Preventing erosion with good drainage
ON THE LOOKOUT: In the second of a 3-part series on slope management, Prof Dr Roslan Zainal Abidin, president and vice-chancellor of Infrastructure University Kuala Lumpur, tells why good drainage is essential to prevent erosion and landslips
ALL man-made slopes and retaining structures need regular maintenance work. Malaysia’s climate features heavy tropical rainfall. In such an environment, a man-made slope or retaining structure that is not maintained will deteriorate, and may become unstable and collapse over time. A failure of a slope or retaining structure could cause injury to people, damage to property and enormous disruption and expense.
Malaysia’s climate is most influenced by the southwest and northeast monsoons, which bring heavy seasonal rainfall every year. In addition, there appears to be a rising number of extreme rainfall events. The rainfall during the northeast monsoon of December 2006/January 2007, saw parts of Johor receive more than three times the normal rainfall for the period, leading to massive floods in the state. Malaysia has also experienced an increasing trend of high-intensity rainfall, or heavy rainfall over a shorter duration.
With seasonal rainfall, it is possible to anticipate when the likelihood of landslides will occur. In Cameron Highlands, data indicate that the months of April, October and December carry a higher risk. In addition, the typical pattern of rainfall overnight and in the mornings mean, generally, landslides are more likely to occur in the afternoon. Due to this, those travelling to Cameron Highlands should ensure they arrive before mid-day during these months.
Heavy rainfall places additional responsibility on the shoulders of those with slopes on the land they own. The responsibility of maintaining slopes and retaining structures rests with the owner of the land, which include joint management bodies and management corporations of apartments and condominium complexes, and indirectly those they employ to manage their property.
Depending on the type of property and size of the land, regular inspections should be carried out during the rainy season, and required maintenance works should be completed during the dry season. Repairing and making a slope safe again after a landslide cost much more than preventing and mitigating slope failure from the outset.
If you have a slope on your property, you should learn how to recognise the risk of impending erosion, landslips or landslides to identify any you see on your regular inspections. Next, your inspections should focus on the condition of your drainage system and the state of any retaining structures.
Landslides are often caused by rainfall runoff saturating soils on steep slopes. Furthermore, rainfall soaking through the ground can cause a rapid rise in groundwater levels. With good drainage, surface water can be diverted away, and should not be allowed to pond on the landslide-prone slope.
When you check the condition of your property, you should look out for signs such as:
CRACKED foundations;
CRACKS or bulges in the ground or retaining structures;
LEANING poles, posts or trees, indicating soil is shifting;
BROKEN pipes, which can cause wet spots or seepage; and,
SOGGY/SPONGY patch of ground that doesn’t dry out.
Should you find these signs, you should consider drainage improvements, which are often the most cost-effective means of reducing the likelihood of landslides. These could range from deepening the drains on your land to simple repairs to patch and line broken drains. The best practice is to inspect the drainage channels and clear any blockage after a heavy rainstorm.
Checking your drainage should be part of your regular inspection, including:
CLEARING accumulated debris from drains;
REPAIRING cracks or damage to any drains or pavements;
REMOVING blockages of drain outlets and weepholes (small openings in retaining structures for drainage);
REPAIRING leaky gutters which are crucial to divert runoff from the roof into the drains instead of into the ground;
REMOVING plants or shrubs which could cause cracks in drains, slope surface covers or retaining structures; and,
CHECKING your swimming pool water use regularly, as unusually high water replenishment could indicate leaks.
Another clue to the state of your slope or retaining structures is if you find you are making repeated repairs to the same area of drain or retaining wall. Should this occur, you should investigate more closely.
Simple repairs are not stop-gap measures, however. It is important to patch broken drains, for example, and not merely fill a crack in the drain with sand or stone. Proper repair must ensure that water does not seep or flow out of any crack in a drain.
The growing number of landslides and the spectacular Bukit Lanjan rockfall in 2009 led to the formation of the Slope Engineering Branch of the Public Works Department, to spearhead a comprehensive National Slope Master Plan. Since then, more criteria have been drawn up for new hillside development for public safety.
Among these were the requirement of a comprehensive infrastructure master plan, where all key infrastructure — from drainage and retaining structures to water and power supply to access roads and sewage facilities — must first be built before any other structures are constructed. However, there remain many issues to be addressed, including the patchwork of regulations and policies on slopes, and the fractured environment of enforcement by different agencies and local authorities.
There has been a widespread expectation of more stringent enforcement against new projects on hillsides. Unfortunately, there has been insufficient focus on how to handle the legacy issues. Frequently, landslides in Malaysia have occurred through the neglect of the retaining structures. Failure to inspect and maintain them can create risk of injury, loss of life and property damage as failing to implement any mitigating measures at all. The cost of regular maintenance is far less than the price of neglect.