There is no short-term solution to mitigate future overtopping of the seawall along the Kitty to Liliendaal stretch of the coast with likely solutions costly and rising sea levels adding another dimension to the threat, according to coastal engineers who said that the authorities face a formidable task.
That area is also likely to experience a prolonged period of erosion due to a natural cycle with stronger and higher intensity waves expected. Unusually strong waves last week lashed that section of the coast leading to flooding with clogged drains also playing a part in hampering drainage. The ferocity of the waves surprised many and engineers Malcolm Alli and Charles Sohan as well as Minister of Transport and Hydraulics Robeson Benn said that erosion of the foreshore along the Kitty to Liliendaal coastal stretch contributed to the severity of the overtopping of the seawall in those areas.
“The situation is indeed serious and in my opinion was caused by scouring of the foreshore from the earlier overtopping, with severe wind,” said Alli, referring to the April 28 overtopping, in a letter to Stabroek News. Alli, who designed this section of the seawall in the 1970s, said that from what he has observed “it is obvious the foreshore has eroded appreciably” and “engineers should have been carrying out soundings to determine the level of the foreshore and come up with solutions to build it back.”
Benn told Stabroek News last week that a “disturbance” in the Atlantic Ocean generated strong waves which rode on top of the regular tide which came parallel to the top of the walls. It was these “swellings,” he said, which came over the seawall and caused the level of flooding seen. He also said that another reason for the overtopping was that the erosional front, marked by the shell bank in the vicinity of the Russian Embassy, which is low in the foreshore, allowing water to come closer to the wall. “Everywhere the shell bank is moving along we’re having this phenomena. We had it three or four years ago at Better Hope and a couple times last year it happened between Ogle and Turkeyen. Now it is here moving along,” he said.
On the morning of April 28, the 1.5 meter swells on top of the 3.11 meter tides easily rose over the sea defences and crashed to the other side with the ferocity raising concerns about future occurrences and whether anything could be done to mitigate the impacts such as raising the seawall.
Alli was not confident that raising the level of the seawall is feasible. He said that raising the concrete coping ‒ the vertical section of the seawall that rises above the ground ‒ higher could result in rotational slips.
The level of the seawall is 62.50 GD and he said that this is “about the highest you build to.” He emphasized that the seawalls in the area cannot be re-aligned any more and the government must hold the present alignment at all costs. “It will be indeed a formidable task,” he said.
The engineer told Stabroek News yesterday that a launch with sounding equipment was normally used to determine the levels of the foreshore. However, he pointed out that the Hydrographic Section that does this work was closed down over 20 years ago. “I see no solution to the problem except more overtopping and flooding and eventually failure. You have to review the levels before coming up with solutions,” Alli said.
Sohan, meantime, told Stabroek News that the authorities have to act. He said that it has to be determined whether it is economic to protect the areas behind the sea defences and in this case, it would be.
“The height of the coping is insufficient so they have to do something,” he asserted. “They have to raise the coping… to cater for the water that is coming over the seawall.” The engineer rebuffed statements that the seawall cannot be raised. “As an engineer, everything is possible but it would not come cheap,” he said.
Sohan also suggested that the seawall is sinking. “The coping is on a clay embankment and the clay is settling slowly,” he added, stating that while the level of the seawall was originally 62.5GD, it is probably now at 51GD though the authorities would have to clarify this figure.
Further, the engineer pointed out that sea levels are rising. Scientists have warned that low-lying coasts of countries such as Guyana are facing threats due to sea level rise as a result of global warming which has caused the melting of the polar ice caps.
Sohan estimated that since the 1950s, the sea level has risen several inches and the seawall has not kept pace with the rise in the water level as well as the settling of the clay embankment. “The sea level is rising but we haven’t raised our seawalls to cater for that,” he said.
In terms of the erosion occurring at the Liliendaal area, Sohan explained that “macro ripples” ‒ ‘sling mud’ banks ‒ are moving along the Guyana coast from east to west at about three quarters of a mile per year. “The macro ripples deposit and consolidate sling mud on the western side (of the bank) and erosion on the eastern side,” he said, noting that it was cyclic. “Between these macro ripples, you have a trough and in the trough you have local waves and swelling occurring… and these are the waves that are coming in,” the engineer explained.
He said that the wave action was stirring up the mud at the foreshore leading to erosion with the greater depth of the water in the trough producing greater and more swelling with high, powerful waves.
Sohan pointed out that a World Bank-funded report done for the government in 1972 had determined that there was a six-year erosion cycle meaning that an area will be subjected to erosion for a six-year period with the area affected usually being between 500 to 2000 feet per cycle. He suggested that this is what is occurring in the Kitty-Liliendaal area and said that the spot will experience erosion with greater or less intensity than is currently occurring.
