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How to provide an uninterrupted sustainable water supply to the Greater Colombo Area

11 April 2019 01:20 am - 0     - {{hitsCtrl.values.hits}}

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Presently the National Water Supply & Drainage Board (NWSDB) is providing water to Greater Colombo area from the following water sources:   

1. Labugama Impounding reservoir with water. treatment facility with average production capacity of 31,651 m3/day.   

2. Kalatuwawe Impounding reservoir with water treatment facility with average production capacity of 65,548 m3/day.   

3. Ambathale Kelani river intake(LB) with water treatment facility with average production capacity of 536,204 m3/day.   

4. Nawagamuwa Kelani river intake(LB) with water treatment facility(CHICO) with average production capacity of 5,375 m3/day.   

5. Biyagama Kelani river intake(RB) ) with water treatment facility- with average production capacity of 186,785 m3/day.   

6. Kaluganga intake(RB) with Kandana water treatment facility with average production capacity of 111,704 m3/day.   

7. Kaluganga intake(LB) with Kethhena water treatment facility with average production capacity of 45,447 m3/day.   

8. Ma Oya river Negombo Bambukuliya water treatment facility with average production Capacity of 33,102 m3/day.   

A large part of water produced from these plants is being distributed to urban and semi-urban population in the western part of the Western Province covering the areas from Negombo to Aluthgama on the western coastal belt including about 20 km to the interior of the country. It is vital to protect these water sources from pollution due to rapid industrialization and unplanned urban development giving rise to other human activities such as releasing untreated effluent including harmful chemicals used in agriculture and other industries. 


  • For a sustainable water supply, it is very important to have adequate raw water storage to meet the required water demand throughout the year
  • When the flood level of the river goes above the minimum supply level of the flood storage ponds, then these ponds would get filled automatically by gravity flow
  • For Colombo, which is a rapidly developing city, a 24-hour storage capacity is desirable in order to avoid inconvenience to the public

Production capacity of water treatment plants   

To maintain an uninterrupted water supply to consumers it is necessary to have a capacity at least 20% over the daily demand capacity of the treatment plant. In order to achieve this, the equipment and other components of the treatment plants need regular and routine maintenance to avoid sudden breakdown of water supply. Normally in rapid sand filters, cracks develop in filter bed due to malfunction of the flocculators or improper maintenance of filters, inadequate mixing of chemicals and frequent fluctuation of turbidity levels in raw water. It is very important to avoid vertical cracks in the filter beds for producing water free of any particles. If cracks appear in the filter media, immediately the relevant filter units should be closed down and commence relaying of filter media. Such repairs may take even months depending on the size of the filter units and the availability of required resources necessary for the repair. If spare filter units are available in the treatment plant, then necessary daily demand could be produced. When designing treatment plants, the required number of filter units has to be decided keeping at least one spare filter unit to be used during repair of such a filter unit, as the repair may take a long time.   

Increasing the Extraction Capacity of the Main Water Source, Kelani River 

The NWS&DB in consultation with the Ministry in charge has been looking at ways of continuing to extract the optimum quantity of water from Kelani River at comparatively low cost. From 1992, several studies were conducted engaging both foreign and local consultants about the possibility of extracting more water’ from the river. After extensive studies carried out over several years it was finally decided to construct a permanent salinity barrier in the river at a location immediately downstream of Ambatale water intake with appropriate protection to prevent collapsing of the banks and also. without hindering the movement of boats. By the year 2006, NWSDB had already spent millions of both local and foreign funds for the study, design, tender documentation and evaluation. Necessary approvals had been obtained from both government agencies and local authorities.   

even by 2016, the construction of the permanent salinity barrier downstream of Ambatale has been only partially completed.

Ironically even by 2016, the construction of the permanent salinity barrier downstream of Ambatale has been only partially completed. The construction of the salinity barrier was delayed due to the proper decision not being taken by the decision makers, ignoring all requests and necessary technical reports submitted to the decision makers by NWSDB and funding arrangement from ADB. In 2006 a Secretary of an outside Ministry tried to divert the funds already allocated for the salinity barrier for the construction of an impounding reservoir in   

Kelani River upstream of Yatiyantota. This same Secretary assured the forum of decision makers that the impounding reservoir can be completed within 2 years, if the funds allocated for the salinity barrier could be transferred to the department under his purview.  

 The benefits of having a permanent salinity barrier are numerous:   

1. Avoiding intrusion of salinity into raw water pump houses and the treatment plant   

2. Making more water available for abstraction   

3. Increasing the depth of the fresh water level upstream of the salinity barrier   

4. Increasing the availability of fresh water for treatment plants   

5. Increasing the efficiency of raw water pumps   

6. Improving the reliability of Greater Colombo water supply system   

7. Reducing the cost of production of potable water from Kelani River   

8. Avoiding bringing expensive Kaluganga water to Colombo District   

9. Providing water demand of south western part of Gampaha District from proposed Kelani River right bank treatment plant   

10.Preventing erosion of Kelani River embankments in Ambatale area   

11.Maximizing the proper usage of available fresh water in Kelani River during drought periods   

12.Possibility of controlling sand extraction close to the upstream of the Barrier.   

The river-bed around Ambatale is 

about 5 m below the Mean Sea Level (MSL). The earlier proposed salinity barrier was to consist of a RCC structure and an inflatable rubber tube dam of 1.5m height fixed on to its crest. The crest level of the RCC structure was to be 0.5 m below MSL. During low flows this rubber tube dam was proposed to be inflated creating a water pond with the overflow level at 1.0 m above MSL, at the same time preventing salinity intrusion towards upstream of the river. During high flows this inflated rubber tube dam could be deflated allowing normal flow in the river.  

However, even today (2016), no proper salinity barrier and no impounding reservoir are available for Greater Colombo water supply. Annually during the dry season, placing of sand bags on the crest of the concrete dam of the partly completed barrier, is being continued, as the inflatable rubber tube dam of 1.5m height was not completed. Continuing such placing of sand bags as a temporary measure is not advisable, as this causes damage to the riverbanks and the public and private property in the vicinity, leading to unnecessary environmental destruction and considerable economic losses. pond with the overflow level at 1.0 m above MSL, at the same time preventing salinity intrusion towards upstream of the river. During high flows this inflated rubber tube dam could be deflated allowing normal flow in the river.   

Another important fact is that the effluent from Biyagama Investment Promotion Zone is released to Kelani River upstream of the present water intake, Action needs to be taken to divert this effluent to downstream of the above mentioned (partially completed) Salinity Barrier to utilize the total flow of the river during low flows. It must be noted that this cost is negligible compared to the advantages of having the barrier.   

 no proper salinity barrier and no impounding reservoir are available for Greater Colombo water supply

Greater Colombo area is growing in all directions and so is the demand for services and what is economically advantageous has to be given priority. No development could be planned without having adequate water supply. To improve the water supply to Greater Colombo area and to satisfy the needs, several important projects were identified by experts and several committees appointed by the subject Ministry and the Board. The present position of these projects is as follows:   

1. Construction of the above explained permanent salinity barrier downstream of Ambatale - partly completed.   

2. Construction of intake and treatment plant in the right bank of Kelani River to serve the areas in the Towns North of Colombo - completed.   

3. Laying of a pumping main of adequate capacity from Ambatale - Presently completed and commissioned under GCWRP.   

Possible ways of meeting the increased water demand in GC area during low flows of the river 

There are two options that could be considered to supplement the additional water requirement during low flows of the river.   

a) Construction of desalination plants along coastal belt of GC area.  

b) Arrange raw water storage (flood water storage) upstream of salinity barrier at Ambatale in Kelani River.   

It is now high time to think about carrying out a feasibility study to create additional raw water storage facilities along Kelani River, as the minimum flow of the river keeps on reducing year after year. Storage of Raw Water.   At present the cost of energy for the desalination of sea water is very high and a country like Sri Lanka cannot maintain these plants viably to supply water to public and therefore, for the moment it is better to consider the option b).   

Raw water storage 

For a sustainable water supply, it is very important to have adequate raw water storage to meet the required water demand throughout the year. During drought period water flows of rivers reduce and water levels of impounding reservoirs also go down due to low precipitation in catchment areas. in such situations it is appropriate to go back to the example of London City along the River Thames and give attention to using the marshlands created by clay burrow pits along the Kelani River as flood storage ponds to be utilized as additional raw water storage.   

some engineers specify river sand for underground filling work without considering the environmental effect to the surface waterways

The Kelani River bed level has gone down more than 10m, compared to river bed level in 1967, due to heavy extraction of sand for development activities. For example, in building construction activities including all types of concrete work, usage of sand is’ compulsory to get a quality product. However, in road construction work and even in underground pipe laying work, for back filling of pipe trenches, specifying the use of river sand is not a very good policy for a country like Sri Lanka. For back filling it is advisable to use trench excavated materials, sieved and mixed with suitable quarry products. As river sand is easily compactable, some engineers specify river sand for underground filling work without considering the environmental effect to the surface waterways. This scarce material should not be misused. As a result of such bad practices, now in 2016, Kelani River minimum flows are getting reduced every year due to additional extraction for development with increasing population in upstream locations of river catchment areas.   

As a solution for this, it is desirable to look for alternative water sources and lands for water storage facilities, which are not being economically used at the moment. To overcome this problem, in the GC water supply a solution could be found if we follow the system adopted in the London city water supply.   

During construction of the London City, gravel had been excavated by the people from the areas along the Thames River that have created large burrow areas. During floods these burrow pits got inundated with flood water and behaved as flood retention areas. Later such individual burrow areas were amalgamated to form large impounding reservoirs and raw water from these reservoirs is being used by the London Water Authority. When the Thames River water level increases during flood season some of these reservoirs get water from the river by gravity through the pipe network connected to the river. Such pipe networks have been placed in a planned manner by the Water Authority to achieve this purpose. Also for use during low water level of the river when the reservoirs do not get water through gravity, raw water pumping stations have been constructed on the river embankment to pump water from the river to these reservoirs. Water treatment facilities of the Water Authority have been constructed very close to these impounding reservoirs. In the past for the urban construction activities in Colombo and suburbs, clay bricks and clay tiles were produced mostly in the Kelani valley. There are thousands of acres of clay burrow pits available in the left bank of the Kelani River, upstream of the Ambatale salinity barrier.   

Most of these burrow areas are inter connected and create low lying marshlands, which get inundated during the rainy season and remain inundated during most of the time of the year, and these lands were used as flood retention areas. It would be feasible to create flood storage ponds by dredging these to increase the capacity and connect them using a pipe network or channel network with controlling devices such as valves or gates. Finally these could be connected to the river at a suitable location, where seasonal average water level of the river should be more than the average water level of these flood storage ponds. The dredged materials generated in the process may be used in other infrastructure development activities in the surrounding areas.   

When the flood level of the river goes above the minimum supply level of the flood storage ponds, then these ponds would get filled automatically by gravity flow. If the flood level does not reach the full supply level of the network.of storage ponds, then pumping arrangements may have to be made.   

Such marshlands suitable for use as flood storage ponds are located mainly between Kaduwela and Hanwella along the Low-level Road spreading from Kelani River left bank. towards west up to Athurugiriya area. These marshlands are shown in Colombo 1: 50 000 to sheet No.66. Most of these marshlands are not economically fully utilized at the moment, yet only used as flood retention areas. Due to the high price of land in Colombo and suburbs there will be a tendency to use these lands for industrial activities, construction of commercial buildings, ‘residential complexes, stores complexes etc. As these marshlands are directly connected to the Kelani River with surface water drains and ground water drains, a proper land use plan has to be prepared and monitored by Central Government authorities, such as Urban Development Authority, National Water Supply & Drainage Board, Sri Lanka Land Reclamation and Development Corporation, Central Environmental Authority, National Physical Planning Department, Irrigation Department etc. Proper guidelines need to be introduced in the Provincial Councils and Local Authorities for the prevention of contamination and pollution, and for preserving the quality of water in Kelani River giving highest priority to same.   

Purified Water Storage Situation in Greater Colombo (GC) 

In urban areas it is necessary to have high elevated storage facilities, such as storage water tanks on top of buildings, elevated water towers and ground storage reservoirs on top of hills or high elevated ground. At present Colombo city uses around 300,000 m3 of water on the average per day. Accordingly, the minimum high-elevated storage has to be 100,000m3 to 125,000m3, and ground-storage facilities should be 200,000 m3 to 175,000 m3. After the completion of the ongoing GCWRP project, the elevated storage in Colombo city will total up to 82,500 m3. After commissioning of Maligakanda new reservoir, a decision has to be taken ‘with regard to the Maligakanda rectangular concrete reservoir, which is around 125 years old, along with piping and valves. For this purpose a panel of experts may have to be appointed by the NWSDB for ascertaining their condition. This would need this reservoir and the components to be isolated from the distribution network for checking the condition for stability, whether they are suitable for further use or, whether they would have to be replaced with a new reservoir along with the other components.   

Most of the recently constructed buildings in Colombo have their own water storage facilities provided for 12 hours to 24 hours requirement. More than 400/0 of population living in Colombo does not have any permanent water storage facilities; they always depend on the central water storage facilities operated by NWSDB.  

When a water interruption is announced, the above category would start collecting water in whatever available containers at their disposal and immediately after water supply is resumed, unused remaining water would be thrown away. This waste is mainly due to the non- availability of proper central storage facilities to meet the operational requirement without stopping water to consumers. The population not having their own water storage facilities belongs to low-income group mostly. Hence it is extremely vital for NWSDB to construct central water storage facilities in whatever high elevated locations that is available in and around Colombo city. Also any high elevated areas owned by NWSDB or the government should be reserved for this purpose and not for any other purpose, where the elevation is not a criterion.   

For Colombo, which is a rapidly developing city, a 24-hour storage capacity is desirable in order to avoid inconvenience to the public that may arise from likely interruptions for repairs of transmission main etc. Apart from this, adequate storage is not available to meet any contingency situation such as fire that may necessitate the use of the storage for domestic consumption.   

The water storage facilities could be in 2 ways:   

1. High elevated storage.   

2. Underground storage (with pumping facilities).   

Elevated storage capacity should be at least 8 to 10 hour requirement and ground storage capacity should be 14 to 16 hour requirement.   

In the Colombo area it is difficult to find large extents of unused lands to construct such underground water storage facilities, but for which some potential locations may be, places such as playgrounds, public grounds, Golf Link, Galle Face Green etc. A study could be carried out in cooperation with the owners of such properties on the possibility of developing underground water storage facilities without harm to their present usage pattern. If these are feasible, then the proposals would include necessary pump houses and pipe networks to fill these storage facilities and to pump water to distribution network and to high elevated storage. It is appropriate to mention here that in some developed countries, roofs of such ground water storage facilities are being used as playgrounds or markets places.   

Concluding Remarks 

Precautionary measures should be taken if necessary with a policy, not to allow production, storage or usage of any chemicals including pesticides and weedicide, in and around marshlands along the Kelani River that could pollute surface or ground water. It is the prime duty of NWS&DB officials to educate the public and especially children through school curriculum, mass media and seminar programs regarding water contamination due to excessive usage of agro chemicals, and also to arrange with the Government to acquire and be in possession of necessary low lying marshlands required for raw water storage at least for the next 200 years.   

In addition it is advisable for the NWSDB to allocate a certain percentage of its income from the GC area for long term water quality monitoring of Kelani River, its tributaries and these marshlands so that the Kelani River will continue as the main water source for the Greater Colombo population, and at the same time remaining unpolluted. Similar appropriate actions will have to be taken by the NWSDB with regard to all other potential water sources in the country to prevent contamination of drinking water by agro-chemicals or any other chemicals and pollution.   

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