Impact of partial confinement of reservoir lotic–lentic transition region on its sediment trapping efficiency.

Reservoir lotic–lentic transition regions typically show significant spatial and temporal hydro-sedimentological variability, highly influenced by their geometry. Previous assessment of the geometrical dependence of reservoir siltation overlooked potential impacts of internal structural alteration of such regions on their sediment trapping efficiency (STE). This study tested a hypothesis that partial confinement of such regions—a relatively inexpensive intervention—can generally increase their STE, thus contributing to sediment management with enhanced siltation control. Three-dimensional computational modelling of hydrodynamics and transport and deposition of suspended cohesive sediments was used to estimate STE for an idealised lotic–lentic transition region with subtropical characteristics under no-confinement and two types of partial confinement. A well-known and tested model was used with a thoroughly verified, mesh independent, numerically stable and mass conserving scenario modelling approach. Over 50 physically sound steady-state scenarios reproduced ranges of hydro-sedimentological and environmental conditions. Wind drove large-scale horizontal and vertical recirculating flow structures with which partial confinement interfered causing hydrodynamic discontinuity between ensuing upstream and downstream portions of the transition region. The study hypothesis was confirmed under such flow-pattern interference, as partial confinement increased STE by up to 16% relatively to corresponding unconfined setups. Transient conditions and other types of transition regions in real reservoirs need further investigation, but this finding is generally in-line with 'pre-reservoir' effects and had not been shown before. Such enhanced localised settling within lotic–lentic transition regions caused by partial confinement can provide additional control over reservoir siltation when considered in combination with other sediment management alternatives. [ABSTRACT FROM AUTHOR]