---
title: "Flow duration curve and its applicability"
subject: "Power Plant Design"
chapter: 3
key: "semester_8/power_plant_design:flow-duration-curve"
source: https://www.bctengineer.com/q/semester_8/power_plant_design/flow-duration-curve
site: BCT Engineering
---

# Flow duration curve and its applicability

Power Plant Design · Chapter 3: Hydro Power Plant Design

- Asked in: 2080 Chaitra, 4 marks; 2078 Chaitra, 4 marks (asked 2 times)
- Video: [Flow Duration Curve Explained with Practical Example #energy #generated #flow](https://www.youtube.com/watch?v=dMetbSB7orw) — Hydropower
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_8/power_plant_design/flow-duration-curve
- Whole chapter: https://www.bctengineer.com/md/semester_8/power_plant_design/chapter-3

## Question

What is flow duration curve? Also discuss about its applicability.

## Short answer

A flow duration curve plots river discharge against the percentage of time it is equalled or exceeded. It is used to select design discharge (e.g., Q40-45 for ROR), compute annual energy, determine firm power, size units, and compare river regimes.

## Answer

A **flow duration curve (FDC)** is a plot of river discharge against the **percentage of time** that discharge is **equalled or exceeded**. It is drawn by arranging recorded (daily or monthly) flows in descending order and plotting each flow against its exceedance probability, $P = \dfrac{m}{n+1}\times 100\%$ (m = rank, n = number of values).

```text
 Q (m3/s)
  |\
  | \
  |  \__
  |     \____
  |          \______
  |                 \_______
  +--------------------------> % time exceeded
  0     45  65        95   100
```

#### Applicability
1. **Design discharge:** the turbine flow is chosen at a set exceedance, e.g. $Q_{40}$–$Q_{45}$ for ROR projects in Nepal.
2. **Installed capacity and energy:** the area under the FDC (limited by the design flow) gives the energy available; it is used to compute annual energy and plant factor.
3. **Firm power:** flow at 90–95% exceedance gives the firm (dependable) power.
4. **Number and size of units:** the shape of the curve shows how long the plant will run at part load, so it guides unit sizing.
5. **Riparian release and water rights:** low-flow end of the curve shows the minimum flow to release downstream.
6. **Comparison of sites:** a flat curve means steady flow (good for ROR); a steep curve means highly variable flow needing storage.

## Check yourself

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

1. A flow duration curve plots discharge against:
   A. The percentage of time the flow is equalled or exceeded
   B. Time of day
   C. Head
   D. Rainfall depth
   Answer: A. Flows are sorted descending and plotted against exceedance probability. (Key point: Definition)

2. The exceedance probability is P =
   A. m/(n+1) x 100%
   B. n/m x 100%
   C. (m-1)/n
   D. m x n
   Answer: A. m is the rank and n the number of values. (Key point: Exceedance formula)

3. For ROR projects in Nepal the design discharge is chosen at about:
   A. Q5
   B. Q40-Q45
   C. Q95
   D. Q100
   Answer: B. Design flow is set at an exceedance of about 40-45%. (Key point: Design discharge)

4. Firm (dependable) power is based on flow at:
   A. 10% exceedance
   B. 90-95% exceedance
   C. 50% exceedance
   D. The mean flow
   Answer: B. Flow available almost all of the time gives firm power. (Key point: Firm power)

## Close variants set in other exams

- [Hydrograph and flow duration curve](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Ahydrograph-flow-duration) — Power Plant Design, chapter 3, asked 3×

## Related in Power Plant Design

- [Flow duration curve and unit sizing](https://www.bctengineer.com/question.md?key=semester_8%2Fpower_plant_design%3Afdc-unit-sizing) — Power Plant Design, chapter 3

## Related in other subjects

- [Flow duration curve and planning parameters](https://www.bctengineer.com/question.md?key=semester_6%2Fhydro_power%3Afdc-planning-parameters) — Hydro Power, chapter 3
- [Flow duration curve and power at mean flow](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aror-capacity-fdc-flow-table-79) — Hydropower Engineering, chapter 3
- [Flow duration curve; firm and secondary energy](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Afdc-firm-secondary-energy-69) — Hydropower Engineering, chapter 3
- [Load duration curve; diesel plant; ROR flow duration](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Aload-duration-diesel-import-fdc) — Hydropower Engineering, chapter 3