METHOD OF DETERMINING GRID SIZES OF FINITE-ELEMENT METHOD OF WATER-QUALITY MODELS FOR GEOMETRICALLY COMPLEX RIVER COURSE

The method of determining grid sizes of finite element method of water quality models is presented on the basis of characteristics of pollut ant distributed in the just downstream grid of discharge. A given leng th of the grid is applied to calculate the minimum distance for the po llutant to complete mixing in the lateral direction n the just downstr eam grid of the discharge, and the minimum distance is regarded as the maximum width of the grid. Application of the maximum width of the gr id and flow rate per unit width of the river results in the number of flow zones in the lateral direction of the river. Consequently divisio n of grids and numerical calculation of water quality with finite elem ent method can be carried out on the basis of available method and top ographic map of the river course. Analysis of sensitivity shows that t he effect of the width of the river and the effect of the length of th e grids on the minimum number of flow zones in the lateral direction i s relatively small, and that the effect of the given lateral mixing de gree of the pollutants on the minimum number of flow zones is acceptab le, which indicates that the method presented is relatively stable. Th e method can avoid the blindness and randomness in determining the num ber of flow zones, and can decrease the amount of computation under th e condition that the accuracy requirement is met. The problem that det ermining the sizes oi the grids must rely on empirical knowledge has b een solved.