This year natural slopes are included for the first time in a landslide-prevention programme aimed at reducing the threat to hillside residents during torrential rain. Under the 'landslip prevention and mitigation programme', the Civil Engineering and Development Department will identify 30 natural slopes and 150 man-made slopes annually to work on. Some 100 slopes on private property will also be inspected each year, and owners required to carry out maintenance if necessary. The programme costs about HK$600 million each year. Last year, work was completed on reducing the landslide risk of more than 8,000 man-made slopes to less than 25 per cent of 1977's risk level. Mak Shu-hei, deputy head of the Geotechnical Engineering Office, said the traditional way to consolidate a man-made slope - such as by soil nailing, which involves inserting metal bars - would not be used on natural slopes because of environmental harm. Debris-resisting barriers, such as check dams or elastic fences, would be installed. 'Soil nailing would cause serious damage to the natural environment, and requires removal of plants,' Mak said. 'So it is not an option for natural slopes. And because natural terrain covers a vast area, it would be more economical to put in place something to trap the debris rather than stabilise the whole slope.' Work on nine natural slopes is complete. A HK$7.8 million 'check' dam, which took a year and a half to complete, has been erected on a natural slope at Nam Chung Village, Tai O, Lantau Island, where a serious landslide took place in 2008 and a few inhabited houses remain. Torrential downpours in June that year triggered more than 400 landslides in and around Tai O, engulfing dozens of houses and cutting off access. Nam Chung Village was one of the worst affected areas. The dam - five metres tall, 10 metres wide and 18 metres long - has a wall made of reinforced concrete and can hold more than 600 cubic metres of debris, the estimated volume of mud that could potentially fall. 'The dam functions like a sieve,' Mak said. 'In the event of a landslide caused by torrential rain, it separates the mud and debris out and reduces the speed of the flood's flow.' There were 101 reported landslides last year, compared with 859 in 2008, and an average of 300 in the past 20 years. No landslide-related casualty was recorded last year. Mak cited the Observatory as saying there would be more extreme weather in the future and cautioned that it implied more landslides.