---
title: "Floods: causes, effects and mitigation"
subject: "Engineering Hydrology"
chapter: 6
key: "semester_5/engineering_hydrology:floods-causes-effects-mitigation"
source: https://www.bctengineer.com/q/semester_5/engineering_hydrology/floods-causes-effects-mitigation
site: BCT Engineering
---

# Floods: causes, effects and mitigation

Engineering Hydrology · Chapter 6: Flood Hydrology

- Asked in: 2066 Magh (old course), 1+5 marks
- Video: [Floods Explained (Episode 1/2): Types, Causes, Impacts, and Case Studies](https://www.youtube.com/watch?v=AVvV_MxNaDs) — Water Edu Hub
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/engineering_hydrology/floods-causes-effects-mitigation
- Whole chapter: https://www.bctengineer.com/md/semester_5/engineering_hydrology/chapter-6

## Question

Define floods. Enumerate the causes, effects and mitigation of floods.

## Short answer

Floods are high flows causing damage. Causes: intense rainfall, snowmelt, deforestation, urbanization. Effects: loss of life, infrastructure damage, erosion. Mitigation includes structural works (dams, embankments) and non-structural measures (forecasting, zoning).

## Answer

#### Definition
A **flood** is a high flow of water in a river, or an overflow onto land normally dry, when discharge exceeds the carrying capacity of the channel and causes damage or threatens life and property. Statistically it is a peak discharge of large magnitude above a chosen threshold (e.g. the annual maximum).

#### Causes
**Natural**
- Heavy, intense or prolonged rainfall, especially the monsoon (June to September in Nepal) and cloudbursts
- Snow and glacier melt, and glacial lake outburst floods (GLOF) in the Himalaya
- Landslides and debris flows blocking rivers, and their sudden breaching
- Steep, young, geologically unstable (Siwalik/Chure) catchments with high runoff
- Backwater, high tides or river-bed aggradation reducing channel capacity

**Man-made**
- Deforestation and overgrazing, which reduce infiltration and raise runoff
- Urbanisation and paved surfaces, encroachment on floodplains and rivers, blocked drains
- Poor design or failure of embankments, dams and weirs
- Unplanned roads and sand/gravel extraction that change river morphology

#### Effects
- Loss of life and livestock, injuries, and water-borne diseases (cholera, typhoid)
- Damage to houses, roads, bridges, irrigation canals, hydropower and drinking water infrastructure
- Destruction of standing crops, loss of fertile topsoil, and sand deposition on farmland, causing food shortage
- Displacement of people, loss of livelihood, and economic loss to the nation
- Erosion of river banks and change of river courses; environmental pollution

#### Mitigation
**Structural**
- Dams and reservoirs with flood storage; detention basins
- Embankments (levees/spurs), flood walls, channel improvement and dredging
- Diversion channels, bypass channels, proper culverts and bridges
- Check dams, bio-engineering and afforestation in the catchment (watershed management)

**Non-structural**
- Flood forecasting and early warning systems, flood-plain zoning and land-use control
- Flood-proofing of buildings, insurance, community awareness and evacuation plans
- Emergency preparedness, relief and rehabilitation; a legal and institutional framework (e.g. Water Resources Act and disaster management acts in Nepal)

## Check yourself

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

1. Which best defines a flood?
   A. Discharge exceeds channel carrying capacity, causing overflow and damage or threat
   B. Water level below normal
   C. A planned release from a dam
   D. Any rainfall above 100 mm
   Answer: A. A flood is high flow or overflow beyond channel capacity that damages or threatens life and property. (Key point: Definition)

2. Which is a man-made cause of floods?
   A. Monsoon rainfall
   B. Deforestation and urbanisation reducing infiltration
   C. Snowmelt
   D. Glacial lake outburst
   Answer: B. Removing vegetation and paving surfaces raise runoff; the others are natural causes. (Key point: Man-made causes)

3. Which is a structural flood mitigation measure?
   A. Floodplain zoning
   B. Early warning
   C. Flood insurance
   D. Embankments and flood storage reservoirs
   Answer: D. Embankments, dams and channel works are structural; the rest are non-structural. (Key point: Structural mitigation)

4. Which is non-structural?
   A. Dredging
   B. Levees
   C. Flood forecasting and warning with land-use control
   D. Check dams
   Answer: C. Forecasting, warning, zoning and preparedness reduce damage without physical works. (Key point: Non-structural mitigation)

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