VSP International     Science Publishers    P.O. Box 346, 3700 AH Zeist Volume 1, 2004, pp. 1$3 The Netherlands Numerical Simulation of Two$Dimensional Dam$Break Flows J.V. Soulis and A.J. Klonidis 1 Department of Civil Engineering, Fluid Mechanics/Hydraulics Division, Democritus University of Thrace, GR$671 00 Xanthi, Greece Received 7 March, 2004; accepted in revised form 10 March, 2004  The present study presents a second order accurate implicit numerical scheme for the calculation of unsteady, two$dimensional depth averaged, free$surface flow problems. The introduction of a non$ orthogonal, boundary$fitted coordinate system allows the accurate simulation of irregular geometries. The model is used to analyze dam$break flow in a converging$diverging flume. The computed results are compared with measurements and satisfactory agreement is achieved.  Τwo$dimensional, unsteady, free$surface, implicit, second order, finite volume    Stokes and Navier$Stokes equations  35Q30   From the most impressive man$made structures, dams become extremely harmful if sudden collapse occurs. Tons of water surge downstream causing very serious environmental catastrophe in an extended area. From the numerical analysis point of view, the simulation of such rapidly varying water flow has always been attracted to the researchers. Katopodes and Strelkoff 5 presented a numerical model for computing two$dimensional dam$break flood waves. They used the characteristics method to approximate the shallow water flow equations. Fennema and Chaudhry 4 utilized the Beam and Warming implicit scheme to solve the equations describing two$dimensional, free$surface flows. Bellos 2 solved the two$dimensional depth$averaged flow equations using the Mac Cormack, two$step, predictor$corrector explicit scheme. An explicit, finite$volume technique adapted to unsteady, depth$ averaged, free$surface flows was presented by Soulis 7 . Extensive comparisons between measured data and numerical results substantiated the validity of the technique. Aureli, Mignosa, and Tomirotti 1 presented a research work where experimental results of 1D dam$break flows with shocks and results found in the literature are compared with those obtained by means of a numerical model based on the well$known McCormack shock$capturing scheme. This paper presents an extension of a previous research work on steady, two$dimensional, free$surface flows developed by Klonidis and Soulis 6 . The model was enhanced in order to be able to solve unsteady flows. The conservative equations of continuity, x$momentum and y$momentum transformed into a non$orthogonal, boundary$fitted coordinate system are numerically solved using a second$order accurate implicit numerical scheme. Numerical results of a two$dimensional dam$break flow simulation are tested successfully against available experimental data.      Under the assumptions of homogenous, 2D, incompressible flow with hydrostatic pressure distribution and with absence of Coriolis and wind forces, the equations used to describe the unsteady flow resulting from the rupture of dam are written in matrix form as 1 Athanasios J. Klonidis. Research Engineer. E$mail: klonidis@otenet.gr