LANDSLIDE HAZARD ASSESSMENTS AND PUBLIC POLICY IN BRITISH COLUMBIA

'Guidelines for Legislated Landslide Assessments for Proposed Residential Development in British Columbia' was published by the Association of Professional Engineers and Geoscientists of BC (APEGBC) in 2006. The document notes several anomalies regarding legislated slope hazard assessments. For example, there are no provincial levels of acceptable landslide safety framed either as slope factors of safety or annual probabilities of partial risk. Therefore, whilst APEGBC members can determine these measures of risk, they should not declare them as acceptable. This issue and others have drawn attention from BC permitting and regulatory agencies.

A few BC permitting agencies utilize threshold levels of partial risk to weigh residential permitting decisions. Professional reports for permit applicants in these jurisdictions must include subjective estimates of landslide probabilities supported by available evidence. The use of such estimates appears to be unique to BC and arises from case law; former guidelines used by the Ministry of Transportation for subdivision approvals; a legislative peculiarity requiring professionals to apply ‘safe for intended use' wording in assessment reports and permitting guidelines developed by the Fraser Valley Regional District, a 14,000 km2 jurisdiction with a number of residential areas proximal to slope hazards. In general, jurisdictions with adopted levels of acceptable landslide risk use the following critical thresholds for partial risk: 10% in 50 years (roughly 500-year return period) for siting and building permits and 0.5% in 50 years (10,000-year return) for subdivision permits in areas exposed to catastrophic hazards. These agencies recognize that many homes are built on previously subdivided land where slope hazards were not recognized earlier. In such cases, threshold permitting criteria are adapted to types of building, the nature of slope hazards and possibilities of protection.

Provincial and APEGBC representatives are preparing to meet to discuss acceptable levels of landslide safety for residential lands and related professional and public-policy matters.