While the importance of woody debris in rivers has been well-studied within the past few decades, the dynamics of the woody debris jam is often overlooked. This study focuses on the woody debris jam treating it as a complex and porous accumulation of heterogeneous material. Accounting for the momentum extracted as flow passes a debris jam, the drag force on three jams with differing structures, representing varying degrees of porosity, is quantified. Utilizing the drag force equation along with the volume and surface area of the jam, the effect of structure on hydraulics is investigated. A theoretical model for debris jam evolution is hypothesized based on these results and a review of the literature. The results from this study show that (1) natural debris jams are porous structures and (2) piece-size distributions may dictate the evolution of a debris jam and therefore its rate of change in hydraulics.
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