Modeling entrainment of waterlogged large wood in stream channels

Abstract

Research has led to an understanding of how floatable wood provides physical complexity and habitat in streams, but little is known about large wood that does not float because it is waterlogged, mineralized, or naturally heavier than water. To better understand the dynamics of this largely unrecognized component of streams, I derived a theoretical model for predicting entrainment of waterlogged logs, then constructed prototype logs to simulate waterlogged wood and used them to measure log friction and critical flow velocities at log entrainment in an alluvial creek. I then modeled critical velocities under the same conditions entrainment was observed. The model underpredicted critical velocities by an average of 24%, with a range of error from 8% to −69%. Sensitivity analyses indicated that reducing lift and drag coefficients improved the average error, but not the error range. Lack of unique drag coefficients for prototype logs, winnowing of bed material from around prototype logs during entrainment tests, error estimating friction for logs approaching neutral buoyancy, and the variability of flow velocity common in streams were probable causes of error. Observed critical velocities that entrained prototype logs averaged 0.70 m s−1. Critical velocities and log friction increased with branch length, bed roughness, and log density, but only friction increased with log length. Friction coefficients averaged 0.62 and increased with branch length and bed roughness. This model and existing equations for predicting movement of buoyant large wood can be used to compute budgets for logs with a range of attributes in streams.