In Italy as abroad, the natural risk due to the hydrogeological catastrophes is a remarkable problem for the produced damages and for the number of victims (e.g. Messina 02/10/2009 and Ischia 10/11/2009). The realization of structural measures for the mitigation of the hydrogeological risk is essential, because of the extreme diffusion of the problem and also to safeguard the exposed people. The detention reservoirs are structural measures for the defense from hydraulic risk. These works allow to contain temporarily part of the flood volume and then return it, getting an attenuation and a delay of the peak discharge, proper to peak reduction phenomenon. A classical structural measures for the mitigation of the hydraulic risk is represented by the off-stream floodplain storage: areas near to water-course used as storages, escaping volumes of upcoming flood through devices of derivation as, for instance, weirs. In the present paper, first results of an experimental survey on a physical model, situated in the Laboratory of Hydraulics of the Department of Hydraulic, Geotechnical and Environmental Engineering of University of Naples Federico II, of off-stream floodplain storage are showed.