---
title: "Steps of graphical flood frequency method"
subject: "Engineering Hydrology"
chapter: 6
key: "semester_5/engineering_hydrology:graphical-flood-steps"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/graphical-flood-steps
site: BCT Engineering
---

# Steps of graphical flood frequency method

Engineering Hydrology · Chapter 6: Flood Hydrology

- Asked in: 2070 Asar, 4 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/graphical-flood-steps
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-6

## Question

Mention the steps for the computation of flood of return period T using graphical method.

## Short answer

Arrange annual maximum floods in descending order, compute return period using Weibull T=(N+1)/m, plot on suitable probability paper, draw best-fit line, and extrapolate to desired T to read flood magnitude. Reliability decreases for T much longer than record length.

## Answer

**Graphical (plotting-position) method for flood of return period T**

1. Collect the annual maximum flood series for N years.
2. Arrange the floods in **descending order** and assign the rank m (m = 1 for the largest).
3. Compute the **return period** of each flood by a plotting-position formula, e.g. Weibull $T=\dfrac{N+1}{m}$ (or the exceedance probability $P=m/(N+1)$).
4. Choose suitable probability paper (semi-log, Gumbel, or log-normal paper) so that the data fall nearly on a straight line.
5. Plot the flood magnitude (vertical axis) against T or P (horizontal axis) for every point.
6. Draw the best-fit straight line (or smooth curve) through the points.
7. Extend the line (extrapolate) to the required return period T and read the flood magnitude from the vertical axis.
8. Report the flood, noting that reliability falls when T is much larger than N.

## Check yourself

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

1. How are the annual maximum floods ordered before assigning rank m?
   A. Ascending, with m = 1 the smallest
   B. Descending, with m = 1 the largest
   C. Chronologically by year
   D. Randomly
   Answer: B. Floods are arranged in descending order so the largest has m = 1. (Key point: Ranking the data)

2. Which plotting-position formula gives the return period of each flood?
   A. T = (N+1)/m
   B. T = N x m
   C. T = N/(m+1)
   D. T = m/(N+1)
   Answer: A. Weibull gives T = (N+1)/m, the reciprocal of exceedance probability m/(N+1). (Key point: Plotting position)

3. After plotting on suitable probability paper, how is the flood of return period T found?
   A. Multiply the largest flood by T/N
   B. Read the nearest observed point only
   C. Fit a best-fit line and extend it to T, then read the flood
   D. Average all plotted floods
   Answer: C. The straight line or smooth curve is extrapolated to the required T and the flood read on the vertical axis. (Key point: Extrapolation to T)

4. What caution applies to the result of this method?
   A. It needs rainfall data
   B. It is less reliable when T is much larger than N
   C. It only works for T less than 2 years
   D. It is always exact
   Answer: B. Extrapolating far beyond the record length gives large uncertainty. (Key point: Reliability)

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