Yacyretá Dam: spillways modification to reduce total dissolved gases concentrations downstream of the dam

The integral project of spillways and energy dissipators has to ensure the pass of the design flood without endangering the stability of the dam. In low ogee spillways, a hydraulic jump stilling basin allows normally the restitution of the flow to the river with a low residual energy that must be compatible with the characteristics of the riverbed. Air-entrainment devices to reduce cavitation risks are often included as an additional safety element. The location of these hydraulic structures in shallow waters with very rich fishes population can present an excellent performance from the hydraulic viewpoint but hazardous for the normal development of those fishes. This situation was reached in Yacyreta dam, where the excessive amount of air into the flow was responsible of the massive fishes mortality as a consequence of having exceeded the standard values of total dissolved gases (TDG). In the present article, the variables considered for the refurbishment of the spillways are presented, having taken as design condition the reduction of air-entrainment in the spillway and the energy dissipator.