Hazards
Senior 1 · Geography
Hazards
Lesson 3 of 4
25 min
The map below was made by the European Union's emergency centre from satellite images taken after the night of 2–3 May 2023. Orange patches are ground that slid. Before reading on, describe their pattern: are they spread evenly, or do they follow something?
The slides follow the steepest valley sides. That night brought two hazards at once: floods in the valleys and landslides on the slopes above them. Floods, droughts, storms and epidemics are the natural hazards Rwandans meet most often. The first three come from the weather (Unit 8); the last comes from living things.
Definition
Flood: water covering land that is normally dry, when a river overflows its banks, rain falls faster than it can drain away, or a lake rises. A flash flood rises within minutes or hours, usually in a steep, narrow valley. Example: the Sebeya, which burst its banks on the night of 2–3 May 2023.
Definition
Drought: a long period with much less rain than normal, so that water in the soil, rivers and wells runs short and crops and animals suffer. Example: failed or late rains in the drier Eastern Province.
Definition
Epidemic: a sudden increase in the number of cases of a disease in one area, far above what is normal there. When an epidemic spreads across many countries or the whole world it is called a pandemic. Example: the Marburg virus disease outbreak in Kigali, September to December 2024; COVID-19 was a pandemic.
Important
Case study: the floods and landslides of 2–3 May 2023
Before. Rain had been heavy since January: by the end of April, weather-related disasters had already killed 60 people and destroyed more than 1,200 homes, so the ground was soaked. On 2 May the Rwanda Meteorology Agency (Meteo Rwanda) forecast above-average rain for the next ten days, with 175 to 200 mm expected in the north-west.
The night. Very heavy rain began at about 6 p.m. and went on all night. On steep slopes the soaked soil gave way; in the valleys, rivers such as the Sebeya rose within hours and burst their banks. Many houses collapsed while families slept.
Effects. At least 135 people were killed and about 110 injured, most of them in Rutsiro, Nyabihu, Rubavu and Ngororero, with Karongi also hit. About 20,000 people lost their homes. Between 5,100 and 6,000 houses were destroyed and about 2,500 more damaged. Floods and slides cut 26 bridges and between 17 and 20 roads, and damaged power installations, eight water treatment plants, five health centres and a hospital. About 4,100 livestock died.
Response. The government set up a command centre joining MINEMA, the Ministry of Local Government and the police; families were sheltered and fed; the Red Cross and UN agencies helped, and the UN released US$1.5 million from its emergency fund. Afterwards, volunteers with OpenStreetMap Rwanda mapped buildings in the affected districts so that high-risk homes could be identified before the next rains (lesson 4).
Worked example 1: how much water is 200 mm of rain?
200 mm of rain falls on one square kilometre of hillside. What volume of water is that?
Step 1 Put everything in metres: and .
Step 2 litres, so this is 200 million litres, the water of about 80 Olympic swimming pools, falling on one square kilometre. On a steep slope much of it runs off within hours.
| Type | Where it happens | Rwandan example |
|---|---|---|
| Flash flood | Steep, narrow valleys where rain runs off fast | The Sebeya and other short rivers of the Western Province |
| River (flood-plain) flood | Wide, flat valley floors in the old stage of a river (Unit 10) | The Nyabarongo and Akagera flood plains in the long rains |
| Urban flood | Towns where roofs and roads stop rain soaking in and drains are too small or blocked | Low-lying streets and valley bottoms in Kigali after storms |
Causes: very heavy or long rain; steep slopes; cleared forest; soil already soaked; river channels filled with silt; houses and roads built on flood plains and wetlands; blocked drains. Prevention: forecasts and warnings, forests and terraces in the catchment, keeping buildings out of flood plains and wetlands, good drains, and wetlands that store flood water.
Rwanda's rain comes in two rainy seasons (Unit 8). When a season's rain comes late, stops early or is much below normal, crops wilt before they ripen. The drier, lower Eastern Province, parts of which receive less than 1,000 mm a year, is most at risk. Effects: crop failure and hunger, rising food prices, wells and streams drying up, cattle dying, and families moving in search of food and work. Prevention: irrigation from lakes and valley dams, rainwater harvesting from roofs, drought-tolerant crops such as cassava and sorghum, storing food, and early warnings from seasonal forecasts. Droughts in 1943 led to the Ruzagayura famine (lesson 4).
Worked example 2: how dry was the season?
A district normally receives 350 mm in the short rainy season. One year it receives 210 mm. Express this as a percentage of normal, and find the shortfall.
Answer. The season brought 60% of its normal rain, a shortfall of 40%: enough to ruin beans and maize planted for normal rains.
Thunderstorms in the rainy seasons bring strong gusts, hail and lightning. Winds lift iron roofs, flatten bananas and maize, and bring down trees and power lines; hail shreds leaves and crops; lightning kills people and cattle in open fields. In August 2026 MINEMA and communities used Umuganda to prepare for the September–December rains: tightening roofs, clearing drains and anti-erosion ditches, and warning people not to shelter under trees during thunderstorms. In Jarama Sector (Ngoma District), which is exposed to strong winds and lightning, residents reinforced roofs and water channels.
Diseases spread through dirty water (cholera, typhoid), mosquitoes (malaria), the air (flu, COVID-19) or contact with body fluids (Ebola, Marburg). Epidemics spread fastest where people are crowded, water and sanitation are poor, or health workers lack protection, and floods often cause outbreaks of water-borne disease.
Important
Case study: Marburg virus disease, 2024
Rwanda declared its first outbreak of Marburg virus disease on 27 September 2024. By the end, 66 people had been confirmed ill and 15 had died. Almost 80% of the cases were health workers, most from two health facilities in Kigali, who had been infected while caring for patients and colleagues before the disease was recognised. The response followed every rule in this unit: detect early (surveillance and testing, with nearly 7,900 tests by early December), cut contacts (tracing and isolating), protect health workers, and treat quickly, including with trial vaccines and medicines. The last case was confirmed on 30 October, and the outbreak was declared over on 20 December 2024, after 42 days with no new cases.
Worked example 3: the case fatality rate
The case fatality rate is the percentage of confirmed cases who die. Find it for Rwanda's Marburg outbreak (66 cases, 15 deaths).
Interpretation. About 23%. In past Marburg outbreaks elsewhere, rates have ranged from about 24% to 88%, so early care kept Rwanda's rate among the lowest recorded.
Warning
Percentage of what? A case fatality rate divides deaths by cases, not by the whole population. 15 deaths out of Rwanda's 13 million people would be a tiny number; 15 out of 66 cases is almost one in four.
Floods 101, by National Geographic (3 min). Watch for the difference between flash floods and river floods, and the safety rule about walking or driving through flood water.
The floods of May 2023 were forecast, yet at least 135 people died. Suggest three reasons why a forecast alone did not prevent the disaster, and say what would have to happen between the forecast and each family for more lives to be saved.
Tip
For every hazard, answer four questions: What is it? What causes it? What does it do (to people, property, the economy, the environment)? How can it be prevented or reduced? Use a named Rwandan case in each answer.
Practice
Remember
Sources and credits
The lesson text, table, worked examples and practice are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
