The contribution contains the reasons for and the methods of flood protection for villages in small valleys attacked by flash floods, which cause high economic, cultural and social damage. One of the possible solutions is to build a detention reservoir, which has to retain the flood wave and prevent an odd amount of water from flowing out of the river bed. In this paper a model example is given. The aim of the example is to show (according to hydrological data) how to design the dimension parameters of the dam, i.e. the height of the dam, the outlet structure capacity, the spillway capacity, the stilling basin and the necessary river bed lining. Attention is paid to stability problems, as well.