The Shallow Canals

George was the assistant resident engineer on a river diversion project. Six canals were to be excavated between seven ponds to divert two watersheds into the headwaters of a new hydroelectric development. One of George’s first tasks was to review survey work required to set out the cut stakes and canal grade boards for the contractor. The first three canals would divert the North-West River waters into the Fourth River watershed, where there was a dam and spillway, and the last three canals would divert this combined flow into the watershed for the powerhouse. To avoid excavating rock, the canals were designed to be wide and shallow.

George met Manuel, the chief surveyor, at the site of the third canal, and he asked Manuel how the survey work on the third canal was coming. Manuel replied that it was fine because the data was identical to that for the first two canals. This reply puzzled George because the invert level at the canal entrance should have been different for each of the six canals. Upon further questioning, Manuel informed George that all six canals had an entrance invert level of 750.12 meters. But George knew this level should at least drop by the energy loss in each canal.

George went to the site office to access the plan and profile drawings for each canal. The sections were identical, as was the profile elevation. It was apparent that the draftsman had copied and pasted the same design onto each drawing, and the design engineers had not noticed the error. The project general layout drawing showed a plan for the six canals but no profile. There was no topographical data for the remote area, so the plans were based on a mosaic traced from aerial photos, with the pond water levels obtained using surveys the previous year. The area was generally undulating, with small low hills between ponds. The canals wandered around to avoid excavating rock, which was identified using hand-excavated test pits and probing with steel rods.

That evening, George spoke with the resident engineer, at the powerhouse, via short-wave radio, the only form of communication in the remote area. Unfortunately, the resident engineer had an electrical background, having recently arrived on site to complete and commission the plant, and he could not understand George’s point.

The next day, George drove to the powerhouse with the drawings. However, the resident engineer insisted George must be wrong because the drawings were all properly signed and sealed by the chief design engineers at their head office. The resident engineer was emphatic that construction continue, following the design. George suggested telephoning or writing to the chief civil engineer, to no avail — a junior engineer should not question work approved by the chief engineers.

George returned to the site, discussed the problem with the construction contractor and reached an agreement to discontinue the surveys for excavation quantity payments and base progress payments on the theoretical quantity in the contract document, instead of the actual excavated volume.

About two months later, the chief civil engineer arrived on site to inspect the completed work with George, the resident engineer and the contractor. The group started at the first canal, and all proceeded well until they arrived at the third canal, where George casually mentioned the constant canal invert levels. Upon seeing the drawings, the chief civil engineer immediately realized the error and suggested the group return to the powerhouse to redesign the canal system. This was done, and the contractor spent the next three months re-excavating the five lower canals.

Canal excavation quantity more than doubled, and expensive rock excavation was needed for the deeper canals. The contractor was pleased because the additional work was undertaken at the same unit prices as quoted for the much smaller contract quantity. The project owner was disappointed, to say the least, because some cost savings realized at the powerhouse disappeared into the extra canal work. However, during the redesign, it became apparent that with the lower canals downstream of the Fourth River, more flood water could be captured, increasing the energy produced and offsetting, to some extent, the higher cost.

Lessons learned

There are several. First, the resident engineer should have allowed George to at least communicate his concerns to the head office. Errors can occur, and any organization where errors discovered by junior engineers are not taken into account will suffer the consequences, and repair costs may affect its profitability. Fortunately, in this case, the client was understanding and did not insist that the consultant bear the cost of the extra work.

Second, recognizing that more attention to detail is required during the drawing approval process, the consultant revised the process by adding more internal reviews by experts in hydraulics, geotechnical and structural engineering within the civil department before approval by the chief civil engineer. Similar revisions were made in the mechanical and electrical departments.

Third, the consultant realized the drawings were not clear, with no general profile showing all the canals. Drawing standards were improved, and the project general design criteria were added to the general layout drawings.

— By James L. Gordon, B.Sc., hydropower consultant

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