CONTENTS-
Abstract
Introduction
Rainfall Runoff and Silting of Ulsoor Lake
Runoff Drainage Management
Eutrophication
Management of Eutrophication
De-Silting the Ulsoor Lake

Restoration of a lake that lies within an urban sprawl is indeed a complex matter, particularly which has been heavily contaminated with incursion of sewage and other forms of pollutants. It is not a task that can be accomplished in   less time, particularly, the de-silting part. De-silting a contaminated lake, saturated with pollutants, needs very careful planning and coordination, particularly the transportation and eventual disposal of the recovered silt. This paper discusses the three major methodologies for de-silting an urban lake, comparing the advantages and disadvantages of each, by taking the example of Ulsoor Lake of Bangalore City. The three methodologies are, firstly, excavation of silt from the drained out lake bed, secondly, dredging the lake with a small road transportable inland cutter suction dredger and pumping the dredged silt to a suitable disposal site, directly to final destination or to an intermediate site before relocating elsewhere by other means, and lastly, dredging the lake with a clam-shell grab dredger, slowly and delicately and transporting the dredged material in tipper trucks to a suitable disposal site.            

The Ulsoor Lake was once a beauty spot of Bangalore. But now, it is almost an eyesore. Years of siltation have reduced the water bearing capacity of the lake, to almost third of the original. Sewage and rotting garbage flow into the lake, day in and day out. In addition, in the recent years, there has been a substantial decrease in the amount of rainwater runoff flowing into the lake. Together, these factors have brought about a serious deterioration in the lake's ecology and aesthetics. While siltation is a natural phenomenon, the rest are consequences of urbanisation. The overloaded sewerage system is literally bursting at the seams spewing its contents into the storm water drains feeding into the lake. Also, the storm water drains have become dumps for garbage, mostly biodegradable, as non-biodegradable garbage is picked off for recycling. The biodegradable garbage quickly rot and blend with the sewage water flowing into the lake. The decrease in inflow of rainwater is due to mismanagement of the catchment area drainage and not necessarily due to a decrease in annual rainfall. The poor drainage is again attributable to haphazard urban growth. Fortunately, there is little evidence of toxic effluent discharge into the lake. The lake's condition is alarming and deteriorating steadily. Yet, there is hope. The lake can be restored and its aquatic life revived. The task is daunting all right, but not impossible. The restoration must be taken up without delay else the lake's ecology may soon go past the point of no return.

There is no point in trying to restore the lake without first tackling the problems of sewage inflow and runoff drainage. It is not a matter that can be handled only by an authority. An active involvement of all residents in the lake's catchment area will be essential. Therefore, before instituting restoration measures, we may have to first organise an emotive advertisement campaign to raise awareness among all the residents of the need and importance of resurrecting the lake's ecology. Without ensuring co-operation of all residents, particularly during the restoration phase, the project could run into serious difficulties, including riots. In addition, restoration process itself must be very carefully managed. For example, de-silting a highly contaminated lake as Ulsoor, if not very carefully controlled, can easily go out of hand with a serious health risk to the residents around the lake.

The lake deteriorated to the present state due to decades of neglect and mismanagement. It is hardly possible to restore the lake to a pristine state in quick time, even with the best technology and know-how. In fact, an approach to rapid restoration could easily prove counter productive. The changes in nature happen ever so slowly; hence rapid reversals can also have deleterious side effects, yet unimagined. Restoration of Ulsoor Lake will be such a reversal process, hence cannot be without pitfalls. It must therefore be very carefully planned and managed. The choice must be for an environmentally safer method, even though it may be much slower or even expensive. The price tag for an ecology-sustaining environment is a stiff one and must be paid for in full, in terms of effort, time and money. There are hardly any success-stories of restoration of contaminated lakes situated amidst urban sprawls. If the restoration of Ulsoor Lake has to succeed, it will require meticulous planning, taking into account all issues however daunting or trivial they may seem. A successful restoration of the lake could serve as a standard for restoring other lakes in and around Bangalore, and probably elsewhere.

Before urbanisation, the runoff reached the lake along the channels it carved itself down the slopes, depending on terrain and obstructions. The runoff constantly altered terrain features through erosive action of flowing water. Such an erosive action is a great leveller, razing hills and filling up depressions, which include lakes. The eroded material, mostly very fine sand or silt ultimately finds its way into the lakes, slowly raising its bed level. Thus, the silting of the lake is a natural ongoing process . A time will come when the lake will be fully silted up in relation to the surroundings and gone forever. The runoff then will course down elsewhere and create a new lake or merge with another lake at a lower elevation. Thus any lake will silt up and eventually disappear over time, unless of course, we de-silt the lake periodically.   

The once open terrain, through which the runoff could course its way to the lake, with relative ease, has since been overrun by urbanisation. Roads and buildings have covered much of the drainage area of the lake. As a result the runoff had to be channelled to the lake, through a network of drains, the storm drains. In this rapid urbanisation, unfortunately, the drainage management did not receive the required priority. The net inflow into the lake has declined sharply due to water logging at various places. Along with this, however, the quantity of silt flowing into the lake also has reduced, substantially. The drains meant to carry runoff to the lake are now silting up compounding the drainage, by the water spreading over a wide area and lost in evaporation.

Map the catchment area of Ulsoor Lake for the following data:  

Progressively, undertake repair/de-silting of all drains meant for rainwater runoff. Also ensure that all drains, minor, intermediary and major, are suitably linked to the main storm water drains.

Connect areas prone to water logging and flash floods to the existing drainage system through additional open drains or pipelines. In some cases pumping up may be necessary.

Eutrophication

The rainwater runoff while bringing in silt into the lake also brings in a large amount of plant and animal matter, including dissolved minerals. Together these make lakes, in general, highly nutritious regions. The lakes are thus termed as biological factories wherein numerous plant and animal species grow and thrive. Ordinarily in tropics, due to siltation and evaporation losses, the water in the lake is reduced. This increases the concentration of nutrients in the lake. When this nutrient concentration increases beyond a certain level, eutrophication sets in. Eutrophication causes algal and/or hyacinth blooms. Such blooms disturb the oxygen balance, secrete toxins of low molecular weight harmful to many aquatic animals and cause clogging of gills of the surface feeding fishes. All other species soon die leaving behind only algae or hyacinth. Either the water turns green laden with algae or is covered by a blanket of water hyacinth. Eutrophication is often naturally managed by periodical inflow of fresh water in sufficient quantities, often leading to flooding. However, when sewage and biodegradable garbage find their way into the lake regularly, combined with a decrease in fresh water inflow, eutrophication sets in very quickly and would never go away. Eutrophication occurred in the Ulsoor lake quite long ago. The recent water hyacinth invasion was only a periodical manifestation of eutrophication. It will happen again. Now that the hyacinth has been removed, the green algal bloom remains. The Ulsoor Lake is actually dead from the biological point of view.

Progressively, restructure the sewerage lines so that it runs well clear of the storm drains, as against the present situation of the sewerage line running either alongside or below the storm drains. All domestic sewage should be let into appropriate area wise treatment plants. The waters from such treatment plants may be let into the storm water drain subject to meeting the requisite standards.

It is nearly impossible to stop garbage, particularly biodegradables, from entering the storm drains. It would require a change in the mindset of nearly entire population within the catchment area. The only option available would be to regularly have them removed from the drains. To enable this, bar filters have to be put up at regular intervals all along the storm drain. This will obviate large accumulation in any particular area and consequent choking of the drain followed by putrefaction. In addition, areas with large garbage entry into the storm drain could be easily identified for other forms of management, including penal action against probable offenders. Fitting bar filters only at the lake's entry points will serve little purpose, because it could end up choking the inflow into the lake. In addition, considerable length of the storm drains at the point of entry into the lake is underground and will make it extremely difficult to clear the rubbish. The last bar filter along the storm drain leading to the lake should be well clear from the entry point. Beyond this bar filter, the portion of the storm drain must either be covered, if not by concrete/stone slabs, at least by wire mesh, so that no garbage can enter into it at this stage.  

Ulsoor Lake, with an area of about 4 lakh square metres and an average depth of 1.5 metres, can hold about 6 lakh cubic metres of water. This water level is maintained in a steady state. Whatever water flows into the lake, even if it is sewage, simply flows out through weir at the south end, into the Shivajinagar storm drain leading to Bellandur Lake. By the time this inflow travels from the inlets at the north to the discharge weir at the south, all material it has been carrying, mostly in suspension, both silt and putrefied garbage is discharged into the lake. Thus we can easily construe that the lakebed is highly contaminated.

As per the earlier studies/estimates, the net siltation in the lake is of the order of 2 to 5 metres. The exact level of silt can be determined only if the original depths of the lake are known to some degree of accuracy. Core samples at regular intervals would be required to get a realistic estimate of siltation across the lake. Such a detailed assessment of siltation through core sampling will be an expensive proposition, and would be required only if the lake has to be completely de-silted. Complete de-silting of the lake will be necessary, only if the lake is going to be used as fresh water storage. If the lake is to be used for only recreational activities and for training of MEG personnel, there is little need to de-silt the lake to the original bed level. Assuming this to be the case, we may have to remove only a metre or so of contaminated silt across the lake. In places, however, we may have to remove silt beyond a metre where the contaminants would have permeated deeper. Most probably, this would be around the inlets at the north. In any case, we may have to de-silt a small area around inlets, to at least 2-3 metres or as deep as possible, below the normal bed level, to serve as a silt trap. This silt trap will prevent spread of siltation across the lake. W e will, therefore, have to remove no more than 5 lakh cubic metres of silt from the lake .

The question now remains; how fast or efficiently should we remove these 5-lakh cubic metres of silt? Before we attempt an answer to this question, we must take a detailed look at the various de-silting options available and their implications from the point of view of cost, speed of operation, and, last but not least, environmental impact. We have before us two basic options for de-silting, the dry-bed excavation of silt, and dredging. The latter is essentially an excavation of bed material underwater through the use of a dredger .

De-silting by Dry-bed Excavation

For dry-bed excavation of silt, the lake must be first de-watered completely. De-watering may be possible only after the complete withdrawal of monsoon rains, most probably, mid-September. De-watering itself may not pose a problem, because the lake can be drained off to the adjoining Shivajinagar storm water drain. Water in the lake can be drained out to this storm drain by progressively lowering the level of outflow weir. There may however be some portions where water would still remain. Water from such portions may have to be pumped out. In addition, we may have to rig up temporary pipelines to convey the inflow of sewage water and occasional rains from the two inlets at the north directly to the Shivajinagar storm water drain. After de-watering, the lakebed should be allowed to dry out for few days before   excavation . The de-watering and drying up would probably take about 30 days. That should leave us with about 270 days before the next onset of monsoon rains sometime in mid-June, the following year. Therefore, we will have to remove 5 lakh cubic metres of the silt from the lake and transport the same to a suitable dumping site or sites, all in just 9 months. Associated computations are indicated in Table-1 below.

Table-1

The advantages and disadvantages of dry bed de-silting are as follows:

Disadvantages  

De-silting by a Cutter Suction Dredger

Dredging is the other option for de-silting the lake. Numerous proposals have been made in the past about dredging the silt from the lake. All these proposals involve the use of a small cutter suction dredger, and pumping the dredged slurry directly to a suitable disposal site. Sites have also been identified and short-listed for slurry disposal, through dredger's own discharge pipelines. But here, there is a problem, and the problem is water. For a cutter suction dredger, whatever be the make or dredging output, the volumetric efficiency is only 20%, at its best, i.e., for every 100 Cu. M of solids dredged and pumped out, 400 Cu. M of water would also get pumped out. Therefore, a cutter suction dredger, by the time it dredges 1.5 lakh Cu. M., would dry out the lake, making further dredging quite impossible. If we have to go on dredging, we will have to make arrangements to bring the water back to the lake. This may not be a simple matter at all, particularly when the nearest disposal site is over 2 kilometres away, with land features of higher elevation between the site and the lake. The slurry must first be allowed to settle down in an enclosed area before the water can be drained to yet another pond or tank. From this draining tank the water will have to be pumped back to the lake. The operation will be costly. Also, we may not be able to bring back sufficient water to the lake to sustain dredging. We can therefore rule out this option of disposing the slurry to a site well away from the lake .

For deploying a cutter suction dredger to de-silt the lake, we may have to first pump the slurry to a site, adjacent to or within the lake, where the water is allowed to drain back into the lake by gravity. After water has drained off sufficiently, the silt may then be transported to the final dumping site, in tipper trucks. Only then can we hope to maintain the required water level within the lake to sustain dredging. We may have to have to select at least two such sites, either in or just outside the lake. Both such sites should be accessible to backhoe-loaders for loading the tipper trucks, for final disposal of silt to a suitable site. Since there is a serious paucity of open land around the lake, one site may be created within the lake, close to the northern end, en-bayed with sandbags or HDPE bags filled with lake's silt, because the general direction of drainage within the lake is north to south. The other site can be created at the open stretch between the lake bund in the south and the Shivajinagar storm drain. We may have to enclose this area securely by raising the sidewall of the storm drain by at least one metre, and also ensure that water does not drain off to the storm drain or anywhere else. Sufficient vents may have to be created along the lake's bund to drain water back into the lake. This area is presently used as a dumping site for construction debris, which will have to be cleared before making it a slurry-disposal site. At each of these sites, we will get about 10,000 square metres of area for disposal. With about one metre increase in level possible at these dumping sites, we will be able to drain out only 10,000 Cu. M of silt at each site, at a time. Thus, after one site is filled up, while the drained silt is being carted away in tipper trucks, the dredger pumps slurry to the second site. By this the dredger can operate continuously and so will be the transportation of drained silt for final disposal. The associated computations are as indicated in Table-2 below.

The advantages and disadvantages of de-silting the Ulsoor Lake using a cutter suction dredger, with local discharge of slurry for draining before onward transportation of drained silt to the final disposal site, are as follows:

Advantages

Disadvantages  

•  Dredging by a cutter suction dredger is a complex operation hence would require specialised manpower, for supervision, manning and operation. Such manpower may not be readily available locally. Specialised training may be required even for all the attended labour during the dredging operation.

•  The operation of cutter suction dredger will result in churning up the bed material causing a serious deterioration in the water quality of the lake.

•  The water drained off from the slurry pumped to the draining sites will cause further deterioration of the water quality. Further, the contaminants in this water will eventually settle down on the de-silted portion of the lakebed. The nutrient level in the lake will continue to remain high and hence there will not be any let up in the eutrophication.

•  The stench from the slurry pumped up to the draining sites with decaying animal and plant wastes would be quite overwhelming, adversely affecting habitability around the lake.

•  There is a serious possibility of the plastics and such other materials choking the suction and discharge lines of the dredger. We can expect frequent hold-ups to clear blockages to the pipelines.

•  The operation of the dredger and tipper trucks at the lake would vastly increase the noise levels in the area, once again adversely affecting the habitability in the area.

•  200 truckloads everyday for 500 days could damage the roads and play havoc on the already busy traffic in the area. The trucks would also increase the levels of air-pollution in the area.

             It does not need an expert to arrive at the conclusion that both the above methods adversely affect the environment. We may achieve only de-silting of the lake but not its restoration, which incidentally should be our real goal . Cutter suction dredging is not particularly environmental friendly, nor is draining out a standing water body, more so, when quality and quantity of inflow cannot be ensured. Yet there is little other option, but to remove the silt in the lake along with the contaminants therein. Such a de-silting, along with contaminants, must be executed very carefully, and cannot be achieved in a timeframe possible through dry bed excavation or cutter suction dredging.

Table-2

De-silting by a Grab Dredger

Grab dredger is the simplest of the dredgers. It can be easily fabricated/assembled locally. The basic components of a grab dredger are:

•  A crane, preferably 5 tonnes, wheeled or tracked, with a reach of at least 5 metres all around.

•  A suitable pontoon on which the crane can be mounted.

•  A 2 cubic metres capacity clamshell grab attached to the crane.

In addition, we would require following ancillaries :

• i. Two diesel or electric powered portable winches for handling hawsers .

• ii. Four pontoons large enough to hold one tipper truck each.

• iii. Sufficient anchors and hawsers for handling the pontoons.

• iv. A suitable ramp on the lake's bank for embarkation/disembarkation of tipper trucks to/from the pontoons.       

• v. Operation of the Grab Dredger

The open clamshell grab is lowered rapidly on to the lakebed by the crane. Upon impact on the lakebed the grab closes taking in the bed material. The grab is then slowly hoisted to the surface and swung on to the pontoon with the tipper. On operating a release mechanism the grab opens discharging the bed material into the tipper. When the tipper is full, the pontoon is hauled to the ramp with the winch ashore, for the truck to drive off to the dumping site, and the next truck takes its place on the pontoon. The pontoon is again hauled back to the dredger pontoon. The dredger pontoon with the crane and grab is anchored to the lakebed while the tipper pontoons are berthed on its either side. When dredging is completed at a particular location, the dredger pontoon is moved to the next location using hawsers and the winches ashore, and anchored again. After loading up one tipper, while the other is being loaded, the former proceeds to the ramp. The pontoon soon returns with an empty tipper to take its place. In this way, continuous operation of the dredger can be ensured, achieving a high production rate. The associated computations are indicated in Table-3 below.

Table-3

The advantages and disadvantages of de-silting Ulsoor Lake using a grab dredger are as follows:

Advantages

•  Grab dredging operation does not disturb the bed beyond the actual impact zone of the grab. The percentage of solids recovered by the grab is almost 80%, with little outflow. The dredged material is loaded straight into the tipper with little spill back into the lake. Thus, spread of contaminants is contained. This is most significant advantage of a grab dredger. Grab dredgers are used all over the world for recovering contaminated bed material with least disturbance or spreading to the adjoining areas in the water body.      

•  The lake would remain functional during de-silting for all activities including training of MEG personnel.

•  De-silting operation will not be impeded by rainfall, however in strong winds pontoon movement may be tricky.

•  Grab dredging is relatively a simple operation, hence will not require specialised manpower, for supervision, manning and operation. The training of personnel for handling hawsers and winch is also quite simple.

•  There is little chance of stench from the dredged materials as these are carted away to the final dumping sites straight away.

•  There is little possibility of the plastic and such other material on the lakebed choking the dredger and holding up the dredging operations.

•  The noise level of the grab dredger is very low compared to that of a cutter suction dredger or excavator.

•  The number of truckloads per day is half that of the cutter suction dredging, hence the damage to the roads and consequent traffic problems will also be much less intense.

•  It is by far the cheapest option and that too; the expenditure is spread over period of nearly four years.

•  The operation neednot be time bound and can absorb delays or even long breaks.

Disadvantages  

The production rate is much too small when compared to other two methods, hence the de-silting will take much longer.

The action plan for de-silting can be drawn up only after approach to de-silt the lake has been finalised. Prior to exercising the option to dredge, either by cutter suction or grab dredger, a detailed hydrographic survey of the lake will be essential for both planning and execution. The scale of survey should be no smaller than 1:1000. The survey should generate the following data:

Also, a similar hydrographic survey would be required after de-silting. This post de-silting survey will serve as a benchmark for monitoring of lake's condition periodically thereafter.  

The measures to restore the lake should invariably employ environmentally acceptable methods. Grab dredging is by far the most suitable method to de-silt the Ulsoor Lake because it does not deteriorate the water quality or causes the lakebed contaminants to spread. However, where there are large tracts of open land just next to a lake for disposal of dredged material and water can drain back into the lake, use of cutter suction dredger may be a viable option, provided the bed material is not contaminated. When dealing with contaminated bed material, great care must be taken to ensure least dispersion of contaminants. This is not possible with a cutter suction dredger, as the cutter churns the bed material before sucking up through suction pipe. Grab dredger may be the only option then, even though it is time consuming and involves great deal more effort. With so little fresh rainwater flowing into the lake in spite of a sizeable catchment area, the dry bed excavation will also not work. Whatever be the de-silting method employed, there is little hope of a full recovery without effectively managing the drainage and inflow of sewage and garbage into the lake. The problems will be much the same with all other lakes and water bodies in and around Bangalore. A successful restoration of Ulsoor Lake, taking care of all aspects of the lake's environment, should serve as a standard for restoration of all other lakes and water bodies, in and around Bangalore or elsewhere.

The silt dredged from the lake will find excellent use as manure for agricultural and horticultural farms. Much of the silt may find use within Bangalore itself, for development of parks and gardens. Before commencing the de-silting operation such end users of silt, as manure, should be informed so that they could make necessary arrangements for collection. They may be allowed to take away the silt from the lake, free of charge, just by meeting the cost of transportation or providing their own tipper trucks. The largest segment of the de-silting cost is the cost of transportation of silt from the lake to the dumping site; hence if there are sufficient such takers for the silt, the overall cost can be brought down substantially. Such a distribution of silt across dispersed uses will also minimise road damage and traffic disruptions along the road between lake and dumping site.    

No lake restoration project will have any lasting effect until all residents in the catchment area are involved. Hence it is essential to launch a campaign to generate awareness. We may also have to send in volunteers to talk to each household, particularly, to the womenfolk, and seek their co-operation. Measures must be instituted to ensure safe disposal of biodegradable garbage by hotels, abattoirs and poultry retailers. We will have to get the people to think long term, so long, as to the quality of the life of their grandchildren and beyond, vis-à-vis the environment. Only those looking for short-term gains have caused much of the environmental degradation. Anything short-term in matters of environmental management may prove dangerous, even if it is a restoration of a lake, based on a limited scope, goal or agenda.

95, AF Golden Jubilee Enclave,
Defence Layout, Vidhyaranyapura,
Bangalore - 560 097. Karnataka, India.
Phone: 3642746
E mail: jjputhur@yahoo.com