Description of Event:
On July 16, 2019, around 16:30, during an operator inspection, a hydro operations employee noticed turbidity in the tailrace of Beta Plant’s powerhouse. The employee proceeded to the power canal forebay where he noticed sloughing and erosion of the canal in progress on the left- and right-hand sides of the canal around canal station 12+50. Recognizing this abnormal condition, he immediately made verbal notifications to the Principal Dam Safety Engineer who in turn notified the acting Chief Dam Safety Engineer. Several supervisors proceeded to the project.
The turbines were taken offline at 16:45 hours and spill established on the spillway. Personnel then proceeded to place a bulkhead at the inlet to the canal. The bulkhead was in place at 18:33 hours, which stopped flow in the canal.
Actions Taken:
The detailed chronological log of key events is provided in Attachment 1.
Power Canal Erosion Event Report – Attachment 1
Method & Findings:
Description of Unusual Occurrences or Operating Circumstances Preceding the Condition
The canal was drawn down over the preceding year for the following:
• From mid‐May 2018 until mid‐November 2018.
• From December 2018 until May 2019.
The canal was in the process of slowly being refilled and monitored under a temporary Dam Safety Surveillance Monitoring Plan (DSSMP) when the incident occurred.
Cause Investigation
The root cause of the condition appears to be a combination of sloughing caused by water surface fluctuations and surface erosion of the canal caused by high velocity water scouring native dry sands behind portions of the deteriorated concrete liner. This cause is supported by the following observations:
• Increased velocity in the canal because of increased river flows due to 1.39 inches of rainfall on July 15th.
• Two-unit trips on July 16th, which caused two separate water level fluctuations within 10 hours prior to the erosion initiation.
• Signs of turbidity in the powerhouse tailrace.
• Signs of undermining of the concrete liner in areas outside of the eroded area.
• Visual inspection of the erosion and sloughing in progress.
• No signs of turbid water in seepage collection points on the hillside below the canal, which may indicate internal erosion.
• No signs of excessive seepage on the hillside below the canal, which may indicate internal erosion.
Supporting Materials:
Corrective Actions:
Measures Taken to Prevent the Worsening of the Condition
An engineered bulkhead was placed at the inlet of the canal and the canal dewatered. The reservoir upstream of the bulkhead was maintained at an elevation of 1190.0 ft NGVD +/‐ .20 ft.
Description of Damage to Project Works and Status of Repair
Erosion along the canal occurred, which required repairs prior to returning to service. It was estimated that 70‐100 cubic yards of sand eroded from the canal banks.
The canal was out of service until December 2019.
Sequence & Completion Dates for Actions Listed:
Lessons Learned:
1. This event demonstrates the benefit of having personnel trained in the aspects of dam safety.
2. Having a defined communications for dam safety events was beneficial in this incident and prevented further damage to the power canal.
Recommendations: