Rip rap versus integrated concrete walls
Experts disagree strongly over best sea defence type for Guyana
November 3, 2000
Former specialist engineer Malcolm Alli has argued again that the rip rap design in sea defence construction be discontinued because it is uneconomic and unsuitable for Guyana but head of the Sea Defence Unit, Mahadeo Persaud disagrees.
Persaud, whose unit is part of the Ministry of Works, is adamant that rip rap is best suited for coastal protective works and is internationally accepted.
Making a presentation on sea defence works in Guyana at a lecture hosted by Guyana Association of Professional Engineers (GAPE) at the Hotel Tower on Tuesday, Alli said millions of US dollars continue to be spent on this type of coastal defence despite many failures.
"There is a school of thought in Guyana which advocates that the rip rap is still suitable for the coast lands. Something is probably wrong with my eyes," Alli stated.
He noted that the European Union has allocated US$20 million for sea defence funding this year. The proposal to use funds to construct two kilometres of sea defence at Capoey, Essequibo could be better used elsewhere for better sea walls, he said.
He said it would have been better to re-align the Essequibo sea defences when the Essequibo road was being constructed but the only choice left now was to confront the sea.
Alli also questioned the amount of 500,000 Euros earmarked to fund the design and preparation for the work. This sum seemed too large, he indicated.
The engineer recalled that the rip rap design was introduced in 1982 as a temporary solution to stem the alarming rate of erosion occurring on the Essequibo Coast. It was considered a quick-fix for the sea defences for the riverain areas.
It was later accepted as a permanent solution for Guyana's sea defence problems throughout the coast, he said.
He recalled that in a 1990 report a senior government engineer stated that there was little cost difference between the rip rap design and the construction of integrated concrete walls and was everlasting. Alli described these statements as misleading and lacking in engineering basis.
Persaud said the world has moved away from construction for sea defences. This type of response is not permanent and when the ground level changed this type of construction would fail completely.
On the other hand, the rip rap is a flexible design that would shift if there is any change in ground level, he stated. No rip rap structure has failed in Guyana, Persaud asserted.
He noted that the concrete construction transfers incoming waves downwards and destroys the foreshore. But with the rip rap, the wave energy is lessened and builds up the foreshore.
Persaud also pointed out that the rip rap structures were simple to build and the materials required were easily available in Guyana.
Giving examples of rip rap failures over the last few years, Alli said there were several slippages in the rip rap shortly after construction at Anna Regina in 1996. There were also slippages at Barnwell, Le Destin, Farm and Ruby on the West Coast Demerara.
Then there was what he described as the total failure at Lusignan, East Coast Demerara in September, 1999. Alli noted that a new rip rap with a different profile was being constructed in this area. He predicted there would be more failures and the entire alignment would have to be abandoned.
At Mon Repos, a new rip rap was built behind a failed wall in 1997. Described as the famous Mon Repos fiasco, Alli said the new slope now appeared to be flat and failing without being subjected to any wave action.
"One can conclude that the rip rap design does not appear to have any engineering basis and was adopted by trial and error or simply because it was easy to construct and that it worked in other countries," Alli stated.
He found it incomprehensible that the experts could not critically examine the causes of the failures in the rip rap design rather than changing the wall profiles then hope this would work.
He said his experience was that continuous exposure of the soft clay to the tides could result in the clay becoming plastic, causing tremendous geo-technical problems.
Alli observed that the rip rap cost about US$3,000 per metre to construct which he said is expensive for a temporary construction. On the other hand, the integrated boulder slope concrete walls built during the 1960s and 1970s cost around US$450 per metre at that time.
He pointed out that the walls were far more extensive than the rip rap and required considerable expertise to design and construct. These walls were still in good condition today despite no maintenance over the years, he stated. Given these facts, Alli questioned why the rip rap design was chosen.
Rip rap was unknown and not hydraulically model-tested for Guyana's conditions, he said, but the concrete walls were well engineered and model-tested for Guyana's conditions.
Alli predicted there would be several breaches in the sea defences in the coming years and a major failure in the Strathspey area where a concrete wall was built to prevent overtopping.
He called on GAPE to evaluate whether the government was pursuing the right course of action for Guyana's sea defences. (Andrew Richards)
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