Mass movement
Frequency: Average of 200-300/yr
Steep terrain and high frequency tropical rainstorms make landslide occurrence on natural terrain a common phenomenon in Hong Kong. Rapid urban development in hilly terrain, coupled with tropical storms with heavy rainfall, has led to the frequent occurrence of disastrous landslides in Hong Kong1, of which more than 90% of Hong Kong’s landslides are the direct result of major rainstorm events (Brand et al., 1984 in Chen and Lee, 2004)1.
Climatic impacts/influences
The climate is subtropical and monsoonal, with mild, dry winters and hot, humid summers. Rainfall is heavy and occasionally intense during the rainstorms and typhoons2, during which precipitation drains rapidly from small steep channelised catchment areas. Annual rainfall levels are around 2,300mm/yr, with up to 500mm falling in one day3. During high rainfall events the Hong Kong Observatory issues landslide warnings in conjunction with the Geotechnical Engineering Office, who jointly maintain a network of over 100 automated rain gauges:
Figure 3 – Locations of automated rain gauge stations (CEDD, 2007a)4
A rainfall intensity of about 70 mm/h appears to be the threshold value above which landslides occur. The number of landslides and the severity of the consequences increase dramatically as the hourly intensity increases above this level. The antecedent rainfall pattern has been found to be of little significance. In addition, a 24 hour rainfall of less than 100 mm is very unlikely to trigger a major landslide event. The above relationship has been adopted as the basis of the Hong Kong Government’s Landslide Warning System that had been in operation since 1984 (Chen and Lee, 2004)1.
On average, a significant event, in which numerous landslide failures occur in 1 day causing considerable disruption and damage, could statistically be expected to take place in Hong Kong once every 2 years (Lumb, 1979 in Chen and Lee, 2004)1.
Further causes of failures
Most of the failures are minor and shallow in depth with a volume of usually less than 50 m3. However, due to population numbers doubling to approximately 7 million from 1965 to 2000, many buildings in Hong Kong are constructed on slopes or close to slopes due the hilly terrain to meet demand for buildings and infrastructure, with slope cutting and infill frequently occurring to produce land (HKSS, 2007)3:
Figure 4 – Lack of flat land for developments (HKSS, 2007)3
Of the 57,000 man made slopes in Hong Kong 39,000 of these are government owned, with the remainder being private slopes, which causes issues with maintaining them as many owners aren’t aware of boundaries, locations, and sizes, a fact that the CEDD have recognised. In response to this they have developed the Slope Information System (SIS), which contains records of the 57,000 slopes and has generated in excess of 300GB of data, much of which can be queried online using a GIS interface (HKSS, 2007)3.
There are also significant widespread issues with illegal squatter settlements on many steep slopes:
Figure 5 – Distribution of squatter settlements in Hong Kong (CEDD, 2007b)5
These settlements tend to occur on the steepest slopes as they are not considered structurally safe for building, which causes further issues with slope stability. The government has offered re-housing options and clearance of the structures since 1980s, however some squatters refuse to move, and not all settlements were covered by the pledge as they existed on private land or government land mixed with private plots (CEDD, 2007b)5.
Prior to the establishment of the Geotechnical Control Office (GCO) by the Hong Kong Government in 1978, constructions of major embankments of loose fill in resettlement estates lacked geotechnical control, particularly in the cases of private development and squatters on steep and deeply weathered hillsides (Chen and Lee, 2004)1. However, the events of the infamous 1972 slides was to have long reaching consequences and lead to the establishment of the GCO.
1 Chen, H. & Lee, C. F. 2004. Geohazards of slope mass movement and its prevention in Hong Kong. Engineering Geology, 76, 3-25
2 Dai, F. & Fan Lee, C. 2002. Landslides on Natural Terrain. Mountain Research and Development, 22, 40-47
3 HKSS. 2007. AGITpro & Intergraph User Conference 2006 - Presentation Material on Slope Information System. Hong Kong Slope Safety
http://hkss.cedd.gov.hk/hkss/eng/download/Intergraoh_and_AGITpro_Oct_19....
4 CEDD. 2007a. Landslip Warning System. Civil Engineering and Development Department
http://www.cedd.gov.hk/eng/publications/information_sheets/info_lws.htm
5 CEDD. 2007b. Reducing Landslide Hazards to Squatters. Civil Engineering and Development Department
http://www.cedd.gov.hk/eng/publications/information_sheets/info_squatter...