Sediment flushing in settling basin

Sediment flushing

Sediment flushing is the removal of the sediment settled in a settling basin (desander) by opening a flushing gate and letting fast-flowing water carry the deposit out through a flushing channel or tunnel back to the river. It keeps the storage space of the basin free, so that the basin can go on trapping sediment and protecting the turbines from abrasion.

Types of flushing systems used in Nepal

1. Intermittent (periodic) flushing
   • The basin has a hopper or a sloping floor (1:20-1:50) with a flushing gate at the downstream end.
   • Sediment is allowed to accumulate for some hours. The chamber is then isolated (inlet closed), drained and flushed through the open gate with water at a velocity of 3 m/s or more. With two or more chambers, one works while the other is flushed.
   • Used in most Nepali RoR plants: simple, and with a lower total water loss.
2. Continuous flushing
   • The floor has longitudinal hoppers or slots (for example, the Dufour type) from which 10-20% of the flow is extracted continuously, carrying the sediment out all the time.
   • Used where the sediment load is very high or the basin cannot be taken out of service. It needs extra water (10-20%), and the basin must be larger.
3. Flushing during operation (hydraulic suction systems): systems such as the Bieri (slotted closure over a flushing channel) and the Serpent Sediment Sluicing System (S4) remove the deposit periodically while the basin stays in operation, so neither draining nor a spare chamber is needed.
4. Gravity flushing / sluicing through a gate in the river bed (undersluice) is used at the headworks and in the gravel trap.
5. Mechanical removal (dredging, sediment-sucking systems) is used in small plants where hydraulic flushing is not possible.