A Slippery Slope Scenario?

A Slippery Slope Scenario?

Posted on January 18, 2014 at 5:00 PM

The District of North Vancouver Parks had informed me they intend to “try” to stabilize the scarp and slopes west of Mountain View Park with some possible tree and native plantings, rather than to discontinue the damaging mountain biking and trail building activities there…

A small Lesson in Geomorphology:

There is a critical relationship between geomorphology, surface drainage and groundwater.  In order to appease the mountain bikers (NSMBA), this critical relationship has been ignored by the District, greatly compromising the stability of the area, leaving us with a “slippery slope” scenario…

Much of the groundwater in the area is the result of small surface streams which disappear into the granular aquifers at the bottom of steep slopes or concentration of groundwater at the contact of ablation and basal till.

**Groundwater is one of the main elements of slope stability since effective stress is controlled primarily by water. Any activity or development that affects the surface or subsurface drainage will therefore influence the effective stress and in turn slope stability.**

(Pretty clear warning against all the mtb riding and digging, drainage tampering, and ongoing development to accommodate off-road mountain dirt biking on the slopes... )

A recommended Geotechnical Report should have been completed long ago.

Geological Legend:

1.      Alluvium: sand, gravel, along present streams.

2.      Colluvium: loose silt, sand, gravel, soil creep and slope wash material, 30-40% slope.

3.      Outwash: bedded sand and gravel terrace, 7% slope.

4.      Till, silty, sandy, boulder, local pockets of gravel, local relief 15 feet, 22% slope.

5.      Till, silty, sandy, boulder, local relief 10feet, 19% slope.

*Representative stratigraphic section

·  Large erratic

n.b. Slump deposits in easterly flowing stream immediately north of boundary.

All colluvial areas are subject to soil creep and other mass movement  processes. The surface should not be disturbed or developed and care should be taken to preserve the cover of vegetation to guard against gully erosion.  Any development should avoid all areas mapped as colluviums and alluvium. (view maps)

(Any mountain bike trail building and riding on greater than a 10% slope grade is not sustainable, in any case – not just on colluvium material.)

The system of natural drainage channels must not be modified, disturbed or diverted without serious consideration of theconsequences of such action. The two main stream channels  in the southern part of the study area should not be disturbed in any way.

The edge of the scarp forming the eastern and northern boundary of the area and other steep slopes are susceptible to mass movement.

Removal of soil, forest undercover and vegetation has the effect  of tremendously increasing the amount of surface run-off and hence creating possible erosion hazards.

(Source: Twin Lakes Study No.2 Site Reconnaissance, 1971)

(Well, so much for the warnings, circa 1970...) 

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Meanwhile, there may be a small glimmer of hope...:

http://www.fonvca.org/letters/2013/18nov-to/John_Sharpe_8dec2013.pdf

...In Ms. Rogers letter to me she writes that "The parks Department is currently in discussion to consider some independent environment review to evaluate the condition of some of the key trails. We agree that it would provide opportunity to benchmark the condition of the trails"...