Water Stewardship Information Sources
| Citation | Kerr Wood Leidal Associates Limited. 2011. Cowichan Lake Erosion Assessment, Draft Report. Prepared for BC Conservation Foundation. |
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| Organization | BCCF |
| URL | http://www.cowichanwatershedboard.ca/sites/default/files/CowichanLakeErosionAssessment_KWL_201102_Draft.pdf |
| Abstract/Description or Keywords | Under the Cowichan Basin Water Management Plan (2007), there is a recommendation to raise seasonal storage on the lake by 30 cm in April – May to increase flow security for the Cowichan River downstream of the Cowichan Lake weir. Several property owners have questioned whether the recommended storage increase would result in significant erosion issues. Given these concerns, there is interest in investigating the nature and root causes of current erosion, as well as in determining whether a higher seasonal lake level above the weir would add to erosion. Cowichan Lake has a range of shoreline types including cohesive materials, uncohesive materials and manmade. The most common shoreline material is uncohesive sediments such as cobble, gravel and sand. Erosion around the lake is primarily due to waves, which can be generated either by wind or vessels. Wind data for the area suggest that the dominant wind direction is westerly or southerly, which reflects the prevailing topography. Several sites which are representative of shoreline and wave exposure conditions were selected for detailed examination. Low severity erosion was observed at all the sites around the lake including locally eroded shoreline profiles and dunes, exposed seawall footings and exposed tree roots. Several potential erosion mechanisms were identified, and their relative importance at each site was quantified and ranked. The most important erosion mechanism is thought to be disruption of sediment transport due to seawall and groyne construction, followed by removal of shoreline vegetation, vessel wake waves, changes in water level regime due to Cowichan Lake weir installation and historical log booming practices. Water levels were analysed using a hydraulic routing model to estimate the effects of the proposed weir raising of 30 cm (from 162.37 m GD to 162.67 m GD). Under the proposed scenario, extreme high water levels are unchanged. For lower water levels, the water level at a given exceedance probability is increased by about 0.25 m on average. The duration of exposure to different water levels was also assessed: elevations between 162.44 m GD and 162.94 m GD would see an 17% percent increase in exposure, while elevations between 161.44 m GD and 162.44 m GD would see a 7% to 12% decrease in exposure. As a result of the proposed weir change, it is likely that some shoreline reshaping due to wave action would occur in the 161.4 m GD to 163.0 m GD elevation range. The shoreline reshaping may take many years and is unlikely to have recreational impacts but could adversely affect some structures and vegetation while providing improved conditions for other structures. Other on-going processes unrelated to the proposed weir raising could also affect future shoreline erosion. These processes include increasing boat-generated waves, removal of shoreline vegetation over an increasingly large proportion of the shoreline, construction of additional sea walls or groynes and climate change. |
| Information Type | report |
| Regional Watershed | Vancouver Island South |
| Sub-watershed if known | Cowichan |
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