**Simplified Method for
the Characterization of the Hydrograph following a Sudden Partial Dam Break
Marco Pilotti; Massimo Tomirotti; Giulia
Valerio; Baldassare Bacchi
M. Pilotti, A. Maranzoni, M. Tomirotti
**DICATA, Università degli Studi di Brescia - Brescia (IT) e-mail:
marco.pilotti@ing.unibs.it

**
versione completa in
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**
Introduction**

According to research on dam safety worldwide Goubet 1979, the overall yearly probability of dam failure can be reckoned around 1/50,000, so that the a priori probability of failure during the lifetime of a dam, supposedly equal to 100 years, can be estimated between 10−2 and 10−3. Anyway, the potential consequences of this type of accident are so severe that every effort must be made in order to reduce them further and to forecast, in a better way, the expected flood extent and effect in the tailwater areas. In order to cope with these important issues, most national legislations prescribe dam safety regulations regarding not only the construction and upgrading of dams but also their operation and maintenance and emergency preparedness plans, so as to minimize the potential harm to the public and damage to property. In this direction, it is of primary importance to take into account the conceivable failure scenarios applicable to the dam in order to compute a realistic flood wave at the dam site that can be routed downstream to outline and characterize the inundated area. The two primary tasks in the hydraulic analysis of a dam break are the prediction of the reservoir outflow hydrograph and the routing of this boundary condition through the tailwater areas. In this paper we will restrict our attention to the first problem for nonerodible dams. The problem of the computation of the hydrograph following the collapse of a dam has been a major concern since the end of 19th century. Whilst the first documented experimental dam break case in a channel was probably investigated by Bazin 1865[1], Ritter 1892[1] derived an analytical solution of the onedimensional 1D[1] De Saint Venant equations for the case of instantaneous removal of a barrage retaining a reservoir in a frictionless, initially dry, horizontal channel with rectangular cross section. Su and Barnes 1970[1] extended Ritter’s solution considering the effects of different channel cross-sectional shapes. The power-type expression that they introduced for the wetted area, depending on the value of an exponent, is suitable to represent cross sections varying from rectangular to parabolic and triangular shapes. To the writers knowledge, very few works proposing new analytic advancement to this problem have been presented afterwards e.g., Sakkas and Strelkoff 1973; Wu et al. 1999[1]. On the other hand, significant advances have been made in the numerical solution of De Saint Venant equations. Several papers have dealt with the detailed reconstruction of floods following important dam failures, such as the Malpasset dam break e.g., Valiani et al. 2002[1], the Gleno dam break Pilotti et al. 2006[1], and the Saint Francis dam break e.g., Begnudelli and Sanders 2007[1]. These modelling efforts show that, albeit under the hydrostatic assumption, it is possible to provide a detailed and reliable description of the hydrograph formation and propagation. The analytic solutions that have been mentioned Ritter 1892; Su and Barnes 1970[1] have limited practical scope for the evaluation of the hydrograph at the breach section. These solutions assume an infinitely long reservoir with the consequence that the discharge at the breach is constant in time. On the other hand, the two-dimensional 2D[1] numerical simulation of a dam break case always requires a great deal of information e.g., the reservoir bathymetry[1] and a considerable level of expertise. Often in practical applications both these requirements cannot be satisfied. [...]