A Landslide Risk Rating System for Baguio Philippines

1 (2007) 85–99www.elsevier.com/locate/enggeo

Engineering Geology 9

A Landslide Risk Rating System for Baguio, Philippines

Artessa Saldivar-Sali a, Herbert H. Einstein b,⁎

a University of the Philippines, Philippinesb Massachusetts Institute of Technology, 77 Massachusetts Avenue, Room 1-342, Cambridge, MA 02139, USA

Received 18 February 2005; received in revised form 9 November 2006; accepted 30 November 2006Available 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 toolwill be made a part of the physical/urban planning process when used by engineers and planners and used to address risks posed bylandslides given the rapidly increasing concentration of population and the development of infrastructure and industry in theBaguio area as well as in other parts of the Philippines.

This systemdescribes the hazard through so called “hazard contributing factors”which in this case are bedrock geology, slope gradientsand 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 extremetopographic relief, underlain by variable bedrock geology, but without significant differences in amount of rainfall from one zone toanother during a major precipitation event.

Although this study and the proposed system are area-specific they have wider application. This is facilitated by emphasizingwhat is specifically local and indicating how adaptation to differing local conditions might be done.© 2007 Elsevier B.V. All rights reserved.

Keywords: Landslides; Risk rating; Geologic factors; Land-use planning

1. Introduction and background

With the rapidly increasing concentration of popula-tion and development of infrastructure, the loss of lifeand property damage caused by landslides in thePhilippines in general and the Greater Baguio area inparticular has grown significantly over the past decades.

Baguio City is located in Luzon Island, approximately200 km north of Manila (Fig. 1). Like many other areas ofthe 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

favoring the occurrence of landslides: the mostly moun-tainous terrain; a humid climate with frequently occurringtyphoons and associated heavy rainfall; earthquakes; defo-restation caused by human activity (mining, heavy deve-lopment, agriculture).

Baguio City's geographical location is 16°24 Nlatitude and 120°36 E longitude. Most of the developedportion lies in the northern half of the city. Baguio Cityextends 8.2 km from east to west and 7.2 km from north tosouth. It has a perimeter of 30.98 km. Baguio is bestknown for its climate, unique in the otherwise tropicalcountry. On the average, the temperature is 8 °C lowerthan the temperature in the lowlands. Generally, themaximum temperature experienced in the area is 26 °C.

mailto:[email protected]

http://dx.doi.org/10.1016/j.enggeo.2006.11.006

Fig. 1. The Philippines and Baguio City.

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

The City owes this climate to its elevation of approxi-mately 1500 m above sea level.

Baguio's climate is characterized by a pronouncedrainy season from November to April and a dry seasonfrom May to October. Average annual rainfall in Baguiois 3648 mm (1950–2003). An average number of fivetropical cyclones, with a maximum-recorded number ofnine tropical cyclones, affect Baguio every year. TheCity receives the highest amount of rainfall in the coun-try, twice the volume experienced in Manila. In 1911,the world record of 1168.1 mm in 24 h (July 14–15) and2009.6 mm in 2 days, 15 h (July 14–17) were recordedin Baguio (Jennings, 1950).

The physical framework of Baguio City integratedroad—and park systems into one. Baguio was envi-sioned to evolve into a compact garden city for 25,000to 30,000 people with Burnham Park at the city center(Fig. 2). Supporting this development plan was theenactment of a charter approved on September 1, 1909that provided administrative autonomy for the city. Soonafter the city's charter was enacted, Kennon Road(Fig. 2) was opened to vehicular traffic. The existence ofan artery to Baguio City, the Cordillera Region'sdistribution center, triggered the gold mining boom inthe surrounding areas in the early to mid 1930s. BaguioCity was the service and operations center for themining industry and, hence, a direct beneficiary of the

economic growth. The events of the Second World Warunfortunately left the city in total devastation. Fastpaced development, however, ensued following the waryears. Such development trends transformed the cityinto what it is today, a premier urban center north ofManila, performing a multiplicity of roles, as aneducational, 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 wasapproximately 128,000. According to a 2000 populationcensus, Baguio is home to 252,386 people with apopulation density of 5151/km2 (http://www.baguio.gov.ph, 2004). This is more than a doubling of thepopulation in only 18 years. The population in 2005 isprojected to reach 280,000. The city's population isgrowing rapidly at an annual growth rate of 4.4% (morethan twice the national population growth rate) or anactual increment of about 7900 individuals a year (http://www.baguio.gov.ph, 2004). This puts immense pressureon residential, commercial, institutional and infrastruc-ture land use developments to expand proportionately.This situation is compounded by inadequate andineffective 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

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

87A. Saldivar-Sali, H.H. Einstein / Engineering Geology 91 (2007) 85–99

population density of over 5000/km2, Baguio isclassified as a highly urbanized city. Attendant negativeresults of recent and ongoing developments in the cityare rapid loss of open space, destruction of the naturalenvironment, and scarring of the landscape. Risks to thesafety of the residents as a result of hillside developmentshave also increased significantly. Parenthetically, thereare far-reaching effects on the city's economy sincetourism, which is the City's basic industry, depends soheavily on the natural environment for its viability.

One of the most important risks associated with theuncontrolled development is the triggering of landslidesand conversely the effect of landslides on common

activities. This paper will propose a system for landsliderisk rating for the Baguio area. Before describing thedevelopment of this system, it is necessary to discuss thedifferent types of land use in more detail. There are fiveland-use categories distinguishing the Benguet PhysicalLand Resources Evaluation conducted by the Bureau ofSoils of the Department of Agriculture; namely: built-upareas, 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 andresidential purposes. Baguio City proper is the majorpopulation center in Benguet province and in the entire


Cordillera region. Two thousand six hundred hectares(approximately half of Baguio City's land area) areclassified as built-up. Built-up areas are found mostly onthe Baguio Plateau (the portion shown in Fig. 2), alongbroad alluvial valleys like La Trinidad, flood plains ofmajor streams, and near access roads and water sources.

In agricultural areas, rice is the primary crop that isplanted (in paddies or terraces). Other crops that areplanted on mountain and hill slopes, and on river terracesare corn, vegetables, legumes, fruits, root crops, andcoffee, among others. Within the Greater Baguio area(Fig. 3), 1557 ha are devoted to agriculture (Rillon, 1992).

Unforested grassland/shrubland areas are generallyfound on steep to very steep slopes. These are usuallyidle lands. “Idle land” means that it is not used for anypurpose except, very rarely as pasture. This type of landcomprises the largest percentage of the Greater Baguioarea at 57.3% or 56,427 ha (Rillon, 1992). Because theseare idle, future development will most likely take place inthese areas, except on the very steep slopes (N50%, seeSection 2.3).

Fig. 3. Landslide map of Greater Baguio, Philippines (a

Woodland/forest areas in Greater Baguio are foundmostly in the Northeast of Baguio City (see e.g. Fig. 3).The total forest area is 33,718 ha or 34.2%. The idealfigure for the entire Greater Baguio area is 60% forestcover (Rillon, 1992). “Ideal” means that any futuredevelopment should maintain at least 60% forest in aparticular area. However, deforestation due to illegallogging and slash-and-burn farming practices has madethis figure very difficult to achieve.

Miscellaneous land-use types include mine pit sites,filling ponds, reservoirs, and riverwash/riverbeds. Intotal, these areas occupy 750 ha, which is less than 1%of Greater Baguio (Rillon, 1992).

Human activity plays a large role in increasing thevulnerability of Baguio's slopes to landslides. Much ofthe Baguio area is denuded due to heavy deforestation.This was brought about by illegal logging and theprevalence of slash-and-burn farming, which is atraditional agricultural practice in the area. Buildingconstruction, road blasting, and blasting associatedwith mining activities disturb the static equilibrium of

fter Mendoza, 1991). (Contour intervals: 100 m).

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


slopes. Undercutting of slopes to make room forconstruction in adjacent areas likewise disturbs slopeequilibrium. These areas with high population density,property, and economic activity that will be adverselyaffected by geologic hazards must be given focus inrisk zonation.

The preceding comments explain why on the onehand landslides occur quite frequently in the Baguioarea and why on the other hand they represent asignificant risk. Fig. 3 shows a map of the landslides inthe Baguio area. Most landslides are rotational, slumptypes. A detailed description of each slide is providedin 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 anessential 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 andfirst applications of systems for landslide hazard/riskmapping, for instance in the Alps (Antoine, 1978;Bonnard and Noverraz, 1984; Einstein, 1988; Noverrazand Bonnard, 1990; Einstein, 1997).

Similarly, if ground conditions, rainfall and topogra-phy combine to produce landslides, correspondinghazard/risk mapping systems have been developedsuch 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 locationis Hong Kong where the above-mentioned naturalconditions combine with high population/buildingdensity. 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 somewhatspecialized area of slope instability hazard and risk arerockfalls affecting roads and railroads. This domain isinteresting since it often involves assessments by peoplewho are not geotechnical or geologic specialists (NewYork State, DOT, 1990; Pierson et al., 1990; OregonDOT, 2002). Given the wide range of landslide or slope


instability hazard and risk assessment approaches, it isnot surprising that generalizations have been attemptedor that reviews of such systems have been made (Varnes,1978, 1984; TRB, 1996; Leroi, 1996; Cruden, 1997; Daiand 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 approachesconsisting of the determination of a hazard and followedby combining the hazard with predicted consequences toestimate risk. The difference is that the proposed systemis based on relatively simple characteristics, which canbe easily assessed in the field or from generally availablerecords. This makes it easily applicable in developingcountries. However, and this will be discussed towardthe end, the system also allows one to extend it byincluding higher level geotechnical information (andmore detailed information on the consequences for thatmatter). This would eventually lead to a “full fledged”system similar to those in the previously mentionedliterature 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 isformulated. The aim for this tool is to contribute to themitigation of the problem as part of the physicalplanning process when used by engineers and urbanplanners.

This Landslide Risk Rating System integratesdifferent hazard contributory factors (bedrock geology,slope gradients, vegetation) and factors contributing torisk (population, land use). In this study, “Hazard” refersto the “probability that a particular danger will occurwithin a given period of time”, while “Risk” refers tothe hazard multiplied by the potential worth of loss(Einstein, 1997).

The analysis performed in this study is based on thespecific attributes of the study area, namely: an area withrelatively extreme topographic relief, underlain byvariable bedrock geology, but without significantdifferences in amount of rainfall from one zone toanother during a major precipitation event.

Although strong precipitation is identified as the maintrigger of these landslides, it is not included as a variable

Table 1Percentage of landslide-affected area by geologic units

Baguio formation(NQbf )

Zigzag formation(PNzf/Mzf )

Kennon lim(Nkl)

Area, (km2) 24.4 10.8 0.6Landslide Area, (km2) 1.4 0.7 0.0Landslides (%) 5.7 6.5 0.0

in this particular risk rating system because heavy rainsassociated with typhoons are almost always uniformthroughout the entire Baguio area. For this reason it isassumed that all slopes, regardless of location, are subjectto the same hazard level due to rainfall.

Three factors are, therefore, identified as beingvariable across geographical locations within Baguioand are the inputs of this risk rating system, viz. bedrockgeology, slope gradient and vegetation. A 54 km2-areawas delineated on the 1:15,000 scale EngineeringGeomorphology Map of Baguio City (after Mendoza,1991, Mines and Geosciences Bureau). All of thelandslides shown on that map fall within the study area(Fig. 3).

2.2. Bedrock geology

Bedrock geology is the basic contributory variableused in this Landslide Risk Rating System. The rationalefor using geology as the base factor is that theengineering properties and behavior of a site varyaccording to the subsurface materials. More importantly,the engineering properties of the residual soils involvedin the landslides can be related to the parent rock fromwhich these soils originated. The result of this is that therelative differences of the residual soils can be expressedby the relative differences between the underlying bed-rock units.

This is adequate at this point of development andapplicable to the risk rating system but it is also anaspect that can be refined. For instance, actual soilproperties (index properties such as Atterberg limits orengineering properties) can be described. Also, thebedrock geology could be refined through inclusion ofstructural aspects (if visible).

There are seven significant geologic units present inGreater Baguio (Fig. 4). The oldest is the CretaceousPugo Formation (KPpf), a stratified sequence of basalticand andesitic volcanic rocks. This is followed by the LateOligocene Zigzag Formation (PNzf), a series of con-glomerates, sandstone, and shale with limestone lenses insome areas. The Late Oligocene Central CordilleraDiorite Complex (Pcdc), is an intrusive consisting of

estone Mirador limestone(Nml)

Klondyke formation(Nkf )

Pugo formation(KPpf )

14.3 2.5 1.40.6 0.02 0.24.2 0.8 14.3

Table 2Classification of landslide percentage areas per geologic unit

% landslides Classification

0–3% Class I3.01–6% Class II6.01–9% Class III9.01–12% Class IV12.01–15% Class VN15% Class VI


diorites and granodiorites. The biohermal Early MioceneKennon Limestone (Nkl) is distinguishable by its karsttopography. The Middle Miocene Klondyke Formation(Nkf) is a thick sequence of conglomerates, tuffaceoussandstone, volcanic and tuff breccia, some siltstone, andmudstone, followed by the Late Miocene MiradorLimestone (Nml), and the youngest is the PlioceneBaguio Formation (NQbf) composed mainly ofpyroclastics.

To determine the relative influence of bedrockgeology on landslide occurrences, the area of eachgeologic unit lying within the study area was measured(Fig. 3, see also Table 4). Within each geologic unit, thearea affected by landslides was also measured (Fig. 3).The percentage (by area) of landslides within eachgeologic unit was computed, with the results shown inTable 1.

Based on the results presented in Table 1, it isproposed that the geologic units be rated as shown inTable 2. Under this proposed classification scheme, thegeologic units would be rated as shown in Table 3.

2.3. Slope gradient

Baguio City proper is located on a relatively flat uplandplateau with small elevation differences. Within the citylimits the topographic conditions are mild to moderatewith rounded hills and mountains. Elevations range from1300 to 1600m above sea level. Changes in elevation andtopographic conditions, however, become extreme outsidethe City proper. The surrounding areas are heavilydissected by young, vigorously eroding river systems,giving the area large elevation differences. For instance: in

Table 3Geologic unit hazard ratings

Baguioformation

Zigzagformation

Kennolimesto

% landslides 5.7 6.5 0.0Class II III I

theNorthwest Plateau 1500masl to 300masl over 7 km; inthe Northeast 1700 masl to 600 masl over 8 km, in theSoutheast 1500 masl to 500 masl over 5 km; and in theSouthwest 1400 masl to 700 masl over 8 km.

Five slope categories are recognized (by theDepartment of Environment and Natural Resources)according to gradients: Class I (0–8%): level to gentlysloping; 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(N50%): very steep. These slope categories super-imposed on the geological units are shown in the mapof Fig. 5.

The five slope categories are also adopted for theLandslide Risk Rating System in order to maintaincongruence with the existing land-use regulations. Theengineer or geologist can then determine the gradient ofthe location being evaluated and assign a class to theslope.

Usually steep slopes are likely to be less stable thanflatter slopes underlain by the same type of rock. Runoffvelocity also increases with slope and so, consequently,does the erosion rate. However, it was not possible toassume independence of the two factors, geology andslope, in their contribution to landslide occurrence in theGreater Baguio area.

• The first step in the procedure employed to determinethe dependence of topography on geology throughthe contribution of the two factors (slope inclinationand bedrock geology) to landslide occurrence isshown in Table 4 as percentage (by area) of eachslope class found in the geologic units.

nne

Miradorlimestone

Klondykeformation

Pugoformation

4.2 0.8 14.3II I V

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 ineach slope class found in the geologic units wasdetermined. These results are shown in Table 5.

• The normalized difference between the percentage oflandslides in each slope class within a given geologicunit (Table 5) and the percentage of each geologicunit occupied by that particular slope class (Table 4)were determined. These normalized differences, ND,
were calculated as follows:
ND ¼ ð% Landslides in Each Slope Class Within Given Geologic Unit−% Geologic Unit Occupied by Slope ClassÞ � 100% Geologic Unit Occupied by Slope Class

ð1Þ

and are shown in Table 6.

given its areal proportion in the geologic unit. Positive

Hence, a normalized difference of zero (0) indicatesthat the expected number of landslides occurred in theparticular slope class, given its areal proportion in thegeologic unit. The normalized difference of −100indicates that no landslides occurred in the geologicunit (within the study area). The negative values ofnormalized difference indicate that fewer landslides thanwere expected occurred in the particular slope class,

Table 4Percentage (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 (N50%)

Geologic unit Total area (km2) Area (%) Area (%) Area (%) Area (%) Area (%)

Baguio formation (NQbf ) 24.36 30.86 52.64 5.70 4.99 5.82Kennon limestone (Nkl) 0.65 6.67 0.00 93.33 0.00 0.00Mirador limestone (Nml) 14.36 15.19 5.70 79.11 0.00 0.00Klondyke formation (Mkf ) 2.49 100.00 0.00 0.00 0.00 0.00Zigzag formation (PNzf ) 10.77 20.55 3.65 40.18 22.83 12.79Pugo formation (KPpf ) 1.40 0.00 0.00 95.24 0.00 4.76Total 54.03


values of normalized difference indicate that morelandslides than were expected occurred in the particularslope class, given its areal proportion in the geologicunit.

Based on the results presented in Table 6, a modifieris applied to the classification of geologic unitspresented in Table 3. The modified geologic formationratings are likewise shown in Table 6. For normalizeddifferences between −50 and +90, the original geologicclass is retained, reflecting the fact that those specifictopographies do not contribute to a significant increaseor decrease of the occurrence of landslides within thegeologic unit. For normalized differences of −50.01 andbelow, the modified geologic class is lowered by oneincrement from the original geologic class. This reflectsthe fact that those particular topographies are associatedwith a significant decrease of landslides within thegeologic unit. For normalized differences of +90.01 andabove, the modified geologic class is increased by oneincrement. This reflects the fact that those particulartopographies contribute to a significant increase oflandslides within the geologic unit. One exception isgradient Class V in the Pugo Formation. In this case, themodified geologic class is increased by two increments

Table 5Percentage by area of landslides in each slope class

Geologic unit Total area oflandslides (km2)

Class I area oflandslides (%)

Class II alandslides

Baguio formation(NQbf )

1.45 62.88 7.63

Kennon limestone(Nkl)

0.00 0.00 0.00

Mirador limestone(Nml)

0.64 51.75 19.30

Klondyke formation(Mkf )

0.02 100.00 0.00

Zigzag formation(PNzf )

0.69 27.24 4.88

Pugo formation(KPpf )

0.17 0.00 0.00

because of the extreme positive value of normalizeddifference.

2.4. Vegetation

The most common vegetational cover in GreaterBaguio is the broadleaf (or short leaf) pine tree that isable 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 land-slides occurring in areas covered by each vegetationtype was determined. The results are shown in Table 7.

As shown in Table 8, hazard classifications areassigned to the different types of vegetation based on theresults presented in Table 7. Areas covered with Class Ivegetation are the least susceptible to landslides, whilethose in Class IV are the most susceptible.

2.5. Faulting/tectonics

The effects of faults on landslides in Baguio were nottaken into consideration in this study for several critical

rea of(%)

Class III area oflandslides (%)

Class IV area oflandslides (%)

Class V area oflandslides (%)

1.55 11.64 16.30

0.00 0.00 0.00

28.95 0.00 0.00

0.00 0.00 0.00

28.45 15.85 23.58

60.29 0.00 39.71

Table 6Modified geologic classes

Geologic class Gradient class Normalized difference Modified geologic class

Pugo formation KPpf (Class V) V 733.91 VIIIV 0 VIII −36.70 VII 0 VI 0 V

Zigzag formation PNzf (Class III) V 84.43 IIIIV −30.58 IIIIII −29.20 IIIII 33.59 IIII 32.57 III

Baguio formation NQbf (Class II) V 180.26 IIIIV 133.49 IIIIII −72.75 III −85.51 II 103.74 III

Mirador limestone Nml (Class II) V 0 IIIV 0 IIIII −63.41 III 238.82 IIII 240.69 III

Klondyke formation Mkf (Class I) V 0 IIV 0 IIII 0 III 0 II 0 I

Kennon limestone Nkl (Class I) V −100 0IV −100 0III −100 0II −100 0I −100 0


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

Table 7Percentage of slides per vegetation type, by geologic unit

Percentage of landslidesoccurring in:

Baguio formationNQbf

Kennonlimestone Nkl

Klondformat

Broadleaf 47.35 ⁎

Broadleaf Mix 2.33 ⁎

Scrub ⁎

Grass 5.05 ⁎

Crop/Agricultural Land 10.87 ⁎

None 34.41 ⁎ 100

*Vegetation data not available.

2.6. Possible refinements

As mentioned earlier, the proposed system isintentionally simple, i.e. relying on easily identifiablecharacteristics and easily distinguishable factors. Futurerefinements can involve the inclusion of factors which arenot considered now such as faulting or river erosion.Already mentioned was the possibility to use more spe-cific ground characteristics. At other locations rainfall

ykeion Mkf

Pugo formationKPpf

Zigzag formationPNzf

Miradorlimestone Nml

100 ⁎ 10.53⁎⁎ 3.51⁎⁎ 3.51⁎ 82.46

Table 8Classification of different types of vegetation

Type Class

Broadleaf mix or bushes/scrub Igrass or crop land/agricultural land IIBroadleaf IIINone IV


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

3. Hazard rating

The resulting hazard ratings based on bedrockgeology and slope gradient (as combined in theModified Geologic Class) and vegetation type areshown in Table 9. This is simply one possible ratingsystem that may be derived from the results presented inthe previous sections. A range of 2 is assigned to each

Table 9Landslide hazard rating of the Greater Baguio Area

Modified geologic class Vegetation type Hazard rating

VII IV 97–99III 95–96II 93–94I 91–92

VI IV 89–90III 87–88II 85–86I 83–84

V IV 81–82III 79–80II 77–78I 75–76

IV IV 73–74III 71–72II 69–70I 67–68

III IV 65–66III 63–64II 61–62I 59–60

II IV 57–58III 55–56II 53–54I 51–52

I IV 49–50III 47–48II 45–46I 43–44

0 IV 41–42III 39–40II 37–38I 35–36

hazard rating in order to allow the user to evaluatespecific sites, taking into account their unique char-acteristics, and use his or her judgment to differentiatewhether the Hazard is relatively high or relatively lowfor its rating class.

Hazard ratings range from a high of 99 for theunstable type of slope to a low of 35 for the most stabletype of slope. The hazard rating numbers in Table 9 arean attempt at expressing the relative hazard. The mosthazardous combination that was found was given thehighest hazard rating and the least hazardous combina-tion that was found was given the lowest hazard rating.Note that no combination was given a hazard rating of100 in recognition of the fact that there may be othercombinations of contributory factors that are even morehazardous. Similarly, Hazard Ratings from 0 to 34 maybe assigned to areas whose combination of contributoryfactors proves to be less hazardous than any found inthis study.

4. Risk rating

4.1. Principle

To define the potential consequences of landslides, atypology of damage related to landslide risk isestablished, namely:

• Loss of life• Loss of property

Hence, hazard can be related to risk by including thepotential consequences of a hazardous event (a threat ordanger, the occurrence of which is uncertain) (Einstein,1988). The Hazard Rating (HR) can be converted to aRisk Rating (RR), by using multipliers based on landuse and on population.

RR ¼ HRTLUMTPM ð2Þ

where:

RR Risk ratingHR Hazard rating

Table 10Land-use multipliers

Land use Land-use multiplier

Built-up 1Grasslands 0.95Agriculture 0.9Miscellaneous 0.85Forest 0.8


LUM Land-use multiplierPM Population multiplier

Table 11Population multipliers

Population per barangay Multiplier

0–1000 0.751000–2000 0.82000–3000 0.853000–4000 0.94000–5000 0.95N5000 1

4.2. Land-use multipliers

For Land Use, a multiplier of 1.0 is applied to themost critical land-use situations. Multipliers less than 1reduce the Risk Rating when land use in the area is lesscritical (Table 10). In this study, multipliers wereselected ordinally (higher position in sequence —higher risk) to reflect the relative reductions to theRisk Rating depending on how critical the land use is inthe area.

The most critical Land Use situations in terms of lossof life and property are in the Built-up areas. These areasare used primarily for commercial and residentialpurposes. For the purposes of this Landslide RiskRating System, roads and highways are also included inthe Built-up classification because of the significantimpact of their closure when affected by landslides. It isclear that Built-up areas contain capital-intensiveequipment and machinery, civil works, and criticaleconomic activities. The effects of a landslide wouldthus have the most import, and the Risk Rating must notbe reduced.

Grasslands are unforested areas covered with grassesand shrubs and are generally found on steep to verysteep slopes. They are usually idle (see Section 1) landvery rarely used for pasture purposes. However, becausethe land is idle, future development will most likely takeplace there. The Land Use multiplier is thus set at 0.95 inrecognition of the fact that grassland areas will mostlikely be tapped for future development (except in thecase of very steep slopes).

A reduction of 10% is applied to the Risk Rating inagricultural areas. Agriculture is a significant componentof the region's economy and, thus, is assigned a LandUse multiplier of 0.9.

Miscellaneous areas include mine pit sites, tailingponds, reservoirs, and riverwash/riverbeds. Since minepit sites and tailing ponds are components of theoperations of mines, their destruction by landslides

would have some economic impact. Landslides nearreservoirs and riverbeds might lead to siltation and/ordamming, and have adverse ecological impact. Theseare 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 facilitatereforestation of the greater Baguio area. A Land Usemultiplier of 0.80 is applied.

Clearly, these Land Use multipliers can be changed ifdesired.

4.3. Population multipliers

For Population, the multipliers range from 0.75–1.0,with high population in the area in question beingassigned a multiplier of 1.0 (Table 11). This reflects thefact that the full Risk Rating must be applied in areaswith the highest population.

4.4. Possible refinements

Similar to the refinements of the factors affecting thehazard (see Section 2.6) it is also possible to describerisk, i.e. the consequences in more detail. For instance,the land use can be further subdivided. A further stepwould be to express the consequences on infrastructure,material property and life in form of utilities as has beendiscussed in Einstein (1988).

4.5. Use of the Landslide Risk Rating System

Practically, the risk rating system is to be used asfollows:

• determine combined slope class/geologic unit (e.g.like map, Fig. 5)

• determine modified geologic class for each of thesecombinations

• for each area as obtained above, subdivide each forvegetation and associate hazard rating (Table 9)

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

Barangay Geologicclass

Gradientclass

Modified geologicclass

Vegetationtype

Hazardrating

Land-usemultiplier

Populationmultiplier

Riskrating

Engineer's Hill NQbf I III IV 65 66 1.00 0.85 55 56Military cut-off NQbf II I III 47 48 1.00 0.8 38 38Puliwes NQbf II I IV 49 50 1.00 0.85 42 43Camp 7 NQbf II I I 43 44 0.95 1 41 42San Vicente NQbf II I IV 49 50 0.90 0.95 42 43Camp Sioco NQbf III I IV 49 50 1.00 0.8 39 40Balsigan NQbf II I IV 49 50 0.95 0.85 40 40Phil-Am NQbf II I III 47 48 0.95 0.75 33 34SLU-SVP PNzf III III IV 65 66 0.90 0.85 50 50Sto. Rosario NQbf III I III 47 48 1.00 0.85 40 41Bakakeng Central NQbf IV III IV 65 66 1.00 0.95 62 63Sto. Tomas proper PNzf III III IV 65 66 0.95 0.9 56 56


• for each area with its particular hazard rating, mul-tiply with appropriate land use/population multiplier.

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

Fig. 6. District V of Baguio City — hazard and risk rating asso

ratings apply are those villages. All this information isalso shown on the map of Fig. 6.

The risk rating system described above has just beendeveloped but has not yet been practically appliedexcept for this example application. One of the majorbenefits of using a tool such as this is that it can serve asa basis for improving building codes. Instead of having ageneralized building code, a code adapted to localconditions can be created. In addition, building regula-tions may be modified to take into account variations in

ciated with villages named in map. (See also Table 12).


geotechnical conditions across a given planning area.Significant savings could, in principle, be achieved byintroducing a certain amount of flexibility into buildingcodes. Another use, quite clearly, is the prioritization ofmitigating measures. Such measures might include:diversion of the flow of stream and river channels awayfrom roads to prevent undercutting of slopes in roadcuts, structural stabilization by installation of piles at thetoe of an undercut slope to increase shear strength of theslope, stringent control of blasting during road con-struction and mining operations in order to minimizevibrations, reforestation of denuded areas with fast-growing trees, improvement of surface drainage sys-tems to reduce downward percolation of rainwater intoslopes, slope-stabilization of existing and proposedslopes, removal of water already present in the groundusing subsurface drains, rock bolting in slopes where theplanes of weakness daylight, review of the strategyfor building road networks, and mapping of existingslide areas in geologic exploration reports to minimizeadverse effects associated with building on old slidematerials.

5. Conclusions

In this study, a Landslide Risk Rating System wasdeveloped for Greater Baguio as a tool for engineers andplanners to delineate land-use and building constraints.Factors that are taken into account in the formulation ofthe rating system are bedrock geology, slope angles, andvegetation as well as land use and population density.The actual procedure consists of associating thesefactors in sequence with particular areas eventuallyresulting, e.g. in a map showing risk ratings for differentareas. This has been shown in an example application tothe District V area of Baguio City.

One of the major benefits of practically using a toolsuch as this is that it can serve as a basis for improvingbuilding codes. Instead of being generalized, buildingcodes may be more effectively adapted to local condi-tions. In addition, building regulationsmay bemodified totake into account variations in geotechnical conditionsacross a given planning area. Significant savings could, inprinciple, be achieved by introducing a certain amount offlexibility into building codes. Another use, quite clearly,is the prioritization of mitigating measures.

References

Antoine, P., 1978. Glissements de terrains et aménagement de lamontagne. Bulletin de la Société Vaudoise des Sciences Naturelles74 (353) (Pasc. 1).

Bonnard, C., Noverraz, F., 1984. Instability risk maps from detectionto administration of landslide risk. Proc. 4th Int'l. Symp. onLandslides. Canad. Geotech. Soc, Toronto.

Brabb, E.E., 1984. Proc. innovative approach to landslide riskmapping. Proc. 4th Int'l. Symp. on Landslides. Canad. Geotech.Soc., Toronto.

Bureau of Soils andWater 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 GéographiePhysique. Colloque de CAEN, pp. 142–151.

Carrara, A., Publiese-Carratelli, E., Merenda, L., 1977. Computer baseddata 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 anddevelopment. Proc. Landslide Risk Management, Vancouver.Balkema.

Cruden, D.M., 1997. Estimating the risks from landslides usinghistorical data. In: Cruden, Fell (Eds.), Proc. Landslide RiskAssessment. Balkema.

Dai, F.C., Lee, C.F., 2002. Terrain-based mapping of landslidesusceptibility using a geographic information system. CanadianGeotechnical Journal 38.

DENR-MGB Task Force, 1990. Preliminary Earthquake and GeologicalHazard Mapping of Baguio City. Department of Environment andNatural 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 toassessment and management. In: Cruden, Fell (Eds.), ProceedingsLandslide Risk Assessment, Honolulu. Balkema.

ERM, 1998. Landslides and Boulder Falls from Natural Terrain: RiskGuidelines Report to Geotechnical EngineeringOffice of HongKong.

Fell, R., Ho, K.K.S., Lacasse, S., Leroi, E., 2005. A framework forlandslide risk assessment. Proc. Landslide Risk Management,Vancouver.

Ho, K., Leroi, E., Roberds, W., 2000. Quantitative risk assessmentapplications. Myths and Future Directions. Proc. Geo Eng.Technomic Publishing Co.

James Montgomery Consulting Engineers, Inc., 1974. Report on theWater Resources Evaluation Program, Baguio City, BenguetProvince. 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 CityQuadrangle. 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. onLandslides, Trondheim.

Leroi, E., Bonnard, Ch., Fell, R., McInnes, R., 2005. Risk assessmentand management. Proc. Landslide Risk Management, Vancouver.Balkema.

Mapping and Reprography Department, 1977. Topographic Map ofBaguio City. National Mapping and Resource InformationAuthority. Sheet 7175 IV.

Mapping and Reprography Department, 1995. Topographic Map ofBaguio City. National Mapping and Resource InformationAuthority.

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. Relativeslope stability and land use planning in the San Francisco Bayregion. U.S.G.S. Prof. Paper, vol. 994.

Noverraz, F., Bonnard, Ch., 1990. Mapping methodology of landslidesand rockfalls in Switzerland. Proc. 6th Int'l. Conference andWorkshop 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 Manage-ment, Vancouver. Balkema.

Pierson, L.A., Davis, S.H., Von Vickle, R., 1990. The Rockfall RatingSystem Implementation Manual, Oregon State Highway Division.

Rillon, E.A., 1992. Engineering Geological Hazard Studies ofEarthquake Affected Areas in Baguio and Benguet. Bureau ofMines and Geosciences.

Roberds, W.J., Ho, K.S., 1997. A quantitative risk assessment and riskmanagement methodology for natural terrain in Hong Kong. FirstInt'l. Conference on Debris Flow Hazard Mitigation. ASCE.

Saldivar-Sali, A. A Landslide Risk Assessment Ranking System forthe Baguio City, Philippines, Area. MIT, M.Eng. Thesis, 2004.

Transportation Research Board (TRB), 1996. Landslides — Investi-gation and Mitigation. Special Report No. 247.

Varnes, D.J., 1984. Landslide hazard zonation: overview of principlesand practice. Natural Hazards 3/63.

Wong, H.N., 2005a. Development and Application of Landslide RiskAssessment. Geotech. Eng. Office, Hong Kong. Special ProjectReport.

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 consequencesof landslides. In: Cruden, Fell (Eds.), Proc. Landslide RiskAssessment. Balkema, Honolulu.

http://www.baguio.gov.ph, 2004. (Official Website of the CityGovernment of Baguio).

Varnes, D.J., 1978. Slope movement types and processes. Landslides,Analysis and Control, Sp. Rep., vol. 176. Transport ResearchBoard, pp. 11–33.

http://www.baguio.gov.ph

A Landslide Risk Rating System for Baguio Philippines

Documents

Transcript of A Landslide Risk Rating System for Baguio Philippines