Hydraulic performance of an enclosed spillway-sluice dewatering scheme: a case study of Cobb Dam, New Zealand

This study examines the hydraulic performance of an enclosed spillway-sluice dewatering scheme at Cobb Dam, New Zealand, where removal of a concrete bulkhead would reactivate a low-level sluice for emergency reservoir drawdown. Laboratory experiments on a 1:25-scale physical model, supplemented by three-dimensional computational fluid dynamics (CFD) simulations, were used to evaluate how bulkhead removal and combined spillway-sluice operation alter discharge capacity, junction hydraulics, and stilling-basin flow. The results show that reactivating the sluice increased peak discharge capacity by approximately 50% under the tested conditions, but shifted pressure concentrations to the enclosed spillway-sluice junction and the stilling basin. Three junction-flow regimes were identified and related to the sluice-to-spillway discharge ratio, which governs local aeration, hydraulic-jump formation, and tunnel pressurisation. Combined operation also generated asymmetric hydraulic jumps and transverse water-level imbalance in the stilling basin, indicating that discharge capacity and structural loading must be balanced in gate operation. Based on pressure, head-loss, and stilling-basin response, a site-specific operating envelope is proposed to support emergency drawdown at Cobb Dam and to provide a reference for similar enclosed surface-intake and bottom-outlet systems.