River water quality assessment: a hydrodynamicwater quality model for propagation of pollutants

A hydrodynamic river water quality model is presented. The model consists of a quantity and a qualitysub-model. The quantity sub-model is based on the Saint Venant equations. The solution of the SaintVenant equations is obtained by means of an explicit scheme based on space-time conservation. Themethod considers the unification of space and time and the enforcement of flux conservation in bothspace and time. On the other hand the quality sub-model is based on the advection dispersionequation. Particularly, the principle of upstream weighting applied to finite difference methods isemployed. Such a method enables us to reduce the numerical dispersion, avoiding oscillationphenomenon. The optimal weighting coefficient has been calculated on the basis of the mesh Pecletnumber. With regards to the quality processes the model takes into account the mainphysical/chemical processes: degradation of dissolved carbonaceous substances; ammoniumoxidation; algal uptake and denitrification; dissolved oxygen balance, including depletion bydegradation processes and supply by physical reaeration and photosynthetic production. To properlysimulate the river water quality four state variables have been considered: DO, BOD, NH4, and NO.The model was applied to the Savena River (Italy), which is the focus of a European-financed projectin which quantity and quality data were gathered. A sensitivity analysis of the model output to themodel input or parameters has been carried out.