22 June 2015
Landslide-induced sediment production after the Sabah earthquake in Malaysia
The Mw=6.0 5th June 2015 Sabah earthquake in Malaysia, which killed 18 people in rockfalls on Mount Kinabalu, generated landslides that have released large volumes of sediment. The heavy tropical rainfall in Sabah means that this sediment is now starting to enter the river systems in the form of mudflows and sediment-rich flash floods. So, for example, last week villages around the town of Ranau were struck by a series of mudflows. In particular, Mesilau was very badly affected, during a period of heavy rainfall last Monday. Mesilau lies on the flanks of Mount Kinabalu, but is some way from the main massif:
The AsiaOne website has an interesting gallery of images of the landslides that occurred last week:
Notable here is the amount of timber in the river – to me this suggests that there must have been quite extensive landslides in the forested areas on the lower slopes of the mountain. I have not seen detailed reports or images of these landslides. The threat that this timber provides to the water supply and to the river itself is clear. The post-seismic landslides from the Sabah earthquake are also moving quite large volumes of sediment, including large boulders. Typically the riverbeds will aggrade (the bed level will rise as sediment is deposited), which will greatly increase the threat of flooding and damage to infrastructure. This slug of sediment will work its way down the river, extending the effects of the earthquake well outside the area affected by the high ground accelerations. In effect this will extend the effects of the earthquake in time and space.
The rainy season in Sabah starts in October, so this will be the main hazardous time. However, in the tropical environment of Sabah heavy rainfall can occur at any point in the year.
Fortunately Malaysia has an excellent slope management organisation, JKR, part of the Public Works Department.