A Landslide Risk Rating System for Baguio Philippines

Engineering Geology 91 (2007) 85 – 99 www.elsevier.com/locate/enggeo

A Landslide Risk Rating System for Baguio, Philippines Artessa Saldivar-Sali a , Herbert H. Einstein b, ⁎ a

b

University of the Philippines, Philippines Massachusetts Institute of Technolog Massachusetts echnology, y, 77 Massachus Massachusetts etts Aven Avenue, ue, Room 1-342, Cambridge, MA 02139, USA

Received 18 February 2005; received in revised form 9 November 2006; accepted 30 November 2006 Available online 29 December 2006

Abstract

This research formulates a Landslide Risk Rating System for the Greater Baguio area in the Philippines. It is hoped that the tool will be made a part of the physical/urban planning process when used by engineers and planners and used to address risks posed by landslides given the rapidly increasing concentration of population and the development of infrastructure and industry in the Baguio area as well as in other parts of the Philippines. Thiss sys Thi system tem de descr scribe ibess thehaza thehazard rd thr throu ough gh so ca calle lled d “haza hazard rd con contribu tributing ting facto factors rs” whic which h in this case are bed bedrock rockgeo geology logy,, slop slopee grad gradient ientss and vegetation. This is then followed by characterizing risk through the “risk contributory factors ”, which are population and land use. The analysis performed in this study is based on the specific attributes of the study area, namely: an area with relatively extreme topographic relief, underlain by variable bedrock geology, but without significant differences in amount of rainfall from one zone to another during a major precipitation event. Although this study and the proposed system are area-specific they have wider application. This is facilitated by emphasizing what is specifically local and indicating how adaptation to differing local conditions might be done. © 2007 Elsevier B.V. All rights reserved. Keywords: Keywor ds: Landslides; Risk rating; Geologic factors; Land-use planning

1. Introduction and background

With the rapidly increasing concentration of population and development of infrastructure, the loss of life and an d pr prop oper erty ty da dama mage ge ca caus used ed by la land ndsl slid ides es in th thee Philippines in general and the Greater Baguio area in particular has grown grow n significantly signifi cantly over the past decades. Baguio City is located in Luzon Island, approximately 200 km north north of Manila Manila (Fig. 1). 1). Like Like man many y other other area areass of the Philippines, Philippines, the Baguio area is subject to many factors

⁎

Corresponding author. E-mail address: [email protected] (H.H. Einstein).

0013-7952/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.enggeo.2006.11.006 doi: 10.1016/j.enggeo.2006.11.006

favoring the occurrence of landslides: the mostly mountainous terrain; a humid climate with frequently occurring typhoon typ hoonss and asso associat ciated ed hea heavy vy rain rainfal fall; l; ear earthqu thquakes akes;; def defoorestation caused by human activity (mining, heavy development, agriculture). Bagui Ba guio o Cit City' y'ss ge geogr ograph aphic ical al lo loca catio tion n is 16° 16°24 24 N latitude and 120°36 E longitude. Most of the developed portion lies in the northern half of the city city.. Baguio City exte ex tend ndss 8. 8.2 2 km fr from om ea east st to we west st an and d 7. 7.2 2 km fr from om no nort rth h to south. It has a perimeter of 30.98 km. Baguio is best known for its climate, unique in the otherwise tropical country. On the average, the temperature is 8 °C lower than tha n the tem temper peratu ature re in the low lowlan lands. ds. Gen Genera erally lly,, the maximum temperature experienced in the area is 26 °C.

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A. Saldivar-Sali, H.H. Einstein / Engineering Geology 91 (2007) 85 – 99

Fig. 1. The Philippines and Baguio City.

The City owes this climate to its elevation of approximately 1500 m above sea level. Baguio's climate is characterized by a pronounced rainy season from November to April and a dry season from May to October. Average annual rainfall in Baguio is 3648 mm (1950–2003). An average number of five tropical cyclones, with a maximum-recorded number of nine tropical cyclones, affect Baguio every year. The City receives the highest amount of rainfall in the country, twice the volume experienced in Manila. In 1911, the world record of 1168.1 mm in 24 h (July 14–15) and 2009.6 mm in 2 days, 15 h (July 14–17) were recorded in Baguio (Jennings, 1950). The physical framework of Baguio City integrated road—and park systems into one. Baguio was envisioned to evolve into a compact garden city for 25,000 to 30,000 people with Burnham Park at the city center (Fig. 2). Supporting this development plan was the enactment of a charter approved on September 1, 1909 that provided administrative autonomy for the city. Soon after the city's charter was enacted, Kennon Road (Fig. 2) was opened to vehicular traffic. The existence of an artery to Baguio City, the Cordillera Region's distribution center, triggered the gold mining boom in the surrounding areas in the early to mid 1930s. Baguio City was the service and operations center for the mining industry and, hence, a direct beneficiary of the

economic growth. The events of the Second World War unfortunately left the city in total devastation. Fast paced development, however, ensued following the war years. Such development trends transformed the city into what it is today, a premier urban center north of Manila, performing a multiplicity of roles, as an educational, trade, tourism and administrative center. Baguio City has 129 barangays, the Philippines' smallest unit of local government (http://www.baguio. gov.ph, 2004). In 1982, the population of Baguio City was approximately 128,000. According to a 2000 population census, Baguio is home to 252,386 people with a population density of 5151/km2 (http://www.baguio. gov.ph, 2004). This is more than a doubling of the population in only 18 years. The population in 2005 is projected to reach 280,000. The city's population is growing rapidly at an annual growth rate of 4.4% (more than twice the national population growth rate) or an actual increment of about 7900 individuals a year (http:// www.baguio.gov.ph, 2004). This puts immense pressure on residential, commercial, institutional and infrastructure land use developments to expand proportionately. This situation is compounded by inadequate and ineffective control over land development. As a result, the traditional role of the city–that of a mountain resort – is jeopardized. With a total land area of 49 km2, and a


87

Fig. 2. Baguio City proper (C. Mapping and Reprography Dept. 1977 and 1995.).

population density of over 5000/km2 , Baguio is classified as a highly urbanized city. Attendant negative results of recent and ongoing developments in the city are rapid loss of open space, destruction of the natural environment, and scarring of the landscape. Risks to the safety of the residents as a result of hillside developments have also increased significantly. Parenthetically, there are far-reaching effects on the city's economy since tourism, which is the City's basic industry, depends so heavily on the natural environment for its viability. One of the most important risks associated with the uncontrolled development is the triggering of landslides and conversely the effect of landslides on common

activities. This paper will propose a system for landslide risk rating for the Baguio area. Before describing the development of this system, it is necessary to discuss the different types of land use in more detail. There are five land-use categories distinguishing the Benguet Physical Land Resources Evaluation conducted by the Bureau of Soils of the Department of Agriculture; namely: built-up areas, agricultural areas, grassland/shrubland areas, woodland/forest areas, and miscellaneous land types (Bureau of Soils and Water Management, 1980). Built-up areas are used primarily for commercial and residential purposes. Baguio City proper is the major population center in Benguet province and in the entire

88


Cordillera region. Two thousand six hundred hectares (approximately half of Baguio City's land area) are classified as built-up. Built-up areas are found mostly on the Baguio Plateau (the portion shown in Fig. 2), along broad alluvial valleys like La Trinidad, flood plains of major streams, and near access roads and water sources. In agricultural areas, rice is the primary crop that is planted (in paddies or terraces). Other crops that are planted on mountain and hill slopes, and on river terraces are corn, vegetables, legumes, fruits, root crops, and coffee, among others. Within the Greater Baguio area (Fig. 3), 1557 ha are devoted to agriculture (Rillon, 1992). Unforested grassland/shrubland areas are generally found on steep to very steep slopes. These are usually idle lands. “Idle land” means that it is not used for any purpose except, very rarely as pasture. This type of land comprises the largest percentage of the Greater Baguio area at 57.3% or 56,427 ha (Rillon, 1992). Because these are idle, future development will most likely take place in these areas, except on the very steep slopes ( N 50%, see Section 2.3).

Woodland/forest areas in Greater Baguio are found mostly in the Northeast of Baguio City (see e.g. Fig. 3). The total forest area is 33,718 ha or 34.2%. The ideal figure for the entire Greater Baguio area is 60% forest cover (Rillon, 1992). “Ideal” means that any future development should maintain at least 60% forest in a particular area. However, deforestation due to illegal logging and slash-and-burn farming practices has made this figure very difficult to achieve. Miscellaneous land-use types include mine pit sites, filling ponds, reservoirs, and riverwash/riverbeds. In total, these areas occupy 750 ha, which is less than 1% of Greater Baguio (Rillon, 1992). Human activity plays a large role in increasing the vulnerability of Baguio's slopes to landslides. Much of the Baguio area is denuded due to heavy deforestation. This was brought about by illegal logging and the prevalence of slash-and-burn farming, which is a traditional agricultural practice in the area. Building construction, road blasting, and blasting associated with mining activities disturb the static equilibrium of

Fig. 3. Landslide map of Greater Baguio, Philippines (after Mendoza, 1991). (Contour intervals: 100 m).


89

Fig. 4. Geologic Map of Greater Baguio (after Lands Geology Division, 1995). (Contour intervals: 100 m).

slopes. Undercutting of slopes to make room for construction in adjacent areas likewise disturbs slope equilibrium. These areas with high population density, property, and economic activity that will be adversely affected by geologic hazards must be given focus in risk zonation. The preceding comments explain why on the one hand landslides occur quite frequently in the Baguio area and why on the other hand they represent a significant risk. Fig. 3 shows a map of the landslides in the Baguio area. Most landslides are rotational, slump types. A detailed description of each slide is provided in the thesis underlying this paper (Saldivar-Sali, 2004). 2. Landslide Risk Rating System

2.1. Framework for the Landslide Risk Rating System

Landslide hazard and/or risk rating systems are an essential tool of land use — and urban planning. Areas, which due to their topography, are subject to landslides,

i.e. mountainous regions have, therefore, seen early and first applications of systems for landslide hazard/risk mapping, for instance in the Alps (Antoine, 1978; Bonnard and Noverraz, 1984; Einstein, 1988; Noverraz and Bonnard, 1990; Einstein, 1997). Similarly, if ground conditions, rainfall and topogra phy combine to produce landslides, corresponding hazard/risk mapping systems have been developed such as in Southern Italy (Carrara et al., 1977; Carrara, 1984) or in the San Francisco Bay area ( Nilsen et al., 1979; Brabb, 1984). A particularly interesting location is Hong Kong where the above-mentioned natural conditions combine with high population/building density. The approaches used there (Roberds and Ho, 1997; Wong et al., 1997; ERM, 1998; Ho et al., 2000; Wong, 2005a,b) are, therefore, exemplary. A somewhat specialized area of slope instability hazard and risk are rockfalls affecting roads and railroads. This domain is interesting since it often involves assessments by people who are not geotechnical or geologic specialists ( New York State, DOT, 1990; Pierson et al., 1990; Oregon DOT, 2002). Given the wide range of landslide or slope

90


instability hazard and risk assessment approaches, it is not surprising that generalizations have been attempted or that reviews of such systems have been made (Varnes, 1978, 1984; TRB, 1996; Leroi, 1996; Cruden, 1997; Dai and Lee, 2002; Leroi et al., 2005; Fell et al., 2005; Cascini et al., 2005; Picarelli et al., 2005). What will be proposed here is somewhat different, although in principle following the standard approaches consisting of the determination of a hazard and followed by combining the hazard with predicted consequences to estimate risk. The difference is that the proposed system is based on relatively simple characteristics, which can be easily assessed in the field or from generally available records. This makes it easily applicable in developing countries. However, and this will be discussed toward the end, the system also allows one to extend it by including higher level geotechnical information (and more detailed information on the consequences for that matter). This would eventually lead to a “full fledged” system similar to those in the previously mentioned literature and do so in a systematic way. In recognition of the situation discussed in Section 1, a Landslide Risk Rating System for the Baguio area is formulated. The aim for this tool is to contribute to the mitigation of the problem as part of the physical planning process when used by engineers and urban planners. This Landslide Risk Rating System integrates different hazard contributory factors (bedrock geology, slope gradients, vegetation) and factors contributing to risk (population, land use). In this study, “Hazard” refers to the “ probability that a particular danger will occur within a given period of time”, while “Risk ” refers to the hazard multiplied by the potential worth of loss (Einstein, 1997). The analysis performed in this study is based on the specific attributes of the study area, namely: an area with relatively extreme topographic relief, underlain by variable bedrock geology, but without significant differences in amount of rainfall from one zone to another during a major precipitation event. Although strong precipitation is identified as the main trigger of these landslides, it is not included as a variable

in this particular risk rating system because heavy rains associated with typhoons are almost always uniform throughout the entire Baguio area. For this reason it is assumed that all slopes, regardless of location, are subject to the same hazard level due to rainfall. Three factors are, therefore, identified as being variable across geographical locations within Baguio and are the inputs of this risk rating system, viz. bedrock geology, slope gradient and vegetation. A 54 km2-area was delineated on the 1:15,000 scale Engineering Geomorphology Map of Baguio City (after Mendoza, 1991, Mines and Geosciences Bureau). All of the landslides shown on that map fall within the study area (Fig. 3). 2.2. Bedrock geology

Bedrock geology is the basic contributory variable used in this Landslide Risk Rating System. The rationale for using geology as the base factor is that the engineering properties and behavior of a site vary according to the subsurface materials. More importantly, the engineering properties of the residual soils involved in the landslides can be related to the parent rock from which these soils originated. The result of this is that the relative differences of the residual soils can be expressed by the relative differences between the underlying bedrock units. This is adequate at this point of development and applicable to the risk rating system but it is also an aspect that can be refined. For instance, actual soil properties (index properties such as Atterberg limits or engineering properties) can be described. Also, the bedrock geology could be refined through inclusion of structural aspects (if visible). There are seven significant geologic units present in Greater Baguio (Fig. 4). The oldest is the Cretaceous Pugo Formation (KPpf), a stratified sequence of basaltic and andesitic volcanic rocks. This is followed by the Late Oligocene Zigzag Formation (PNzf), a series of conglomerates, sandstone, and shale with limestone lenses in some areas. The Late Oligocene Central Cordillera Diorite Complex (Pcdc), is an intrusive consisting of

Table 1 Percentage of landslide-affected area by geologic units

Area, (km2) Landslide Area, ( km2) Landslides (%)

Baguio formation (NQbf )

Zigzag formation (PNzf/Mzf )

Kennon limestone (Nkl)

Mirador limestone (Nml)

Klondyke formation (Nkf)

Pugo formation (KPpf)

24.4 1.4 5.7

10.8 0.7 6.5

0.6 0.0 0.0

14.3 0.6 4.2

2.5 0.02 0.8

1.4 0.2 14.3

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Table 2 Classification of landslide percentage areas per geologic unit % landslides

Classification

0–3% 3.01–6% 6.01–9% 9.01–12% 12.01–15% N 15%

Class Class Class Class Class Class

diorites and granodiorites. The biohermal Early Miocene Kennon Limestone (Nkl) is distinguishable by its karst topography. The Middle Miocene Klondyke Formation (Nkf) is a thick sequence of conglomerates, tuffaceous sandstone, volcanic and tuff breccia, some siltstone, and mudstone, followed by the Late Miocene Mirador Limestone (Nml), and the youngest is the Pliocene Baguio Formation (NQbf) composed mainly of pyroclastics. To determine the relative influence of bedrock geology on landslide occurrences, the area of each geologic unit lying within the study area was measured (Fig. 3, see also Table 4). Within each geologic unit, the area affected by landslides was also measured (Fig. 3). The percentage (by area) of landslides within each geologic unit was computed, with the results shown in Table 1. Based on the results presented in Table 1, i t i s proposed that the geologic units be rated as shown in Table 2. Under this proposed classification scheme, the geologic units would be rated as shown in Table 3. 2.3. Slope gradient

Baguio City proper is located on a relatively flat upland plateau with small elevation differences. Within the city limits the topographic conditions are mild to moderate with rounded hills and mountains. Elevations range from 1300 to 1600 m above sea level. Changes in elevation and topographic conditions, however, become extreme outside the City proper. The surrounding areas are heavily dissected by young, vigorously eroding river systems, giving the area large elevation differences. For instance: in

I II III IV V VI

the Northwest Plateau 1500 masl to 300 masl over7 km; in the Northeast 1700 masl to 600 masl over 8 km, in the Southeast 1500 masl to 500 masl over 5 km; and in the Southwest 1400 masl to 700 masl over 8 km. Five slope categories are recognized (by the Department of Environment and Natural Resources) according to gradients: Class I (0–8%): level to gently sloping; Class II (9–18%): gently sloping to undulating, Class III (19–30%): undulating to moderately steep, Class IV (31–50%): moderately steep to steep, Class V (N 50%): very steep. These slope categories superimposed on the geological units are shown in the map of Fig. 5. The five slope categories are also adopted for the Landslide Risk Rating System in order to maintain congruence with the existing land-use regulations. The engineer or geologist can then determine the gradient of the location being evaluated and assign a class to the slope. Usually steep slopes are likely to be less stable than flatter slopes underlain by the same type of rock. Runoff velocity also increases with slope and so, consequently, does the erosion rate. However, it was not possible to assume independence of the two factors, geology and slope, in their contribution to landslide occurrence in the Greater Baguio area. •

The first step in the procedure employed to determine the dependence of topography on geology through the contribution of the two factors (slope inclination and bedrock geology) to landslide occurrence is shown in Table 4 as percentage (by area) of each slope class found in the geologic units.

Table 3 Geologic unit hazard ratings

% landslides Class

Baguio formation

Zigzag formation

Kennon limestone

Mirador limestone

Klondyke formation

Pugo formation

5.7 II

6.5 III

0.0 I

4.2 II

0.8 I

14.3 V

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Fig. 5. Slope categories superimposed on geologic units in study area. (The map section corresponds to the marked area in Fig. 4).

•

•

The percentage (by area) of landslides occurring in each slope class found in the geologic units was determined. These results are shown in Table 5. The normalized difference between the percentage of landslides in each slope class within a given geologic unit (Table 5) and the percentage of each geologic unit occupied by that particular slope class (Table 4) were determined. These normalized differences, ND, were calculated as follows:

ND ¼

Hence, a normalized difference of zero (0) indicates that the expected number of landslides occurred in the particular slope class, given its areal proportion in the geologic unit. The normalized difference of − 100 indicates that no landslides occurred in the geologic unit (within the study area). The negative values of normalized difference indicate that fewer landslides than were expected occurred in the particular slope class, given its areal proportion in the geologic unit. Positive

ð% Landslides in Each Slope Class Within Given Geologic Unit −% Geologic Unit Occupied by Slope Class Þ Â 100

and are shown in Table 6.

% Geologic Unit Occupied by Slope Class

ð1Þ

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Table 4 Percentage (by area) of slope classes per geologic unit within study area Slope classification

Class I (0–8%) Class II (9–18%) Class III (19–30%) Class IV (31–50%) Class V (N 50%)

Geologic unit

Total area (km2) Area (%)

Area (%)

Area (%)

Area (%)

Area (%)

Baguio formation (NQbf ) Kennon limestone (Nkl) Mirador limestone (Nml) Klondyke formation (Mkf ) Zigzag formation (PNzf ) Pugo formation (KPpf ) Total

24.36 0.65 14.36 2.49 10.77 1.40 54.03

52.64 0.00 5.70 0.00 3.65 0.00

5.70 93.33 79.11 0.00 40.18 95.24

4.99 0.00 0.00 0.00 22.83 0.00

5.82 0.00 0.00 0.00 12.79 4.76

30.86 6.67 15.19 100.00 20.55 0.00

values of normalized difference indicate that more landslides than were expected occurred in the particular slope class, given its areal proportion in the geologic unit. Based on the results presented in Table 6, a modifier is applied to the classification of geologic units presented in Table 3. The modified geologic formation ratings are likewise shown in Table 6. For normalized differences between − 50 and +90, the original geologic class is retained, reflecting the fact that those specific topographies do not contribute to a significant increase or decrease of the occurrence of landslides within the geologic unit. For normalized differences of − 50.01 and below, the modified geologic class is lowered by one increment from the original geologic class. This reflects the fact that those particular topographies are associated with a significant decrease of landslides within the geologic unit. For normalized differences of + 90.01 and above, the modified geologic class is increased by one increment. This reflects the fact that those particular topographies contribute to a significant increase of landslides within the geologic unit. One exception is gradient Class V in the Pugo Formation. In this case, the modified geologic class is increased by two increments

because of the extreme positive value of normalized difference. 2.4. Vegetation

The most common vegetational cover in Greater Baguio is the broadleaf (or short leaf) pine tree that is able to grow in the area because of the cool weather (James Montgomery Consulting Engineers, 1974). Other types of vegetation include bushes/scrub, grass, or crop land/agricultural land. Within each geologic unit, the percentage of landslides occurring in areas covered by each vegetation type was determined. The results are shown in Table 7. As shown in Table 8, hazard classifications are assigned to the different types of vegetation based on the results presented in Table 7. Areas covered with Class I vegetation are the least susceptible to landslides, while those in Class IV are the most susceptible. 2.5. Faulting/tectonics

The effects of faults on landslides in Baguio were not taken into consideration in this study for several critical

Table 5 Percentage by area of landslides in each slope class Geologic unit

Total area of landslides (km2)

Class I area of landslides (%)

Class II area of landslides (%)

Baguio formation (NQbf ) Kennon limestone (Nkl) Mirador limestone (Nml) Klondyke formation (Mkf ) Zigzag formation (PNzf ) Pugo formation (KPpf )

1.45

62.88

7.63

0.00

0.00

0.64

Class III area of landslides (%)

Class IV area of landslides (%)

Class V area of landslides (%)

1.55

11.64

16.30

0.00

0.00

0.00

0.00

51.75

19.30

28.95

0.00

0.00

0.02

100.00

0.00

0.00

0.00

0.00

0.69

27.24

4.88

28.45

15.85

23.58

0.17

0.00

0.00

60.29

0.00

39.71

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Table 6 Modified geologic classes Geologic class

Gradient class

Normalized difference

Modified geologic class

Pugo formation KPpf (Class V)

V IV III II I V IV III II I V IV III II I V IV III II I V IV III II I V IV III II I

733.91 0 − 36.70 0 0 84.43 − 30.58 − 29.20 33.59 32.57 180.26 133.49 − 72.75 − 85.51 103.74 0 0 − 63.41 238.82 240.69 0 0 0 0 0 − 100 − 100 − 100 − 100 − 100

VII V V V V III III III III III III III I I III II II I III III I I I I I 0 0 0 0 0

Zigzag formation PNzf (Class III)

Baguio formation NQbf (Class II)

Mirador limestone Nml (Class II)

Klondyke formation Mkf (Class I)

Kennon limestone Nkl (Class I)

reasons (DENR-MGB, Task Force, 1990). Tectonically, the whole area is very active and therefore practically all the geologic units are highly faulted. Secondly, considering the very high density of faulting, it is doubtful that all the faults in Baguio have been mapped. To use the mapped faults in risk rating would give bias to the well-mapped areas. Therefore, like precipitation, faults are treated as a common denominator for the whole of Greater Baguio.

2.6. Possible refinements

As mentioned earlier, the proposed system is intentionally simple, i.e. relying on easily identifiable characteristics and easily distinguishable factors. Future refinements can involve the inclusion of factors which are not considered now such as faulting or river erosion. Already mentioned was the possibility to use more specific ground characteristics. At other locations rainfall

Table 7 Percentage of slides per vegetation type, by geologic unit Percentage of landslides occurring in:

Baguio formation NQbf

Broadleaf Broadleaf Mix Scrub Grass Crop/Agricultural Land None

47.35 2.33 5.05 10.87 34.41

*Vegetation data not available.

Kennon limestone Nkl

Klondyke formation Mkf

⁎

Pugo formation KPpf 100

Zigzag formation PNzf

Mirador limestone Nml

⁎

10.53

⁎

⁎

⁎

⁎

⁎

⁎

⁎

⁎

⁎

⁎

100

3.51 3.51 82.46

95


Table 8 Classification of different types of vegetation Type

Class

Broadleaf mix or bushes/scrub grass or crop land/agricultural land Broadleaf None

I II III IV

may not be uniform and would have to be distinguished. All this is simply an indication as to how the system may be expanded if desired and possible. 3. Hazard rating

The resulting hazard ratings based on bedrock geology and slope gradient (as combined in the Modified Geologic Class) and vegetation type are shown in Table 9. This is simply one possible rating system that may be derived from the results presented in the previous sections. A range of 2 is assigned to each Table 9 Landslide hazard rating of the Greater Baguio Area Modified geologic class

Vegetation type

Hazard rating

VII

IV III II I IV III II I IV III II I IV III II I IV III II I IV III II I IV III II I IV III II I

97–99 95–96 93–94 91–92 89–90 87–88 85–86 83–84 81–82 79–80 77–78 75–76 73–74 71–72 69–70 67–68 65–66 63–64 61–62 59–60 57–58 55–56 53–54 51–52 49–50 47–48 45–46 43–44 41–42 39–40 37–38 35–36

VI

V

IV

III

II

I

0

hazard rating in order to allow the user to evaluate specific sites, taking into account their unique characteristics, and use his or her judgment to differentiate whether the Hazard is relatively high or relatively low for its rating class. Hazard ratings range from a high of 99 for the unstable type of slope to a low of 35 for the most stable type of slope. The hazard rating numbers in Table 9 are an attempt at expressing the relative hazard. The most hazardous combination that was found was given the highest hazard rating and the least hazardous combination that was found was given the lowest hazard rating. Note that no combination was given a hazard rating of 100 in recognition of the fact that there may be other combinations of contributory factors that are even more hazardous. Similarly, Hazard Ratings from 0 to 34 may be assigned to areas whose combination of contributory factors proves to be less hazardous than any found in this study. 4. Risk rating

4.1. Principle

To define the potential consequences of landslides, a typology of damage related to landslide risk is established, namely: • •

Loss of life Loss of property

Hence, hazard can be related to risk by including the potential consequences of a hazardous event (a threat or danger, the occurrence of which is uncertain) (Einstein, 1988). The Hazard Rating (HR) can be converted to a Risk Rating (RR), by using multipliers based on land use and on population. RR ¼ HR TLUMTPM where: RR HR

Risk rating Hazard rating

ð2Þ

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Table 10 Land-use multipliers Land use

Land-use multiplier

Built-up Grasslands Agriculture Miscellaneous Forest

1 0.95 0.9 0.85 0.8

LUM PM

Land-use multiplier Population multiplier

would have some economic impact. Landslides near reservoirs and riverbeds might lead to siltation and/or damming, and have adverse ecological impact. These are given a Land Use multiplier of 0.85. The largest reduction is applied to forested areas. This is because these areas are largely undeveloped and, ideally, will remain undeveloped in order to facilitate reforestation of the greater Baguio area. A Land Use multiplier of 0.80 is applied. Clearly, these Land Use multipliers can be changed if desired. 4.3. Population multipliers

4.2. Land-use multipliers

For Land Use, a multiplier of 1.0 is applied to the most critical land-use situations. Multipliers less than 1 reduce the Risk Rating when land use in the area is less critical (Table 10). In this study, multipliers were selected ordinally (higher position in sequence — higher risk) to reflect the relative reductions to the Risk Rating depending on how critical the land use is in the area. The most critical Land Use situations in terms of loss of life and property are in the Built-up areas. These areas are used primarily for commercial and residential purposes. For the purposes of this Landslide Risk Rating System, roads and highways are also included in the Built-up classification because of the significant impact of their closure when affected by landslides. It is clear that Built-up areas contain capital-intensive equipment and machinery, civil works, and critical economic activities. The effects of a landslide would thus have the most import, and the Risk Rating must not be reduced. Grasslands are unforested areas covered with grasses and shrubs and are generally found on steep to very steep slopes. They are usually idle (see Section 1) land very rarely used for pasture purposes. However, because the land is idle, future development will most likely take place there. The Land Use multiplier is thus set at 0.95 in recognition of the fact that grassland areas will most likely be tapped for future development (except in the case of very steep slopes). A reduction of 10% is applied to the Risk Rating in agricultural areas. Agriculture is a significant component of the region's economy and, thus, is assigned a Land Use multiplier of 0.9. Miscellaneous areas include mine pit sites, tailing ponds, reservoirs, and riverwash/riverbeds. Since mine pit sites and tailing ponds are components of the operations of mines, their destruction by landslides

For Population, the multipliers range from 0.75–1.0, with high population in the area in question being assigned a multiplier of 1.0 (Table 11). This reflects the fact that the full Risk Rating must be applied in areas with the highest population. 4.4. Possible refinements

Similar to the refinements of the factors affecting the hazard (see Section 2.6) it is also possible to describe risk, i.e. the consequences in more detail. For instance, the land use can be further subdivided. A further step would be to express the consequences on infrastructure, material property and life in form of utilities as has been discussed in Einstein (1988). 4.5. Use of the Landslide Risk Rating System

Practically, the risk rating system is to be used as follows: •

•

•

determine combined slope class/geologic unit (e.g. like map, Fig. 5) determine modified geologic class for each of these combinations for each area as obtained above, subdivide each for vegetation and associate hazard rating (Table 9)

Table 11 Population multipliers Population per barangay

Multiplier

0–1000 1000–2000 2000–3000 3000–4000 4000–5000 N 5000

0.75 0.8 0.85 0.9 0.95 1

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Table 12 Hazard and risk ratings for selected villages in the Greater Baguio area Barangay

Geologic class

Gradient class

Modified geologic class

Vegetation type

Hazard rating

Engineer's Hill Military cut-off Puliwes Camp 7 San Vicente Camp Sioco Balsigan Phil-Am SLU-SVP Sto. Rosario Bakakeng Central Sto. Tomas proper

NQbf NQbf NQbf NQbf NQbf NQbf NQbf NQbf PNzf NQbf NQbf PNzf

I II II II II III II II III III IV III

III I I I I I I I III I III III

IV III IV I IV IV IV III IV III IV IV

65 47 49 43 49 49 49 47 65 47 65 65

•

for each area with its particular hazard rating, multiply with appropriate land use/population multiplier.

To show how the application of the proposed hazard and risk system would look, we have applied it in the District V area of Baguio City. Table 12 lists all the entries (geology, etc.), the resulting hazard rating and then with the population multiplier the resulting risk rating. The ratings apply to the “ barangays”, i.e. villages named in Table 12. Hence, the land units to which the

66 48 50 44 50 50 50 48 66 48 66 66

Land-use multiplier

Population multiplier

Risk rating

1.00 1.00 1.00 0.95 0.90 1.00 0.95 0.95 0.90 1.00 1.00 0.95

0.85 0.8 0.85 1 0.95 0.8 0.85 0.75 0.85 0.85 0.95 0.9

55 38 42 41 42 39 40 33 50 40 62 56

56 38 43 42 43 40 40 34 50 41 63 56

ratings apply are those villages. All this information is also shown on the map of Fig. 6. The risk rating system described above has just been developed but has not yet been practically applied except for this example application. One of the major benefits of using a tool such as this is that it can serve as a basis for improving building codes. Instead of having a generalized building code, a code adapted to local conditions can be created. In addition, building regulations may be modified to take into account variations in

Fig. 6. District V of Baguio City — hazard and risk rating associated with villages named in map. (See also Table 12).

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geotechnical conditions across a given planning area. Significant savings could, in principle, be achieved by introducing a certain amount of flexibility into building codes. Another use, quite clearly, is the prioritization of mitigating measures. Such measures might include: diversion of the flow of stream and river channels away from roads to prevent undercutting of slopes in road cuts, structural stabilization by installation of piles at the toe of an undercut slope to increase shear strength of the slope, stringent control of blasting during road construction and mining operations in order to minimize vibrations, reforestation of denuded areas with fastgrowing trees, improvement of surface drainage systems to reduce downward percolation of rainwater into slopes, slope-stabilization of existing and proposed slopes, removal of water already present in the ground using subsurface drains, rock bolting in slopes where the planes of weakness daylight, review of the strategy for building road networks, and mapping of existing slide areas in geologic exploration reports to minimize adverse effects associated with building on old slide materials. 5. Conclusions

In this study, a Landslide Risk Rating System was developed for Greater Baguio as a tool for engineers and planners to delineate land-use and building constraints. Factors that are taken into account in the formulation of the rating system are bedrock geology, slope angles, and vegetation as well as land use and population density. The actual procedure consists of associating these factors in sequence with particular areas eventually resulting, e.g. in a map showing risk ratings for different areas. This has been shown in an example application to the District V area of Baguio City. One of the major benefits of practically using a tool such as this is that it can serve as a basis for improving building codes. Instead of being generalized, building codes may be more effectively adapted to local conditions. In addition, building regulations may be modified to take into account variations in geotechnical conditions across a given planning area. Significant savings could, in principle, be achieved by introducing a certain amount of flexibility into building codes. Another use, quite clearly, is the prioritization of mitigating measures. References Antoine, P., 1978. Glissements de terrains et aménagement de la montagne. Bulletin de la Société Vaudoise des Sciences Naturelles 74 (353) (Pasc. 1).

Bonnard, C., Noverraz, F., 1984. Instability risk maps from detection to administration of landslide risk. Proc. 4th Int'l. Symp. on Landslides. Canad. Geotech. Soc, Toronto. Brabb, E.E., 1984. Proc. innovative approach to landslide risk mapping. Proc. 4th Int'l. Symp. on Landslides. Canad. Geotech. Soc., Toronto. Bureau of Soils and Water Management, 1980. Present Land Use Map, Province of Benguet (Philippines). Department of Agriculture. Carrara, A., 1984. Landslide hazard mapping: aims and methods. Mouvements de Terrains. Association Francaise de Géographie Physique. Colloque de CAEN, pp. 142–151. Carrara, A., Publiese-Carratelli, E., Merenda, L., 1977. Computer based data bank and statistical analysis of slope instability phenomena. Zeitschrift fur Geomorphologie N.F. 21 (2), 187–222. Cascini, L., Bonnard, Ch., Corominas, J., Jibson, R., Montero-Olarte, J., 2005. Landslide hazard and risk zoning for urban planning and development. Proc. Landslide Risk Management, Vancouver. Balkema. Cruden, D.M., 1997. Estimating the risks from landslides using historical data. In: Cruden, Fell (Eds.), Proc. Landslide Risk Assessment. Balkema. Dai, F.C., Lee, C.F., 2002. Terrain-based mapping of landslide susceptibility using a geographic information system. Canadian Geotechnical Journal 38. DENR-MGB Task Force, 1990. Preliminary Earthquake and Geological Hazard Mapping of Baguio City. Department of Environment and Natural Resources—Bureau of Mines and Geosciences, Manila. Einstein, H.H., 1988. Special lecture — landslide risk assessment. Proc. 5th, Int'l. Symposium on Landslides. Balkema. Einstein, H.H., 1997. Landslide risk — systematic approaches to assessment and management. In: Cruden, Fell (Eds.), Proceedings Landslide Risk Assessment, Honolulu. Balkema. ERM, 1998. Landslides and Boulder Falls from Natural Terrain: Risk Guidelines Report to Geotechnical Engineering Office of Hong Kong. Fell, R., Ho, K.K.S., Lacasse, S., Leroi, E., 2005. A framework for landslide risk assessment. Proc. Landslide Risk Management, Vancouver. Ho, K., Leroi, E., Roberds, W., 2000. Quantitative risk assessment applications. Myths and Future Directions. Proc. Geo Eng. Technomic Publishing Co. James Montgomery Consulting Engineers, Inc., 1974. Report on the Water Resources Evaluation Program, Baguio City, Benguet Province. Prepared for Local Water Utilities Administration. Jennings, A.H., 1950. World's greatest observed point rainfalls. Monthly Weather Review 78, 4–5. Lands Geology Division, 1995. Geological Map of Baguio City Quadrangle. Bureau of Mines and Geosciences. Sheet 3169 III. Leroi, E., 1996. Landslide hazard-risk maps of different scales — objectives, tools and developments. Proc. 7th Int'l. Symp. on Landslides, Trondheim. Leroi, E., Bonnard, Ch., Fell, R., McInnes, R., 2005. Risk assessment and management. Proc. Landslide Risk Management, Vancouver. Balkema. Mapping and Reprography Department, 1977. Topographic Map of Baguio City. National Mapping and Resource Information Authority. Sheet 7175 IV. Mapping and Reprography Department, 1995. Topographic Map of Baguio City. National Mapping and Resource Information Authority. Mendoza, C.V., 1991. Engineering Geormorphology Map of Baguio. Bureau of Mines and Geosciences. New York State, DOT, 1990. Rockfall Hazard Rating Procedure.


Nilsen, T.H., Wright, R.H., Vlasic, T.C., Sprague, N.E., 1979. Relative slope stability and land use planning in the San Francisco Bay region. U.S.G.S. Prof. Paper, vol. 994. Noverraz, F., Bonnard, Ch., 1990. Mapping methodology of landslides and rockfalls in Switzerland. Proc. 6th Int'l. Conference and Workshop on Landslides. Milan. Oregon DOT, 2002. Rockfall Report. Picarelli, L., Oboni, F., Evans, S.G., Mostyn, G.,Fell, R., 2005. Hazard characterization and quantification. Proc. Landslide Risk Management, Vancouver. Balkema. Pierson, L.A., Davis, S.H., Von Vickle, R., 1990. The Rockfall Rating System Implementation Manual, Oregon State Highway Division. Rillon, E.A., 1992. Engineering Geological Hazard Studies of Earthquake Affected Areas in Baguio and Benguet. Bureau of Mines and Geosciences. Roberds, W.J., Ho, K.S., 1997. A quantitative risk assessment and risk management methodology for natural terrain in Hong Kong. First Int'l. Conference on Debris Flow Hazard Mitigation. ASCE. Saldivar-Sali, A. A Landslide Risk Assessment Ranking System for the Baguio City, Philippines, Area. MIT, M.Eng. Thesis, 2004.

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Transportation Research Board (TRB), 1996. Landslides — Investigation and Mitigation. Special Report No. 247. Varnes, D.J., 1984. Landslide hazard zonation: overview of principles and practice. Natural Hazards 3/63. Wong, H.N., 2005a. Development and Application of Landslide Risk Assessment. Geotech. Eng. Office, Hong Kong. Special Project Report. Wong, H.N., 2005b. Landslide risk assessment for individual facilities. Proc. Landslide Risk Management, Vancouver. Balkema. Wong, H.N., Ho, K.S., Chan, Y.C., 1997. Assessment of consequences of landslides. In: Cruden, Fell (Eds.), Proc. Landslide Risk Assessment. Balkema, Honolulu. http://www.baguio.gov.ph , 2004. (Official Website of the City Government of Baguio). Varnes, D.J., 1978. Slope movement types and processes. Landslides, Analysis and Control, Sp. Rep., vol. 176. Transport Research Board, pp. 11–33.

A Landslide Risk Rating System for Baguio Philippines

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