Channelling Turbid Density Currents in Dam Reservoirs

Abstract

Dam reservoirs form a crucial part for human society storing water, controlling floods, providing hydropower, water for irrigation and drinking. Annually 1% of the worldwide dam reservoirs storage capacity is lost, caused by sedimentation. The inflow of sediment and reduction of flow velocity and turbulence in the reservoir pro- vides favourable conditions for settling. Several sediment transport mechanisms are responsible for this, one of these is the turbid density current. The turbid density currents settle as the reservoir becomes wider, and it is affected by forces along the top and the bottom of the current. Recently, focus on reservoir engineering has shifted from primarily structural dam design towards complete sediment management strategies. In order to improve the sustainability of dam reservoirs, many management techniques are developed that inhibit or mitigate sedimentation. However, the effectiveness of these techniques is not yet known. This thesis provides an additional concept for sediment management in dam reservoirs consisting of channelling of turbid den- sity currents in dam reservoirs. The channel provides controllable parameters. The aim is to study the effects of channelling turbid density currents.
The study starts with a literature review, to describe sedimentation, sediment transport, and turbid den- sity currents in dam reservoirs, including their analytical and numerical descriptions.
Two computational models study the concept: a steady-state model and a numerical model. The steady- state solution and is based upon an equation for open channel flows modified for turbid density currents. This model is used to investigate the effects of hydraulic radii and slope of the channel on the turbid density current — secondly, the dynamic numerical solution. An analytical description is provided using the one- dimensional shallow water equations, consisting of the continuity, momentum and particle conservation equations. The solution includes four sources: deposition, erosion, gravity and friction. It omits water en- trainment and bed deformation. The model is discretised using the Generalised Lax Friedrichs method. First validation and investigation of the quality of the source terms are done. Subsequently, the model, including the four source terms, is used to study the effect of slope, hydraulic radii, concentration and sediment size in the channel. Expanding the numerical study by a Water Injection Dredging case in which local velocity, concentration and height are increased along a certain length to study possible effects.
To conclude, channelling turbid density currents is a viable solution to improve sediment transport. The slope and depth of the channel have the most significant effects. The generalised Lax Friedrichs method provides a valid and straightforward discretisation method for the numerical model. Furthermore, the model provides an easy, quick and simple to use tool to make first estimations of the effects of channel dimensions.