Hydraulic Resistance of Large Woody Debris in Step-Pool Channels

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9902440WohlThe stability of headwater channels with step-pool morphologies exerts an important control on downstream water and sediment discharge, and aquatic and riparian habitat. Recognition of this role has prompted increasing attention to management and restoration of step-pool channels by resource managers, particularly through the manipulation of large woody debris. However, our present understanding of the basic hydraulics and sediment transport mechanisms operating in these channels is limited. The complex hydraulic resistance and turbulence associated with coarse clasts, large woody debris, and plunging flow over the steps creates velocity distributions that are poorly approximated by the standard log-velocity profile, for example. Detailed characterization of hydraulic resistance and associated velocity distributions is a key first step in understanding the mechanics of step-pool channels.We propose to use flume experiments to manipulate the variables contributing to flow resistance in step-pool channels in order to directly measure the proportion of total resistance produced by each of three components. These components are grain roughness, which is provided by the skin friction of the bed material; form roughness, which is related to the drag produced by pressure differences on the upstream and downstream sides of an obstacle to flow that occurs at regular intervals along the channel (such as bed-steps); and debris roughness, produced when individual pieces of debris that are not regularly spaced along the channel create hydraulic resistance. Recent field research indicates that existing equations for partitioning flow resistance associated with these three components do not accurately characterize resistance in step-pool channels. We have developed an experimental design involving a series of basic flume configurations for which total resistance will be measured under varying conditions of channel gradient and discharge. By systematically altering flume configuration, we can test hypotheses regarding the relative importance of each component of hydraulic resistance.The results from the proposed research should allow quantification of the hydraulic resistance produced by large woody debris in step-pool channels. This quantification will be very useful to resource managers attempting to design and manage rates of wood recruitment and retention in headwater channels in response to increasing concern over the potential impacts of land use on debris loading in headwater streams.