Abstract
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.
Highlights
First efforts to understand the solute transport subject led to a longitudinal dispersion theory which is often referred to as the classical advection–dispersion equation (ADE; Taylor, 1954)
∂x where A is the flow area, C the solute concentration, Q the volumetric flow rate, D the dispersion coefficient, λ the firstorder decay coefficient, S the source term, t the time and x the distance. When this equation is used to simulate the transport in prismatic channels and rivers with relatively uniform cross-sections, accurate results can be expected; but field studies, in mountain pool-and-riffle streams, indicate that observed concentration–time curves have a lower peak concentration and longer tails than the ADE equation predictions (Godfrey and Frederick, 1970; Nordin and Sabol, 1974; Nordin and Troutman, 1980; Day, 1975)
The spatial derivatives are discretized by the QUICK scheme, which is based on quadratic upstream interpolation of discretization of the advection–dispersion equation (Leonard, 1979)
Summary
First efforts to understand the solute transport subject led to a longitudinal dispersion theory which is often referred to as the classical advection–dispersion equation (ADE; Taylor, 1954). Transient storage zones mainly include eddies, stream poolside areas, stream gravel bed, streambed sediments, porous media of river bed and banks and stagnant areas behind flow obstructions such as big boulders, stream side vegetation, woody debris and so on (Jackson et al, 2013) These areas affect pollutant transport in two ways: on the one hand, temporary retention and gradual release of solute cause an asymmetric shape in the observed concentration–time curves, which could not be explained by the classical advection–dispersion theory; on the other hand, it is affected by the opportunity for reactive pollutants to be frequently contacted with streambed sediments that indirectly affect solute sorption, especially in low-flow condi-
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