Sediment management in reservoirs situated in mountainous regions is a critical operational concern with direct implications in live storage sustainability and therefore in production revenues. The paper presents the main results of a physical model study of a sediment evacuation system foreseen for a large hydropower scheme in Ecuador, carried out at the Laboratory of Hydraulic Constructions (LCH) of the Ecole Polytechnique Fédérale de Lausanne (EPFL). The system comprises a sediment bypass tunnel (SBT) and a submerged weir nearby the gated tunnel inlet, conceived mainly to bypass bed load sediments. Four operation modes were identified and documented. The final sediment bypass efficiency is almost 100% for frequent floods under upstream drawdown, reducing to 20% for larger flood events above the capacity limit of the bypass tunnel. The tests resulted in an improved configuration of the system in all operational modes, including reverse flushing. Guidelines for prototype operation were proposed to consider, among others, the sequential combination of high and low reservoir levels and different gate openings, during and after floods events, in order to improve the overall sediment bypass efficiency.