ID:272 FORECASTING EROSION INDUCED LANDSLIDE Roslan
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ID:272 FORECASTING EROSION INDUCED LANDSLIDE Roslan Zainal Abidin1 & Mohamad Ayob2 1Deputy Vice Chancellor, Kuala Lumpur Infrastructure University College, Malaysia E-mail: [email protected] 2Senior lecturer, School of Engineering & Technology Infrastructure, Kuala Lumpur Infrastructure University College E-mail: [email protected] In Malaysia, erosion induced landslide poses enormous threats and over the past years as well as in the present scenario have caused severe damages. Apart from claiming lives of the humanity, it destroys residential and commercial properties, arrests development in urban and rural areas and impairs water quality of rivers and streams. The problem of erosion induced landslide is not unique as it occurs in most countries throughout the world. From the engineering perspective, soil erosion includes the process of detachment of soil particle from the soil mass as a function of rainfall erosivity and soil erodibility. When raindrops fall on the bare surface of a slope, it would result in the slope to be eroded and exhibiting erosion features of sheet, rill or gully. With increasing external stimulus of intense rainfall, this would gradually cause slope failure or landslide as commonly being known. Slope failure due to soil erosion phenomenon that leads to landslide accurrences had entirely been referred under the standard classification system of shallow translational type of movement of debris slide and flows. Identification of potential erosion locations is substantially crucial as it would lead to the determination of landslide prone areas. At present, there are about 50 numbers of major landslides event in Malaysia since 1993 that have been identified as erosion induced landslide mainly due to its geographical location in the area of destruction which recorded an average annual rainfall of 2500 mm. The need to take appropriate mitigating measures against erosion is essential in planning new development projects. As erosion induced landslide constitute a major socio-economic problem, information on erosion risk locations would supplement a reliable landslide hazard map in the country. This in many ways would reduce the number and impact of landslide accurrence, thus mitigating economic and social losses. Keywords: Soil Erosion, Landslide, Rainfall Erosivity, Soil Erosibility. Introduction Soil erosion is universally recognized as a serious threat to the human’s well- being. The development of this matter which was almost unknown more than 102 years ago is now gaining worldwide attention. Studies of the effect of soil erosion on early civilizations have shown that a major cause of the downhill of many flourishing empires was due to soil degradation. The 2 main agents of soil erosion are water and wind. By consideration of the conditions under which each will be active, a pattern can be built up of the areas of the world where either water or wind is likely to be particularly serious. The factor which most influenced soil erosion by water is the mean annual rainfall. In regions of very low, there can naturally be little soil erosion caused by rain. Further, what little rain does fall is mainly taken up by vegetation permanently short of water. At the other extreme, an annual rainfall of more than 1000mm usually leads to dense forest vegetation. The most severe soil erosion will tend to be associated with the range of rainfall when the vegetation is largely distributed, higher rainfall and the natural forest is removed. However, it is not the amount of rainfall that matters, but also the kind of rain. The intensive downpour common in equatorial and tropical climates has a very much ID:272 more damaging effect that the gentler rain of temperate climates and the approximate limit of the area of destructive rain are latitudes 4° North and 4° South. The Erosion Process Erosion is essentially a two part process. One part is the loosening by raindrop impact involving wetting and drying cycles. The other part of the process is the transportation of soil particles, largely by flowing water. In its physical aspect, erosion is an accomplishment of certain amount of work in tearing apart and transporting soil material largely by flowing water. In mathematical terms, erosion is a function of the erosivity of the rain and the erodibility of the soil, (Hudson, 1979); Erosion = f {Erosivity} {Erodibility} (1) Landslides The word ‘landslide’ refers to the geomorphic features that result from the event as defined by Cruden (1991). Other terms used to refer to landslide events include ‘mass movements’, ‘slope failures’, ‘slope instability’ and ‘terrain instability’ (Ministry of Sustainable Resource Management, 2003). In spite of the simple definition, landslide events are complex geological/geomorphological processes and are therefore difficult to classify. Although it is not the vital aim to make a detailed study concerning the morphology and typology of landslides, nevertheless, it is important to distinguish different kinds of slide forms. The most widely used and useful classification is that of Varnes (1978), which classified landslides based upon material type and the type of movement that took place. The basic types of slope movements in the classification are summarized in Table 1. Table 1: Classification of Slope Movement (After Varnes, 1978) TYPE OF MATERIAL Engineering Soils TYPE OF MOVEMENT Bedrock Predominantly Predominantly Coarse Fine Rock Falls Debris fall Earth fall fall Rock Topples Debris topple Earth topple topple Rock Rotational Debris slump Earth slump slump Few Rock units Debris block Earth block Slides block slide Slide Translational slide Many Rock Debris slide Earth slide units slide Rock Lateral spread Debris spread Earth spread spread Rock Debris flow Earth flow Flows flow (soil creep) (soil creep) ID:272 (deep creep) Combination of two or more principal Complex types of movement Factor influencing erosion There are 4 factors that have been identified would contribute in either expediting or inhibiting the soil erosion process namely soil characteristics, topography, ground cover and climate. Soil characteristics There are four soil characteristics that are important in determining the level of soil erodibility, namely the soil texture, organic matter content, soil structure and permeability.Soil Texture is one of the most significant soil physical properties since it influences the behavior of soil hydraulic properties like infiltration and hydraulic conductivity (Golson, Tsegaye, Rajbhandari, Green, Mays and Crosson, 2001). Obviously, soil texture refers to the sizes and proportions of the particles making up the particular soil. Sand, silt, and clay are the three major classes of soil particles whereby the existence of these three different components in any particular soil would lead to the classification of soil series and their textural classification. Rengam series for instance are made up of about 20% - 50% sand, 5% - 10% silt and 20% - 40% clay is classified as a sandy clay loam while sandy loam soil, e.g Serdang series, is one with either 20% clay or less, 10% silts and about 70% - 80% sand. Topography Slope length and slope steepness are critical factors in erosion potential, since they determine in large part of the velocity of runoff. The increasing velocity of runoff that results from higher gradients makes the water a better transporting agent, causing more soil removal. The potential erosive energy of flowing water increases as the square of the velocity (Hudson, 1979). As slopes get steeper, erosion increases and long continuous slopes allow runoff to build up momentum. Due to the increased runoff, the film of water on the surface becomes thinner thus allowing raindrops to hit the ground more directly. As a result, the impermeable layer that is formed on soil during rainstorms is eroded away, exposing the easily detachable materials in the subsoil (RRIM, 1980). As the high velocity runoff prolongs, it tends to concentrate in narrow channels and produce rills and gullies. Ground cover Ground cover refers principally not only to vegetation, but it also includes surface treatment placed by man such as shot-crete, jute netting, wood chips and crushed rock. Tropical country like Malaysia, although receives high rainfall, experiences suitable climate for the growth of vegetation if compared to the deserts environment. Vegetation grows rapidly and provides a complete ground cover, which protects the soil from ID:272 erosion. However, although rainfall is very infrequent in deserts, but when it does occur, it is typically very intense and the erosion rates are often high because there is little ground cover to protect the soil. Climate Climate acts in several ways to promote the occurrence of erosion and landslides but rainfall is the most significant influence, both long, soaking rains and short-duration, high-intensity rainstorms can promote failure (Walker et al, 1987). For the case of Peninsular Malaysia with an equatorial monsoon type of climate and experiencing a distinct seasonal monsoon rainfall with mean annual precipitation ranges from 1750 – 2510 mm, high erosion occurrences throughout the country is anticipated as it is also known that high erosivity of tropical rains is attributed to its intensity, big drop size and to wind velocity that increases the energy load (Lal, Lawson and Anastase, 1978). In order to make general prediction of landslides occurrence for an area solely based on precipitation, the availability of large amount of rainfall data is important