Unstable slopes in our unstable world

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IT was three Malaysian newspaper articles last November and December which prompted this article. Yet another landslide occurred on Dec 7 at Canada Hill, Miri, followed by a subsequent article in The Borneo Post of Dec 27 entitled, ‘Mitigating landslides at Canada Hill underway’. A full page spread in a Kuala Lumpur tabloid of Nov 14, provided by the Ministry of Works, was entitled, ‘CHECK for signs of LANDSLIDES in your neighbourhood’. Through diagrammatic representations, this spread detailed the signs we should look for ‘On and around slopes’, ‘Around your house’, and ‘Around your community’, together with very wise advice on how to cope with a landslide during and after the event.

Bornean Malaysia and Indonesia, in an equatorial climate, are subjected to slope failures, which are amplified by periods of drought followed by sudden and intensive rainfall inputs. Kuching International Airport, at only 27 metres above sea level, annually records an input of just over 4,000ml of rain. In the highland areas of Borneo, rainfall far exceeds this amount, albeit in intensity upon steeper slopes which will in turn react to the changes, and now climate changes, in the once seasonal patterns of the monsoons. Local farming communities in the highlands will bear witness to this.

One thing is certain – very few people can accurately estimate the angle of a slope with the naked eye, apart from a near vertical rock-face. For many a year on fieldwork expeditions with my geomorphology students, never once did any of them give me an acceptable approximation of a slope’s angle. Thus, they were given clinometers to measure segments of a slope, usually in glaciated valleys or limestone gorges and even on giant landslides without in those days wearing hard hats to cope with possible rock-falls.

Slopes, be they on hillsides, sea cliffs, or on riverbanks are all in a state of dynamic equilibrium, constantly internally responding to natural and manmade changes in their inputs and outputs. Slope collapses must not always be viewed in terms of ‘natural disasters’ but better viewed as responses to change in the inputs and outputs of their system – most often occasioned by man – to allow slopes to return to new angles of rest.

Responses to slope movements

We tend to be more responsive to slope failures rather than being more proactive in anticipating and preventing them. We deforest stable slopes, drive road and rail cuttings through hillsides by blasting through mountains, build houses and hotels with panoramic views on the sides of hills and near sea cliffs without a single thought that such man-induced actions may, in the short- or long-term, suffer structural damage and disasters through slope failures.

Photo shows tarpaulin sheets covering a section of Canada Hill affected by a landslide last month.

A more proactive approach to such developments in an equatorial climate would be to acknowledge that the chemical decomposition, through the tropical weathering rates of bedrock, is five times faster than in temperate climes. New structures thus need piling and bolting deeply into the solid rock beneath, let alone stabilising the slopes above and below new creations.

Worldwide slope adjustments

For over 50 years, I have studied the events leading to the destruction of people and property throughout the UK and overseas. In 1966, on the very day that I celebrated my small Oxford College’s hockey team’s victory away against Cardiff University, all students were asked to come as volunteers to rescue victims from a massive landslide in the small coal mining village of Aberfan, a few kilometres away in South Wales. The disaster of the collapse of an overloaded coal shale tip heap was caused by a river emerging at its foot as a spring and cutting back into the slope. This ‘spring sapping’ caused instability on the shale slopes until they collapsed onto the village, killing 116 pupils in the primary school and another 28 adults in that village. The blame for this disaster lay in the lap of the government of that day.

In the early 1970s, on a very steep hillside in the New Territories, Hong Kong, high rise apartment blocks were shunted downhill in a domino-like effect as those above collapsed onto those below as the rotted granite regolith on which their foundations were built expanded with heavy rainfall inputs, thus causing the slopes to bulge and collapse in order to meet their new angles of rest.

In the UK’s Peak District National Park, in October 1976, the main A625 road linking Sheffield with Manchester vanished downslope overnight and forever. It had been constructed on a circuitous route uphill over a multiple landslip at Mam Tor in Derbyshire. The previous summer had recorded the longest drought in British history, followed by an exceptionally wet autumn. The slopes swelled and bulged and moved downhill. This road, like the others I’ve mentioned were all disasters waiting to happen for, in all cases, adequate drainage systems under the slopes had not been installed.

Landslides recorded in Borneo

Most landslides are triggered by a combination of factors which I have tried to show in my accompanying diagram. The most disastrous are caused by a lack of ingenuity on the part of planners, developers, and civil engineers in the construction of roads and settlements in often precarious places. Canada Hill in Miri, for example, has suffered numerous landslides in the last few years forcing the local authorities together with Sarawak and federal funding, to address the stabilisation of its slopes. The Kapit area is frequently hit by landslides, which dislodge houses and block roads. With the ever-increasing sizes of lorries and their loads, it is not surprising that their vibrations in low gears cause trigger landslides in hilly rural areas.

Visitors walk along the broken road at Mam Tor in Derbyshire. – Photo by Eamon Curry

For very many years I have driven from Kota Kinabalu to the National Park of Mount Kinabalu along its uphill meandering road. On my earlier drives the road was often reduced to one lane only as landslides had blocked sections of this road, as well as shifting downslope to devour houses below. In more recent drives along the same road, often in torrential rain, I have observed how civil engineers have stabilised steep slopes with mesh structures allowing vegetation to recolonise bare slopes and anchor them, together with drainage systems to take the rainfall into rivers below. Such ingenuity needs to be applauded and I trust that the lessons learned there will be taken into account in the construction of the Pan Borneo Highway.

Careful planning together with the resettlement of folk can overcome catastrophe. Slope failures resulting in landslides and inevitable loss of life exist in all corners of our world and will still occur, but with stringent planning most can be avoided. Sea cliffs collapse onto holidaymakers on beaches below, while hotels and houses are built high up in threatened coastal areas offering sea views. Building developers and their land agents need to be aware of potential slope failures as well as of their constructions in lowland river delta areas where sea levels are rising with increasing risks of flooding. Local authorities, too, need to be ahead of proposals submitted for planning applications in suspected areas of potential slope failure or flooding risks.

In short, all slopes are part of an ecosystem peculiar to the area in which they are located. Should we upset the balance between input and output in a stable system, then we are asking for trouble, should that slope reach a state of entropy and thus ‘crash out’. With foresight and truthful assessments of the probability of slope collapses many potential ‘accidents’ can be avoided or averted. Such assessments need the input of geomorphologists and geologists, and to recognise their values in the future planning of new housing developments. Such specialists exist through civil and military engineering courses at first degree level and beyond. It is only through the employment of these specialists at both national and local government levels that slope failures leading to catastrophes and the loss of human lives and, indeed, animal habitats, can be avoided.