Dear Editor,
In articles which appeared in the news media on Jan. 17, 2018, the Chief Executive Officer (CEO) of the National Drainage and Irrigation Authority (NDIA) stated that three pump stations are now under construction at Buxton and other communities on the East Coast of Demerara (ECD) and these are expected to be completed and operational before commencement of the May/June rainy season. Photos accompanied the articles showed construction of one of the stations adjacent to the Buxton sea defence.
Photos of the Buxton station showed it in an advanced stage of construction with a steel sheet piling revetment (SSPR) supporting a clay embankment with two fuel storage tanks and a cottage built on top of the embankment a few feet away from the revetment.
The clay embankment in addition to the loaded fuel tanks and other structures will impose large horizontal soil pressures on the SSPR which will have a pre-disposition for outward movement if it is not anchored and restrained firmly in the underlying soil. Experience has shown that at least two-thirds of the length of the steel sheet piling (SSP) driven in the type of soil shown should be underground. However, the ultimate depth to which it should be driven will depend on the soil cohesive/shear strength and its horizontal pressure on the SSPR which the site soil tests should provide. In addition, the tie-back rods should be adequately sized, spaced, corrosion resistant and anchored onto restraints outside the embankment’s slip circle to withstand the tensile forces imposed upon them. It would be prudent therefore for the NDIA to review the design of this project to ensure that the design engineer has satisfied these basic technical requirements required for the design of this project failing which, there could be another costly revetment failure similar to those which occurred at Craig on the EBD and Mabaruma, NWD waterfront in recent times.
Finally, the CEO has stated that the newly installed pumps will prevent flooding of the ECD communities by increasing the drainage coefficient of these areas from 1.5 inches of rainfall in 24 hours to some unknown figure. The newly determined coefficient was not stated because no one knows but it is generally accepted that the coefficient for residential areas should be the minimum removal of 3 inches of rainfall falling over a specified area in 24 hours and it is hoped that the new pumps together with existing facilities will achieve this desired objective.
It should be further noted, that drainage of the coastland is primarily based on an integrated system – outfall channels, sluices, primary and secondary drainage channels. To operate efficiently, all these components have to be adequately sized and in good working condition free of debris, sediment and obstructions if their design requirements are to be met. If not, the pumps and sluices will be starved of water as the rainfall backs-up, inundating the properties they are intending to drain of excess water.
Yours faithfully,
Charles Sohan
