--- title: "Control of bed load and floating debris at intake" subject: "Hydropower Engineering" chapter: 5 key: "semester_7/hydropower_engineering:ror-intake-bedload-debris-control" source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/ror-intake-bedload-debris-control site: BCT Engineering --- # Control of bed load and floating debris at intake Hydropower Engineering · Chapter 5: Headworks of Run-of- River (RoR) Plants - Asked in: 2069 Chaitra, 6 marks - Video: [RoR Headworks Intake Mastery #ror #water #diversion #weir](https://www.youtube.com/watch?v=7NTjkrUl8Bg) — Hydropower - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/ror-intake-bedload-debris-control - Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-5 ## Question How is the control of bed load and floating debris in a ROR intake done? Explain with appropriate plan and sectional drawings of the system. ## Short answer Bed load is excluded by placing the intake sill 1.0–1.5 m above the river bed, an undersluice with divide wall to flush gravel, and a gravel trap. Floating debris is controlled by an angled trash boom, a trash rack with 50–100 mm spacing, and low approach velocity (0.6–1.0 m/s) to prevent clogging. ## Answer In a RoR plant on a steep Himalayan river, bed load (gravel, boulders) and floating debris (logs, leaves, trash) must be kept out of the intake. This is done by the arrangement shown below. ```text PLAN river ====================================> scour sluice (undersluice) ||||||| weir +---+====================+ | flood divide wall | | wall --> gravel trap |--> canal to | rack|gate | settling basin +---------------------------+ trash boom o-o-o-o (angled) SECTION (through intake) pond ~~~~~~~~~~~~~~~~~~~~~~~~~~~~ boom o | rack |gate| |###\ | |==> canal sill 1.0-1.5 m above undersluice floor //// undersluice floor (low) //// --> flushing ``` #### Control of bed load 1. **High intake sill**: the intake floor is placed 1.0-1.5 m (or more) above the undersluice crest or river bed, so the bed load rolls past. 2. **Undersluice (scouring sluice)** next to the intake, with gates, creates a flushing channel in front of it; it is opened during floods to scour deposits, and the **divide wall** separates the intake from the sluice. 3. **Intake location** on the outer bank of the bend, where the secondary flow moves bed load to the inner bank. 4. **Curtain (skimming) wall** or **bed-load excluder**: only the upper clear water is drawn. 5. **Gravel trap** after the intake with flushing arrangement, and a **settling basin** for sand. 6. **Guide bunds and training walls** keep a stable flow line near the intake. #### Control of floating debris 1. **Trash boom** (floating or fixed), set at an angle to the flow, deflects logs and trash towards the weir spill or sluice. 2. **Trash rack** (clear spacing about 50-100 mm) with an automatic or manual **rake**, inclined at about 75°. 3. **Skimming walls and a surface spillway** pass the floating material over the weir. 4. Low velocity at the rack (0.6-1.0 m/s) so the debris does not hold on the bars. 5. Periodic inspection and cleaning in the monsoon. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why is the intake floor placed 1.0-1.5 m or more above the undersluice crest or river bed? A. So that bed load rolls past below the intake instead of entering B. So that floating debris is trapped on the sill C. To increase the head across the trash rack D. To let sand settle inside the canal Answer: A. Gravel and boulders move along the bed, so a raised sill prevents them from entering the intake. (Key point: High intake sill) 2. What are the undersluice (scouring sluice) and the divide wall for? A. The sluice stores water for the dry season; the wall supports the rack B. The sluice is opened in floods to flush deposits in front of the intake; the divide wall separates the intake from the sluice C. The sluice passes the turbine discharge; the wall reduces seepage D. The sluice traps floating logs; the wall raises the sill Answer: B. Opening the gated undersluice creates a flushing channel that keeps the intake front clear of deposits. (Key point: Undersluice and divide wall) 3. Where is the intake best located on a river bend? A. On the inner bank, where flow is slow and sediment settles B. On the straight reach far downstream of the weir C. On the outer bank, because secondary flow carries bed load to the inner bank D. At the middle of the river on the bed Answer: C. In a bend, secondary currents sweep bed load toward the inner bank so the outer bank draws cleaner water. (Key point: Intake location) 4. How do the trash boom and the low rack velocity (0.6-1.0 m/s) control floating debris? A. The boom stores the debris permanently, and low velocity helps sand to settle B. The boom lifts debris into the canal, and high velocity cleans the bars C. The boom stops bed load, and low velocity increases head loss D. The angled boom deflects logs and trash towards the weir spill or sluice, and low velocity stops debris from holding on the bars Answer: D. A boom set at an angle guides floating material away from the intake, and a low rack velocity keeps debris from being pinned on the rack. 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