SECTION-6 Limnology, Watershed Hydrology and Monitoring

BATHYMETRIC ANALYSIS AND THE CHARACTERISATION OF THE HEBBAL LAKE TO EXPLORE RESTORATION AND MANAGEMENT OPTIONS
V.G.Ranjani1 and T.V.Ramachandra1


ABSTRACT
INTRODUCTION
URBAN WETLANDS
STATUS OF WETLANDS IN BANGALORE
EXPERIMENTAL PROCEDURE
BATHYMETRIC ANALYSIS
ESTIMATION OF CATCHMENT YIELD
PHYSICO - CHEMICAL ANALYSES
PHYSICAL PARAMETERS
CHEMICAL PARAMETERS
SOCIO-ECONOMIC SURVEY
RESULTS AND DISCUSSIONS
TABLE 1: RESULTS OF THE PHYSICO-CHEMICAL ANALYSIS OF THE LAKE WATER
TABLE 2: RESULTS OF THE GROUND WATER ANALYSIS
TABLE 3: RESULTS OF THE SOCIO-ECONOMIC, BATHYMETRY AND CATCHMENT YIELD STUDY
CONCLUSIONS
ACKNOWLEDGEMENT
REFERENCES


ABSTRACT: first topic previous topic next topic last topic

Wetlands in India support crucial needs such as drinking water, protein production, fodder, water purification, wildlife habitat and flood storage. Due to anthropogenic stress and urbanisation, quality deterioration and loss of wetlands continue to occur. This study was undertaken to analyse the impact of various human activities that have contributed to the degradation of one of Bangalore's most important wetlands - the Hebbal Lake. A detailed investigation was undertaken to assess the catchment characteristics, bathymetric aspects of the lake and physico-chemical characterisation. Based on this investigation, best management practices are proposed for conservation and management of the lake.

INTRODUCTION: first topic previous topic next topic last topic

Wetlands, as the term implies are 'Wet' lands and they exist because the inflow of water exceeds the outflow for brief to extended periods of time during the growing season. Inland wetlands receive water from precipitation, snow melt, river outflow, surface overland flow, ground water discharge, lake seiches and seepage from streams, lakes, ponds and irrigation systems. The fact that lakes occupy such a small fraction of the landscape belies their importance as environmental systems and resources for human use. They are major recreational attractions for the people. Large lakes and reservoirs are also used as drinking water supplies. Besides their importance for human use, lakes have intrinsic ecological and environmental value (Mishra.S.R and Saxena.D.N, 1992). Stress to lakes arise from easily identifiable point sources such as municipal and industrial wastewater, and non-point sources such as urban and agricultural run-off within a lake's watershed, and insidious long-range atmospheric transport of contaminants. Major categories of stress include excessive eutrophication from nutrient and organic matter loading, siltation from inadequate erosion control in agricultural, construction, logging and mining activities, introduction of exotic species; acidification from atmospheric sources and acid mine drainage; and contamination by toxic (or potentially toxic) metals such as mercury and organic compounds such as poly-chlorinated biphenyls (PCB's) and pesticides.

URBAN WETLANDS: first topic previous topic next topic last topic

Wetlands near urban centres are under increasing developmental pressure for residential layouts, industrial and commercial facilities. Increasing population and economic growth create high demands for real estate in sub-urban localities. As suitable upland becomes exhausted, pressure intensifies to develop wetlands for residential layouts, manufacturing plants, business office complexes and similar uses. Thus urban wetlands are vulnerable to developmental changes.

STATUS OF WETLANDS IN BANGALORE: first topic previous topic next topic last topic

Bangalore has no natural wetlands, they were built mainly for various hydrological purposes and mainly to serve the needs of irrigated agriculture. Prior to 1896, the water demand of the city was met by surface water-bodies and dug wells. The unplanned development of the city has engulfed many water bodies and also blocked these catchment basins. The number of man-made wetlands in the existing BCC and BDA area has fallen from 262 in 1960, to around 81 lakes at present. About 35% of the water bodies of Bangalore were lost in the past 20 years (Deepa et al, 1997). Remaining lakes are on the verge of extinction, as they are filled with solid wastes and untreated sewage.

EXPERIMENTAL PROCEDURE: first topic previous topic next topic last topic

The study concentrated on analysing the depth of the lake, computing its catchment yield and volume. The study also concentrated on the quality of lake water and ground water in the surrounding areas. Another component of this study was the evaluation of the economic dependency of the people living in the neighboring areas of the lake, which gives an idea about the necessity to restore the lake.

BATHYMETRIC ANALYSIS: first topic previous topic next topic last topic

The analysis is carried out dividing the lake into different zones using transects. The depth at various points is found using a rope to which a weight is tied at one end. This rope is lowered into the lake till it touches the bottom of the lake. The length of the rope submerged in the water is measured, which gives the depth of the lake at that point. Similar analysis is carried out at various points where transects intersect. The points where the depth measurements are made must be more in number to get the depth at various points of the lake. This would give a true representation of the depth of the lake. The length of the boundary is measured and the volume computed from the data obtained. In this study, the bathymetric analysis was carried out from the data obtained by the Forest Department, Government of Karnataka.

ESTIMATION OF CATCHMENT YIELD: first topic previous topic next topic last topic

The contour lines for the area surrounding the lake were digitised using a guide map of Bangalore of scale 1:20000. Following the ridges depicted by the contour lines marked the catchment boundary. The sub-catchments and their area were found using the land-use land-cover pattern in the area. The monthly and daily rainfall data were analysed and the average annual rainfall determined, which was used to calculate the catchment yield using the formula given below (Muthreja, K.N, 1995).
CATCHMENT YIELD = C * A * P,
where C = co-efficient of run-off, A = catchment area, P = precipitation.

PHYSICO - CHEMICAL ANALYSES: first topic previous topic next topic last topic

The lake water was sampled at the following points:

  • Areas adjoining the bank near the forest nursery.
  • Near the point that was formerly the input channel.
  • Near the output channel.
  • Along the bank adjoining the highway.
  • At the centre of the lake.
  • Point on the opposite end of the input point.
  • Ground water sampling was done in areas lying within 1-km radius from the lake. About five areas were chosen and water was sampled from five wells in each area. These grab samples were immediately transported to the lab for analysis.

    PHYSICAL PARAMETERS: first topic previous topic next topic last topic

    The physical parameters analysed were colour, temperature, total solids, total suspended solids and total dissolved solids. The physical parameters analysed for ground water were temperature and total dissolved solids. Colour was determined by visual comparison method and temperature was determined using a thermometer. The other parameters were analysed according to the APHA (American Public Health Association) standard methods.

    CHEMICAL PARAMETERS: first topic previous topic next topic last topic

    The chemical parameters analysed in the lake water were pH, electrical conductivity, dissolved oxygen, total alkalinity, nitrates, nitrites, total hardness, calcium, magnesium, chlorides, sulphates, phosphates, BOD, COD, sodium and potassium. The heavy metals analysed were iron, zinc and copper.

    In the ground water, the chemical parameters analysed were pH, electrical conductivity, total alkalinity, nitrates, total hardness, chlorides, sulphates, phosphates, fluorides, COD, sodium and potassium. The heavy metals analysed were iron, zinc and copper.

    Electrical conductivity was measured using a conductivity meter, pH using a pH meter and temperature using a thermometer. D.O. was estimated using the Winkler's method. Total solids, total suspended solids, total dissolved solids, total hardness, alkalinity, chlorides, sulphates, nitrates, phosphates, B.O.D. and COD were determined according to APHA (1985) methods. Fluorides were determined using the Eriochrome-cyanin-R-method. Sodium and potassium were determined by flame-photometry, heavy metals using an Atomic absorption spectrophotometer and coliforms using the method prescribed by APHA (1985).

    SOCIO-ECONOMIC SURVEY: first topic previous topic next topic last topic

    To assess the importance of the lake, a random survey of the houses situated within 1 km radius from the lake was carried out using a questionnaire designed for the purpose. The study was carried out in four areas around the lake, three of which were semi-urban and the other completely urban.

    RESULTS AND DISCUSSIONS: first topic previous topic next topic last topic

    The results are shown in Tables 1 to 3. The bathymetric analyses showed the present depth of the lake between 1.5 m (towards periphery) and 1.75 m (towards centre). The lake is fully sedimented. By desilting, the Forest Department aims to achieve a depth of about 3 m in the centre of the lake, and 1.69 m in the periphery. The area of the restored lake would be 143 ha, far more than the present water spread of 75 ha. The catchment area was found to be 3750 ha. Of this, 200 ha were covered with vegetation and 150 ha were cultivated soil. The remaining 3400 ha were built-up land in the catchment. From the monthly rainfall data of 70 years, the annual yield from the catchment was found to be 15.2 million cubic metres. The yield was found to be 3.04 million cubic metres during the N.E. monsoon, 10.12 million cubic metres during S.W. monsoon and 3.28 million cubic metres during the other seasons. The volume of the tank was calculated and found to be approximately 2.38 million cubic metres. In case, the silt of depth 1.25 m is removed, then the storage capacity would be 4.07 million cubic metres. From this result, it is evident that the yield is enough to fill the lake in all seasons. There is no other source of water to the lake.

    The result of the physicochemical analyses showed that pH was higher than the desirable range (9.44) in some areas of the lake. This could be due to the presence of alkaline effluents in the lake, and also the nature of the soil, containing a high concentration of carbonates and bicarbonates. The electrical conductivity was found to range from 1.68-2.5 mS/cm. This was due to the presence of salts in the lake. The high value for solids (suspended and dissolved) indicated a high rate of sedimentation in the lake. The D.O was found ranging form 3.1-10 mg/l. The high D.O was found only in the areas lying close to weed bed.

    TABLE 1: RESULTS OF THE PHYSICO-CHEMICAL ANALYSIS OF THE LAKE WATER first topic previous topic next topic last topic

    Parameters

    Sam 1/5

    Sam 2/5

    Sam 3/s

    Sam 4/5

    Sam 5/5

    Sam 6/5

    Sam 1/6

    Sam 2/6

    Sam 3/6

    Sam 4/6

    Sam 5/6

    Sam 6/6

    Color

    Greenish

    Green

    ish

    Green

    Ish

    Green

    ish

    Green

    Ish

    Green

    Ish

    Greenish

    Greenish

    Green

    Ish

    Black

    Green

    Ish

    Green

    ish

    Temperature#

    25.5

    25.4

    25.5

    25.5

    25.5

    25.6

    26.5

    26.3

    26.6

    26.3

    26.4

    26.2

    PH

    8.97

    8.9

    8.79

    8.78

    8.8

    8.8

    9.28

    8.52

    8.75

    9.44

    9.22

    9.02

    E.C*

    1.69

    1.7

    1.68

    1.76

    1.74

    1.71

    2.4

    2.5

    2.5

    2.5

    2.5

    2.5

    TS

    1358

    1408

    1376

    1530

    1412

    1362

    2600

    2200

    2200

    3400

    2400

    2200

    TSS

    98

    100

    106

    80

    82

    94

    120

    112

    124

    139

    192

    120

    TDS

    1260

    1308

    1270

    1450

    1330

    1268

    2498

    2102

    2088

    3275

    2280

    2066

    D.O

    5.48

    3.1

    3.8

    7.8

    7.8

    9

    10

    5.5

    6.6

    6.4

    8

    7.2

    Total Alkalinity**

    478

    456

    387

    284

    322

    361

    524

    536

    316

    268

    312

    332

    Nitrates

    0.6

    0.8

    0.5

    0.7

    0.4

    0.5

    1

    0.6

    0.7

    1.3

    0.8

    0.7

    Nitrites

    NA

    NA

    NA

    NA

    NA

    NA

    47.4

    75.8

    83

    33.2

    23.7

    35.7

    T.Hardness**

    500

    506

    530

    508

    538

    520

    804

    800

    848

    820

    820

    844

    Calcium**

    143

    156

    104

    123

    105

    149

    230

    203

    200

    213

    210

    230

    Magnesium**

    80

    85

    84

    86

    82

    88

    96

    93

    95

    104

    95

    89

    Chlorides

    248

    248

    256

    248

    256

    250

    476

    328

    328

    336

    316

    320

    Sulphates

    232

    212

    136

    248

    264

    212

    768

    610

    660

    600

    590

    660

    Phosphates

    5.84

    5.16

    4.3

    4.81

    5.85

    4.12

    5.68

    5.5

    3.5

    3.4

    4.64

    4.98

    BOD

    14

    15

    14.5

    12

    12.5

    13

    8.9

    8.75

    8.8

    9

    8.6

    8.9

    COD

    264

    256

    236

    228

    216

    272

    209

    189

    185

    607

    289

    193

    Sodium

    92

    98

    97

    98

    96

    90

    86

    92

    97

    132

    82

    90

    Potassium

    32

    36

    41

    42

    30

    33

    40

    46

    33

    42

    42

    40

    *mS/cm     **mg/l as Calcium carbonate    rest in mg/l     #degrees celsius
    Heavy Metals

    Sam 1

    Sam 2

    Sam 3

    Sam 4

    Sam 5

    Sam 6

    Iron

    0.8

    0.9

    0.9

    0.6

    0.4

    0.8

    Zinc

    0.1

    0.1

    0.2

    0.6

    0.1

    0.3

    Coliforms - above 1000/100ml. Faecal coliforms present.

    TABLE 2: RESULTS OF THE GROUND WATER ANALYSIS first topic previous topic next topic last topic

    Parameters

    SAM-1

    SAM-2

    SAM-3

    SAM-4

    SAM-5

    SAM-6

    SAM-7

    SAM-8

    SAM-9

    SAM-10

    SAM-11

    SAM-12

    SAM-13

    Temperature

    25

    25.1

    24.8

    25

    25.2

    25

    24.9

    25

    25

    24.6

    26

    25.5

    26

    PH

    6.58

    6.67

    6.68

    6.86

    6.16

    7.2

    7.5

    6.9

    7.3

    6.8

    6.7

    6.7

    6.9

    EC*

    1.8

    1.98

    2.45

    1.52

    0.74

    0.73

    0.9

    0.97

    0.52

    0.54

    1.51

    1.89

    1.27

    T.Hardness**

    460

    480

    644

    352

    184

    232

    264

    256

    160

    164

    420

    640

    428

    TDS

    1804

    1832

    2864

    1440

    720

    752

    786

    832

    544

    472

    2000

    2600

    1200

    Sulphate

    48

    68

    68

    44

    32

    39.2

    33.6

    45.6

    23.2

    22.4

    37

    64

    32

    Chlorides

    264

    260

    332

    268

    128

    80

    96

    104

    40

    56

    95

    80

    75

    Nitrates

    3

    2.78

    7.8

    1.45

    0.6

    2.6

    1.11

    1.28

    0.27

    0.8

    0.5

    0.5

    0.6

    Phosphates

    NDL

    NDL

    NDL

    NDL

    NDL

    NDL

    NDL

    NDL

    NDL

    NDL

    1.45

    2.35

    1.37

    COD

    23.9

    35.8

    71.7

    43.28

    39.8

    24

    32.1

    36.1

    32.1

    36.1

    39.5

    52.7

    65.85

    Potassium

    1

    1

    2

    1

    2

    3

    1

    1

    3

    4

    1

    1

    1

    Sodium

    50

    63

    65

    53

    42

    52

    52

    67

    68

    51

    75

    80

    82

    Fluorides

    0.69

    0.57

    0.57

    0.52

    0.52

    1.09

    1.5

    0.78

    1.24

    0.78

    0.55

    0.48

    0.38

    Alkalinity**

    290

    296

    252

    268

    272

    258

    258

    256

    274

    279

    281

    240

    265

    Parameters

    SAM-14

    SAM-15

    SAM-16

    SAM-17

    SAM-18

    SAM-19

    SAM-20

    SAM-21

    SAM-

    22

    SAM-23

    SAM-24

    SAM-25

    Temperature

    26

    25.8

    25.5

    24

    26

    26

    24

    24.6

    23.8

    23.8

    23.6

    23.6

    PH

    6.7

    6.8

    7.04

    7.18

    7.47

    6.89

    7.04

    7.6

    7.7

    7.06

    7.7

    6.8

    EC*

    1.37

    0.91

    1.38

    1.66

    0.98

    0.9

    0.96

    0.5

    0.8

    0.9

    0.4

    0.7

    T.Hardness**

    460

    300

    1004

    1300

    1040

    1170

    1050

    400

    820

    650

    280

    580

    TDS

    2000

    1200

    962

    1232

    774

    702

    820

    544

    962

    1066

    518

    914

    Sulphate

    26.8

    25.6

    54.4

    40.8

    28.4

    40.8

    44

    73.6

    65.6

    80

    59.2

    60.8

    Chlorides

    65

    88

    260

    168

    128

    120

    140

    160

    230

    310

    120

    230

    Nitrates

    0.7

    0.5

    1

    1.3

    0.64

    0.4

    1.3

    0.2

    0.4

    1.3

    0.4

    1.3

    Phosphates

    1.94

    1.34

    NDL

    NDL

    NDL

    NDL

    NDL

    NDL

    NDL

    NDL

    NDL

    NDL

    COD

    36.51

    70.24

    80

    40

    8

    8

    16

    8

    8

    12

    8

    32

    Potassium

    3

    1

    17

    6

    1

    1

    2

    2

    7

    5

    3

    6

    Sodium

    80

    86

    164

    154

    91

    85

    85

    44

    50

    72

    43

    60

    Fluorides

    0.29

    0.48

    0.36

    0.41

    0.32

    0.23

    0.18

    0.69

    0.19

    0.31

    0.4

    0.38

    Alkalinity**

    289

    265

    396

    3.28

    252

    240

    188

    320

    500

    450

    250

    330

    *mS/cm     **mg/l as Calcium carbonate     rest in mg/l
    Heavy metals

    Zinc

    0

    0

    0

    0

    0.2

    0

    0

    0.1

    0.3

    0

    0.7

    1.6

    TABLE 3: RESULTS OF THE SOCIO-ECONOMIC, BATHYMETRY AND CATCHMENT YIELD STUDY first topic previous topic next topic last topic

    Area

    Domestic

    (per 1000 people per day)

    Agriculture

    (per ha. per day)

    Livestock

    (per 1000 animals every 2 days)
         

    Water use

    Fodder

    Hebbal

    Rs. 1937.50

    Rs. 19

    Rs. 2170.80

    Rs. 33500

    Guddadahalli

    Rs. 1603.30

    Rs. 19

    Rs. 1250

    Rs. 37500

    Bhoopasandra

    Rs. 1547.80

    Rs. 5

    Rs. 1270.80

    Rs. 16666.70

    UAS Layout

    Rs. 1547.00

         

    Present water spread area 75 ha

    Area after restoration

    143 ha.

    Present depth of the lake

    1.5-1.75 m

    Depth after restoration

    3 m

    Present volume* of the tank

    2.38

    Volume after restoration

    4.07

    * Volume in million cubic metres

    Total Catchment area

    3750ha

    Vegetated area

    200ha

    Cultivated soil

    150ha

    Built up land

    3400ha

         
     

    Daily rainfall data

    Monthly rainfall data

    Season

    Yield

    Annual

    17.1

    15.2

    SW monsoon

    10.12

    7.6

    NE monsoon

    3.54

    3.8

    Other

    3.28

    3.04

    Catchment yield (in million cubic metres)

    The alkalinity ranged from 284-536mg/l. Water is very hard and unfit for any use. The nitrate concentration was found to be between 0.4 and 1.3mg/l, whereas nitrites were found to range from 33.2-83 mg/l. The chlorides were also found to be high, making the water unfit for irrigation purposes also. Sulphates ranged from 136-768 mg/l, much higher than the standard of 150 mg/l. Phosphate is an indicator of eutrophication and was found to be between 3.64 and 6.5 mg/l. The BOD was not very high, but was however higher than the standard (3 mg/l). This could be attributed to the inflow of sewage into the lake, which has stopped now. COD was found to be much higher than the standard indicating the presence of various chemical compounds in the water. Sodium and potassium were also high, being the essential nutrients along with phosphates to the microbial biota causing eutrophication of the lake. Heavy metals like iron and zinc were found in the lake. The coliform count was very high, more than 1000 per 100ml of water. This result indicates sewage pollution and eutrophication of the lake.

    The hardness was found to be very high (160-1170 mg/l as CaCO3) in the ground water in some areas. These were rural areas, and the water was not being used regularly. This might have led to accumulation of salts in the well. The observed result of the total dissolved solids also correlates to the hardness result. Chlorides in all areas except Guddadahalli were found to be well within the standards. It was the highest value (332 mg/l) in this area. Nitrates were highest in the Guddadahalli area indicating a possibility of organic pollution in the area. The area is agricultural, therefore, nitrates from fertilisers could also have leached into the soil. Phosphates were so less in some areas that it could not be detected, it was found only in Nagashettihalli ranging between 1.34-2.35 mg/l. All the other parameters were found to be within the limits prescribed for drinking water.

    The dependency of the area on the lake was the highest for livestock maintenance. It is so because, the fodder for the cows is completely obtained from the lakes. Since the usual feed for cows is expensive, they make use of the weeds in the lakes to forcibly feed the cows. The agricultural dependency is low in these areas ranging from Rs.5 per ha to about Rs.19 per ha. When the lake is cleaned, the agricultural dependency will definitely rise, as the people will then use the clean water of the lake. Even in the present situation, people who do not have access to clean water use the lake water to irrigate their fields. The dependency of the domestic sector ranged from Rs.1547-1938 per 1000 people per day. This dependency was highest in Hebbal, as the people there depended on the ground water for their daily needs. An interesting thing that came to notice was that people here used more water for bathing than others did in the surrounding areas. Though Guddadahalli is also dependent on ground water for its daily needs, the dependency there was less compared to Hebbal. The other areas (Bhoopasandra and U.A.S. Layout) have access to treated water, and people do not depend much on the ground water to satisfy their daily needs.

    CONCLUSIONS: first topic previous topic next topic last topic

    The following conclusions were arrived at, based on the study.

  • The bathymetric study revealed that the lake was highly sedimented.
  • The catchment yield was more than sufficient to fill the lake all through the year.
  • The sewage inflow has severely polluted the lake.
  • The ground water is not fit for drinking in many areas.
  • The people in the area are dependent on the lake for their daily supply of water.
  • Some people are conscious of the need to preserve the lake.
  • The people opine that the lake should be converted into a recreation spot, after restoration.

  • The lake has degraded due the following reasons:
  • Excessive input of domestic sewage and industrial effluents from the surrounding areas (particularly BEL).
  • Excessive sedimentation due to the absence of erosion checks.
  • Dumping of organic wastes (e.g. carcasses and the like).

  • As a result, the lake has turned eutrophic. The following steps should be taken care of before any restoration work is carried out.
  • The water already present must be completely drained before any restoration work is implemented.
  • Dredging of the sediments in the lake has to be carried out. The sediments so removed must be transported elsewhere after de-watering to prevent them from re-entering the lake. These can be used as manure if they are detoxified suitably.
  • Water should be let into the lake only after treating it suitably.
  • The lake bottom should be covered with some material (could be fresh mud) to prevent recontamination of the water by contact with the already existing soil.
  • The wastewater could also act as a source of water to the lake after effective treatment to the water.
  • The weeds (like water hyacinth) should be removed and fishes and native species of plants introduced in their place.
  • The conveyance of run-off water into the lake could be covered with grass in order to trap the sediments in the run-off.
  • People should be taught the importance of preserving the lake, so that dumping of wastes and carcasses is stopped.
  • A good vegetative cover and maintenance of a riparian area around the lake is also effective in filtering the run-off.

    ACKNOWLEDGEMENT: first topic previous topic next topic last topic

    We acknowledge the cooperation of Mr. Kiran R. in conducting the socio-economic survey and thank him for lending valuable suggestions in completion of the same. We are grateful to the Karnataka Pollution Control Board authorities for granting us permission to carry out the chemical analysis of samples. We also acknowledge the help rendered by Mr. Akshay Heblikar for the successful completion of the work. Karnataka Forest Department officials, provided us the data required for the bathymetric analysis.

    REFERENCES: first topic previous topic next topic last topic

  • Deepa.R.S, Ramachandra.T.V. and Kiran.R., 1997. Anthropogenic stresses on wetlands of Bangalore. In Proceedings of the National Seminar on Environmental Pollution and Remedies.
  • Mishra.S.R and Saxena.D.N. 1992. Aquatic Ecology, Ashish Publishing House, Punjabi Bagh, Delhi.
  • Muthreja.K.N, 1995. Applied Hydrology, Tata McGraw-Hill Company Limited, New Delhi.
  • Address: first topic previous topic

    1.) Energy & Wetlands Research Group,
    Centre for Ecological Sciences,
    Indian Institute of Science,
    Bangalore 560 012,
    India