--- title: "Settling basin design principle and components" subject: "Hydro Power" chapter: 5 key: "semester_6/hydro_power:settling-basin-principle-components" source: https://www.bctengineer.com/q/semester_6/hydro_power/settling-basin-principle-components site: BCT Engineering --- # Settling basin design principle and components Hydro Power · Chapter 5: Components of Hydropower System - Asked in: 2072 Asoj, 7 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_6/hydro_power/settling-basin-principle-components - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-5 ## Question Explain the design principle and criteria of settling basin with neat sketches of basin and discuss the different component of the basin. ## Short answer A settling basin reduces velocity to settle out sediment; length is L ≥ h v / w. Design criteria target particle size (0.2 mm), horizontal velocity (0.2–0.4 m/s), and at least two chambers. Components: inlet transition, settling and storage zones, flushing system, outlet weir, and spillway. ## Answer A **settling basin (desander)** is an enlarged chamber in the water conductor system where the flow velocity is reduced so that suspended sediment particles larger than a chosen size settle out before the water reaches the turbines. In Nepalese Himalayan rivers the silt load is very high and quartz-rich, so desanders are essential to prevent turbine abrasion. #### Design principle The basin works on the **settling theory**: a particle entering at the water surface at depth $h$ moves forward with the flow velocity $v$ and falls with its fall (settling) velocity $w$. It is trapped if it reaches the bed before the end of the basin. $$ \begin{aligned} \text{Time to settle } t &= \frac{h}{w}, \quad \text{time to pass } = \frac{L}{v} \\ \text{Condition: } \frac{L}{v} &\ge \frac{h}{w} \;\Rightarrow\; L \ge \frac{h\,v}{w} \end{aligned} $$ Width $B = \dfrac{Q}{v\,h}$. In practice turbulence reduces the effective fall velocity, so the length is increased using Velikanov's or Sakhmin's methods, e.g. Velikanov: $$ L = \frac{\lambda^2 v^2 (\sqrt{h} - 0.2)^2}{7.51\,w^2} $$ where $\lambda$ depends on the required trap efficiency. #### Design criteria 1. **Particle size to be removed:** usually 0.2–0.3 mm for medium heads; 0.1–0.15 mm for high-head Pelton plants (Nepal practice: 0.2 mm, or 0.15 mm for heads above about 250 m). 2. **Trap efficiency:** generally 90% or more for the design particle size. 3. **Horizontal velocity:** low enough to avoid resuspension; by Camp, $v = a\sqrt{d}$ cm/s ($a = 44$ for $0.1 < d < 1$ mm), usually 0.2–0.4 m/s. 4. **Uniform flow distribution:** gradual inlet transition (expansion about 1:5 to 1:8) and baffles/guide walls. 5. **Depth:** usually 3–6 m plus extra depth for sediment storage. 6. **Flushing:** sediment must be removed periodically or continuously (hydraulic flushing, Bieri system, Serpent system); flushing discharge must be available. 7. **Number of chambers:** at least two chambers so that one can be flushed while the other runs. 8. **L/B ratio:** generally 8–10 for good flow; a large width is split into several chambers. #### Components of a settling basin ```text PLAN inlet settling zone outlet from ________ ___________________ ________ intake \ | | | | / to ------> > ---| | (two chambers) | |--- < headrace ______ /_____| |___________________| |_____\ transition weir/transition L-SECTION ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ WL \ inlet | settling zone |outlet \________|________________________| sediment storage -> flushing canal/gate ``` 1. **Inlet zone (transition):** gradually expanding section with guide walls or baffles that slows the water and spreads it uniformly over the basin width. 2. **Settling zone:** the main chamber of required length, width and depth where particles settle. 3. **Sediment storage zone:** extra depth below the settling zone, with sloping or hopper-shaped bed, where deposits collect. 4. **Flushing system:** flushing gates, conduits or channels with sufficient slope to return deposits to the river. 5. **Outlet zone:** converging transition and overflow weir that collect clear surface water into the headrace without disturbing the settled sediment. 6. **Spillway/escape:** a side spillway to release excess water and protect the basin. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. A particle is trapped if: A. L/v >= h/w, i.e. L >= h v / w B. L/v <= h/w C. w >= v D. h = 0 Answer: A. Time to pass the basin must be at least the time to fall through depth h. (Key point: Settling condition) 2. The basin width follows from: A. B = w h B. B = L/10 only C. B = Q/(v h) D. B = hv/w Answer: C. Continuity: Q = B h v. (Key point: Width) 3. Why is length increased using Velikanov or Sakhmin methods? A. Turbulence reduces effective fall velocity B. Depth is zero C. Water is denser D. Particles float Answer: A. Eddies keep particles suspended, so a longer basin is needed. (Key point: Turbulence correction) 4. The outlet zone consists of: A. A sediment hopper B. A flushing gate C. A converging transition and overflow weir that collect clear surface water D. An inlet baffle Answer: C. Surface clear water passes without disturbing deposits. (Key point: Outlet zone) 5. The flushing system is to: A. Return deposited sediment to the river B. Store water C. Increase velocity D. Add sediment Answer: A. Gates and channels with enough slope remove deposits. (Key point: Flushing) ## Related in Hydro Power - [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 - [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× - [Desander and its design criteria](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Adesander-design-criteria) — Hydro Power, chapter 5, asked 4× - [Surge tank functions and design criteria](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asurge-tank-functions-design) — Hydro Power, chapter 5, asked 2× ## Related in other subjects - [Single chamber settling basin design (15 m³/s)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asettling-basin-single-chamber-15cumec) — Hydropower Engineering, chapter 5 - [Settling basin 60 m³/s, 0.15 mm, head 300 m](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asettling-basin-60cumec-300m-015mm-turbulence) — Hydropower Engineering, chapter 5, asked 2× - [Continuous settling basin (10 m³/s)](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asettling-basin-continuous-10cumec-015mm) — 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×