Abstract
ABSTRACT The initial stages of instantaneous dam-break waves are here evaluated spatially and temporally through 36 physical experiments. Different conditions were tested for downstream (J) and upstream (M) water depths and their ratios (r) to approach realistic conditions for prototype dams. Two non-dimensional parameters are proposed – effective height (HEF) and effective velocity (VEF) – to evaluate water depths and velocity peaks along the dam-break wave evolution. The maximum wave height is estimated as a function of r, whereas the HEF is inversely related to r. The maximum VEF peak is registered for r between 0.1 and 0.2, considered a critical description for real dams. The presence of downstream water depth also modifies the dam-break wave frontal shape and types of wave break features. Previously published classifications of the moving wave based on those features are now expanded with a first tested r = 0.8 in which no jet was identified (undulated movement).
Highlights
The dam-break wave is described as an unsteady translatory wave, moving on the water free surface, with net mass transport in the direction of wave movement (Le Méhauté, 1976)
The observed flows were classified by shape – jet type and the wave front break shape, if present
We suggest for prototype dams that this relationship indicates that the more impactful and catastrophic flooding consequences of a dam failure may occur in cases of low water levels (J) in the downstream river in relation to the reservoir water depth (M)
Summary
The dam-break wave is described as an unsteady translatory wave, moving on the water free surface, with net mass transport in the direction of wave movement (Le Méhauté, 1976). A positive wave has water depth behind the wave (as defined by the direction of translation) higher than the undisturbed flow depth. A violent and turbulent spray of water escapes (Wu, 2014), causing an instantaneous adjustment of the pressure field because the fluid is incompressible. This adjustment of the pressure field causes the resulting unsteady motion, eventually producing a bore as observed experimentally (Stansby et al, 1998). The mass of water flows horizontally (like a jet) under gravity, representing the non-Boussinesq limit in which air is the fluid of smallest density (Mariño & Thomas, 2011)
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