KUANTAN: Multiple landslides and detritus flows are said to be the main cause of the debris flood phenomenon in the highland valleys of the Sungai Bentong basin in Pahang and Sungai Langat in Selangor.
Academy of Professors Malaysia environment and sustainability cluster head Prof Datuk Dr Ibrahim Komoo said the triggering factor for the landslides and debris flows that occurred almost simultaneously was the heavy rain that lasted for several days before the disaster on Dec 18 and 19.
"Aerial surveys over more than two hours along the basins of Sungai Benus, Sungai Perdak, Sungai Timbul, Sungai Kerau, Sungai Telemong and Sungai Bentong proved that there were landslides and streams of medium to very large pieces of debris scattered widely.
"The large landslides can be estimated to be 150m long, 50m wide and 5m to 10m thick while some of the debris streams that moved from the tops of the mountains to the foot of the highland valleys reached up to 5km in length.
"All of this became a source of debris that produced an unusually large flood. We estimate the rubble that formed the flows reached up to 10 million cubic metres," he said in a statement on Tuesday (Jan 4).
He explained that the soil layer became oversaturated because of extreme rainfall while steep slopes lost stability and collapsed at about the same time.
Additional reasons included geological factors such as soil and rock type, basin shape and rock structure as well as non-geological factors such as the clearing of forests in steep cliff areas, cutting of slopes to build infrastructure including roads and clearing of land for agriculture, he added.
"Aerial surveys showed that all types of land usage in steep slope areas were involved in the occurrence of landslides and debris flows.
"Among them were water catchment forests, secondary forests and agricultural areas," he said.
The Pahang Forestry Department recently said it had not found evidence of any logging activity in the Lentang forest.
Logging was alleged to have been the cause of a flow of wood debris during the recent floods.
The department said a review on the issue of timber piles in Jalan Bentong-Karak near the Karak Road Transport Department weighing station and the Sri Telemong bridge instead found that there were two water surge incidents.
Ibrahim, who is also a Universiti Kebangsaan Malaysia principal fellow, said the disaster that occurred this time was triggered by a linked geological process, namely landslides, debris flow, debris flood, mud flood and river flood.
"Debris floods are different from normal river floods because of the presence of excessive debris.
"During normal floods, water can rise up to several metres above the river bank. However, the increase is gradual with not much suspended material except the mud content and (it) causes less damage.
"Debris floods involve a sudden rise in water with a very large amount of debris content, having a very high destructive power beyond human expectations," he said.
He called for unity in the face of the extraordinary disaster.
"Don't point fingers but instead learn and find the best solution for a happier life," he said.
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