Brush accumulation behind logjams with a lower gap: formation and impact on backwater rise

Logjams with a gap at the bed form naturally in small channels and are used in engineering practice to reestablish the hydraulic and ecological benefits associated with logjam presence in riparian systems while maintaining river connectivity at base flow. The accumulation of wood pieces acts as a porous obstruction. The backwater rise and velocity beneath the jam are key factors in determining the ecologic, geomorphic, and flood risk impact of a jam. We present recent results demonstrating that the distribution of flow through and beneath the jam satisfies a two-box, momentum-based model constrained by drag generated in the jam, momentum loss in flow through the lower gap, and net pressure force. The fraction of discharge beneath the jam increases with jam resistance. Backwater rise follows established sluice gate models, increasing with discharge beneath the jam and thus with jam resistance. The effect of brush accumulation upstream of the jam on backwater rise and velocity beneath the jam is examined using flume experiments.