Effects of wood on debris flow runout in small mountain watersheds

Debris flows have typically been viewed as two‐phase mixtures of sediment and water, but in forested mountain landscapes, wood can represent a sizable fraction of total flow volume. The effects of this third phase on flow behavior are poorly understood. To evaluate whether wood can have a significant effect on debris flow runout in small mountainous watersheds, we used a landscape‐scale model combining empirical, stochastic, and physical submodels of storms, fires, forest growth, tree fall, wood decay, soil production and diffusion, landslide initiation, debris flow runout, and fluvial sediment transport. We examined changes in the cumulative distribution function of debris flow runout lengths in a small (2 km 2 ) watershed in the Oregon Coast Range due to presence or absence of two hypothesized effects of wood: (1) velocity reduction due to entrainment of wood in the runout path and (2) velocity reduction due to changes in flow direction angle. The model was calibrated such that the distribution for simulations including both effects was similar to that measured in the study basin, and amounts of wood in the simulation and the field, both fallen in small valleys and incorporated by debris flows, were comparable. Removal of either effect, or both, significantly shifted runout length distributions to longer lengths. Simulations and field observations indicate that with wood, fluvial transport is a significant source of sediment output, few debris flows reach the outlet, and debris flow deposits are widely distributed throughout the network. Simulations indicate that without wood, basin sediment yield greatly increases, that yield is dominated by longer‐runout debris flows, and that debris flow deposits are concentrated in the low‐gradient reach near the outlet.