Hydraulic Resistance of Large Woody Debris in Step-Pool Channels

[9902440woh]阶梯池型水源通道的稳定性对下游的水沙流量以及水生和河岸生境具有重要的控制作用。认识到这一作用，促使资源管理人员日益注意管理和恢复台阶池通道，特别是通过处理大型木屑。然而，我们目前对在这些渠道中运行的基本水力学和泥沙输运机制的了解是有限的。例如，复杂的水力阻力和湍流与粗碎屑、大型木质碎屑和台阶上的急流有关，造成了标准对数速度剖面难以近似的速度分布。水力阻力和相关速度分布的详细表征是理解阶梯-池通道力学的关键的第一步。我们建议使用水槽实验来操纵影响阶梯池通道流动阻力的变量，以便直接测量这三个组成部分所产生的总阻力的比例。这些成分是颗粒粗糙度，它是由床料的表面摩擦提供的；形成粗糙度，这与沿通道（如床阶）有规则间隔的流动障碍的上游和下游两侧的压力差产生的阻力有关；碎片的粗糙度，当碎片沿着河道没有规律地间隔时产生的水力阻力。最近的现场研究表明，现有的与这三个分量相关的分配流动阻力方程不能准确地表征阶梯池通道的阻力。我们已经开发了一个实验设计，涉及一系列基本水槽配置，在不同的渠道梯度和流量条件下，将测量总阻力。通过系统地改变水槽结构，我们可以检验关于水力阻力各组成部分相对重要性的假设。拟议研究的结果应该可以量化台阶池通道中大型木质碎屑产生的水力阻力。这种量化对资源管理人员非常有用，他们试图设计和管理水源渠道的木材吸收和保留率，以回应人们对土地使用对水源溪流中碎片负荷的潜在影响的日益关注。

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.