---
title: "Sediment yield, reservoir life and remedies"
subject: "Hydropower Engineering"
chapter: 4
key: "semester_7/hydropower_engineering:reservoir-sedimentation-yield-life-remedies"
source: https://www.bctengineer.com/q/semester_7/hydropower_engineering/reservoir-sedimentation-yield-life-remedies
site: BCT Engineering
---

# Sediment yield, reservoir life and remedies

Hydropower Engineering · Chapter 4: Headworks of Storage Plants

- Asked in: 2072 Kartik, 1+3 marks
- Video: [Sediment Science: Source, Sediment Yield in Reservoirs, and Sediment Yield in Rivers #dam](https://www.youtube.com/watch?v=xQt-5DSX5qs) — Hydropower
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_7/hydropower_engineering/reservoir-sedimentation-yield-life-remedies
- Whole chapter: https://www.bctengineer.com/md/semester_7/hydropower_engineering/chapter-4

## Question

What do you mean by sediment yield and life of a reservoir? Explain various remedial measures that help to reduce reservoir sedimentation.

## Short answer

Sediment yield is the total sediment leaving a catchment per unit area per year (t/km²/yr). Reservoir life is the time to fill dead storage, T = C_dead/(η V_s). Remedial measures include catchment treatment, upstream sediment traps, sluicing, flushing, dredging, and design provisions like large dead storage and bottom outlets.

## Answer

#### Sediment yield
**Sediment yield** is the total quantity of sediment that leaves a catchment and passes a given point (such as a dam site) in a given time. It is expressed as tonnes (or m³) per km² of catchment per year. It depends on rainfall, slope, geology, soil, land cover and land use; in the Himalayan rivers of Nepal it is very high (several thousand t/km²/yr).

#### Life of a reservoir
The **life of a reservoir** is the time taken for the sediment to fill the dead (or useful) storage so that the reservoir can no longer serve its purpose:
$$
T=\frac{C_{dead}}{\eta\,V_s}
$$
where $C_{dead}$ is the storage to be filled, $V_s$ is the annual sediment inflow (volume) and $\eta$ is the trap efficiency (Brune's curve; large reservoirs trap more than 90%).

#### Remedial measures to reduce reservoir sedimentation
1. **Catchment treatment**: afforestation, terracing, contour farming, check dams and gully control, bank protection; avoid unplanned roads and quarrying.
2. **Upstream sediment traps**: silt-trapping dams or small check dams that hold back coarse material before the reservoir.
3. **Sediment routing in the reservoir**:
   - *Sluicing / bypassing*: pass the sediment-laden flood flow through low-level outlets or a bypass tunnel before it settles.
   - *Venting of density currents* through bottom outlets.
4. **Flushing**: draw the reservoir down and open large low-level gates so that high velocity scours the deposits (works best in narrow reservoirs).
5. **Dredging and mechanical removal** of deposits near the intake.
6. **Design measures**: provide large dead storage; place intakes high above the bed; raise the dam if needed; install bottom outlets and sediment excluders.

## Check yourself

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

1. What is sediment yield?
   A. The volume of sediment that a dam can store
   B. The fraction of sediment trapped by the reservoir
   C. Sediment leaving a catchment past a point per unit area per year (t/km^2/yr)
   D. The sediment removed by flushing each year
   Answer: C. Yield is the total sediment outflow from the catchment at a point, expressed per km^2 per year. (Key point: Sediment yield)

2. In T = C_dead/(eta Vs), what does eta represent?
   A. Trap efficiency of the reservoir
   B. Turbine efficiency
   C. Porosity of the deposited sediment
   D. Fraction of storage that is live
   Answer: A. eta is trap efficiency (Brune's curve); only the trapped fraction of inflowing sediment fills storage. (Key point: Reservoir life)

3. Which is a catchment treatment measure for reducing sedimentation?
   A. Drawdown flushing
   B. Afforestation, terracing and check dams
   C. Dredging near the intake
   D. Raising the intake level
   Answer: B. Catchment treatment cuts sediment at the source; flushing, dredging and intake design act in or near the reservoir. (Key point: Catchment treatment)

4. When does flushing work best?
   A. With the reservoir full and gates closed
   B. In wide shallow reservoirs at low velocity
   C. By pumping water over the dam crest
   D. Drawdown with large low-level gates in narrow reservoirs
   Answer: D. Flushing needs high velocity through low-level outlets to scour deposits, which is best in narrow reservoirs. (Key point: Flushing)

## 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 deposition in storage reservoir; management](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Astorage-sediment-deposition-management) — Hydropower Engineering, chapter 4

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