J. Fluid Mech. (1998), vol. 374, pp. 407–424. Printed in the United Kingdom c  1998 Cambridge University Press 407 The initial stages of dam-break ﬂow† By P. K. STANSBY 1 , A. CHEGINI 1 AND T. C. D. BARNES 2 1 Hydrodynamics Research Group, School of Engineering, The University, Manchester M13 9PL, UK 2 School of Mathematics, University of Bristol, Bristol BS8 1TW, UK (Received 10 March 1997 and in revised form 21 January 1998) Experiments have been undertaken to investigate dam-break ﬂows where a thin plate separating water at diﬀerent levels is withdrawn impulsively in a vertically upwards direction. Depth ratios of 0, 0.1 and 0.45 were investigated for two larger depth values of 10 cm and 36 cm. The resulting sequence of surface proﬁles is shown to satisfy approximately Froude scaling. For the dry-bed case a horizontal jet forms at small times and for the other cases a vertical, mushroom-like jet occurs, none of which have been observed previously. We analyse the initial-release problem in which the plate is instantaneously removed or dissolved. Although this shows singular behaviour, jet-like formations are predicted. Artiﬁcially smoothing out the singularity enables a fully nonlinear, potential-ﬂow computation to follow the jet formation for small times. There is qualitative agreement between theory and experiment. In the experiments, after a bore has developed downstream as a result of highly complex ﬂow interactions, the surface proﬁles agree remarkably well with exact solutions of the shallow-water equations which assume hydrostatic pressure and uniform velocity over depth. 1. Introduction Dam-break ﬂows are an important practical problem in civil engineering and their prediction is now a required element in the design of a dam and its surrounding environment. The idealized two-dimensional problem of the instantaneous removal of a barrier between two bodies of water at rest with diﬀerent levels above a horizontal bed has long been a test case for numerical simulations. This is probably because analytical solutions exist if the assumption of hydrostatic pressure is made so that the problem reduces to a one-dimensional problem. This may be generalized to a two- dimensional horizontal plane problem to provide the basis for practical numerical simulations. The equations are known as the shallow-water equations. Simulations based on the full Navier–Stokes/continuity equations have also been made, where the surface is tracked through a ﬁxed mesh, either using particles (the marker-and-cell method), e.g. Harlow & Welch (1965) and Nichols & Hirt (1971), or through surface water concentrations (the volume-of-ﬂuid method), e.g. Hirt & Nichols (1981). The results, at least in terms of the variation of depth proﬁles with time, have appeared broadly similar to those for the shallow-water solutions. In practice the release of water will be more gradual than this idealization and depend on water/soil interaction or concrete fracture at a breach. However the † This article ﬁrst appeared in volume 370, pp. 203–220 but without p. 213. This reprinting replaces that version and it will be the one that is referenced.