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Sediment deposition in storage reservoir; management

Short answer

Coarse sediment forms a delta at the reservoir head, medium sediment forms foreset beds, and fine silt/clay travels as density currents to settle as bottomset beds near the dam. Sediment management includes reducing inflow (catchment treatment), passing sediment (sluicing, bypassing, venting), removing deposits (flushing, dredging), protecting intakes, and operating with low reservoir during high-sediment floods.

Describe the sediment deposition mechanism with a neat sketch in a storage type hydropower plant. Briefly discuss sediment management techniques.

Sediment deposition mechanism

When a river enters a reservoir, its velocity and transporting power fall, so the sediment settles in a typical order:

  • Coarse sand and gravel settle at the head of the reservoir and form a delta (topset beds, nearly horizontal).
  • Medium sediment moves down the delta front and forms the steeper foreset beds.
  • Fine silt and clay travel as turbidity (density) currents along the old river bed to the dam, where they settle as bottomset beds, or are removed if the bottom outlets are opened.
 inflow --> pivot point
  ~~~~~~~~~~~~\~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ water level
   topset beds \________
   ============ \ foreset beds          dam
   (coarse)      \_______ bottomset __  |
 ///////////////////////////(fine)////|||/// old bed
          turbidity current ----------> low-level outlet

The deposits reduce the live storage, raise the upstream flood levels, block the intake and abrade the turbines.

Sediment management techniques

  1. Reduce the inflow: watershed management, check dams and sediment traps upstream.
  2. Pass the sediment through: sluicing during floods with the gates fully open; a bypass tunnel or channel; venting of turbidity currents.
  3. Remove the deposits: drawdown flushing through bottom outlets and dredging near the intake.
  4. Protect the intake and plant: an intake high above the bed with sediment excluders, trash racks, and a settling basin on the power conduit.
  5. Operating rules: keep the reservoir low during the sediment-heavy flood season (the monsoon, about June to September in Nepal) and fill it after the peak sediment flows have passed.
  6. Increase capacity by providing a large dead storage or raising the dam.

✓ Answer checked by Claude Opus 5.5 (high effort)

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