D-CASCADE: a basin-scale sediment (dis)connectivity model to quantify sediment budgets and explore reservoir sediment management strategies

Dam development projects alter natural river sediment connectivity, resulting in cumulative negative externalities across different spatio-temporal scales. Designing optimal reservoir management strategies that account for economic objectives and sediment connectivity is rarely done due to the lack of specifically designed modelling tools to properly quantify the hydro-morphological response of river systems. Models designed for this purpose must retain both a basin-scale perspective (to capture cumulative impacts of multiple dams) and a dynamic time representation (to capture dynamic reservoir operations). This work presents the D-CASCADE model, a process-based basin-scale dynamic sediment transport model, which in this new version includes specific add-ons components to integrate reservoirs and dynamically simulate dams’ water and sediment management operations. D-CASCADE can be used to extract indicators of sediment connectivity alteration in response to multiple dam constructions and joint operations of water and sediment management in reservoirs, including sediment flushing. In this way, D-CASCADE can explore alternative dam sites and optimize water and sediment management strategies to reduce trade-offs between hydropower and sediment. The study focuses on the 3S river system, a data-scarce tributary of the Mekong river, where major dam development is ongoing. First, (1) modelled network sediment transport scenarios matching field data measurements are identified and used as a baseline to which to compare dam impacts on sediment delivery. Then, (2) the effect of reservoir management is explored for different, pre-defined dam development portfolios, showing a reduction of sediment delivery to the outlet up to 57%. Finally, (3) sediment management (through drawdown flushing) is optimized by including parameters specific to the timing, frequency, and design of drawdown flushing into the operation rules, showing how reservoir sedimentation and downstream sediment starvation can be mitigated via well-designed flushing operations, albeit at a significant loss in energy generation.