Will the rise of groundwater table affect Colombo Fort? | Sunday Observer

Will the rise of groundwater table affect Colombo Fort?

Rising groundwater level in the Colombo Fort area, once a marsh land reclaimed by successive Dutch and British Governors, poses a danger to some of the heritage buildings inside the Colombo Fort area, a study done by a leading geo technical expert and a private company, has shown.

Dr Godakuru P Karunarathne, a geotechnical expert who had worked on sea-bed reclamation projects across Asia, including the Changi Airport reclamation in Singapore, presenting his findings at a meeting convened by the Institute of Engineers Sri Lanka (IESL), last week, said, data collected by a Sri Lankan geotechnical engineering company, Soiltech Ltd, over a period of time from boreholes and tubewells in the Fort area, is showing an alarming trend in the rise of ground water level after the Port City reclamation started.

Dr Karunarathne attributes this trend to what possibly could be poor drainage along the Chaithya Road interface to the Port City reclamation area west of Colombo Fort, coupled with other factors related to the land reclamation.

Sector experts say, as the biggest development project currently underway in the country, Port City, built on 269 hectares of reclaimed land adjacent to the Colombo Fort area, it is to be expected that such a massive change in the original coastline can cause unseen chain reactions, and therefore, it is imperative that relevant stakeholders and professionals do proper initial assessments and studies to ensure that any negative consequences of the development of a bigger scale is completely eliminated or reduced.

At the presentation, Colombo Municipal Council Engineers contested a statement made by a Port City representative that a drainage channel running Southwards from the Chaithya flowing to the sea near the Beire Lake outlet in front of the Presidential Secretariat is adequate to cater to the current drainage needs of Colombo Fort.

While there is some contention between the Port City Construction Company and the Colombo Municipal Council over the adequacy of the channel, sector experts who choose to remain anonymous, say, the bigger problem is the sand bank that’s already blocking the Beira outlet, just in front of the Presidential Secretariat.

“It appears that despite many lessons from the past, we have not taken into consideration the effect of the littoral drift along our coastline and the wave patterns. Sand is being pushed upwards into the channel mouth, completely blocking the mouth, necessitating daily removal of sand using earth moving equipment. We saw this happening before in the Oluvil Harbour, which is now abandoned due to a permanent sand bank that’s completely blocking the harbour entrance,” one commentator observed.

“There are some permanent solutions that are being examined. One is, building breakwaters extending up to the natural reef, which is about 500 metres away from our current coastline, from around Dehiwala to Bambalapitiya, which would also create new urban beaches and retain the sand that’s being pushed northwards by the littoral drift. The Megapolis Ministry has already obtained approval from the Cabinet to extend the beach area from Bambalapitya downwards to about Ratmalana, which could alleviate the issue and provide a permanent solution for the channel mouth being blocked,” he said. "Colombo Fort is a unique geological feature. Manmade out of a marshy area, it was once bounded on all sides by canals and the sea, virtually an island.

Just before the Port City reclamation commenced, a canal running north from the Colombo Royal Yacht Club, alongside the Telecom building and another running east to west below the Finance Ministry and the Presidential Secretariat divided the Fort area from the rest of the city. Drainage pipes ran into the sea across Chaitya Road and through the Port area. Some of these drainage were blocked during the construction of the SAGT terminal”, a commentator present at the meeting said.

“Port City reclamation is done adjacent to the Fort mainland area. We are trying to find out the influence of this reclamation on the groundwater level in the area. Our study is that when you do reclamation adjacent to the Chaithya Road area what would its impact be on the groundwater level?” Dr Karunarathne initiating the discussion, asked.

Explaining the research mechanism, Dr Karunaratne said, several assumptions had to be made, but the trend lines prove the water level is rising at whatever assumption, other than the base assumption of no rainfall, no reclamation and drainage remaining as was prior to the commencement of the reclamation. The highest rise is predicative when an assumption is made where reclamation is completed, drainage is poor and there is rainfall above 100mm per hour. According to the initial EIA conducted by the CCDA it was said that the rainfall to this particular area can be 50- 100 mm per hour, however, this can vary. For example, last week’s rainfall was much more than that 100 mm.

Once the reclamation is completed, which is the physical entity that is equivalent to the Fort area, taking into consideration factors such as, rainfall intensity, drainage provision and sea level, the ground level is expected to respond accordingly.

“We should consider the effects on the groundwater level with such an intensity of rainfall. With the on-going reclamation, once completed, heavy intensity of rainfall, 50 to 175 mm/hour, can increase groundwater level to numbers beyond imagination,” he said.

One of the main factors that can impact this change in groundwater level, he said was the efficiency of the drainage at Chaitya Road.

Therefore, will the rise of the groundwater table affect the buildings and other infrastructure of the mainland; Fort?

“If the water table rises, structures built on simple footings (apart from piled foundations) may be subjected to loss of bearing capacity, which in turn reflects as building settlement – mostly as differential settlement,” Dr Karunarathne said.

Even though according to this study the effect of the increase in water level can mainly damage the old buildings in and around the Fort area, since modern buildings are built on piling, it doesn’t mean there is no effect on new buildings as well.

“The only way to handle this situation is to take preventive and mitigating action,” he said showing the areas in which the impact can have disastrous results, such as, on building foundations, basement walls and floors, tunnels, surface drains, sewers, cable ducting, towers, and chimneys, roads and pavements and lunging (bump) at the bridge approach due to differential settlement between (Eg. Indonesian volcanic clay).

Groundwater, in response to large scale reclamation can change the course of the environment. Our littoral drift has changed due to the reclamation carried out. The Kelani river mouth can get choked or you may find changes far away down south as a chain reaction to this.

As solutions the forum discussed components of such buildings need to be strengthened with in-situ ground modifications ranging from grouting to micro-piling for ground strengthening, or seepage cutoffs for basements and ground floors.