Influence of the method of determination of the longitudinal dispersion coefficient (D_L) in the simulation of water quality in rivers due to instantaneous releases of potentially polluting constituents

摘要

The use of mathematical models for the simulation of water quality in rivers is among the most important watershed management tools. For this purpose 1D mathematical models are commonly used due to their simplicity. In this case the longitudinal dispersion coefficient (D_L) is taken as the main representative hydrodynamic parameter of the spreading of potentially polluting constituents in water. Several methods are available in the literature for prompt evaluation of D_L, although it is recognised that most of them are inapplicable to natural water bodies. In this study a number of semiempirical methods of determination of D_L was used. The results obtained from their application on simulation of instantaneous release of constituents in a watercourse were compared to those obtained with the estimation of D_L using Fischer's Routing Procedure, which is based on a tracer technique. A case study was undertaken in Muqui do Sul River, in the state of Espirito Santo, Brazil. Eight simulations were carried out through the implementation of a routine to obtain the analytical solution of the one-dimensional dispersion equation. For each simulation it was assumed a different value for D_L obtained from different selected methods. The utilisation of different methods of evaluation of D_L resulted in significant differences in time and concentration parameters considered in this work related to the passage of a plume of constituents through a river control section. These results warn environmental numerical modellers that care should be taken when choosing a semiempirical method of evaluation of D_L with the purpose of assessing the impact of accidental releases of pollutant loads, taking place in short periods of time, in the river water quality.