--- title: "Silt excluder, silt ejector, sediment control" subject: "Irrigation and Drainage Engineering" chapter: 5 key: "semester_6/irrigation_and_drainage_engineering:silt-excluder-ejector-sediment-control" source: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/silt-excluder-ejector-sediment-control site: BCT Engineering --- # Silt excluder, silt ejector, sediment control Irrigation and Drainage Engineering · Chapter 5: Diversion Headworks - Asked in: 2074 Bhadra, 4 marks; 2066 Bhadra (old course), 6 marks; 2065 Shrawan (old course), 8 marks; 2065 Kartik (old course), 4 marks; 2062 Kartik (old course), 4 marks; 2072 Asoj, 4 marks; 2068 Chaitra (old course), 4 marks; 2063 Baisakh (old course), 4 marks; 2063 Asoj (old course), 4 marks; 2078 Poush, 5 marks (asked 10 times) - Video: [silt excludore and ejector video lecture](https://www.youtube.com/watch?v=6raI2-Tcxqs) — D DAWAR - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_6/irrigation_and_drainage_engineering/silt-excluder-ejector-sediment-control - Whole chapter: https://www.bctengineer.com/md/semester_6/irrigation_and_drainage_engineering/chapter-5 ## Question Differentiate between a silt excluder and a silt ejector (extractor). Explain, with sketches, how they control the bed load (sediment entry) into the irrigation canal at the headworks. ## Short answer A silt excluder is placed in the pocket before the head regulator and diverts the bed-load layer through tunnels back to the river, preventing silt entry. A silt ejector is located in the canal downstream and extracts already-entered bed silt via bed tunnels, returning it to the river. ## Answer #### Silt excluder and silt ejector A **silt excluder** prevents silt from entering the canal; a **silt ejector** removes silt that has already entered the canal. | Point | Silt excluder | Silt ejector (extractor) | |---|---|---| | Location | In the river pocket in front of the head regulator, at the undersluice floor level | In the canal, a short distance downstream of the head regulator | | Action | Diverts the silt-laden bottom layer to the river before it enters the canal | Extracts the bed silt from the canal water and returns it to the river | | Principle | Tunnels below the regulator crest; the clearer upper layer enters the canal | Tunnels on the canal bed under a diaphragm slab: the upper water goes on, the bottom layer passes through the tunnels | | Position of regulator crest | Raised above the sluice crest so only the upper water is drawn | Normal regulator crest; silt is already in the canal | | Efficiency | Better (keeps silt out) | Less (deals with silt already in the canal, and the canal loses water) | | Water used | About 20% of the canal discharge passes through the tunnels to the river | About 10-20% of the canal flow is escaped back to the river | | Cost | Moderate | More, and needs a good outfall into the river | #### Working and sketches Silt excluder (plan and section through tunnels): ```text PLAN River flow --> ____________________________________ | under- | tunnels -> | river | sluice |==============| (d/s) | pocket |==============| |____________| head | | regulator |-> canal ------------------------------------- SECTION (through the pocket) water level ~~~~~~~~~~~~~~~~~~~~~~~~~ regulator crest (high) +-----+ ------------------------ | |-> canal silty bottom layer --> [==tunnel==]-> river undersluice bed level ____|_________ ``` The tunnels have their roofs at the level of the canal regulator crest. Clear water from the upper layers flows over the tunnel roof into the canal. The bottom layer, which carries the bed load, passes through the tunnels and is released through the undersluices into the river. Silt ejector (on the canal): ```text canal --> [ ejector at ] --> canal continues | ===tunnels===|--> escape to river section: bed silt enters the tunnel because the tunnel is placed on the canal bed, with a lower level outfall into the river. ``` The canal water passes over the tunnel roofs; the silt-laden bottom layer enters the tunnels (formed below a horizontal diaphragm slab a little above the canal bed) and goes through an escape channel back to the river. The tunnels are designed for about 10-20% of the canal discharge, with a velocity of about 2.5-3 m/s so that the silt keeps moving. #### Control of bed load - The **undersluices** are kept at a low level so that the flow in the pocket scours the deposits and sends the bed silt downstream. - The **canal head regulator** crest is placed about 1.2-2.0 m above the sluice crest, so mainly the upper layer, with less silt, is drawn. - The **silt excluder** diverts the bed load in the pocket, and the **ejector** removes what still enters, so silting of the canal is reduced and the sediment is returned to the river. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the basic difference between excluder and ejector? A. Excluder keeps silt out of the canal; ejector removes silt already in the canal B. Excluder is on the canal; ejector is in the river pocket C. Excluder removes suspended silt; ejector removes bed silt D. They are the same device at different places Answer: A. An excluder prevents entry; an ejector extracts silt that has entered. (Key point: Basic difference) 2. Where is the silt ejector located? A. In front of the head regulator B. In the canal a short distance downstream of the head regulator C. Under the weir crest D. At the tail of the canal Answer: B. The ejector is on the canal, a little below the head regulator. (Key point: Location) 3. Which device wastes about 20-25% of the canal flow? A. Silt excluder B. Undersluice C. Silt ejector D. Fish ladder Answer: C. Ejector tunnels are designed for 20-25% of canal discharge, which is lost back to the river. (Key point: Water wasted) 4. Why is the head regulator crest placed 1.2-2.0 m above the sluice crest? A. To reduce the cost of gates B. To increase the canal flow C. To allow fish to pass D. So that mainly the upper, less silty layer is drawn into the canal Answer: D. The raised crest draws the clearer upper layer; the bottom layer goes through the undersluices. (Key point: Role of regulator crest) ## Related in Irrigation and Drainage Engineering - [Silt excluder design, Q 200 cumec](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asilt-excluder-design-q200) — Irrigation and Drainage Engineering, chapter 5 - [Design steps of a silt excluder](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Asilt-excluder-design-steps) — Irrigation and Drainage Engineering, chapter 5 - [Functions of divide wall, undersluice, silt devices](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Adivide-wall-undersluice-silt-devices-functions) — Irrigation and Drainage Engineering, chapter 5 - [Components of canal irrigation system](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acanal-components-sketch) — Irrigation and Drainage Engineering, chapter 3, asked 7× - [Canal head regulator design, Q 200](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acanal-head-regulator-design-q200) — Irrigation and Drainage Engineering, chapter 7 - [Possible alignments of an irrigation canal](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Acanal-alignment-types-sketch) — Irrigation and Drainage Engineering, chapter 3, asked 5× ## Related in other subjects - [Control of bed load and floating debris at intake](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-intake-bedload-debris-control) — Hydropower Engineering, chapter 5 - [Differentiate GVF, RVF and SVF with sketches](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Adifferentiate-gvf-rvf-svf-sketch) — Hydraulics, chapter 5, asked 5× - [Irrigation canal earthwork with vertical drop](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Acanal-a-drop-0-400) — Estimating and Costing, chapter 6 - [Irrigation canal earthwork, 0-600 m](https://www.bctengineer.com/question.md?key=semester_7%2Festimating_and_costing%3Airrigation-canal-k-drop) — Estimating and Costing, chapter 6