Hydraulic Resistance of Large Woody Debris in Step-Pool Channels

9902440Wohl梯级-池型水源渠道的稳定性对下游水沙流量、水生生物和河岸生境具有重要的控制作用。 认识到这一作用，促使越来越多的注意管理和恢复的步骤池渠道的资源管理人员，特别是通过操纵大型木质残体。 然而，我们目前的基本水力学和泥沙输运机制在这些渠道的理解是有限的。 复杂的水力阻力和湍流与粗糙的碎屑，大的木质碎片，并在台阶上的暴跌流创建的速度分布，是很差的近似标准的对数速度分布，例如。 水力阻力和相关的速度分布的详细表征是关键的第一步，在理解的力学步骤池channel.We建议使用水槽实验来操纵的变量，有助于在步骤池渠道的流动阻力，以直接测量的总阻力的比例，由三个组件。 这些组成部分是：颗粒粗糙度，由河床物质的表面摩擦力提供;形式粗糙度，与沿通道沿着规则间隔出现的流动障碍物的上游和下游侧的压力差产生的阻力有关（如床阶）;以及当不沿通道沿着规则地间隔开的单个碎片产生液压阻力时产生的碎片粗糙度。 最近的现场研究表明，现有的方程划分与这三个组成部分的流动阻力不准确的阶梯池渠道的阻力特性。 我们已经开发了一个实验设计，涉及一系列的基本水槽配置，总阻力将在不同的条件下测量渠道梯度和流量。 通过系统地改变水槽配置，我们可以测试假设的相对重要性，每个组件的水力阻力。从拟议的研究结果应该允许量化的水力阻力产生的大型木质残体在步骤池渠道。 这种量化将是非常有用的资源管理人员试图设计和管理率的木材招聘和保留在源头渠道，以应对日益关注的潜在影响，土地利用的碎片装载在源头流。

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.