Flood protection of Colombo Metropolitan Region-An alternative scheme

By Dr Janaka Ratnasiri

I thank Eng. Anton Nanayakkara (AN)’s write-up in The Island of 28.07.2020, responding to my article on Flood Protection of Colombo Metropolitan Region which appeared in The Island of 21.07.2020. The main purpose of my article was to highlight the fact that the government after getting Japanese Consultants to formulate a Master Plan for flood protection of Colombo Metro Region at great cost, what is being implemented as a priority project is only a clean-up of the Weras Ganga basin, making a mockery of the word Master Plan. This area is totally outside the Greater Colombo area with no impact on its flooding. AN has failed to comment on this issue.

METRO COLOMBO URBAN DEVELOPMENT

PROJECT

With the failure of the Master Plan to address the flood situation within the city and its suburbs, Sri Lanka Land Development Corporation (SLLDC) has taken the initiative to develop a separate project titled Metro Colombo Urban Development Project (MCUDP) to address this issue. This project expected to be executed during 2012 – 2020 is estimated to cost USD 104 Million (SLLDC Website). It will address flood mitigation in areas covered by the Colombo Municipal Council, Sri Jayewardenapura, Battaramulla, Rajagiriya, Madiwela and Dehiwala. Activities described under “Improvements to existing drainage systems” in my previous article of July 21st were in fact carried out by SLLDC under this project.

COMMENTS ON ENG. NANAYAKKARA’S

RESPONSES

In his response, AN has made certain remarks on some statements appearing in my article and questions their validity. What I have said are totally based on material extracted from other sources including the JICA reports and the website of the SLLDC and not my own suggestions. It appears that AN seems to be unaware of the latest situation in this regard, and hence they need clarification. My comments are given against each of AN’s statements which are given below using material extracted from SLLDC website – Special Projects pages.

1. “The Madiwela East Diversion (MED), remaining dry most of the time, as mentioned, may be due to its wrong location, too far upstream of the Kelani Ganga about 10 miles above the historic Nagalagam Street outfall”.

COMMENT: MED was established by constructing a new canal from the Thalangama Tank up to the origin of the existing natural canal flowing through Malabe paddy fields parallel to Chandrika Kumarathunga Mawatha. It has its natural outfall at Ambatale. The topography of the area does not permit shifting of this outfall further downstream.

2. “Even during floods of the Kelani Ganga, this outfall No 1 (See plan) has to be closed, long before Nagalagam Street outfall closing at +5.00 ft MSL, the accepted minor flood level for Colombo, negating the very purpose for which this canal was built”.

COMMENT: The SLLDC is currently building a pumping station at Ambatale across the MED canal to pump water to the river when its water level rises during heavy rainfall, at a cost of USD 5.85 Million and LKR 1,181 Million (SLLRDC website).

3. “The learned doctor has not noticed the extent to which the Thalangama Tank had silted up, reducing the capacity to retain flood water (about 50 ac.ft) entering the Parliament lake”.

COMMENT: In a project carried out by the SLLDC during 2016 – 2018, the tank was dredged to increase its water holding capacity and remove unnecessary growth on the tank bund, at a cost of LKR 107 million. In any case, I wonder how even a professional hydrologist could notice the extent of silting of the tank just by looking at it.

4. “This gate was constructed at ID’s flood control premises to pump water from the Kelani Ganga to the Beira Lake, for the purpose of cleaning the lake. The project ceased soon after the flood. Strangely, no inquiry was made. It was all swept under the carpet”.

COMMENT: According the SLLDC website, it has built three gates across Kolonnawa Canal, Heen Ela and St. Sebastian canal at the crossing of New Kelani Bridge Road to isolate the canal system enabling water to be pumped back into the canal system from the river by operating the pumps installed at St. Sebastian outfall in the reverse direction. This work to be carried out during 2018 – 2020 is estimated to cost of USD 5.85 million and LKR 1181 million. So, it is not a case of sweeping under the carpet.

5. “Dr. R’s reference to the Beira Lake, too, needs some clarifications. The Beira Lake is not a natural lake. It is an artificial lake also kept at an artificial level, of 6.00 ft above mean sea level, by the Beira Spillway”.

COMMENT: A pumping station is being built across St. Sebastian Canal at Maradana for pumping water from the canal to Beira Lake during periods of high rainfall in Colombo. This work to be carried out during 2019 -2020 is estimated to cost of USD 5.93 million and LKR 165 million. (See also the last paragraph).

6. “Ignoring many other references, contained in Dr R’s article, let me now say a few words about narrowing of bridges, mentioned in it. This is not a matter of life and death, as made out to be. Any hydrologist will agree that within the narrowed section, the velocity will increase to make up for the constriction”.

COMMENT: Widening of the canals and removing bottleneck were not proposals that I made, but what are actually executed by SLLDC as described in its website. Kolonnawa Canal Diversion Stage III says “the canal has become very narrow at certain sections due to encroachment. Some resettlement and land acquisitions are undertaken to remove bottlenecks”. This work to be carried out during 2018-2020 will cost of LKR 1,000 million. Diversion Stage IV also refers to removing two bottlenecks near the outfall.

7. “If, as proposed, the southern diversion takes place, such a canal would become a “trans-basin diversion” let alone the new outfall getting pushed about 20 miles, down south, to Panadura; not to mention reversing the natural flow direction, within the Madiwela catchment, and aggravating the already existing problems, within Bolgoda”.

COMMENT: The proposed diversion is not the first trans-basin diversion in Sri Lanka. Under the Mahaweli Scheme, there are trans-basin diversions. There are even such diversions among ancient works including diversion of Kala Oya to Malwathu Oya basin and Amban Ganga to Yan Oys basin. More recently, Kalu Ganga (Matale) was diverted to Amban Ganga basin under Moragahakanda Project, Uma Oya is being diverted to Kirindi Oya basin. It is also proposed to divert Gin Ganga to Nilwala basin. If Madiwela South diversion is the only practical option available to protect Sri Jayewardenapura area from flooding, it should be pursued after addressing whatever environmental issues that it may cause.

8. “The proposals (which) I have been making for more than 30 years, do not go against nature, no damage to environment by digging new canals, no underground tunnels of large diameter, no widening of bridges, and no pumping”.

COMMENT: If AN’s proposal with no digging of new canals, no tunneling or no widening of canals had merit, why wasn’t it accepted by authorities for implementation all these 30 years?

OPTION WITH NO DIGGING, TUNNELING AND PUMPING

As mentioned in my previous article, the Diyawannawa Lake has two draining outlets, one via Kolonnawa Canal and the other via Wellawatta Canal. The Kolonnawa Canal branches into three canals with outfalls to the Kelani River at Grandpass, Kotuwila and Ambatale which need pumping during heavy rainfall days. Hence, only the Wellawatta Canal is available for draining direct into the sea without resorting to digging new canals, or building tunnels or installing pumping stations. Under the MCUDP project, the stretch of Wellawatta Canal beyond the Galle Road was dredged, widened and the outfall improved at a cost of LKR 111.6 Million. It is to be seen whether this outlet together with the improved outfalls to Kelani River could handle the draining of Diyawannawa Lake during an extreme rainfall event.

ALTERNATIVE PROPOSAL TO DRAIN FLOOD WATER

AN has expressed his reservations about using the Beira Lake as an outfall for flood water as the level of the spillway cannot be adjusted. Though a sum of LKR 1,350 million is spent on building a pumping station at Maradana to divert flood water coming along the Dematagoda Canal into the Beira Lake and then to the sea, there is a doubt as to whether this diversion will work. If it works, it will take flood water from Kotte diverted to St. Sebastian Canal first to the Floating Market and then to the Beira Lake before the water enters the spillway near Galle Face. This will invariably raise the water level of Beira Lake which is presently maintained at 1.8 m above mean sea level to prevent buildings constructed on wooden piles along the lake from collapsing. However, according to an environment screening study on a project for rehabilitation of the Beira Lake carried out by Moratuwa University in 2011, any changes to the water level of the Beira lake can have an adverse effect on the stability of these foundations.

There is however, another alternative option available to improve the draining of Kotte flood water flowing along Dematagoda Canal into the river without posing any of these problems. That is by diverting water flowing in Dematagoda Canal direct into Kiththamphuwa Ela (KE) before it joins with St. Sebastian Canal, by constructing a new canal branching off from the Dematagoda Canal just before it crosses the railway line. This canal could run parallel to the railway line and join with the KE where it makes a U-turn near Welewatta Road. This link canal is only about 0.5 km long and this area comes mostly under railway reservation. The stretch of KE which runs parallel to the railway line up to the river outfall is being widened and dredged under the Kolonnawa Canal Diversion Stage IV at a cost of LKR 1,432 Million. Hence, construction of this new link canal could be undertaken as a part of this project.

The distance to the existing river outfall along St. Sebastian Canal from this branching point is 3.0 km while the distance to the Beira Lake outfall via St. Sebastian Canal in the opposite direction 5.2 km, whereas the distance to the river outfall along the proposed link canal and KE is only 1.7 km. Further, the present St. Sebastian Canal route has six road crossings and several bends while the route via Beira Lake has eight road crossings. Also, the stretch of St. Sebastian Canal behind the Technical College passes through a narrow passage cut through a hill with no room for widening. On the other hand, the proposed route via the link canal and KE is short and straight with only one road crossing at Orugodawatta and is a better option to drain the Kotte flood water into Kelani River, than the proposed scheme via Beira Lake.

CONCLUSION

The SLLDC has already executed several projects worth LKR 1,165 Million with World Bank funding to improve the drainage in several canals in the city and its suburbs. Several more projects estimated to cost over LKR 4,500 Million and USD 44 Million are on-going. This includes a project to take flood water from Kotte all the way to Beira Lake and then to spillway at Galle Face for discharging into the sea by reversing the flow in St. Sebastian Canal. However, this does not appear sensible even to a layman like myself. It is more sensible to drop this proposal and instead develop the link canal to take flood water flowing in Dematagoda Canal direct to KE stretch running parallel to the railway line and thereafter to the Kelani river. The pumping equipment intended for diverting flood water via Beira Lake could be installed at the outfall of KE near Kalu Palama, enabling it to remove the flood water during heavy rainfall.