Leakage Mysteries at Kootenay Canal Generating Station, British Columbia

The Kootenay Canal Generating Station diverts water from the Kootenay River and conveys it along a 4.5 km long canal. The canal terminates in a concrete lined forebay section that is contained on its downstream end by conventional concrete gravity dam structures. From here the flow drops about 82 m through steel penstocks and a 560 MW powerhouse before being discharged back into the Kootenay River. The canal is a cut and fill structure, incised into the southern flank of the Kootenay River valley. The spoil from the necessary excavations was used to construct concrete faced rockfill dam embankments that contain the canal on its northern, downslope side. In 2003, a leak of about 250 litres/s (3,963 USGPM) emerged from the toe of the forebay section of the canal embankment and lasted for 3 days. ROV inspection determined that the leak originated from the last concrete liner slab on the north side of the forebay. This slab, immediately adjacent to the north concrete gravity dam, had settled 175 mm. Later review of historical records showed that this slab had been the source of a similar leak which occurred shortly after first filling of the canal in 1976. Mysteriously, the leaks appeared some 85 m upstream of the source. Also curious, a weir connected to a seepage collection system built into the north forebay embankment did not respond to either leak. The canal was dewatered in 2009 to repair the settled north side slab as well as to treat the suspected source of leakage originating on the south side of the forebay. The north side repairs were successful but upon re-watering, the south side leakage continued and for several weeks flowed with renewed vigour – Kootenay Canal’s third mystery. In response, a detailed geologic model of the forebay was constructed; construction records were reviewed; additional instrumentation was installed; new and existing performance data was carefully scrutinized and drain pipe camera inspections were carried out. These investigations resolved the specific questions concerning the Kootenay Canal’s performance and also shed light on concrete faced rockfill dam (CFRD) performance more generally. 31 pp, 1 reference.