Simulation of Dam Failures in Multidike Reservoirs Arranged in Cascade

Abstract

A majority of river development projects that imply the creation of a lake or reservoir for hydroelectric generation, water supply, and/or recreational purposes are very frequently the object of studies that relate to the consequences of dam failure. The objective of these studies is to assess the impact of such a catastrophic event on any populated areas downstream that may be in the path of the resulting surge wave. Also, the results may be used to design protective measures so as to assure the security of the areas likely to be affected, as well as to develop emergency evacuation procedures. These studies, which frequently rely on numerical simulation, cannot be accomplished with existing models when the reservoir is composed of multiple dams and embankments disposed along its perimeter. This paper describes a simulation methodology that was developed to evaluate the response of a chain of reservoirs to a catastrophic input, generated as a result of extreme floods or failure of an upstream dam. The methodology is based on several numerical tools that assess, as accurately as possible, the risks associated with the possible failure of a reservoir within the system. Each reservoir is modeled as being composed of a main dam with several surrounding dikes or embankments. The dikes and embankments may fail at water surface elevations lower than the crest height of the main dam thus providing a measure of protection rather like a “fuse” in electrical systems. The simulation package comprises three separate analyses; the first uses the standard level pool routing procedure based on the continuity equation and a storage—outflow relationship, whereas the second and third are based, respectively, on the one (1D)- and two-dimensional (2D) unsteady formulations of the full dynamic equations. The simulation procedure incorporates the effects of crest erosion and subsequent formation of a breach in a dike leading to reservoir outflow, reduction of water surface elevations, and (possibly) protection of the main dam structure. The overall velocity field and water surface elevations as well as the discharge through the breach as a function of time have been computed. Comparison between the numerical simulations and data obtained from a reduced-scale laboratory model have, in general, validated the approach used. Varible geometry dikes in which time-dependent trapezoidal breaches could be simulated were incorporated into the physical model. This allowed different hypotheses regarding erosion and breach development to be simulated, so that a sensitivity analysis of the parameters affecting the behavior of the reservoir could be performed. Implementation of the methodology proposed here could provide useful results that would assist engineering personnel at the design stage, while also being of use to civil protection authorities who have the responsibility for the development of emergency evacuation procedures.