This article is about the calculation of flexible dam structures used for protection of populated areas from flooding, which is actual problem because of the climate changes and inadequate conditions of existing river dykes, which frequently do not satisfy ecological requirements in Russian Federation. Issue of shape calculation of water-filled membranes is considered, depending on internal pressures and external forces. Stability of water-filled membranes under hydrostatic pressure and uplift forces is analyzed. Explanations and requirements for creating new structures of flexible dams are given; they satisfy requirements for ecological infrastructure and provide reliability in terms of construction on biogenic soils and in crowded urban places. Designed structures are biopositive and can be erected without using massive equipment in emergency conditions.