Phillips, J., Veldhuisen, C. and Olis, M., 2009. Using channel geometry to simplify the use of the sizing table for culverts on Type N streams. Skagit River System Cooperative, La Conner, WA. pp. 15.

Although forest roads can adversely impact watershed processes by adding sediment and
constraining stream channels, adequately sized road crossing structures can reduce such
impacts. According to Washington State Forest Practice Rules, Type N Water crossing
structures must be sized to accommodate the “100 year flood event with consideration for
the passage of debris” (WAC 222-24-040). The Forest Practices Board Manual describes
three approved methods for determining culvert sizing: A) the Sizing Table Method; B)
the Bankfull Width Method and C) the Hydraulic Design Method. In this paper, we
address the Sizing Table Method (Table 1), which we observe to be commonly used in
the Skagit Basin.