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Report: Final Cowichan Sediment Management Review - Stoltz Slide and Block 51.

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The purpose of the project was to evaluate critical erosion sites on the Cowichan River, to review and comment on previous conceptual restoration designs that were developed for Stoltz Slide, to recommend an action plan for quantifying the relative contributions of sediment from each of the significant sources, and to recommend a potential stabilization strategy for the Stoltz Slide including a preliminary implementation budget.

Author:  Marc Gaboury, M.; LGL Environmental Research Associates and Murray, D.; Kerr Wood Leidal Associates Ltd. Prepared for BC Conservation Foundation

Date Published:  Apr 2005

Report ID:  23902

Audience:  Government and Public

Following the office and field assessments that were conducted during the summer of 2004, a memo outlining preliminary findings for the Stoltz Slide was prepared by Kerr Wood Leidal (KWL) and sent to Craig Wightman of the BC Ministry of Water, Land and Air Protection. The memo identified the need to quantify the relative contribution of sediment from Stoltz Slide in order to know how effective slide stabilization would be at reducing overall suspended sediment loads in the river. A preliminary strategy for stabilization for Stoltz Slide was also presented that included: 1) in-river structures, 2) gully stabilization treatments, and 3) bioengineering measures. A 2 preliminary budget (Class D level) for the implementation of the sediment monitoring and three stabilization components ranged from $895,000 to $1.35M. An overview memo on the findings of the aerial photograph analysis, office review and field investigations for the Stoltz and Block 51 sites was prepared by KWL. The memo provided detailed comments / recommendations on the remedial options submitted by nhc (1996) and Newbury (1996). In summary, it was concluded that the options identified by nhc and Newbury together contain elements that would be partially successful in stabilizing the slide. One of the key issues is to reduce the amount of flow against the toe of the Stoltz Slide. Without a diversion of some of the flow and the establishment of a terrace at the base of the cutbank, it would be extremely difficult to stabilize the base of the cutbank and thus minimize fine sediment inputs. A proposed strategy for stabilization of Stoltz Slide, which included a number of elements previously proposed by nhc and Newbury, was presented. The strategy as developed would be implemented in phases, as follows: Phase 1: Fine sediment source tracking to determine the relative contribution from Stoltz and other significant erosion sites to the overall suspended sediment load in the river. Phase 2: River based measures that include excavating or enlarging a channel on the inside of the meander bend, constructing a partial weir in the mainstem, and constructing a terrace with bank protection at the toe of the eroding cutbank. Phase 3: Gully stabilization options that include placing an inverted filter of sand and gravel over the face of the silt, constructing horizontal drains on the face of the slope, and flattening the overall slope by bulldozing sand and gravel down from the crest of the slope. Phase 4: Bioengineering treatments, to be used in conjunction with other measures, include: gully check dams, live staking and willow wattles, willow brush layers, and anchored large woody debris. A brief investigation of erosion problems in the Block 51 area found that the site represents a less suitable stabilization project in comparison to Stoltz Slide. The relative contribution of fine sediments to the river from this site are believed to be significantly smaller than the Stoltz site for most river flows and the channel remains highly susceptible to lateral instability. Further work on the fine sediment source tracking project in 2005, under Cowichan Treaty and Pacific Salmon Commission funding, will determine definitively what the sediment contribution is from Block 51. If the sediment analysis indicates that stabilizing the cutbanks 3 in Block 51 is feasible, then a phased approach that includes river based measures and bioengineering treatments is also recommended. Based on our preliminary review of the Stoltz Slide and Block 51 areas, we have provided a strategy to move forward on this long-standing sediment management issue. Further assessments and field work will establish the feasibility for stabilization measures at the Stoltz Slide and Block 51 areas, with initial rehabilitation designs likely proceeding in Summer 2005. This project was co-funded by Living Rivers-Georgia Basin/Vancouver Island (2006-11), a program of the BC Living Rivers Trust Fund.

Report Type
  Fish and Aquatic Habitat Information
  Fish and Fish Habitat - Channel Assessment
  Fish and Fish Habitat - Habitat and Stream Assessment
  Fish and Fish Habitat - Restoration
  Fish Species - Brown Trout - Salmo trutta
  Fish Species - Chum Salmon - Oncorhynchus keta
  Fish Species - Coastal Cutthroat Trout - O. clarki clarki
  Fish Species - Coho Salmon - Oncorhynchus kisutch
  Fish Species - Cutthroat Trout (General) - Oncorhynchus clarki
  Fish Species - Dolly Varden Char - Salvelinus malma
  Fish Species - Rainbow Trout - Oncorhynchus mykiss
  Fish Species - Steelhead - Oncorhynchus mykiss
  Region - Vancouver Island
  Watershed Groups - 920 - Vancouver Island (East) Rivers
  Water Information - Channel Morphology
  Water Information - Channel Surveys
  Water Information - Hydrometric / Hydrology
  Water Information - Restoration
  Water Information - Water Quality

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