Dear Editor,
In a SN article of October 29 on the rehabilitation of coastal sea defences being undertaken by Contractor BK International (BKI) and funded by the European Development Fund (‘BK plugs rip-rap instead of wave wall’), BKI has expressed the view and concern that the existing ‘wave wall’ design as compared to ‘rip rap’ has proved to be unsuccessful, costly and technically useless. Hence the need for the Ministry of Works to revise its contract to accommodate the new found rip rap design philosophy developed by the contractor to make the sea defences more responsive to arrest erosion and the sea level rise due to climate change, and enhance the growth of mangroves.
It is evident that BKI is comparing mangoes and oranges. The mass concrete seawalls defined as ‘wave walls’ by BKI are over 30 years old, and given the fact that little or no maintenance was done on them over the years, they have performed their function admirably in protecting coastal Guyana. Further, the design philosophy of these walls was to deflect and return the waves 180 degrees after impact by means of their curved coping. These walls also had piling at their toes to prevent scouring and undermining of the slope. Rip-rap to prevent run-up or erosion was never a design consideration. However, as with most designs, mass concrete seawalls had their flaws, the chief being that a rigid structure should not have been built on a flexible foundation unless there were adequate safeguards.
The inadequately compacted clay embankment on which they were built settled over the years which left the structures with little or no support causing cracks and cavities to develop on and under the concrete slabs. With lack of proper maintenance, sections of the walls rapidly disintegrated under the impact of heavy wave action. The older concrete sea walls as at Stewartville had a 4 ft vertical and not a curved coping wall. Further, the height of many of the existing copings are now below design requirements because of rising sea level and settlement of the clay embankment, and raising them to the required elevations poses challenging technical and financial options.
Sea wall design comprises several and distinct components such as the toe, slopes, berm, coping and embankment. BKI’s contract for rip-rap placement at specific locations on the sea defences is intended to address problems relating to particular segments of the defences in order that the structure could carry out its design function, and not to ferret out design flaws in the system.
The basic design purposes of rip-rap when placed at critical locations on the sea defences are to prevent erosion and absorb kinetic energy generated by the waves’ impact, and impede the run-up of water on the slope and over the coping which could cause damage to the earthen embankment supporting the coping. As claimed by BKI, rip-rap does not have the capacity to deflect the impact of waves in predetermined directions or protect the toe of the sea wall from the type of erosion being experienced along coastal Guyana when foreshore level dips to 40 GD.
Yoursfaithfully,
Charles Sohan
