OUTBREAK FLOODS AT CANYON CREEK - A HAZARD ASSESSMENT

Canyon Creek is a 79 km² watershed that discharges to the North Fork of the Nooksack River near the town of Glacier, Washington (about 65 km east of Bellingham). Development on the 100 ha fan includes more than 40 houses and the Mount Baker Highway. The fan reach is about 1.1 km long with an average channel gradient of 2.5%. In November 1989, a large debris flood occurred on Canyon Creek destroying one house. A smaller event in November 1990 destroyed three additional houses. The extent of damage in 1990 was caused by up to 8 m of aggradation from the 1989 debris flood. To protect existing development, Whatcom County constructed an armoured berm in 1994 that extended from the fan apex to the creek mouth. The USFS estimated that the 1989 event had a peak flow of about 455 m³/s, which is about three times larger than the estimated Q₁₀₀ of 170 m³/s. In contrast, the peak flow about 7 km upstream of the fan apex was estimated at 87 m³/s. The significant increase in discharge between the two reaches, especially in the absence of major tributaries, indicates that the 1989 event was the result of an outbreak flood from a landslide dam or the collapse of a large log jam.

Whatcom County felt that continued maintenance of the existing berm would not provide adequate long-term protection and obtained funding to acquire lots on the hazardous portion of the fan. Kerr Wood Leidal Associates was retained to determine whether the proposed buyout reduced the debris flood risk to an acceptable level. The investigation concluded that the most probable location for a large landslide dam is about 5 km upstream of the fan apex where two large earthflows are being undercut by Canyon Creek. The 500-year return period was selected as the design criterion for debris floods at Canyon Creek. FLDWAV software was used to model outbreak floods for different landslide dam heights. The study concluded that the design debris flood could have a peak discharge of 710 m³/s. This event was further modelled with FLO-2D software to estimate maximum flow depth and velocity on the fan. Based on the modelling results, it was concluded that the proposed buyout would be an effective risk reduction measure as it incorporates most of the highest hazard land on the fan. Site-specific land use regulations were developed for the remainder of the fan.