Document Details

Windthrow Modelling
Mitchell, Stephen J.
Damaging winds seriously impact the intent and implementation of integrated resource management planning. There has been a history of damage from recurring storm winds in coastal British Columbia associated with extra-tropical cyclones. These large scale Pacific low pressure systems interact with local geography and terrain and produce widespread damage to forest edges recently exposed by harvesting. By building large datasets of cutblock edge segments, it is possible to analyze the relationship between edge windthrow probability, and wind, topographic, stand and management variables. Windthrow occurrence along cutblock boundaries was mapped using aerial photographs and GoogleEarth images. Using ArcView Geographic Information System (GIS), buffers 25m deep were created adjacent to the edge of cutblocks harvested between 1994 and 2004 for 11 sample mapsheets within the Chilliwack Forest District. Each of these buffers was then divided into 25m long segments. A total of 6198 forested segments were obtained and used to study the relationship between cutblock edge windthrow and other stand level variables. Five percent of segments had greater than 30% of segment area damaged and 20% loss of canopy. Segments at mid-elevations, windward facing boundaries, and taller stands were more likely to be wind damaged. Douglas-fir dominated stands suffered higher loss from windthrow than other species, and early mature stands of this species were most vulnerable. Logistic regression models were fit using wind, topographic, stand and management variables to predict probability of damage. The best-fit model had a predictive accuracy of 73%. Models fit in earlier studies for locations in coastal BC gave good relative predictions for the Chilliwack dataset. This indicates a general consistency in the factors that contribute to windthrow probability. These models and the resulting hazard maps can be used for landscape and stand level analysis and planning. Detailed cutblock design and windfirming prescriptions should be based on field inspection.
Report Number
Final Report Chilliwack
Final Report Queen Charlottes
Final Report Squamish

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