In the face of dramatic meteorological events, the institutions dedicated to the flood safety push for the adoption of two relatively new measures: social participation and the creation of reservoirs in which temporarily store the excess water. The article aims to verify how really innovative the two measures are, and how these relate to each other according to an approach that insists on «socio-technical packages». The hypothesis is that change is temporally and spatially punctuated. Three Italian projects of water detention basin have been analysed. The research shows innovation happened mainly within the public administration (civil engineering) with a limited inclusion of planning and ecological expertise. The construction of detention basins is fruit of a contingent combination of factors opening «windows of opportunity».