Overtopping of seawall along Rupert Craig Highway

May 15, 2013 Letters

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Dear Editor,
In a release by the European Union expressing grave concern over the recent overtopping of the sea defences along the Rupert Craig Highway (RCH), it appears that Minister of Public Works (MPW), Robeson Benn, has suddenly decided to do something tangible to contain the flooding of residential areas south of RCH in the vicinity of Sheriff Street.
Since this segment of seawall was constructed in the 60’s, there has been a significant rise in sea level and its clay embankment has consolidated and deformed. Therefore the top elevation of its coping is no more at its design 62.50 GD but considerably less and at high tides, significant wave heights with swells cause sea water to easily overtop the seawall and flood adjoining low-lying lands.
In a 1972 Report on the sea defence by NEDECO, Dutch Consultants, it was recommended that the public should not be allowed access to the sea defences in order to protect them from damage of all sorts.
Unfortunately, Minister Benn with tacit support from the Georgetown Chamber of Commerce and Industry (GCCI) has allowed ‘liming’ on the seawall along RCH for many years now and the traffic has destroyed the turf, eroded the clay embankment thereby reducing its effectiveness to arrest erosion from the elements which if not addressed could eventually threaten stability of the clay embankment. He has further allowed building on the sea defence reserve in contravention of the Sea Defence Ordinance.
During his term, Minister Benn has shown scant interest in a planned and executed programme to maintain the sea defences such as having MPW carry-out regular and detailed surveys of the foreshore, seabed, settlement of the structures/embankments, inspection of constructed works and so forth and so on, since the changes observed over time with respect to these categories would have alerted MPW of the situation it now faces and with some advance time for appropriate action to be taken, some initiative could have been taken to prevent the impact of the disaster which recently occurred and is likely to do so again.
After all, doing nothing and waiting for the overtopping phenomena with resulting flooding of coastal areas from one location to another should not be the philosophical view Minister Benn should be taking and certainly not under the pretext of cost as the damage was extensive and many people have suffered much and are likely to have a repeat of the same any time soon.
It is inconceivable that Minister Benn should state that the placement of sandbags along RCH as is currently being done is the best solution to deal with overtopping. At high tide, sea water flows over the top of the coping down the sloping clay embankment into a drain and then overflows onto RCH. Sandbags placed on the south side of this drain (which is inadequate since it was intended to drain the road and not for overtopping) and on top of the divide of the E-W carriageways of RCH would probably help to channel some flood water towards the Liliendaal pumps but most of it will still find its way through the many gaps of the sandbagging and into the residential areas south of RCH and flooding them as has been happening and will continue to happen.
A cost effective engineering solution to prevent overtopping of the seawall is still possible.
In hindsight and having regards to hydraulic and stability considerations, if this segment of seawall was built with a top elevation of 65.0 GD and a curved inner coping (as at Georgetown seawall) to return the breaking waves back into the ocean and not over the coping there is no doubt that the present overtopping would have been greatly reduced.
However, the placement of sandbags in a seemly unplanned way to contain overtopping from the sea will not solve the problem but exasperate it.
Finally, Minister Berm stated that MPW was looking to build groynes as he noticed that these were assisting to break the waves as was evident at the Carifesta Avenue groyne (CAG) just north of the Kitty pump station. Construction of groynes on oceanic coasts were not intended to break wave action but to trap/prevent the drift or movement of material (sand and shell) along the coast due to the action of littoral current.
Breaking/dampening of waves as was observed at CAG was due primarily to the presence of sand/shell banks trapped around the groyne because of the accretion taking place at this location as well as the movement of sling mud westwards. There is no data per se to corroborate Minister Benn’s claim that groynes are the proper means to prevent large scale erosion or dampen/break waves and he should wait to make an informed judgement when the present erosion cycle reaches the CAG.
Charles Sohan

 

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