---
title: "How LDOF occurs"
subject: "Engineering Geology II"
chapter: 5
key: "semester_4/engineering_geology_ii:ldof-occurrence"
source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/ldof-occurrence
site: BCT Engineering
---

# How LDOF occurs

Engineering Geology II · Chapter 5: Geological Hazards

- Asked in: 2076 Baisakh, 2 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/ldof-occurrence
- Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-5

## Question

How does a landslide dam outburst flood (LDOF) occur?

## Short answer

A landslide dam outburst flood occurs when a landslide blocks a river, forming a loose debris dam. Water impounds upstream and eventually the dam fails by overtopping, piping, or slope collapse, releasing a destructive flood wave that can erode its bed and grow downstream.

## Answer

A **landslide dam outburst flood (LDOF)** is the sudden flood produced by the failure of a natural dam formed when a landslide blocks a river.

#### How it occurs
1. A large landslide, rock avalanche or debris flow (triggered by heavy rain or an earthquake) falls into a narrow river valley and **blocks the river**, forming a dam.
2. Water **accumulates upstream** and forms a lake.
3. The dam is made of loose, unsorted, poorly compacted material and has no spillway.
4. The dam fails by **overtopping and erosion**, **piping/seepage**, or sudden slope collapse. Failure may occur within minutes to months, sometimes years.
5. A large volume of water and debris is released as a **destructive flood wave** that travels downstream and may grow by eroding its bed and banks.

```text
   landslide
      \\  
  =====\\|dam|------> breach
  river  [lake]         flood wave
```

Examples: the 2014 Jure (Sunkoshi) landslide dam and the landslide dams formed after the 2015 Gorkha earthquake.

## Check yourself

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

1. What starts the sequence that leads to a landslide dam outburst flood?
   A. Heavy rain raising the river level without any blockage
   B. A landslide, rock avalanche or debris flow blocking a narrow river valley
   C. Melting of a glacier lake moraine
   D. Failure of a concrete dam's spillway gate
   Answer: B. A large mass movement falling into a narrow valley blocks the river and forms a natural dam. (Key point: Dam formation)

2. Why is a landslide dam prone to failure?
   A. It is made of loose, unsorted, poorly compacted material and has no spillway
   B. It is made of well-compacted clay with a spillway
   C. It is reinforced by vegetation and rock bolts
   D. It is thin concrete that cracks under load
   Answer: A. The dam is loose and uncompacted with no outlet, so water pressure and flow can easily erode it. (Key point: Dam weakness)

3. Which is a way a landslide dam typically fails?
   A. Gradual drying of the lake
   B. Overtopping and erosion, piping/seepage, or sudden slope collapse
   C. Only by deliberate blasting
   D. Only by earthquake shaking of the lake bed
   Answer: B. Overtopping with erosion, piping through the dam body, or slope collapse are the stated failure mechanisms. (Key point: Failure modes)

4. What happens after the dam breaches?
   A. The lake level rises further
   B. Water slowly seeps downstream with no debris
   C. A large volume of water and debris is released as a destructive flood wave that may grow by erosion
   D. The river returns to its normal flow immediately
   Answer: C. The stored water and debris surge downstream as a flood wave that can erode and enlarge itself. (Key point: Flood wave)

## Related in other subjects

- [Gravity dam equilibrium and overtopping discharge coefficient](https://www.bctengineer.com/question.md?key=semester_7%2Fhydropower_engineering%3Agravity-dam-reservoir-capacity-curve-hrwl) — Hydropower Engineering, chapter 4