GSM Technical talk (in collaboration with IGM and the Geology Programme, UKM): “Tanah runtuh pendam-lama – Daripada geobahaya kepada geobencana (Old-dormant landslides - from geohazards to geodisasters)” - Tajul Anuar Jamaluddin, Ph.D. (UKM)

Event Code : GSM TT 11-27-2019 Venue Name : Bilik Mesyuarat Program Geologi, Bangunan Geologi, Fakulti Sains & Teknologi, Universiti Kebangsaan Malaysia, Bangi Host Name : Coordinator Name : Date From : 2019-11-27

Talk begins at 10:30am. Tea and refreshments will be provided at 10am.

Abstract

Tanah runtuh pendam-lama yang berskala besar dan berkedudukan dalam yang tersembunyi di terrain perbukitan dan tanah tinggi tropika lembab kini telah diketahui dan dikenalpasti sebagai suatu geobahaya terrain semulajadi yang serius. Fenomena geobahaya tanah runtuh pendam berskala besar amat jarang diperkatakan di negara ini. Maklumat literatur mengenainya dalam konteks negara beriklim tropika lembab seperti di Malaysia amat terhad. Justeru, kesedaran mengenainya di kalangan pihak berkepentingan amat rendah dan ancamannya sering diabaikan. Kewujudannya pula sering terlepas pandang kerana morfologinya dikaburi oleh tindakan proses-proses eksogen (luluhawa, hakisan, pengenapan, tanah runtuh susulan) dan tutupan tumbuh-tumbuhan hutan hujan tropika yang padat dan tebal. Pengecamannya amat bergantung kepada fitur-fitur geomorfologi yang menjadi tanda pengenalan utamanya. Antaranya ialah fitur-fitur seperti kerawang, tubir utama yang curam atau cekung, kepala, lekukan di bahagian tengah dan morfologi beralun di bahagian kaki cerun, serta perlencungan pada sistem saliran semula jadi di hilir kaki cerun. Di bahagian tengah hingga atas jasad runtuhan, saliran biasanya bercabang dan berbentuk-Y ke hulu. Pengalaman mencerap dan mengkaji tanah runtuh baharu merupakan suatu aset yang amat membantu dalam penyiasatan tanah runtuh pendam-lama. Tanah runtuh pendam juga mempunyai kitaran geomorfik tersendiri, samada peringkat muda, dewasa atau tua. Biasanya semakin tua sesebuah tanah runtuh pendam, semakin sukar untuk mengecaminya. Walaupun berada pada kitaran peringkat-tua, pengaktifan semula boleh berlaku terutama apabila diganggu oleh aktiviti manusia dan tindakan cuaca yang ekstrim. Tanpa penyiasatan dan pemetaan yang terperinci, kewujudannya sering terlepas pandang kerana saiznya yang terlalu besar hingga mencapai skala ratusan hingga ribuan meter lebar dan panjang. Kini, dengan pelbagai kemudahan teknologi pengimejan terrain dan penderiaan jauh (e.g. LiDAR, IFSAR, Google Earth, dsb) proses pencerapan, pengecaman dan pemetaan tanah runtuh pendam-lama semakin mudah dan meyakinkan. Sebaik sahaja dikenalpasti, kawasan tanah runtuh pendam-lama seharusnya dielakkan daripada diganggu atau dibangunkan. Ini kerana pengalaman di Malaysia menunjukkan bahawa pembangunan infrastruktur di dalam kawasan seperti ini seringkali menimbulkan banyak masalah, daripada masalah kejuruteraan semasa hinggalah selepas pembinaan. Kerosakan dan kemusnahan yang timbul selalunya tidak dapat diselesaikan dengan mudah kerana saiz dan isipadu kegagalan yang terlalu besar dan tidak dapat diatasi dengan sebarang kaedah kejuruteraan berstruktur yang biasa. Beberapa contoh geobencana yang disajikan dalam kertas ini menjadi bukti betapa gelinciran tanah pendam-tua adalah isu geobahaya yang perlu diambil berat, kerana ia boleh berubah menjadi geobencana yang amat merugikan. Kaedah pencegahan risiko bencana yang paling baik adalah dengan mengenalpasti kewujudannya terlebih dahulu dan mengelakkan diri daripada membangun di kawasan terbabit.

Large-scale, deep-seated, old-dormant landslides are normally hidden under thick and densely forested, wet tropical hilly terrain and highlands. With the increasing number of costly infrastructural damages and disruptions to human activities, they have now been increasingly recognised as a serious natural terrain geohazard. In this country, old-dormant landslide geohazards have been widely ignored. Literatures on them in the contexts of wet tropical terrain such as Malaysia, are very limited and hence awareness amongst the stakeholders is generally very low and their threats are often neglected. Their existences are often overlooked because their morphology is often mollified by exogenic processes (weathering, erosion, sedimentation, successive landslides) and obscured by rapid growth of and dense vegetation cover. Their identification have to be relied heavily on the associated signature geomorphic features such as the crown, arcuate head scarp, steep and concave main scarp, head, depression at mid slope, and hummocky topography at the foot slope, as well as sometimes with sudden diversion in the natural drainage along the toe. At the mid- and upper-slope sections, the natural drainage tends to bifurcate upstream into Y-shaped tributary. Experience in observing and studying new landslides is a valuable asset in investigating old-dormant landslides. Dormant landslides are also subjected to geomorphic cycle; either young, mature or old. The older the landslide, the more difficult for it to be discerned. Even though a dormant landslide is at very old stage, landslide reactivation is admissible notably when disturbed by human activities and extreme climatic events. Without detailed geological investigation and mapping, the existence of old-dormant landslides might be simply overlooked because of their enormous sizes, reaching up to hundreds to thousands of meters in width and length. With recent technological advancements in terrain imaging and remote sensing (e.g. LiDAR, IFSAR, Google Earth, etc.), their observation, identification and mapping have become easier and more convincing. Once they are detected and identified, the old-dormant landslide area should be avoided from disturbance or development activities. The Malaysian experiences indicate that infrastructural developments built on or across them often resulted in various problems, entailing from engineering difficulties during and after constructions. The untoward property damages and destructions usually so difficult to overcome due to the enormous failure scale and could not be resolved with conventional hard engineering structures. Some examples of geodisaster incidents presented in this paper are proof that old-dormant landslides are serious geohazard and could cause very costly consequences. Identification of the old-dormant landslides and avoiding development on them on the first place would be the best option for disaster risk prevention when dealing with old-dormant landslides.