Dear Editor,
I have read with interest the numerous letters and articles written about the proposed design of the relief structure(s) for the EDWC. Both engineers Malcolm Alli and Charles Sohan must be commended for their points of view. Hope bidders took advantage of those opinions. The tenders are open and it is time to evaluate.
To the designer, the hydrological model entails basins, nodes and reaches. Basins flow to nodes. Reaches (also called links) connect nodes. That translates to storing rain in a conservancy (node) and discharging it through a few structures and/or canals (reaches) to the outfalls (node). Of course, numerous software can perform this function, so what is the problem?
The answer is obtaining the input parameters: determining the design storm, defining the drainage basins areas/divides, calculating the time of concentrations, calculating soil hydrologic curve numbers, computing the conservancy stage-storage relationship, determining tailwater conditions to the boundary nodes (outfall, maybe tidal), the length and cross section of canals, existing or proposed (reaches), discharge structures geometrics, head loss coefficients, other discharge coefficients, base flow characteristics (if any), evapo-transpiration losses, etc. Easy to read, but very time consuming to determine. Remember in modelling, garbage in is garbage out (the GIGO rule).
In principle the model is attenuating the rainfall (called runoff) for release at a later time, in engineering terms to control the outflow hydrograph. I would like to briefly expand on the difficulties that the designer may have to accurately determine just two of the above parameters: drainage basin areas/divides and the conservancy stage-storage relationship. Drain-age basin divides give areas that define the watershed. How can these be verified? From old contour maps, maybe, or the use of areas from past reports. If we do not want to use the past records then what? Remember if less area is modelled than actually exists, the runoff will be underestimated. If there is insufficient time to verify areas then assumptions will have to be made.
The conservancy stage-storage (or stage-area) relationship is required. How can this be determined? From a past report, but will this be a reflection of the existing situation? The questions can go on and on.
The bottom line is that I foresee a lot of assumptions will have to be made to run the model.
A risk analysis may be required to assess the impact of the assumptions. The output of the model will therefore be used to determine the capacity of the canal. The canal and conservancy will have to be checked against some predetermined constraints and varying tailwater conditions. My point is a number of iterations and scenarios would have to be developed. Locating the canal at either Hope or Land of Canaan can be scenarios to be considered and recommended.
Are our local consultants capable of undertaking such a modelling exercise? I doubt it. It is my opinion that the GOG is skimming the surface on this one and has only a vague idea as to the depth of expertise required for this undertaking.
Again I reiterate the model and risk analysis are everything to this project. Surely the local engineers can design a canal or is this what the project is all about? I also think the GOG should consider hiring Independent Technical Reviewers (ITR) to review the consultant’s work, ie model − structural, geotechnical, etc. Investing in ITR will be money well spent. My fellow engineers may have heard this before, ‘Doctors bury their mistakes, engineers live with theirs.’ I hope my view helps in some way in coming up with the best design approach.
Yours faithfully,
Ralph V. Seegobin