On the validity of the British Columbia Forest Practices Code guidelines for stream culvert discharge design

In the stream culvert discharge design guidelines of the Forest Practices Code (FPC) of British Columbia (BC), the 100-year instantaneous flood (Q 100 ) is assumed to be three times as large as the mean annual flood (Q 2 ) regardless of basin characteristics and location in the province. A regional linear moment analysis of annual maximum flows is used to demonstrate that this assumption is invalid and that Q 100 /Q 2 ratios vary substantially with basin area and climate. For the snowmelt-dominated peak flows in the Columbia and southern Rocky Mountains, Q 100 /Q 2 decreases slightly with increasing drainage area, from 2.3 (1 km 2 ) to 1.9 (100 km 2 ). For the flood peaks generated by rainfall and rain on snow in coastal BC, this range is 3.1–2.6. In the semi-arid Interior Plateau region, variability in Q 100 /Q 2 ratios is most dramatic. For a 10-km 2 basin, the calculated Q 100 /Q 2 ratio of 4.9 is 1.6 times the assumed factor of 3, while for a 1-km 2 basin Q 100 /Q 2 is 7.5 or 2.5 times this factor. Underestimating Q 100 /Q 2 may lead to underdesign and early failure of road culverts, and therefore, current FPC guidelines for estimating the 100-year instantaneous flood may have serious adverse economic and environmental consequences in small Interior Plateau watersheds.