Sunday, April 27, 2008

A survey launch would allow monitoring of coastal erosion

A survey launch would allow monitoring of coastal erosion -engineer Philip Allsopp says in letter
Stabroek News article. April 27, 2008

Engineer Philip Allsopp says he does not believe that impending doom lurks on the coast of Guyana in terms of rising seas and erosion if coastal management is tackled intelligently and continuously and he pointed to the need for a survey launch so hydrographic surveys could be done.
In an April 6 letter to the Stabroek News, Allsopp said that there was a cycle of erosion and accretion along the Guyana coast whereby there were zones of erosion alternating with zones of accretion (ie sling mud) and the whole pattern moving from east to west at a speed of approximately three quarters of a mile per year.
Allsopp explained that there were protective mud bands (sling mud) alternating with troughs of erosion (lowered foreshores) moving steadily westwards. He said that the studies indicated that every section of the sea defences was exposed to erosion about three times a century.
“It is wrong to state that the cycle of erosion is now starting – it has been going on as long as the Amazon discharged mud into the Atlantic, and it is wrong to state that the cycle is 40 years – it is closer to 33 years,” Allsopp wrote.
He said that this pattern had first been observed by a Director of Public Works (then called the Colonial Engineer) around 1920 and confirmed and quantified by the Delft Hydraulics team in 1960-61. “I have records to prove this. If this pattern is identified and studied then it is possible to predict where and when erosion will occur and plan your maintenance accordingly. This was done in the 1960s very effectively,” Allsopp said.
He said that at that time the ministry had had a hydrographic survey launch and two excellent hydrographic surveyors in the persons of ‘Sammy’ Luck and Paul Kranenburg who had surveyed the coast continuously and reported their foreshore measurements to the Sea Defence Board and to Barry Manson Hing, the Executive Engineer on a monthly basis. “This practice ceased some years ago; there is no survey launch and it has not been possible to monitor the areas of erosion to permit of forward planning by the Sea Defence Board. In the present age of satellite scanning this should be achievable,” he said.
Allsopp said that overtopping of the walls is not an unusual occurrence, but it would happen only in the zones of erosion such as Montrose, during the high tides which were aggravated by an unusual swell in the Atlantic coupled with high winds.
He said that a similar case was that of Canvey Island in the Thames estuary, “which I observed as a research student in 1953.” He went on to say: “It was made inhabitable.
It should not be difficult to repair the washed out portions of the embankment on the back slopes of the walls, but this must be done immediately or the unsupported walls would collapse. The important point here is that this condition could have been anticipated if proper hydrographic surveys had been in effect. At least one hydrographic survey launch is necessary for effective coastal management.”
But in a letter on April 11, 2008 in response, Charles Sohan said that the scant data available made it difficult to predict accurately when and where erosion would affect the sea defences, since this depended on a number of unknown variables such as the width of the tidal flats, the type of material making up the foreshore, the intensity of wave action and the direction of wave attacks.
He said moreover that the general pattern was disturbed near the mouths of the rivers by local currents and by local deposition of riverain silt and sand.
“Similarly, man made (Demerara Harbour Bridge) or natural obstructions to tidal and other currents can and have caused unexplained local erosions (Craig, EBD), which is almost as severe as the cyclic attack but which may stop as unpredictably as it began,” Sohan said.
Sohan said that if Allsopp’s staff in the 1960s were able to predict when and where erosion would have occurred there would not have been any breaches of the sea defences at Bladen Hall, Lusignan and other places in the 1960s.
“Mr Allsopp is of the opinion that if the coastal cyclic phenomenon of erosion and accretion is identified and studied it is possible to pinpoint coastal areas of erosion and plan effective maintenance accordingly. These statements are all well and good, but in recent years the government has failed to provide the resources, manpower and equipment needed to collect the relevant data to make this possible,” Sohan said in his letter.
Scientists have projected that at current levels of melting ice caps, seas can rise up to 30 cm during this century because of global warming. Local experts said that Guyana’s sea defences needed to be strengthened to deal with the challenges of climate change, but this would be an expensive undertaking.

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