---
title: "Different methods of flood prediction"
subject: "Engineering Hydrology"
chapter: 6
key: "semester_5/engineering_hydrology:flood-prediction-methods"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/flood-prediction-methods
site: BCT Engineering
---

# Different methods of flood prediction

Engineering Hydrology · Chapter 6: Flood Hydrology

- Asked in: 2082 Baisakh, 4 marks
- Video: [Flood Frequency Analysis Basics](https://www.youtube.com/watch?v=Nlzjk8um65E) — Anne Jefferson
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/flood-prediction-methods
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-6

## Question

What are the different methods of flood prediction?

## Short answer

Methods include envelope curves, empirical formulae (e.g., Dickens, Ryves), rational method Qp=CiA/360, unit hydrograph, flood frequency analysis (Gumbel, log-Pearson III), and watershed models/routing.

## Answer

The main methods of estimating (predicting) floods are:

1. **Envelope curves:** the maximum observed floods of many catchments are plotted against area for a region, and the upper envelope is used for a quick estimate.
2. **Empirical formulae:** regional relations such as Dickens, Ryves, Inglis, Fuller and the WECS/DHM equations of Nepal, $Q=CA^n$, where A is the catchment area.
3. **Rational method:** $Q_p=CiA/360$ (A in ha, i in mm/h), for small catchments.
4. **Unit hydrograph method:** the design storm's rainfall excess is applied to a unit hydrograph (observed or synthetic) to get the flood hydrograph.
5. **Flood frequency studies (statistical methods):** a probability distribution (Gumbel, log-Pearson III, log-normal) is fitted to the annual flood series, and the flood of a given return period is extrapolated. Both analytical and graphical (plotting position) approaches are used.
6. **Watershed models and routing:** rainfall-runoff models (HEC-HMS etc.) for gauged and ungauged catchments, and regional flood frequency analysis for ungauged sites.

## Check yourself

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

1. What are envelope curves?
   A. Regional upper envelope of maximum observed floods plotted against area
   B. Unit hydrograph shapes
   C. Curves of rainfall against time
   D. Rating curves
   Answer: A. They give a quick regional estimate from the maximum floods of many catchments. (Key point: Envelope curves)

2. For which catchments is the rational method used?
   A. Only snow-fed rivers
   B. Small catchments
   C. Very large basins
   D. Only gauged large rivers
   Answer: B. Q = CiA/360 is for small catchments. (Key point: Rational method)

3. What does the unit hydrograph method apply?
   A. Empirical C A^n
   B. Rating curve
   C. Rainfall excess of the design storm to a unit hydrograph
   D. Gumbel to monthly flows
   Answer: C. The design storm's rainfall excess gives a flood hydrograph. (Key point: Unit hydrograph)

4. Which are flood frequency distributions?
   A. Thiessen, isohyetal
   B. Gumbel, log-Pearson III, log-normal
   C. Dickens, Ryves, Inglis
   D. HEC-HMS, SWMM
   Answer: B. These fit annual floods and extrapolate; the others are empirical formulae, models or rainfall averaging methods. (Key point: Statistical methods)

## Related in Engineering Hydrology

- [Rational and empirical flood equations](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Arational-empirical-eqns) — Engineering Hydrology, chapter 6