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FORECASTING EROSION INDUCED LANDSLIDE

Roslan Zainal Abidin1 & Mohamad Ayob2

1Deputy Vice Chancellor, Infrastructure University College, 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 (Lumb, 1975), as rainfall data can provide useful information in determining temporal probability of occurrence of potential damaging events. In this context, the term climate would mostly refer to rainfall, where rain is always seen as the driving force of erosion and concentrated heavy rainfall usually the main factor that induces landslides.

The universal soil loss equation In soil erosion study, the universal soil loss equation or USLE (after Wischmeier and Smith, 1960) as shown in Figure 1 is used in quantifying the interaction of the erosion factors to estimate the tonnage of soil loss per year. Soil characteristics, topography, ground cover and climate are the four principal factors in soil erosion that form the basis of the USLE. The equation includes the rainfall erosivity factor, the soil erodibility factor, the topographic factors and the cropping management factors. The equation takes the simple product form of:

A = R K L S C P (2) Where, A = estimated average annual soil loss (tonens/ha/yr) R = rainfall erosion index (J/ha) K = soil erodibility factor (tons/J) LS = slope length and steepness factor (dimensionless) C = vegetative cover factor (dimensionless) P = erosion control practice factor (dimensionless)

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UNIVERSAL SOIL LOSS EQUATION

EROSIVITY ERODIBILITY

PHYSICAL MANAGEMENT RAINFALL CHARACTERISTICS

ENERGY LAND CROP MANAGEMENT MANAGEMENT

A = R x K x L S x P x C

Figure 1: Factors governs by Universal Soil Loss Equation

As there have been substantial developments in establishing means and methods of identifying, classifying and assessing potential (degree of soil erodibility) hazard due to erosion and landslides occurrences, the “ROM” Scale and ‘ROSE Index (degree of rainfall erosivity) can be considered as one of the latest developments in soil erosion hazard classification.

Rainfall Erosivity Erosivity can be defined as the potential ability of the rain to cause erosion thus poses a triggering factor in most of the erosion and landslides problems. The characteristics of the erosive properties of rainfall such as rainfall amount, duration, intensity, raindrops (size velocity and shape), kinetic energy and seasonal distribution of the rain are the factors that have a great influence on soil erosion (Roslan, et. al., 1996). For all climatic regions, any rainstorm with an intensity of more than 34 mm/day is liable to create erosion. However, for Malaysia, 20 mm/day is taken as a critical daily amount of rainfall that is liable to cause erosion (Roslan and Tew, 1997). The erosive power of rain is determined by its rainfall intensity (mm of rain per hour) and droplet size. A highly intense rainfall of relatively short duration can produce far more erosion that a long-duration storm of low intensity and storms with large raindrops are much more erosive than misty rains with small droplets. Rainfall intensity may be classified as shown in Table 2.

Table 2: Classification of rainfall intensity (after Roslan, 1997) Rainfall Intensity Remarks (cm/hr) < 0.65 Low 0.65 – 1.3 Medium 1.3 – 5.0 High > 5.0 Severe

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By applying the USLE standards (Wischmeier & Smith, 1978) and ‘ROSE’ Index (Roslan & Shafee, 2005) calculation procedures, the 10 minutes raw rainfall data provides all the significant rainfall erosivity values. There are few equations that needed for the completion of the rainfall erosivity assessment which has to be calculated accordingly and follows the designated steps. Figure 2 shows the flow chart diagram for rainfall erosivity analysis.

RAINFALL EVENT (AMOUNT & DURATION)

RAINFALL PARAMETERS ANALYSIS

RAINFALL RAINFALL MAXIMUM 30 MINUTES INTENSITY, I ENERGY, E RAINFALL AMOUNT, I30

RAINFALL EROSIVITY, R

Figure 2: Rainfall erosivity analysis flow chart

The following equation is used to calculate the rainfall intensity, I of a certain rainfall event:

I = ∑ Amount of rainfall (3) Period of rainfall

By obtaining the rainfall intensity of a rainfall event, the particular rainfall kinetic energy, E can be calculated by using equation:

E = 210 + 89 log10 I (4) where, I = rainfall intensity (cm/h)

The rainfall erosivity value for certain rainfall event will need the maximum 30 minutes, I30 rainfall amount to be taken into account. This 30 minutes rainfall amount is defined as the most critical amount of rain that falls with 30 minutes interval which will generated the maximum rainfall intensities in a rainfall event. By multiplying the product of rainfall kinetic energy and maximum 30 minutes rainfall amount of the rainfall event, the rainfall erosiveness can be finally determined from the equation as follows:

R = E x I30 (5) Where, E = rainfall kinetic energy (tonne m/ha.cm), and I30= 30 minutes maximum rainfall amount (cm) ID:272

The final outcome of this analysis would be the rainfall erosivity value in tonne.m2/ha.hr which represents the amount of soil loss caused by the rainfall effect on the surface of the soil. This method clearly shows the significant threshold of rainfall that can contribute to landslide occurrence. The rainfall erosivity categories with respect to ‘ROSE’ Index (Roslan & Shafee, 2005) are as shown in Table 3.

Table 3: Rainfall erosivity category (‘ROSE’ Index)

‘ROSE’ Index CATEGORY (tones.m2/ha.hr) < 500 Low

500 – 1000 Moderate

1000 – 1500 High 1500 – 2000 Very High > 2000 Critical

By knowing the quantum of rainfall erosivity, the degree of rainfall erosiveness categorized according to ‘ROSE’ Index with regards to erosion induced landslide incidents can be made known. This valuable information can be used as an early warning to all sensitive sloping areas in the country besides taking early preventive measures. The index can also be applied universally since rainfall erosivity can be theoretically evaluated from the available automatic rainfall gauge station throughout the world. Table 4 shows the results of accumulated rainfall erosivity values for landslide tragedies in Malaysia.

Table 4: Rainfall erosivity values for landslide tragediesin Malaysia RAINFALL No. DATE TRAGEDY LOCATION EROSIVITY (tones m2/ha.hr) 1 11 December, 1993 Highland Tower 868 2 7 December, 1994 Kg.Raja, Tanah Rata 1169 3 30 June, 1995 KM 39, KL - Karak Highway 2712 4 6 January, 1996 KM 303.8, Gua Tempurung 403 5 29 August, 1996 Pos Dipang Kampar 649 KM 49, Jalan Tapah- Cameron 6 9 October, 1996 419 Highland 7 16 November, 1998 Jalan Tun Sardon, Balik Pulau 1137 8 28 November, 1998 Bukit Awana, Paya Terubong 1875 , Ulu 9 15 May, 1999 1157 Kelang 10 5 September, 1999 Jalan Tun Sardon, Balik Pulau 1113 11 20 November, 2002 , Hulu Kelang 2574 12 6 November, 2004 Taman Harmonis, Gombak 2870 ID:272

13 8 January, 2006 Taman Pusing, Ipoh 1326

Taman Desa, Jalan Kelang 14 15 January, 2006 733 Lama , 15 17 February, 2006 1469 Rawang 16 13 April, 2006 Tmn. Beringin, 870 17 16 April, 2006 Kg. Sg. Bukit Putih, Ampang 3886

18 1 June, 2006 Kg. Pasir, Hulu Kelang 1469

19 25 June, 2006 Karambunai 556

20 3 October, 2006 Wangsa Maju, Kuala Lumpur 2354

21 7 November, 2006 Gunung Jerai, Sg. Petani 2600 22 10 November, 2006 Kg. Sg. Bukit Putih, Ampang 1095

23 27 February, 2007 Taman Pelangi, Rawang 977

24 22 March, 2007 Persint 9,Putrajaya 136 25 2 June, 2007 Jalan Duta, Kuala Lumpur 1434

Lorong Ayer Panas, Kuala 26 8 June, 2007 3560 Lumpur

27 26 September, 2007 , Gombak 2045

28 23 November, 2007 Jalan Kuala Kangsar - Taiping 968

Tmn Bkt Mewah, Bkt 29 6-Dec-08 1356 Antarabangsa

Soil Erodibility Erodibility defines the resistance of the soil to both detachment and transport, although many other factors such as topography and soil management may affect soil erodibility. Any property that prevents or makes soil detachment or soil transportation difficult can reduce soil erodibility. Texture and structure certainly affect the size of grains exposed to erosive elements. As runoff must occur for rapid erosion to take place, soil properties that affect infiltration rate and permeability also affect the rate of erosion. Therefore, soil texture and structure must be the most important determinants of soil erodibility (Stallings, 1986). The role of soil texture has been indicated earlier where large particles are resistant to transport because greater force is required to move them and that fine particles are resistant to detachment because of their cohesiveness. The least resistant particles are silts and sands, thus soils with high silt and sand content are erodible. Many attempts have been made to devise a simple index of erodibility based either on the properties of the soil as determined in the laboratory or the field, or on the response of the soil to rainfall. Theoretically the use of clay content as an indicator of erodibility was found to be more satisfying, when the clay particles combine with organic matter to form soil aggregates or clods. It is the stability of these which determines the resistance of the soil (Morgan, 1971). Middleton (1930), used dispersion ratio for its erodibility index, where silt + clay content of undispersed soil were compared with that of soil dispersed in water. Bouyoucos (1935), used clay ratio as in Equation 11 to get the Bouyoucos erodibility index, which has led to the development of EIROM equation.

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Bouyoucos erodibility index = % Sand + % Silt (6) % Clay An advance and new improved soil erodibility index was then developed by the name of EIROM or ‘ROM’ Scale (Roslan & Mazidah, 2001). This Index is the modification of the original Bouyoucos equation as mentioned earlier. This new equation is still using the original principal of Bouyoucos which is analyzing the soil physical properties such as sand, silt and clay. The new equation clearly had shown the significant value and threshold for soil erodibility demarcation. From the previous equation, the obtained value will only provide an index of soil erodibility but did not demarcate any threshold. With the new EIROM equation as in Equation 7, the more realistic and significant value of soil erodibility index can be used simultaneously with its risk category as shown in Table 5 to indicate the degree of soil erodibility. EIROM = % Sand + % Silt (7) 2(% Clay)

Table 5: ‘ROM’ Scale with regards to soil erodibility category ‘ROM’ Scale Soil Erodibility Category

< 1.5 Low 1.5 ~ 4.0 Moderate 4.0 ~ 8.0 High

8.0 ~ 12.0 Very High > 12.0 Critical

The ‘ROM’ Scale The establishment of the ‘ROM’ scale was based merely on soil grading characteristics and it uses EIROM equation in order to obtain the soil erodibility index. In other words, the ‘ROM’ scale is used to measure the degree of soil erodibility based on EIROM equation. This is the first ever scale that has been developed to grade the degree of erosion with regards to soil erodibility index. In the initial stage of ‘ROM’ scale development, a number of areas and locations in Peninsular Malaysia where erosion induced landslide had occurred were identified. Physical reconnaissance and observation of soil erosion features with basic information about soil classification at the identified areas were recorded. The impact of soil erosion process on the affected areas recognized from the slopes physical features that are commonly known as sheet, rill and gully were critically observed. The identification of the soil physical properties is crucial in establishing erosion induced landslide risk. The EIROM equation as shown earlier was created to obtain an acceptable sound erodibility values compared to other scale of measurements. When any slopes experienced landslides, its erodibility is analysed with respect to the ‘ROM’ Scale. Table 6 shows the degree of landslide incidents with regards to ‘ROM’ scale classification. With its simple application and user-friendly nature, prediction can be made precisely on the susceptibility of the soil or slopes before they fail. As the scale is very easy to understand, the public would be able to understand better the level or degree of landslide occurrences.

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Table 6: Degree of Soil Erodibility at landslide tragedy site Landslide Tragedy Erodibility ‘ROM’ Scale No. Date Location Values Category 1 11 Dec 93 Highland Tower 15 Critical 2 26 Dec 93 Km 72 Gerik – Jeli 14 Critical Km 29 Tapah – Tanah 3 8 Dec 94 13 Critical Rata 4 30 Jun 98 Genting Highland 4 High Km 63.8 Kuala Lumpur - 5 01 Jul 98 5 High Karak Bukit Awana, Paya 6 28 Nov 98 5 High Terubong Penang Bukit Antarabangsa, Ulu 7 15 May 99 7 High Kelang Paya Terubung, Pulau 8 07 Dec 99 5 High Pinang Kampung Ruan 9 28 Jan 02 24 Critical Changkul 10 20 Nov 02 Taman Hillview 4 High 11 24 Feb 04 Cameron Highland 38 Critical 12 11 Oct 04 KM 303 Gua Tempurung 20 Critical KM 42 Cameron 13 17 Dec 05 Highland (Tapah- 28 Critical Ringlet) 14 08 Jan 06 Taman Pusing, Ipoh 2 Moderate Taman Desa, Jalan 15 15 Jan 06 2 Moderate Kelang Lama Kg. Sg. Bukit Putih, 16 16 Apr 06 4 High Ampang Taman Bukit Belimbing, 17 17 May 06 10 Very High 18 01 June 06 Kg. Pasir, Ulu Kelang 3 Moderate 19 25 June 06 Karambunai, Sabah 10 Very High Wangsa Maju, Kuala 20 03 Oct 06 21 Critical Lumpur 21 07 Nov 06 Gunung Jerai, Kedah 40 Critical Kg. Sg. Bukit Putih, 22 11 Nov 06 13 Critical Ampang 23 18 Nov 06 Jaya 2 Moderate 24 23 Nov 06 Taman Bukit Serdang 4 High 25 28 Nov 06 KM 262.6 21 Critical 26 29 Dec 06 Kapit, Sarawak 9 Very High 27 12 Jan 07 Jalan Lela, Sandakan 17 Critical ID:272

Taman Pelangi, 28 27 Feb 07 15 Critical Rawang Wilayah Persekutuan 29 21 Mar 07 11 Very High Putrajaya Jalan Duta, Kuala 30 02 June 07 2 Moderate Lumpur Lorong Ayer Panas 31 02 Aug 07 12 Critical Baharu, Kuala Lumpur 32 30 Nov 08 Perdana 11 Very High Taman Bukit Mewah, 33 6 Dec 08 23 Critical Bukit Antarabangsa

The ‘ROM’ scale value can be readily used to grade any slopes where predictions are intended. If the degree of erosion at a particular location is found to be high or critical, early measures can be taken into action in order to stabilize it or to prevent it from failure especially before the pressure of imminent and significant triggering factor such as heavy rainfall season or abnormal rain. Rainfall – Soil Chart With regards to erosion induced landslide events and rainfall erosivity as well as soil erodibility relationship, a chart named as Rainfall-Soil Chart (RS Chart) as in Figure 3(a), 3(b), 3(c), and 3(d) is developed to forecast the erosion induced landslide risk level. By knowing the risk level of rainfall erosivity and soil erodibility, prediction on future landslide areas especially at sloping areas in the country can be made known. The rainfall soil chart not only can be used locally but also can be applied globally since rainfall erosivity and soil erodibility values can be determined.

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Figure 3(a): Rainfall-Soil Chart for forecasting erosion induced landslide

Figure 3(b): Landslide incidence at Highland Tower, Ulu , recorded landslide risk of CL3

Figure 3(c): Landslide incidence at Taman Mewah, Bukit Antarabangsa, , Selangor recorded landslide risk of CL5

Figure 3(d): Landslide incidence at Putrajaya, Federal Territory recorded landslide risk of VHL1 ID:272

Conclusion Soil erosion is globally recognised as a serious threat to the human well-being. The basic definition of the word ‘Soil Erosion’ commonly means the destruction of soil by the dual action of water and wind. It is essentially a smoothing process with soil particles being carried, rolled or washed down by the gravitational force. Erosion induced landslide is fundamentally a continuous process caused by two prominent means of disturbance either geologically or accelerated that affect the geological strata and the surface of the earth. The severity or impact on the soil strata depends significantly on the rainfall intensity, energy and magnitude of the rainfall erosiveness besides the degree of soil erodibility itself which scour away, loosen and breakdown the soil particles and carry them away. A combination of these two main factors namely rainfall erosivity and soil erodibility can be used as predictive tool in forecasting erosion induce landslide. By knowing the level of rainfall erosivity and soil erodibility impact of the area, the potential risk of erosion induced landslide can be made known.

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