An unusually large debris flow at Hummingbird Creek, Mara Lake, British Columbia

On 11 July 1997 a very large debris flow occurred at Hummingbird Creek, Mara Lake, British Columbia. Long-term antecedent precipitation was record breaking, whereas short-term precipitation at Salmon Arm did not exceed the 2 year return period for intensity. A 25 000 m(3) debris avalanche was initiated downstream of a forest road culvert, which drained a small catchment that had been artificially increased by a factor of three. The debris avalanche entered the channel of Hummingbird Creek and triggered a debris flow. Velocities were back-calculated using the forced vortex equation and multiplied by the cross-sectional area to obtain peak discharge estimates that ranged from 600 to 1000 m(3)/s. Approximately 92 000 m(3) of sediment was deposited during this event, which makes it the largest nonvolcanic debris flow recorded in British Columbia to date. A three-dimensional runout model was used to simulate this event. Results are in fair agreement with the observed behaviour. This study emphasizes the need to carefully manage forest resources with high downstream risks and the need for hazard assessments prior to development on alluvial fans.