Consultants proposed Flagstaff Relief as cheaper alternative to Hope option to drain conservancy

Dear Editor,

I refer to a letter by Mr Chandan of NDIA in KN dated April 19 as well an article in SN dated the previous day quoting Minister Robert Persaud where both claimed that the main consultants’ preferred choice to drain the EDWC was the Hope Canal.  I am asking that both these gentlemen read the Draft Report of the Engineering Design of this project with particular reference to Section 8 ‘Conclusions and Recom-mendations,’ rather than relying on second-hand information from their local engineering advisers.

The consultant as part of their terms of reference for the project mentioned the Hope canal as a possible choice to drain the EDWC whenever there is a heavy rainfall, but stated more sophisticated modelling of the conservancy itself is required and this needed to be supported by improved data on bathymetry/topography of the conservancy as well as a survey of the interconnecting channels throughout the conservancy. Simulated head losses carried out towards the proposed canal are at best approximate, and can only improve with a more physically relevant model of the conservancy.

In other words more modelling work has to be done to determine the quantity of the water flowing from the Flagstaff end to the Hope canal, and this should be carried out before embarking on the construction of the Hope Canal Project.   I might add that the last dry spell with the conservancy at very low levels would have been the ideal time to get all the above bathymetry and surveys within the conservancy completed.

Many Guyanese may not know it but the  EDWC captures the flows from two main tributaries of the Mahaica River – the Lama River, which intersects the eastern conservancy dam 10 miles upstream of Flagstaff, and the Maduni River, that intersects the dam a further 3 miles upstream of Lama.   During any flood event the main flows entering the conservancy will be at these points.  The flows will therefore have to be conveyed a significant distance in the conservancy itself before getting to the proposed Hope relief canal entrance.  This aspect of hydraulic conveyance capacity of the internal channels, sheet flow passages over the submerged areas and the conservancy internal borrow trench has not been given enough study and analysis.  This should have been undertaken before finalizing any relief canal design and rushing to spend money on the relief canal construction.

A 4-door sluice at both ends of the sea defence and the EDWC was also recommended for the project in the report, but this has now been changed to 8 and 3 doors respectively.

The consultants went on to state that a much shorter and cheaper channel relief is possible between Flagstaff and the Mahaica River, and stated that with improvement of the channel cross section in the lower 7 km of the Mahaica River (the mouth needs to be dredged) it could be possible to discharge in excess of nearly 3200 cusecs, which is far more than the Land of Canaan Sluice (LOC) working at 100% efficiency can provide. The Mahaica option would also need maintenance, but that requirement may not be greater than the maintenance required for the Hope canal option. The capital cost of the Mahaica relief would be significantly lower than the Hope option.

The consultants also stated that there is a lack of knowledge about the prime causes of the flooding from the Mahaica River when water is discharged from the EDWC via the Maduni and Lama Sluices, and went on to state that improvement in the lower reaches of the Mahaica would improve upstream flood conditions and reduce the risk of the EDWC overtopping.

The Minister’s statement that the Mahaica Relief would require the banks of the Mahaica River to be raised is, therefore, incorrect.  A bar also develops at the mouth of the Mahaica River and impacts its flow capacity at low tide.

It is obvious from the above that the Mahaica Relief is the better and cheaper choice to drain the EDWC, and that there will be no fear of flooding of the residents in the upper and lower reaches of the Mahaica when water is released into the Mahaica River.

The Mahaica River at its mouth was modelled and flow established as 3200 cusecs whilst the flow from the conservancy to the EDWC 3-door sluice is unknown and yet to be established. This will determine the flow into the canal and ultimately the 8-door sluice at the sea defence end. In other words what comes in goes out, hydrologically speaking.

Lastly, I agree with Mr Chandan that the Consultants Design Report is a feasibility report but so far an EIA of the project is yet to be carried out.  This should be followed by a formulated flood relief project-alternative and finally, definitive designs of the relief works for bidding.

Yours faithfully,
M. Alli

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