Abstract: Open-source code ELCIRC(Eulerian-Lagrangian CIRCulation) solves the shallow water equations using a semi-implicit Eulerian-Lagrangian finite volume/finite difference method with horizontally unstructured grids and vertically unstretched z-coordinates. ELCIRC source code aside from the governing equation solution is analyzed in this paper. The operation of interpolation,topological structure,definite condition,hierarchy information,and wetting and drying method are described in detail. Backtracking,as the key technology of ELCIRC,is expounded comprehensively. Moreover,some detailed problems are discussed,and a few references and suggestions are given to avoid the errors caused by similar number subtraction and by employing small number as the denominator. ELCIRC has been applied in practice and could serve as a valuable reference for developing hydrodynamic models.

Key words: ELCIRC , backtrack , source code , 3-D hydrodynamic model , algorithm