rainfall thresholds for landslide prediction in loano

Journal of Applied Geology, vol. 3(1), 2018, pp. 23–31DOI: http://dx.doi.org/10.22146/jag.40001

Rainfall Thresholds for Landslide Prediction in Loano Subdistrict,Purworejo District Central Java Province

Farma Dyva Ferardi*1,3, Wahyu Wilopo1,3, and Teuku Faisal Fathani2,3

1Department of Geological Engineering, Faculty of Engineering, Gadjah Mada University, Yogyakarta, Indonesia2Civil and Environmental Engineering Department, Faculty of Engineering , Universitas Gadjah Mada, Yogyakarta,Indonesia3GAMA-InaTEK, Centre of Excellence of Technological Innovation for Disaster Mitigation, Universitas Gadjah MadaYogyakarta, Indonesia

ABSTRACT. Purworejo Districtis a district with high disaster risk index in Indonesia, es-pecially landslide. The main factor that triggers the landslide is rainfall. However, therehas been no comprehensive research on the intensity of rain that triggered the landslidein Purworejo District, especially in the Sub district of Loano. Huge landslide occurred inLoano Sub district at 2016, causing 46 deaths and damages in several houses. Therefore,it is necessary to study the prediction of landslide based on rainfall data and geologicalconditions of Loano Sub district, Purworejo District. The objective of this research is toanalyze the mechanism of landslide, to analyze the rainfall that triggers the landslide andto estimate the intensity of rain that can triggers the landslide. The research method iscollecting historical data of landslides in Purworejo District along with rainfall data, geo-logical observation that includes the condition of lithology, geomorphology and hydroge-ology, undisturbed soil sampling in Loano Sub district. The prediction of landslide usesempirical methods which were then simulated by Geostudio 2012 Software. The resultsshow that the type of landslide is sliding, the main control factor is the steep slope and thethickness of the soil. The rainfall threshold triggering landslide are as follows for lithologyandesite breccia I = 81.782 D−1.197, sandy clay I = 92.579 D−0.13, and andesite intrusionI = 145.32 D−0.338.

Keywords: Landslide · Prediction · Rainfall thresholds · Loano Subdistrict · Central Java.

1 INTRODUCTION

Purworejo District is ranked 19th out of 496 dis-tricts and cities throughout Indonesia with adisaster risk index score of 215 or high category(BNPB, 2013). In 2016, a large-scale of landslidedisaster occurred in Caok village, Karangrejovillage, Loano sub-district claimed 46 casual-ties, 15 missing people and dozens of damagedhouses (Widiastutik & Buchori, 2018).The rain-fall and geological condition are the main fac-tors causing the landslide (Cruden et al., 1996;Varnes, 1978; Varnes, 1984). Rainfall intensity

*Corresponding author: F.D. FERARDI, Depart-ment of Geological Engineering, Gadjah Mada Univer-sity. Jl. Grafika 2 Yogyakarta, Indonesia. E-mail:[email protected]

as a trigger for landslide varies depending onthe geological conditions of each region (Kar-nawati; 2005). There are several ways to ana-lyze rainfall threshold, one of them is empiri-cal methods (Guzzetti et al., 2005).An empiricalthreshold is composed by geological condition-ssuch as; soil moisture, thickness of soil, andhydrology. When rainfall reaches or exceeds thethreshold, it will trigger landslide (Reichenbachet al., 1998; Cepeda et al., 2010). By understand-ing the rainfall thresholds, the casualties dueto landslide can be reduced and as an illustra-tion for the relevant agencies in performing themost appropriate mitigation actions (Peruccacciet al., 2017).The application of rainfall thresh-olds is then developed as early warning system

2502-2822/© 2018 Journal of Applied Geology

http://dx.doi.org/10.22146/jag.40001

FERARDI et al.

for landslide (Aleotti, 2004; Huang et al., 2015;Luca et al., 2017). Therefore, it is necessary tostudy the prediction of landslide based on theanalysis of rainfall data and geological condi-tions, especially in Loano District generally inPurworejo District.

This research is aim to: (i) Evaluate the mech-anism of landslide, (ii) Analyze the rainfall thattriggers the landslide, (iii) Estimate the inten-sity of rain that triggers the occurence of land-slide.

2 RESEARCH METHODS

There are 2 sources of data used in this study;primary data and secondary data. Primarydata include geological data (morphology, ge-ological structure, hydrology and hydrogeol-ogy, lithology, and land use). Secondary datainclude regional geological data, history oflandslide in 2008–2018, and daily rainfall dataof 2008–2018. The rainfall data was preparedbythe Irrigation Department, Public WorksAgency of Purworejo District. The history oflandslide was prepared by the Disaster Man-agement Agency Purworejo District, Ministryof Health,and local newspaper.

The research was conducted in 4 stagesmethod, starting from the preparation phasecovering problem formulation, literature study,and secondary data gathering including dailyrainfall data of 2008–2018 and history of land-slide of 2008–2018. The second stage is field-work and laboratory analysis. This stage in-cludes collecting engineering geology aspectssuch as morphology, lithology, undisturbedsoil sampling, geological structure, depth ofgroundwater level, land use and observationon the location of landslide. In addition, it alsoincludes testing undisturbed soil sample in thelaboratory (ASTM, 2000). The third stage is dataanalysis of geological maps, geomorphology,rainfall thresholds in each geological condition,and slope stability analysis. The geologicalcondition is represented by the lithological unitin Loano District. Therefore, each geologicalcondition has each rainfall threshold. Then, thefinal stage is the conclusions and advice fromresearch.

3 DATA REPORTING

3.1 Geomorphology of the research areaThe geomorphology zone of research area isbased on morphography aspect or slope valueby Karnawati (2005). The geomorphologyof the research area is divided into 4 zones,namely; Plain (0–10°), Low Slope Zone (10–20°),Medium Slope Zone (20–40°), and High Den-sity Zone (>40°). The study area is dominatedby medium slope zones and the location of thelandslide is mostly found in the medium slopezone and in the high slope zone.

3.2 Geological of research areaThe geological mapping in the study area re-sulted in 3 geological units, from the oldest un-til the youngest, namely andesite breccia units,andesite intrusion, and alluvial deposit (sandyclay deposit). In this study area,the geologi-cal structures are normal fault and shear joint.The normal fault is found in Glagah Riverwhile the direction of the shear joint isrelativelynorthwest-southeast. From the remote sensinganalyzis, a dextral and sinistral fault. The geo-logical map of study area is shown in Figure 1.

3.3 Rainfall of research areaThe main triggering factor of landslide espe-cially in Indonesia is due to high rainfall (Kar-nawati, 2005). Rain can reduce the tension be-tween rock pores, weathering rocks so that thevalue of rock cohesion decreases (Hardiyatmo,2012). To predict landslide, needs daily rainfalldata within a period of 10 years from 2008 to2018.

Statistics of rain events in the study area in-dicate that the rain always drops in 4 consec-utive months in November–April. Peak of therainy season occurs in December and January.Anomalies occur in 2014 and 2016 where in thatyear the peak of the rainy season occurred inJune. The data is coming from Irrigation De-partment, Public Work Services, Purworejo Dis-trict from 2008–2018. Figure 2 shows the rainintensity graph for 10 years.

3.4 Characteristics of soil mechanicsSoil technical parameters were obtained fromlaboratory tests. This test aims to determine thesoil properties used as a parameter of slope sta-bility and the landslide mechanism that occurs.

24 Journal of Applied Geology

RAINFALL THRESHOLDS FOR LANDSLIDE PREDICTION IN LOANO SUBDISTRICT, PURWOREJO DISTRICT

Figure 1. Geological map of research area Figure 1. Geological map of research area.

Figure 2. Graphic of rainfall intensity in study area from 2008-2018 Figure 2. Graphic of rainfall intensity in study area from 2008–2018.

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Soil sampling is focused on Caok Village whichis the most vulnerable area of landslide and isconsidered to represent the research location.The location of soil sampling is shown in Fig-ure 1. An original rock from soil samples isinthe form of andesite breccia that has undergoneintensive weathering. The tested parametersinclude grain size distribution, specific grav-ity, water content, atterberg limit, and triaxialtest.Table 1shows the result of undisturbed soilsample testing in each parameter.

4 ANALYSIS

4.1 Landslide mechanismIn the research area, there is a geological struc-ture, shear joint, with the relative direction ofthe northwest, N 353°E and N 302°E. At this lo-cation,is indicated the present of normal faultdue to sudennly change in the topograpich ele-vation.

The observed landslide is rotational slidewith slides in the form of curved curves orhorseshoes. This type of landslide often oc-curs on steep slopes (>40°). Landslide is con-trolled by a steep slope of 43° so that the grav-itational force produced when the soil expe-riences movement is even greater. The con-stituent rocks, andesite breccia, which are vol-canic products, are susceptible to weatheringdue to the dominance of the feldspar mineralcontent. Feldspar is easily weathered whichmakes the andesite breccia altered into a thicklayer of soil.

The presence of shear joint becomes a discon-tinuity for rocks so that rainwater can enter andinteract with feldspar minerals. This mineralfeldspar is susceptible to alter into clay mineralwhen interacting with rainwater. This clay min-eral can be a slip surface of landslide. On theobserved location, the slip surface is in the formof lapsed andesite breccia that has been alteredinto clay soil size with direction of N 43°E andthe direction of landslide is N 55°E. Both the di-rection of avalanche and the slope of the slipsurface have the same relative direction thatis northeast, thus increasing the occurrence oflarge-scale landslide. Figure 3 shows the mor-phological appearance of landslide at the studysite.

4.2 Rainfall thresholds and lithological unitRainfall intensity that can trigger landslide ineach region is different. This difference is cer-tainly caused by the geological conditions, es-pecially rock types, compactness, and weather-ing rates. Therefore, each geological condition,represented by lithological unit, will have dif-ferent rainfall threshold.

Collecting the landslide history data is an im-portant step that must be carried out in thisresearch. The landslide history data have in-formation about rainfall intensity and durationwhen the landslide occurs, type of landslide,and type of lithological unit. In addition, thevalue of rainfall threshold is obtained by plot-ting rainfall intensity and rainfall duration.

In this research, there are several lithologicalunits thatare sandy clay, andesite intrusion, andandesite breccia (Rahardjo et al., 1995). The plot-ting of rainfall threshold is done on each litho-logical unit.

4.2.1 Alluvium deposit (sandy clay)The value of rainfall threshold is obtained byplotting the rainfall intensity and duration oflandslide that occurs on alluvium deposit. Byusing this method, the rainfall threshold valuesis represented by equation I = 92.697 D−0.13.The value of I means maximum rainfall inten-sity while D means the duration of rainfall cal-culated in number of days. This rainfall thresh-old value is very suitable to be used in the de-velopment of early warning systems with geo-logical conditions in the form of alluvium de-posits. Figure 4 shows the rainfall thresholdcurve in the geological condition of the clay de-posits. Landslides that occur in geological con-ditions in the form of avalanches on the cliffs isdue to increased flow of river currents.

When there is rainfall with the intensity of85 mm/day with the duration of 1 day or abovethe curve line, then it is predicted that a land-slide will occur. When there is rainfall with anintensity of 60 mm/day for 4 days, it is pre-dicted that a landslide will occur. Meanwhile,when it is exactly at the line of the rainfallthreshold, the soil is predicted to be in criticalcondition.

4.2.2 Andesite intrusionWhen the geological conditions are in andesiteintrusion, the rainfall threshold values is repre-



Crown

Crack

Arah Longsor N 55 E

Escarpment

Direction of Landslide N 55 E

Figure 3. Landslide morphology on study area, Dusun Caok, Karang Rejo Figure 3. Landslide morphology on study area, Dusun Caok, Karang Rejo.

0

20

40

60

80

100

120

140

160

180

200

0 1 2 3 4 5 6 7 8

Axis Title

Axis Title

I = 92,679 D -0,13

Rai

nfal

l In

tens

ity

(mm

/day

) Legend : Landslide History : Rainfall Thresholds

Antecedent Rainfall Duration (day)

Figure 4. Rainfall thresholds curve for sandy clay Figure 4. Rainfall thresholds curve for sandy clay.

sented by the equation I = 145.32 D−0.338. Thevalue of I means maximum rainfall intensitywhile D means the duration of rain calculatedin number of days. This rainfall threshold valueis very suitable for the development of earlywarning system in Purworejo District. Figure 5shows the rainfall threshold curve in the geo-logical conditions of andesite intrusion. Land-slides that occur in this geological unit isslidingtype. Landslides occur in weathered andesite.

When there is rainfall with intensity of100 mm/day with the duration of rain 1 dayor above the curve line then a landslide willoccur. When there is rainfall with intensity of70 mm/day for 2 days, then a landslide will

0

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0 1 2 3 4 5 6 7

Axis Title

Axis Title

Rai

nfal

l In

tens

ity

(mm

/day

)

I = 145,32 D -0,336

Legend : Landslide History : Rainfall Thresholds


Figure 5. Rainfall thresholds curve for andesite intrusionFigure 5. Rainfall thresholds curve for andesite in-trusion.

occur. Moreover, if it is exactly at the line of therainfall threshold, the land is predicted to bein critical condition. The intensity of rain trig-gering landslide for this geological conditionis greater than other geological conditions.Thisis because andesite lithology has the highestand freshest compactness compared to otherlithologyin the study area.

4.2.3 Andesite brecciaThe rainfall threshold values of andesitebrecciais represented by the equation I =81.782 D−1.197. Value I means the maximumrain intensity whereas D means the duration


FERARDI et al.

0

50

100

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0 1 2 3 4 5 6 7 8

Intensitas Hujan rerata (mm)

Durasi Hujan (hari)

I = 81,782 D -1,197

Rai

nfal

l In

tens

ity

(mm

/day

)

Legend : Landslide History : Rainfall Thresholds


Figure 6. Rainfall thresholds curve for andesite breccia Figure 6. Rainfall thresholds curve for andesite brec-cia.

of rain calculated in number of days. Thisrainfall threshold value is very suitable for thedevelopment of the early warning system inPurworejo District. Figure 6 shows the rainfallthreshold curve in the geological conditions ofthe andesite breccia.

When there is rainfall with intensity of80 mm/day with the duration of rain 1 dayor above the curve line then a landslide willoccur. When there is rainfall with intensity of30 mm/day for 2 days hence predicted land-slide. Meanwhile, when it is still in the rainfallthreshold curve, then the soil is predicted to bein a critical condition.

4.3 Landslide predictionThe prediction of landslide occurrences in thisstudy is related with rainfall thresholds andslope stability analysis. In this research, theanalysis of slope stability and prediction oflandslide occurrence using Geostudio 2012 soft-ware aid that is Seep/W and Slope/W. TheSeep/W software is used to determine the in-filtration of rainwater that enters the soil duringrainfall under certain conditions and its relationto groundwater rise. Slope/W is used for theslope stability analysis by considering the con-dition of the ground water level. The Parame-ter input in Geostudio 2012 is water content, co-hesion, angle of friction, bulk density, and drydensity (Table 1).

This simulation is performed on soil samplesfrom intensive weathering of andesite brecciaon Table 1 noted as S2. The location of sam-ples had experienced landslides in 2016. How-ever, the sampling is carried out on soil thathas never experienced landslides. The Sim-ulation was performed only on lithology an-

desite breccia which represents all of litholog-ical units and considered as a verification ofrainfall thresholds in each lithological unit. Therainfall thresholds of andesite breccia is repre-sented by the equation I = 81.782 D−1.197 whereI is intensity in mm and D is time in day.

The prediction of the landslide occurrencein this study includes 4 simulations. The firstsimulation was performed in a normal condi-tion where there was no rise in ground waterlevel, no rain, and no decrease in soil mechani-cal properties, the value of cohesion, and fric-tion angle. The simulation, which is the ini-tial condition of slope, shows the safety factoris 1.442 (Figure 7A). This safety factor indicatesthe slope is stable or safe (Bowles, 1984).

In the second simulation, the rainfallwas below the rainfall threshold,which was50 mm/day with 1 day rain duration. In thissimulation therewas an increase in ground wa-ter level, a decrease in cohesion values, and anincrease in weight of slopes. The rainfall inputin Slope/W was still under the rainfall thresh-olds. The second simulation shows the safetyfactor is 1.371 (Figure 7B). This safety factorindicates the slope is still stable or safe (Bowles,1984).

The third simulation waswhere rainfall wasexactly on the rainfall threshold, which was50 mm/day with 1.5 days duration of rain. Inthis simulation, there was an increase in groundwater level compared to the initial conditionand there was a decrease in cohesion value andincrease weight of slope. When the rainfall in-put in Slope/W reachesthe rainfall thresholds,the safety factor is 1.171 (Figure 7C) which isthe slope is critically to move (Bowles, 1984).

The fourth simulation was when the rain-fallwas above the rainfall threshold, which200 mm/day with 1 day rain duration. Inthis simulation, there was a significant rise ingroundwater level compared to the initial con-dition, the decrease of soil cohesion value andthe increase weight of slope. When the rainfallexceeds the rainfall thresholds, the safety factoris 0.874 (Figure 7D). This safety factor indicatesthat the slope is unstable or critical. Figure 7shows the illustrations of each simulation.



Table 1. The result of undisturbed soil sample testing in each parameter.

Testing Parameter Notation Unit Sample of Andesite BrecciaS2 S4 S7

Water Content Water content w % 45145 39063 46041

Volumetric Weight Bulk density γb gram/cm3 2.43 1905 21.39Dry density γd gram/cm3 1.67 1.37 14.65

Specific Gravity Specific gravity Gs 2654 2656 2677

Atterberg Limit

Liquid limit LL % 43.96 43.65 43.36Plastic limit PL % 30.14 30.92 37.84Plasticity Index PI % 13.82 12.73 5.53Liquidity Index LI 1.09 0.64 1.48

Grain SizePercentage of gravel % 0 0 0Percentage of sand % 9.7 10.01 23.01Percentage of silt/clay % 90.3 89.99 76.99

Triaxial CU Angle of internal friction ϕ ° 24 6 13Cohesion c kPa 20885 60.12 17.45

Figure 7. The illustration of landslide by each rainfall thresholdsFigure 7. The illustration of landslide by each rainfall thresholds.


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5 CONCLUSION

Based on the results of the research and the dataanalysis, several conclusions can be drawn.Themechanism of landslides that occurs is domi-nated by the type of sliding type, namely ro-tational sliding or slump.Landslides will occuralong the discontinuity plane. The main con-trolling factors arethe steep slope and the thicksoil. The triggering factor is high rainfall inten-sity.

Rainfall threshold values that trigger land-slides vary in each type of lithology. The rain-fall threshold valuefor the sandy caly depositrepresented by the equation I = 92.679 D−0.13.The rainfall threshold values fo the andesiteintrusion represented by the equation I =145.32 D−0.338, andthe rainfall threshold for the-andesite breccia represented by the equationI = 81.782 D−1.197. While I is the rain intensity(mm/day) and D is the duration of rain (days).Landslides will occur with rainfall intensity ofmore than 95 mm/day for sandy clay deposit.However for andesite intrusion and andesitebreccia are more than 145 mm/day and morethan 80 mm/day, respectively.

ACKNOWLEDGEMENTS

The authors would like to thank to GeologicalEngineering Department UGM for supportingall expense inthis research.

REFERENCES

Aleotti, P. (2004) A Warning System for Rainfall-Induced Shallow Failures. Engineering Geology,73, pp. 247–265.

American Standard Testing and Material (2000)D2487: Standard Practicefor Classification ofSoils for Engineering Purposes (Unified Soil Clas-sification System), 100 Barr Harbor Drive: UnitedStates.

Bowles, J.E. (1984) Physical and Geotechnical Prop-erties of Soils, McGraw-Hill Education, NewYork, USA.

Badan Nasional Penanggulangan Bencana (2014) In-deks Risiko Bencana Indonesia (IRBI) tahun 2013.Direktorat Pengurangan Risiko Bencana, DeputiBidang Pencegahan dan Kesiapsiagaan BNPB,332p.

Badan Penanggulangan Bencana Daerah KabupatenPurworejo (2016) Bencana Tanah Longsor Kabu-paten Purworejo dan Rencana Kontijensi; unpub-lished

Cepeda, J.M., Hoeg, K., Nadim, F. (2010) Landslide-triggering rainfall thresholds: A conceptual

framework. Quarterly Journal of EngineeringGeology and Hydrogeology, 43(1), pp 69-84.

Cruden, D.M., Varnes, D.J. (1996) Landslide Typesand Processes, Transportation Research Board,National Academy of Science.

Dearman, W.R. (1991) Engineering Geological Map-ping, Oxford: Butterworth-Heinemann, Ltd.

Geostudio (2012) Computation Laboratory of Civiland Environmental Engineering Department,Faculty of Engineering, Universitas GadjahMada.

Guzzetti, F., Peruccacci, S., Rossim M. (2005) Rain-fall Thresholds for the Initiation of Landslides inCentral and Southern Europe. Risk-AdvancedWeather Forecast System to Advise on RiskEvents and Management (RISK-AWARE). Me-teorology and Atmospheric Physics, 98(3-4), pp.239-267.

Hardiyatmo, H.C. (2012) Penanganan Tanah Long-sor dan Erosi.Edisi pertama. Gadjah Mada Uni-versity Press, Yogyakarta.

Huang, J., Ju, N.P., Liao, Y.J., Liu, D.D. (2015)Determination of Rainfall Thresholds for Shal-low Landslides by a Probabilistic and EmpiricalMethod. Natural Hazard Earth Systems Science,15, pp. 2715-2723.

Irrigation Department, Public Work Services of Pur-worejo, (2008–2018), Daftar Laporan Curah HujanBagian Bulan Januari-Desember, Purworejo; un-published.

Karnawati, D. (2005) Bencana Alam Gerakan MassaTanah di Indonesia dan Upaya Penanggulangan-nya. Jurusan Teknik Geologi, Fakultas Teknik,Universitas Gadjah Mada, Yogyakarta.

Luca, D.L., Versace, P. (2017) Diversity of Rain-fall Thresholds for Early Warning of Hydro-geological Disaster. Advances in Geosciences, 44,pp. 53-60.

Peruccacci, S., Brunetti, M.T., Gariano, S.L.Melillo,M. Rossi, M., Guzzetti, F. (2017) Rainfall Thresh-olds for the Possible Occurrence in Italy. Geomor-phology, 290, pp 39-57.

Rahardjo, W., Sukandarrumidi, Rosidi, H.M.D.(1995) Peta Geologi Lembar Yogyakarta, Jawa.Pusat Penelitian dan Pengembangan Geologi,Bandung.

Reichenbach, P., Cardinali, M., Vita, P.D., Guzetii,F. (1998) Regional Hydrological Thresholds forLandslides and Floods in the Tiber River Basin(Central Italy). Environmental Geology, 35(2-3),pp. 146-159.

Varnes, D.J. (1984) Landslide Hazard Zonation: AReview of Principles and Practice,The UNESCOPress, Paris, 60p.

Varnes, D.J. (1978) Landslides: Analysis and Con-trol, Transportation Research Board Special Re-port, Issue 176, pp. 11-33; Transportation Re-search Board.



Widiastutik, R., Buchori, I. (2018) Kajian Risiko Ben-cana Longsor Kecamatan Loano Kabupaten Pur-

worejo. Jurnal Pembangunan Wilayah dan Kota,14(2), pp.109–122.


rainfall thresholds for landslide prediction in loano

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