Water Stewardship Information Sources
| Citation | Miles, M. 2007. Hydrotechnical hazard assessment, Cottonwood Creek Fan: Youbou, BC. Prepared for Youbou Land Group. |
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| Organization | Youbou Land Group |
| URL | http://cvrd.bc.ca/DocumentCenter/Home/View/1140 |
| Abstract/Description or Keywords | A mixed residential and commercial development is proposed for a site located on the north side of Cowichan Lake near Youbou, BC. The property consists of a 2.6 km² fan formed by Cottonwood Creek. This report provides an initial assessment of the hydrotechnical hazards which could result from flooding or channel shifting on Cottonwood Creek. Historical air photo analyses do not show any indication that debris torrents (or channelized high velocity mixtures of water, sediment and debris) have occurred on lower Cottonwood Creek over the period since 1938. This supports similar conclusions reached in the companion geotechnical report prepared by Thurber Consultants Ltd. However, the air photo analysis does indicate that large volumes of coarse textured sediment are periodically transported from the upstream watershed and can cause aggradation of the river bed and lateral channel instability. Modest sized flood flows in November 2006 and January 2007 (return periods are #10 years) mobilized large quantities of sediment, locally eroded the channel banks and caused the formation of sizeable log jams. High water levels locally overflowed into low-lying areas on both sides of the creek. However, these flood waters were of shallow depth and were generally restricted to depressions or remnant channel traces. The higher elevation area west of Cottonwood Creek was not threatened by the winter 2006/2007 events as it was well above flood stage (typically 3 to 4 m). In contrast, flood waters overflowed into depressions or old road beds on the left bank. This resulted in a shallow and generally confined flow of water across the east side of the Cottonwood Creek fan which extended towards the former mill site. The Cottonwood Creek fan consists of three areas which are exposed to varying levels of flood-related hazards. The central portion of the Cottonwood Creek fan consists of the active river channel and adjacent low-lying areas. This corridor conveys flood waters and provides an area for storing sediment and debris being delivered from the upstream watershed. It also provides important biological functions. It is therefore best suited as a riparian reserve as currently proposed on the land use plan. Hydrologic and hydraulic analyses indicate that the area of high ground located to the west of the Cottonwood Creek corridor is located 3 to 4 m above potential flood levels. This terrain is, with one local exception, sufficiently far back from the creek that channel shifting and sediment accumulation will not result in bank erosion concerns. Some additional surveying and hydraulic calculations are required to confirm these results. The initially proposed development plan will need to be slightly modified, such that it is set back (i.e. to the west) from the edge of this higher terrain. The eastern portion of the Cottonwood Creek fan is lower in elevation than the western portion of the fan. This area was subject to local flooding in the winter of 2006/007 due to overbank flow being channelized along abandoned channels and abandoned roads. Flood and erosion protection measures will be required to ensure the proposed residential and commercial properties in this area can be safely developed. This appears to be feasible but will require modifying the initially proposed development plan and additional design work to develop appropriate flood protection measures. |
| Information Type | report |
| Regional Watershed | Vancouver Island South |
| Sub-watershed if known | Cottonwood Creek |
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| Comments | |
| Project status | complete |
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