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The Morphotectonic Characteristic of Erosion and Flood Zones ofCisangkuy Watershed in Bandung Basin, West Java

Emi Sukiyah1, AgungMulyo1, Pradnya P. Raditya2

1Faculty of Geology, Padjadjaran University, Jalan Raya Bandung-Sumedang Km 21, 45363, Bandung, Indonesiaemisukiyah@gmail.com

2Magister Program of Geological Engineering, Padjadjaran University, Jalan Dipati Ukur No. 35 Bandung, IndonesiaE:Mail:Radityarendra90@gmail.com

Abstract

Cisangkuy watershed which is part of the upstream Citarum watershedlocated in the southern part of BandungRegency. This area is controlled by active faults trending west – east. Flooding phenomenon always occurs in thedownstream region of Cisangkuy watershed such as residential and industrial area, Dayeuhkolot. The river inability toaccommodate runoff is one of the causes of flooding, particularly when rainfall intensity is quite high.Morphometriccharacteristics of watershed may reflect tectonic conditions of a region. Different morphotectonic features will givedifferent erosion responses. It will have implications for the distribution of flood-prone area.

This research is based on the mindset that past geological events, along with its development produces a distinctivelandform.This response is reflected in the diverse landform dimension ratio. Probabilistic approach is also used in thedata analysis to obtain measurable research results.

Research resultsshows that the aspect ofwatershed morphotectonic, geological feature, and climate play a role in thedetermination of areas prone to erosion and flooding. An increase of average erosion in the upstream area willincrease the distribution of flooded areas in the downstream area.Narrowing and silting up of river flow occurs rapidlydue to the uncontrolled erosion and rapid land use conversion. Verification result of the correlation models proved thatthe flood-prone areas are generallylocated in downstream of rivers have a high to very high erosion rate.The frequentlyflooded areas are Banjaran, Sepen, Baleendah, and Dayeuhkolot. This flood-prone area occupies the depressionlandform.

Keywords: Cisangkuy, erosion, flood, morphotectonic, watershed.

Introduction

A watershed has limitation in managing waterbalance. Water supply balance will be maintained ifthe land uses in a watershed consider naturalcarrying capacity aspects. Geological featuressuchas rocks and tectonic have a role in the assessmentof natural carrying capacity. If the water balance isdisturbed between the input and output of water inwatershed, the negative impact of environmentaldamage cannot be avoided such as flood.

Cisangkuy watershed which is a part ofupstream Citarum watershed is located in thesouthern part of Bandung regency. This area hascomplex geological aspects. The upstream areas areplantations, farms, and PLTP Wayang-Winduinstallation. The extent of cultivation area has

implications for the expansion of land use changefor development of supporting facilities. Land use ina watershed that does not consider the carryingcapacity ability can lead to environmentaldegradation. This condition can cause erosion thatexceeds the limit. Erosion occurred in an area willcause material deposition in other places that havelower elevation. Sedimentation processeswill form anew land, such as sand bar, point bar, and delta. Inaddition, sedimentation processes can also result insilting of the river channel. This will affect theinability of river to accommodate runoff resulting inflooding (Ilyas, 1987).

A Floodis an incident where the land which isusually dry becomes submerged due to heavyrainfall. Topographic factor such as lower elevation

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to concave area and low soil infiltration abilitywhich causes the inability to absorb water are otherfactors causing floods. In addition, floods can alsobe defined as an overflow of river water due toinability river channel to accommodate runoff.

Flooding phenomenon always occurs in thedownstream region of Cisangkuy watershed,Dayeuhkolot area. Flood that occurred in this area iscaused by the inability of Cisangkuy river channelaccommodate runoff, especially when rainfallintensity relatively high. Narrowing and silting ofriver channel occur so fast due to the uncontrollederosion as a result of rapid land use conversion.

This research aims to determine the quantitativecharacteristics of landform as a result of geologicalstructure can be used to predict erosion and floodprone area in Cisangkuy watershed (Figure 1).Furthermore, this research aims to determine thecharacteristics of watershed morphotectonicdelineate spectral response through inter-channelcomposite on Landsat imagery, delineate erosionand flood prone area, and correlate between thesevariables. The results are expected to be useful tohelp society and local government in findingsolution to erosion and flood problem.

Figure 1. Research area location

Methodology

Material and tools used in this research are:

- Geological map of Garut and Pameungpeukquadrangle (Alzwar et al., 1992).

- Geological map of Bandung quadrangle(Silitonga, 2003).

- Toporaphic map of Barutunggul, Pangalengan,Lebaksari, Pakutandang, Soreang, Pasirjambu,Cililin, and Bandung (Bakosurtanal, 2000).

- Landsat imagery ETM+ 2001.- Rainfall data.- Geological survey and documentation tool.- GIS software.- Image processing software (Earth Resource

Mapping Ltd, 2003).

Some of the variables that are not measured inthe field is determined through satellite imagery andtopographic map. Variables of landform andtectonic include slope gradient, subwatersheddimension, river order, and bifurcation ratio (Rb).

Secondary data used are from scientificpublication of some previous researchers equippedwith field observation data and analysis inlaboratory. The data obtained from the field consistsof geological structure, rocks, morphometry, andlandform elements.

Prediction of the material eroded amount canalso use erosion equation. USLE (Universal SoilLost Equation) is a calculation formula forpredicting annual sheet erosion (Wischmeier andSmith, 1962; in El-Swaify et al., 1982), which isformulated as follows:A = RKLSCP ..................................................... (1)with A = the average annual soil loss (tons/acre), R= run off erosivity index, K = soil erodibility factor,LS = topographic factor (L = slope length factor, S= slope steepness factor), C = cropping-managementfactor, P = erosion control practice factor.

Zoning of flood prone area is done throughcalculation using spatial analysis and GIS softwarewhich is supported by simple grid method (Sukiyahet al., 2004; Sukiyah et al., 2007). Various aspectsrelated to flood are given weight value (0-5)according to its priority and value (0-5) according toits condition. Furthermore, the weight value andvalue are multiplied to get scores in every relatedaspect. Total score is obtained by adding up all the

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analyzed aspect scores. Statistical approach used toobtain significant result. Research framework isshown in Figure 2.

Figure 2. Research framework

Result and Discussion

Geology of CisangkuyWatershed

Geology of Cisangkuy watershed is part of theregional geology of Bandung basin. Landform ofBandung area is an elongated ellipsoidtrendingnorthwest-southeast. The basin has a length of 60km and width 40 km.

Bandung basin can be divided into eastern,central, and western area (Bronto et al., 2004).Cisangkuy watershed can be classified into centralpart of Bandung basin. Composition of rocks thatexist in the Cisangkuy watershed is dominated byQuaternary volcanic rocks. The rock formationconsists of lake deposits (Ql), Malabar-Tilu volcanicrocks (Qmt), Mount Tilu lava (Qtl), Mount Kendenglahar and lava (Ql-kw), undifferentiated volcanicrocks (Qopu), Waringin-Bedil andesite of Malabarproducts (Qwb), and Beser Formation (Tmb) as theoldest rocks (Alzwar et al., 1992 and Silitonga,2003; Table 1).

Lineament and fault pattern generally trendingnorthwest-southeast, northeast-southwest, andnorth-south (Figure 3). Developing faults generallyare strike-slip fault and normal fault. These faultsare active based on the evidence of landslide area,

deformed and unstable area that mostly coincideswith fault zone.

Table 1. Rocks formation in Cisangkuy watershed

Code Rock Formation Age

Ql Lake deposits Holocene

Qmt Malabar-Tilu volcanic rocks Late Pleistocene

Qtl Mount Tilu lava Late Pleistocene

Ql (k,w) Mount Kendeng lahar and lava Late Pleistocene

Qopu Undifferentiated volcanic rocks Middle

Pleistocene

Qwb Waringin-Bedil andesite of

Malabar products

Early Pleistocene

Tmb Beser Formation Late Miocene

Figure 3.Distribution of erosion in Cisangkuy watershed and theexistence of active faults

Morphotectonic of CisangkuyWatershed

Several active faults control Bandung basin,including Cisangkuy watershed (Figure 4 and Figure5). Ciwidey-Patrol-Wangisegara fault is fault thataffects the downstream of Cisangkuy watershed.The fault plane is contact between Mount Malabar(Qwb) and Mount Malabar-Tilu (Qmt) rocksproduct. Several smaller active faults areTanjaknangsi, Cisondari-Cisangkuy, Waringin-Haruman-Barugbug, and Arjasari-Baleendah faults.

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Those faults control Cisangkuy watershedparticularly in the upstream area. Based on thisphenomenon, it is indicated that Cisangkuywatershed deformed complex. This phenomenonalso has implication for further geomorphologyprocesses such as erosion.

Figure 4.Bandung basin in Landsat imagery: (1) Forest, (2)Cultivation area, (3) Paddy field, (4) Residential, Industrial, and

Office area (Sukiyah, 2009)

Figure 5.(a) Escarpment in Mount Wayang-Windu(Pangalengan), (b) Patrol Valley

Indication of geological structure existence asan indication of fault is difficult to find on theQuaternary volcanic rocks that composeddominantly in Cisangkuy watershed. This conditionis caused by imperfect compacted rocks so theability to record geological structure traces is lowerthan older rocks. In addition, volcanic rocks in thisarea tend to have intermediate to maficcharacteristic that are prone to weathering process.Some specific landforms can also be an indicator tofind active fault. Besides, drainage pattern, landformlineament, escarpment, and landslide area can alsobe used to find active fault.

Ciwidey-Patrol-Wangisagara fault seemed veryclear on the composite channel 1, 4, and 7 ofLandsat imagery (Figure 4). It is because this fault isquite large and some of its traces recorded on thehard volcanic rocks (lava) from Beser Formation.Based on the imagery analysis result, deformed areacan be identified and land use can be classified sothe erosion and flood prone area can be estimated.

Tectonic condition in an area can be determinedby morphometryor morphotectonic analysis ofwatershed. Cisangkuy watershed is divided intosubwatersheds to simplify calculation process, bothin spatial and tabular format. The calculation resultis shown in Table 2 and Table 3. Rivers that flow inCisangkuy watershed have order 1 to 6. Spatialdistribution of the rivers is forming patterns whichcan reflect the lithology and tectonic condition.Bifurcation ratio (Rb) distributed in 5 levels.Completeness of Rb level varies in eachsubwatershed. Some watersheds that are not quitelarge only have Rb1-2, namely Citanjung,Ciparingga, Babakankiara, andWangunsubwatershed. Rb value is dominated by arange of values less than 3 which showsdeformation control due to tectonic activity. Somesubwatersheds have value of Rb 7 in thedownstream, Cibintuni and Cilakisubwatershed.This phenomenon indicates imbalance ofgeomorphology processes in the upstream anddownstream of watershed as a result of tectonicactivity.

Table 2. Distribution of river order in Cisangkuy watershed

Code SubwatershedRiver Order

1 2 3 4 5 6

Csky-01 Cibintuni 49 29 11 7 1

Csky-02 Citalugtug 52 23 21 5

Csky-03 Citanjung 5 4

Csky-04 Ciparingga 2 1

Csky-05 Cigeureuh 27 21 13 3

Csky-06 Ciseureuh 6 2 3

Csky-07 Cisangkuy 220 113 52 13 12 14

Csky-08 Cilaki 28 11 14 2

Csky-09 Ciburuy 7 5 1

Csky-10 Babakankiara 3 2

Csky-11 Wangun 2 1

Table 3. Bifurcation ratio (Rb) in Cisangkuy watershed

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Code Subwatershed Rb1-2 Rb2-3 Rb3-4 Rb4-5 Rb5-6

Csky-01 Cibintuni 1.69 2.64 1.57 7.00 -

Csky-02 Citalugtug 2.26 1.10 4.20 - -

Csky-03 Citanjung 1.25 - - - -

Csky-04 Ciparingga 2.00 - - - -

Csky-05 Cigeureuh 1.76 1.62 4.33 - -

Csky-06 Ciseureuh 3.00 0.67 - - -

Csky-07 Cisangkuy 1.95 2.17 4.00 1.08 0.86

Csky-08 Cilaki 2.55 0.79 7.00 - -

Csky-09 Ciburuy 1.40 5.00 - - -

Csky-10 Babakankiara 1.50 - - - -

Csky-11 Wangun 2.00 - - - -

Erosion Dynamics in CisangkuyWatershed

Erosion calculation is done for eachsubwatersheds in Cisangkuy watershed (Table 4,Figure 6). The areas that have very high annualaverage erosion are located in Cilaki, Ciburuy,Babakankiara, and Wangunsubwatershed (Figure 7).These subwatersheds are located in the western ofCisangkuy watershed and occupy a steep slopeforming morphotectonic unit (escarpment). On theother hand, the areas that have very low annualaverage erosion are located in Citanjung andCiparinggasubwatershed. These subwatersheds havebird feathers shape with low drainage density.Tectonic activity is almost not identified in thesetwo subwatersheds.

Table 4. Distribution of erosion in Cisangkuy watershed

Code Subwatershed

Area(ha)

Averageerosion(ton/ha/

yr)

TotalErosion(ton/yr)

ErosionProne Class

Csky-01 Cibintuni 2,267.0 186.2 421,993.0 IV (High)

Csky-02 Citalugtug 4,208.0 40.7 171,137.3 II (Low)

Csky-03 Citanjung 721.9 12.3 8,872.1I (Very Low)

Csky-04 Ciparingga 516.6 10.4 5,381.8 I(Very Low)

Csky-05 Cigeureuh 2,904.0 26.6 77,254.0 II(Low)

Csky-06 Ciseureuh 1,208.0 24.5 29,575.3 II(Low)

Csky-07 Cisangkuy 12,260.0 181.9 2,229,812.0 IV(High)

Csky-08 Cilaki 1,309.0 1,272.0 1,665,061.1V(Very High)

Csky-09 Ciburuy 393.5 1,993.1 784,296.7V(Very High)

Csky-10 Babakankiara 192.7 2,272.0 437,822.1V(Very High)

Csky-11 Wangun 298.7 1,858.2 555,050.3V(Very High)

Total erosion is determined by the amount ofannual average erosion and watershed area, forexample a large subwatershed will indicate a highamount of annual average erosion.Cisangkuysubwatershed has the highest erosiontotal whereas Cilakisubwatershed has lower erosiontotal than Cisangkuysubwatershed. These twosubwatersheds have fairly clear differences, wherethe size difference of subwatersheds is very high aswell as the annual average erosion. Combination ofthese two variables makes the two subwatersheds asthe contributor of high annual erosiontotal in theresearch area.

Figure 6.Graph of annual average erosion in Cisangkuywatershed

Figure 7.Erosion prone area in Cisangkuy watershed

Relationship between erosion andmorphotectonic is evidenced by correlation betweenthe range of bifurcation ratio as a reflection ofmorphotectonic and annual average erosion (Figure8). Correlation between tectonic activity level andaverage erosion total can be expressed by the linearregression equation: y = 458.86x + 230.61 with

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correlation coefficient (r) = 0.25. Tectonic activity isdirectly proportional to the increase of erosion total.The higher of tectonic activity that controls awatershed, the higher of annual average erosiontotalmayoccur. Low correlation coefficient (r =0.25) shows that there are many variables thatcontrol the annual average erosion total such as landuse (CP) and rainfall (R). Impact of tectonic activitywill be reflected in the soil erodibility index (K) andgeomorphology index (LS). This phenomenon isproved in Cisangkuy watershed, where thesubwatersheds which have very high averageerosion such as Cilaki, Ciburuy, Babakankiara, andWangunsubwatershed also have high Rb value.

Figure 8.Graph of correlation between tectonic activity leveland annual average erosion

Flood Prone Area in CisangkuyWatershed

Some aspects related to the possibility of floodin Cisangkuy watershed are lithologies (rock typesand textures), land use, rainfall intensity, slopegradient, river characteristic (river order),morphotectonic (land deformation due to tectonicactivity). Based on the result of statisticalcalculation using normal distribution pattern methodto spatial analysis result, classification of floodprone area is obtained: Frequently flooded area (score 101 - 120) Potentially flooded area (score 81 - 100) Rarely flooded area (score 61 - 80) Safe area (score 0 - 60)

Spatial distribution of flood prone area is shownin Figure 9. Frequently flooded area occupyCisangkuy river valley in the southern of Banjaran(Cigeureuhriver) to Baleendah and Dayeuhkolot(Figure 10). Potentiallyflooded area occupy aroundCilaki to Ciseureuhriver, and Cisangkuy river valleywhich is above the frequently flooded area. In

addition, other areas in the research area can beclassified into rarely flooded area and safe area.

Verification result to the phenomenon oftectonic-erosion-sedimentation shows that the floodprone areas are located around river downstreamthat has a high to very high erosion rate in upstream.Based on morphotectonic aspect, flood prone areasoccupy the depression landform. The landformcharacter allows water to stagnate. Flood prone areausually is passed by river segments that have highorder, more than 3. However, some anomalies canoccur, for example on the low order rivers (less than2) that occupy specific area. Besides, bifurcationratio (less than 3 and more than 6) also can reflectflood prone area, especially in the downstream areaof watershed.

Figure 9.Flood prone area in Cisangkuy watershed

Figure 10.(a) Cisangkuy river debouches into Citarum river, (b)The House Located in Frequently Flooded Area

Distribution of flood area in each subwatershedlocated in Cisangkuy watershed is shown in Table 5.The largest flood prone area is in the downstream of

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Citalugtugsubwatershed (209 km2). Flood proneareas tend to be decreased to the upstream ofCisangkuyriver. It is associated with thegeomorphology conditions that do not allow waterto stagnate.

Table 5. Distribution of flood area in Cisangkuy watershed

CodeSub

watershed

Luas (km2)Safearea

Rarelyarea

Potential area

Pronearea

Csky-01 Cibintuni 1,558.0 547.9 114.6 42.8

Csky-02 Citalugtug 3,113.0 704.5 181.7 209.1

Csky-03 Citanjung 607.5 68.5 38.5 7.4

Csky-04 Ciparingga 406.5 70.7 23.4 16.0

Csky-05 Cigeureuh 2,489.0 342.0 62.3 12.1

Csky-06 Ciseureuh 853.4 324.5 30.4 -

Csky-07 Cisangkuy 8,615.0 3,574.0 52.7 -

Csky-08 Cilaki 1,064.0 235.5 9.6 -

Csky-09 Ciburuy 330.7 62.8 - -

Csky-10 Babakankiara 177.3 15.4 - -

Csky-11 Wangun 257.5 32.9 8.3 -

Relationship between the amount of erosionand distribution of flood prone area is proved by thestatistical analysis result on both these variables(Figure 11). The graph shows that the correlationbetween the amount of erosion and the floodpotential area indicates a positive correlation withthe equation: y = 44.321x– 16.161 and r = 0.75(Figure 11a). Similarly, the amount of annualaverage erosion to the distribution of flood pronearea also indicates positive correlation with theequation: y = 25.441x – 17.343 and r = 0.92 (Figure11.b)

Figure 11.Graph of correlation between the amount of erosionand distribution of flood prone area

Conclusion

Result of the research in Cisangkuy watershedindicates that some subwatersheds controlled byactive tectonic. This phenomenon is reflected by

fault scarp of Ciwidey-Patrol-Wangisagara thatcrossed the downstream Cisangkuy watershed. Thisphenomenon quantitatively also reflected bybifurcation ratio less than 3 and more than 5.

Existence of active fault can result in deformedvolcanic rocks and steep slope landform. Thiscondition has implication for erosion andsedimentation phenomenon in Cisangkuywatershed. Statistical analysis of morphotectonicand erosion variables, and also flood distributionshows that these variables interrelated andcorrelated statistically. This correlation is reflectedby erosion prone area distribution that coincideswith faulted landform. Erosion prone areas arelocated in the upstream Cisangkuy watershed whichcoincides with Waringin-Haruman-Barugbug faultand Gambung-Sadatapa fault, and also in thedownstream Cisangkuy watershed which coincideswith Ciwidey-Patrol-Wangisagara fault scarp.

High level of erosion in the upstream watershedhas implication for sedimentation process in thedownstream watershed. This correlation can be usedto predict flood prone area. Verification result ofcorrelation model proved that flood prone areagenerally located in downstream of rivers which hasa high to very high erosion rate. The frequentlyflooded areas are Banjaran, Sepen, Baleendah, andDayeuhkolot. This flood-prone area occupies thedepression landform.

Acknowledgments

We thank Faculty of Geological Engineering,PadjadjaranUniversity, whichhave helped theresearch activity so this scientific article can bemade. In addition, we would like to thank all thosewho have helped in this research activity.Hopefully, this scientific article can be useful bothfor the science development and society.

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