NDIA does not have the data to estimate the 10 000-year storm

Apr 12, 2009 Letters

---

Dear Editor,
The CEO (ag), NDIA, Lionel Wordsworth must be commended for his letter on the Hope canal design, which he circulated to all the leading newspapers in Guyana, namely Guyana Chronicle, SN and Kaieteur News.
Lionel attempted to justify the Hope Conservancy Relief Channel, and to sanction that the consultancy acquisition process followed the government procurement policy. On both issues, he is far from convincing.
His letter left us to think that there is a misinterpretation of the required hydrologic methods/thinking that dictates the design of the drainage infrastructure. And, with respect to the procurement process, there is no proper explanation as to when and how Cemco became involved in the design consultancy award.

Let’s look at the design flood for the canal. Lionel stated that the Hope Conservancy Relief Channel “is to evacuate the 10,000-year storm”. It leaves some doubt that Lionel comprehends the significance of using a 10,000 year design storm and what is required to predict/quantify such a storm?
After speaking to a few engineering colleagues, the general consensus was that the 10,000 year event stated was a typo; and the CEO will quickly correct the design storm in a subsequent letter. This never happened; and hence, we treated the 10,000 year storm as a fact, thus the need for this letter.

For those who are not hydrologists or statistically minded, the 10,000-year storm event translates to: a storm having a chance (on average) of being equalled or exceeded once in 10,000 years and has a 0.01% probability of occurrence.
Similarly, a 10-year design storm will have a chance of being equalled or exceeded once in every 10 years or a 10% probability of occurrence.
Since events are random, the particular storm event may happen more than once in the stated number of years.
The storms that Guyana experienced during 2005 to 2008 are definitely not the 10 000 year events. To expect the 10 000 year event is absolutely in-comprehendible. Then to adopt this as a design storm is astounding.

The EDWC is a low dam in comparison to other dams in other countries. The design storms of the spillways for those dams are definitely not the 10,000 year event.
Major dykes and spillways are designed to withstand the 100 year storms, and where the failure would be catastrophic, the level of protection is sometimes raised to the 200 year event.
Engineers commonly use the Probable Maximum Flood (PMF) as this event accounts for the climatic variations and some cumulative impacts.

The PMF is far less than the 10,000 year event (if it can be determined). This suggests to us that the design criterion for the Hope Relief Canal is questionable and may be flawed.
For one extraordinary moment, let’s entertain the fact that the CEO is correct in abstracting this value from the model /reports done by his team of international and local experts.
Is he or the NDIA aware of the 10,000 year criterion, and the injudicious statistical projection required to make such a prediction possible based on limited recorded hydrological data? Remember Christopher Columbus – recorded data would have been available long before his arrival in Guyana to provide a reasonable sample for such a prediction. If we apply the general rule that you need ‘x’ data to predict/estimate ‘2x’ storm (for example, 50 years data to predict the 100 year storm), the NDIA would require 5000 years of data, and that is before Christ.

This brings a follow-up question – what part did the Hydrometeorological Section played in advising and determining this prediction, and did they review and approve the hydrologic aspects of the reports/models?
It is therefore imperative that the CEO (ag) and the NDIA consider posting these reports mentioned in Lionel’s letter on the World Wide Web so that interested persons can provide some assessment of the appropriateness, assumptions made, scientific applications and judgment for the benefit of the people of Guyana.

Now that we are hearing of a design storm of 10,000 years, the confidence in the models has vanished?
We also wonder if the NDIA took the time to determine what storm was used in the design of the existing channels and the conservancy. Certainly, if the risk in the design is to manage the 10,000 year occurrence, it is questionable if we still need an outlet to the Atlantic Ocean. Would that outlet be effective as it would be subjected to the tidal cycle and the sediment accumulations that affect similar outfalls?
The NDIA is spending $3 B in constructing a canal to bring flood relief to the costal urban areas with a 10 000 year flood design. Why can’t the NDIA implement a 100 -200 years design (typical design risk for spillways) and save millions?

Like others who have written on this subject, we are of the opinion that the efficiencies within the drainage system, coupled with the implementation of scientific reservoir operating rules may show that there is no need for consultants and contractors to build another outlet. Since the conservancy dam is not being raised, can the NDIA say at what storm event we would expect the overtopping of the conservancy dam to occur?
Further, with the existing outlets operating at 80% efficiencies, would there be any overtopping?
The other aspect of this letter raises questions about the design consultant acquisition process. The Consultancy service for the Hope Conservancy Relief Channel was awarded to Cemco/SRKN’gineering in association with the UK-based Mott MacDonald Company.

At the time of the opening of the tenders, GINA in press releases said that two bids were received. Cemco was not mentioned by the press as part of a joint bid. It was surprising to see Cemco signing the agreement for the joint team, obviously acting as the prime consultant.
Why did Cemco not correct the press release to say they are the prime consultant? At what stage was Cemco introduced into the bidding race and why was the inclusion of Cemco not mentioned in subsequent press releases by GINA?
MOA should have the past ‘models’ and reports reviewed by unbiased independent technical experts other than the team that was awarded the consultancy contract.
Ralph V. Seegobin
Ram Dharamdial

---

---