Forestry on fans: a problem analysis

Abstract

Forested fans are often crossed by roads and their high-volume stands are attractive for harvesting. Gentle slopes of fans belie the fact that hydrogeomorphic hazards can be present. Fans can be the run out zones for debris flows and they can be subject to floods and debris floods. This study assessed the effect of natural hydrogeomorphic processes on forest practices that were undertaken on 55 fans in west central British Columbia. Forest practices aggravated these processes on 41 (74%) fans, leading to increased erosion and destabilization of fan surfaces and stream channels. Identification of hydrogeomorphic hazards is needed to avoid the adverse effects of forestry practices on fans.Keywords: forested fans, forestry on fans, hydrogeomorphic processes, forest practices on fans, forest practices Les cônes d'alluvion boisés sont souvent parcourus de routes et leurs peuplements à fort volume de bois sont attrayants pour l'exploitation forestière. Les pentes douces des cônes d'alluvion cachent le fait que des accidents hydrogéomorphologiques puissent survenir. Les cônes d'alluvion peuvent être des zones de débordement pour les écoulements de débris et peuvent subir des inondations et déverse-ments de débris. Cette étude a évalué l'effet des processus hydrogéomorphologiques naturels sur les pratiques forestières qui ont été entreprises sur 55 cônes d'alluvion dans le centre-ouest de la Colombie-Britannique. Les pratiques forestières ont aggravé ces processus sur 41 (74 %) des cônes d'alluvion, menant à une érosion accrue et une déstabilisation de la surface des cônes d'alluvion et des canaux des cours d'eau. L'identification des risques hydrogéomorphologiques est requise afin d'éviter les effets négatifs des pratiques forestières sur les cônes d'alluvion.

Influence of landslides on biophysical diversity — A perspective from British Columbia

“…Relatively few studies, however, explore plant succession on landslides -most of the work relates to landslide dating (e.g., Corominas et al, 1994;Lang et al, 1999;Wilford et al, 2003). Globally landslide-vegetation studies have been reported in Africa (Lundgren, 1978), the Caribbean (Guariguata, 1990;Dalling, 1994;Walker et al, 1996;Myster and Walker, 1997), Japan (Sakai and Oshawa, 1993;Yamamoto et al, 1995;Yajima et al, Fig.…”

Hydrogeomorphic processes and vegetation: disturbance, process histories, dependencies and interactions

“…The impact of forest harvesting on peak flow response is undisputed; its quantification is challenging because of the confounding influence of roads that have a different, and generally larger, influence on the hydrology than does the felling of trees (Wemple et al , ; Sidle et al , ). Forest management activities commonly aggravate natural hydrogeomorphic processes and ultimately lead to increased erosion (Figure A) and destabilization of fan surfaces and stream channels (Wilford et al , ). In coastal British Columbia (Canada), approximately one‐half of all landslides occur in clearcuts and are not road‐related (Jakob, ), this is why proposed harvesting should be assessed and managed accordingly.…”

“…Woody debris derived from forests stabilizes the bed and banks of channels such that floods and debris floods transport less sediment (Lisle, ), thus limiting the extent of disturbance along the transport path of hydrogeomorphic events and markedly enhancing sediment deposition (Wilford et al , ). Standing vegetation, such as dense alder stands, within the riparian zone increases channel roughness – Manning's roughness coefficient n – by an order of magnitude and thereby significantly reducing sediment transport capability (Ogrosky and Mockus, ).…”

Landslides impacting linear infrastructure in west central British Columbia

“…Debris flows intergrade to debris floods (hyperconcentrated flows) and floods, with decreasing sediment concentrations. Many fans in west central BC are the product of all three processes (Wilford et al 2003). All three processes can destroy infrastructure.…”