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Hillslope stability in tower karst at Ipoh, Perak State, Malaysia
Residential development in the tower karst of the Kinta valley, Perak, Malaysia is proceeding at a rapid pace, and many developments have been subject to damage and loss of life from landslides and rockfalls. Urban and rural planning in the region requires a reliable methodology for evaluating the stability of limestone hillslopes. This hillslope stability study was conducted at Gunung Tempurung- Gajah, a 600-metre high limestone tower to the south of the city of Ipoh, for an ecotourism development partially funded by the Perak State government.
Radarsat image of the Kinta valley showing karst towers on the east side
The evaluation of hillslope stability can be made by an hierarchy of techniques:
Eastern side of Gunung Tempurung with mining ponds in foreground.
The tower was divided into a sequence of sectors, each of which was evaluated, and for each of which the following ratings were obtained: hillslope type, landslide activity and rock mass stability. The hillslope type was defined according to a scheme initially used by Joe Jennings in the Bukit Batu area closer to Kuala Lumpur, and modified for the Kinta valley. The landslide activity was assessed using the methods developed by Crozier in NZ. The rock mass stability was defined using a rating method modified to take account of evidence of karst water movement in the footslope zone. Mapping and recording proofs of stability of the hillslopes was another important positive aspect of this study. The entire perimeter of the Gunong Tempurong - Gajah massif was surveyed on foot – about 26km of tower base. Wherever possible an inspection of individual rockfaces and mass movement deposits was made, and sections of the cliff base and lower faces were traversed.
The likely distance to which debris from rockfall and landslides will be dispersed from the base of a cliff is of fundamental concern to planners. A number of landslides were studied and their gross morphology measured. From these data, a highly significant relationship was developed explaining 95% of the variance in debris extent in terms of cliff height. The regression equation was used to develop the minimum buffer zone for each hillslope sector of Gunung Tempurung - Gajah.
Over geological timescales, periodic landslides and rockfalls are a normal and expectable part of the geomorphological processes in the tower karst of the Kinta valley. The expected frequency of landslides today is difficult to determine but recourse can be made to data on the frequency of high-intensity rainfall, and examination of revegetation on landslip debris. From these data, it seems probable that minor landslides can be triggered every 2-3 years in the area, with major phases of landslide activity occurring every 20 years.
Landslide stability zones for part of the tower
The study identified a number of active landslide sectors on the slopes of Gunung Tempurung-Gajah. These sectors (about 10% of the perimeter) should be avoided in any development and a wide buffer zone left near them. There are a number of sectors in which landslide activity has taken place within the last five years, and a number of sectors which are potentially active following intense rainfall (26 and 35% of the perimeter respectively). No buildings should be erected within these zones or close to their boundaries. There are also a number of sectors which show evidence of some landslide activity but not in the recent past. These may fail in the future, but some development may be appropriate as long as the indicated buffer zones are adhered to. In many places old mining ponds and fish farms separate the alluvial plain from the karst towers.
These should be retained as amenity zones which have potential to absorb the mass and energy of any mass movement debris which is released as a result of landslide or rockfall activity.