DEBRIS FLOWS FROM FOREST LANDSCAPES ALONG THE MAINLAND BRITISH COLUMBIA COAST

Numerous studies have examined land instability along the mainland coast of British Columbia (the ‘Inside Passage’). Typically these studies focused on specific sites and events such as timber harvesting, weather, and/or triggering processes. This study provides an overview of land stability along the entire BC coast; more than 1800 distinct debris flows are documented and categorized in terms of ecological, geographic, and land-form conditions. The data incorporates direct observations, satellite imagery, aerial photographs, and precipitation records.

The largest of these flows exceeds 24 ha, but most are 1.5-5 ha (avg 3 ha). Soils along these waterways are young, thin, and rest upon glacially polished granitic and gneissic bedrock. The forest floor comprises a thick layer of organic matter due to low rates of organic decomposition in these cold, wet environments. Collectively these conditions create an ideal environment for flows, which occur primarily on well-established forest sites on slopes averaging 27°. More than 66% are on southern aspects (SW/S/SE). Approximately half the flows are bounded by latitude 51°N; the average drop is 279 m with a run of 538 m. The flows overwhelmingly start in mature conifer forests, appear to be triggered by storm events, and typically originate within undisturbed forest areas (93%). Sixteen percent of the flows terminate in the coastal waterway; 43% in streams, rivers, lakes, and valley bottoms; the remaining 41% on forested slopes or within a harvest area. On average, about 90 debris flows occur each year, primarily in winter when precipitation adds to hill slope mass, causing the supersaturated soil to become unstable and enabling any slight hill slope disturbance to trigger a debris flow with its subsequent cascading avalanche.