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Natural Terrain in Hong Kong

View of Kowloon

Urban expansion in Hong Kong is gradually encroaching further upon the steeper natural hillsides that fringe the urban area. Landsliding on these hillsides during intense rainfall is quite common and widespread, leading to extensive studies of natural terrain landslide processes, and the implementation of control procedures to ensure safe development. This web page contains a summary of current knowledge.

Natural Terrain

Natural Terrain

Facts about Natural Terrain

Natural terrain covers about 60% of the total land area of Hong Kong. The boundary between development area and natural terrain is about 2,000 km long.

Natural Terrain in Hong Kong

  • - Hong Kong has a total land area of about 1100 km2
  • - About 650 km2, or 60%, is natural terrain
  • - There is a severe shortage of easily developed level land in Hong Kong
    • About 40% of natural hillsides slope at less than 20 degree
    • About 40% of natural hillsides slope between 20 degree and 30 degree
    • About 20% of natural hillsides slope between 30 degree and 45 degree , with less than 1% sloping at more than 45 degree

Natural Terrain and Urban Development in Hong Kong

  • - Most developed areas extend as far as slopes that are too steep to develop
  • - The development boundary is the line where urbanisation meets natural terrain
  • - The urban perimeter is more than 2000 kilometres long
  • - Debris avalanches are quite common on natural hillsides
  • - Debris is commonly deposited on upper slopes, therefore rarely affects developments
  • - Larger events tend to be more mobile and can extend onto lower slopes
  • - Natural terrain failures may be triggered by human activity
  • - Some become channelised along drainage lines and can reach the development boundary
  • - Less common are rock falls and boulder falls

Natural Terrain landslides in Hong Kong

In the past 50 years, more than 470 people have been killed by failures of man-made slopes (fill slopes, cut slopes and retaining walls). In contrast, the number of fatalities due to landslides and boulder falls on natural hillsides is very much lower. During the same period, four people are known to have been killed by boulder falls from natural hillsides in two separate incidents. Records on natural hillside landslide incidents in the past are few, and there has been only one documented fatality caused by landsliding on natural hillsides in the past 15 years. The Government has therefore given priority to man-made slopes and retaining walls, and the on-going Landslip Preventive Measures (LPM) Programme has been concerned almost entirely with upgrading substandard man-made slopes and retaining walls.

Landslip preventive works on natural hillslides are often likely to be exceedingly expensive, and such works may not be justified to achieve what could be only marginal reduction in risk. Also, widespread works on natural slopes could cause considerable environmental damage.

Although to date there have been few casaulties from landslides and boulder falls on natural hillsides, the risk to the community as a whole could be increasing as the population of Hong Kong grows and development spreads further into areas adjacent to natural hillsides. The existing policies and procedures on natural hillsides are therefore under constant review.

Since 1994 the GEO has been carrying out systematic research and development on natural hillside hazards in Hong Kong. An inventory of landslides (the Enhanced Natural Terrain Landslide Inventory) has been compiled and is available for public viewing on this website (Slope Information System). Methodologies for applying quantitative risk assessment (QRA) techniques to both landslides and boulder falls are being assessed. In addition, studies are being carried out to examine landslide mechanisms, debris travel distance and the correlation between rainfall and landsliding. Guidance documents on hazard mitigation methods and debris barrier design have been completed and a guidance document which will present suggested methods of assessing natural terrain hazards is complete.


Landslides

Landslides

Facts about landslides

Contains information about 109,000 natural terrain landslides interpreted from 1945-2009 aerial photographs.

Natural Terrain Landsliding

  • - Mostly shallow failures on the middle and upper hill slopes
  • - Commonly in groups on 30o to 40o slopes
  • - Landslides are concentrated in areas affected by intense rainstorms
  • - Densities of more than 10 landslides per km2 have been recorded
  • - On average, about 320 natural terrain landslides occurred each year from 1945 to 1994
  • - Typical volumes range between 50 m3 and 2,000 m3
  • - Very large landslides of thousands of cubic metres volume are rare

Classification

  • - Landslides are classified according to two criteria:
    • Type of material: rock, boulder, or debris
    • Movement mechanism: fall, slide, topple, slump, flow or avalanche
  • - Names combine the two terms: e.g. rock fall, debris flow and debris avalanche
  • - Most common types in Hong Kong: rock falls, boulder falls, slumps, debris slides, debris avalanches and channelised debris flows.

Slumps, Slides, Avalanches and Flows

Represent a continuum of increasing mobility of generally granular materials on slopes

Slumps

Slides

  • - Slides are the next stage of mobilisation
  • - The displaced material moves beyond the plane of rupture, but remains intact
  • - Only represents a small proportion of natural terrain landslides in Hong Kong

Debris Avalanches

  • - Debris avalanches are the most common types of natural terrain landslide in Hong Kong
  • - Most of the displaced material breaks up and becomes remoulded
  • - The debris commonly spreads out as a debris lobe
  • - Open slope debris avalanches are usually small with limited runout
  • - However, the debris may be funneled into channels
  • - This results in a channelised debris avalanche, in which the trail is narrower than the source

Debris Flows

  • - A Debris flow occurs when landslide debris is mixed with surface water and slurry flow develops
  • - Most commonly occurs when the debris enters a stream and the flowing debris is channelised along the drainage line
  • - Channelised debris flows may increase in volume as they erode and entrain loose deposits from the stream bed and banks
  • - For example, the 1990 Tsing Shan Debris Flow had an initial volume of about 2,000 m3, but eroded and entrained an additional 18,000 m3 of material containing large boulders
  • - As dilution increases the debris slurry will become more mobile until hyperconcentrated stream flow develops
  • - A debris flood develops when further dilution causes the mechanism to grade into stream flow, which is an alluvial process

Ancient Deep Seated Landslides

  • - Scars of ancient, more deep-seated landslides can be recognised on aerial photographs
  • - Many of these larger landscape features, and their related deposits, probably formed during periods of wetter climate, such as between 8,000 to 10,500 years ago
  • - The source areas are usually degraded by more recent small landslides and erosion
  • - Failure material forms fans and aprons of colluvium, e.g. the Sham Wat debris lobe comprises several large rock and soil failures
  • - Provisional dating indicates that the events may have occurred within the last 10,000 years

Boulder Falls

Detailed mapping of boulder fields on Happy Valley



Name Area (m²) Boulder Density Boulder Size Boulder Type
Corestone or Tor
Boulder Shape
556 121295.891 20-50% <2m wide 0% angular
439 83937.094 v20-50% <2m wide 0% angular
441 25719.098 20-50% <2m wide 0% angular
440 28685.965 10-20% 2-5mwide 0% angular
442 22972.184 10-20% 2-5m wide 0% angular
522 52211.871 50-75% >5m wide 0% angular
518 14216.853 <10% <2m wide 0% angular
448 25725.613 10-20% 2-5m wide 0% angular
521 25640.650 50-75% >5m wide 0% angular
427 43701.066 10-20% <2m wide 0% angular
449 53517.516 <10% <2m wide 0% angular
435 52757.371 <10% 2-5m wide 0% angular
519 30715.998 10-20% <2m wide 0% angular
438 93869.133 10-20% <2m wide 0% angular
249 24212164.000 >75% >5m wide 0% angular

Name Area (m²) Boulder Density Boulder Size Boulder Type
Colluvial Boulder Field
Boulder Shape
556 121295.891 20-50% <2m wide 0% angular
439 83937.094 v20-50% <2m wide 0% angular
441 25719.098 20-50% <2m wide 0% angular
440 28685.965 10-20% 2-5mwide 20% angular
442 22972.184 10-20% 2-5m wide 0% angular
522 52211.871 50-75% >5m wide 0% angular
518 14216.853 <10% <2m wide 0% angular
448 25725.613 10-20% 2-5m wide 0% angular
521 25640.650 50-75% >5m wide 0% angular
427 43701.066 10-20% <2m wide 0% angular
449 53517.516 <10% <2m wide 0% angular
435 52757.371 <10% 2-5m wide 0% angular
519 30715.998 10-20% <2m wide 0% angular
438 93869.133 10-20% <2m wide 0% angular
249 24212164.000 >75% >5m wide 0% angular

Name Area (m²) Boulder Density Boulder Size Boulder Type
Cliff or Rock Outcrop
Boulder Shape
556 121295.891 20-50% <2m wide 20% angular
439 83937.094 v20-50% <2m wide 30% angular
441 25719.098 20-50% <2m wide 20% angular
440 28685.965 10-20% 2-5mwide 30% angular
442 22972.184 10-20% 2-5m wide 50% angular
522 52211.871 50-75% >5m wide 0% angular
518 14216.853 <10% <2m wide 30% angular
448 25725.613 10-20% 2-5m wide 50% angular
521 25640.650 50-75% >5m wide 10% angular
427 43701.066 10-20% <2m wide 30% angular
449 53517.516 <10% <2m wide 100% angular
435 52757.371 <10% 2-5m wide 70% angular
519 30715.998 10-20% <2m wide 30% angular
438 93869.133 10-20% <2m wide 0% angular
249 24212164.000 >75% >5m wide 0% angular

Name Area (m²) Boulder Density Boulder Size Boulder Type
Scree or Talus Deposite
Boulder Shape
556 121295.891 20-50% <2m wide 80% angular
439 83937.094 v20-50% <2m wide 70% angular
441 25719.098 20-50% <2m wide 80% angular
440 28685.965 10-20% 2-5mwide 50% angular
442 22972.184 10-20% 2-5m wide 50% angular
522 52211.871 50-75% >5m wide 100% angular
518 14216.853 <10% <2m wide 70% angular
448 25725.613 10-20% 2-5m wide 50% angular
521 25640.650 50-75% >5m wide 90% angular
427 43701.066 10-20% <2m wide 70% angular
449 53517.516 <10% <2m wide 0% angular
435 52757.371 <10% 2-5m wide 30% angular
519 30715.998 10-20% <2m wide 70% angular
438 93869.133 10-20% <2m wide 100% angular
249 24212164.000 >75% >5m wide 100% angular

 

Boulder falls facts

Rock and Boulder Falls

  • - Rock falls and boulder falls are less common than landslides
  • - Rock falls originate from jointed rock faces
    • Occur as falls, slides or topples of angular, joint-bounded rock fragments
    • Fragments may accumulate to form a scree slope
    • May continue downslope by rolling, bouncing and sliding
  • - Boulder falls originate from corestones or colluvial boulders on slopes
    • Occur when boulders are dislodged by undercutting or landslides
    • Boulder fields most commonly occur on granites and coarse ash tuffs
    • Deposits commonly accumulate as boulder lines in valleys and on coast


Controlling Factors

Relief Map

Landforms

Topography Facts

  • - The topography of Hong Kong is characterised by rugged uplands and steep slopes
    • About 55% of the land has an elevation of less than 100 mPD
    • About 10% is higher than 300 m in elevation
  • - 23 peaks are higher than 500 mPD
  • - The highest peaks are Tai Mo Shan (957 m), Lantau Peak (933 m) and Sunset Peak(869 m)
  • - Landforms in the west are generally low lying and subdued
  • - Landforms in the east are steeper and more rugged
  • - Extensive areas of flat land are restricted to northern and northwestern New Territories
Coastal Features
  • - Hong Kong has the appearance of a drowned landscape
  • - The coastline is highly indented with 232 offshore islands
  • - The coastline in the west is generally low lying with long sandy beaches
  • - The coastline in the east is higher, highly indented and fringed with cliffs and boulders
Landscape Processes
  • - Main landscape forming processes are landslides on hillsides and river erosion in valleys
  • - Weathering produces a weathered mantle over the surface, which is incised by fluvial valleys
  • - Erosion removes the soft material leaving slopes covered in boulders
  • - Deeply weathered profiles, failure scars, catchment erosion, stream valleys, rock faces, and rocky ridgelines characterise different elements of the land surface

Rivers

Rivers

Drainage Facts

  • - There are no large rivers in Hong Kong
  • - Streams are highly seasonal with high summer (rainy season) flows
  • - During the wet season, flooding is common in low lying areas
  • - Most of the drainage basins are small with short tributary streams
  • - Most of the master streams appear to be truncated, with drowned lower courses
  • - River deposition has formed distinctive alluvial flats in many coastal embayments

Geology

Geology

Geology Facts

  • - Volcanic, granitic and sedimentary rocks underlie the landscape of Hong Kong
  • - Igneous (volcanic and granitic) rocks are exposed over about 70% of the area
  • - Volcanic rocks tend to form the steeper, higher and more angular topography
  • - Granitic rocks generally form a lower, more rounded and subdued landscape
  • - Sedimentary rocks produce the distinctive scenery of the northeastern New Territories
  • - Transported superficial deposits form a discontinuous cover over the landscape
  • - These deposits mainly consist of river alluvium and hillslope colluvium
  • - The main geological structures are northwest- and east-northeast- to east-trending faults
  • - Preferential weathering and erosion along faults has produced distinctive linear landforms
  • - Faults largely determine the orientations of major valleys and ridges

Weathering

Weathering
A three-dimensional model of the deeply weathered bedrock surface below the Tuen Mun valley

Weathering Facts

  • - Weathering is the breakdown and alteration of rocks at the earth's surface
  • - Physical and chemical weathering processes weaken and disintegrate rock materials
  • - Weathering produces a mantle that covers most of the rocks in Hong Kong
  • - The weathered mantle consists of a layer of soft, granular or clayey material containing blocks of harder rock termed corestones
  • - Feldspar, biotite and hornblende are chemically transformed, mainly into clay minerals
  • - The main factors that control weathering are the rock type, rock structure, slope angle, hydrogeology and the nature and extent of erosion
  • - Biological factors, such as root penetration, are important locally
  • - Weathering depths are usually greatest over the granitic and coarse-grained volcanic rocks

Climate

Climate

Climate Facts

  • - Hong Kong is situated in the subtropical zone
  • - The subtropical monsoonal climate is hot and humid with a markedly seasonal rainfall
  • - Tropical depressions bring intense summer rain storms
  • - Hong Kong has a mean annual rainfall of about 2,200 millimetres
    • The heaviest recorded hourly rainfall was 110 millimetres on the 8 May 1992
    • Heavy rainfalls result in large fluctuations in runoff and grounwater levels
  • - Hong Kong is affected by typhoons between the months of June to October
  • - On average, there are 5 or 6 tropical cyclones with accompanying rainstorms each year
  • - The mean annual temperature is 28.6oC in July and 15.6oC in January
  • - The highest recorded temperature was 36.1oC on the 18 August 1990
  • - Lowest recorded temperature was 0.0oC on the 18 January 1893

Process of Evaluating Natural Terrain Landslide Hazards

Process of Evaluating Natural Terrain Landslide Hazards

Development Control for Natural Terrain

Natural Terrain Hazard Screening

Objection Criteria      Study Criteria
  • - Two sets of screening criteria have been developed
    • The first determines if a Natural Terrain Hazard Study is required
    • The second identifies sites where potential natural terrain hazards may render the development proposal impractical
  • - In January 2000 the GEO issued an "Advice Note to S16 Applicants"
  • - This is issued to developers applying under Section 16 of the Town Planning Ordinance
Natural terrain hazards need to be studied:
where there is natural terrain outside the site, but within the same catchment, which is at an angular elevation of 20¢X or more from the site and where there is natural terrain sloping at more than 15¢X within 50m horizontally upslope of the site, provided that there is a credible debris flowpath to the site. Topographic maps of 1:5000 scale shall be used to judge whether these criteria are met.
There are occasionally cases where a natural terrain landslide hazard study may also be required for development sites which do not meet the above criteria, as for example for development sites which are either intersected by or adjacent to a natural drainage course.
- If the following criteria are met, the GEO will object in principle to the proposal until such time that an Assessment have been accepted as demonstrating that the consequential risk is acceptable or that the hazard can be effectively mitigated.
Natural terrain hazards are expected to be severe:
where there is natural terrain outside the site, but within the same catchment, which is at an angular \ elevation of 35¢X or more from the site and at an elevation of 50m or more above the proposed site formation level. Topographic maps of 1:5000 scale shall be used to judge whether these criteria are met.
Sites which are located on or immediately below terrain that is known to be affected by active, large-scale, deep-seated movement are also expected to be subject to severe natural terrain hazards.

Natural Terrain Hazard Review (NTHR)

  • - A NTHR is required if Initial Screening indicates that the potential hazards are significant
    • For Private Developments a NTHR would be carried out at the Building Plan stage
    • For Public Works Projects a NTHR would be done at the Preliminary Project Feasibility Study stage
  • - A NTHR should include a desk study of the catchment above the site
  • - The NTHR should examine the low-level 1963/1964 and most recent aerial photographs
  • - A field visit is essential for an overview of the site and to determine the conditions near the uphill site boundary
  • - Any need for further study should also be identified
  • - A Natural Terrain Hazard Assessment will be necessary if significant hazards are identified

Natural Terrain Hazard Assessment (NTHA)

  • - A NTHA is a detailed study of the natural terrain hazards
    • For Private Projects a NTHA is carried out at the Design stage
    • For Public Works Projects a NTHA is carried out at the Engineering Feasibility Study and Detailed Design stages
  • - Methods include reviewing all available aerial photographs, and detailed field mapping of the terrain and active slope processes
  • - Maps should show the type and location of natural terrain hazards affecting the site
  • - The study should determine the likely magnitude of the hazards (design events)
  • - The NTHA should conclude if the risk is acceptable, or if mitigation measures are required to reduce the risk

Natural Terrain Hazard Mitigation Strategy (NTHMS)

  • - A NTHMS has to be developed if the risk is unacceptable
  • - Experienced judgement is an essential element for assessing natural terrain hazards and for formulating suitable mitigation strategies
  • - A NTHMS should identify mitigation options
  • - Mitigation can be Active or Passive
    • Active Mitigation strategies can be preventative, such as stabilizing the source area, or protective, such as constructing energy dissipation or deflection structures and containment measures along the travel path or in the deposition area
    • Passive Mitigation refers to avoiding developments in areas exposed to natural terrain hazards

Risk Assessment

  • - Risk is a function of hazard and consequence
  • - Therefore, an unacceptable level of risk can be mitigated by reducing the hazard, or reducing the consequence, or by reducing both
  • - Decisions can be supported by quantitative risk assessment (QRA) techniques
    • QRA offers a formalised methodology to gauge the relative importance of different parameters and the associated uncertainties
    • QRA also enables the acceptability of a calculated risk level to be determined by comparing it to established guidelines

Booklet on "Natural Terrain Landslide Hazards in Hong Kong" Booklet on Natural Terrain Landslide Hazards in Hong Kong

  • We have prepared a booklet entitled "Natural Terrain Landslide Hazards in Hong Kong”. This booklet introduces the natural terrain landslide hazards to the public by asking five Ws and one H :"WHY are natural terrain landslides a hazard in Hong Kong?”; "WHAT happened to our natural terrain?”; "WHERE were these natural terrain landslides?”; "WHEN should I be concerned with natural terrain landslide hazards?”; "HOW are natural terrain hazards assessed/dealt with?”; and "WHO should act?”. We hope that this booklet could serve to enhance public awareness of natural terrain landslide hazards as well as to remind all of us the need to stay vigilant and be prepared.

  • The booklet can be downloaded from: http://www.cedd.gov.hk/eng/publications/geo/naturalterrain.html