Dear Editor,
Reference is made to Mr R Seegobin’s letter in SN dated February 3, (‘The model and risk analysis are everything to the Hope Canal project’) which identifies the essential parameters for hydrological modelling of the EDWC to determine the proposed additional capacity of the conservancy flood relief as well as where to locate the canal.
We are not sure if the writer has actually worked with reservoir modelling for low-level lands. Modelling for multiple outlets into tidal and non-tidal areas is not simple. Although many of the parameters could be gathered from old maps and existing data if they can still be found, a lot of new surveys will have to be undertaken. A key parameter is the storage-elevation curve of the conservancy, which must be revised to account for sedimentation over the last 100 years and fix the various capacity limits of its volume.
An old capacity curve from 1950 was used by the Hydraulics Division in the 1970s, but this needs to be thoroughly revised. Hydrographic surveys as well as a Lidar survey of the catchment areas above water will also have to be done.
Not much is mentioned about his risk analysis details, an exercise which has rarely (if ever) been used in Guyana. A lot of dam safety protocols have to be included in this, but where is the dam safety analysis for this aging liability? Management of this conservancy is also essential in any flood hazard reduction measures, and nothing has really been achieved in this sector to establish rule curves on supply and demand of water. No one has a grasp on how much water is really needed for this conservancy and what its inter-year distribution is. Can its full supply level be reduced substantially year round? Has the conservancy ever been annually cleaned of all weeds and silt to facilitate east-west water passage?
The condition of the reservoir must be a realistic input to the model since it impacts on output.
In addition to hydraulic modelling of the conservancy there must be hydraulic analysis of the water conveyance works throughout the conservancy to the river and the sea. Some hydrologic models can handle channel routing in their routines, but the conservancy in the coastal plain cannot be modelled in sufficient detail using nodes and storage points. The complementary hydraulic analysis of the system is the key to a search for the least-cost solution to the flooding problem. Substantial surveys have to be undertaken to establish the variables needed to design any improvement works. Surprisingly, work has yet to be completed to establish the hydraulic capacity of the existing Land of Canaan (LOC) canal and its conservancy supply. This may be a bit complicated for local engineers and most of it should have been done since the problem started.
The UN in 2006 made some excellent recommendations about measures needed to decrease the hazard level of the conservancy for the short, medium and long terms, but implementation progress in this area has been dismal.
Pictures of the conservancy dam show a badly deteriorated eastern section of the dam compared to the 1970s. Successive government engineering interventions since the 1990s have apparently aggravated the dam condition. In short, these so called improvements have been based on poor engineering judgement and are now liabilities. No new information has really been collected about the physical characteristics of the reservoir, despite all the problems encountered to date. The hydraulic capacity of the burrow trench, the nature of any internal cross dams and the capacity of the relief outlets have not been firmly established.
The Minister of Agriculture has decided that the Hope Canal is the best option for additional relief, but perhaps he ought to be cautious about his decision and the technical advice on which it is based. High water problems in the sector can be caused by several factors, but generally if the burrow trench and surface weeds are cleared then internal water slopes can be very shallow. If he thinks this should be a solution then see what will happen when the government tries to tack on the Mahaicony conservancy to the Abary conservancy and then discharge the total spill flows of both into the Berbice River.
A relief canal to the Atlantic is not without problems and liability. How much pegasse is in the land between the conservancy and the crown dams as well as Hope Estate, and can it support a large canal? This will have to be a ‘fill’ canal since land levels along the canal alignment are as low as 52.00 GD. Once such a canal is built then its embankments become an extension of the conservancy dam. One of the authors worked in the then British Guiana during construction of the Land of Canaan sluice and questioned the technical soundness and safety of constructing a 2000 cusecs relief structure through the existing conservancy dam in the Hope area. Few Guyanese would remember the Bonasika sluice fiasco of the Boeraserie conservancy. Hydraulic losses along the Hope canal route will have to be accounted for and include friction losses and head losses at the intake structure, a bridge at the public road and the outlet structure through the sea defences. Their sum amounts to the total head that will drive the canal flow. The objective is to make sure that the relief canal at the sea is independent of high tides and its discharge is 24 hours per day like the LOC. If the Hope Canal has to work with even a short tide lock period (where spring tides are higher than the canal tail water level) then this will become a white elephant.
A quick estimate by the authors indicates that with a 65 ft canal bottom and a conservancy flood level of 58.00 GD after deducting all losses, the water level at the outlet could be 54.50 GD and this could require some tidal closure.
Our point is that a decision for a Hope Canal outlet instead of adopting a holistic approach and analysis, can worsen the conservancy problem instead of improving it, especially after spending some $3B plus.
Yours faithfully,
Malcolm Alli
Mike Ragwen
