--- title: "Why settling basins are essential in Nepal" subject: "Hydro Power" chapter: 5 key: "semester_6/hydro_power:settling-basin-necessity-nepal" source: https://www.bctengineer.com/q/semester_6/hydro_power/settling-basin-necessity-nepal site: BCT Engineering --- # Why settling basins are essential in Nepal Hydro Power · Chapter 5: Components of Hydropower System - Asked in: 2075 Baisakh, 3+3 marks; 2074 Bhadra, 3+3 marks; 2082 Chaitra, 3 marks; 2070 Magh, 3 marks (asked 4 times) - 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-necessity-nepal - Whole chapter: https://www.bctengineer.com/md/semester_6/hydro_power/chapter-5 ## Question Why settling basin is essential in hydropower electric project in Nepal? Draw a layout plan and section of typical settling basin along with its design criteria. ## Short answer Nepal's young Himalayas cause high sediment with hard quartz, leading to severe turbine abrasion and headrace filling in run-of-river plants. Settling basins remove particles (0.15-0.5 mm) by reducing velocity to 0.2-0.4 m/s with ≥90% efficiency, using two chambers for continuous flushing. ## Answer #### Why a settling basin is essential in Nepal A **settling basin (desander)** is an enlarged chamber after the intake where flow velocity is reduced so that suspended sediment settles and is flushed back to the river. In Nepal it is essential because: 1. **Young, fragile Himalaya:** rivers carry very high sediment loads, especially in the monsoon (often several thousand ppm). 2. **Hard, sharp quartz particles:** quartz and feldspar content is high, so turbines (especially high-head Pelton and Francis) suffer severe **abrasion/erosion** of runners, needles, guide vanes and seals. 3. **Most plants are run-of-river:** there is no large reservoir to trap sediment. 4. Sediment **fills headrace canals and tunnels**, reducing capacity and increasing head loss. 5. Wear causes **loss of efficiency, frequent shutdowns and costly repairs** (e.g. Jhimruk, Khimti, Kaligandaki A). #### Typical layout ```text PLAN inlet settling zone outlet from +-----+--------------------------+-----+ to intake| / | ==== chamber 1 ==== | \ |headrace ----> |< |--------------------------| >|----> | \ | ==== chamber 2 ==== | / | +-----+--------------------------+-----+ transition |flushing| spill transition v channel v weir SECTION (longitudinal) WL ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ \ inlet outlet| \___ ____weir__| \______________________/ bed slope 1:20-1:50 --> flushing gate/pipe ``` #### Design criteria - **Particle size to remove:** 0.15–0.2 mm for high-head plants (> 100 m) and 0.2–0.5 mm for low/medium heads. - **Trap efficiency:** usually ≥ 90% of the design particle size. - **Flow velocity:** less than critical (scour) velocity, by Camp: $V = a\sqrt{d}$ cm/s (d in mm, a = 44 for 0.1–1 mm), normally 0.2–0.4 m/s. - **Size:** settling length $L = \dfrac{hV}{w}$ (w = fall velocity), increased for turbulence (Vetter/Camp efficiency); $L/B$ about 4–10. - **Inlet and outlet transitions:** gradual (about 1:5 to 1:8) to give uniform flow; baffles or flow-straightening walls. - **Number of chambers:** at least two, so one can be flushed or repaired while the other works. - **Flushing:** continuous or intermittent (hydraulic flushing gates, Serpent/SSSS systems, pipes); bed slope 2–5% towards the flushing outlet. - **Spillway:** to pass excess water during floods or load rejection. - Provision for sediment monitoring and bypass of flood flows. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the main job of a settling basin after the intake? A. Reduce velocity so suspended sediment settles and is flushed back to the river B. Store water for the dry season C. Increase flow velocity D. Generate pressure for the penstock Answer: A. Slowing the flow lets sediment settle, and it is flushed out, protecting waterway and turbines. (Key point: Purpose of settling basin) 2. Why is the settling basin especially essential in Nepal? A. Plants have large reservoirs B. Turbines are all low head C. Himalayan rivers carry high loads of hard quartz sediment that abrades turbines D. Rivers are clear and slow Answer: C. Young Himalaya gives heavy, hard, sharp sediment that erodes runners and needles, especially in high-head Pelton and Francis turbines. (Key point: Why essential in Nepal) 3. Why does run-of-river type make desanders more necessary? A. There is no large reservoir to trap sediment B. Large reservoirs trap sediment before the intake C. They have no headrace D. They use only low-pressure pipes Answer: A. Without storage, sediment goes straight into the waterway unless a desander removes it. (Key point: Run-of-river factor) 4. Which flow velocity is typically adopted in the settling zone? A. 5-8 m/s B. Above the scour velocity C. 0.2-0.4 m/s, below the critical scour velocity D. 2-4 m/s Answer: C. The velocity must stay below the critical (Camp) velocity, normally 0.2-0.4 m/s, to avoid resuspension. (Key point: Design criteria) ## Related in Hydro Power - [Settling basin design principle and components](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Asettling-basin-principle-components) — Hydro Power, chapter 5 - [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 - [Layout of hydropower scheme components](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Ahydropower-scheme-layout) — Hydro Power, chapter 5 - [Stages of hydropower project study](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Ahydropower-study-stages) — Hydro Power, chapter 3, asked 3× ## Related in other subjects - [Typical job layout of a building project](https://www.bctengineer.com/question.md?key=semester_8%2Fconstruction_management%3Atypical-job-layout-building) — Construction Management, chapter 6, asked 4× - [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× - [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 100 m](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Asettling-basin-60cumec-head-100m-015mm) — Hydropower Engineering, chapter 5