Galiano Island Geological Hazard Mapping Assessment

Extracted text (beginning)

[[page 1]]
C.N. RYZUK & ASSOCIATES LTD.
Geotechnical/Materals Engineering

28 Crease Avenue Victoria, B.C. VéZ1S3 Tel (250) 475-3131 Fax: (250) 475-3611

March 31, 2010
File No: 8-4764-2

Islands Trust

#200 - 1627 Fort Street
Victoria, B.C.

V8R 1H8

Attn: Mr. R. Kojima, Island Planner
Dear Sir:

Re: Geological Hazard Mapping Project
Galiano Island, B.C.

We have undertaken an assessment of available geological hazard (geohazard) mapping
methodologies and have provided associated guidance for development of a mapping program for
Galiano Island. Our objective was to identify land that may be potentially hazardous to proposed
development because of a combination of steep slopes and geologic conditions, and provide
recommendations for consideration to restrict development that may increase or exacerbate such
hazardous areas.

Background

We have undertaken similar work for two other Gulf Islands, with North Pender Island being the
pilot project and Salt Spring Island following a similar methodology. In the course of our
development, we reviewed alternative GIS based approaches for developing geohazard mapping,
including the probabilistic infinite slope analysis (PISA) mapping program. These methods all rely
on the same elevation information that has been obtained using recent aerial photography to
generate 2 m contour interval topographic mapping, from which a Digital Elevation Model (DEM)
was created using 5 m pixilation. However, mathematical models are entirely dependent on the
input variables. In such models, slope gradient, geology type and rock fracture characteristics, soil
type, occurrence of surface and ground water, and presence of vegetation are all considered as input
variables, along with seismic acceleration when considered. Of this information, only slope
gradient is readily available from the existing data capture systems at a resolution that would be
appropriate and suitable for development of mapping to be used in prediction and assessment of
geohazards at the scale of most residential properties.

It is also important to note that a model that uses an increased number of variables would inherently
require more aerial photo interpretation and associated field work/ground truthing to confirm that

C.N. Ryzuk & Associates Ltd.

[[page 2]]
C.N. RYZUK & ASSOCIATES LTD.

Islands Trust March 31, 2010
Geological Hazard Mapping — Galiano Island, BC

the model is reasonably representative of the actual ground conditions. With multiple parameters
affecting the selected simulation model in different ways, it becomes more complex to determine
which parameters have the greatest influence on the results. In addition, inclusion of seismic

effects further compounds the problem of identifying the predicted driving mechanism of failure.

Accordingly, we have recommended adopting a form of slope classification on the basis of the
known occurrence of terrain failure on increasingly steep slopes as inclination itself is believed to
be the most reliab

First 3,000 characters. Open the original for the whole document.