Detention basin Grimmelshausen – Flood spillway The significant increase of flood hydrographs, known from the hydrological expertise and the new version of DIN 19700 requires to review the flood safety of the construction. In the present study the capability of the existing discharge constructions (bottom outlet, slide gate, drainage tunnel) were determined. In direct comparison with the new design-basis floods the discharge capability of the constructions was not enough to prevent an overflow of the dam. This results in the need to develop a separate flood spillway for the detention basin Grimmelshausen. A discussion of different variants for hydraulic improvement was conducted. For all three variants, among other things, the required overflow width, the length of the spillway, the dimensions of the stilling basin and the water levels were calculated.