Dams in Slovakia were built mostly in the second half of the last century. Many of these structures are currently showing signs of aging. An example is the dam Harmónia, in which, after many years of operation took multiple damage and failure. They are both external signs of aging concrete structures, as well as more serious disorders in filter flow and excessive settling of functional object. The aim of this study is to evaluate the current state of the dam in Harmónia and create a numerical model of filter flow through the body of dam and subsoil of present dam. Calculations have been addressed in the program GeoStudio. The program was modeled ofr current state of filtration flow, possible reason of filtration failure and design of the drainage system that would ensure reliable discharge of leachate.