Referring to the write up in the Sunday Observer of December 10, 2017 titled, “Will the Rise of Groundwater Table Affect Colombo Fort?” I wish to comment and state that a rise of water table to any alarming level is most improbable. The cause for this rise of water table is the Port City reclamation work.
The concerned area is bounded on the North by the harbour front, West by the Marine Drive, South by Beira Lake and East by the Harbour Canal from the McCallum Locks up to the harbour.
This land mass is fairly confined to the Marine Drive side with extensive rock outcrops from the Galle Buck point where the Battenburg battery was located, then coming down to the Varaya Chaitiya structure where the two arches are founded on bedrock near ground level and further down to the South encountering Galbokka Point and the famous Governor`s Bath now buried with all rock outcrops, and running further South closer to the Beira Lake outfall.
Beira Lake water level is maintained at +6 ft MSL by this Galle Face spillway whose crest is at +6 ft MSL and by St Sebastian Locks (at Hulftsdorp) and McCallum Locks (near Lake House) constructed under the Beira Lake scheme of the 1920s.
This Fort area land mass and the Port City reclamation are not contiguous but distinctly separate and so the assumed water table profile would not be correct.
It is essential that the existing drainage system running along the Marine Drive from the Chaitiya area running south and flowing to the sea near the Beira outfall should be maintained by the Port City developers. There are two storm water outlets side by side each about 1m in diameter from the Presidential Secretariat side flowing to the sea about 10m North of the Beira outfall.
I was familiar with it as we were told to protect it from any intruding terrorists who could travel inwards and cause major disaster. I had a photograph of these two outlets, which I sent to the Presidential Secretariat about a year back suggesting to take up the matter with the Port City developers.
On the southern side behind the Presidential Secretariat and the Treasury buildings there is the Beira Lake providing reliable drainage for this sector.
Drainage on the eastern side is into the Harbor Canal at MSL falling into the harbour basin near the Hartal bridge of the Port and the drainage is efficient.
On the North all drainage go into the harbour basin with several large outlets to the sea and some coming from President’s House area. This good drainage system will prevent the rising of groundwater level.
It is pertinent to state after seeing several excavations in the Fort area over the years that the sub-surface material is coarse, like the construction of the basement of Galadari Hotel where the soil is coarse Lateritic, so that the drainage has to be efficient.
Port City reclamation extends to a depth of about 14m in the Ocean and filling material is coarse sand whose mean grain size is width 50 around 0.8mm as I remember.
In addition in the reclamation area there are internal canals connected to the sea. Thus the drainage in this area will be very effective without the buildup of water table. As regards rainfall causing floods in Colombo there had been 440mm rain in November 2010 causing havoc, flooding the New Parliament as well. Then in May 2016 in the outskirts of Colombo, the Kelani valley was severely flooded due to river upstream rains but rain in Colombo had been 256mm.
The presence of a good drainage system will not cause any alarm even with flash peak rain intensities.
It is important for Port City developers to provide and maintain an efficient reliable Marine Drive side drainage system from the Chaitiya area up to the Beira Lake outfall end. It is also essential to maintain the Beira Lake outfall to the sea without getting it choked due to siltation from northbound littoral drift. It is the responsibility of the Megapolis Ministry to ensure that these requirements are fulfilled. The above observations are made with my knowledge of the area for about 50 years, the Marine Drive, Beira Lake scheme including the Locks and the Harbour Canal coming under the control of the Port engineers.
A suggestion to the researchers of the geotechnical engineering company, Soiltech Ltd is to have a properly designed systematic measurement scheme to be carried out on a regular basis to measure the groundwater table in the concerned area if it is their intention to proceed with this subject. Of course, my personal contention is that there will be no danger of a rise of groundwater table in the Colombo Fort area resulting from the Port City reclamation works provided the above mentioned requirements are fulfilled.
- D.Godage
Chartered Engineer.