Sediment yield, reservoir life and remedies

Sediment yield

Sediment yield is the total quantity of sediment that leaves a catchment and passes a given point (such as a dam site) in a given time. It is expressed as tonnes (or m³) per km² of catchment per year. It depends on rainfall, slope, geology, soil, land cover and land use; in the Himalayan rivers of Nepal it is very high (several thousand t/km²/yr).

Life of a reservoir

The life of a reservoir is the time taken for the sediment to fill the dead (or useful) storage so that the reservoir can no longer serve its purpose:

$$T=\frac{C_{dead}}{\eta\,V_s}$$

where $C_{dead}$ is the storage to be filled, $V_s$ is the annual sediment inflow (volume) and $\eta$ is the trap efficiency (Brune's curve; large reservoirs trap more than 90%).

Remedial measures to reduce reservoir sedimentation

1. Catchment treatment: afforestation, terracing, contour farming, check dams and gully control, bank protection; avoid unplanned roads and quarrying.
2. Upstream sediment traps: silt-trapping dams or small check dams that hold back coarse material before the reservoir.
3. Sediment routing in the reservoir:
   • Sluicing / bypassing: pass the sediment-laden flood flow through low-level outlets or a bypass tunnel before it settles.
   • Venting of density currents through bottom outlets.
4. Flushing: draw the reservoir down and open large low-level gates so that high velocity scours the deposits (works best in narrow reservoirs).
5. Dredging and mechanical removal of deposits near the intake.
6. Design measures: provide large dead storage; place intakes high above the bed; raise the dam if needed; install bottom outlets and sediment excluders.