George and the two other members of the Dam Safety Board (DSB) were looking at longitudinal cracks on the side of the road, adjacent to a canal. The cracks, which were about 400 meters long, had been the subject of considerable discussions over many years. DSB had inspected the canal annually for the past 15 years. Despite several measures aimed at stabilizing the canal side slope, the cracks reappeared every year.
The 4 km-long, 25 year-old canal provided water to a three-unit hydroelectric plant. The canal was mostly in a side hill, except for a 400 meter stretch over a height of land, where the canal was in a deep cut between two hills. In this stretch, the canal side slopes had been steepened by 0.25 in 1 to reduce excavation volume. The entire length of the canal was lined with a clay blanket covered with sandy gravel, as well as riprap near the water surface.
In this 400 meter stretch, the right bank of the canal had shown a persistent tendency to crack a few meters above the water level. The owner’s investigation of the cracks, using test pits, revealed that they were less than 2 meters deep. Survey monuments in the crest of the slope on the downstream side of the cracks revealed a slow downhill movement of the embankment. The riprap above the water level also appeared to be creeping down the slope.
Despite the symmetry of this canal section, the cracks only appeared on the right side, where the hillside was higher and steeper than it was on the left bank. Mitigation measures undertaken included providing additional drainage in the hillside above the canal and widening the road that paralleled the canal to reduce load. Regardless, the cracks appeared each spring and were graded over to prevent water ingress.
This year, DSB decided a permanent fix was needed because of the high risk of a local slope failure. Such a failure would shut down the units, and the value of the lost energy greatly exceeded the estimated cost of the remedial work. Thus, DSB asked the consultants who designed the hydro development to produce a report outlining the reasons for the cracking and plans for a fix.
Nine months later, the completed report concluded that the cracking was a result of excessively rapid drawdown of the canal water, caused by plant operators exceeding the maximum “turbine ramping rate” stated in the plant operating manual! The repair was estimated to cost several millions of dollars and consisted of improved drainage and more side hill excavation work.
This conclusion astounded DSB and the plant operators because the consultant had never mentioned “ramping rates” during the commissioning work. Ramping rate is the rate at which generating units can be loaded. If the units are at the end a canal and are loaded quickly, the water level at the downstream end of the canal is temporarily lowered rapidly as the water slope increases to provide the required water acceleration. Once the required velocity is attained, the water level at the downstream end slowly rises to a level corresponding to the flow and friction losses in the canal. Rapid drawdown can cause slope failures.
The operating manual, produced when the plant was first commissioned, contained a brief statement to the effect that drawdown on the canal was in fact limited to 15 cm per hour. However, this restriction was not mentioned in the summary. The operating manual did not mention the corresponding rate of load acceptance.
The consultants had gone through the detailed operating records and found many instances where the canal drawdown was about 50 cm per hour, far in excess of the allowable rate. However, DSB pointed out that if this was the cause, cracking should have appeared on both banks of the canal.
Later, when George discussed the report unofficially with one of the consultant’s engineers whom he had known for many years, the reason for the report’s conclusions became apparent. If the consultant admitted that the canal slope was too steep, the plant owner could claim that this was a design error and that repair work should be carried out at the consultant’s cost.
However, DSB pointed out to the owner that when the project was constructed, the consultant had made a design decision based on risk versus cost and had steepened the canal slope to save excavation quantities and cost. Also, a rough calculation indicated that the money saved by this decision, if invested over 25 years, would have now reached a value several times the estimated cost of repairs. On this basis, the original design decision was correct. DSB recommended that all parties drop the cost issue and work together to produce a repair program. The plant owner accepted this recommendation.
The least cost option was found to consist of flattening the slope by 0.25 in 1, which would be attained by placing riprap in a triangular section from a narrowed canal base. The resulting head loss was calculated to be less than 0.1% of the gross head, which was an acceptable amount.
The next year, the contractor completed repair work using a barge-mounted backhoe equipped with a global positioning system, allowing the operator to determine the bucket location relative to the canal and desired underwater riprap slope, to within 5 cm. The cost of this repair was well under $1 million. The canal has now operated for several years with no evidence of further slope creep.
Two lessons can be learned as a result of this incident.
First, it is difficult for operators to abide by ramping rate restrictions. This is particularly the case when there is a wide variation in the reservoir level at the canal entrance. Ramping rates will be fast when the reservoir is full and much slower when relatively empty. In this case, the original operators could not recall any restrictions on turbine loading, and the plant owners were not aware of the restrictions either. Also, the increased ramping rate did not appear to affect the canal slope integrity on both sides. In any event, ramping rate restrictions should be avoided by hydro project owners and operators.
Secondly, blaming the consultant for design issues many years after the project has been commissioned may not be in the owner’s best interest and may even be counterproductive. A cooperative attitude often can produce a more cost-effective solution.
— By James L. Gordon, B.Sc., hydropower consultant