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UBC Theses and Dissertations

Forest stand characteristics as indicators of hydrogeomorphic activity on fans Wilford, David John


Forested fans are common landforms in west central British Columbia. They can be subject to hydrogeomorphic processes ranging from debris flows to floods carrying bedload and woody debris. These processes represent hazards for forest management activities such as road construction and harvesting. Conversely, forest management activities can exacerbate the effect of natural hydrogeomorphic processes, increasing the level of disturbance on fan surfaces and in stream channels. This thesis presents the scientific basis for a hazard classification of fans for forest management. The classification is based on forest stand characteristics, airphoto and site features, and basic watershed attributes. Forest stands are used to determine the power of hydrogeomorphic events (floods, debris floods, and debris flows). High-power events clear swaths through a forest stand, which lead to the establishment of distinctive cohorts. Low-power events lead to the deposition of sediments under a forest canopy, which generally results in growth responses rather than mortality or removal of the forest stand. In cases where there is mortality, tree stems remain in situ rather than being cleared from the affected area. Site features of sediment deposits are used to determine the hydrogeomorphic process. Watershed attributes were determined for identifying hydrogeomorphic processes, power and disturbance extent level. The attributes are basic morphometric parameters such as watershed area and relief, and most can be measured directly from topographic maps without the use of geographic information systems (GIS). Dendroecological techniques were used to determine the number of events occurring on study fans. Regression equations were developed to predict the number of events in the past 50 years. The independent variables were watershed attributes that can be determined by a combination of topographic map measurements and GIS. An examination of 55 fans with forestry activities provides a comprehensive hazard perspective on key fan and watershed attributes, and on the influence of road construction and harvesting prescriptions in zones of fans that are subjective to active hydrogeomorphic processes. The hazard classification and the forestry activity review provide the first comprehensive basis for sustainable forest management on fans in British Columbia.

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