Mass conservative transport scheme for the application of the ELCIRC model to water quality computation

An attempt was made to couple the water quality model of Danshuei River to the three-dimensional unstructured-grid hydrodynamic model [Eulerian-Lagrangian circulation model (ELCIRC)]. The Eulerian-Lagrangian scheme for the solution of the transport equations of salt in ELCIRC was demonstrated to be not mass conservative. The scheme was replaced with a finite-volume/finite-difference upwind scheme to ensure mass conservation both locally and globally. The same scheme was also used for the scalar transport equation in the water quality model. The representation of mass flux in the scalar transport equation is carefully formulated to be consistent with that of volume flux used in the continuity equations of ELCIRC. It was demonstrated that the newly revised scheme (1) conserved mass locally and globally; (2) conserved mass for both conservative and nonconservative substances subjected to biogeochemical transformation; and (3) preserved the integrity of the wetting-and-drying scheme. Further, the baroclinic simulation using the newly revised scheme showed a better result in terms of salt intrusion and salinity distribution in the Danshuei River estuary.