Cambridge O Level Environmental Management (5014): Natural hazards – Study Guide

Natural hazards are extreme natural events that threaten people, property and the environment. Topic 6 of Cambridge O Level Environmental Management 5014 covers four groups: earthquakes and volcanoes, tropical cyclones, flooding and drought. For each, the examination logic is the same: what causes it, what are its effects, and how can people predict, prepare for and recover from it. Learn that pattern and you can answer questions on any hazard, including one you have not studied in detail.

Where this fits in the specification

This topic builds on the earlier topics about water and land, because floods and droughts are changes in the water cycle and both affect soil and crops. It also links to climate change and to human population: more people living on floodplains, coasts and slopes means that the same hazard causes greater harm.

A hazard is the natural event; a disaster happens when a hazard seriously damages a vulnerable community. Risk increases with the size of the hazard and the number of people exposed, and it decreases with preparedness and wealth.

6.1 Earthquakes and volcanoes

The Earth’s crust is broken into tectonic plates that move slowly on the mantle below. Most earthquakes and volcanoes occur at plate boundaries.

• Earthquakes happen when stress builds up where plates lock together, then releases suddenly as the rocks slip. The point underground where this happens is the focus; the point on the surface directly above is the epicentre. Shaking is strongest near the epicentre. Magnitude is measured on the moment magnitude scale (older questions mention the Richter scale), and a seismometer records the waves.
• Volcanoes form where magma reaches the surface, typically at destructive boundaries (one plate sinks beneath another) and constructive boundaries (plates move apart), or over a hot spot.

Effects: collapsed buildings, fires from broken gas pipes, landslides, tsunamis after undersea earthquakes, and loss of life. Volcanoes add lava flows, ash fall that collapses roofs and ruins crops, pyroclastic flows, lahars (mudflows) and gases that can affect climate.

Management: earthquakes cannot be predicted accurately, so the emphasis is on preparation: earthquake-resistant buildings (deep foundations, flexible frames, cross-bracing), land-use planning, drills, emergency supplies and rescue teams. Volcanoes can often be monitored (gas emissions, small tremors, ground swelling) so evacuation is possible. Benefits to remember: volcanic soils are very fertile, and geothermal energy is a resource.

6.2 Tropical cyclones

A tropical cyclone (also called a hurricane or typhoon, depending on region) is a large, rotating storm with very low pressure and strong winds.

Formation needs: sea surface temperature of about 26 to 27 degrees Celsius or more, a large area of warm ocean, and a location away from the equator so the Earth’s rotation can spin the system. Warm, moist air rises, condenses, and releases heat that powers the storm. When it moves over land or cold water it loses its energy supply and weakens.

Effects: strong winds that destroy buildings and crops, heavy rain causing flooding and landslides, and storm surge (a rise in sea level driven by wind and low pressure) that floods coastal areas and causes most deaths.

Management: satellite tracking and forecasts, early warnings, evacuation, cyclone shelters, strengthened buildings, coastal defences such as mangroves and sea walls, and insurance and aid for recovery. Compare the death tolls in richer and poorer countries to show how preparation changes outcomes.

6.3 Flooding

Floods occur when water exceeds the capacity of a river channel or drainage system, or when the sea overflows the coast.

Physical causes: prolonged or intense rainfall, rapid snowmelt, steep impermeable slopes, saturated soil, and storm surges.

Human causes: deforestation (less interception), urbanisation (concrete and tarmac stop infiltration so water runs quickly into drains and rivers), building on floodplains, and poorly maintained drainage.

Effects: deaths, damage to homes and infrastructure, contaminated drinking water and disease, lost crops and livestock, and disruption to schools and businesses. A positive effect is the fertile silt left behind on a floodplain.

Management falls into two types:

• Hard engineering: dams and reservoirs, embankments (levees), channel straightening or widening, and flood barriers. They protect strongly but are expensive and can move the problem downstream.
• Soft engineering: afforestation, floodplain zoning, wetland restoration, warning systems and flood plans. They are cheaper and more natural but may give less protection against extreme events.

Worked example: reading a flood hydrograph

A town on a river records the following. Heavy rain peaks at 14:00 on Monday. The river reaches its highest level at 19:30 the same day, with a discharge of 85 cubic metres per second. Normal (base) flow is 12 cubic metres per second.

Lag time = time from peak rainfall to peak discharge = 19:30 - 14:00 = 5 hours 30 minutes.

Rise above normal flow = 85 - 12 = 73 m3/s.

Interpretation: a short lag time and high peak suggest fast runoff, typical of a steep, urban or deforested catchment. A longer lag and a lower peak would suggest a wooded or permeable catchment where water soaks in and moves slowly. A sensible suggestion for this town is to plant trees upstream and add retention ponds, which would lengthen the lag time and lower the peak.

6.4 Drought

A drought is a long period with much less rainfall than normal, leading to a shortage of water.

Causes: natural variation in weather patterns, persistent high-pressure systems, and climate change altering rainfall. Human factors make drought worse: over-abstraction from rivers and aquifers, deforestation, overgrazing and wasteful irrigation.

Effects: crop failure and livestock deaths, food shortages and higher prices, dried-up wells and rivers, reduced hydroelectric power, wildfires, migration, conflict over water, and in extreme cases famine.

Management: water conservation and rules on use, rainwater harvesting, drip irrigation, drought-resistant crops, recycling waste water, building reservoirs, improving the efficiency of supply pipes and early-warning systems that monitor rainfall and soil moisture.

Common mistakes

• Mixing up the focus and the epicentre of an earthquake.
• Saying that cyclones form over any warm area. They need warm ocean and distance from the equator.
• Treating hard engineering as always the best answer. Evaluation requires cost, side effects and who benefits.
• Calling a hazard a disaster without mentioning vulnerability.
• Forgetting the human contribution to floods and droughts.

Exam technique

Questions with explain why need a reason chain: cause, process, consequence. Suggest questions ask you to use ideas on unfamiliar information, such as a map or table. For evaluate questions, give at least one advantage and one disadvantage of the strategy, then a conclusion that depends on the community’s wealth and the hazard’s size. Case studies earn marks for named places and specific figures, so prepare one for each hazard type.

Check your understanding

1. Define the epicentre of an earthquake. The point on the Earth’s surface directly above the focus.
2. Give two conditions needed for a tropical cyclone to form. Sea temperature of about 26 to 27 degrees Celsius or more, and a location away from the equator.
3. Why is storm surge so dangerous? It floods low-lying coasts with sea water, and it causes most cyclone deaths.
4. Rainfall peaks at 09:15 and the river peaks at 13:45. State the lag time. 4 hours 30 minutes.
5. Name one hard and one soft engineering flood strategy. Hard: a dam or levee. Soft: afforestation or floodplain zoning.
6. Explain how urbanisation increases flood risk. Impermeable surfaces stop infiltration, so water runs off quickly through drains into rivers, increasing the peak.
7. State two ways of managing drought. Rainwater harvesting and drip irrigation, or drought-resistant crops.
8. Why can richer countries often reduce the impact of a hazard more than poorer ones? They can afford monitoring, strong buildings, warnings, shelters and rescue services.

Frequently asked questions

What is the difference between a hazard and a disaster? A hazard is the natural event; a disaster happens when a hazard seriously damages a vulnerable community. Calling a hazard a disaster without mentioning vulnerability is a common mistake.

What sea temperature is needed for a tropical cyclone to form? Tropical cyclones need a sea surface temperature of about 26 to 27 degrees Celsius or more over a large area of warm ocean. They do not form over just any warm area.

How do humans increase flood risk? Deforestation reduces interception of rainfall, and urbanisation covers the ground with concrete and tarmac that stop infiltration, so water runs quickly into drains and rivers. Building on floodplains and poorly maintained drainage add to the risk.

How do you calculate lag time on a flood hydrograph? Subtract the time of peak rainfall from the time of peak river discharge. A short lag time with a high peak suggests fast runoff, typical of a steep, urban or deforested catchment.

Is hard engineering always the best way to manage floods? No. Hard engineering such as dams and embankments can be effective but is costly, while soft engineering such as afforestation and floodplain zoning works with natural processes. Treating hard engineering as always the best answer is a common mistake.