The Swiss utility Kraftwerke Oberhasli AG (KWO) currently implements "KWO plus", an upgrading program involving a large number of economical and ecological improvements of their hydropower schemes. A major project, the modification of the Handeck 2 power plant intake, requires total emptying of the Räterichsboden reservoir having an active volume of 25 million m3. In order to define the most efficient and ecological emptying process for these circumstances, KWO commissioned a hybrid modelling program of the sediment remobilisation processes during the planned reservoir drawdown. Hydraulic model tests were conducted at the Laboratory of Hydraulics, Hydrology and Glaciology (VAW) by means of a 1:35 scaled Froude model. Numerical simulations of sediment transport were performed by the Laboratory of Hydraulic Constructions (LCH) using the software FLOW-3D. A good agreement between both models was found for total values of the sediment concentration and its variation in time. Both the physical and the numerical model were validated by prototype data obtained during an annual functional test of the bottom outlet. These measurements were conducted at three downstream positions in the Aare River. The involved solution procedures provide the (1) sediment flux during pressure flushing, (2) influence of outlet gate opening height and opening velocity on sediment concentration, and (3) time-dependent distribution of sediment concentration along the Aare River.