--- title: "Sediment flushing in settling basin" subject: "Hydropower Engineering" chapter: 5 key: "semester_7/hydropower_engineering:sediment-flushing-types-nepal" source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/sediment-flushing-types-nepal site: BCT Engineering --- # Sediment flushing in settling basin Hydropower Engineering · Chapter 5: Headworks of Run-of- River (RoR) Plants - Asked in: 2073 Shrawan, 2+4 marks - Video: [Design Of Settling Basin(Tank) Part I | Hydropower Engineering | IOE, Pu, PoU](https://www.youtube.com/watch?v=NAE_aVTGJRw) — Parash Joshi - Civil Construction and Tutor - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/sediment-flushing-types-nepal - Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-5 ## Question What do you mean by sediment flushing in a settling basin? Briefly explain the different types of flushing systems used in hydropower in Nepal. ## Short answer Sediment flushing removes settled sediment by opening a gate to let fast water carry the deposit out through a channel. In Nepal, common systems are intermittent flushing with a hopper/sloping floor and gate; continuous flushing via longitudinal slots (Dufour) for high loads; and hydraulic suction systems (Bieri, S4) that flush during operation. ## Answer #### 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is sediment flushing in a settling basin? A. Removing settled sediment by opening a flushing gate and letting fast water carry it through a channel back to the river B. Adding chemicals to coagulate fine silt C. Passing sediment through the turbines to scour them D. Raising the water level to float sediment out through the spillway Answer: A. Flushing clears the storage so the basin keeps trapping sediment and protects turbines from abrasion. (Key point: Meaning of sediment flushing) 2. How is intermittent flushing carried out? A. Sediment is extracted continuously through slots with 10-20% of the flow B. The chamber is isolated by closing its inlet, drained, and flushed at 3 m/s or more through the open gate C. The plant is shut down for the whole season D. Sediment is dredged mechanically each day Answer: B. Sediment accumulates for hours; with two or more chambers one is isolated and flushed while the other works. (Key point: Intermittent flushing operation) 3. Which describes continuous flushing systems such as Bieri or Serpent? A. They need no extra water and a smaller basin B. They extract 10-20% of the flow continuously, needing extra water and a larger basin C. They flush only once a year D. They are used only at the river bed undersluice Answer: B. Continuous systems carry out sediment all the time using 10-20% additional flow, suited to very high sediment loads. (Key point: Continuous flushing characteristic) 4. What is the typical arrangement for hydraulic flushing in a chamber? A. Flat floor with an inlet gate only B. Hopper or sloping floor (1:20-1:50) with a flushing gate at the downstream end C. Floor sloping towards the inlet D. Vertical shaft in the middle with no gate Answer: B. The sloping floor guides sediment to the flushing channel and the downstream gate releases it. (Key point: Flushing gate and floor slope) ## Related in Hydropower Engineering - [Frequency of flushing of settling basin](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asediment-flushing-frequency-10-percent-storage) — Hydropower Engineering, chapter 5 - [Continuous flushing settling basin, 95% efficiency](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asettling-basin-continuous-60cumec-velikanov) — Hydropower Engineering, chapter 5 - [Settling basin with Hazen coefficient and sediment depth](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asettling-basin-hazen-sediment-depth-5000ppm) — Hydropower Engineering, chapter 5 - [Settling basin 60 m³/s, 0.2 mm, w = 1.5 cm/s](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asettling-basin-60cumec-02mm-w15) — Hydropower Engineering, chapter 5, asked 4× - [Settling basin 50 m³/s, 0.2 mm](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asettling-basin-50cumec-02mm-w15) — Hydropower Engineering, chapter 5, asked 4× - [Settling basin 40 m³/s, 0.15 mm](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asettling-basin-40cumec-015mm-w15) — Hydropower Engineering, chapter 5, asked 4× ## Related in other subjects - [Settling basin design for 0.25 mm sediment](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asettling-basin-design-0-25mm) — Hydro Power, chapter 5 - [Why settling basins are essential in Nepal](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asettling-basin-necessity-nepal) — Hydro Power, chapter 5, asked 4× - [Settling basin design for 0.2 mm particles](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asettling-basin-design-0-2mm) — Hydro Power, chapter 5 - [Settling basin design for 0.15 mm particles](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asettling-basin-design-0-15mm) — Hydro Power, chapter 5