25

CHAPTER 3

STUDY AREA

3.1 GENERAL

The South-Eastern peninsular Rivers are affected by monsoon rains

and physiographic characteristics. Chennai Basin is one such area which

comprises of ephemeral to intermittent streams along with a good

groundwater resource. The distance between hills to coast is on an average

160 km along the River course. The groundwater resources spanning a

distance of 72 km across the Rivers provide the needed water resources for

the Chennai Metropolitan and its neighborhoods. The details of the selected

study area are presented in the following sub headings.

3.2 STUDY AREA – CHENNAI BASIN

The Chennai Basin comprises of the four Rivers namely: Araniar,

Kosasthalaiyar, Coovum, and Adyar River. The index map of Chennai Basin

is presented in Figure 3.1 below. The Rivers originate from North to South on

the Western side from the hills of altitude ranging from 100 m to 200 m and

flows to the Bay of Bengal on the East to North-East. Chennai city is usually

considered as a plain land surface with a gentle slope towards Bay of Bengal.

The land elevation is within 10 m and the maximum elevated lands are

located on the South-Western part of the City.

26

On an average the topography is even and the land throughout the

districts renders subdivisions into natural regions rather difficult. Surprisingly

the Rivers though originating at different locations have a tendency to merge

on the surface or below the surface. This is justified by the fact that though

the Rivers remain dry at the surface yet they are able to provide the

groundwater supply for a period of 2 years even after the worst monsoon

season.

27

Figure 3.1 Index map of study area

28

The Chennai metropolitan is located in the North-Eastern corner of Tamil

Nadu State. The drainage basin taken up for study is located between latitudes

12º 40' N to 13º 40’ N and longitudes 79o 15’ E to 80o 50’ covering an area

of 5542 sq km. It is bound by Bay of Bengal in the East, Thiruvallur district in

the North and West and Kancheepuram district in the South.

The River basin authority has designated this River basin 4C. The sub-

basins of this River basin are given in Table 3.1. The first three components

refer to River Araniar which drains into the Pulicat lagoon. The next three

refers to the watersheds between River Kosasthalaiyar and Coovum. While

the last watershed represents River Adyar and the previous last two the upland

areas of River Kosasthalaiyar. The three components of each River except

that of River Adyar respectively represent the head, middle and tail reaches.

The head reach of River Adyar is included as it represents a transition from

storage to dynamic flows. This concept is utilized in this research for

providing the augmented head using the attenuated flood.

Table 3.1 Watershed code

S.No Watershed code Watershed name

1 4C2D1 PULICAT

2 4C2C7 ARANIAR

3 4C2B6 ARANIAR

4 4C2C4 KOSASTHALAIYAR

5 4C2A7 POINI

6 4C2C3 COOVUM

7 4C2B2 KAIGAL,MALATTAR

8 4C2C5 NANDI

9 4C2C2 ADYAR

29

The administrative classification of the Chennai basin is shown in table

3.2. The Vellore district with four Taluks and blocks forms the source area of

River Kosasthalaiyar and Coovum. The Thiruvallur district with eight Taluks

and 13 blocks provides the base for the integrated water resources

management. Therefore, a detailed description of this is given under the

section conceptualization of the study area. The Kancheepuram district with

three taluks and two blocks feeds the River Adyar. The Chennai district with

5 taluks forms the tail end areas of Rivers Coovum and Adyar.

30

Table 3.2 Tamil Nadu state list of districts, taluks and blocks

S.No District Name Nos. Taluk Name Nos. Block

1

Chennai - coastal 1 Kottai – Tondiarpet -

Tanks 2 Purasawalkam – Perambur -

Coovum 3 Egmore – Nungambakkam -

Coastal 4 Mylapore – Tiruvallikeni -

Adyar 5 Mambalam - Guindy -

2 Kancheepuram – Adyar

6 Kancheepuram

1 Kancheepuram 7 2 Walajabad 8 Tambaram 4 Thomas malai

13 Sriperumpudur 12 Sriperumbudur 13 Kundrathur

3

Thiruvallur – Kosasthalaiyar 14 Thiruvallur

14 Thiruvallur

Coovum 15 Kadambathur Nandhi

15 Tiruthani 16 Tiruthani

Kosasthalaiyar 17 Tiruvelangadu Nagari

16 Pallipattu 18 Pallipattu

Nandhi 19 R.k.pet Coovum

17 Ambattur 20 Villivakkam

Coovum 21 Puzhal Kosasthalaiyar

18 Uthukottai 22 Poondi

Araniar 23 Ellapuram

Kosasthalaiyar 19 Ponneri 24 Minjur 25 Sholavaram

Adyar 20 Poonamallee 26 Poonamallee Araniar 21 Gummidipoondi 27 Gummidipoondi

4

Vellore 22 Walajapet

28 Walajapet 29 Sholinghur

Kosasthalaiyar and Coovum 23 Arakonam

30 Arakonam 31 Nemili 32 Kaveripakkam

Source: Groundwater atlas of Tamil Nadu, State Ground and Surface Water Resources Data Centre, Taramani, Chennai-113

31

3.3 HYDROLOGICAL SCENARIO

The total basin area is 7282 km2, with 1740 km2 in Andhra Pradesh

and the remainder in Tamil Nadu. Chennai receives rain from South-West

monsoon and North-East monsoons. The former is usually not sufficient to

increase the surface storage; however, it does contribute in improving the

groundwater table during the crucial period after summer. The North-East

monsoon manages to fill many of the surface storage tanks and helps to

recharge the groundwater. The basin receives 943 mm rainfall each year with

75% dependability. The average annual rainfall in Chennai basin is presented

in Table 3.3 below.

Table 3.3 Average annual rainfall in Chennai region

No. Region Raingauge Station

Average annual rainfall (mm)

1 Hills

Sholingur 952 Tirutanni 1047 Pallipet 895 2

Plains

Thiruvallur 1088 Sholavaram 1289 Minnal 959 Poondi 1292 Arakkonam 1070 3

Coast

Tambaram 1424 Meenambakkam 1324 Saidapet 1286 Nungambakkam 1215 Chepauk 1112

Generally, the average annual rainfall is high in coastal areas and low

in hills. However, within the coastal areas, the hills in Tambaram and

Meenambakkam have higher rainfall compared to the plains. This may be

32

considered as the main cause of floods in Chennai. If this rain water is not

harvested, it will drain off as flood discharge to the Bay of Bengal. Therefore,

the need for integrated management of flood with groundwater basin

resources becomes necessary for efficient flood management. The dependable

rainfall values of the River basins, which indicates the assured amount of

rainfall for 50% to 75% of the considered time period (years), are presented in

Table 3.4.

Table 3.4 Dependability analysis of rainfall of Chennai region

RIVER BASIN

Rainfall

50%

Dependable

75%

Dependable

ADYAR 1385 mm 1211 mm

ARANIAR 1274 mm 1099 mm

COOVUM 1246 mm 1030 mm

KOSASTHALAIYAR 1072 mm 818 mm

River basins can also be considered as a single basin of Thiruvallur

district as indicated in Figure 3.2 and Tables 3.5 and 3.6. The Thiruvallur

district is bounded by the Chembarampakkam as a storage zone that recharges

the River Coovum below the Korattur diversion. Between the diversions of

Kesavaram and Koratur of the River Coovum can be considered as a drainage

zone towards Chembarampakkam. Below Korattur diversion across the River

Coovum, heavy pumping for catering to the city requirements is taking place.

The sustainable groundwater in the zone makes one realize the concept that

the hard rock areas between Sriperampudur and Chembarampakkam are

33

contributing to the ground in the zone. Therefore, the alluvial belt with a

thickness of 25 m can be considered as spanning the diversions of Korattur

and Thamaripakkam and extending to Periyapalayam. Surprisingly the

surplus level of Chembarampakkam, the Korattur diversions, Thamaripakkam

diversion and Periyapalayam are almost at the same altitude above mean sea

level. This has led to the concept of considering the area between the Pallipat

to Ramakrishnarajpet (RK pet) as a high alluvial plain that allows sub-surface

flow to the Korattur diversion on the western side. From the Korattur

diversion, the alluvial soil extends from southwest to northeast spanning

Thamaraipakkam and Periyapalayam. This area is heavily exploited for its

groundwater use. The three districts of Kancheepuram, Thiruvallur, and

Chennai form a single groundwater basin though the origin of the Rivers may

be considered as independent units.

34

Table 3.5 River basin and sub basin of Chennai

Name of the River basin Name of the sub basin

ARANIAR

KOSASTHALAIYAR

BASIN

1.Pulicat

2. Araniar III

3. Araniar II

4. Araniar I

5. Sholavaram Tank

6. Redhills lake

7. Mettupalayam

8. Poondi

9. Allikuzhi canal

10. Nagari River

11. Arungulam

12. Murukkampattu

13. Pallipattu

14.Nandhi River

15. Srikalipuram

16. Kosasthalaiyar-1

17. Coovum -1

18. Mappedu

19. Chembarampakkam

20. Nemam

21. Coovum –II (Source: State Ground and Surface water resources Data centre, Chennai)

35

3.4 GEOLOGY

In the Chennai region there are extensive outcrops of the granitic

basement and of the Gondwanas represented by shales, sandstone and

conglomerates. In places all of these are overlain unconformably by much

younger strata, namely some tertiary but mainly quarternary alluvial and

marine littoral deposits, generally related to the present surface drainage

regime and coastline. In the area under discussion, the upper catchments of

the Rivers (west of the Poondi Reservoir and the Kesavaram Diversion) are

predominantly uncovered basement, whereas the lower catchments lie mainly

on sedimentary outcrops, in the case of the Kosasthalaiyar, Coovum and

Adyar mostly on alluvium.

Despite the range of rock outcrops, the lower catchments are in the

form of a flat, featureless plain; even the upper catchments are of subdued

topography and might be described as sloping plains. The most dramatic

topographic features are those caused by igneous intrusions and recent

geomorphic processes: some post Archean intrusions form rocky hillrocks,

which although mainly less than 33m in height stand out in the otherwise

featureless topography. Two ridges running parallel with the coast are

probably the result of normal geomorphic processes (such as are responsible

for the accumulation of sand dunes in many coastal areas) in the presence of

an advancing shoreline, either because of sea level changes related to the

Pleistocene ice age, or due to tectonic movements of the margin of the sub-

continent. The crystalline basement rocks have no primary porosity and

permeability when fresh and unaltered, but groundwater occurs in small

quantities in the weathered zone, normally 10 to 20m thick and occasional

deeper fissure zones.

Large diameter wells in such strata may yield as much as 5 l/s, though

normally the discharges are less. The Gondwana sediments, particularly the

36

sandstones, probably possess some primary porosity, but permeabilities are

likely to be dominated by secondary features such as weathering, fissures and

joints; in fact their water bearing properties are not vastly different from those

of the Basement. The Quarternary and Tertiary strata are extremely variable

in geometry and lithology, but the alluvium forms highly productive aquifers,

in some places suitable for high yielding drilled wells. The coastal sands also

form a good aquifer, but the capacity of this for groundwater development is

limited by the pressure of sea water in the immediate vicinity and the threat of

saline intrusion into wells with high discharges.

37

Figure 3.2 Geological formations encountered in Chennai basin

38

Table 3.6 Hydro-stratigraphy of Chennai basin

Age of rock Rock type Description of different

units Groundwater bearing capacity

Recent Alluvium Sandy, sandy silts and beach formations High to Very high potential

Tertiary Sedimentary deposits

Sand, sandstones, clay and shales

Very high capacity in sand and sandstones (good to moderate potential)

Gondwana Sandstone, Siltstone, Grit, Claystone

Clay stone compact rock, grit with fine compact fine sand clayey rocks with yellow ochre

Poor to very poor groundwater availability is seen in sandstone, grit, clay stone, siltstone region.

Archaen Crystalline complex, hard basement

Granites, Charnockites, different metaforms of gneiss's and transformed granites and gneiss.

Very poor to poor yielding capacity

39

3.5 HYDROGEOLOGY

The total surface area of the Araniar-Kosasthalaiyar Basin is about

1000 km2 of which some 275 km2 is underlain by a buried channel system;

probably that of the Palar; its geometry and hydraulic properties have been

studied by drilling and pumping tests. the listed transmissivity range from 80

to 6535 m2/day and the storativity values are from 6.5 x 10-5 to 0.105.

The main aquifer in the area is the Quarternary alluvium, reflecting a

buried channel system. Here the alluvium is up to 50m thick, and much of it is

sandy, though clays are sufficiently persistent in places to make its lower part

behave like a confined aquifer.

The Coovum – Adyar Basin covers an area of about 350 km2 and is

underlain by some 20m of alluvial strata, some of them sandy. However,

drilling at few some sites penetrated predominantly clayey deposits. Two

pumping tests were carried out by the UN project to evaluate the properties of

the aquifers. Yields from both wells were estimated to be about 25l/s.

In addition to these inland basins, significant exploitable groundwater

occurs in the Quarternary deposits near the city and along the coastal sand

ridge to the south of the city. These aquifers are probably partly alluvial and

partly of marine littoral origin. In both cases the saturated thicknesses are

probably small and their proximity to the sea limits, the productive potential.

In Chennai there are thousands of dug wells and bore wells most of which are

still used, because supply from the public water supply system is inadequate.

Recharge conditions in the whole area are generally favorable. The

land is mainly flat with poorly developed surface drainage; soils are often

light with a high potential infiltration intake and the Rivers and the reservoirs

40

can be expected to have high losses, though the Rivers carry flow only for

limited periods at best. Relatively high recharge confirmed by large seasonal

water table fluctuations, often in the range of 2 to 5m. From experience

elsewhere, natural recharge to the alluvial aquifers would be expected to be

about 25 to 30% of average rainfall. This compares well with the UN project

computation of annual recharge to the Araniar-Kosasthalaiyar basin of 170

Mm3 during a wet year.

The present status of these rivers at various locations is shown in

Figures 3.4 to 3.14.

Figure 3.3 Upstream sand deposits at River Nagari

Figure 3.4 Upstream reach of River Kosasthalaiyar

41

Figure 3.5 Sandy areas of Kosasthalaiyar

Figure 3.6 Diversion weir across River Coovum at Korattur

Figure 3.7 Head regulator of New Bangaru channel at Korattur

42

Figure 3.8 Hydraulic particulars of diversion and regulator

Figure 3.9 New Bangaru channel at Thirumazhisai

Figure 3.10 New Bangaru channel near Chembarambakkam

43

Figure 3.11 Chembarambakkam reservoir

Figure 3.12 River Adyar at Chembarambakkam

Figure 3.13 Upstream reach of River Adyar

44

Groundwater quality is variable. In the inland basins it is mainly good,

with overall mineralization of less than 1000 mg/l (in the deeper, high

permeability layers often less than 500 mg/l). Nevertheless there are potential

quality problems in several areas:

Sea water intrusion of the coastal aquifers and part of the Arani

Korattalayar Basin

Increasing salinity of some aquifers through groundwater

recirculation by irrigation

Severe pollution of the aquifers underlying urban and industrial

areas, particularly Chennai City itself

Long term pollution of potable supply well fields by agricultural

chemicals; with relatively thin phreatic aquifers and extensive use

of chemical fertilizers and insecticides, some index compounds,

such as nitrate and indane, should be monitored in wells used to

provide drinking water.

3.6 DEMAND AND SUPPLY SCENARIO OF CHENNAI

METROPOLITAN AREA

The demand and supply scenario of Chennai Metropolitan area

comprising of water supply agencies requirement and the demand-supply

requirements of Chennai City is shown in Tables 3.7 to 3.9 and Fig. 3.15

45

Table 3.7 Water consumption in Chennai city

S. No. Agencies Summer (%)

Winter (%)

1 Metrowater pipe supply 32.80 40.40

2 Metrowater supply by tanker lorries 5.70 3.20

3 Private tanker lorry supply 0.09 0.06

4 Private Open wells 26.07 26.40

5 Private Bore wells 34.09 28.47

6 Other sources 1.25 0.97

Total 100 100

Suppy pattern by various agencies - Summer

32.8

5.7

0.0926.07

34.091.25

Metrowater pipe supply

Metrowater supply by tankerlorries

Private tanker lorry supply

Private Open wells

Private Bore wells

Other sources

Supply pattern by various agencies - Winter

40.4

3.2

0.0626.4

28.470.97

Metrowater pipe supply

Metrowater supply by tanker lorries

Private tanker lorry supply

Private Open wells

Private Bore wells

Other sources

Figure 3.14 Seasonal water supply patterns by various agencies

46

Table 3.8 Estimates of total water demand for Chennai (1991-2021) (Mm3)

S. No

Agency 1991 1995-96 2001 2011 2021

1 Murugappa Chettiar Research Centre

…… 312 347 440 560

2 Chennai Metropolitan Development Authority (CMDA)

344 …… 438 549 ……

3 Chennai Metropolitan Water Supply and Sewerage Board (CMWSSB)

332 …… 418 526 ……

4 S.N. Ayyadurai

(Chennai city) …….. …… 354 485 565

5 Institute for Water Studies (for Chennai Basin)

…… 315 (1994) 536 …… ……

Table 3.9 Estimates of total water supply for Chennai (1991-2021) (Mm3)

S.

No. Agency 1991 1994 2001 2011 2021

1 Chennai Metropolitan Development Authority

158 ……. ……. 514 …….

2 Central Groundwater Board 169 ……. ……. 374 …….

3 S.N.Ayyadurai (Chennai City) 127 ……. 294 294 498

Source: Nigam et al (1998)