data that covers different processes governing on transport, cross-section types and flow regimes. Comparing results of the model with two common contaminant transport models show good accuracy and numerical stability of the model than other ones.

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