---
title: "Sediment deposition in storage reservoir; management"
subject: "Hydropower Engineering"
chapter: 4
key: "semester_7/hydropower_engineering:storage-sediment-deposition-management"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/storage-sediment-deposition-management
site: BCT Engineering
---

# Sediment deposition in storage reservoir; management

Hydropower Engineering · Chapter 4: Headworks of Storage Plants

- Asked in: 2081 Baishakh, 3+3 marks
- Video: [Loss of Reservoir Capacity due to Reservoir Sedimentation | Loss of useful storage](https://www.youtube.com/watch?v=6Ort8jrjfPc) — Let's GROW!
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/storage-sediment-deposition-management
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-4

## Question

Describe the sediment deposition mechanism with a neat sketch in a storage type hydropower plant. Briefly discuss sediment management techniques.

## Short answer

Coarse sediment forms a delta at the reservoir head, medium sediment forms foreset beds, and fine silt/clay travels as density currents to settle as bottomset beds near the dam. Sediment management includes reducing inflow (catchment treatment), passing sediment (sluicing, bypassing, venting), removing deposits (flushing, dredging), protecting intakes, and operating with low reservoir during high-sediment floods.

## Answer

#### Sediment deposition mechanism
When a river enters a reservoir, its velocity and transporting power fall, so the sediment settles in a typical order:

- **Coarse sand and gravel** settle at the head of the reservoir and form a **delta** (topset beds, nearly horizontal).
- **Medium sediment** moves down the delta front and forms the steeper **foreset beds**.
- **Fine silt and clay** travel as **turbidity (density) currents** along the old river bed to the dam, where they settle as **bottomset beds**, or are removed if the bottom outlets are opened.

```text
 inflow --> pivot point
  ~~~~~~~~~~~~\~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ water level
   topset beds \________
   ============ \ foreset beds          dam
   (coarse)      \_______ bottomset __  |
 ///////////////////////////(fine)////|||/// old bed
          turbidity current ----------> low-level outlet
```
The deposits reduce the live storage, raise the upstream flood levels, block the intake and abrade the turbines.

#### Sediment management techniques
1. **Reduce the inflow**: watershed management, check dams and sediment traps upstream.
2. **Pass the sediment through**: *sluicing* during floods with the gates fully open; a *bypass tunnel or channel*; *venting* of turbidity currents.
3. **Remove the deposits**: *drawdown flushing* through bottom outlets and *dredging* near the intake.
4. **Protect the intake and plant**: an intake high above the bed with sediment excluders, trash racks, and a settling basin on the power conduit.
5. **Operating rules**: keep the reservoir low during the sediment-heavy flood season (the monsoon, about June to September in Nepal) and fill it after the peak sediment flows have passed.
6. **Increase capacity** by providing a large dead storage or raising the dam.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Where do coarse sand and gravel settle in a reservoir?
   A. Next to the dam as bottomset beds
   B. At the head, forming the delta topset beds
   C. On the foreset slope only
   D. They stay in suspension to the outlet
   Answer: B. Velocity falls as the river enters the reservoir, so the coarsest material drops first at the head. (Key point: Delta formation)

2. How do fine silt and clay reach the dam?
   A. As turbidity (density) currents along the old river bed
   B. By rolling along the delta top
   C. By dropping at the pivot point
   D. They never reach the dam
   Answer: A. Fines travel as density currents to the dam, settling as bottomset beds or being vented through bottom outlets. (Key point: Turbidity currents)

3. Which is an effect of sediment deposition?
   A. Increased live storage
   B. Lower upstream flood levels
   C. Less intake blockage
   D. Reduced live storage and abrasion of turbines
   Answer: D. Deposits take up storage, raise flood levels, block the intake and abrade turbines. (Key point: Effects of deposits)

4. What operation rule helps manage sediment in Nepal?
   A. Keep the reservoir full in monsoon
   B. Close bottom outlets in floods
   C. Lower the reservoir in monsoon and fill after the sediment-heavy flow has passed
   D. Fill the reservoir at peak flood
   Answer: C. Passing sediment-laden floods at low level and filling afterwards reduces trapping. (Key point: Operation rule)

## Related in Hydropower Engineering

- [Remedial measures for sediment deposition in RoR](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-sediment-deposition-remedial-measures) — Hydropower Engineering, chapter 5
- [Sediment yield, reservoir life and remedies](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Areservoir-sedimentation-yield-life-remedies) — Hydropower Engineering, chapter 4
- [Pumped storage plant: power, turbine, setting, surge](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apumped-storage-turbine-surge-numerical) — Hydropower Engineering, chapter 7
- [RoR plant layouts; storage over RoR](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-layouts-storage-importance) — Hydropower Engineering, chapter 2
- [Pumped storage plant and benefit to Nepal](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Apumped-storage-benefit-nepal) — Hydropower Engineering, chapter 2
- [Diversion type storage plant characteristics](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Adiversion-storage-plant-characteristics) — Hydropower Engineering, chapter 2

## Related in other subjects

- [Reservoir type hydropower plant schematic](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Areservoir-plant-schematic) — Power Plant Design, chapter 3
- [Reservoir storage for wheat, rice, maize](https://www.bctengineer.com/question.md?key=semester_6%2Firrigation_and_drainage_engineering%3Areservoir-supply-40000ha-wheat-rice-maize) — Irrigation and Drainage Engineering, chapter 2
- [Short note: Pumped storage hydropower](https://www.bctengineer.com/question.md?key=semester_7%2Fenergy_environment_and_society%3Apumped-storage-hydropower) — Energy Environment and Society, chapter 5
- [Pumped storage plant and feasibility in Nepal](https://www.bctengineer.com/question.md?key=semester_7%2Fpower_plant_equipment%3Apump-storage-nepal-feasibility) — Power Plant Equipment, chapter 1, asked 5×