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LETTER: Slocan River Road damage— an example of Perry Ridge instability

From reader Marilyn Burgoon…
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One lane has slumped and the cut-slope above is moving. There are additional cracks in this vicinity. Water is directed through black plastic pipes.

The Geological Hazards Mapping of the Slocan Valley states that, “Mass movement processes involve the downslope movement of surficial material due to gravity. Material moves downhill by rolling, sliding and slumping. In most cases the failures appear to be caused by:

1) High pore water pressure between sediment layers (primarily clay) or possibly along pre-existing fractures or faults within the clay and,

2) Over-steepened scarp faces in places aggravated by recent undercutting of the scarp by the river or road construction.”

The Perry Ridge Water Users Association has requested a copy of the geotechnical assessment from Ben Tanasichuk, Highways Area Manager.

Main roads are monitored by the citizens driving to and from their homes and the highways contractor YRB. Once resource extraction roads are constructed and resources removed, these roads can have similar earth movements that can descend to the valley bottom.

Geologist June Ryder, has said, “Even the most careful planning of roads and cutblocks cannot guarantee that accidents (slides, debris flows, floods) will not occur, a single such event on Perry Ridge, where steep slopes and steep creeks descent to highly sensitive terrain with dense rural settlement, could have disastrous effects.”

Dr. Lee Benda, a geologist at the University of Washington, states harvesting can increase soil water “on the order of 20 to 35 per cent.”

Tara DeCourcy, B.C. government Woodlands Manager, says that B.C. Timber Sales are not planning to go back to extract resources on Perry Ridge for five to seven more years. She stated they do not intend to collect precipitation data and plan to continue using the Equivalent Clearcut Area (ECA) methodology to assess risk.

Hydrologist Allen Isaacson, the co-author of the ECA method has written, “The ECA methodology was not designed for this type of terrain. Without the basic data, how is a statistical analysis of risk possible? What is a 100 year event in this area? What is the annual precipitation or runoff? The basic information needed for any type of risk analysis is lacking. In summary, the whole process (planning) should be done with proper data collection and analysis, or it should be admitted that Perry Ridge is a unique area and not suitable for development.”

PRWUA insists that BC Timber Sales collects the data.

Marilyn Burgoon

Perry Ridge Water Users Association