Environmental Conservation and Tourism

Lessons

Senior 6 · Geography

Environmental Conservation and Tourism

5 lessons

Lesson 3 of 5

Catastrophes: Causes, Effects and Solutions

45 min

The night of 2 to 3 May 2023

After days of heavy rain, floods and landslides struck Rwanda's Western and Northern provinces, and parts of the Southern Province, during the night of 2 to 3 May 2023. The Sebeya River burst its banks in Rubavu, and hillsides collapsed in Ngororero, Nyabihu, Rutsiro, Karongi and Musanze. The Ministry in Charge of Emergency Management (MINEMA) recorded 135 deaths, 106 injuries and 10,942 houses destroyed or severely damaged, along with 20 national roads, 21 district roads and 47 bridges (MINEMA, 2023). Across Lake Kivu, in Kalehe in the DR Congo, the same rains killed hundreds more.

The rain was a natural hazard. It became a catastrophe because many people lived on steep, deforested slopes and in the floodplain of the Sebeya, where the water reached them at night. This lesson explains how hazards become disasters and how the risk can be reduced.

Definition

Hazard: a natural or human-made event that could cause loss of life, injury or damage, such as heavy rain, an earthquake or a chemical leak.

Catastrophe (disaster): a serious disruption of a community, with widespread human, material, economic or environmental losses that exceed its ability to cope with its own resources.

Vulnerability: how easily people, buildings or systems are harmed by a hazard, depending on factors such as poverty, location, building quality and preparedness.

Risk: the likelihood of loss when a hazard meets a vulnerable community, often summarised as $\text{Risk} = \text{Hazard} \times \text{Vulnerability}$ .

Disaster risk reduction (DRR): action to prevent new risks, reduce existing ones and strengthen resilience.

Kinds of catastrophe

Group	Catastrophe	Causes	Example
Geological	Earthquakes and tsunamis	Sudden movement along faults, especially at plate boundaries and in rift valleys; undersea quakes displace water	Indian Ocean tsunami, 26 December 2004 (about 228,000 deaths); Türkiye–Syria earthquakes, 6 February 2023 (more than 50,000 deaths); Lake Kivu earthquake, 3 February 2008
	Volcanic eruptions	Magma rising through weaknesses in the crust	Nyiragongo, 22 May 2021: lava reached the edge of Goma and tremors cracked buildings in Rubavu
	Landslides	Heavy rain on steep slopes, deforestation, cutting into slopes, earthquakes	Western and Northern Rwanda, May 2023
Climatic	Floods	Prolonged or intense rain, blocked drains, building on floodplains, loss of wetlands and forest	Sebeya River, May 2023
	Droughts	Failure of rains, linked in East Africa to El Niño and La Niña	The Horn of Africa, 2020–2023
	Tropical cyclones and storms	Low pressure over warm oceans	Cyclone Idai, March 2019 (Mozambique, Zimbabwe, Malawi)
Biological	Epidemics and pests	Spread of disease; swarms of insects	COVID-19 (from 2020); desert locusts in East Africa, 2019–2021
Human-made	Industrial and technological accidents, fires, wars	Human error, poor safety, conflict	Chernobyl nuclear accident, 26 April 1986; Beirut port explosion, 4 August 2020

Effects of catastrophes

Human: deaths, injuries, trauma, disease from dirty water, displacement and homelessness.
Economic: destroyed houses, roads, bridges, power lines and crops; lost jobs and income; the cost of rebuilding, which may take years of a poor country's budget.
Environmental: eroded hillsides, silted rivers and lakes, damaged forests and wildlife habitats, pollution from damaged factories.
Social and political: schools and health centres closed, families separated, pressure on governments and sometimes conflict over aid.

Effects are usually worse in poorer countries and poorer communities, because more people live in exposed places, buildings are weaker and there is less money for warnings, rescue and rebuilding.

Worked example 1: comparing earthquakes

Problem. On the moment magnitude scale, each increase of 1 means about $10^{1.5} \approx 32$ times more energy released. The main Türkiye–Syria earthquake of 2023 had a magnitude of 7.8; the Lake Kivu earthquake of 2008 had a magnitude of about 5.9. About how many times more energy did the larger one release?

Solution.

10^{1.5 \times (7.8 - 5.9)} = 10^{1.5 \times 1.9} = 10^{2.85} \approx 708

Interpretation. About 700 times more energy. Yet energy alone does not decide the death toll: the depth of the quake, the distance to cities, the time of day and, above all, the quality of buildings make the difference between damage and catastrophe.

Worked example 2: how much water does a storm deliver?

Problem. In an illustrative storm, 80 mm of rain falls in six hours over a catchment of 300 km². If 40% of it runs off the surface instead of soaking in, what volume of runoff reaches the rivers?

Solution.

V_{\text{rain}} = 0.080\,\mathrm{m} \times 3.0 \times 10^{8}\,\mathrm{m^2} = 2.4 \times 10^{7}\,\mathrm{m^3}

V_{\text{runoff}} = 0.40 \times 2.4 \times 10^{7} = 9.6 \times 10^{6}\,\mathrm{m^3}

Interpretation. Nearly 10 million cubic metres pour into the rivers in a few hours. On a forested, terraced catchment more water soaks in, so the runoff share falls and floods are smaller. On bare, steep slopes the share rises, which is why catchment management of the Sebeya is part of flood control.

Reducing the risk

Figure 1. The disaster management cycle. Drawn by Ganzaa.

Phase	Measures	Examples
Mitigation	Reduce the hazard or the exposure: land-use planning, moving people out of high-risk zones, earthquake-resistant buildings, terraces, tree planting, river training and drainage	Rwanda's relocation of households from high-risk zones; terraces and check dams in the Sebeya catchment
Preparedness	Early warning systems, monitoring of volcanoes and rainfall, evacuation plans and drills, stockpiles of food, tents and medicines, insurance	Weather warnings from Meteo Rwanda; monitoring of Nyiragongo by the Goma Volcano Observatory
Response	Search and rescue, first aid, temporary shelter, clean water, food, restoring roads and power	Government, Rwanda Red Cross and partners after May 2023
Recovery	Rebuilding to safer standards ("build back better"), restoring farms and jobs, counselling	New housing on safer sites for displaced families

Internationally, the Sendai Framework for Disaster Risk Reduction 2015–2030, adopted on 18 March 2015, sets targets to reduce disaster deaths, the number of people affected, economic losses and damage to infrastructure.

Warning

Common mistakes

Calling every hazard a disaster. Heavy rain on an empty mountainside is a hazard; it becomes a disaster only when it seriously harms a community.
Treating disasters as purely natural. Where people live, how they build and how they use the land decide how much damage a hazard causes.
Writing only about response. Examiners reward mitigation and preparedness, which save the most lives.

UN Office for Disaster Risk Reduction (UNDRR): "What is the Sendai Framework for Disaster Risk Reduction?". As you watch, note its targets and priorities and match each one to a phase of Figure 1.

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Practice

Distinguish between a hazard and a disaster, using the May 2023 floods as an example.
Classify these catastrophes as geological, climatic, biological or human-made: drought, earthquake, cholera epidemic, factory explosion, landslide, cyclone.
An earthquake of magnitude 6.5 is compared with one of 5.5. About how many times more energy does the larger release?
50 mm of rain falls on a catchment of 120 km² and 60% runs off. Calculate the runoff in m³.
Explain three reasons why landslides are common in Rwanda's Western and Northern provinces.
Describe the four phases of the disaster management cycle with one example each.
"Disasters in poor countries are caused more by poverty than by nature." Discuss.

Show answersHide answers

A hazard is an event that could cause harm (the heavy rain of early May 2023); a disaster is the serious disruption that results when it hits a vulnerable community (135 deaths and over 10,000 houses destroyed or badly damaged).
Drought: climatic; earthquake: geological; cholera epidemic: biological; factory explosion: human-made; landslide: geological (often triggered by climatic rain); cyclone: climatic.
10^1.5 ≈ 32 times.
Rain = 0.050 m × 1.2 × 10⁸ m² = 6.0 × 10⁶ m³; runoff = 0.60 × 6.0 × 10⁶ = 3.6 × 10⁶ m³.
Any three: steep slopes; heavy rainfall in the rainy seasons; deep, weathered soils that become saturated; deforestation and farming on slopes; houses and roads cut into slopes; earthquakes in the Western Rift.
Mitigation (moving households out of high-risk zones, terracing); preparedness (early warnings from Meteo Rwanda, drills, stockpiles); response (rescue, shelter, first aid after May 2023); recovery (rebuilding on safer sites, restoring farms).
For: poverty forces people onto steep slopes and floodplains, into weak houses, without warnings or insurance, so the same hazard kills far more people in poor countries. Against: some hazards are so extreme (the 2004 tsunami, major earthquakes) that they devastate rich places too, and climate change is increasing extreme rainfall. Judgement: nature supplies the hazard, but vulnerability, largely shaped by poverty and planning, decides how great the disaster becomes.

Summary

A hazard becomes a catastrophe when it meets a vulnerable community; risk depends on both hazard and vulnerability.
Catastrophes may be geological, climatic, biological or human-made, with human, economic, environmental and social effects.
Risk is reduced through the cycle of mitigation, preparedness, response and recovery, and internationally through the Sendai Framework (2015–2030).
Rwanda's May 2023 floods and landslides (135 deaths, MINEMA) show how slope, rainfall and settlement combine.

Sources and credits

Ministry in Charge of Emergency Management (MINEMA), assessment of the disaster-hit regions, May 2023; ReliefWeb, Rwanda: Floods and Landslides, May 2023. Figures cited.
UNDRR, What is the Sendai Framework? Cited.
2023 African Great Lakes floods, 2021 Mount Nyiragongo eruption, 2004 Indian Ocean earthquake and tsunami and Moment magnitude scale: Wikipedia contributors, CC BY-SA 4.0, adapted by Ganzaa.

Map: ECDM 20230510 FL DRC Rwanda (ERCC, European Union; public domain), and photo: The Nyiragongo volcano erupted on Saturday, May 22 (MONUSCO Photos, CC BY-SA 2.0), via Wikimedia Commons.

Video: "What is the Sendai Framework for Disaster Risk Reduction?", UNDRR, embedded from YouTube.

Figure 1, the worked examples and the exercises were drawn and written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1The night of 2 to 3 May 2023

2Kinds of catastrophe

3Effects of catastrophes

4Reducing the risk

The night of 2 to 3 May 2023

Kinds of catastrophe

Effects of catastrophes

Reducing the risk