Long-term modelling of reservoir sedimentation with turbid underflows

Abstract

This paper presents the development of a turbid density current module (TDCM) for a one-dimensional quasi-steady reservoir sedimentation model suitable for long-term simulations and an example of its application. The module determines the plunging point of a density current based on the criterion of the densimetric Froude number. Following plunging, simulation continues in a supercritical regime as long as the energy of density flow and bed slope are sufficient to support transport in this mode, or until the dam is reached. A muddy pond is then formed. An adapted version of the theory of Toniolo et al. (2007) is used to compute water and sediment balance in a muddy pond. If the level of muddy pond exceeds the lowest level outlet, sediment is vented from the reservoir. The model was applied to one of the largest reservoirs in the world, the Nurek reservoir in Tajikistan. Historical data on bed levels and sediment inflow as well as a recent survey from 2015 were used to calibrate and validate the model. The model showed good results both in terms of modelled bed levels and composition of deposited sediment. For comparison purpose, the model was also calibrated without the TDCM module. However, the discrepancy between the observed and modelled data was significantly higher, confirming that the good match in the case of the TDCM module was not achieved solely by calibration, but rather by a more appropriate approach to modelling. These results support the choice of using the model with TDCM for modelling a general pattern of long-term deposition in the Nurek reservoir, in which turbid density currents play an important role.