BANGKOK'S ENVIRONMENTAL PROBLEMS ASSOCIATED WITH ITS LOWLAND CONDITION

Transcription

1 BANGKOK'S ENVRONMENTAL PROBLEMS ASSOCATED WTH TS LOWLAND CONDTON Dr. Winai Liengcharernsit ' ABSTRACT Rapid growth n industrialization and urban expansion in Bangkok the capital of Thailand in the past two decades resulted in deterioration in environmental quality. Covering about 1560 square kilometers with more than 5.6 million inhabitants Bangkok has faced several problems. The daily demand of water supply and the amount of wastewater generated are quite high. Besides surface water source a number ofgroundwater wels were driled for subsidiary water source. Since Bangkok is located on the alluvial deposits in the flood plain of the Chao Phraya river which drains its runoff into the Gulf of Thailand over- pumping of groundwater has resulted in intrusion of saline water from the sea and many wells have been abandoned. Over-pumping of groudwater also resulted in falling of piezometric heads of groundwater and as a result land subsidence in Bankok area and vicinity has occurred. The investigation conducted by Asian nstitute of Technology (AT) revealed the land subsidence rate was about 5.0 centimeters per year in the central part of Bangkok and was as high as 1 0 centimeters per year during in the southeastern part. To solve this problem the government has decided to reduce groudwaterextraction to a level below natural aquifer recharge rate. To cope with increasing water demand in the lower Chao Phrayaplain a transbasin development project was implemented to devert excess water from the Mae Klong rive to the Chao Phraya river. A network of canals were excavated 1 Associate Professor Departnment of Civil Engineering Faculty of Engineering' Kasetsart University

2 "" LUl'l'l 19 lj'i::"l1lj 2536 for navigation and water usage in teh past. Nowadays. these canals are utilized for draining flood water in the rainy season and for disposal of wastewater from various sources. However. draining canal water out to the receiving river is not easy. since the river water level during the high-tide period is even higher than the ground level in the city. Several water gates with large pumping stations were installed at the mouths of the canals for water level control and for dilution of canal water. Some nood protection programs have been implemented and some longterm measures are being considered. Nowaday Bangkok has no central wastewater treatement system. The generated wastewater is discharged directly or indirectly into the canals which finally drains into the Chao Phraya river. This has resulted in deteriorated water quality. Several plans for Bongkok's sewerage system have been proposed but none of them have been implemented. due to the lack of political will and financial support. However. more allention has been given to this issue. and detailed designs are being undertaken in some inner zones of the city. 1. NTRODUCTON Bankok. the capital of Thailand. is located on alluvial deposits in the delta flood plain of the Chao Phraya river. some 25 kilometers from the Gulf of Thailand (Figure 1). Covering approximately square kilometers of lowland area with more than 5.6 million inhabitants Bongkok has faced many environmental problems. some of which are closely related to its low land condition. These include saline water intrusion. land subsidence. flooding. wastewater disposal. and control of water level in the canal network. 2 GROUNDWATER EXTRAC TON AND LAND SUBS DENCE Rapid growth in industrialization and urban development in Bangkok in the past two decades has resulted in an increase in water demand. Two major sources of water supply are available in the area; tap water supplied by the Metropolitan Water Works Authority (MWA) and groudwater from the underlying aquifers. Until now. the MWA is not able to supply adequate water to cope with the increasing demand and its service area does not cover the whole city yet. Therefore large amount of groundwater has been extracted by both private and public sectors. n 1986 total extraction of groundwater in Bankok and its two adjacent provinces Nonthaburi and Samut Prakan was approximately 1.1 million cubic meters per day. which far exceeded the estimated safe yield of 0.8 million cubic meters per day. t should be noted that the above mentioned extraction rate did not include the unlicensed use of groundwater which be as high as 50% of the licenced usage.

3 15' f lf-f~----'~. 1 \ gbu'ri ~Lopburi '- ngthong / " " 14' r---'l:'-;:-::;:; ~'r-----=1_tc =~ f=~-...j " ~~ " \ ' / ' \ / Gulf of Thailand : ; o Km. 101 Figure 1 Lower Chao Phraya Plain and Bankok

4 oj. "" Lfl~V1 19 1J'i~'l11J 2536 Over-pumping of groundwater has resulted in falling of piezometric heads in the underlying aquifers which subsequently resulted in land subsidence in Bangkok area and nearby provinces. An investigation on land subsidence caused by deep well pumping in Bangkok area was conducted by Asain nstitute of Technology (AT) during and it was concluded that excessive groundwater extraction caused the decline in groundwater level with the rate of 2.5 to 3.0 meters per year and the drop in water level was the main cause of widespread land subsidence. The rate ot land subsidence was about 5.0 centimeters per year in the central part of Bangkok and the maximum rate was as high as 1 0 centimeters per year in the southeastern part of the city during (AT 1980). n order to solve the problems on land subsidence the extraction ofgroundwater must be reduced. The government has set a plan to decrease groundwater extraction rate to the level below natural aquifer recharge. Attempts have been made to cut the loss in the MWA's water supply system from 40% to 30% and to increase water production capacity. To solve the problem of water deficiency in the lower Chao Phraya plain a transbasin development project was implemented to divert excess water from the Mae Klong river to the Chao Phraya river. 3. SALT WATER NTRUSON The falling in piezometric heads of groundwater aquifers caused by overpumping also resulted in intrusion of saline water from the sea especially those walls located in the southern part of Bangkok which is close to the sea. Many wells could supply fresh water for some period time but later when more wells were developed and pumping continued the withdrawn water became more and more brackish. Several wens have been abandoned and new wells are nonnally drilled to the deeper aquifers. Besides seepage of saline water into groundwater aquifers intrusion of sea water to the upstream sections of th Chao Phraya river has occurred. This is mainly caused by the increase in river water use. n the drought years salt water could intrude farther upstream from the river mouth and caused damage to fruit trees and crops grown in the vicinity of the river and its connected canals. 4. FLOODNG n the fonner days there existed many canals in Bangkok. These canals were utilized for navigation agricultural and domestic water uses as well ws stonn drainage. Due to rapid development in the past two decades. a number of canals have been replaced by roads and some sewers but with much less drainage capacity. This reason together with occurrence of land subsidence due to over-pumping of groundwater has resulted in more frequent flooding in Bangkok area especially during

5 L~lntl\j - n'in!l1f1l~ 1f'l1n'i'i~'l1'i ~n the period of high tied and high discharge in the Chao Phraya river. Several studies were conducted to determine the most effective means for preventing flooding in Bangkok area (e.g. CDM 1968; NEDECO et a 1984; JCA 1986; NEDECO et ai 1987; and JCA 1988). n the City Core Project it was suggested that approximately 98 square kilometers in central Bangkok be divided into 6 polders and the recommended flood control and drainage system included improvement of canals drainage capacity and installation of pumping stations at the mouths of some main canals (Figure 2). n the Eastern Suburban Project it was proposed that the eastern suburban area covering about 260 square kilometers be protected by a system of polders and storm water would be discharged by pumping into major canals (Figure 3). n the Thonburi and Samut Prakan West Project a total of 108 kilometers of flood barrier with 45 regulators and 95 cross dams were proposed to protect approximately 340 square kilometers on the western part of Bangkok and the northwest ern part of Samut Prakan province (Figure 4). A number of pumping stations were planned to discharge storm water during the period of high tide. 5. WASTEWATER DSPOSAL At present Bangkok has no central wastewater treatment system. Only 2% of the population are connected to the community sewer networks and treatment plants. Most buildings have individual septic tank and seepage pit. Since groundwater table in the top unconfined aquifer is quite high only 1.0 meter below ground lever! and the upper soil layer consists mainly of clay with very low permeability the seepage rate of wastewater into the ground is quite low causing frequent operational problems. Thus most buildings have connected the seepage pits with the storm drains which in turn discharged into the nearby canals. As a result water in these canals and in the lower section of the Chao Phraya river has very poor quality. Several plans for Bangkok's sewerage system were proposed e.g. CDM (1968) andjca (1981) (Figures 5 and 6). However none of them has been implemented due to lack of political will and financial support. The government is now aware of this environmental issue and willing to invest in the central wastewater treatment plants and sewerage systems. Detailed designs in some inner zones of the city are being conducted.

6 L'lilJ;i 9 l.h~~lij 253b LEGEND: -- Flood pr01kllo/\ 0119""'.'' land lublldlllu../r Figure 2 Proposed Flood Protection System for Bangkok City Core (NEDECO et ai 1984)

7 N / Seala D 1 2 ::.. s(l'"'l ~ Figure 3 Proposed Flood Protection System for Eastern Suburban (JCA. 1986)

8 .. t 0"" L'iUJ'V 19 '\'::~11J 2536 Paddy ~ Paddy LEGEND.: MASTER PLAN AREA STUDY AREA TO 8 SPW PUMP STATON AS PROPOSED BY RD rlood BARRER BMA /SAMUT PRAKAN OTHER PROPQSED EMBANKMENTS EMBANKMENtS PROPOSED BY RO CHAO PHYA Z DVERSiON CANAL. Figure 4 Proposed Flood Protection System for Thonhuri and Samut Prakan West Area (NEDECO et ai 19 B7)

9 ! ~ ~ <5 ~ g ~ Figure 5 Proposed Sewerage System by CDM (1968)

10 ... L'illJ'V 19 ' 0 "" lj'iil:"lllj ~ i!~~! ~ / / ( < U C; \ \ Figure 6 Proposed Sewerage System by JCA (1981)

11 REFERENCES CDM (1968). Sewerage Drainage and Flood Protection Systems Bangkok and Thonburi Thailand February. JCA (1981). Bangkok Sewerage System Project Master Plan Report August. JCA (1981). Feasibility Study on Flood Protection/Drainage Project in Eastern suburban-bangkok February. JCA (1988). Basic Design Study Report on the Project for the Procurement of Equipment for Flood Control Center in Bangkok and ts Vicinity September. NEDECO NECCO LM/SPAN (1984). Bangkok Flood control and Drainage Project (City Core) June. NEDECO and SPAN (19B7). Master Plan for Flood Protection and Drainage of Thonburi and Samut Prakan West September.