Graduate Thesis Or Dissertation

Entrainment, transport, and deposition of large woody debris in streams : results from a series of flume experiments

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  • Although there has been extensive research on the geomorphic and ecologic role of large woody debris (LWD) in streams, the dynamics of LWD in streams are poorly known due to various measurement difficulties during floods. In this thesis I present the results of two flume experiments on the dynamics of wood in streams. The first experiment examined the effects of piece interaction on wood transport, while the second evaluated a theoretical model predicting the threshold of movement, and factors controlling the deposition of individual logs. Flume experiments allow us to model the wood movement under a variety of piece geometries, piece concentrations, and hydraulic conditions. The first experiment showed that logs moved in three distinct transport regimes; congested, semi-congested, and uncongested transport. During congested transport the logs move as a single mass and occupy > 33 % of the channel area. Uncongested transport occurs when the logs are moving as individuals and occupy < 10 % of the channel area. Semi-congested transport is intermediate between these two transport regimes and occurs when wood moves in clumps of 2-3 logs. Transport regime was dependent upon the ration of the volumetric input rate of pieces to the flow, and to a lesser degree, the ratios of the piece length to channel width and piece diameter to channel depth. The transport regime was reflected in the deposit. Congested transport deposits have a higher portion of their pieces oriented parallel to flow than uncongested and semi-congested transport. We expect that congested transport will occur in low-order channels where input rates are high and channel geometry is small relative to piece size. Uncongested transport will dominate large channels where input rates are lower relative to flow and channel geometry is large relative to piece size. Our theoretical model and these experiments indicate that the entrainment of individual logs was dependent upon the angle of the piece, and the presence/absence of rootwads. Although previously noted as a first-order control on piece movement, piece length had little effect on the entrainment threshold, but did affect the distance transported. The distance transported decreased with increases in the ratios of the piece length to average channel width (L[subscript log]/w[subscript av]), the piece length to the radius of curvature (L[subscript log]/R[subscript c]), and the piece diameter relative to average depth (D[subscript log]/D[subscript av]). These three ratios comprise the debris roughness. Increased debris roughness caused a general decrease in distance transported. Pieces with high debris roughness can travel further than predicted if they have high momentum, and over 50 % of their channel area deeper than the depth at which the piece floats. These results indicate that flume experiments and theoretical models, tools that have been extensively used to study sediment dynamics, are a useful in examining wood dynamics.
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