Long-term sediment management for sustainable hydropower

Since the second half of the 20th century and for decades to come, sedimentation in reservoirs is challenging our ability to maintain the storage capacity needed for water supply, flood mitigation and hydroelectricity production. This complex issue needs to be accounted for from the early stages of the design of dams and reservoirs, until the definition of operation rules oriented towards sustainable management of water and sediments. With reference to the widely-used Driver, Pressure, State, Impact, Response (DPSIR) analytical framework (European Commission 2003), the present chapter reviews key figures and trends in worldwide reservoirs sedimentation, as well as analyzes contemporary sediment management strategies.
Besides setting some fundamentals of reservoir sedimentation and summarizing up-to-date background information on rate and distribution of loss of storage in the world, we identify some key driving forces of soil erosion, such as land use, urban development, agriculture and deforestation. Main mechanisms of watershed erosion and sediment yield to reservoirs are also discussed, with a focus on their spatial and temporal variation. Means for quantifying the magnitude of this pressure on reservoirs sustainability are presented, including reservoir surveys, fluvial measurements and combined approaches with numerical modelling.
After a review of sedimentation impacts, both locally and downstream as well as upstream of the reservoir, we describe current experience and future opportunities of response to reservoir sedimentation. Possible measures are classified into three broad categories: sediment yield reduction, sediment routing (bypass and pass-through) and sediment removal (dredging and flushing). Pro and contras are presented for the different modes of reservoir operation, as well as their expected efficiency and some specific constraints.
Predicting the sedimentation pattern as well as grain sorting processes is a prerequisite for developing optimal sediment evacuation strategies, involving for instance flushing operations. Sound modelling of sediment transport and deposition in reservoirs is highlighted as highly beneficial to support sediment management in the perspective of achieving reservoir sustainability. The chapter also illustrates how spatially distributed numerical modelling may succeed in enhancing decision-making for selecting optimal sediment management measures.