Citation | MWH. 2014. Dam safety reviews for the middle and lower Chase River dams. Prepared for City of Nanaimo. |
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Organization | City of Nanaimo |
URL | https://www.nanaimo.ca/assets/Departments/Engineering~Public~Works/Water~Supply/Publications~and~Forms/DSR%20MandL%20Chase%20River%20-%20Final%20(28%20Feb%202014).pdf |
Abstract/Description or Keywords | MWH has prepared this report for the Dam Safety Reviews (DSRs) of the Middle & Lower Chase River Dams for the City of Nanaimo (the City) in accordance with the scope and budget outlined in our proposal dated March 7, 2013. The 2013 DSRs for the Middle & Lower Chase River Dams were carried out in three phases namely: Phase 1 - Document Review including all pertinent documents on the dam history, construction records, where available and previous inspections and investigations Phase 2 – Site Visits & Detailed Work Phase 3 – Report Finalization The Middle Chase River Dam is located in the southern part of the City of Nanaimo and was constructed around 1911 to provide water for coal washing in the early part of the century when coal mining was in full production in Nanaimo. The dam is 13m high and 50m long and is constructed of earth/rockfill shells on either side of a concrete core wall extending from the dam crest and keyed into bedrock. The concrete core is 0.6m thick at the crest elevation and was raised by 0.3m in 1980. The upstream shell is constructed to a slope of 1.5H to 1V and the downstream to a slope of 2H to 1V and was re-constructed in 1980 with the incorporation of a gravel filter drain to intercept seepage. Concentrated seepage was observed in 1992 and an additional gravel filter drain installed near the right abutment to intercept this. The spillway comprises a channel with concrete side walls and a natural rock base on the left side of the dam. The Lower Chase River Dam is located downstream of the Middle Chase River Dam. Like the Middle Chase River Dam it was originally constructed in 1910 to supply water for the nearby Harewood Colliery when this was in production. It was believed that this function was finished around 1945 and now is part of the recreational area called Colliery Dam Park. The Lower Chase River Dam is an earth/rockfill structure approximately 77m long and with a maximum height of about 24m. The dam lies in a narrow steep sided ravine with both abutments founded on what is believed to be till material. Bedrock might be overlain by a veneer of till or possibly channel fill in the center of the ravine. The downstream shell was stabilized in 1980 by a compacted sand and gravel berm to a slope of 2H to 1V and the original upper slope was left at 1.5H to 1V. Drainage ditches were also formed at the abutment and a gravel filter drain to intercept seepage through the dam and abutments. A concrete wall provides the impervious barrier and also forms the front face of the dam. The top 0.6m or so of the wall is 0.3m thick and it then reportedly thickens to 1.2m. It is believed that the wall is founded on till at the base of the dam. The upstream support of the wall is believed to be by a rockfill shell. The spillway is a rectangular concrete lined channel which bifurcates into two channels upstream of the spillway structure. A double span footbridge is constructed over the channel. The spillway is a free overflow structure with the water level controlled by the spillway lip elevation. There are no offtake works from the dam; the two original low level outlet pipes passing through the body of the dam were capped in concrete, the valves removed and the control chamber backfilled in 1980. Both Middle and Lower Chase River Dams have “Extreme” consequence categories in accordance with the 2011 BC regulations. |
Information Type | report |
Regional Watershed | Vancouver Island South |
Sub-watershed if known | Chase River |
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Comments | |
Project status | complete |
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