CONTRIBUTION OF GROUNDWATER ABSTRACTION TO …

VOLUME 13, NO. 2, EDISI XXXII JUNI 2005

MEDIA KOMUNIKASI TEKNIK SIPIL 27

CONTRIBUTION OF GROUNDWATER ABSTRACTION TOLANDSUBSIDENCE AT THE NORTH COAST OF SEMARANG

Suripin1

ABSTRACT

Banyak kota-kota di Indonesia terletak di kawasan pantai atau dataran banjir yang terbentukdari endapan alluvialr. Ketika kota berkembang permasalahan penurunan muka tanah munculakibat penambamhan beban bangunan yang meningkat yang diperparah oleh pengambilan airtanah yang berlebihan. Tulisan ini membahas dampak pengambilan air tanah yang berlebihandan peningkatan beban bangunan terhadap penurunan muka tanah di Kota Semarang.Berdasarkan kecenderungan pengambilan air tanah dan penambahan beban bangunan, potensipenurunan muka tanah ke depan dapat diprediksikan. Hasil prediksi menunjukkan lajupenurunan yang cukup akurat dibandingkan dengan hasil pengukuran. Penurunan muka airtanah mempunyai pengaruh yang lebih dominan dibandingkan dengan penambahan bebanpemabangunan terhadap penurunana muka tanah.

Kata kunci : groundwater abstraction, uverburden load, land subsdeince

1 Pengajar Jurusan Teknik Sipil Fakultas TeknikUniversitas Diponegoro Semarang

BACKGROUND

Land subsidence resulting from groundwaterover-abstraction and un-controlleddevelopment is a serious hazard which hasaffected many cities in the world. It causesextensive and costly damage to urbandrainage, infra-structure and buildings, andother operational problems. The landsubsidence is occurring beneath severalmajor cities in Indonesia and developslowly, silent, and unknowingly.

The land subsidence is also causing costlydamage in many big cities. However, it isdifficult to measure convincingly so thatthere has often been a prolonged debateand resistance to control groundwaterpumping before taking any action. Thestudy, therefore, has also an objective toprevent the land subsidence from occurring.An example on the handling of landsubsidence is taken in Tokyo, Shanghai, andBangkok where groundwater pumping were

controlled and consequently, the soilsettlement rates were also reduced.

It is therefore very important and critical forany development in an area, especially inthe area sited on the alluvial deposits suchas at the North Coast of Semarang, to beaware of any land subsidence occurrences.Early long-term anticipation to control andprevent land-subsidence from occurring isthe key issue to be addressed appropriately.

The study is aimed to analyse the effect ofgroundwater abstraction and thedevelopment scale to land subsidence.

DESCRIPTION OF STUDY AREA

Semarang, the capital city of Central Java,issituated on the North Coast of Java, atSouthern Latitude 6o56’08” to 7o06’57”,Eastern Longitude 110o16’17” to 110o30’31”.(Figure 1). The city covers an area of373.73 sq. kilometers, and acts as a centerfor regional government, industries, trade,

Contribution of Groundwater Abstraction to Landsubsidenceat the North Coast of Semarang

MEDIA KOMUNIKASI TEKNIK SIPIL28

education and tourism. The population ofthe Semarang City at the year of 2000 is1,309,667 people, with the annual growth of1.51 %.

The north coastal plain is characterized bydelta sediment deposit composed of verysoft clay, soft clay, soft to me

dium sandy silt, and mixed of sand and lossto rigid small shells. All sediment materialsare originated from Garang River.

The alluvial deltaic sediments that underlinemost of the coastal plain aquifer, are a thicksequence of clays. However, at shallowdepth within the clays there are sandylayers forming a minor semi unconfinedaquifer that is widely used for water supplyby means of dug wells. Groundwater withinthe shallow aquifer is generally brackish orsaline, but there is a broad zone withfresher water alongside the Kaligarang-Semarang, where it flows through the city(PPLH, 1996).

]

Nevertheless, in general the groundwater isnot potable due to its high salinity. Manylocal inhabitants in these areas still use dugwells for general sanitation but buy theirdrinking water, as the total urbanpopulation served by the system underPDAM in 1995 was estimated only 40% by

direct connection and 10% by publicstandpipe. The shallow aquifer, in general,is contaminated by domestic wastewaterand it is also vulnerable to pollution fromindustrial wastewater or leachates. Thus,the shallow semi-unconfined aquifer of thecoastal plain is not suitable for water supply.

JAVA SEA

LEGEND :

Figure 1. Study Area



LITERATURE REVIEW

In general, land subsidence or settlementcan occurred as a result from various causessuch as (1) Normal or natural consolidationof recent clayey sediments under their ownweight. This process is usually very slowand may takes a few decades afterdeposition, (2) Earthquake vibrations, whichare mainly have influence on fine sands andsilts, (3) Compaction by rollers, for example,to strengthen the foundation beneath anembankmenT, (4) Dessication or drying outof soils due to the water table drops,probably as a result of deep drainage canalsand mainly affecting soft organic clays andpeats, (5) Hydrocompaction, (6)Dewateringby pumping to lower the water table infoundation excavation or by electro-osmosisin silty clays, and (7) Groundwater pumping.

In all of these processes, the compaction orconsolidation of the soil increases its densityand reduces its pore spaces. Soils are elasticmaterial to only a limited extends. Soilcompaction which beyond the elasticdeformation, i.e., become a plasticdeformation will results in the groundsurface remains deformed and will notrebound even if the causes of thatcompaction is removed.

Land subsidence due to groundwaterabstraction has to be differentiated fromother consolidation processes, i.e., fromglobal eustatic effects and from rainyseason flooding. An eustatic sea level rise,such as from global warming, will be muchslower than the consolidation processes dueto groundwater abstraction. Storm flooding,caused by poor drainage and the blockageof canals, is a problem on coastlines withrapid sedimentation.

The process that will be pursued in thisstudy is the interaction on the susceptibilityof land subsidence due to groundwaterabstraction and the increase in thedevelopment scale in the study area. Thesegroundwater abstraction and increase thedevelopment have resulted in an increase

load to the soil. These processes have beenvery common phenomenon occurring in allcontinents. To emphasize the important ofthe proposed study, it is relevant to describethe overall dynamic interaction betweengroundwater abstraction, the increase infuture development, land subsidence, andother environmental impacts.

Land subsidence is a subject within thescope of both geology and soil mechanics.However, the scope of the study withinwhich the groundwater abstraction and thedevelopment master plan are the primarycontributing factors have attracted thewater resources, groundwater, andenvironmental specialist to be involvedintensively.

The settlement resulted from increase inoverburden can be estimated based on thefollowing formula:

o

o

o

cPPP

e1HC

S log ................... (1)

where:S = the settlement (cm),Cc = coefficient of soil compressibility,H = soil thickness layer (cm),eo = pore number,Po = the initial effective overburden

pressure (kg/cm2),ΔP = the change in effective overburden

pressure caused by increase inoverburden load from thedevelopment (kg/cm2).

Total settlement that will result from theincrease in effective pressure, i.e., eitherdue to the groundwater abstraction or theincrease load from the development iscalculated from the following formulae.

Mh.PS

...................................... (2)

where :S = the total settlement in mm,h = the individual layer thickness in

meters.



Meanwhile, the rate of consolidationdepends on the consolidation coefficient, Cv,and can be calculated through the followingexpression.

vCHTt2.

...................................... (3)

where :t = the total time in years for the whole

settlement to occur,T = dimensionless time factor for

various degrees of consolidation,H = the total thickness of the clay

deposits.The value of Cv is calculated from thefollowing expression.

vwv m

kC ...................................... (4)

Equivalently, the value of Cv can also beestimated as a function of Liquid Limit(Lambe and Whitman, 1969). Themagnitude and the rate of aquifersettlement due to well pumping can beevaluated based on the following equation(Lohman, 1972):

wa

w

cu nDSS

................ (5)

if the water compressibility, βw is neglected,then

w

cu

SS .................................. (6)

where:Sc = coefficient of storage of an aquifer,γw = unit weight of water,Da = depth of aquitard, n is porosity,βw = water compressibility.

ANALYSIS AND DISCUSSION

The recharge at the Ungaran aquifer can beestimated by various methods as the data isavailable. Those are:

Water recharge

Precipitation Percentage : with the averagerecharge area rainfall is 2885 mm/year,total recharge area is 110.9 km2, and theinfiltration factor 35%, the rechargecalculated is then 2.885 x 110.9 x 106 x35% = 112,000,000 m3/year or 307,000m3/day.

Base flow Analysis of Garang River: Basedon available data for 1986 – 1995 atPanjangan, the mean annual discharge ofthe river is 9.6m3/s and the base flow is2.72m3/s. The direct abstraction from tubwells and spring piped by PDAM toSemarang and Ungaran cities is 0.73m3/sec,thus total actual base flow is then3.45m3/sec, or 298.000m3/day.

Soil Moisture Balance : The calculated totalriver flow is found to be 10.8 m3/sec, andthe estimate Ungaran recharge is 5.25m3/sec, or 450,000 m3/day.

The above analysis indicates that therecharge in Gunung Ungaran averages some300,000 m3/day to 400,000 m3/dayalthough the infiltration occurs duringNovember to April, and is released throughthe year from the underground reservoir.This recharge is calculated for an averageyear but the reliable yield has to base on adry year so that the usable groundwaterresource may be only about 60% of therecharge, say 210,000 m3/day to 240,000m3/day.

Water Supply

The population of Semarang City increasedfrom 1,232,931 people in 1995 to 1,309,667people in 2000. It means that the annualgrowth is 1.21%. The projected populationin 2005, and 2010, and 2025 is



consecutively 1,376,627; 1,417,167; and1,443,803 people.

The present water supply system ofSemarang City is originated from varioussources; spring, deep wells, and surfacewater. Total production is 1.853 lt/s,composed of 951 lt/s (51,32%) surfacewater and 902 lt/s (48,68%) groundwater.

The total urban population served by thesystem under PDAM in 1995 was estimatedonly 40% by direct connection and 10% bypublic standpipe. The remaining inhabitantsare relying on dug or drilled private wellsand public water taps supplied by watertrucks.

Figure 2. Schematic Hydrological section at Semarang

The water demand of Semarang City formunicipal and industrial use is increasing inline with the rapid population growth andindustrial development in Central Java.Considering to the population growth, andincreasing water use percapita, water forindustry, as well as for other purposes, it ispredicted that the water demand will haveto increase 3.374 m3/s (106,402,464m3/year) in 2010, 3.077 m3/s (125,418,672m3/year) in 2015, and 5.183 m3/s(163,451,088 m3/year) in 2025.

The current available water supply of0.276m3/s will have to be increased to5.426m3/s by the year 2025. If the surfacepotential water source, i.e.: Garang River(1.000 l/s), Babon River (50 l/s), and Kudu(2.250 l/s) have been fully developed, thewater demand in Semarang City would besupplied sufficiently until the year 2025.However, the fact shows that the PDAMproduction was always far below thedemand, and so far the deficit of waterdemand was fulfilled from groundwater,

Java Sea

faultGFp

Sea

Clay

Sand

BFGFu

GFh

GFh

IrrRO2

ET

IF

RF

Water table

Quaternary-TertiaryDamar volcanics-sedimentsNotopuro brecciasTertary marine clays

Holocene-Pleistocenevolcanics

fault

Coastalplain

Candi-Srondolhils

Key:RF = rainfallET = evapotranspirationIF = infiltrationRO = surface runoffIrr = irrigation diversionBF = baseflowGF = groundwater flow, Ungaran (u), hills (h),

& plain (p)

Mount Ungaran



particularly that was pumped from deepaquifer.

Groundwater Abstraction

Groundwater is difficult aspect to control inIndonesia because of many private tubewelloperated. The discharge may be small butthe cumulatively is very big, specialize for alarge city like Semarang as study area. Themonitoring of groundwater abstraction isfundamental analysis in this research. Thisis related with water sources supply and soilproperties characteristic especially landsubsidence in the study area. The otherresearch support this statement thatdecrease of groundwater elevation impactedto the land subsidence in Semarang about0,6 cm/year until 1,2 cm/year ( MineralTechnology Faculty of ITB, 1995).

Both number tube wells and discharge ofgroundwater abstraction in Semarang City isincreased markedly, from 127 tube wells in1982 to 776 tube wells in 1998, with thedischarge of 13.49 million cubic meter in1982 to 35.64 million cubic meter in 1997(Table 1).

Table 1. Increasing number on discharge oftube wells in Semarang City between 1900

and 1998

Volume ofabstractionNo Year

Numberof tubewells m3/day million

m3/year1 1900 16 1.300 0,472 1910 18 1.310 0,473 1920 18 1.400 0,504 1932 28 1.610 0,585 1982 127 37.460 13,496 1985 150 44.064 15,867 1990 260 61.570 22,178 1995 316 74.130 26,699 1996 659 80.954 29,1410 1997 745 96.798 34,8511 1998 776 98.998 35,64

Source: Sukrisno – DGTL (1999). Konservasi AirTanah Daerah Semarang-Demak dan Sekirtarnya.(1999).

Land Subsidence

Land subsidence in the study area ispresuming to be caused by soilconsolidation and groundwater pumping.Measurement of Land Subsidence theindicator base on the leveling ground, asmentioned above.

Based on the compilation of the data,measurement carried out between 1984 and2000), it was identified that there were only7 (seven) stations (bench marks) whichwere always measured, while the otherstations were intermittently measured. Thedata is combined with the latestmeasurement, which was carried out in thisstudy (October 2002). Analysis of thosedata found that the rate of land subsidencevaries from 0.79 cm/year at the ElizabethHospital (upper area) to 5.07 cm/year atDTK 135 located at Jl. Imam Bonjol. Ingeneral, there land subsidence rate tends tobe higher at the coastal region then at theupper area.

Considering to the topographic andgeological situation, the study area can bedivided into three regions; lower, middle,and upper or hilly area. Land subsidence inthe lower area is highest than the others.The average land subsidence in the lowerarea is three to four times at the upperarea, and four to six times in the middlearea (see Table 2).

Table 2. Average Land Subsidence inSemarang City

TypeArea

AverageLand

Subsidence(cm/year)

Location study(Station)

Hilly 1.17 Sultan Angung(Akpol)

Middle 0.90 Kalisari/Dr.Sutomo Street

Lower 3.3-5.07 TuguMuda,Pelabuhan



y = 0.1594x2.5069

R2 = 0.9954

0

10

20

30

40

50

0 2 4 6 8 10 12

Overburden (m)

Land

sub

side

nce

(m)

y = 3.436x0.2589

R2 = 0.9801

0

2

4

6

8

10

12

0 5 10 15 20 25 30 35 40 45

Ground water drop (m)

Overb

urden

(m)

Land subsidence due to consolidationprocess

There are two kinds of consolidation:primary consolidation, and secondaryconsolidation. The primary consolidation isoccurred in a certain duration time. It isdepended on the various factors, such as;overburden on the top of the consolidatedsoil, the thickness and the number of softsoil layers, effective soil stress due to load,and contact area between load andconsolidated soil. The value of primaryconsolidation can be estimated based onequation (1), where the overburden is equalto the land filling thickness. The secondaryconsolidation is occurred slowly, and it isusually neglected is the calculation.

Based on the available soil data, and theincreasing overburden due to land filling,the rate of land subsidence due tooverburden can be calculated. Some ofconstants say Cc=0.3-0.8, 1.7-1.8

t/m3 , eo = 1.6 - 1.8, and the upper andlower clay layer thickness are consecutively23.2 and 9.60 meters. It can be shown thatthe land subsidence increases withincreasing overburden. The increasing landsubsidence is power function of theoverburden with the power of 2.5. It isexactly fit until overburden thickness of 8meters (Figure 4).

Figure 3. Correlation between overburdenand land subsidence

Land Subsidence due to GroundwaterAbstraction

As stated by ITB (1995) that over pumpingin Semarang caused ground water drop andland subsidence. By implementing the sameequation for the land subsidence due tooverburden (eqn 1), the rate of landsubsidence due to ground water drop canbe estimated. In this case, the increasingoverburden is generated by decreasingground water level. Therefore the analysisconsists of two steps; first estimate theground water drop against ground waterpumping, and then estimate the landsubsidence against ground water drop.

The overburden generated by ground waterdrop can be estimated by using equations(5) or (6). The results are presented inFigure 5. There is simple correlationbetween ground water drop andoverburden, i.e.:

2589.0436.3 ww SH ......................... (7)

where:Hw = overburden (m),Sw = ground water drop (m).

Figure 4. Correlation between aquifersettlement (groundwater drop) and

overburden in Semarang City



y = 6.5267x0.6394

R2 = 0.9904

0

15

30

45

60

75

90

0 10 20 30 40 50

Aquifer Settlement (cm)

Lan

d su

bsid

ence

(cm

)

Figure 5. Correlation between AquiferSettlement and Land Subsidence

As the correlation between groundwaterdrop and overburden has been found, theland subsidence generated by groundwatercould also be estimated. The results arepresented in Table 3. The correlation

between aquifer settlement and landsubsidence then could be generated in theform of:

6394.05267.6 SuS .......................... (8)

where S is land subsidence in centimetre,and Su is aquifer settlement in centimetre.

Total Land Subsidence

Total land subsidence, combination betweenconsolidation process, and land subsidencedue to ground water drop, then could beestimated by using Figure 6. As examples,Table 3 shows the comparison between landsubsidence obtained from direct groundlevel measurement and analysis for somestation within Semarang.

Figure 6. Total land subsidence, combination between consolidation process and landsubsidence due to ground water drp



Table 3. Comparison between measured and estimated and subsidenceof some stations in Semarang

Estimated land susidence due to (cm)No Station Address Measured Land subsidence

Oct97-Dec98 (cm/year) Overburden Groundwaterdrop Total

1 TTG 447 SPBU Kaliwiru 0.00 0 0.260 0.2602 DTK 341 Akpol 1.17 0 0.807 0.8073 DTK 340 Rs. Elizabeth 0.79 0 0.807 0.807

4 DTK 009 Ps. Bunga, Jl. Dr.Sutomo 0.90 0 1.765 1.765

5 TTG 446 Tugu Muda 2.30 0 5.110 5.1106 DTK 135 JL. Imam Bonjol 5.07 0.02 3.189 3.209

7 DTK019 Jembatan Banjirkanal Barat 3.30 0.46 5.110 5.570

8 DTK368 Madukoro Street 4.27 0.46 5.110 5.570

CONCLUSION AND RECOMMENDATION

Conclusion

1). Land subsidence occured in SemarangCity composed of two processes:consolidation of soft soil layer due tooverburden, and land subsidence dueto ground water pumping. The rate ofland subsidence varies spatiallydepended on the overburden load,ground water pumping and thegeological characteristics. In generalthe rate is higher in the lower (coastal)area than in the upper area. Maksimumland subsidence occurred in TanjungMas Harbor, about 4 – 5 cm/year.

2). Land filling and reclamation on thecoastal region contributed to the landsubsidence. It increases overburdenload. The higher the land filling, thehigher the rate of land subsidence tendto be.

3). Groundwater pumping tends toincreases continuously, as the watersupply production from PDAM does notmeet the water demand. The groundwater pumping increases markedly inline with the increasing water supplydeficit. Ground water pumping isalready higher than the naturalgroundwater recharge.

Recommendation

1). The centre of development in theSemarang City should be moved fromlow land area at the north to the hillyarea at the south. It is aimed tomaintain or even reduce overburdenload as well ground water pumping atthe coastal area. To maintaingroundwater balance, many effortshave to be carrout to increasegroundwater recharge, such asmaintain recharge area, and developartificial recharge.

2). Increasing overburden load, due toeither land filling and buildings,increasing land subsidence rate. Landfilling, therefore, is not the answer tosolve the tidal inundation problem inthe low land-soft soil area.

3). The existing deep wells have to bestrickly controled and monitored, toensure that meterings are installed onthe all wells, and the pumping rate isnot over the permitted discharge (safeyield).

4). Application of additional new deepwellsshould be strickly limited for a veryurgent only, while PDAM should fastenits development to meet the waterdemand, whithout this efford limitationof ground water pumping would beineffectrive.



ACKNOWLEGMENTS

Part of this paper is the result of theResearch Grant supported by TPSDP CED-UNDIP Batch I, ADB Loan No. 1792-INO.This support is greatfully aknowledged.

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