---
title: "Need of engineering hydrology in bridge design"
subject: "Engineering Hydrology"
chapter: 1
key: "semester_5/engineering_hydrology:hydrology-need-bridge-design"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/hydrology-need-bridge-design
site: BCT Engineering
---

# Need of engineering hydrology in bridge design

Engineering Hydrology · Chapter 1: Introduction

- Asked in: 2080 Baisakh, 4 marks
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/hydrology-need-bridge-design
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-1

## Question

As a civil engineer how do you justify the need and importance of Engineering Hydrology for the design of Bridge?

## Short answer

Engineering hydrology supplies design flood discharge, high flood level, waterway opening, and scour depth. These determine bridge span, deck elevation, and foundation depth. Without hydrologic data, the structure would be unsafe or uneconomical, risking failure from floods.

## Answer

A bridge crosses a river, so its safety depends on how much water the river carries and how high it rises. Engineering hydrology supplies this information.

#### Need and importance
1. **Design discharge:** The peak flood for a selected return period (for example 100 years for major bridges) is found from rainfall-runoff relations, rational method, unit hydrograph or flood frequency analysis of gauged data.
2. **Design high flood level (HFL):** From the discharge and the cross-section (rating curve), the HFL is found. The bridge deck level is fixed above HFL with a freeboard.
3. **Waterway (linear opening):** The span and number of piers are chosen so that the flood passes without excess afflux, using the discharge and regime width (Lacey).
4. **Scour depth:** Scour depends on discharge per unit width and silt factor, and decides the foundation depth of piers and abutments.
5. **Afflux and backwater:** Narrowing the channel raises the upstream level; this needs hydrologic and hydraulic computation to avoid flooding upstream.
6. **River training and protection:** Guide bunds, spurs and pitching are based on flow and flood data.
7. **Economy and safety:** An underestimate causes overtopping, scour and failure (common in Nepal's monsoon), and an overestimate wastes money.

A civil engineer therefore cannot fix the span, deck level or foundation depth without hydrologic data.

## Check yourself

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

1. The design discharge of a major bridge is the peak flood for:
   A. A selected return period, e.g. 100 years
   B. The average flow
   C. The minimum flow
   D. The last year's flow
   Answer: A. Found by rational method, unit hydrograph or frequency analysis. (Key point: Design discharge)

2. Deck level is fixed:
   A. At river bed
   B. Above HFL with a freeboard
   C. Below HFL
   D. At average water level
   Answer: B. Found from rating curve. (Key point: HFL)

3. Scour depth depends on:
   A. Deck colour
   B. Traffic only
   C. Discharge per unit width and silt factor, and decides foundation depth
   D. Span only
   Answer: C. Used for pier foundation depth. (Key point: Scour)

4. Narrowing the channel causes:
   A. A fall in upstream level
   B. No change
   C. Reduced scour
   D. A rise in upstream level (afflux)
   Answer: D. Needs backwater computation. (Key point: Afflux)

## Related in Engineering Hydrology

- [Significance and uses of hydrology in design](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Asig-hydrology-civil) — Engineering Hydrology, chapter 1, asked 2×
- [Hydrological cycle and uses of engineering hydrology](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Ahydrologic-cycle-uses) — Engineering Hydrology, chapter 1
- [Return period for bridge design at 5% risk](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Abridge-return-period-5pct-risk) — Engineering Hydrology, chapter 6
- [Gumbel design flood for highway bridge, 4% risk](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Agumbel-bridge-risk4pct) — Engineering Hydrology, chapter 6
- [Hydrologic cycle with sketches and its need](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Ahydrologic-cycle-sketch-need) — Engineering Hydrology, chapter 1
- [Importance of hydrology in Nepal, global water balance](https://www.bctengineer.com/question.md?key=semester_5%2Fengineering_hydrology%3Ahydrology-importance-global-balance) — Engineering Hydrology, chapter 1

## Related in other subjects

- [Scope and importance of geology in civil engineering](https://www.bctengineer.com/question.md?key=semester_3%2Fengineering_geology_i%3Ascope-and-importance-of-geology) — Engineering Geology I, chapter 1, asked 24×
- [Important properties of civil engineering materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aimportant-properties-civil-materials) — Civil Engineering Materials, chapter 1, asked 8×
- [Importance and scope of civil engineering materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aimportance-scope-subject) — Civil Engineering Materials, chapter 1, asked 8×
- [Classification of civil engineering materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aclassification-civil-materials) — Civil Engineering Materials, chapter 1, asked 2×