An overview of Kolkata city from the drainage aspect:
In the present topic, my main concern is to describe the existing drainage system which takes care of Kolkata proper. SWF system is a major artery responsible for city drainage. But, there are some other adjoining independent khals which receive drainage water yeiding from the extended part of KMC area along with their own drainage catchment. These khals also take very important role in sharing the drainage problem in a large area of the congruent part of city where rapid urbanization is taking place. Some of the key points of these khals also have been highlighted in the present topic for the understanding and assessment of the drainage issue in a comprehensive manner.
A: Introduction:
Before going to our subject matter, we may have a look about basic feature about Kolkata city proper to understand the problem and its limitation.
Some 315 years ago, British merchants had dropped anchor at the small hamlet of ‘Kolikata’ on the eastern corner of the Indian sub-continent. Soon after, they were granted trading rights and the area saw spectacular growth – the merger of the three hamlets of Kolikata, Sutanuti and Govindpur led to the creation of a hub for British commercial and business interests and activities in the sub-continent: the city of Calcutta. Regarded today as one of the biggest cities in the world, it is also the fourth largest in India in terms of population. It was later renamed Kolkata. Over the years, Kolkata has grown sporadically and haphazardly, not conforming to any master plan.
At present, Kolkata Municipal Corporation area is bounded by river Hoogli in the Northwest, South 24 Parganas district in the south and southwest, Salt Lake City in the east and North 24 Parganas district in the north.
The area falls between north latitudes of 22°28¢00¢¢ and 22°37¢30¢¢ and east longitudes 88°17¢30¢¢ and 88°25¢00¢¢
KMC ( Kolkata Municipal Corporation ) covers an area of 187.3 sq.km.( 72 sq.mile) and are divided into 141 wards and 15 nos. of borough. The east–west dimension of the city is comparatively narrow, stretching from the Hooghly River in the west to roughly the Eastern Metropolitan Bypass in the east — a span of 9–10 km (5.6–6.2 mile).
The north–south distance is greater, and its axis is used to section the city into North, Central, and South Kolkata.
Initially the city covered an area of 104 sq.km. with 100 nos. of wards, which subsequently extended to the present area after including more wards from the municipalities of South Suburban, Garden Reach and Jadavpur in 1983.
Two planned townships in the greater Kolkata region are Bidhannagar, also known as Salt Lake City and located north-east of the city and Rajarhat, also called New Town and sited east of Bidhannagar.
Kolkata is the 13th most populous and 8th largest urban agglomeration in the world with a population of 14.11 million (Census 2011) .
The city has lost green cover between 1980 and 2010 more than any metro in the country.
During the period its urban sprawl has increased 4.3 times from 71 sq.km to 314 sq.km expanding 8 sq km every year.
It is revealed from a report from the Indian Institute of Science ( IISc) that the vegetation has been declined from 33.6 % to 7.36% in last 30 years. This escalates run of factor drastically.
The city’s population density has nearly doubled from 2039 persons per sq km in 1971 to 3879 persons per sq km townships to Kolkata’s east, the dominant growth in the past decade has been in the south-west covering Behala, Haridevpur, Maheshtala, Garden Reach . As per census report in 2011 , K.M.C has population 44.87 lakhs persons and the density of population stands as 23, 995 persons per sq.km.
Kolkata is close to becoming the single largest urban patch with no vegetation. This will affect its ecological integrity and alter the micro-climate. The moderate temperatures that the city has had in the past may become extreme. The loss of greenery is extremely worrying.
Spread roughly north–south along the east bank of the Hooghly River, Kolkata sits within the lower Ganges Delta of eastern India; the city’s elevation is 1.5–9 m (5–30 ft). The general elevation of kolkata is nearly 14.26 ft. GTS near.
B: Discussion on the drainage Basin:
The management of drainage system of Kolkata has for long been far from satisfactory is well known. Even at some places, a little precipitation is creating a good inconvenience to the inhabitants.
Initially, the inhabitants were located on high lands and during rains the intervening low pockets served as temporary reservoirs, from which water slowly found its way through natural drains.
But, the situation deteriorated as the town increased in size with the increase in population and consequent to this , the natural drainage failed to function.
In the last couple of years, a massive development work has been started, not only in the city, but all through the adjoining areas and different municipalities in the form of construction of Hospitals, Hotels, high rise residential and commercial buildings including housing complexes coming up in large vacant low lying areas undertaken either by the public undertaking or big private firm apart from other individual personal level.
The city of Kolkota and added areas including different Municipalities of north & south 24 pgs have different outfall drainage channels.
Each outfall drainage channel has particular design discharge capacity and full drainage level (F.D.Level ) .
Therefore, any change in the land used plan in the Basin area of the respective Channel will impart excess pressure to particular drainage system to accommodate such additional discharge.
Moreover, the change in land pattern resulted higher rise in the runoff index with reference to the existing one for which the pumping stations have been originally conceived in the city of Kolkata and suburban areas
The areas of high lands were small and were rapidly filled up and the town extended to different low areas.
Thus, these low areas gradually ceased to function as temporary detention reservoirs and they became problem areas and obstructed the drainage of high lands also.
The core city of Kolkata had its underground drainage system introduced as back 1859–1875 as a combined sewerage system prior to the introduction of underground piped water supply from the surface water sources.
When the underground drainage system was introduced, the city population was to the tune of 0.63 million and there were plenty of marshy lands, ponds, open spaces etc.
Subsequently, the population explosion has been phenomenal which could not be anticipated by our town planners.
As a result, the marshy lands, ponds etc. wherever available have been filled up indiscriminately and habitation started. This presented a great problem to the future town planners for affording desired civic amenities to the people
Hemmed between Hooghly River on the west and Salt Lake to the east, the drainage of Kolkata and its adjoining areas are mainly dominated by three major basins:
i) the portion that caters directly into the river Hooghly in the west and
ii) Kultigong of Matla estuary system in the east , being carried by the S.W.F and D.W.F channel mainly which also receives drainage water from T.P basin area at Chowbhaga.
iii) On the north, the drainage of Bagjola basin area is also draining into the Kultigong.
Whereas, the basins of Tolly’s nullah, Monikhali, Churial along with other minor drainage basins are draining directly into the Hooghly river in the west.
B( i) : Rainfall:
We considered the rainfall records for the last few decades in drainage pumping stations; it is observed that the average rainfall is to the tune of 1650 mm (record from 1967 to 1989).
Out of that, more than 80% rainfall occurs normally between June to September
It is also noticed that the rainfall in a day even may exceed 200 mm occasionally.
The total rainfall on successive three days estimated during 1978, 1984, 1986, and 1999 is up to the tune of 554.75 mm, 499.25 mm, 443.00 mm and 334.10 mm respectively.
Whereas the drainage index adopted varies from 75 mm to 150 mm in a day.
B(ii): Drainage capacity:
The northern part of the network (T.H.C. System) which started functioning during 1876 has been designed with a rainfall of 1/4th inch per hour (150 mm per day) with 100% run off and
The southern part (S.H.C System) established in 1890 or so has been designed to a 1/6th inches rainfall per hour (100 mm per day) with 100% run off.
On average, the city experiences about 4 to 6 water logging in a year. It is reduced after eighties.
Nearly 80% to 85% of the core city area and 50% of the total city area is covered with underground sewage and drainage system. Apart from these, the box drain of different sizes, katcha and pucca surface drains (open drains) of big sizes in some areas were constructed.
However, the drainage disposal was entirely dependent on the network of main pumping stations linked with small pumping stations and associated boosting pumping stations.
In the seventies, attempts were made to augment the capacity of the system to ½ inch rainfall per hour, but the target could not be achieved due to various technical and financial constraints.
A greater part of the core city of Kolkata is served by underground drainage system of the combined type in which both sewage and storm water flows through the same conduit.
The slope of the ground of the city area is west to east in general. The sewers are also sloped in that direction. The area is very flat and low like the shape of saucer with scattered and localized pockets.
The drainage water is lifted and discharged ultimately into the tidal river further down to the east through outfall channels.
This was necessary to prevent pollution of the river Hooghly water which is used by the dwellers in the city for bathing and religious purpose.
B(iii) : Topographical feature & necessity of pumping stations:
The general elevation of kolkata is nearly 14.26 ft. GTS near the river bank. But, there is many low pockets which are below 10 ft . The average slope though not uniform, but in general it is from west to east i.e. from the east bank of river Hooghly to the Salt Lake.
It is pertinent to mention here that river Hooghly flows down along the western bank of the city. It has no spilling over effect of water over the city’s western bank , even when the river is flowing accompanied with upland discharges in the monsoon period. However, there is tidal effect in the west bound khals , for which, the drainage is blocked in the flow tide period for a period of 12 to 13 hours a day.
Therefore, the city’s drainage problem is usually linked with high downpour during the monsoon.
Collected drainage and sewage from sewers laid under the streets, flow down to the pumping station located on the eastern side of the city.
In some cases, the flows are to be boosted by pumps at intermediate pumping stations and the sewage and drainage water from the different pumping stations, ultimately passes down through the major arteries like SWF and DWF system to river Kultigong
Due to its unique physical feature of Kolkata city (bowl shaped), the center of the city is somewhat in lower elevation and as such every drop of waste water / storm water in 80% city core area requires pumping to relieve water-logging.
The situation is worsened in low pocket area, where the inundation takes for prolonged period because of earlier drainage in the higher elevated areas.The worst effected areas are around Rabindra Sarani, Amherst St., Thanthania, Ballygunge Phari, Park Circus, Lansdown etc. where even a small amount of rainfall brings tremendous suffering to people.
Furthermore , due to unexpected growth of population, more and more people are settling in low lying marshy area around fringes of city which were originally vacant marshy land and played a vital role in the drainage system of the city instead now becomes chronically inundated zone.
Besides, the city sewer system has no separate dry weather and storm weather arrangement, as a result, almost all the major and minor underground trunk sewer lines are badly silted up for the absence of cleaning at regular interval and this has made the reduction in flowing capacity of storm weather flow to a considerable extent
The main pumping stations which dispose a major part of drainage flow are located at Palmerbazar, Dhapa, Ballygunge and Chowbhaga. Apart from it, there are also small pumping station i.e. Birpara, Duttabagan, Lake Town, Topsia, Kulia, Tangra, Chingrighata, Pagladanga etc. which discharge directly to different outfall channel. The overall capacity of these pumps is 5100 cusecs and drains for an area of about 88 sq. km .
Recently, the existing capacity of Chowbhaga pumping station has been increased to 2200 cusecs by adding 900 cusecs of pumps . Thus , the revised overall capacity of the pumping stations are at present about 6000 cusecs.
In addition to it, there are small lifting/ boosting pumping stations for localized pocket area at Belgachia, Ultadanga, Maniktola, Nimakmahal, Mominpur, Chetla Lock, Jodhpur Park etc. which are ultimately connected to the major pumping stations.
C: S.W.F system:
The Kolkata drainage outfall system mainly consists of S.W.F. & D.W.F. channels. The S.W.F. system which include both Town Head Cut (T.H.C.) and Suburban Head Cut (S.H.C.) channel, after receiving storm water discharge , flows down further eastward direction for out falling into river Kulti, approximately 35 km away from the Ballygunge pumping station.
It may be mentioned that the S.W.F. system caters discharge not only from the city of Kolkata, but, also Suburban and rural areas , where as , Kolkata proper with contiguous suburban areas of about (121 sq.km ) and that of contiguous rural area is 177 sq.km and together with it comes to around 300 sq.km.
It accommodates a discharge of about 5100 cusecs from the K.M.C & contiguous Suburban areas at Bantala point and additional 1500 cusecs from the contiguous rural areas during its en-route to the outfall into the river Kultigong at Ghusighata . The existing sectional capacity of the S.W.F channel at the upper reaches is 5100 cusecs and at the tail end 5550 cusecs .
But as the Kultigong is a tidal river , the S.W.F. channel can effect drainage only for 5.5 hour to 6 hours in every 12 hr cycle, on an average.
Therefore , if we consider the tide lock-age period of 12 hours , the accumulated water will make the channel capacity inadequate. Thus , at the moment , both the pumps and the arterial drainage networks are under performing.
To ensure the drainage & to reduce the excess accumulated water above the F.D line in the S.W.F drainage system during the blockage period , we may have to think for some structural modification to increase the disposal, i) by modifying the existing bed slope with higher gradient , keeping parity with the outfall river bed level & also ii) providing a suitable pump to remove the excess water , the initial capacity of which may be decided after observing the performances of the proposed pumping station for a capacity of 2000 cusecs ( in the 1st phase ). This is considered as 40 % of the accumulated excess water ,likely to be around 5024 cusecs for the tide lock age period , at the outfall point.
It was assessed that the discharges as yielding from 80% of original city area (104 sq.km) i.e 83 sq. km, is dispersed through the city major and small pumping stations or by directly through inlets /lead channels to ultimately SWF system and that of from the rest part, comprising of 21 sq.km area, is catered through Bagjola khal , Circular canal Tolly,s Nullah( partially) and the river Hooghli directly.
We may see that though S.W.F system originally mooted to disperse the drainage water from majority of city area , gradually it is becoming pressurized to receive more drainage water, yielding from Jadavpur and large part of newly developing area spread over either side of EM bypass. The contribution of drainage water from this part is quite heavy. The S.W.F system receives water from Chowbhaga pumping stations under the scope of T.P canal & Intercepting channel drainage basin System . The overall basin area has 42.34 sq. km with seven sub-basins. The T.P system has its own basin of about 31.15 sq.km. The entire basin areas are becoming rapidly urbanized, linked with different lateral branches. Out of that, about 17 sq.km under KMC area within the Borough no VII , X & XII , spread over adjoining the EM by pass , is drained by forced pumping through Chowbhaga pumping station , for disposing into S.W.F System . Recently, the existing capacity of Chowbhaga pumping station has been increased by more than 900 cusecs.
D : Tolly’s Nullah , Churial and Monikhali khal :
These three important independent drainage channels e.g Tolly’s Nullah , Churial and Monikhali take an important role to drain out water from K.M.C urban areas (i.e 66 sq.km) and other adjoining municipalities, like Mahesthala and Sonarpur -Raipur Municipalities . These channels discharge directly in to the River Hooghli river, either through gravitational flow or forced pumping . One new pumping station of 900 cusecs has been constructed recently at the outfall of New Monikhali canal to ensure the drainage during the tide lock age period. This is very important as maintaining of the water level within the F.D Line of the drainage khal , will help the continuation of drainage flow from the basin area , through out the day without any blockage .
E : Observation & Conclusion:
In the last decades, the city had witnessed a rapid growth in population for urbanization and commercialization activities in the entire area to meet up huge housing demand for both residential and commercial purposes and as a result, a number of housing projects either in the government level or public-government undertakings or individual level have come up with the result of massive demand of vacant land in this area and by this way, the existing land used plan has been changed drastically. Many of the low lying areas have been reclaimed and developed for the purpose of different residential complexes.
Moreover, due to unexpected growth of population, more & more people are settling in low lying marshy areas covering not only around fringes of city, but also in different Municipalities areas in adjoining south & north 24-pgs districts. Originally, all the vacant marshy land used to play vital role in the drainage system of the entire area, instead of, now becomes chronically inundated zone.
The city of Kolkata and added areas including different Municipalities of north & south 24 pgs have different outfall drainage channels.
Each outfall drainage channel has particular design discharge capacity and full drainage level (F.D.Level). Therefore, any change in the land used plan in the Basin area of the respective Channel will impart excess pressure to particular drainage system to accommodate such additional discharge. As a result, due to saturated condition of the respective drainage channel, the situation in the low pocket areas are becoming worsened day by day and for obvious reason, an inundation takes place for a prolonged period because of earlier drainage in comparative higher elevated areas with an effect of tremendous suffering to the people even for a moderate intensity of rainfall.
More over, the change in land pattern resulted high rise in the runoff index with reference to the existing one for which the pumping stations have been originally conceived in the city of Kolkata and suburban areas. Besides, the low lying areas which previously acted as temporary detention basin now for its changed condition, makes an immense pressure in the overall drainage system.
In consideration of such development activities, it appears to be absolutely necessary to frame up some guidelines so that the water congestion in the newly developed areas can be averted as far as possible. Here, we may refer the present condition of Bangur, Laketown and other areas which were developed in the low lying areas without complying the basic needs for outfall drainage channel (Bagjola khal) to maintain their smooth drainage flow where the water logging is a chronic problem in every monsoon period. Recently, installation of an pumping station has improved the local water congestion to some extent , but it has caused an additional pressure to the existing carrying capacity of the outfall Bagjola khal with an deterrent effect in the gravitational drainage flow of the other adjoining basin areas .
Moreover , we have also noticed that a large scale of housing construction work at various levels are coming up along the EM by pass stretching at its both sides , changing the land use plan abruptly with the result of huge demand to improve the basic need for human requirements like providing zero water logging facility & water supply etc. This will pressurize the outfall drainage system in a large extent to accommodate an excessive discharges yielding from such newly & future built up area.In turn, it will set back the flowing of drainage water from the upstream side of S.W.F system with the result of further water logging in the city drainage system. Thus, any significant addition of drainage water in the downstream side of S.W.F system, need to be counter addressed in the overall perspective of drainage system in the city proper.
It may be reiterated here that the discharge from any individual sub-basin ultimately passes down through the outfall drainage channel. Therefore, any localized excess drainage water has to be accommodated in the existing carrying capacity of the particular drainage channel. But, the flowing of drainage water along the channel is guided by F.D Level line(full drainage level line) and also restricted for several hours in a day due to the tidal effect in the outfall river. Apart from that, lack of gravitational flow in the drainage basin, will demand more lifting pumps to negotiate such additional drainage flow.
Therefore, to cope up the situation, entire aspect is to be looked into with the overall perspective. All the link development works/ activities should be equally stressed, right from the receiving stage to disposal stage of the drainage water within the present scope of hydrological and topographical parameters in the K.M.C drainage area. Any development work in the chain of work, dealing in a isolated manner, perhaps may not serve the desired result.
The present system is capable of catering discharge with 1/4 inch drainage index per hour on and average. As such, any concentrated rainfall with one inch per hour or more , will definitely invite water logging in a particular sub-basin and likely to continue , depending on both i) the capacity of pumping station and ii) the drainage level outfall channel .
In the seventies, attempts were made to augment the capacity of the system to ½ inch rainfall per hour, but the target could not be achieved due to various technical and financial constraints.
Finally, I like to emphasize that if the initial infrastructural development work is not carried on commensurating with the existing full design level of the outfall canal system, a major water logging problem in the particular area is inevitable and of course, in such process of development works, a realistic approach requires to be made correlating all the related factors , such as , a) the ground level of the built up area , b) the localized lifting pumping stations, if needed, c) other drainage parameters of outfall channel system related to respective basin area & d) also , the economic aspects to make it a final shape.
Such measures are very much essential due to limited sustainability of the existing drainage system which can not combat the additional surface flow , with the result of i) either for changes in land use plan or ii) more intensive storm.
