River and sea flood risks
A flood is when water covers land that is normally dry. River floods happen when a lot of rain or melted snow arrives upstream and the river cannot carry it. Sea floods happen with storm surges, high tides and rising sea level. Low land near the sea or a river, called a floodplain or delta, is most at risk.
Risk = chance of flood × damage. A flood in an empty field is a small risk; the same flood in a city is a big risk.
Heavy rain and water surplus
Heavy rain can be too much for drains and rivers. Hard surfaces (roads, roofs) stop water soaking in, so it runs off fast. Rainfall volume = depth × area. For example, 50 mm (0.05 m) on 2 km² (2 000 000 m²) gives 100 000 m³. Fixes: green roofs, parks that can hold water, storage ponds and bigger drains.
Drought and water shortage
Drought is a long time with much less water than normal. Rivers carry less water, soil dries, crops suffer and salt water can move into river mouths. Fixes: store water in dams and groundwater, save water, reuse water and grow crops that need less water.
Water safety policy and spatial planning
Governments decide a safety standard, for example "the dike must hold a flood that comes once in 1000 years". Tools:
- Hard defences: dikes, dams, storm barriers, pumps.
- Room for the river: widen the channel or make a flood basin so the water level falls. Flow Q = width × depth × speed, so with more width the same flow needs less depth.
- Spatial planning: do not build homes in the most dangerous places; keep floodplains as fields or parks.
- Early warning and evacuation plans.
Climate change and sea-level rise
When the climate warms, the sea water expands and ice on land melts, so the sea level rises. Warmer air holds more moisture, so heavy rain can get heavier. A dike that was high enough before may not be enough later, so plans must allow for extra height and use a margin of safety.
Try it
In a bowl, build a small sand wall (a dike) along one side and pour water slowly on the other side. Predict the level at which water goes over. Then widen the water side and pour the same amount. What happens to the level?
Key formulas and definitions
- Rain volume V = depth × area
- Flow Q = width × depth × speed
- Risk = chance × damage
- Safe if water level < dike crest (top) level
Worked examples
1. 50 mm of rain falls on 2 km². Find the volume of water.
0.05 m × 2 000 000 m² = 100 000 m³.
2. A river is 100 m wide, 5 m deep and flows at 2 m/s. Find its flow. If it is widened to 150 m with the same speed and flow, what is the new depth?
Q = 100 × 5 × 2 = 1000 m³/s. New depth = 1000 ÷ (150 × 2) = 3.33 m. So the water level is lower.
3. A dike crest is 5.0 m above a mark. The water is 4.8 m. The sea rises by 0.5 m. What happens?
New water level = 5.3 m, which is higher than 5.0 m. The water goes over the dike (overtopping), so the dike must be raised by more than 0.3 m.
Common mistakes
- Thinking only high dikes solve everything. Giving the river room and planning where to build matter too.
- Forgetting that drought is also a water problem.
- Mixing rainfall depth (mm) and volume (m³). Multiply by area and convert to metres.
- Believing risk depends only on chance. Damage matters too.