Bulkley Valley Research Centre - Science in the Public Interest

Sustainable Forest Management on Alluvial and Colluvial Fans

Project Reference Number: 2003-03

Project Status: Complete

Led by: Dave Wilford, PhD, Research Hydrologist, Ministry of Forests and Range, Smithers

Matt Sakals, Graduate Student UBC

Funder: Ministry of Forests

Fans are the moderately low-gradient, conical-shaped landscape features formed by the deposition of sands, gravels and cobbles when confined streams enter a larger valley.  A central feature of most fans is one or more unconfined stream channels. Characteristically, forested fans are productive sites with gentle gradients and easy access that appear to offer low-cost harvesting opportunities – but they are subject to episodic floods, debris floods and debris flows (collectively termed hydrogeomorphic processes), shifting stream channels and unanticipated sediment deposition that make harvesting and road-building problematic.

This project was started by the B.C Forest Service in 2000 with the objective of developing a hazard classification for forestry on fans.  In an assessment of 65 fans between Terrace and Burns Lake, a range of site and watershed features were identified that can be used to predict hydrogeomorphic hazards as well as a series of appropriate prescriptions for sustainable forest management on fans.

In 2003, fieldwork assisted by the Bulkley Valley Research Centre found that hydrogeomorphic activity was present on 83% of the 53 additional fans sampled.  This fieldwork confirmed that fans must be considered as geomorphically active and special forest management recognition must be given to them.  Results from the 2003 field season are being used to improve the predictive models. The fieldwork confirmed that site and aerial photograph features are valid, in the study area and in other areas around the province.  Results have been presented in workshops and conferences around B.C. and in a series of publications from professional newsletters to peer-reviewed scientific journals.  In 2004, further extension activities are planned, and a series of collaborations with other scientists to apply results and undertake regionally focused research in other parts of the province.

Fans are identified as a special feature in the new Forest and Range Practices Act, which replaced the Forest Practices Code in 2004.  This project is providing the scientific support for such policies and the extension is providing field practitioners with the tools to undertake forest management on fans in a sustainable manner.

Abstract:

Debris flows, debris floods and floods in mountainous areas are responsible for loss of life and damage to infrastructure, making it important to recognize these hazards in the early stage of planning land developments.  Detailed terrain information is seldom available and basic watershed morphometrics must be used for hazard identification.  An existing model uses watershed area and relief (the Melton ration) to differentiate watersheds prone to flooding from those subject to debris flows and debris floods.  However, the hazards related to debris flows and debris floods are not the same, requiring further differentiation.  Here, we demonstrate that a model using watershed length combined with the Melton ratio can be used to differentiate debris-flow and debris-flood prone watersheds.  This model was tested on 65 alluvial and colluvial fans in west central British Columbia, Canada, that were examined in the field.  The model correctly identified 92% of the debris-flow, 83% of the debris-flood and 88% of the flood watersheds.  With adaptation for different regional conditions, the use of basic watershed morphometrics could assist land managers, scientists and engineers with the identification of hydrogeomorphic hazards on fans and elsewhere.

A copy of the following article may be obtained from the senior author by contacting

Dave Wilford at dave.wilford at gov.bc.ca

"Fans with Forests: contemporary hydrogeomorphic processes on fans with forests in west central British Columbia, Canada."

 

Related Reports

Publication Date Report Title Authors
2005 Dendroecology: A Guide for Using Trees to Date Geomorphic and Hydrologic Events - LMH 58 David Wilford, Paolo Cherubini and Matt Sakals
2005 Forest Management on Fans: Hydrogeomorphic Hazards and General Prescriptions - LMH 57 D.J. Wilford, M.E. Sakals and J.L. Innes