Sediment transport and riverbed evolution of sinking streams in a dammed karst river

Abstract

Little knowledge has been gained about the mechanics of the sediment transport and riverbed evolution of sinking streams in a karst river, because of inadequate data of field investigation and laboratory experiments. Numerical simulation of mixed flow and sediment transport on moveable riverbed in sinking streams is seldom reported, due to difficulties in modeling irregular closed cross-sections (CS). In this study, a unified hydrodynamic and sediment transport model is developed for the free-surface and the pressurized mixed reaches of a karst river with sinking streams, where a technique of symmetric shadow subgrid is proposed to describe and account for an irregular closed CS and its riverbed evolution. Mechanics of riverbed evolution of sinking streams in a dammed karst river are studied using a real karst-river reservoir (Jiayan reservoir), which is still conducted in a data-scarce environment. Under damming effects, a delta deposition of sediment develops at the end of reservoir backwater and steps forward. It is found that the deposition delta is able to go through the sinking streams while a narrow passage is finally reserved in sinking streams and keeps conveying flow. The riverbed elevation of a closed CS, mainly determined by the lower one of CS ceiling and reservoir water level, is dynamically stable. In a dammed karst river, a fully developed (shrunk) sinking stream has periodic morphological dynamics, one cycle of which can be described as a successive process of “siltation → reduction of conveyance area → flow lag upstream of sinking streams → large hydraulic gradient → increase of velocity → large sediment transport capacity → erosion of closed CS → increase of conveyance area”. Sensitivity studies of the parameters of the hydrodynamic and the sediment transport models are performed, whose results indicate that the periodic morphological dynamics of a fully developed sinking stream in a dammed karst river is not changed. The sinking stream in a dammed karst river will not be silted completely, when the condition for forming a large gradient of piezometric head along a sinking stream is not broken by special geomorphologies or anthropogenic influences. This fact is supported by both the numerical simulations and our field investigation of sinking streams in an existing karst-river reservoir.