Floods and Landslides
FoundationHeavy rain makes rivers overflow (floods) and soaks slopes until soil slips (landslides). Both are common in Hong Kong's rainy season — both stem from 'too much water, too fast'. Solutions: drainage, storage and slope engineering.
- Flood = water can't drain; Landslide = slope can't hold
- Urbanisation amplifies flood risk
- Fixes: drainage/storage + slope engineering + warnings
§Text Passage
1Hong Kong's summers bring frequent rainstorms, and floods and landslides are two of the city's most familiar hazards. Floods arise from natural and human causes together: days of relentless rain swell the rivers, and the larger the catchment, the more water gathers into them; meanwhile urbanisation has sealed the city's ground with concrete and asphalt, so rain cannot soak in and all of it becomes surface runoff rushing into the drains — once the drains are overwhelmed, water spills onto the streets, and low-lying districts are hit first.
2The main cause of landslides is also heavy rain. Water soaks into hillside soil; once saturated, the soil grows distinctly heavier while friction between its particles keeps falling, until the whole mass loses support and slides down the slope. Vegetation matters just as much: tree roots bind the soil and draw off water, so on slopes stripped of plants the soil takes the full force of the rainwash and the risk runs higher than on natural hillsides.
3Hong Kong's most painful lesson came in the June 1972 rainstorm, when huge landslides struck Sau Mau Ping and Kotewall Road in Mid-levels one after another, killing well over a hundred people; the tragedy led the government to establish the Geotechnical Engineering Office in 1977, which registers, grades and regulates every man-made slope and schedules regular inspections and reinforcement works. For flooding, the government also builds interceptors and underground storage tanks — the Tai Hang Tung tank, for example, holds floodwater during a storm and releases it only after the rain eases, easing the load on downstream drains.
4Climate change is making rainstorms sharper and more concentrated, and the September 2023 'rainstorm of the century' proved it: the Observatory recorded 158.1 millimetres of rain in a single hour, the highest on record, flooding many urban districts. When a Black Rainstorm Warning or Landslip Warning is in force, keep well clear of hillsides, rivers and retaining walls, and check the weather before setting out on a hike; more extreme downpours also mean that today's drainage designs and slope standards must be reviewed and raised continuously against the latest climate data.
1Summary 重點摘要
Heavy rain makes rivers overflow (floods) and soaks slopes until soil slips (landslides). Both are common in Hong Kong's rainy season — both stem from 'too much water, too fast'. Solutions: drainage, storage and slope engineering.
2Vocabulary cards Vocabulary
Water covering land that is normally dry.
Soil and rock sliding down a slope.
The area whose rain drains into one river.
Stabilising slopes with reinforcement, drainage and shotcrete.
Rain that cannot soak in and flows over the ground.
Rain soaking down from the surface into the soil.
Soil holding all the water it can — heavier, less friction, prone to slipping.
Holds water during cloudbursts and releases it later, e.g. Tai Hang Tung.
3Step by step Steps
- Flood causesFloods arise from natural and human causes together. Naturally, days of relentless rain swell the rivers, and the larger the catchment, the more water gathers into them. In cities, urbanisation seals the ground with concrete and asphalt: rain cannot soak in and all of it becomes surface runoff, rushing into the drains within minutes. Once the drains are overwhelmed, water spills onto the streets, and low-lying districts are hit first and hardest.
- Landslide causesThe main cause of landslides is heavy rain. Water soaks into hillside soil; once saturated, the soil grows distinctly heavier while friction between its particles keeps falling, until the whole mass loses support and slides down the slope together. Vegetation is equally decisive: tree roots bind the soil and draw off water. On slopes stripped of vegetation the soil takes the full force of the rainwash, and the risk runs even higher than on natural hillsides.
- Hong Kong's responsesHong Kong answers both hazards with engineering. For flooding, the government builds interceptors and underground stormwater storage tanks — the Tai Hang Tung tank, for example, holds floodwater during a rainstorm and releases it only after the rain eases, lightening the load on downstream drains. For slopes, the Geotechnical Engineering Office registers every man-made slope, grades its risk, and schedules regular inspections and reinforcement works, with the riskiest slopes dealt with first.
- Personal readinessWhen a Black Rainstorm Warning or Landslip Warning is in force, keep well clear of hillsides, rivers, cuttings and retaining walls, and never linger in low-lying or dangerous spots. Always check the weather before hiking, and cancel the trip if conditions look poor. The commonest mistake is heading for the hills anyway during warnings — extremely dangerous. Residents living close to slopes should also watch for government evacuation arrangements and act quickly on instructions.
- Historic events and lessonsHong Kong's most painful lesson came on 18 June 1972, when days of relentless rain triggered huge landslides at the Sau Mau Ping resettlement area and Kotewall Road in Mid-levels one after another, killing well over a hundred people and shocking the whole city. The tragedy led the government to establish the Geotechnical Engineering Office in 1977 to register and regulate every slope in Hong Kong under strict standards — today's slope-safety system was built on those bitter lessons.
- A changing climate, new challengesClimate change is making rainstorms sharper and more concentrated. The September 2023 'rainstorm of the century' proved it: the Observatory recorded 158.1 millimetres of rain in a single hour, the highest since records began; urban districts flooded badly and several landslides struck the slopes. More extreme downpours mean today's drainage designs and slope standards may no longer suffice, so they must be reviewed and raised continuously against the latest climate data.
4Diagrams Diagrams
5Worked example Example
Why do urban Hong Kong floods, while rural slopes collapse — in the same storm?
- Urban: concrete seals the ground → rain becomes surface runoff → drains overloaded → flood
- Rural slopes: rain soaks in → soil saturates → slips
- One storm, two mechanisms
- Different fixes: drains/storage for cities; slope engineering for hillsides
- Answer: One storm, two mechanisms: city ground is sealed with concrete, so rain cannot soak in and all becomes surface runoff rushing into drains — once they are overwhelmed, streets flood. Rural slope soil, meanwhile, saturates, grows heavier and loses friction, sliding as a landslide. The fixes differ accordingly: drainage and storage tanks for the city, slope engineering for the hillsides — both are indispensable.
6Common mistakes Mistakes
7Quick check Quick check
Q1Why do cities flood more easily than countryside?
Q2What directly causes a landslide?
Q3Name two of Hong Kong's flood/landslide measures.
Q4What key change followed the 18 June 1972 rainstorm?
Q5How does global warming raise Hong Kong's flood and landslide risks?
8Recap Recap
- Flood = water can't drain; Landslide = slope can't hold
- Urbanisation amplifies flood risk
- Fixes: drainage/storage + slope engineering + warnings
- Cities flood from sealed ground; slopes fail from saturated soil
- During rainstorm or landslide warnings, stay clear of slopes and rivers