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PotentialSeismicityofYangonRegion(geologicalApproach)

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May 26, 2011 16:27 AOGS-SE 9in x 6in b1146-ch12

Advances in GeosciencesVol. 26: Solid Earth (2010)Ed. Kenji Satakec© World Scientific Publishing Company

POTENTIAL SEISMICITY OF YANGON REGION(GEOLOGICAL APPROACH)

HLA HLA AUNG

Member, Myanmar Earthquake Committee,MES Building, Hlaing University Campus, Yangon, Myanmar

[email protected]

Yangon sits on the southeast corner of Ayeyarwady Delta Basin, 35 km fromthe west of Sagaing fault and on the southern spur of NNW–SSE trending Bagoanticlinal ridge. Yangon is mostly covered with alluvial deposits. Bago anticlineis threatening Yangon with seismic disturbances. This disturbance might notbe a significant one but the movement along Sagaing fault that was believed tobring severe damage to buildings and loss of human lives in Bago and Yangonin 1930 may have potential of causing a catastrophic earthquake in the future.The prevailing geological structures, along with surface geological condition,soil characteristics, and tectonic setting have made Yangon an earthquakeprone area. In this paper, an effort is made to examine Yangon region with

respect to geological knowledge, existing historical earthquake records, recentinvestigation of seismic activity and seismotectonic of Yangon region to giveinformation on earthquake hazard for the region. Geological knowledge is veryimportant for analyzing geological site characteristics to consider for urbandevelopment. To-date Yangon has annual increase in population and expandingurban development. If an earthquake of magnitude 7.0 on Richter scale occursin Yangon, there would be higher damage to the buildings and more loss ofhuman lives.

1. Introduction

This paper is the first attempt to give relevant information about potentialseismicity of Yangon region from the point of view of geological knowledge.Owing to spare population and traditional construction of buildings, nohistorical earthquake records had shown a catastrophic earthquake inMyanmar. A basic element to mitigate the effect of potential damagingearthquakes is the geological understanding of built environment, whichinvolves potential earthquake source areas related to rupture mechanismand surface geology. Geological aspects are also important for earthquake

139


140 H. H. Aung

zonation mapping, which can provide reliable and practical outcomes fornatural disaster planning projects for future earthquake, land-use planning,and building code revision.

2. Location

Yangon is located between latitudes 16◦ 45′N–17◦ 4′N and longitudes 961′E–96 20′E, on the southeastern corner of the Ayeyarwady Delta basin,at the mouth of three rivers: Yangon, Ngamoyeik and Bago rivers and34 km from the sea in the coastal area. It has a tropical monsoon climatewith annual precipitation of 2366mm. The average temperature is 27◦C. Ithas population of about six million people. Owing to the annual increasein population, the size of the city has expanded several times than itsprewar size. Yangon’s pride: the Shwedagon Pagoda was built on the topof Singuttara Hill, on the southern spur of Bago Yoma (Fig. 1). Town planmap of Yangon is shown in Fig. 2.

16˚-

17˚-

18-

Fig. 1. Location and general geological map of the Ayeyarwady Delta Basin (adaptedfrom Geological Map,1:1,000,000).1


Potential Seismicity of Yangon Region 141

Fig. 2. Town plan map of Yangon City.

3. Tectonic Setting

Yangon region is tectonically located on the southern spur of the NNW–SSEtrending Bago anticlinal ridge, which lies immediately on the western siteof Sagaing Fault. Bago Yoma is a ridge of both geological and geomorphicalprominence ridge with 400 miles long and 40 miles wide and is composed ofMiocene rocks. Bago Yoma extends toward south into the gulf of Motammaand might be connected to Alcock Rise.2 Yangon is 35 km in the westof Sagaing fault. The Bago Yoma, Sagaing fault, and Central Andamanspreading center are the most significant structures of shear band of Sagaingfault with 100 km width.3


142 H. H. Aung

4. Geology

Yangon area is underlain by alluvial deposits (Pliestocene to Recent), thenon-marine fluvialtile sediments of Irrawady formation (Pliocene), and hard,massive sandstone of Pegu series (early–late Miocene). Alluvial deposits arecomposed of gravel, clay, silts, sands and laterite, which lies upon the erodedsurface of Irrawaddy formation at 3–4.6m above sea level. The central partof Yangon area is occupied by the anticlinal ridge as a backbone, 30m abovemean sea level and covered with sands, sand rock, soft sandstones, shale,clays, and laterite of Irrawaddy formation. The hard compact sandstoneand shale of Pegu series can be found at the northwest corner of Hlawgalake with NNW–SSE strike dipping to the east.4 Alluvial deposits are foundin the surrounding areas of the ridge (Fig. 3), whereas lateritic soils can befound along the ridge (Fig. 4).

5. Structure

In the geological map (Fig. 2), two anticlines can be seen trendingNNW–SSE and are cut by NNE–SSW trending transverse fault. Thefolds of Bago Anticlinorium plunge gradually to the south and finallydisappear under the deposits of Ayeyarwady delta.5 Eastern fold approachesBago whereas western fold extends south to Yangon and further southinto the Mottama basin. The structural trends here include Twante,Kawhmu, Yangon, and Hlegu-Thanlyin trends. They are NNW–SSEtrending and are double-plunging anticlines, cut by transverse faultstrending NNE–SSW. Folds are aligned with axes parallel to the directionof maximum extension and are arranged as en-echelon and oblique tothe main Sagaing fault zone (Fig. 5). These structures are the southernmost continuation of the Bago Yoma and are located quite close tothe Gulf of Mottama. Twante anticline is a symmetrical and double-plunging anticline with gentle dip 7–15◦ on both flanks. It is made up ofIrrawaddian rocks and alluvium in places. Kawhmu anticline is an elongated,asymmetrical and doubly plunging anticline with NNW–SSE strike. NNE–SSW trending en-echelon tranverse faults cut the anticline into slices.Sabagyisan anticline is a symmetrical anticline with dips 5–20◦ plungingto NNW.

Miocene and Pliocene rocks are folded and quarternary pebbles andterraces are uplifted. These deformation found in Yangon region should be



Fig. 3. Geological map of Yangon.6

considered due to the mobility of Bago anticline. Two terraces are foundnear Yangon with 10m thick of alluvial clays. They are situated 70 kmnorth from Yangon and raised 20m above the sea level due to the upliftingconnected to the development of Bago anticline.


144 H. H. Aung

Fig. 4. Soil map of Yangon. (Source: Land Use Bureau of Yangon).

6. Seismicity Background

In 17 December 1927, a six-grade earthquake hit Yangon and caused certainamount of damages. It was felt 15,000 sq.km from Kyangin to Dedayealong the western slope of Bago Yoma. In July 1930 Bago earthquakewith M = 7.3 affected Yangon, vibration spread caused damage to thebuildings and 500 persons and 50 persons were killed in Bago and Yangon,respectively.6 The last record of the earthquake that struck Yangon is 1978,M = 5.7. In the recent seismicity map (Fig. 6) two significant clusters of



Fig. 5. Structural trends in Yangon Region (derived from Oil map).1

epicenters draw our attention: one is along N–S trending Sagaing fault andsecond one is along NNW–SSE trending Bago anticline. These distributionsof epicenters imply the tectonic movement along these structures, whichare tectonically active. The Yangon earthquake in 1927 probably originatedfrom the uplifting of Bago Yoma caused movement along the lines ofweakness below the deltaic alluvium and Bago earthquake in 1930 wasoriginated from the displacement on Sagaing fault. As seen in this seismicintensity map, seismicity is high in the south of Yangon area, which


146 H. H. Aung

Fig. 6. Seismic intensity map of Myanmar region. (Source: NEIC).

indicates that the Andaman sea region is a zone of high seismicity zoneoriginated at shallow depth of less than 30 km. In seismic intensity map ofModified Mercalli Scale (U.S.G.S. earthquake catalog 1970–1973) (Fig. 7)and earthquake zonation map of Bago–Yangon region (Fig. 8), there arethree earthquake hazard zones according to their relevant magnitude, inwhich Yangon falls in seismic zone VI whereas Bago falls in seismic zoneVIII.

Based on the lithology and the structure of the area, two areas aredivided in the micro-zonation map (Fig. 9). The area along fault and foldcovered with sand rock is a critical area and the area covered with loosesand and alluvial deposits are the most critical area because such alluvial



Fig. 7. Seismic intensity map of Bago–Yangon region. (Source: USGS earthquakecatalog).

soil are the most vulnerable area for earthquake hazard. As earthquake cantrigger landslides, slope stability studies are very important for future urbandevelopment. In Yangon area, most of the areas are flat-lying lowland in thedeltaic region where slope gradient is gentle so that landslide can only betaken account along the river bank (Fig. 10). To define which area in Yangon


148 H. H. Aung

Fig. 8. Seismic zone of Bago–Yangon.

has the highest risk is super-imposing the seismic hazard micro-zone mapon the slope stability map. For Yangon area, the most suitable area forfurther urban development sits outside the most vulnerable seismic zoneand landslide-prone area.



Fig. 9. Microzonation map of Yangon Area.

7. Active Structures and Seismicity

The historical seismicity background along the Sagaing fault, shown inGeology of Burma by Chibbher (1983), and recent seismic investigation3

show that Myanmar lies within the broad, which is seismically activeSagaing transform belt between India and Indochina plate. A seriesof pull-apart basins from Central Andaman Basin in the south toHukawng Basin in the northernmost part of Myanmar and other related


150 H. H. Aung

Fig. 10. Landslide hazard map of Yangon area.

structures such as NW–SE trending thrust faults, NW–SE and NNW–SSE trending en-echelon folds, the basin bounding faults of ENE–WSWtrending normal faults, and N–S trending strike-slip faults are formed bythe NNW-oriented extension and ENE-oriented compressive deformations.Within through-going deformation zone, the structures formed by thesedeformations as Neogene is active and these active structures are capableof generating future earthquakes and these are the potential source areas inMyanmar.7



8. Conclusions

The aim of this brief paper is to give a profile of seismic hazard in Yangonregion from a geological approach. Geo-morphologically speaking, Yangonlies in a coastal area of Ayeyarwady delta region, at the mouth of threerivers and mostly covered with alluvial deposits. Tectonically, it is locatedon the southern extension of Bago anticline and 35 km from the west ofSagaing fault. Structurally, spur of Bago anticlinal ridge passes through thecenter of Yangon city as a backbone and extends to the south. There aremany en-echelon folds in Yangon region trending NNW–SSE and are cut byNNE–SSW trending transverse faults. On the seismic aspect, Yangon fallsin seismic zone VI. The prevailing geological structures along with surfacegeological condition, soil characteristics, and tectonic setting have madeYangon an earthquake prone area. As the population increases in Yangon,urban development has been taking place, at present, mostly on alluvialdeposits. Now there are many high-rise buildings in many parts of Yangon.Damage potential to the buildings and loss of lives in a future earthquakewith magnitude of 6 or 7 on Richter scale in Yangon would be much largerthan that in 1927 and 1930.

References

1. F. Bender, Geology of Burma (Gebruder Borntraeger, Berlin Stittgart,Germany, 1983).

2. J. R. Curray, J. Asia Earth Sci. XX (2005) 1–42.3. C. Rangin, GIAC Conf. Yangon, Myanmar (1996–1999).4. W. Naing, M. Sc. Thesis, Univ. of Yangon (1970), unpublished.5. G. P. Gorshkov, Byull. Sovj. Seim. 12 (in Russ.) (1959).6. H. L. Chhibber, The Geology of Burma (Macmillan and Co. Limited,

St. Martin’s Street, London, 1934).7. H. H. Aung, Advance in Geosciences 13 (2009).

YANGON RIVER GEOMORPHOLOGY IDENTIFICATION AND ITS ENVIROMENTAL

IMAPACTS ANALSYSI BY OPTICAL AND RADAR SENSING TECHNIQUES

Aung Lwina, Myint Myint Khaingb

aRemote Sensing Department, Mandalay Technological University, Myanmar - [email protected]

bRemote Sensing Department, Mandalay Technological University, Myanmar - [email protected]

Working Group VIII/4: Water

KEY WORDS: Fluvial, Sedimentology, LULC, Hydrologic process, Environmental impacts

ABSTRACT:

The Yangon river, also known as the Rangoon river, is about 40 km long (25miles), and flows from southern Myanmar as an outlet

of the Irrawaddy (Ayeyarwady) river into the Ayeyarwady delta. The Yangon river drains the Pegu Mountains; both the Yangon and

the Pathein rivers enter the Ayeyarwady at the delta. Fluvial geomorphology is based primarily on rivers of manageable dimensions.

The emphasis is on geomorphology, sedimentology of Yangon river and techniques for their identification and management. Present

techniques such as remote sensing have made it easier to investigate and interpret in details analysis of river geomorphology. In this

paper, attempt has been made the complicated issues of geomorphology, sedimentation patterns and management of river system and

evolution studied. The analysis was carried out for the impact of land use/ land cover (LULC) changes on stream flow patterns. The

hydrologic response to intense, flood producing rainfall events bears the signatures of the geomorphic structure of the channel

network and of the characteristic slope lengths defining the drainage density of the basin. The interpretation of the hydrologic

response as the travel time distribution of a water particle randomly injected in a distributed manner across the landscape inspired

many geomorphic insights. In 2008, Cyclone Nargis was seriously damaged to mangrove area and its biodiversity system in and

around of Yangon river terraces. A combination of digital image processing techniques was employed for enhancement and

classification process. It is observed from the study that middle infra red band (0.77mm - 0.86mm) is highly suitable for mapping

mangroves. Two major classes of mangroves, dense and open mangroves were delineated from the digital data.

1. INTRODUCTION

1.1 Landforms formed by rivers

Running water in fixed channels is the most widespread agent

of land sculpturing working on earth's surface. Therefore, the

landforms created are more important than those formed by

other agents. Flow of water takes place in rivers under the

influence of gravitation. The type of flow can be laminar or

turbulent. `Laminar' flow is a flow in which the streamlines

remain parallel to the axis of the flow. In a `turbulent' flowing

river, a mixing of water by turbulent eddies takes place.

A river can erode when it transports material. The transport can

take place in different ways:

- in solution

- in suspension - these are the small particles carried in

suspension.

- in saltation - sand grains hop over the bottom, the

sand grain reaching the bottom gives an impulse to

another sand particle.

- shoving: coarse material rolls over the river bed.

Coarse material is often deposited as riffles and bars in the

riverbed, these bars are placed alternating in the left and right

side of the river and form bank bars. In braided channels with

criss crossing waterways, channel-bars and islands develop

between the water courses. Laboratory experiments have shown

that the cross section of a channel transporting the same volume

of water is dependent on the type of bed material. Fine material

gives a deeper bed, coarse material a flatter, broader river bed.

A river can have a straight, a sinuous, meandering, or

a braiding channel. A meandering river flows in sinuous curves.

Meanders are arbitrarily confined to a ratio of channel length

to valley length. The water in the meander moves as a

corkscrew, the so called helicoidal flow, that means that the

flow is downstream, but besides that a movement in

perpendicular direction occurs, formed by the centrifugal force

on the water in the bend. This type of flow causes erosion in the

outer(concave) side of the meander and deposition in the

inner(convex) side. The strongest erosion takes place a short

distance after the central part of the bend. This causes "point

bars" to develop on the inner side, and the meander to migrate

downstream. A meander tries to broaden and to move

downstream. When meanders attain extreme looping, a cutting

of the meander can be formed during avulsions. In the cut-off

part an oxbow-lake is formed. In aerial photographs old cut-off

meanders, meander scrolls or point bars etc. can be easily

distinguished.

The zone where the meanders are formed is called

"meander-belt". Sometimes a relation between the width of the

channel and the width of the meander belt exists, according to

different authors the relation varies between 1:12 and 1:18. A

'braiding' river is characterised by different criss-crossing

channel ways around alluvial islands. The growth of an island

begins as the deposition of a central bar starts. The bar grows

downstream and in height and forces the water to pass through

the flowing water channels.

1.2 Remote sensing techniques for landform Analysis

Remote sensing techniques have opened new vistas for

landform analysis (both static and dynamic aspects), coupled

with field verification surveys. Landforms can be directly and

best viewed using remotely sensed data, since relief forms are

well expressed on the surface of the earth and recorded in

International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B8, 2012 XXII ISPRS Congress, 25 August – 01 September 2012, Melbourne, Australia

175

images. The combination of systems (DIGITAL IMAGE

PROCESSING, multi-date and multi-scale data analysis)

increases information generation capability and thematic map

generation facility. These modern techniques have contributed

tremendously towards terrain analysis, understanding of site

conditions, spatial distribution of features, and resources.

Analysis of remotely sensed data using standard

interpretation techniques is particularly useful in channel

change detection, identifying palaeo-channels, regional

landform distribution, as well as detection of shallow buried

channels and buried valleys under special conditions using

thermal IR and radar imagery. In radar imagery over extremely

dry sands of desert areas of Sahara in northern Sudan, buried

valleys at 1.5 meters depth below surficial cover have been

detected (SIR-A data, 1981). Dynamical aspects of

geomorphology, landslides etc. can also be monitored. Digital

enhancement techniques are useful for improved interpretation

of terrain features. The development of landforms depends on

the climatic regime, the operative processes of denudation and

sedimentation during and after their formation as well as their

intensity in time and space, and the rocks and materials (their

composition, nature, and structure) acted upon. Man-made or

anthropogenic causes also affect landform development.

The identification of landforms and geomorpholoical

domain on remotely sensed data is based on area association

(arid, mountainous, glacial, coastal, flood plain, tropical etc.),

association of features, landform shape and size, drainage

patterns/ dissection, relief, tone, texture, land use/land cover,

erosion and other patterns etc. leading to "convergence of

evidence" upon logical inductive and deductive reasoning.

Analytical "Keys" can also be developed for an area of study

based on field criteria and a priori knowledge of typical forms

as seen on images.

Remote sensing provides a regional, synoptic view and permits

recognition of large structural patterns and landforms over

contiguous geomorphic domains. It enables the location and

delineation of extent of identified features observed over large

areas. The repetitive coverage of terrain in multispectral

mode provided by satellite mounted sensors enables

comparison of scenes of the same location in different periods/

seasons. This is extremely valuable for monitoring change, as

well as extracting more information about significant earth

features from scenes by viewing under seasonal conditions

(temporal and spectral resolutions).

2. REGIONAL GEOLOGY AND TECTONICS

2.1 Study area and its existing conditions

The present study area covering the Yangon and its surrounding

region falls in 96° and 96° 15’E and 16° 45’and 17° N as

referred as map index of UTM Sheet No. 1969-01. The central

part of the Yangon comprises Miocene consolidated sediments

overlain by the Quaternary sands, silts and clay. Win Naing

(1972) stated the uppermost part of the Mingalardon Ridge as

the Irrawaddy Formation of Pliocene age. But, thinly laminated,

weathered shale exposed in Shwegondaing area during

excavation for motor road extension works in 2003 and

completely weathered sandstone during excavation for the

foundation of the Yanshin Centre at the Shwegondaing Junction

reveal that the lithological character is resemble to that

Miocene sediments exposed in the Taikkyi Taungnio area (Tint

Lwin Swe, 2002). Kyaw Htun (1996) explained that Thadugan

sandstone and Besapat alternations in the Thadugan area were

belonged to the Upper Pegu Group of Miocene age; namely, the

Kyaukkok and Obogone formations. In addition, some rock

exposed in the left and right abutments of Inyar Lake and

geological drilled data for water well at the junction of the

Inyar and the Damazete roads (Tint Lwin Swe, 1998) show that

the lithological type is especially similar to that of the

Thadugan.

The Quaternary sediments widely distributed at the

outskirt of the Yangon, consisting of thick, high plastic, stiff

clay underlain by sand and silt. Win Naing (1972) classified

generally the Quaternary sediments into valley-filled deposit

and the alluvium. The valley-filled deposit includes the

Pleistocene older alluvium of a particular type of terrace

deposit (Leicester, 1959 and Kyaw Htun, 1996) of

unconsolidated gravels, sands and silts and the alluvial is

younger age clayey deposit. The pattern and distribution of rock

basement and soil deposit are depicted in Figure (1).

Figure 1. Soil and rock distribution of the Yangon area

(Win Naing, 1972)

Tectonically, the Yangon is situated in the southern

part of the Central Lowland, which is one of three major

tectonic provinces of Myanmar. The Taungnio Range of the

Gyophyu catchments area of Taikkyi District, north of Yangon,

through the Thanlyin Ridge, south of Yangon forming a series

of isolated hill is probably resulted from the progressive

deformation (Ramsay, 1967) of the Upper Miocene rocks as the

eastern continuation of the subduction or stretching and

compression along the southern part of the Central Basin and

regional uplifting of the Pegu Yoma.

2.2 Yangon river in and around soil investigations

The different varieties of the individual soil characteristics are

Meadow and Meadow Alluvial Soil, Gley and Gley swampy

soils, Swampy soils, Lateritic soils, Yellow brown forest soils,

Dune forest & Beach sand, Mangrove forest soils and Saline

swampy meadow gley soils. The meadow soils which occur

near the river plains with occasional tidal floods are non-

carbonate. They usually contain large amount of salts. Meadow


176

Alluvial soils (fluvic Gleysols) can be found in the flood plains.

They have the texture of silty clay loam and they have the

neutral soil reaction and are rich in available plant nutrients.

nMeadow Gley soils (Gleysol) and Meadow swampy (Histic

Gleysol) occur in the regions of lower depressions where the

lands are inundated for more than 6 months in a year. The

texture of these soils is clayey to clay and usually having very

strong acid reaction, and contain large amount of iron.

Figure 2. Soil map of the Yangon area (copyright of Land use

division, Myanma Agriculture Service (Feb 11, 2002)

Dune forest and Beach sand can be found only at the

coastal line of Myanmar. The areas of their occurrence are

insignificant. The coastal line should be under wind and water

erosion control. Mangrove forest soils occur in very small area

along the coastal line of Myanmar, especially in the region of

Ayeyarwady Delta. These are marine flat lowlands, which are

affected by daily tides. Saline swampy meadow gley soils in

Ayeyarwady Delta and along the river bands of the Gulf of

Motama and the marine flat lowlands influenced by the tidal

sea water, which is always salty.

2.3 Typical Drainage Patterns

This area almost fluvial food plain, other is lower coastal plains

where there may be few surface drainage channels. In and

around Yangon river areas, the water table is often high;

relatively young and subjected to a minimum of dissection. A

high water table minimizes runoff and restrict system that may

from between floods.

Many major streams in level regions are constructional. They

build up their own flood plains and have little contact with the

underlying material of the area. Some major streams in level

areas, however, are engaged in eroding and are, therefore

destructional. Examples of such streams may be found in

coastal plains and in lakebeds.

Figure 3. Typical Tidal Flood Pattern in Myanmar

3. METHODOLOGY APPROACH

The methodology used in this study involved distinct steps of

digital processing of individual remote sensing data, multi-

sensor data integration, and visual interpretation of the

geomorphological products. The processing of remote sensing

images was done using ENVI 4.7 and Sufer version 10.7.972

software, following schemes for enhancements and integration

of optical and SAR images successfully used for Yangon river

geomorphology and terrain analysis. The corresponding

information was acquired on the terrain based on a ground

positioning system (GPS) campaign and used as ground control

points (GCPs). Since the area presents low relief and no digital

elevation model (DEM) was available, an ortho-rectification

scheme, assuming a flat terrain model.

4. RESULTS AND DISSUSION

4.1 Interpretation and terrain analysis from optical data

Long ago back from more than 10 years, AVNIR imagery

taken by Japan Advanced Earth Observation Satellite (ADEOS)

Figure 4. ADEOS/AVNIR 432 FCC Color Composite Image

acquisition at December 25, 1996


177

at December 25, 1996. In this imagery, we easily interpreted by

visually for land use land cover condition of Yangon river in

and around and City area.

Figure 5. Landsat 432 Color Composite Ortho-rectified Image

acquisition at Feb 25,2006

In Figure 5, Landsat Satellite acquired with ETM+ Sensor for

the study area. After composite of FCC 432 combination was

done and carefully analyzed for landuse landcover extended

and urban, sub-urban sprawled areas.

Figure 6. Landsat 432 Color Composite Ortho-rectified Image

acquisition at March 3,2009

In May 2 of 2008, Myanmar was seriously hit by Cyclone

Nargis and there was damaged to coastal mangrove areas and

its biodiversity system in and around of Yangon river terraces

(see figure 6).

4.2 Interpretation and terrain analysis from RADAR data

Figure 7. JERS 1 SAR Multi Temporal image of study area

In Figure 7, Japan Earth Observation Satellite was taken

Synthetic Aperture Radar (SAR) imagery for 3 different

seasons of around 1996. Coastal surveillance and

environmental monitoring has motivated the development of

automatized feature extraction tools using remote sensing data.

Target detection by Synthetic Aperture Radar (SAR) has been

extensively studied in recent years. In carefully interpretation

from SAR Imagery, river boundary and coast line field give

high radar backscattered energy due to their high surface for

roughness. Strong waves and tides (surfing in particular) make

seawater very rough which leads to very high radar

backscattered energy at places. Coastline is therefore masked at

places between land and water boundary.

Figure 8. SRTM data of Yangon river rings


178

In Figure 8, Shuttle Range Topographic Mission (SRTM) data

was prepared for shaded and relief map for terrain conditions.

This study area is almost flat and fluvial flood plains. The

product generated from SRTM data to topographic analysis is

important for descriptions of soil contacts and structural

features. The perspective of the relief, through the simulations

of different angles of illuminations, gave the shadow of the

relief, giving the impression of concavity and convexity,

allowing the identification of structural features, soil contacts,

erosion zones and other geomorphological features of the study

area.

4.3. Gemorphological Map generation

Figure 9. Geomorphological Map of Yangon river in and

around area.

The landform classification system is based on geomorphologic

principles, i.e., classification on the basis of landforms, and the

dominant processes in operation related to historical processes.

Additional factors, including land use and land cover, were also

used for classification. The final geomorphological map is

presented in Figure 9. Integration of both optical and radar data

was implied for geomorphic landform mapping, in details of

terrain conditions, manmade features and lanuse land cover

around Yangon river bed and around Coastal flood plain

terraces.

5. CONCLUSION

The contribution of TM band 4 was related to the

discrimination of dense mangrove forest from secondary

vegetation of the coastal plateaus, whose spectral response is

mixed with exposed soil produced by human activity and

disaster affected. The JERS-1 SAR data have contributed to the

enhancement of distinct coastal vegetation height, geometry,

water content, and degraded and regenerating mangrove

regions. The Multi temporal SAR product was fundamental in

providing consistent information about the geo-botany

(vegetation and coastal sedimentary environment relationship)

and emerged and submerged coastal geology that cannot be

accomplished from field investigations alone..

6. REFERENCES

References from Books:

Bushnell, T.M et al., 1955. Air Photo Analysis. Newyork, USA

p.p 12-13

Garde, R.J., 2005. River Morphology. New Age International

Publisher, India, p.p 71-72.

Lecture Notes, Geosciences Division, Indian Institute of

Remote Sensing, India, p.p 103-104.

References from Other Literature:

Aung Lwin, R. S Chatterjee and Myint Myint Khaing, 2010.

Analysis of Change Detection on Coastline using ERS SAR

tandem pair. Myanmar Engineering Society Annual

Conference, Yangon, Myanmar

Kyaw Htun. 1996. Sedimentology and Petrography of South-

Western Part of Thadugan, Shwe Pyi Tha Township, M. Phil.

Paper, Geology Department, Yangon University, Myanmar

Pedro Walfir M. Souza Filho and Waldir Renato Paradella

2005. Use of RADARSAT-1 fine mode andLandsat-5 TM

selective principal component analysis for geomorphological

mapping in a macrotidal mangrove coast in the Amazon Region

Can. J. Remote Sensing, Vol. 31, No. 3, pp. 214–224,

Tint Lwin Swe, 2004. Determination of Peak Ground

Acceleration for Yangon and Its Surrounding Areas. Staff

Report, Yangon Technological University, Myanmar.

Win Naing. 1972. The Hydrogeology of the Greater Rangoon,

M. Sc.Thesis, Geology Department, University of Rangoon.

Myanmar

7. ACKNOWLEDGED

The authors would like to thank the National Space

Development Agency of Japan (NASDA). In the case of JERS-

1 SAR data and ADEOS/AVNIR imagery were kindly provided

by the Ministry of International Trade and Industry of Japan

(MITI) and NASDA for research purposes.

Special thanks are extended to USGS, Google Earth and Global

Land Cover Facilities (GLCF) Teams for free provision of

Landsat 7 ETM+ Imagery and SRTM images. In many depth

are due to my colleagues from Remote Sensing Department,

Mandalay Technological University, Mandalay for their kind

patience and encouragement to finish this work.


179

Ceylon Journal of Science: Physical Sciences, 4(1), 47-59 (1997)

RELATIONSHIP BETWEEN SUBSURFACE GEOLOGY AND GROUND SUBSIDENCE OF BANGKOK METROPOLIS, THAILAND

N.W.A.M.M.K.N. BANDARA* Asian Institute of Technology, Bangkok, Thailand.

ABSTRACT

Bangkok city, the capital of Thailand which has many engineering and environmental problems due to ground subsidence was selected as the main study object in this research study. The study included data collection, bore hole logging and investigations on some important underground geotechnical parameters to prepare thickness maps, static water level maps, ground elevation and subsidence maps of Bangkok subsoil. Thickness of both fine grained compressible clay layers and that of coarse grained non compressible sand layers are highly varying from place to place and they are highly deformed. Area of eastern Bangkok is affected by the highest ground subsidence and this area is underlain by the thickest portion of both first and second compressible clay layers. The lowest static water levels of upper most aquifers is also overlain by this area. The uppermost two compressible clay layers contribute more percentage for ground subsidence.

1. INTRODUCTION

General Situation

The study area is located within the latitudes 13° 29' 32" - 13° 57' 45" and the longitudes 100° 24 ' - 100°45' Bangkok Metropolis covers an area about 1569 square kilometers. The area is extremely flat and the relief is less than 0.5 m. The elevation is ranging from 0 to 1.5 m while the average elevation is less than 1.0 m above mean sea level (Fig. 1).

The ground subsidence is the most serious threat to the development of Bangkok and its suburbs. Ground water withdrawal from the deep well pumping is the main reason for this. However, the current rate of ground water pumping cannot be reduced by a considerable amount because of the high demand. In addition, the surcharge load of engineering structures, weight of over layers and vibration due to traffic and pile driving are significant. Differential settlement of the ground surface creates engineering, environmental and social problems. However, flooding is the most disastrous result of ground subsidence because of the lower ground elevations and smaller gradient of slopes. There are some places where the ground elevation has gone down beyond the mean sea level. *

* Current Address: Department of Geology, University of Peradeniya, Peradeniya.

47

it li li l i J4 ii ii ir it i t i» it I I I I I I I I I I if I I I I n it rt n I I n ri rr u n 1 1 1 1 n it 11 n I I if 11 •• i l

Fig. 1: Ground Elevation above Mean Sea Level

48

Bangkok and its suburbs are already developed but, the geology of greater Bangkok has been largely ignored in land use planning and development. In addition, a proper land use planning was not used in its development. Unplanned infrastructures and low land reclamation in vulnerable areas are some examples.

The magnitude and the rate of subsidence are directly related to the change in effective stress in the various compacting beds which is a result of piezometric level changes and the thickness and compressibility of soil.

OBJECTIVES

Determination of the relationship between subsurface stratigraphy, ground water withdrawal and ground subsidence.

2. GEOTECHNICAL CONSIDERATION OF GROUND SUBSIDENCE

Geology and Structure

The area is underlain by thick Quaternary and Tertiary deposits consisting of alluvial and deltaic sediments. Subsoil within the uppermost 200 m consists of two types of alternative layers, coarse grained sand with high permeability and low compressibility and fined grained clay with low permeability and high compressibility. According to Brown et al. (1951) and Sodsri (1978), three types of sediments underlain by the Bangkok plain.

I. Unconsolidated silt, sand, clay and gravel in the flood plain, stream channel or terrace.

II. Beach and esturine clay, sand and gravel IU. Residual layers of laterite or creator capping stabilised surfaces.

According to Nutalaya and Rau (1981), Quaternary and Tertiary sediments of the Bangkok delta represent a complex sequence with a thickness of more than 2,000 m but, only the uppermost 200m is explored. In the lower central plain, sedimentation was controlled during the Tertiary and Quaternary times by a combination of tectonic movements both within the plain and in the adjacent mountains. Further, they pointed out that the Bangkok basin had been continuously filled with alternative layers of sand and clay throughout the Quaternary time.

These sediments are underlain by a highly fractured and faulted basement rock consisting of quartzite, gneisses and granitic gneisses. A series of active faults and structural blocks occupied the basement.

Ground Water Consumption

Ground water is extracted from all the sand layers within the uppermost 200 m of Bangkok subsoil. There are number of ground water monitoring stations installed to monitor the ground water level after identifying the critical zones and areas of heavy ground water

49

development in the Bangkok area. It is estimated that, about 1.2-1.4 million cubic meters per day of well water is pumped from ground water aquifers in the Bangkok Metropolitan region. Due to the unplanned ground water consumption, a number of environmental problems have arisen such as salt water encroachment, ground subsidence, ground water depletion etc.,.

Flooding

Flooding is one of serious hazards in Bangkok metropolis. The flood season in Bangkok generally begins in September but rainstorms can cause immediate flooding at almost any time between May and October. However, the most severe floods occur in October when river draining from northern Thailand brings water to Bangkok. In the spring tidal period, flooding is more severe as the high water level in the sea retards the river flow, resulting the water level to rise in the flood plains. In addition, hundreds of canals which receive runoff from the large sub urban area also flow in to the river. These canals have with negligible gradients or are concave in some locations. Tidal action sends water back into the canals during high tide periods. Water gates are designed to prevent flood water but when the city is flooded and the river is at its highest state, this measure is not sufficient to prevent flood. Since the gradient is extremely flat, the river flowing across the city called Chao Phraya can not keep the water within its own when a certain gauging height is reached. However, the Chao Phraya river can discharge 1,500 m 3/sec through the city without flooding low lying areas.

Ground Subsidence in Bangkok

As mentioned elsewhere the ground subsidence is a serious problem in Bangkok area. Sodarathit (1989) found that the average ground subsidence in the eastern part of Bangkok metropolitan region was 5 - 10 cm per year and some parts of the area is below the mean sea level. However, it is difficult to determine how much of the annual flood damage is due to the ground subsidence.

Brand and Balasubramaniuum (1976) showed that the consolidation of the soft clay contributes the major part of the subsidence. According to Nutalaya et al. (1989), the ground subsidence of Bangkok area affects more than 4,500 km 2 area.

Effect of Ground Subsidence in Bangkok

According to Nutaliya et al., (1989) differential settlement occurs when structures located at different foundation depth cause cracking and bending of structures, floor slabs, concrete walkways and steps, detachment of septic tanks and sidewalks or steps from buildings. Sinking of benchmarks is another serious problem and hence, the benchmarks of Bangkok area can not be used as reference datum. In addition, as a result of lowering of the piezometric level, saline water replaces fresh water in the aquifers located next to the sea and therefore, the amount of saline water content increases in the ground water. The deterioration of ground water quality is also caused by the penetration of mineralized water from the higher pore pressure area in the clay layers in to the sand layers.

50

When ground subsidence continues, the gradient of ground surface decreases and therefore, drainage of flood water by gravity flow will be reduced and at the same time, water accumulates in the centre area of subsidence bowl. As a sequence, septic tanks in the flooded area will become water logged and the foul mass of night soils tearing with virulent bacteria and water borne diseases will become a potential health hazard. Therefore, ground water of Bangkok is now highly polluted and not suitable for consumption at all.

3. MECHANICS OF SOIL CONSOLIDATION AND GROUND SUBSIDENCE

The annual rate of subsidence varies greatly in direct response to seasonal pumping. Subsidence at a given location will continue as long as declining water levels continue to cause increased effective stress.

Das (1994), summarized the theoretical expressions related to ground settlement in three ways that is deformation of soil particles, relocation of soil particles and expulsion of water or air from void spaces. In general, the soil settlement caused by load is divided into three categories, i.e. immediate settlement, primary consolidation settlement and secondary Consolidation Settlement. However, primary consolidation settlement of soil is the most relevant type since it is the result of a volume change in saturated cohesive soils due to expulsion of water occupied by the void spaces.

Empirical expression for one dimensional primary consolidation of saturated cohesive soils is as follows,

For normally consolidated soil

S = primary consolidation settlement Cc = compression index (slope of the e-log p plot) T - thickness of the Layer e 0 = initial void ratio at the initial volume Po = initial average overburden pressure Ap = increase of vertical pressure Pc = maximum past pressure

Terzarghi and Peck (1967) described the mechanics of subsidence due to deep well pumping. According to their interpretations subsidence or settlement can result from consolidation of soil deposits due to deep well pumping because of the lowering of ground water level or piezometric pressure. As a result, a descending water flow occurs from compressible clay layers to aquifers. Force generated by this flow will compress the clay and other compressible deposits.

(1)

Where,

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They further introduced a relationship between water level changes and effective stress(Ao) i.e.,

Aa = Yw(H-h) (2) where,

H = current water table h = original water table y w = unit weight of water

According to Terzaghi and Peck (1967), if the clay strata are soft and thick, and if the water level is lowered over a considerable distance, the settlement is likely to be occurred over a large area. Further, the general pattern of subsidence can be expected to be characteristics bowl shape, with the greatest subsidence at the centre of the well field.

Declining of piezometric pressure in a water bearing deposit imposes an increase in effective stress in all the soil strata above it and in itself. Increased effective stress causes consolidation and hence ground settlement.

Ground subsidence generally corresponds to the piezometric pressure fall. According to Dowson (1963), in many areas, subsidence is directly proportional to the elevation of the ground water level that is directly related to the quantity of water removed.

4. METHODOLOGY

The ground water well log data and records of ground water monitoring stations in Bangkok were used to identify the subsurface geological charateristics. The thickness of sand and clay layers were individually calculated for each station and transferred them as separate layers based on their depths.

The static water level variation from point to point were estimated by studying the static water level from many stations in the vicinity of Bangkok metropolis.

5. RESULTS AND DISCUSSIONS

Projected Ground Elevation in the Year 2005

From Fig. 2, more than 50% of the study area which is below the latitude 13°45' N will be submerged by the sea if the current rate of subsidence is allowed to continue. The most important zone of the city is situated within this limit.

52

H t m - - , " J , I U MMI _ ' " IUUI . ' .]»»'.»._,, ' l*Ji

u n 11 I I I I I I n ir li i» » I I I I I I I I I I ii it i« I I n n r> ri n n ri ir ft 1111 •> 11 M n if 11 IT I I I I I I

Fig. 2: Projected Ground Elevation AMSL (m) in the Year 2005

Stratigraphy of the Study Area

The Chao Phraya basin comprises alternative layers of sand and clay. However, during this study, these alternative layers were examined only up to 200 m depth from the ground surface because of two reasons. First one is the depth of bore holes since most of them penetrated up to the depth around 200 m while the other one is geotechnical importance of uppermost subsoil within this range.

Five sand layers and another five clay layers could be identified within this limit and the variation of subsoil characteristics of these strata and their depth range is shown in Table 1 and a N-S cross section over the study area is shown in Fig. 3.

53

From the Fig. 4, the rate of ground subsidence is varying from place to place. According to the maps produced in this study, the highest ground subsidence rate takes place in the area south to the Bangkok Metropolis where the first compressible clay layer has its thickest section (Fig.5). This layer has the highest compressibility when compared to the other lower lying compressible layers. In addition, as mentioned above, the static water levels of top most two aquifers (Pra Phradaeng and Nonthaburi) are at their lowest level beneath the same area (Fig. 6 and 7).

Furthermore, the rate of subsidence varies from place to place making depressions. In addition to that, the rate increases towards the centre of each depression. Six major depressions can be seen over the study area.

With the increasing of ground water withdrawal from sand layers, water contained in compressible layers drains off from both top and bottom in to the overlaying and underlying sand layers resulting a heavior. This is because of the hydraulic gradient developed due to the ground water withdrawal.

Table 1: Description and the Depth Ranges of Soil Strata within the Upper-Most 200 m of Bangkok Sub Soil

Name of the Strata Soil Description Depth Range (m) Thickness (m) Name of the Strata Soil Description

Top of Layer

Bottom of Layer

Range Average

Bangkok Soft Clay Gray colored soft clay with shell fragments

0 11-19 11-19 15

First Clay Layer Stiff to medium clay with shell fragments

11-19 13-65 1-45 13

First Sand Layer Fine to coarse sand in association with clay patches.shell fragments

13-65 25-75 5-45 12

Second Clay Layer Clay with shell fragments 25-75 35-82 5-20 14

Second Sand Layer Fine to coarse sand with clay layers

35-82 44-90 5-30 16

Third Clay Layer Silty clay with shell fragments

44-90 60-102 5-40 17

Third Sand Layer Sand, fine to coarse with clay layers and gravel

60-102 85-123 6-40 25

Fourth Clay Layer Hard clay with inter beds of sand, shell fragments

85-123 98-157 5-25 19

Fourth Sand Layer Silt sand with shell fragments

98-157 118-185 5-36 15

Fifth Clay Layer Clay, sandy with shell fragments

118-185 124-210 5-68 14

Fifth Sand Layer Fine to coarse sand with thin clay layers

124-210 - - -

54

M*: t . city h f m : I - M Uta-i

Fig.3: North-South Vertical Cross Section of Bangkok Metropolis

n I I I I I I n I I ii ir I I I I •• it M I I I I I I •• if 1 1 I I fi FI ri rt if »i n if it »i •• « « I I • • • • " • • • •

Fig.4: Rate of Ground Subsidence in the Year 1996 (cm/yearj

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I I I I I i l l I I l l l l ir « • • • • i l i l I I l< i l i l I f I I • • I I ri f i l i l l l l l « If M i l I I l l I I M l l l l l l If l i l l l l

Fig. 5: Thickness of the Uppermost Clay Layer (m) •_

l l i f I I » l i I I l i i r I I i f i t I I I I tt I I I I l l i f I I I I I I I I n ri f i » ri i f f l l l i f l i • • I I I I I I • • I I i l i f i l

' Fig.6: Static Water Level of the Pra Pradeang Aquifer in 1996 (meter below mean sea

56

6. CONCLUSIONS

1. Six compressible and five non-compressible layers were identified in the strata within uppermost 200m depth (table 1). Basically, Bangkok metropolis consist of five sand layers. The five sand layers are found at an approximate distance of 30m, 60m, 90m, 135m and 170m depth from the surface. Although these values will give some idea of the sand layer locations, there can be high variations to the above figures depending on the locality due to the presence of synclines, anticlines, connections of layers etc. In some instances, sand lenses are found at various depth levels in the Bangkok area. The first sand layer which is situated at a level of 20-40m takes almost a planner and horizontal profile throughout the study area.

The Bangkok soft clay layer with an approximate thickness of 10-20m is the top most layer. Other five layers are interbeded with sand layers.

2. The present highest subsidence rate takes place in the area of eastern Bangkok where both the two top most compressible clay layers have their thickest sections.

3. The static water levels of all the top most aquifers are at their lowest, where the rate of subsidence is highest.

4. Ground subsidence makes depressions throughout the area.

ACKNOWLEDGEMENTS

The author first expresses his profound gratitude and sincere appreciation to his advisor Professor Prinya Nutalaya, for his persistent guidance, invaluable suggestions, generous help and friendly discussions, all of which enabled the author to accomplish this study.

Author also wishes to extend his sincere appreciation to Professor A.S.Balasubramanium and Dr. Noppadol Phein Wej for their valuable guidance, suggestions.

A very special word of thanks goes to Dr. Vachi Ramnarong, Director of the Mitigation of Ground Water Crisis and Land Subsidence in Bangkok Project (MGL), Department of Mineral Resources, Thailand for providing valuable data related to this study work.

REFERENCES

1. Research Report, Division of Geotechnical and Transportation Engineering, AIT, Bangkok, Thailand, Vol. 2 (1978b).

2. Research Report no 82, Division of Geotechnical and Transportation Engineering and Division of Water Resources Engineering, AIT, Bangkok, Thailand (1978 c).

3. Research Report No.91, Vol. 2, AIT, Bangkok, Thailand (1981). 4. A Report submitted to the Japan Internationa] Co-operation Agency by School of Civil

Engineering, AIT, Bangkok, Thailand (1993). 5. Brand, E. W. and Balasubramanium, A. S., Proceedings of the Second International

Symposium on Land Subsidence, Anaheim, California, pp. 365 - 374 (1976).

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6. Das, B. M , Principles of Geotechnical Engineering, Third Edition, Ch. 8, pp. 253- 315 (1994).

7. Dawson, R.F., J. Surveying and Mapping Division, ASCE, V.89, No.SU2, pp 1-12 (1963).

8. Department of Mineral Resources and Ministry of Industry (DMR), 1992, Ministry of Industry, Records of Groundwater Monitoring Wells in Bangkok and Adjacent Provinces, Report No. 1, Bangkok, Thailand (1992).

9. Heley and Aldrich, Report of Haley andAldrich, Inc. (1969) 10. Metcalf and Eddy Inc., Report on Ground Water Monitoring, Well construction and

future programs, The Metropolitan Water Work Authority, Bangkok, Thailand (1972). 11. Nutalaya, P. and Rau, J.L., Episodes, Vol. 4, pp. 3-8 (1981). 12. Nutalaya, P., Yong, R.N., Chumnankit, T. and Buapeng, S., The Report Presented at the

Workshop on Bangkok Land Subsidence, 22-23 June (1989). 13. Rau, J. L., The Geology of Bangkok Metropolis and Adjacent Areas, ATT, Bangkok,

Thailand (1981). 14. Terzaghi, K., and Peck, R. B., Soil Mechanics in Engineering Practices, Wiley, New

York (1967). 15. Worayingyong, K., Preliminary Predictions of Subsidence in the Bangkok Area, M.Eng.

Thesis, AJT,Bangkok,Thailand (1975). 16. Nutalaya, P., Proc. of the Conference on the Geology of Thailand, Department of

Geological Sciences, Chiangmai University, Thailand (1973).

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different between yangon and bangkok subsidence study

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